摘要:

Presidential Address Transition-metal Complexes of Some Perfluoro-ligands By Sir Ronald Nyholm F.R.S. DEPARTMENT OF CHEMISTRY UNIVERSITY COLLEGE LONDON GOWER ST. LONDON W.C.1 (Delivered at Nottingham 16th April 1969) Introduction The past two decades have seen a rapid development in organic fluorine chemistry. This has resulted in the availability of suitable intermediates from which fluoro- and perfluoro-ligands can now be synthesised readily and those interested need not be specially skilled in synthetic organo-fluorine chemistry as such. This has led to the preparation and use of many new fluorinated ligands and the range of these is now quite extensive. We use the term ‘fluoro-ligand’ to mean one in which some of the hydrogen atoms have been replaced by fluorine e.g. (CF,CH,),As; a ‘perfluoro-ligand’ is one in which all hydrogen atoms have been replaced by fluorine e.g.or The complexes obtained between these fluorinated ligands and transition metals in particular have often been found to display marked stability and to possess unusual properties. Our interest in fluoro-ligands was derived initially from a desire to incorporate in a chelate molecule the equivalent of a C1- ion. Thus the compound isformally the equivalent of one mole of PhAsMe and one mole of a weak acid. Ligands such as F F F F F F F F have therefore been investigated as the equivalent CI- i.e. anions of strong acids. The properties of chelate ligands such as 1 Transition-metal 1. 2. 3. 4. 5. 6. Complexes of Some Perfluoro-ligands c which combine a strong acid and a neutral donor should prove of considerable interest.We discuss here the following aspects of this subject Relevant properties of fluorine. Classification of perfluoro-ligands. Survey of complexes of PF3 and other ligands in which F is attached to the donor atom. Synthesis and properties of perfluoro-alkyl and perfluoro-aryl complexes. The nature of the metal-carbon bond in perfluoro-alkyl and -aryl metal complexes. Complexes of ligands in which fluorine is substituted on atoms distant from the donor atom. Particular attention is paid to perfluoro-ligands but some reference will be made to partially substituted ligands (e.g. tetrafluorodiarsine). 1 Relevant Properties of Fluorine The properties of fluorine are dependent partly upon its strong electron attract- ing power expressed by the highest electronegativity of all elements partly upon the presence of lone pairs of electrons potentially available for n bonding to other atoms and partly upon its size.It is useful to summarise in advance certain properties of fluorine in relation to the other halogens and hydrogen in Table I. Table 1 Properties of the halogens and hydrogen Ionisation potential (eV) 13.595 17-42 13.01 11.84 10.45 Electron affinity (eV) 0.7 3.48 3.69 3.45 3.15 Pauling electronegativity 2.20 3.98 3-16 2.96 2.66 Dissociation of X,l(kcal/mole) 103.3 37.0 57.1 45.4 35.6 Covalent radius (A) 0.30 0.64 0-99 1.14 1.33 Bond energy of H-X1(kcal/mole) 103.3 134.9 102-2 86-5 70.5 Element H F el Br I The high electronegativity of fluorine explains the strong inductive effect which leads to an increase in the acid strength of compounds such as perfluorophenol as compared with phenol.Also since the strength of a covalent bond for bonds of the same type is roughly proportional to the product of the electronegativities of the bonded atoms covalent bonds to fluorine are specially strong (cf the hydrogen halides in Table 1). The inductive effect of the fluorine atoms diminishes A. G. Gaydon ‘Dissociation Energies’ Chapman and Hall London 3rd edn. 1968. 2 Nyholm as the position of substitution from the donor atom increases as discussed below. The lone pairs on fluorine atoms combined with the small covalent radius of fluorine can increase or decrease the strength of the bond to the atom to which fluorine is attached depending upon whether there are vacant orbitals of the correct symmetry on the second atom (as in PF,) or lone pairs thereon (as in F,) respectively.The higher inter-electron repulsion in the 2s2p3 shell of the fluorine atom as compared with that in the 3s3p3 shell of chlorine is generally assumed to explain why the electron affinity of fluorine is lower than chlorine. On the other hand the proximity of the lone pairs on the two adjacent fluorine atoms in Fz explains its low bond energy compared with that of Cl, even though the electronegativity product of fluorine is larger than that of chlorine. 2 Classification of Fluorine-substituted Ligands It is convenient to classify these with three features in mind. First the position of the fluorine substituent with respect to the donor atom; secondly whether the donor atom carries a charge; and thirdly the extent to which n delocalisa- tion occurs in the ligand.Table 2 shows how such a classification enables ligands Table 2 Types of' perfluoro-ligands Examples Uncharged R-F H-F (H Bonding only) Charged F- (Class A Ligand) Uncharged NF3 PF3 SF. Charged -CF3- -PF,- Uncharged N(CF 3 P(CF,) { { 1. Fluoride Ion 2. FonDonor Atom -_____--_-* 3. F on Atom 4. F o n Atom TWO Removed from Donor F3 c,n,s- A n- t * Ligands shown below this line have delocalised ?r systems. t The (unstable) CF,S- anion derived from MeSH is also in this category. 3 Transition-metal Complexes of Some Perfzuoro-ligands to be subdivided in this way. In this survey we consider (T bonded perfluoro- ligands only. Only a few wtype perfluoro-complexes e.g. of C,F4 are known.2 3 Survey of Complexes of PF8 and Other Ligands in which F is Attached to the Donor Atom (i) Direct attachment of the fluoride ion occurs in covaIent metallic fluorides.The fluoride ion behaves as a good ligand towards Class A metals (e.g. Mg2+ A13+ Th4+) even in aqueous solution; for these metals stability constant data indicate that the halide ions are attached in the order F- > Cl- > Br- > I-. However F- can also form strong bonds to Class B metals in the absence of water and similar solvents as evidenced by the stabilisation of PtVI in PtF,. It should be borne in mind that if a Class B metal ion forms a covalent M-F bond this will usually be stronger than an M-CI an M a r or an M-1 bond but in the presence of water the highest solvation energy of the F- ion often leads to a lower stability constant in solution as compared with the other halide ions.Except for hydrogen bonding between the fluorine atom and a proton in the [HF& ion fluorine in alkyl or aryl halides and in hydrogen fluorides shows virtually no tendency to donate its lone pairs to metal atoms. (ii) When fluorine is attached to the donor atom its effect is to decrease the donor capacity of the lone pair in (J bond formation but when there are vacant orbitals on the donor atom it can enhance the acceptor capacity of that atom provided that the acceptor vacant orbital is of the appropriate size and symmetry to overlap with a filled d orbital of the metal. Considering first uncharged ligands the Group V and VI fluorides only need concern us. Substitution of hydrogen by fluorine in ammonia or water greatly diminishes (T donor capacity to metals and of course there are no vacant d orbitals to receive electrons in dn bond formation.(The effect of antibondingp orbitals on the N or 0 appears to be negligible in saturated compounds but they may be important in complexes of pentafluoropyridine.) Of the later Group V elements the fluorides of PF3 only appear to have been investigated in detail although AsF, SbF, and BiF may behave similarly unless they act as fluorinating agents. Group VI fluorides of the type SF4 with one lone pair might be expected to behave like PF but so far SF has been found to behave mainly as a reactive fluorinating agent.3 Following the first preparation of Ni(PF3)4 by Wilkinson? we are indebted to Kruck for most of the detailed investigation of the chemistry of PF com- plexes summarised in his review? This ligand is capable of stabilising transition metals in low oxidation states and derivatives corresponding to complete replacement of carbon monoxide from the equivalent metal carbonyl can be prepared as shown in Table 3.It is noteworthy however that PF forms deriva- tives such as Pd(PF,)4 and Pt(PF3)4 for which the corresponding metal tetra- carbonyls are unstable or unknown at least at room temperatures. .a G. Parshall and R. Cramer J. Amer. Chem. SOC. 1965,87 392. W. Sheppard E. I. du Pont de Nemours and Co. 1969 persona1 communication. G. Wilkinaon J. Amer. Chem. SOC. 1951,73 5501. 4 Nyholm Table 3 Triifluorophosphine metal (0) compounds VI VII Cr(PF3)li m.p. 193" (-a (-CO Sublimes) m.p. 154') (-CO (-CO Sublimes) m.p. 160') (-CO (-CO Sublimes) m.p. 177") (-CO (-CQ m.p.-20") m.p. 51") (-a (-CO m.p. -22") m.p. 76") (-CO (-a m.p. -15") m.p. 2) VIIIC Ni(PF3)* m.p. -55" b.p. 70.5" (-CO m.p. -25") b.p. 43") Pd(PF3)a m.p. -41" decomp. -20" (-CO unknown) Pt(PF*)* m.p. -15" b.p. 86" (-CO unknown) A study of the P-F stretching frequency of PF complexes6 enables one to infer much about the nature of the metal-P bond in the same way that the G O stretching frequency in metal carbonyls has been used. However inter- pretation of the results is made more difficult because FGP or M+P double bonding or both types may take place. If we assume that there is some PFF / F QF QF / F 'F \F \F \F viz :P-F :P-F M t P - F M-P-F double bonding in free PFs then an increase in P-F stretching frequency in a complex such as Ni(PF3)4 is expected simply because donation of the 'non- bonding' lone pair of the P atom to the metal should increase the capacity of the P atom to attract electron pairs from the fluorine atoms.This is presumed to occur in the first group of compounds shown in Table 4; these consist of mono- meric PFJ complexes of the Cr Mo and W congeners and the hydrometal PF derivatives of Co Rh and Ir because P-F stretching frequencies higher than in PF8 are observed. When however one places a negative charge on the complex anion as in [Co(PF,) J- the maximum P-F stretching frequency is observed at a lower frequency. The formal negative charge on the metal ap- parently increases the size of the non-bonding d orbitals thus enhancing metal + phosphorus 7r bonding as in metal carbonyl anions; this in turn decreases the P-F bond order as is observed.Force constant calculations indicate that the 6Th. Kruck Angew. Chem. Internat. Edn. 1967,6 53. 5 Transition-metul Complexes o j Some Perfuoro-ligat ids Ni-P bond in Ni(PF,)4 (2.71 mdynel8i) is little different from that of Ni-C in Ni(C0)4 (2.52 mdyne/A). The powerful electron withdrawing power of PF3 is also indicated by the fact that the Re-Cl stretching frequency in Re(PF3),C1 is 37 cm-l higher than in Re(CO),C16. It will be noted that when one replaces one or more PF groups in a neutral complex [e.g. Ni(PF,),] by a weaker v-bonding ligand e.g. Ph3P there is a decrease in the P-F stretching frequency as compared with free PF,. As with the [Co(PF,)J- anion this suggests that greater metal-PF double bonding has decreased the donation of lone pairs from fluorine to phosphorus.Table 4 Phosphorus-fluorine stretching frequencies of PF and its complexes P-F Frequency (cm-l) 892,848 961,917,907 867 949,912,904 863 965,925,910 877 916 851 911 853 914 852 915,901 873 864,851 926,906 876 856 904,864 910 860 906 867 820 804 814,794 830,811 856 824 815 800 [Co(PF,)a (PPh,)]- 873 821 788 775 749 4 Synthesis and Properties of Perfluoroalkyl and Perfluoroaryl Complexes A considerable number of perfluoro-metal complexes involving charged ligands of carbon (CF,- CF3CH2- CF,=CF-) and of phosphorus (PF2-) e.g. [(PF,),CoPF,], are now known' but the perfluoro-carbon derivatives of the transition metals have been prepared only during the past few years. Indeed Lagowski* in his review of perfluoroalkyl derivatives of metals and non-metals Th. Kruck A. Engelmann and W.Lang Chem. Ber. 1966,99,2473. Th. Kruck and W. Lang Angew. Chem. Internat. Edn. 1967 6,454. J. J. Lagowski Quart. Rev. 1959 13 233. 6 Nyholm in 1959 did not mention any transition-metal compounds. More recently in 1963 H. C. Clarkg states that ‘there have been only a few brief reports of the preparation of such (i.e. transition metal) derivatives and full details have yet to be published. . . . Obviously this field of perfluoro-alkyl chemistry is as yet virtually untouched.’ The main reason for the absence of these compounds was undoubtedly the lack of suitable preparative methods. This deficiency has been remedied in recent years using Grignard and other techniques; these have been summarised by Stone and TreichePo and by Stonell in reviews of the subject. These methods are summarised below for both perfluoro-alkyl and perfluoro- aryl (C,F,-) derivatives.(See also appendix p. 19.) Reaction of a Grignard or organo-lithium reagent with a complex halide e.g. THF (i) trans-(Et,P),NiC12 + CF,=CFMgBr -+ Br / \ trans-(Et,P),Ni(CF=CF,) + trans-(Et,P),Ni CF=CFg (ether) (ii) (n-C,H,),ZrC12 + 2C6F5Li -+ (n-C,H,),Zr(C,F,) Double decomposition via the reaction of an alkali-metal salt of a metal metal carbonylanion with a perfluoroacyl halide followed by loss of CO e.g. (i) 2C,H,COCI + Na,Fe(CO) -3 (C,H,),Fe(CO) + 2CO (ii) C6F5COC1 + NaMn(CO) - C,F,COMn(CO) - THF A THF C,F,Mn(CO) -I- co Insertion of a perfluoro-olefin in an M-H bond of a carbonyl metal hydride or a metal-alkyl or -aryl bond of a metal carbonyl alkyl or aryl. Strictly speaking this does not yield a perfluoro-alkyl derivative but the carbon atom attached to the metal is fully fluorinated benzene e.g.HCo(CO) + CF,=CF - HCF2=CF2Co(C0) MeRe(CO) + CF,=CF -4 MeCF,CF,Re(CO) PhMn(CO) + CF2=CF2 - PhCF,CFZMn(CO)5 pentane pentane @ H. C . Clark Adv. Fluorine Chem. 1963 3 19. lo P. M. Treichel and F. 0. A. Stone Advances in Organometallic Chemistry 1964 1 143. loa See also F. J. Hopton A. J. Rest D. T. Rosevear and F. G. A. Stone J. Chem. SOC. (A) 1966 1326. l o b M. G. Hogben and W. G. A. Graham J. Amer. Chem. SOC. 1969,91,283; M.G. Hogben A. S. Gray A. J. Oliver J. A. J. Thompson and W. G. A. Graham J. Amer. Chem. SOC. 1969 91 291. l1 F. G. A. Stone Endeavour 1966 25 33; M. I. Bruce and F. G. A. Stone Angew. Chem. Znternat. Edn. 1968 7 747; see also references therein. 7 Transition-metal Complexes of Some PerpUoro-ligands (4) Direct reaction of a perfluoro-olefin with a metal carbonyl in effect an oxidation of the metal CF2-CFa / I I \ I e.g.Fe(CO) + 2CF2=CF2 - (CO),Fe CF2-CF2 [(n-C5H5) (CO)~CO] + 2CFz=CF -+ CFZ-CFa / I (n-C,HJ (CO) CO 1 \ I CF2-CF2 ( 5 ) Oxidation of a metal carbonyl compound with a perfluoro-alkyl iodide benzene The effective electronegativity of the perfluoroalkyl group and the stability of perfluoroalkyl compounds generally will be discussed below. (iii) Next we consider perfluoro-complexes in which the fluorine atom is attached to the atom(s) one removed from the donor. Uncharged ligands of this type include N(CF3)3 and P(CF3),. The former is expected to be a very poor (T donor and like NF3 unable to accept ?T electrons from the metal atom (except via anti-bondingp,* orbitals).No derivatives of N(CF3)3 appear to have been reported. Metal complexes of P(CF,) has been studied by Emeleus12 and others. It behaves as a poor (T donor and a reasonably good ?T acceptor but few com- plexes have been investigated in detail. In general it forms less stable complexes than those of PF3. In view of the enhancement of stability arising from chelation bidentate groups such as e.g. (T-C,H,)CO(CO)~ + CF31 -+ (.rr-C,H,) (CO)ICoCF Q R/PA 3 should yield complexes of much greater stability but the ligands have not yet been reported. Charged fluoro-ligands such as CF3CH2- in which the fluorine atom exerts its effect through a CH2 group appear not to have been studied but presumably ligands such as this will be intermediate in behaviour between the ethyl and perfluoro-ethyl group probably resembling more the ethyl group.The charged ligand C6F,- is of special interest because it combines two unique features. Although the nearest fluorine atom is on an atom one removed from the donor carbon atom there are five fluorine atoms around the ring exerting l2 H. J. Emelkus and J. D. Smith J. Chem. SOC. 1958 527. 8 Nyholm an inductive effect. Also there exists here the delocalisation effect which ensures that the effect of the fluorine atoms at the donor carbon is greater than in say the CF,CH,- group. As shown by Stonelo and his collaborators per- fluorophenyl bromide or iodide conveniently prepared from hexafluorobenzene can be used to prepare a Grignard or a lithio-compound; these react with covalent metal halide bonds such as in trans-[Et,P],PtCl to yield derivatives such as frans-[Et,P],Pt(C,F,)Cl and cis-[Et,P],Pt(C,F,),.This type of reaction has yielded a considerable number of transition-metal and post transition-metal complexes. Another general method has recently been reported13 from our laboratories which involves the use of bis(pentafluorophenyl)thallium(m) bromide as an oxidising agent for the transfer of C,F,-groups to a transition metal in a ‘low’ oxidation state. A simple example is the oxidation of triphenylphosphine gold(1) chloride viz. 2Ph,P+AuCl + [(C,F,),TIBr] 3 2Ph3P 3 Au(C,F,)~C~ 4- TlBr. The reaction is carried out in benzene solution and on warming or in some cases under reflux a nearly quantitative transfer of the C,F,-groups occurs with the precipitation of thallium(1) bromide.In the square complex This work arose out of an investigati~nl~ of the reduction of thallium(rrr) compounds in an attempt to prepare derivatives with a TI-T1 bond. Simple alkyl and aryl derivatives of thallium(1rx) are usually salt-like derivatives of 2-co-ordinate thallium e.g. [Et-TI-Et]+Cl-. In order to obtain a 4-co-ordinate thallium(1rr) complex a more electronegative ligand than C2H is needed; the dimeric C,F compound [(C,F,),TlBr] was prepared for this purpose. The structure is unknown but is presumably similar to that of the 2-co-ordinate bridged thallium(1Ir) hydroxide obtained by treatment with alkali; AU(PPhJ(CcF5)zCl the two C6F5-grOUpS are the crystal structure of this has been determined by Bright and Truter.16 Earlier Nesmeyanov16 had shown that (C,H,),TlBr is reduced by stannous chloride to yield (C,H,),SnCI,; but similar reactions have not been reported with other metallic reducing agents.We have now been able to prepare C,F,-derivatives of the metals shown in Table 5 in the oxidation states indicated either by oxidation with [(C6F,),TlBr]2 or by oxidation with this reagent followed by reduction with hydrazine or other R. S. Nyholm and P. Royo Chem. Comm. 1969,421. laaR. Baker and P. Pauling Chem. Comm. 1969 745. l4 G. B. Deacon J. H. S. Green and R. S. Nyholm J. Chem. Sac. 1965,4367. l6 D. Bright and M . R. Truter Chem. Comm. 1969 in the press. l6 A. N. Nesmeyanov A. E. Borisov and N. V. Novikova Izvest. Akad. Nauk S.S.S.R. Otdel Khim. Nauk. 1959 644. 9 Transition-metal Complexes of Some Per-uoro-ligcmds reducing agents.This process preferentially removes one halide ion and one C6F5-group rather than two C6F5-groups e.g. N2H4 Ph3P AU(C,F,),CI -+ PhaP AU-CGF~ Table 5 Metals and oxidation states (underlined) yielding C,F,-complexes by [(C6FS),TIBr] oxidation Fe COIII NiII and IV c u Ag co Ni = a RhI and I11 IrI and I11 ptII and IV ~~1 and 111 pd11 and IV - a c = - Oxidation of mercury(r) chloride to Hg(C,F,) and SnCl to SnC12(C6F5)2 by this general reaction has also been achieved. Deacon and co-worke~s~~ have extended the reaction for the oxidation of the elements in the free state for Groups IIB IIIB IVB VB VIB and VIIB to form derivatives such as Zn(C6F6)2 from Zn Cd and Hg to C1 Br and I have been shown to react except Ga Si Pb and Bi. So far we have not observed reactions of the elements of Group VIII and IB with (C6Fs),TlBr but the following oxidations have been carried out with (C6Fs),TIBr.The properties of the complexes are shown in Table 6. (C6F6)3In(PPh3) (C6F5)& (CsF5)3AS (C,F,),Se and C6FsBr. AII eIementS (a) Zerovalent Metal Complexes (C6Fd2T1Br (Ph8P)2Nio(CO) -+ (Ph3P) 2Ni11( C,F,) (Ph3P)*Pd0 (Ph3P) 2Pd11(C6F5)2 (Ph3P)4Pto -P (Ph3P)2R11(C6F5)Z (h) Univalent Metal Complexes (PPh3) SRh'CI - --t (PPh3)!4fi(C6F6)!,3CI The formation of a 5-co-ordinate rhodium(m) complex is noteworthy. This is presumably due to the bulky nature of the Ph3P group. Indeed Wilkinson18 has suggested that dissociation of a Ph3P group takes place even from 4-co- ordinate (Ph3P),RhCl when the latter behaves as a hydrogenation catalyst with olefins. The rhodium(1) analogue of Vaska's compound reacts similarly (Ph3P)&h1(CO)CI -+ (Ph3P)2(CO)Rh111(C6Fs)&1 (Ph,P),IrI(CO)Cl -+ (Ph,P),(CO~Ir111(C,,F5),C1 (c) Bivalent Metal Complexes (Ph3P),Pt11Cl2 reacts with (C6F5),T1Br without deposition of thallium(1) (Ph3P),PdI1Cl2 (Ph,P),PdIV(C,F5),C1 l7 G.B. Deacon 1969 personal communication. J. A. Osborn F. H. Jardine J. F. Young and G. Wiikinson J. Chern. SOC. (A) 1966,1711. 10 Nyholm bromide to yield a more deeply coloured compound. The structure of this is uncertain but the products of its reactions with bromine yielding (Ph3P)2PtIV- (C,F,)Br, and on reduction which gives (Ph,P),Pt(C,F,)Br suggest the following possible structure Ph3 P This possible structure is based on the products obtained by halogen oxidation and hydrazine reduction and on the observed molecular weight and molecular conductivity.The compound is monomeric and a non-electrolyte; the crystal structure is at present being investigated by Professor H. M. Powell. Oxidation of CoII and NiII complexes can be effected also L As As Br2 + Br- This COIL' complex is green and hence presumably the CsF6- and the Br-groups are in trans-positions. The corresponding bis-diarsine nickel@) dibromide is oxidised by (C,F,) ,TIBr to the following nickel(rv) complex 11 Transition-metal Complexes of Some Perfluoro-ligands This derivative is thus similar to the corresponding di-chloro- and di-bromo- nickel(rv) cations described some years ag0.le As pointed out below the C,F,- group has an electronegativity about the same as that of bromine and the preparation of metal complexes involving C6F6- in place of bromine or chlorine is thus in principle feasible given a similar reaction path to facilitate the mech- anism of C,F,-addition.Attempts to synthesise an Fe1I1-c6F5 complex by oxidising [FeI1(diarsine),C1Jo were unsuccessful. With one or two exceptions e.g. (Ph,P),Ni(CO), addition of C,F,- takes place most readily when the metal ion being oxidised is co-ordinatively un- saturated.* This seems to throw some light on the mechanism of the reaction. It is possible that a weak metal-thallium bond is first formed as the reaction intermediate; formation of this is then followed by transfer (presumably step- wise) of the two C6F6-groups. The process for the oxidation of Ph,P-+AuCl would then involve the following intermediate Br W The alternate free radical mechanism is not favoured because if [(C,F,),TlBr] in benzene is heated to the temperatures used for oxidation no C6F6-C6F6 is isolated; this would be expected if C6F free radicals were formed.The reduction of these perfluorophenyl metal complexes for example with hydrazine gives rise to products containing C@,- attached to the metal atom in the lower oxidation state. There is preferential loss of halogen rather than of the two C6F groups. Thus reduction of Ph,P4Au(C6F,)2C1 yields Ph,P-+ Au-C6F,. (Ph3P)2Pd1V(C6F6)2CIa forms (Ph3P),Pd1I(C6F6)C1 which incident- Is R. s. Nyholm J . Chem. SOC. 1951,2602. * Even with (Ph,P),Ni(CO) three-co-ordination may arise owing to the loss of a CO group. 12 Nyholm ally may be re-oxidised with chlorine to form a PdIV complex with only one C,F5-grOUp attached i.e.(Ph3P),Pd'V(C6Fs)CI,. As mentioned above reduction of the adduct between (Ph3P),Pt11C12 and (C,F,),TIBr yield~(Ph,P),Pt~~(C,F~)C1 which can like the PdII compound be re-oxidised with chlorine to yield the -C6F5 complex (Ph3P),PtIV(C6F5)C19. Table 6 summarises the properties of these compounds. Table 6 Properties of perfluorophenyl transition-metal complexes prepared using [(C6F&TIBr] as oxidant [Co(diars),a (C,F,)Br]Br Co(diars) ,Br [Ni(diars),(C,F6)2IBr Ni(diars),Br Compound Starting material Ni(PPh3) Z(C6F6) 2 * Ni(PPh3) 2(C0)2 trans-Rh( PPh,),(C,F,) ,( C0)CI trans-R h( PPh3) ,( C0)CL frans-Ir(PPh ,) ,(C6F5) ,(CO)CI trans-Ir(PPh ,) ,(CO)CI cis-Pd(PPh3) z(caFs)2 Pd(PPh3) 1 cis-P t (PPh,) &F5) Pt(PPh3h trans-Pd(PPh,) 2(GF5)2Cl2 trm-Pd(PPh,),CI 'PtTI(C6F5) ,C1 ,Br(PPh ,) a) cis-Au(PPh,) (C~F,)ZC~ Au(PPh,)CI cis-Pt(PPh,) ,CI rAu(PPh 3)C6F cis-Au(PPh,) (C6F5) ,C1 Colour Green Yellow Green- black Yellow- red Yellow Yellow White White Pale White White White yellow m.p.("C) 194 203 185 115 189 196 235 245 250 276 160 171 dec. A Pd(PPh3)2(C,Fs)CI trans-Pd(PPh,) ,(C6F5) ,CIZ White 230 IPt(Pph3)2(C6Fdclc 'PtTl(C6F5)2ClzBr(PPh3)a) White 286 (PPh 3) dCBF 5) c13 Pt(PPh3)2(C6F5)C1 Yellow 180 B { or PtTI(C,F,) ,C1 %Br(PPh s) a Diarsine is o-phenylenebisdimethylarsine. b First reported by J. R. Phillips D. T. Rosevear and F. 0. A. Stone J. Orxanometallic Chem. 1964,2,455. First reported by D. T. Rosevear and F. G. A. Stone J. Chem. SOC. 1965 5275. Also reported by L. G. Vaughan and W. A. Sheppard J. Amer. Chem. SOC. 1969,91 in the press who have also prepared (Ph,P)Au(C,F,) (a) by NaHl reduction and (b) by C1 oxidation.The reactivity towards various small molecules of these perfluoro-compounds of metals in higher oxidation states has been relatively little studied and offers an interesting area for research. The behaviour of carbon monoxide with some derivatives has however been investigated. Thus when (PPh3),Rh(C,F5),CI is treated with carbon monoxide one obtains the perfluoro-ketone (C&&CO according to the reaction (PPh3)2Rh(C6F6)2CI + 2CO -+ (PPh,),Rh(CO)CI 4- (C,F,),CO. The extension of this reaction to the study of perfluoro-alkyl complexes is being followed up. The preparation of a wide range of C,F,-complexes of metals in various oxida- 13 Transition-metal Complexes of Some Perpuoro-ligands tion states has provided a unique opportunity for the study of the behaviour of the C,F,-group in different environments.In general one observes that as the oxidation state of the metal to which the C,F,-group is attached rises the stretch- ing frequency of the C-F band of highest intensity (ca. 1050 cm-’) increases significantly. A similar increase in stretching frequency occurs as one passes along the series C6FS-I C,F,-Br C,F,-CI to C,F,-F. Presumably metals of high oxidation state or groups of high electronegativity increase the positive charge on the C,F,-group thereby enhancing the strength of attachment of the fluorine to the individual carbon atoms. The 19F n.m.r. spectra of pentafluorophenyl metal complexes have been studied by Stone and co-workerslOa by Vaughan and Sheppard (see footnote dto Table 6) and by Graham and collaborators.lOb Taking the chemical shift of the 19F in hexafluorobenzene as 163.0 p.p.m.upfield from CFCI as the basis then in general the meta fluorine atoms in a C6F5-transition metal complex are the same or a shade smaller the para fluorine shifts are about 2 p.p.m. lower and the ortho shifts are about 40 p.p.m. lower. In complexes such as (C,F,),Au(PPh,) there are small differences between the ortho fluorine atoms according as to whether one is concerned with a C,F,-group cis or trans to the PPh group the chemical shift of the ortho-fluorine in the CiS-C,F being slightly less (2 p.p.m.) than in the trnns-isomer. In general the more electronegative the group attached to the C6F5- the larger is the chemical shift of the ortho-fluorine; the n.m.r.spectra of C,F,-complexes of Au(I) and AU(III) are in agreement with this generalisation. However whilst good qualitative correlations such as the above are available a quantitative separation of the diamagnetic and paramagnetic terms in the chemical shift is not yet possible. Also the proximity of the ortho- fluorine atoms to the metal atom undoubtedly leads to field effects which cannot be assessed quantitatively at present. A wealth of chemistry remains for investigation among compounds of the earlier transition elements. We are currently extending our studies to the lower oxidation states of complexes ranging from the Ti Zr and Hf triad across to Fe Ru and 0s. 5 The Nature of the Metal-Carbon Bond in Perfluoro-alkyl and -my1 Metal Complexes The greater ‘stability’ of perfluoro-alkyl and -awl transition-metal complexes as compared with the unsubstituted alkyl and aryl derivatives calls for comment.The word ‘stability’ is used here in the rather loose sense that the compound can be prepared and does not readily decompose spontaneously or in the presence of air and moisture. This use of the word ‘stability’ implies a combination of thermodynamic factors (free energy of formation i.e. a strong M-C bond) and a relatively large energy of activation for reaction with reagents such as air moisture solvents etc. The main factors which lead to this ‘stability’ are ( i ) A high M-C bond energy. Other factors being equal this is dependent on the product of the electronegativities of the bonded atoms. Lagowski20 has *O H. B. Powell and J. J. Lagowski J.Chem. Sac. 1965 1392. 14 Nyholm estimated effective electronegativity values for various perfluoro-alkyl groups using pK values of the perfluoro-alkyl and -aryl mercury(I1) hydroxides and i.r. stretching frequencies of the Hg-Cl bond in substituted mercury(@ chlorides. Stone2' has estimated values of electronegativity by studying the C-0 stretch- ing frequencies of compounds of the type X-Mn(C0)5 where X = a halogen C,F,- Me etc. Stone has noted the variation in electronegativities depending on whether one studies RfSnR3 or RfMn(CO) compounds. The following sequence of electronegativities is then obtained as a mean of the various estimates F(4.0) > Cl(3.2) - CF3(3.2) N C2F,(3-2) > C,F,(3.1) > C6F5(3.0) - Br(3.0) > Ph-(2-8) > I(2.7) > Me(2.6) (no useful purpose is served by attempting to give values to more than one decimal place).In terms of electronegativity product alone one would espect that perfluoroalkyl bonds will be stronger than perfluoro- aryl bonds; both in turn are stronger than unsubstituted metal-alkyls and -aryls. As a simple illustration CF,Co(CO) distils without decomposition at 91" but MeCo(CO) is stable only below -30". The effect of possible double bonding is discussed in (ii) below. For a particular carbon ligand the metal- carbon bond strength should also increase with increasing electronegativity of the transition metal. This arises with increasing oxidation state of the metal (e.g. PtIV > PtII) and in general as one moves vertically down the periodic table especially to the third transition series ( P P > NiIV and PdIV) and as one passes to the right in any transition series.It is understandable therefore that very stable metal-carbon (T bonds are commonly found with PtIV and AuIII. (ii) Double bond character. Unfortunately there are not many data available yet to permit a comparison of perfluoro- and unsubstituted alkyl and aryl transition metal bonds but a survey of these data by Churchill and Mason22 indicates the following (a) There is a shortening of the M-C bond of ca. 0.1 A in a perfluoroalkyl transition-metal bond as compared with an unsubstituted alkyl-metal bond. (6) There is less difference between perfluoroalkyl-metal bonds and un- substituted alkyl metal bonds but both appear to be 0.05-4.1 A shorter than the bond between a metal and an unsubstituted alkyl-metal bond.Churchill and O'BrienZ3 have summarised recent data on simple aryl compounds. These data suggest that some double bonding occurs between the transition metal and a perfluoroalkyl group; this probably involves the overlap of a filled f 2 g orbital of the metal with a vacant antibonding C-F orbital of suitable symmetry. As pointed out by Mason2* use of an antibonding orbital of the carbon atom alone is unlikely because of the way in which the C-F stretching frequency in the -CF group changes on co-ordination to a metal. has shown that in (CO),MnCF the C-F stretching frequency is lowered by J. Dalton I. Paul and F. G. A. Stone J. Chem. Soc. (A) 1968,1212. M. R. Churchill and R. Mason Adv. Organometdic Chem. 1968 5 125. as M. R. Churchill and T. A. O'Brien J. Chem. SOC. (A) 1969,266.24 R. Mason 1969 personal communication. 25 F. A. Cotton and J. A. McCleverty J . Organometallic Chemistry 1965 5 490. 15 Transition-metal Complexes of Some Perfluoro-ligands ca. 100 cm-l as compared with CF3X (X = Cl Br or I). This is consistent with a weaker C-F bond and a stronger and shorter M-C bond. Comparing the normal aryl and perfluoroaryl groups it seems that the double bonding which apparently occurs in both cases is little enhanced by the sub- stitution of all five hydrogen atoms by fluorine. However this substitution presumably increases the strength of the 0 M-C bond. (iii) A large energy separation between the CT bonding orbital and the lowest empty anti-bonding orbital is desirable to minimise bond breaking by electron promotion from the bond.26 In general the separation between these molecular orbitals increases as the separation between atomic orbitals increases in the free atom and this in turn depends upon the effective nuclear charge on the metal- or its effective electronegativity.Hence the effect of this should be most apparent with metals such as Pt and Au. (iv) Minimum M-C polarity will help to enhance stability in the kinetic sense since a highly polar Ti8+-@- bond for example is more likely to be attacked by a nucleophilic reagent than a less polar Pt-C bond. ( v ) The absence of vacant d orbitals on the metal (or indeed donor atom) will inhibit nucleophilic reagent attack. This favours stability of PtIv-C bonds as compared say with TiIV-C bonds. (vi) The absence of lone pairs on the metal or ligand will increase stability by minimising attack by an electrophilic reagent.These conditions are satisfied ideally in bonds between PtlV and a saturated carbon atom. (vii) Finally reference should be made to intramolecular reactivity. It has been noted that F,C-metal complexes are less stable than the corresponding C,F,-metal derivatives. Here the proximity of the fluorine atoms to metal in the case of the -CF derivatives assists the formation of M-F and :CF2 radicals ; alternatively if there are reactive groups on the metal these can react with a nearby fluorine atom of a -CF group in a cis-position. Such a reaction is of course much less likely with the ortho-fluorine atoms of a co-ordinated C6F,- grOUP. 6 Complexes of Ligands in which Fluorine is Attached to an Atom Two Removed from the Donor Atom In the cases of both charged and uncharged Iigands we need to consider perfluoroaryl types only.because it is reasonable to assume that in ligands such as CF3CH2CH,- or (CF,CH,),As the inductive effect of the fluorine atoms would be largely neutralised by the intervening CH2 group. Examples of charged ligands of the aryl type include pentafluorophenol and pentafluoro t hiophenol. These ligands and the corresponding pen tachloro- derivatives have been studied to obtain ligands which are the organic equivalent of halide ions. For this purpose one needs a relatively polarisable ligand (e.g. ae J. Chatt and B. L. Shaw J. Chem. Soc. 1960 1718. 16 Nyholm C,F,-S-) which forms a strong acid. From these pseudo-halide ligands it is possible to prepare the equivalent of tetrahalogenometallate(n) anions from a range of bivalent metals of the first transition series by the following type of rea~tion.~' The silver or thallium salt of the pseudo-halide Iigand is prepared and allowed to react with Ph4As],MCl4 in a suitable non-aqueous solvent.Thus with [Ph,As],CoCl metathesis occurs with precipitation of silver or thallium(1) chloride and the [Ph,As],[Co(ligand) J can be isolated by concentration of the solvent. The complexes are deep blue and have spectra and magnetic moments characteristic of tetrahedrally co-ordinated CoII complexes. The corresponding reactions take place with NiII CuII and ZnII. Table 7 shows the properties of a selection of these complexes. The values of lODq and fi (Jorgensen's nephel- auxetic covalency parameter) are similar to those of the halogens.For the usual halides and the ligands studied values of lODq decrease in the order -NCO > -NCS > pentafluorophenol > pentachlorophenol > thiophenol > penta- fluorothiophenol > C1- > Br- > I-. The p value in the nephelauxetic series decreases in the order pentafluorophenol > C1- > -NCO > pentachlorophenol > -NCS > Br- > I- > pentafluorothiophenol.28 A study of the bidentate group y y H F F SH shows2B that it behaves like other thiolene type ligands much of the negative charge residing on the ring. This chelate group dissolves readily each of the first transition metals if they are finely divided. However to simulate say a tertiary phosphine and a C1- ion in a chelate group ligands such as F F K2 PPh2 F SH should prove of considerable interest.Finally we refer to uncharged ligands with fluorine substitution to the donor atom. These include ligands of the type (C,F,),As. These are known to co- F&AsMe2 F F AsMe2 27 B. Hollebone and R. S. Nyholm 1969 unpublished observations. as A. Kernmit A. Nicholas and R. D. Peacock Chem. Comm. 1967,599. A. Callaghan A. J. Layton and R. S. Nyholm Chem. Comm. 1969,399. 17 Transition-metal Complexes of Some Perfuoro-ligands Table 7 Spectral and magnetic properties of tetrahalide and pseudohalide metal- late(@ complexes lODq B pe.v (B.M.) 2 - (cm-l) (cm-l) " co-t-9 ] 3920 770 4.75 [ "-64 3 1 * - 3850 r 9 2 - 71 5 631 4.76 4.69 3870 4300 600 4-42 695 3.65 - Diamag. * The Nin derivative Ni(ligandIa is octahedrally co-ordinated. ordinate to transition metals less strongly than the corresponding (C,H,),As derivatives but detailed studies are lacking.The tetrafluorodiarsine ligand in which the aromatic ring only has been fully substituted with fluorine has been studied in some detail.3O In general it differs from the unsubstituted diarsine in two main ways. Overall it is a weaker donor. With the ds metals Ni" PdII m N. V. Duffey A. J. Layton R. S. Nyholm D. Powell and M. L. Tobe Nature 1966,212 177. 18 Nyholm and PtIr it forms derivatives of the type M(F-diarsine),Hal,. The Pd” and PtII derivatives display the usual diamagnetic behaviour ; but whereas the di-iodide of NiII is brown and diamagnetic in the solid state the chloride and bromide are green and paramagnetic. Secondly it shows a greater tendency than diarsine to attach itself by one donor arsenic atom only e.g.in complexes of the type RhCI,.(F-diarsine),. This can be isolated as a non-electrolyte in which one ligand is chelated and the other attached as a unidentate group. The presence of one unco-ordinated -AsMe group is shown by the reaction of CH,I with the complex. A 1 :1 electrolyte is formed. It is clear from these studies that even when as far removed from the donor atom as the 18 atom the fluorine exerts a marked effect in co-ordination provided that it is attached to an aryl group. Appendix. In the discussion on p. 7 we inadvertently omitted the method for synthesising fluorocarbon-metal complexes which involves the attack of a metal carbonyl anion on a fluorocarbon although it is implicit in method 2 (p. 7). Thus hexafluorobenzene reacts with the [Re(CO),]- anion (but not the corresponding [Mn(CO),-ion) to yield CsFs Re(CO), The procedures have been summarised by Bruce and Stone32. Finally a useful review of the preparation and properties of fluorocarbon-metal complexes has been published by Acknowledgements. The author is indebted to his collaborators for much of the work referred to before publication. Thanks are due also to Professors F. G. A. Stone and R. Mason and Dr. W. A. Sheppard3I for valuable discussions. *l W. A. Sheppard and C. M. Sharto ‘Organic Fluorine Chemistry’ W. A. Benjamin New York 1969. 38 M. I. Bruce and F. G. A. Stone ‘Preparative Inorganic Reactions’ ed. W. L. Jolly Inter- science 1968 4 177. 33 W. J. Bland Leicester Chem. Rev. 1967 8 15. 19

ISSN:0009-2681    

DOI:10.1039/QR9702400001

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Ami no-acids Peptides and Proteins Volume I The Chemical Society announces the publication of Volume 1 in this the third title in their series of Specialist Periodical Reports. This volume reviews and evaluates progress reported during 1968 to a depth and comprehensiveness not available elsewhere. The scope is indicated by the principal chapter headings Amino-acids ; Structural investigation of peptides and proteins ; Peptide synthesis ; Peptides of abnormal structure; The relationship between structure and biological activity of peptides and proteins ; Metal derivatives of amino-acids peptides and proteins. The coverage includes naturally-occurring synthetic and chemically modified materials and chemical physical stereochemical analytical structural and synthetical studies. Publication will be annual authorship being undertaken by a team of eight scientists led by Dr.G. T. Young of Oxford as Senior Reporter. Specialist Periodical Reports are designed to assist the research worker or specialist in his own field t o give the non-specialist a concentrated but complete view of the topic being reported and to provide libraries with a useful source book. Size 83" x 53" Pages xii + 308 Cloth Bound SBN 85186 004 4 Price per volume Fellows of The Chemical Society f 3. 0.0 (US $7.20) Non- Fellows f4.10.0 (US $1 0.80) This publication may be ordered from the Publications Sales Officer The Chemical Society Blackhorse Road Letchworth Herts England. Ami no-acids Peptides and Proteins Volume I The Chemical Society announces the publication of Volume 1 in this the third title in their series of Specialist Periodical Reports.This volume reviews and evaluates progress reported during 1968 to a depth and comprehensiveness not available elsewhere. The scope is indicated by the principal chapter headings Amino-acids ; Structural investigation of peptides and proteins ; Peptide synthesis ; Peptides of abnormal structure; The relationship between structure and biological activity of peptides and proteins ; Metal derivatives of amino-acids peptides and proteins. The coverage includes naturally-occurring synthetic and chemically modified materials and chemical physical stereochemical analytical structural and synthetical studies. Publication will be annual authorship being undertaken by a team of eight scientists led by Dr. G. T. Young of Oxford as Senior Reporter. Specialist Periodical Reports are designed to assist the research worker or specialist in his own field t o give the non-specialist a concentrated but complete view of the topic being reported and to provide libraries with a useful source book. Size 83" x 53" Pages xii + 308 Cloth Bound SBN 85186 004 4 Price per volume Fellows of The Chemical Society f 3. 0.0 (US $7.20) Non- Fellows f4.10.0 (US $1 0.80) This publication may be ordered from the Publications Sales Officer The Chemical Society Blackhorse Road Letchworth Herts England.

ISSN:0009-2681    

DOI:10.1039/QR97024BX003

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Molecular Complexes of Water in Organic Solvents and in the Vapour Phase By Sherril D. Christian Ahmed A. Taha and Bruce W. Gash DEPARTMENT OF CHEMISTRY THE UNIVERSITY OF OKLAHOMA NORMAN OKLAHOMA 73069 1 Introduction Much of the literature of aqueous solution chemistry concerns solvation or hydration phenomena and the interpretation of these phenomena in terms of water-dissolved solute interactions. Previous reviews have treated the inter- action of water molecules with ionic salts polar organic molecules and com- pounds of biological imp0rtance.l The interaction of water molecules with each other in liquid water and in aqueous solutions has been investigated by virtually all the applicable methods of physical chemistry.1b,2 That formidable problems arise in interpreting physical and chemical information about concentrated solutions is evident from the variety of mutually contradictory models which have been proposed for liquid water and aqueous ~olutions.~ It has long been recognised that solute-solute molecular interactions can more readily be elucidated than solvent-solvent or solvent-solute interactions.Fortunately non-ionic solute species ordinarily follow Henry’s law at concen- trations up to at least several mole per~ent.~ When spectral methods are employed Beer’s law generally applies to the individual solute species in a comparable concentration range. Similarly other specific properties of a dissolved solute are constant in the dilute region e.g. partial molar volume energy and heat capacity; dipole moment; chemical shift in proton magnetic resonance o R.A. Robinson and R. H. Stokes ‘Electrolyte Solutions’ 2nd edn. Butterworths Scientific Publications London 1959; b J. L. Kavanau ‘Water and Solute-Water Interactions’ Holden-Day Inc. San Francisco 1964; c F. Franks ed. ‘Physico-Chemical Processes in Mixed Aqueous Solvents’ Heinemann Educational Books Ltd. London 1967; d F. Franks and D. J. G. Ives Quart. Rev. 1966 20 1 ; e R. W. Gurney ‘Ionic Processes in Solution’ McGraw-Hill Book Co. Inc. New York 1953;fH. E. Whipple ‘Forms of Water in Biologic Systems’ Ann. New York Acad. Sci. 1965 125 (2)’ 249. a C. Pimentel and A. L. McClellan ‘The Hydrogen Bond’ W. H. Freeman and Co. San Francisco 1960. a H. S . Frank and Wen-Yang Wen Discuss. Faraday Soc. 1957 24 133; b H. S. Frank Proc. Roy. SOC. 1958 A . 247 481; c L. Pauling and R. E. Marsh Proc.Nat. Acad. Sci. U.S.A. 1952 38 112; d W. F. Claussen J Chem. Phys. 1951,19,259 1425; e G. Nkmethy and H. A. Scheraga ibid. 1962,36 3382 3401 ; f G. Nemethy and H. A. Scheraga J. Phys. Chem. 1962,66 1773; g H. S. Frank and A. S . Quist J. Chem. Phys. 1961,34,604; h M. D. Danford and H. A. Levy J. Amer. Chem. SOC. 1962,84,3965; i J. A. Pople Proc. Roy. Soc. 1951 A 205 163; j B. E. Conway Ann. Rev. Phys. Chem. 1966,17,481; k W. A. P. Luck in ‘Physico-Chemical Processes in Mixed Aqueous Solvents’ ed. F. Franks Heinemann Educational Books Ltd.. London 1967. J. H. Hildebrand and R. I,. Scott ‘The Solubility of Non-electrolytes’ 3rd edn. Dover Publications Inc. New York 1964 29. 20 Christian Taha and Gash (lH n.m.r.) studies and many others. Advantage has been taken of the constancy of parameters such as these in numerous studies of molecular interactions in dilute non-electrolyte solutions and in the vapour phase.lbBa As early as 1890 Beckmann inferred from cryoscopic data that benzoic acid dissolves in benzene primarily as the dimer;6 in contrast there is still considerable discussion regard- ing the size and nature of the kinetic units in pure associated liquids.It should be emphasised that investigations of molecular complex formation in dilute solution yield information about the interactions of solute molecules which are solvated not free as they would be in the vapour phase. For example the equilibrium 2PhC0,H = (PhC02H)2 is strongly affected by solvents;s as the medium is changed from vapour to cyclohexane to chloroform solvation more and more effectively opposes the formation of the dimer.In comparing hydrogen-bonding interactions in dilute organic solutions with those in either aqueous solutions or the vapour state methods are needed for predicting the influence of solvents on hydrogen-bonding equilibria. The present Review will summarise what is known about the molecular complexity of water in the vapour state and in dilute solution in non-polar and polar organic solvents; both the behaviour of water as an individual solute and its interactions with other polar solutes will be considered. In order to relate information about the solute properties of water to results for aqueous solutions a discussion will be given of methods for correlating effects of solvation on molecular complex formation reactions. 2 The Molecular Complexity of Water in Dilute Solution Water vapour is acknowledged to be virtually ideal at least at pressures less than ca.90% of saturation in the vicinity of room temperature.' High-tem- perature PVT8 and i.r. spectrala data show however that water vapour deviates considerably from ideality and that aggregates exist in significant concentrations at temperatures above loo" near saturation. Statistical mechanical calculations used to correlate the second virial coefficient of water vapour and the lattice energy of ice indicate that the distance of contact between water molecules in the vapour is comparable to the intermoIecular distance of water molecules in ice.l0 Two other methods have provided information related to the association of water in vapour or in an inert matrix.Mass spectrometric measurements of irradiated water vapour have led to the determination of relative concentrations of the hydrates H+(H20)n and thermodynamic constants for the reactions E. Beckmann 2. phys. Chem. (Leipzig) 1890 6 437. a Y . I'Haya and T. Shibuya Bull. Chem. SOC. Japan 1965 38 1144; b G. Allen J. G. Watkinson and K. H. Webb Spectrochim. Acta 1966 22 807. ' E. N. Dorsey ed. 'Properties of Ordinary Water-Substance' Rheinhold Publishing Corp. New York 1950 p. 54. * a Cr. S. Kell G. E. McLaurin and E. Whalley J. Chem. Phys. 1968,48 3805; b G. S. Kell G. E. McLaurin and E. Whalley ibid. 1968 49 2839. lo J. S. Rowlinson Trans. Faraday SOC. 1949,45 974; J. S. Rowlinson ibid. 1951,47 120. W. A. P. Luck and W. Ditter Ber. Bunsengesellschaft Phys. Chem. 1966 70 11 13. 21 Molecular Complexes of Water in Organic Solvents and in the Vapour Phase H,O + H+ (H20)n-1 = H+ ( H 2 0 ) n in which 1 < n < 8.11 The matrix isolation method (in which an associating vapour is diluted with an inert gas quickly frozen and examined spectrally) indicates that water dimers (probably cyclic) and higher polymers exist in a nitrogen matrix at 20K.l2 it is generally believed that water dissolves in the aliphatic and aromatic hydrocarbons and CCl primarily as the monomer.Cryo~copic,~~ spectral,16 vapour pressure,le dielectric constant,17 tracer,ls and partial molar volume1B studies indicate that only small concentra- tions of associated species are present even at concentrations approaching saturation. Activity data for water dissolved in several hydrocarbons and chlorinated hydrocarbons are shown in Figure 1.The activity aw is the ratio of water partial pressure to the saturation pressure of pure water at 25"; fw represents the total or analytical concentration of water. Data are displayed in a log-log plot; the fact that the points for the non-polar solvent systems fall nearly on straight lines drawn with unit slopes implies that water is essentially monomeric in these systems. There is evidence for positive curvature in the plots for aromatic solvents near saturation but only a few percent of the water molecules appear to be in associated forms. In slightly polar organic solvents such as the partially chlorinated hydro- carbons water is somewhat polymerised. Vapour pressure,labP2O i.r.,21 and lH n.m.r.22 data permit calculation of self-association constants for water in 1 ,Zdichloroethane (DCE) 1,1,2,2-tetrachIoroethane (TCE) and 1,2,3-trichIoro- propane.Spectral evidence indicates that trimeric species are favoured over dimers although one of the two vapour pressure investigations of water in DCE yielded data which can equally well be interpreted as indicating the presence of dimers or trimers,20a and the other vapour pressure results indicate that the most IikeIy associated species are trimers or tetramers.lsb Ca. 8-15% of the dissolved water is associated at saturation at 25" ; trimer formation constants vary in the range 3-5 1 mole-%. Water activity data for DCE and TCE are included in Figure 1. In spite of some reports to the l1 a P. Kebarle S. K. Searles A. Zolla J. Scarborough and M. Arshadi J. Amer. Chem.Soc. 1967,89,6393; b M. G. Inghram and R. Gomer Z. Naturforsch. 1955,10a 863. l2 M. Van Thiel E. D. Becker and G. C. Pimentel J. Chem. Phys. 1957,27,486. 13a M. Gordon C. S. Hope L. D. Loan and Kyong-Joan Roe Proc. Roy. SOC. 1960 A 258 215; b T. Ackerman Z . phys. Chem. (Frankfurt) 1964 42 119; c A. Risbourg and R. Liebaert Compt. rend. 1967 264 C 237. l4 J . M. Peterson and W. H. Rodebush J. Phys. Chem. 1928,32,709. l5 a E. Greinacher W. Luttke and R. Mecke 2. Elektrochem. 1955,§9,23; b S. D. Christian H. E. Affsprung J. R. Johnson and J. D. Worley J. Chem. Educ. 1963,40,419. leS. D. Christian H. E. Affsprung and J. R. Johnson J . Chem. SOC. 1963 1896; b J. R. Johnson S. D. Christian and H. E. Affsprung ibid. (A) 1966 77. l7 M. D. Gregory H. E. Affsprung and S. D. Christian J. Phys.Chem. 1968,72 1748. l8 J. W. Roddy and C. F. Coleman Talanta 1968 15 1281. ID W. L. Masterton and H. K. Seiler J . Phys. Chem. 1968,72,4257. 2o a W. L. Masterton and M. C. Gendrano J . Phys. Chem. 1966 70 2895; b R. L. Lynch Ph.D. Dissertation University of Oklahoma in preparation. 21 C. Jolicoeur and A. Cabana Canad. J . Chem. 1968,46 567. zeT. F. Lin S. D. Christian and H. E. Affsprung J. Phys. Chem. 1965. 69 2980. 22 Christian Taha and Gash 0.10 0.08 0.06 0.04 0.03 0.02 2 0.0 1 0.008 0.006 0.004 0.003 0.002 0.00 1 0.1 0.2 0.3 0.4 0.6 0.8 1.0 QW Figure 1 Formal solubility of water at 25" in 1 ,ZdichIoroethane 1,1,2,2-tetrachIoroethane benzene toluene cyclohexane-J. R. Johnson Ph.D. Dissertation University of Oklahoma 1966; diphenylmethane 1,2,3-trichloropropane-R. L .Lynch Ph.D. Dissertation in prepara- tion carbon tetrachloride-R. D. Grigsby Ph.D. Dissertation University of Oklahoma 1966 23 Molecular Complexes of Water in Organic Solvents and in the Vapour Phase Activity-concentration data have been obtained for water in numerous slightly volatile polar organic solvents for which the saturation concentration of water exceeds 1 mole The solvents fall into two classes those for which an assumed monomer-dimer equilibrium is adequate to correlate activity data and those for which monomer-trimer equilibrium provides a better fit. Introducing equilibrium constants for larger aggregates does not improve the correlation. Table 1 summarises formation constants at 25" for the dimer and trimer in Table 1 Solubility and association of H20 in organic solvents Solvent Tributylamine NN-Dimethylaniline 1,1,2,2-Tetra- doroethane 1,2-Dichloroethane Nitrobenzene Di butylphthalate NN-Dimethy lcyclo- hexylamine Methylphenylketone Cyclohexanone Aniline Benzyl alcohol KH (torr 1 mole- 504.0 435.1 256.2 218.8 182.3 128.3 51.9 43.6 21.8 18.1 11.9 fw" r Kw,* -l) (mole 1-l) (1 mole- 0-0522 10 0.0668 19 0.1010 8-4 0.1262 14.2 0.1666 21 1.054 0.2440 20 0.68 13841 75 0-9840 41 2.4590 50 2-9240 50 3.812 355 Kw,* Ref.16-11 23 26-39 23 -l) (1 mole-a) 3.60 16b 4.60 16b 23 23 4.92 23 0.776 23 0-289 23 0.194 23 0.046 23 *Formation constant for associated species from monomeric water. several solvents. Included are Henry's Law constants for the water monomer KH; the approximate formal concentration of water at saturation in each solvent fw0; and the approximate percent of the dissolved water in associated forms at saturation r.As KH decreases and fwo increases the association con- stants tend to decrease but in spite of this r is generally greater in the more reactive solvents. Thus mass action is more effective in promoting association than is solvation effective in preventing it. Little is known about water association in polar solvents which are completely or nearly completely miscible with water. Approximate values of dimerisation constants for water in several proton-accepting solvents have been inferred from limiting slopes of plots of proton chemical shift vs. water c0ncentration.~4 To explain proton-exchange rate data for water in proton-accepting solvents it has been proposed that solvated cyclic trimers are present at higher water concentrations and that protons transfer within the ring by a concerted mech- ani~m.*~b lH N.m.r.and molar volume data are available for the system H,O- ethylene oxide (EO) at 0" over the entire concentration range.as Water protons tend to bond to lonepair electrons of other water molecules in preference to the 23 J. R. Johnson S. D. Christian and H. E. Affsprung J. Chem. SOC. (A) 1967,1924. 24a G. Mavel Compt. rend. 1959 248 1505; b J. R. Holmes D. Kivelson and W. C. Drinkard J . Amer. Chem. SOC. 1962,84,4677. D. N. Glew H. D. Mak and N. S. Rath Canad. J. Chem. 1967,45,3059. 24 Christian Taha and Gash EO oxygen lone-pair electrons. The linear water trimer appears to be an important associated species at mole fractions of water less than ca. 0.11. At higher water concentrations chain-branching cross-linking and formation of cyclic 'aggregates predominate until ultimately the three-dimensional water networks characteristic of pure liquid water are formed.Density and Raman spectral data for H,O-acetic acid have been interpreted by assuming the presence of water dimers as well as hydrated acid species.26 3 Water-Polar Solute Interactions in Dilute Solution Early evidence for polar solute-water interactions was provided by Bodtker who noted that the solubility of water in ether increased as oxalic acid was added;27 he surmised that a compound forms between water and the acid. Before 1940 there were several reports of the effect of dissolved water on results of cryoscopic,2s partition,2a and s o l ~ b i l i t y ~ ~ ~ ~ ~ ~ studies of polar organic mole- cules.sl In particular S z y s z k o ~ s k i ~ ~ ~ calculated dimer formation constants for several aromatic carboxylic acids in benzene from partition and acid solubility data; he showed that the apparent association constant calculated by ignoring the effect of dissolved water was ordinarily considerably smaller than the corrected constant in one case by a factor of nearly 1:50.Later investiga- indicated that hydrates of both the acid dimer and the monomer are probably present in wet benzene solutions of chlorinated aliphatic carboxylic acids. Lassettre31 reviewed the early partition results and cautioned that the technique should not be employed for determining association constants unless corrections are made for water-polar solute interactions in the organic phase.In spite of this warning there have been numerous recent attempts to use the uncorrected partition method to study self-association; for certain systems however reasonable formation constants are obtained with the method,33 presumably owing to a fortunate cancellation of errors. The techniques employed to investigate molecular complexes of water in dilute solution resemble those usually employed in hydrogen bond studies2 However complications arise in using solvents such as benzene carbon tetra- chloride and the aliphatic hydrocarbons which dissolve less than 0.5 mole 28 J. J. Kipling J. Chem. SOC. 1952 2858. 27 E. Bodtker 2. phys. Chem. (Leipzig) 1897 22 505. a M. Rozsa 2. Elektrochem. 1911 17,934; b R. P. Bell and M. H. M. Arnold J. Chem. SOC. 1935 1432. as a S. Horiba Mem. Coll.Sci. Univ. Kyoto 1914,1,49; b B. U. Szyszkowski Z.phys. Chem. (Leipzig) 1928 131 175. 30 a 0. N. Lewis and G. H. Burrows J. Amer. Chem. SOC. 1912,34,1515; b L. A. K. Staveley J. H. E. Jeffes and J. A. E. Moy Trans. Faraday SOC. 1943,39,5; c E. Cohen and W. D. J. van Dobbenburgh 2. phys. Chem. (Leipzig) 1925 118 37; d E. Cohen and S . Miyake Verlag Akad. Wetenschapen Amsterdam 1925 34 933 ; e J. H. Hildebrand Science 1936 83 21. 81 E. N. Lassettre Chem. Rev. 1937 20 259. 32 R. P. Bell 2. phys. Chem. (Leipzig). 1930,150 A 20. 33 a M. Davies and H. E. Hallam J. Chem. Educ. 1956,33 322; b M. Davies and D. M. L. Griffiths 2. phys. Chem. (Frankfurt) 1954 2 352; c M. Davies P. Jones D. Patnaik and E. A. Moelwyn-Hughes J . Chem. Soc. 1951 1249. 25 Molecular Complexes of Water in Organic Solvents and iii the Vapour Phase percent water.It is difficult to prepare homogeneous solutions of water in a non-polar solvent by direct mixing of the liquid components; even when con- siderably less water is added than is required to saturate the solvent vigorous stirring for 24 h will not promote dissolution of all the drops.l7 This problem may account for some of the discrepancies among literature reports on the properties of water in dilute s ~ l u t i o n . ~ ~ ~ ~ @ The solute isopiestic method has been employed to prepare homogeneous solutions of water in organic solvents at known water activities; vapour phase equilibration of benzene or CCI with an aqueous solution of known water activity occurs within only a few hours even without stirring and without removing air from the equilibrator v e s ~ e l .~ ~ ~ J Manometric techniques are convenient for measuring activities of water in the very dilute solution range (formal water concentrations 0.00005 to 0-00300); one method incorporates a mercury-covered sintered-glass disc inlet valve formerly used in mass spectrometer ~ y s t e m s . ~ ~ ? ~ ~ A. Treatment of Data.-Simultaneous partition water solubility and isopiestic measurements for a given organic solvent-polar solute system have been used to investigate hydration equilibria. To illustrate the rationale of the method we shall outline the computations involved in studying a hypothetical compound which is present in the organic phase as both monomers and dimers (hydrated and unhydrated). Assume that the solute A is distributed between water and the organic solvent and that the concentration of A in the aqueous phase C A ~ is directly proportional to the activity of A.The analytical or formal concentration of A in the organic phase may be written f A = CA (1 + K11CW + K12W2 + - . ) 2CA2 (K20 -I- & ~ C W + K2zw2 . . ) (1) or f A = KDcAw (1 + K1lm f K12Wa + ) f 2KD2CAw2 (Kzo + K ~ ~ c w + K22m2 + . - ) (2) where CA and cw are concentrations of monomeric A and water (W) respectively in the organic phase; K2 is the equilibrium constant for formation of the unhydrated dimer from the unhydrated monomers; KI1 K12 . . . KZl K22 . . . are formation constants for the hydrates AW AW2 . . . A2W A2W2 etc. from monomers of A and W; and KD = CA / C A ~ is the distribution constant for monomeric A between water and the organic phase.The concentration of water monomer CW may be calculated from the expression cw = awcwo where -0 is the limiting value of cw at unit water activity. Henry's Law is assumed to apply to each hydrated and unhydrated species individually. The measured concentration of water in the organic phase is greater in the presence of A than it would be if only water and the solvent were present; the excess water solubility dfw is due to the formation of hydrated species and may be expressed as s4 a A. A. Taha R. D. G-rigsby J. R. Johnson S. D. Christian and H. E. Affsprung J . Chem. Educ. 1966,43,432; b E. E. Tucker Ph.D. Dissertation University of Oklahoma 1969. 26 Christian Taha and Gash Afw = CA ( K ~ ~ C W + 2K1acwa + . . . ) + C A ~ ( K ~ ~ c w + 2K22cw2 + . . . ) (3) Equations (1) and (3) apply equally to the partition systems or to solutions of A in solvent equilibrated at known reduced water activities.If sets of distribution data VA CA* CW) and water solubility data (fA CW dfw) are available Km and the hydration constants may be inferred by non-linear least-squares analysis.36 Initial values of KD K2 and the hydration constants are chosen and equation (1) is solved for each point to yield trial values of CA. The mean square deviation E given by E = CWS(AfWexpt1. - Af+alc.)a + CWpdfAexptl. - fAcalc.)2 (4) all points partition data points is then calculated using expressions (3) and (2) to obtain dfwcalc. and f~calc. respectively. (The weight factors WS and WP are determined from the precision of the water solubility and partition data respectively.) After each calculation of E the procedwe is repeated with a set of modified values of all the constants generated with a numerical optimum-seeking technique.The process is continued until the absolute minimum in E and the corresponding least-squares values of the constants are located. Standard errors in the constants may be calculated by the method of Sillen.36 Ordinarily no more than 2 or 3 hydration constants are needed to fit a given collection of data. Equations similar to (1-4) may be developed to fit i.r. and lH n.m.r. data for systems in which hydrates of polar solutes are p r e ~ e n t . ~ ~ . ~ ~ It is commonly assumed that both Henry’s Law and Beer’s Law apply individually to solute species and that the proton chemical shift of each species is concentration inde~endent.~~ Important qualitative information about hydrated species can be obtained from spectral data even when it is not possible to calculate hydrate formation constants.Bands in the 3 pm region have been attributed to 1 :1 and 2:l hydrates (B H-0-H and B * * - H-0-H * * - B) of organic bases (B) in CCl,.39 The relative strengths of water-polar solute interactions have been inferred from proton chemical shifts24 and OH-stretching f r e q ~ e n c i e s ~ ~ ~ of water dissolved in hydrogen-bonding solvents. B. Thermodynamic Results for Water-Polar Solute Complexes.-Before the past decade not much was known about specific molecular complexes of water. as R. Van Duyne S. A. Taylor S . D. Christian and H. E. Affsprung,J. Phys. Chem. 1967,71 3427. 38 L. 0. Sill& Acra Chem. Scand. 1964 18 1085.37 a F. Takahashi and N. C. Li J . Amer. Chem. SOC. 1966,88 11 17; b J. D. Worley Ph.D. Dissertation University of Oklahoma 1964; c A. Fratiello and D. C. Douglas J. MoZ. Spectroscopy 1963 11 465; d J. M. Sorensen Ph.D. Dissertation Case-Westem Reserve University 1967. 88 N. Muller and P. Simon J. Phys. Chem. 1967 71 568. as a S. C. Mohr W. D. Wilk and G. M. Barrow J. Amer. Chem. SOC. 1965,87 3048; b P. Saumagne and M. L. Josien Bull. Soc. chim. France 1958 813 40 E. D. Becker J. Chem. Phys. 1959 31 269. 27 Molecular Complexes of Water in Organic Solvents and in the Vapour Phase This section summarises recent research which has yielded thermodynamic parameters for specific complexes of water with carboxylic acids phenols and alcohols ketones and ethers amines amides and various poly-functional polar solutes.Since 1960 several specific complexes of water (W) with monocarboxylic acids (A) have been reported based on spectral,4l vapour vapour pressure,43 and partition-water solubility44 measurements. 1.r. spectra of moist CClp solutions of three highly acidic halogenated acetic acids and three weaker aliphatic acids have been obtained in the acid C=O stretching and the water hydroxyl-stretching regions.4f At very low water activities new bands attributed to A W and AaW appear in both spectral regions. Formation constants for A W and A2W as well as AW2 have been r e p ~ r t e d . ~ ~ ~ ~ ~ The 1 :2 and 2:l complexes of benzoic acid and salicyclic acid in several non-polar and slightly polar solvents have formation constants (from the monomers) of the order of several hundred in la mole- units.A vapour phase constant at 25" of 0.010 torrs (equivalent to 3.5 x lo6 1 mole-2) has been obtained for the formation of the dihydrate of trifluoroacetic Values of the formation constant for A W for aromatic acids in CC14 benzene and 1,2-dichloroethane are in the range 1-12 1 mole-2. The large magnitudes of the A2W and AW formation constants as well as the acid self-association constants indicate the difficulty of determining accurate parameters for the A W formation constant; i.e. it is difficult to reach a con- centration region in which a major portion of the hydrated acid is in the form of the 1:l complex. Similar but less severe complications arise in studies of hydrates of other polar solutes having both donor and acceptor sites. Relatively little is known about specific hydrates of phenols and alcohols in spite of the fact that numerous studies have been made of aqueous and moist organic solutions of these mmpounds.ld~* Hydration studies are complicated by a lack of reliable values of self-association constants.Early p a r t i t i ~ n ~ ~ ~ ~ ~ and cryoscopic2sa investigations showed that strong interactions occur between dissolved water and phenols or alcohols in organic solvents. Partition-water solubility data at unit water activityYc6 and water solubility data at reduced water activities,46b-d have indicated the presence of PW Paw and PW in solutions of phenol (P) and water (W) in several organic solvents. In benzene 1,1,2,2- tetrachloroethane and 1 ,2-dichloroethaneY the trirneric hydrates are more 41 J.de Villepin A. Lautie and M. L. Josien Ann. Chim. 1966,1 365. 42 S. D. Christian H. E. Affsprung and C. Ling J. Chem. SOC. 1965 2378. 4s G. 0. Wood D. D. Mueller S. D. Christian and H. E. Affsprung J . Phys. Chem. 1966 70 2691. *4 a R. Van Duyne S. A. Taylor S. D. Christian and H. E. Affsprung J . Phys. Chem. 1967 71 3427; b R. Van Duyne Ph.D. Dissertation University of Oklahoma 1969. 45 a R. D. Vold and E. R. Washburn J. Amer. Chem. SOC. 1932,54,4217; b E. R. Washburn V. Hnizda and R. D. Vold ibid. 1931,53,3237; c E. R. Washburn and H. C. Spencer ibid. 1934 56 361. 46 R. M. Badger and R. C. Greenough J. Phys. Chem. 1961 65 2088; b J. R. Johnson Ph.D. Dissertation University of Oklahoma 1966; c J. R. Johnson S. D. Christian and H. E. Affsprung J. Chem. SOC. (A) 1967 764; d J. R. Johnson S.D. Christian and H. E. Affsprung ibid. 1965 1. 28 Christian Taha and Gash important than PW and account for most of the bound water. In CC14 solutions of phenol Afw rises rapidly with increase in phenol concentration and aw indicating the presence of hydrates involving at least 2 water molecules and several phenol molec~les.~~d Partition and water solubility data for alcohols and 1-naphthol in toluene indicate that 2:l alcohol-water and 1 :1 naphthol-water species are probably present,,' although the stoicheiometry of water in these complexes has not been confirmed by solubility measurements at reduced water activities. Similar data for 1-decanol (D) provide evidence for the existence of DW in 1 ,2-dichloroethane and D2W in i~o-octane.~~ Conductance measurements for solutions of several nitro-substituted phenols and water (fw > 0-3 mole 1-l) dissolved in acetonitrile indicate that the phenols form hydrates PWn in which n is one greater than the number of nitro-groups in the substituted phenol.*@ Ethers and ketones are not highly associated in organic solvents at concentra- tions less than several tenths molar.Therefore the analysis of hydration data for these compounds does not involve large corrections for solute self-associa- tion. The acetone monohydrate in 1 ,Zdichloroethane has been investigated by dielectricto lH n.m.r.,22 and partition-water solubilityZ2 methods; a formation constant of 0.85 1 mole-' at 25" and a dipole moment of 3.4 D have been reported for the complex. The dipole moment of the hydrate nearly equals the vector sum of the moments of water and acetone.Table 2 lists formation constants Table 2 Equilibrium constants for formation of hydrates of ketones ethers and dimethylsulphoxide at 25 O Ketone or Ether Solvent Cyclopen t anone Acetone 2,3-Butanedione Acety lacet one Acetone Tetrahydrofuran Dioxan Dimethylsulphoxide CCl CCI CC14 CC14 Acet one-cyclohexane THF-cyclohexane Cyclohexane Dimethylsulphoxide-CCI KBW KB,w* Ref. 2.2 1.4 51b 2.4 1.6 51a 1.2 2.0 51a 1.5 0.86 51a 0-08t 37a 0-13t 37a (I mole-l) (1 mole-') 1*3t 0.85t 38 0-26t 52 *For the reaction BW + B = B2W. ?Converted from mole fraction units and interpolated to 25" for the ketone (B)-water complexes BW and B,W in CC14 and cyclohexane at 25°.37aJj1 Hydration equilibria of ethers have been investigated by lH n.m.r. measurements;37as38 constants for dioxan and tetrahydrofuran complexes are 47 D.J. Turner A. Beck and R. M. Diamond J. Phys. Chem. 1968,72,2831. 48 D. J. Turner and R. M. Diamond J . Phys. Chern. 1968,72,3504. '@ A. D'Aprano and R. M. Fuoss Proc. Nat. Acad. Sci. US. 1968,61 1183. 6o T. F. Lin S. D. Christian and H. E. Afhprung J. Phys. Chem. 1967,71 1133. 61 a T. F. Lin S. D. Christian and H. E. Affspmg J. Phys. Chem. 1967,71 1968; b R. L. Lynch S. D. Christian and H. E. Affsprung ibid. 1969,73 3273. 29 Molecular Complexes of' Water in Organic Solvents and in the Vapour Phase included in Table 2 as well as the B2W formation constant for dimethyl sul- phoxide.6a AH for the addition of the first water to acetone is ca. -3.5 k a l / whereas values in the range - 1.6 to - 2-8 kcal/mole have been obtained for addition of a second base unit to the monohydrates of ketones and ethersg7V* and dimethyl s~lphoxide.~~ Presumably B2W has the bridged structure B 9 HOH - - - B proposed earlier on the basis of i.r.It is obvious that weaker hydrogen bonding is involved in ketone- or ether-water complexes than in hydrates of phenols and carboxylic acids. The 1 :1 complex formation constants reported in Table 2 are comparable in magnitude to those for com- plexes of similar bases with Considerable effort has been expended in studying the hydration of amines aniides and biologically important compounds containing the amide group and peptide linkages. The fact that amines are known to form strong hydrogen bonds with proton donors of many types and the tendency of amines to obey Henry's Law throughout considerable ranges of concentration have made these compounds logical choices as solutes in hydration studies.There is spectral evidence for the aromatic amine (D) hydrates DW and D2W in organic solvent~,39,~~ although some aliphatic amines apparently form DW but not D2W owing to the strong inductive effect of the polar D W hydrogen bond.65 Table 3 summarizes formation constants for DW DW2 and D2W (from the monomers) as well as selected values of these parameters for methanol-amine complexes. Dipole enhancements for 1 1 complexes of hydroxylic proton donors and amines have been reported; the excess of the complex dipole moment over the vector sum of the donor and acceptor moments ranges from less than 0.3 D for aliphatic alcohol complexes with pyridine and triethylaminess to 0.4-1.3 D for aliphatic amine complexes with ~ a t e r l ' e ~ ~ and phenols.68 1.r.spectral evidence for the separation of charge in various n-propylamine-proton donor com- plexes50 complements the dielectric results; the amine N - H and the OH a e N stretching bands are significantly altered in complexes with proton donors having pKa values < ca. 10. Nevertheless the charge separations indicated by the dipole moment enhancement for water-amine complexes are unexpectedly large compared to those reported for the aliphatic alcohol-amine complexes. Table 4 gives formation constants for the amide (M) hydrates MW and M2W. Difficulties again arise in treating hydration data for lactamsso and acetamides61 in that accurate self-association constants are required in calculating hydrate S.F. Ting S. Wang and N. C. Li Canad. J. Chem. 1967,45,425. 6a E. S. Hanrahan and F. Deskins Proc. West Virginia Acad. Sci. 1967,39 371. 64 A. N. Sidorov Optics and Spectroscopy 1960 8 24. 6s H. E. Affsprung J. Derkosch and F. Kohler Discuss. Furaday Soc. 1965 40,224. sB J. W. Smith J. Chim. phys. 1964 61 125. M. D. Gregory Ph.D. Dissertation University of Oklahoma 1968. J. R. Hulett J. A. Pegg and L. E. Sutton J. Chem. SOC. 1955 3901. T. Zeegers-Huyskens Spectrochim. Acta 1965,21 221. 8o a J. D. Worley Ph.D. Dissertation University of Oklahoma 1964; b D. Mueller Ph.D. Dissertation University of Oklahoma 1966. a R. D. Grigsby Ph.D. Dissertation University of Oklahoma 1966; b R. D. Grigsby S. D. Christian and H. E. Affsprung J . Phys. Chem. 1968,72,2465. 30 Christian Taha and Gash Table 3 Equilibrium constants for formation of amine-water a d amine-methanol complexes at 25" Amine Diethy lamine Diethy lamine Diethylamine Pyridine Pyridine Pyridine Pyridine Pyridine C yclohexy lamine N-Methylcyclo- hexy lamine NN-Dimethylcyclo- hexy lamine Triethy lamine Triethylamine Triethy lamine Diethy larnine Diethylamine Diethy lamine Pyridine 2-Picoline 2-Ethylpyridine 2-Isoprop ylpyridine 2- t-Butylpyridine 2.6-Lu tidine Water Complexes Solvent KDW Hexadecane 11.0 Diphenylmethane 8.5 Benzyl Ether 2.9 Cyclohexane 5.3 CC14 2.6 Toluene 1.5 Benzene 1.2 1-2 Dichloroethane 1.0 Benzene 6.7 (1 mole-l) KDW~* KD,w* Ref.(12 mole-2) (P mole-2) a a 9-7 a 6-8 b 18.3 3-0 b 7.3 1.6 b 6.3 1-5 b 1 -0 1.0 b C Benzene 5.3 C Benzene 3.9 Benzene 3.5 Cyclohexane 7.0 Toluene 3.7 Methanol Complexest Hexadecane 8.7 Diphenylmethane 4.8 Benzyl Ether 2.7 CC14 2.3 CCL 3.1 CC14 2.6 CC14 1-7 CC14 1.4 CCl 4-4 300 22-5 8-5 C C C c sEthyl-6-methylpyridine CCl; 4.1 d 2,6-Diethylpyridine CC14 3.0 d 2,6-Di-isopropylpyridine CCl 0.8 d a ref.34b; b ref. 74b; C ref. 17; T. Kitao and C. H. Jarboe J. Org. Chem. 1967,32,407. *For the reactions from the monomers fIn the case of the methanol systems KDW and KDW indicate formation constants for the 1 1 and 1 2 amine -methanol complexes respectively. Table 4 Equilibrium constants for formation of amide hydrates at 25" Amide Solvent KMW KM~w* Ref. (1 mole-') (1 mole-') 2-Pyrrolidone CC14 9.0 9 a N-Methyl-2-pyrrolidone CCI 9.2 39 b N-Methyl-2-pyrrolidone Benzene 6.0 4.3 b N-Methyl-2-p yrrolidone 1,2-Dichloroethane 2.4 b NN-Dimethylacetamide 1,2-Dichloroethane 2.2 b N-Methylacetamide cc14 11 C NN-Di me t h ylacet amide NN-Dime t hylacetamide- cyclohexane 0.3 d a ref.37b; b ref. 606; ref. 616; ref. 37a *For the reaction MW + M = M2W 31 2 Molecular Complexes of Water in Organic Solvents and in the Vapour Phase formation constants. N-Methylacetamide (M) in CC& for example associates extensively in dilute solution to form chain polymers. The formation constant for the reactions M + W = MW Mz + W = MzW . . . Mn + W = MnW appears to be nearly constant (ca. 11 1 mole-’) and independent of chain length which suggests that each unit in the chain contributes an equal number of nearly equivalent hydration sites.B1b 2-Pyrrolidone is extensively dimerized in CCl, and the equilibrium constant for the reaction M + W = MW nearly equals that for MP + W = MPW.The species MIW probably consists of water attached to the cyclic 2-pyrrolidone dimer ; whereas N-methyl-2-pyrrolidone which associates to a lesser degree probably forms the bridged hydrate Several reviews have treated the extensive literature on the energetics dynamics and structure of water molecules bound to proteins and other macro- molecules;1bJf~s2 no attempt will be made to summarise research in this area. In the context of the present Review a series of studies of the interactions of water with various polar groups of deoxyribonucleic acid (DNA)s3 and its molecular constituentsB4 is of interest. 1.r. spectra of thin films of Na and Li salts of DNA have been measured at various water activities (relative humidities).Bab As aw increases from 0 to 0.60 first the POz- group and then the POC and COC oxygens become hydrated.At a water activity of ca. 0.65 a hydration shell of 5 or 6 water molecules probably surrounds the phosphate group; above aw = 0.65 the C-0 and ring nitrogen atoms become hydrated. Polarised i.r. and U.V. spectra indicate that between aw = 0.55 and 0-75 the structure of DNA changes from the disordered lower humidity form to the ordered B configuration in which the base pairs become stacked one above another and oriented perpendicular to the axis of the helix.s3c Gravimetric results for numerous constituents of DNA show that most of the solid purines and pyrimidines and their nucleosides and nucleotides are little hydrated at water activities <0.93 whereas several salts of the nucleotides and other com- pounds containing the ionic phosphate group absorb >20 molecules of water per molecule of compound at aw = 0.93.These studies underline the importance of the ionic phosphate groups as sites for hydration in nucleotides and poly- nucle~tides.~~ Tributylphosphate (TBP) and other organophosphorus compounds have been widely used as extracting agents for metal ions.6s There have been several reports of the hydration of these compoundS,sa although in most studies the M . . W . . M.606 6a J. Steinhardt and S. Beychok in ‘The Proteins’ ed. H. Neurath Academic Press New York and London 1964 vol. 2 p. 276. 63 a M. Falk K. Hartman and R. C. Lord J. Amer. Chem. SOC. 1962,84,2843; b M. FaIk K. Hartman and R. C. Lord ibid. 1963,85 387; c M. Falk K. Hartman and R. C. Lord ibid. 1963 85 391.64 M. Falk Canad. J. Chem. 1965 43 314. 66 H. Freiser Analyt. Chem. (Ann. Rev.) 1968 40 522R. a L. Kuca Coll. Czech. Chem. Comm. 1967,32,720; b D. C. Whitney and R. M. Diamond J. Phys. Chem. 1963 67,209; c T. J. Conocchioli M. I. Tocher and R. M. Diamond ibid. 1965 69 1106; d J. J. Bucher and R. M. Diamond ibid. 1969,73,675; e J. J. Bucher and R. M. Diamond ibid. 1969 73 1494. 32 Christian Taha and Gash water activity was not varied systematically so that the stoicheiometry of water in the hydrate formation reactions could be inferred. A IH n.m.r. investigations7 yielded equilibrium constants equal to 6.1 and 0-73 1 mole-1 (converted from mole fractions to molarity units; interpolated to 25") for the reactions TBP + W = TBP-W and TBP-W + TBP = (TBP),.W respectively.Enthalpy values of -4.1 and -2.0 kcal/mole respectively were given for the two hydration reactions. Water solubility-partition data for TBP yield 1 1 hydrate formation constants of 34 in iso-octane,66d 18 in C14,86b 9-1 in benzene,66e and 2.6 1 mole-1 in CHC13.66e IH N.m.r.68 and partition-water solubilityss studies indicate that HN03 H20 and (HN03)2 H20 exist in benzene and toluene. Recent reviews provide numerous references to investigations of the hydration of ions in organic medias5 and the extraction of metal salts and chelate~.~~ There is ample evidence for the formation of strong complexes between water and extracting agents (including ions ion pairs and polar molecules) but accurate thermodynamic data for well-characterised hydrate species are generally lacking. 4 Solvent Effects Variation in polarity and reactivity of solvents strongly affects values of the thermodynamic functions characteristic of hydrogen bond formation reactions.Even the so-called 'inert' solvents influence molecular complex formation equilibria in general reducing the tendency for aggregates to form as compared to the vapour More polarisable and more polar solvents ordinarily lead to a further decrease in the percentage of associated species present at fixed total concentration of donor and acceptor. Henry's Law constants,'l X-H stretching frequencies,7oa solubility parameter~,~~C and IH n.m.r. chemical shifts of dissolved solutesa4 have been related to the reactivity of solvents; in a general way changes in these parameters parallel variations in thermodynamic constants for association reactions of the solutes in the various media.Quantitative treatments of the effect of solvent on complex formation reactions have falIen into two main classes:70C972 (i) methods in which corrections are made for presumed competitive equilibria involving the formation of complexes between solvent molecules and one or more of the dissolved solute species; and (ii) techniques in which solvation of the donor acceptor and complex molecules is considered to occur in a non-specific way. Neither approach by itself has 67 S. Nishimura C. Ke and N. C. Li J. Amer. Chem. SOC. 1968 90 234. g9 a E. Hogfeldt and B. Bolander Arkiv Kemi 1963,21 161 ; b C. J. Hardy B. F. Greenfield and D. Scargill J. Chem. SOC. 1961 90. 70 u A. R. H. Cole and A. J. Michell Austral. J. Chem. 1965,18,102; b T.Gramstad Specfro- chim. Acru 1963 19 1363; c H. Buchowski J. Devaure P. V. Huong and J. Lascombe Bull. SOC. chim. France 1966 2532. 71 M. L. Josien and N. Fuson J. Chem. Phys. 1954,22,1169. 78 a R. S. Drago T. F. Bolles and R. J. Niedzielski J. Amer. Chem. SOC. 1966 88 2717; b P. J. Trotter and M. W. Hanna ibid. 1966,88 3724; c S. Carter J. N. Murrell and E. S. Rosch J. Chem. SOC. 1965 2048; d G. Kortiim and W. M. Vogel Z. Elektrochem. 1955 59,16; e S. D. Christian J. R. Johnson H. E. Affsprung and P. J. Kilpatrick J. Phys. Chem. 1966,70,3376; f J. Barrio1 and A. Weisbecker Compr. rend. 1967,265 C 1372. J. C. Eriksson L. Odberg and E. Hogfeldt Acra Chem. Scand. 1967,21,1925. 33 Molecular Complexes of Water in Organic Solvents and in the Vapour Phase proven to be entirely satisfactory.Methods of type (i) require the assumption that equilibrium constants for specific molecular complexes involving the solvent and other species are truly constant over wide ranges of concentration and in different media. On the other hand type (ii) methods tend to underestimate the effect that formation of specific molecular complexes between solute and solvent has on the energetics of the dissolved donor acceptor or complex. We shall limit our discussion of solvent effects to some recent applications of non-specific solvation theories. In treating data for reactions such as A(so1vated acceptor) + D(so1vated donor) = AD(so1vated complex) it is convenient to define the dimensionless parameter a = LIEADO / (~EAO + LIED*) in which ~EADO ~EAO and represent internal energies of transfer of the individual species from vapour to the sohent s at infinite d i l ~ t i o n .~ * e ~ ~ ~ The parameter a may be interpreted as the fraction of the energy of solvation of the free donor plus acceptor that is retained by the complex AD; in the absence of strong dipole enhancement in the complex it is expected that a < 1 since the process of bringing the reactive groups of donor and acceptor together to form the complex should 'squeeze out' solvent molecules. Apparently a is not strongly dependent upon choice of medium; a similar parameter a' = ~GADO 1 (AGAO + AGDO) (where the AGO terms represent solvation Gibbs free energies for IM ideal dilute solution standard states) is also ntarly constant from solvent to solvent. Procedures have been developed for calculating a from a lattice theory utilising group interaction energies,74 and for predicting E' from v 4 v-+s v-+s v+s v-bs V+S v-4 v4s v+s Several useful relations involving a and a' are - ~ E I O = (a - 1) (AEA' + LIEDO) I-tII I+II AGIIO - AGIO = (a' - 1) ( ~ G A ' +AGDO) I411 I+II KADII I KADI = (KI),AKD,D)"-~ 8 in which ~EIIO and ~EIO are standard internal energy changes for the reaction A + D = AD in solvent I1 and solvent I respectively; ~ G I I O anddGIO are the corresponding standard free energy changes; KADII and KADI are equilibrium constants for the formation reaction in the two solvents in 1 mole-* units; and &,A and KD,D are distribution constants between solvent I and solvent I1 for the acceptor and donor respectively.(KD,A for example may be equated to the ratio of the Henry's Law constant of the acceptor in solvent I to the Henry's Law constant of acceptor in solvent 11; a large value of KD for a given solute indicates that solvent I1 is much more effective than solvent I in solvating the species.) 79 a S.D. Christian and J. Grundnes Acta Chem. Scand. 1968 22 1702; b S. D. Christian and H. E. Affsprung ?3olute Properties of Water' U.S. Government Printing Office Wash- ington D.C. 1968 p. 57. 74 a T. L. Stevens Ph.D. Dissertation University of Oklahoma 1968; b J. R. Johnson P. J. Kilpatrick S. D. Christian and H. E. Affsprung J. Phys. Chem. 1968,72 3223. 34 Christian Taha and Gash If a’ is constant equation (7) requires that for a given complex formation reaction in a series of solvents a plot of log KAD vs. log (KD,AKD,D) will be linear with slope a’ - 1.Such a plot is shown in Figure 2 for the reaction water (A) + pyridine (D) = pyridine monohydrate (AD).72e The points lie approxi- mately on a straight line having a slope of - 0-29 corresponding to a’ - 0-71. For the solvents employed lattice calculations lead to predicted values of a and a‘ of ca. 0-75. Strong complexes for which the dipole moment is considerably greater than the vector sum of the moments of the unreacted donor and acceptor exhibit 35 Molecular Complexes of Water in Organic Solvents and iit the Vapour Phase anomalous solvation effects.75 To account for the large values of a and a’ obtained for amine-water complexes it has been proposed that the solvation energy of the complex includes an extra contribution due to excess dipole- induced dipole interactions not properly accounted for in lattice energy calcula- tion~.~’ Table 5 lists values of a and a’ for the 1 :1 hydrate of diethylamine (DEA) Table 5 Eflect of solvent on donor acceptor and complex for the reaction DEA + W = DEA-W or(experimenta1) a’(experimenta1) ~EODEA*H,O (kcal/mole) ~ E O D E A (kcal/mole) AEOH,O (kcal/mole) v+s v4s v+s d@DEA’H,O (kcal/mole) v4s v+s v-bs d G’DEA (kcal/mole) dGO13,o (kcal/mole) n-Hexadecane 1.01 1-05 - 7.80 - 5.60 -2.13 - 3.80 - 3.16 - 0.45 Dipheny lmethnne 0-85 1 -01 - 8.92 - 6.29 -4.17 - 5.20 - 3.36 - 1.79 Benzyl ether 0.80 0.90 - 10.18 - 6.43 - 6.33 - 5.26 - 3.33 - 2.54 in several solvents as well as solvation energies and free energies for the in- dividual species involved in the Predicted values of a and a’ for n-hexadecane and diphenylmethane obtained from lattice calculations again lie in the range 0-7-0.8.The difference between the energy of solvation of the complex in each solvent and that predicted from the solvation energies of the unreacted components using a = 0.75 is ca. - 1 to - 2 kcal/mole. A major part of this extra solvation energy is probably due to the interaction of the excess dipole of the complex with the polarisable solvent. It is clear that non- polar and slightly polar solvents are less effective in preventing the formation of highly polar complexes than they are in opposing complex formation between weakly-interacting donor and acceptor molecules. The properties of aqueous solutions and of dilute solutions of water in non- polar media are apparently little related. However an understanding of the progressive changes which occur in the solvation of water and other polar molecules in various media including dipole-solvent intera~tions~~f and inductive effects,”a should form the basis for building a bridge between these two interesting areas of chemistry. l6 J. Grundnes and S. D. Christian J. Amer. Chem. SOC. 1968,90,2239. 36

ISSN:0009-2681    

DOI:10.1039/QR9702400020

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Photochemical Reactions in Natural Product Synthesis By P. G. Sammes DEPARTMENT OF CHEMISTRY IMPERIAL COLLEGE LONDON S.W.7 1 Introduction One of the most rewarding areas for synthetic innovations in recent years has been in the field of organic photochemistry. In general absorption of light by a molecule can produce three types of activated molecule not accessible by normal thermal means. These are the electronically excited singlet and triplet states and often a vibrationally ‘hot’ ground state. Each of these excited states may undergo different chemical reactions in proceeding back to the ground state. The triplet excited state which generally has a relatively long lifetime is frequently encoun- tered in photochemical reactions. The potential complexity of photochemical reactions has deterred many chemists from exploiting them in syntheses.However with the increased under- standing of the nature of photochemical processes the chemist is now often able to quench undesirable reaction paths and to sensitise the required course of reaction. Furthermore because excited states possess potential energy levels above that of the ground state photochemical reactions often lead to products with strained structures. The controlled release of such strain energy can provide a suitable driving force for subsequent reactions as illustrated below in the synthesis of caryophyllene alcohol (p. 48). Although related reviews1 have been written and several excellent textbooks2 have appeared the present review is intended to illustrate recent examples of the scope and preparative significance of photochemical reactions as applied to the synthesis of natural products.As a consequence many important photo- chemical reactions such as halogenation nitrosationag and alkylati~n,~ have been omitted. However because of the importance of protecting groups in any synthetic work a final brief section on photosensitive protecting groups has been included. (a) K. Schaffner Fortschr. Chern. otg. Nu?ws?ofe 1964 22 1 ; (b) P. de Mayo and S. T. Reid Quart. Rev. 1961,15,393; (c) P. de Mayo Adv. Org. Chem. 1960’2,367. * (a) A. Schonberg ‘Preparative Organic Photochemistry’ Springer Verlag Berlin 1968; (b) J. G. Calvert and J. N. Pitts ‘Photochemistry’ J. Wiley New York 1965; (c) R. 0. Kan ‘Organic Photochemistry’ McGraw-Hill Book Co. New York 1966; (d) D. C. Neckers ‘Mechanistic Organic Photochemistry’ Reinhold Publishing Corp.New York 1967; (c) ‘Organic Photochemistry’ ed. 0. L. Chapman Marcel Dekker Inc. New York 1967; (f) ‘Advances in Organic Photochemistry’ eds. W. A. Noyes G. S. Hammond and J. N. Pitts Interscience New York 1963; G. Sosnovsky ‘Free Radicals in Preparative Organic Chemistry’ Macmillan Co. New York 1964. a D. Elad Forrschr. Chem. Forsch. 1967.7 528. 37 Photochemical Reactions in Natural Product Synthesis 2 cis-trans Isomerisation The light induced isomerisation of cis and trans olefins is well do~umented.~ For example irradiation readily establishes an equilibrium between maleic (75 %) and fumaric acid (25 %).s Similarly photolysis effects isomerisation ( 1 ) (2) between cis- and trans-cinnamic acids6 and between tiglic (1) and angelic (2) acids.? cis-trans Isomerisation is of extreme importance in the visual processes associated with vitamin A8 and in carotenoid chemistry.3b The conversion of HO (3) (4) trans-vitaminD (3) into the active cis-isomer (4) proved crucial in the total synthesis of the vitamin.B The mechanism of cis-trans isomerisation has been investigated in detail.3,10 In the excited triplet state of simple olefins the lowest energy conformation is produced by rotation of 90" about the carbon-bon bond.ll It is believed that collapse to the ground state occurs from this orthogonally disposed excited ' ( a ) G.M. Wyman Chem. Rev. 1955 55 625; (b) L. Zechmeister 'Cis-tram Isomeric Carotenoids Vitamins A and Arylpolyenes' Academic Press New York 1962. bA. R. Olsen and R. F. Hudson J. Amer. Chem.SOC. 1933,55 1410. ' S. W. Pelletier and W. L. McLeish J. Amer. Chem. SOC. 1952,74,6292. * (a) H. H. Inhoffen G. Quinkert H. J. Hess and H. Hirschfield Chem. Ber. 1957,90,2544; (b) H. H. Inhoffen H. Burchardt and G. Quinkert Chem. Ber. 1959 92 1564; (c) I. T. Harrison and B. Lythgoe J. Chem. Soc. 1958 837; ( d ) I. T. Harrison R. A. A. Hurst B. Lythgoe and D. H. Williams J. Chem. Soc. 1960 5176. lo (a) R. B. Cundall and P. A. Griffiths J. Amer. Chem. Soc. 1963,85,1211; (b) G. S. Ham- mond N. J. Turro and P. S. Leermakers J. Phys. Chem. 1962,66 1144. l1 R. S . Mulliken and C. C. J. Roothan Chem. Rev. 1947,41,219. R. Stoermer Ber. 1909,42,4865 and 1911,44,637. M. Mousseron Adv. Photochem. 1966,4 195. 38 Sammes state to form either the cis- or the trans-isomer. The isomerisation is often sensitised since direct photolysis to the excited singlet state is often difficult to achieve the energy required lying in the far ultraviolet region and because intersystem crossing to give the required triplet level is inefficient.In sensitised reactions the triplet level can be reached directly by energy transfer from an excited triplet dOnor.la For stilbene Hammond‘s group found that high energy donors those capable of transferring energy in excess of that required to promote either the cis- or tram-olefin to its triplet level all afforded the same equilibrium ratio of cis- to trans-stilbene as expected for diffusion controlled energy transfer.l2U As lower energy sensitisers were used different ratios of the two isomers formed since the excitation energy required by either the cis- or the trans-stilbene differ with the consequence that the sensitiser preferentially excites one of the two isomers in this case the trans-isomer so that the ratio of the cis-isomer in the equilibrium mixture increases.Isomerisation even occurred when the sensitiser energy was below that required to attain the usual triplet excited state of either of the starting olefins. This effect was explained by postulat- ing that non-Franck-Condon processes occur in which excitation occurs with T1 (trans) &’ So (trans) S (cis) 0 trans 90 180 cis Figure 1 Schematic representation of cis-trans isomerisation a. Sensitised excitation. b. “on-vertical’ excitation. c. Internal conversion to the ground state. T. Triplet state. S. Singlet state. 8. Angle of twist ‘*(a) G. S. Hammond J.Saltiel A. A. Lamola N. J. Turro J. S. Bradshaw D. 0. Cowan R. C. Counsell V. Vogt and C. Dalton J. Amer. Chem. SOC. 1964,86,3197; (b) P. J. Wagner and G. S. Hammond Adv. Photochem. 1968 5 21. 39 Photochemical Reactions in Natural Product Synthesis rotation about the olefin carbon-carbon bond to give the so-called ‘phantom’ triplet state i.e. the orthogonally oriented triplet (see Figure 1). For sensitisers of very low energy no isomerisation is possible. 3 Photolytic Electrocyclic Reactions Conjugated olefinic systems can often undergo direct photocatalysed electro- cyclic processes recently reviewed.13 Such reactions are extremely useful in natural product synthesis because they generally proceed along well defined stereochemical paths. An example is in the synthesis of vitamin D2 from ergo- sterol (5).14 The cyclic diene is isomerised photolytically into the acyclic triene previtamin D2 (6) by cleavage of the 9,lO-carbon-carbon bond.The latter compound can undergo photocatalysed recyclisation to give back either starting material or lumisterol (7) in both of which the cyclisation has occurred in a conrotatory mode as well as cis-trans isomerisation to (8). On heating pre- vitamin D2 establishes an equilibrium with pyrocalciferol (9) and isopyro- calciferol (lo) the thermal cyclisations occurring in a disrotatory manner. At the same time a thermal sigmatropic rearrangement occurs to give vitamin D2 (11). The photoisomerisation of a cyclic diene into a triene has also been used in an elegant synthesis of dihydrocostunolide (1 2).16 Photolysis of the bicyclic lactone diene (14) derived from a-santonin (13) gives an equilibrium mixture with the medium ring triene (15) again by a conrotatory process.Because of strain in the medium ring triene conjugation is restricted and selective reduction of the central cis-disubstituted bond with Raney nickel is readily effected to yield dihydrocostunolide (12). A simple extension of this reaction has allowed the conversion of the trans-fused bicyclic precursor (1 6) into the cis-fused compound occidentalol (17).l6 Cyclisation of the intermediate triene occurs thermally by a disrotatory process. Since the hexatriene-cyclohexadiene isomerisation is reversible suitable trienes are readily cy~1ised.l~ For example cis-stilbene is cyclised on photolysis to 9,lO-dihydrophenanthrene the trans-isomer forming.Mild oxidation of the latter with air iodine,l* or cupric chloride,lg produces phenanthrene. This very useful reaction has been applied to the synthesis of aporphine alkaloids.20 For example the substituted stilbene (18) on photolysis can lead to (&)-nu& ferine (19).21 The reaction is general and other triene systems will also cyclise. Thus the anil (20) cyclises upon irradiation provided the nitrogen atom is l3 G. B. Gill Quart. Rev. 1968 22 338. l4 (a) B. Lythgoe Proc. Chem. Soc. 1959 141; (b) H. H. Inhoffen Angew. Chem. 1960,72 875; (c) E. Havinga and J. L. M. A. Schlatmann Tetrahedron 1961 16 146. l5 E. J. Corey and A. G. Hortmann J. Amer. Chem. SOC. 1963,85,4033. l6 A. 0. Hortmann Tetrahedron Letters 1968 5785. l7 F. R. Stermitz ‘Organic Photochemistry’ ed.0. L. Chapman Marcel Dekker Inc. New York vol. 1 1967 p. 247. ln D. J. Collins and J. J. Hobbs Chem. and Znd. 1965 1725. 2o N. C. Yang G. R. Lenz and A. Shani Tetrahedron Letters 1966 2941. 21 M. P. Cava S. C. Havlicek A. Lindert and R. J. Spangler Tetrahedron Letters 1966,2937. C. S . Wood and F. B. Mallory J. Org. Chem. 1964,29 3373. 40 t- * c n W h 2 r- e h 2 W n E. 41 Photochemical Reactions in Natural Product Synthesis I g e 1 4 I I g n 2 W 0 ,o h 2 W n M - W 0 42 Sammes Reduction M e 0 Meo3YMe (19) protonated to give calycanine (21),22 a degradation product of the alkaloid ~alycanthine.~~ Under acidic conditions amides such as (22) can also be cyclised by irradiation. Presumably the acid protonates the amide group to give the immonium alcohol (23) which has increased carbon to nitrogen double bond character so that it behaves as a hexatriene system.The cyclised isomer can re-aromatise by loss of water. Borohydride reduction of the salt (24) yields the protoberberine alkaloid /3-coryaldine (25).24 The observed mode of cyclisation is of interest in that little of the competing reaction leading to the aporphine skeleton was obtained. It appeared therefore that cyclisations were preferred which aromatise without the need of oxidation. Using this argument Kupchan considered that a more efficient way of effecting similar conversions was to replace one of the hydrogen atoms normally removed by oxidation by a good leaving group. The iodo- 22 V. M. Clark and A. Cox Tetrahedron 1966,22 3421. and N. Sheppard Proc. Chem. SOC. 1960,76.24 G. R. Lenz and N. C. Yang Chem. Comm. 1967 1136. R. B. Woodward N. C. Yang T. J. Katz V. M. Clark J. Harley-Mason R. F. J. Ingleby 43 Photachemical Reactions in Natural Product Synthesis n m tf. n N N W 44 Sarnmes group was found to be useful for this Thus the iodostilbene (26) cyclised to give aristolochic acid (27) whereas classical methods for its synthesis had failed. In a detailed study of the mechanism of this reaction however it was concluded that the reaction does not proceed via a dihydrophenanthrene type of intermediate but instead by prior homolysis of the carbon to iodine bond.2s Nevertheless this alternative method of cyclisation is of general syn- thetic application. It has been used in an alternative synthesis of (&)- nuciferine (19) via photolysis of the precursor (28) as its hydrochloride sa1t.l' Further ramifications of the photocyclisation reactions have been exploited in syntheses.Brockmann has studied the cyclisation of bianthrone derivatives,28 the work culminating in the synthesis of hypericine (30) from protohypericine (29).29 This process is believed to be involved in the biosynthetic route to hypericine. Diphenylamine and its derivatives form the corresponding carbazoles on irradiati~n.~~ The presence of a mild oxidising agent is beneficial but does not appear to be essentiaL31 The alkaloid glycozoline (32) was readily prepared from the amine (31).32 25 S. M. Kupchan and H. C. Wormser Tetrahedron Letters 1965 359. 2e S. M. Kupchan and H. C. Wormser J . Org. Chem. 1965,30 3792. a7 S. M. Kupchan and R. M. Kanojia Tetrahedron Letters 1966 5353.28 H. Brockmann R. Neef and E. Miihlmann Chem. Ber. 1950,83,467. 28 H. Brockmann and H. Eggers Angew. Chem. 1965 67 706. 'l E. J. Bowen and J. H. D. Eland Proc. Chem. SOC. 1963,202. 32 W. Carruthers Chem. Comm. 1966,272. C. A. Parker and W. J. Barnes Analyst 1957 82 606. 45 Photochemical Reactions in Natural Product Synthesis OH 0 OH OH 0 OH OH 0 Oil hv Me0 Me0 OMe According to theory the electrocyclic process hexatriene to cyclohexadiene is one of a series of reactions.13 Conjugated butadienes should also photoisomerise to the corresponding cyclobutenes by a concerted disrotatory process.33 Such 38 R. Srinivasan Adv. Photochem. 1966 4 113. 46 Sammes a reaction is realised in the conversion of colchicine (33) by sunlight to P-lumi- colchicine (34) and its y-isomer (35).34 The #komer is further converted by light to give the dimer a-lumicolchicine (36).36 These processes have been achieved in ~ i t r o .~ ~ 4 Cycloaddition Reactions The addition of an olefin to another double bond can be catalysed by light.37 However simple olefins absorb in the far ultraviolet region which is difficult to reach experimentally particularly for preparative work. This problem can be overcome by either using sensitisers in which case reactions can proceed via the triplet manifold,12 or by using suitable derivatives of olefins which absorb at longer wavelengths i.e. either by conjugation or suitable intramolecular interaction. Thus germacrene D (37) in which there is a strong transannular effect has )cmsx at 259nm ( E 4500). Direct photolysis gives mainly (-)$- bourbonene (38).38 (37) A similar example is the photolysis of myrcene (39) which gave besides the more favoured cyclobutene derivative (41) some /3-pinene (40).a@ Better yields of the latter would be expected at higher temperatures where the cycIobutene could thermdly reform starting diene.In contrast the sensitised photolysis gives neither (40) nor (41) but instead the bicyclobutane (43).40 The sensitised cyclisation proceeds via a triplet excited state. This state behaves as a diradical and the two bond forming steps occur consec~tively.~~ Primary bond formation has a choice as to where it will occur. In such cases the preferred initial reaction will tend to form a five-membered ring where possible in preference to a smaller or bigger size.42 This empirical guide is known as the 'rule of five'.43 34 F.Santavy Coll. Czech. Chem. Comm. 1950,15 552. 85 0. L. Chapman H. G. Smith and R. W. King J. Amer. Chem. SOC. 1963 85 806. 36 (a) E. J. Forbes J. Chem. SOC. 1955 3864; (b) P. D. Gardner R. L. Brandon and 0. R. Haynes J. Amer. Chem. SOC. 1957,79 6334. s7 J. S. Swenton J . Chem. Educ. 1969 46 7. s8 K. Yoshihara Y. Ohta T. Sakai and Y. Hirose Tetrahedron Letters 1969 2263. 38 K. J. Crowley Proc. Chem. SOC. 1962 245. 40 R. S. H. Liu and G. S. Hammond J. Amer. Chem. SOC. 1967 89,4936. 48 (a) R. C. Lamb P. W. Ayers and K. M. Toney J . Amer. Chem. SOC. 1963 85 3483; (b) N. 0. Brace J. Amer. Chem. Soc. 1964 86 523; (c) C. Walling and M. S. Pearson J. Amer. Chem. SOC. 1964 86 2262. 48 R. Srinivasan and K. H. Carlough J. Amer. Chem. SOC.1967 89,4932. Cf. P. S. Skell and R. C. Woodworth J. Amer. Chem. SOC. 1960 82 3217. 47 Photochemical Reactions in Natural Product Synthesis For myrcene the primary step probably yields the most stable diradical (42) followed by cyclisation to the bicyclobutane (43). Sensitised Itv \ -1 (42) (43) On irradiation conjugated ketones can add to olefhs to give a cyclobutane deri~ative.~~ One of the first reported examples of this extremely important reaction was the intramolecular photocyclisation of carvone (44) to carvone- camphor (45).46 Corey was the first to realise that intermolecular applications of this reaction could be useful for the synthesis of natural products.4s In an elegant demonstration of its potential 4,4-dimethylcyclopentene (46) and 3- methylcyclohex-Zenone (47) were photolysed to give a mixture of cis- and trans-fused strained tricyclic ketones.The main isomer was the cis-anti-cis form (48). Addition of methyllithium gave the corresponding tertiary alcohol (49). Treatment of the alcohol with acid catalysed loss of water to give a carbonium ion followed by 1,Zbond rearrangement to relieve strain from the cyclobutane moiety. Quenching of the rearranged carbonium ion with water afforded directly a-caryophyllene alcohol (50). 44(u) P. E. Eaton J. Arner. Chern. SOC. 1962 84 2344 and 2454; (b) R. Criegee and H. Furrer Chern. Ber. 1964 97 2949. 46(u) G. Ciamician and P. Silber Ber. 1908 41 1928; (b) G. Biichi and I. M. Goldman J. Amer. Chem. SOC. 1957 79 4741. 46 E. J. Corey and S. Nozoe J. Amer. Chern. SOC. 1964 86 1652. 48 Sammes H+ (47) (48) )+ ! (49) H I L H+ In a related isobutene was added to cyclohexenone to give a mixture of (4,2,0)-bicycloketones.Mild base yielded mainly the &isomer (51) which was used as starting material in a synthesis of (j-)-caryophyllene (52) and (&)-isocaryophyllene (53). Y 4- Q 0 -3 4 The addition of substituted olehs to cyclohexenones is often stereoselective. The direction of addition is that expected for a stepwise reaction with formation of an intermediate diradical. A probable explanation for the stereoselectivity involves initial excitation of the enone by an n -+ m* transition to give a triplet state which is polarised (e.g. 54).48 An approaching olefin (e.g. 55) would also be polarised to give an oriented complex possibly even a charge transfer com- E. J. Corey R.B. Mitra and H. Uda J. Amer. Chem. SOC. 1964,86,485. '' E. J. Corey J. D. Bass R. LeMahieu and R. B. Mitra J. Amer. Chem. SOC. 1964,86,5570. 49 Photochemical Reactions in Natural Product Synthesis plex followed by bonding at the 2-position of the enone with the nucleophilic end of the olefin and final collapse to the observed product (56). Several approaches to the synthesis of the bourbonenes have been reported. In photolysis of a mixture of 2-cyclopentenone and the cyclopentene (57) gave a 1 :1 mixture of cis-anti-cis head to tail (58) and head to head (59) adduct s. Treatment of the latter ketone with methylenetriphenylphosphorane yielded p-bourbonene (38) which could be equilibrated with acid to give a- bourbonene (60). An alternative route started with the bis-enone (61) which was irradiated to give the diketone (62) that could be condensed and converted into a-bourbonene.60 ** J.D. White and D. N. Gupta J. Amer. Chem. SOC. 1968 90 6171. M. Brown J . Org. Chem. 1968,33 162. 50 Sarnrnes In a related studys1 the cyclodecadienone (63) was photolysed to give sub- stantial amounts of the ketone (a) an effective precursor of copaene (65)51 and the ketone (59). In contrast germacrene D (37) gave only small amounts of (64 x = 0) (65 X = CHa copaene on photolysis (see above). It is probable that the cyclisation of germ- acrene D proceeds by a singlet excited state in a concerted manner. Sensitised photolysis of germacrene would be expected to give more copaene. The synthetic utility of such photocycloaddition reactions has been con- siderably extended by de may^.^^ Irradiation of cyclic 1,3-diones or their en01 acetate esters with olefins gives a strained cyclobutane of the p-hydroxyketone type.These may spontaneously deketolise to give the ring expanded cyclic system by the addition of two carbon atoms. The reaction is exemplified by the synthesis of y-tr~polone.~~ Dichloroethylene and the en01 acetate of 173-cyclo- pentadione (66) were photolysed and the reaction product immediately hydro- lysed with methanolic base to give directly y-tropolone (67). c1 c*j +Jj AcO hv ‘*’’ “d AcO OH’- - 0 Cycloaddition of the enol acetate (66) to dimethyl chloromaleate (68) gave a mixture of isomeric adducts (69) which could be brominated with pyridinium perbromide followed by treatment with silver oxide and oxidation with sodium bismuthate to give the bicyclic dione (70).Mild acid hydrolysis removed the acetate group and the resulting #%hydroxyketone spontaneously deketolised 61 C. H. Heathcock and R. M. Badger Chem. Comm. 1968 1510. (a) V. H. Kapadia B. A. Nagsampagi V. 0. Naik and S. Dev Tetrahedron 1965,21,607; (b) P. de Mayo R. E. Williams G. Buchi and S. H. Feairheller Tetrahedron 1965 21 619. 68 P. de Mayo Pure Appl. Chem. 1964,9 597. “H. Hikino and P. de Mayo J. Amer. Chem. SOC. 1964 86 3582. 51 Photochemical Reactions in Natural Product Synthesis to give the naturally occurring tropolone stipitatonic acid (71) a synthesis that would be difficult to achieve by classical means.6s By the use of the appropriate precursors fused bicyclic systems containing a seven membered ring can also be synthesised.Thus /3-himachalene (76) has been prepared from the en01 acetate (72) and the acetal (73). Photolysis gave a good yield of the required adduct (74) but as an alternative to dealdolisation the ketone moiety was reduced to the alcohol followed by formation of the mesylate 0 9 u (73) H AcO’ OAc 0 m 0 P (74) + Q=@ O w 0 OAc (77) 65 0. L. Lange and P. de Mayo Chem. Comrn. 1967,704. 52 Sammes ester and then hydrolysis. Fragmentation to the keto-olefin (75) occurred which was eventually converted into the desired hydrocarbon.66 A study of the factors affecting the initial photocyclisation in the latter case showed that for non-polar solvents addition was highly stereoselective giving mainly (74) but as solvent polarity increased this preference decreased and more of the isomer (77) formed.This result again points to the importance of dipolar interactions between reactants.s7 That these reactions may also be sensitised indicates operation of a triplet mechanism.68 Various ramifications of these additions to enones have recently been applied to synthetic work. A route to the prostanoic acids e.g. (81) started from the cyclopentenone (78) and the chloro-olefin (79).69 This approach takes advantage of the fact that strained 1,4-diketones such as the adduct (80) are readily reduced by zinc dust with opening of the cyclobutane ring to give the diketone (81). de Mayo has recently discovered that enol esters of cyclopenta-1,2-diones &YR* Zn > & R l + c1 k hv > c1 R2 H 0 178) (79) (SO) (81) R ' = (CH2)6<:02CHj R2 =(CHz )4CH3 also add to olefins. Thus the acetate (82) adds to the cyclopentene (83) to give an adduct (84).This particular cyclopentene was chosen because of its ready 0 ( 8 5 ) I OH (87) B. D. Challand G. Kornis G. L. Lange and P. de Mayo Chem. Comm. 1967 704. 67 B. D. Challand and P. de Mayo Chem. Comm. 1968,982. 58 H. Nozaki M. Kurita T. Mori and R. Noyori Tetrahedron 1968,24 1821. 5D J. F. Bagli and T. Bogri Tetrahedron Letters 1969 1639. 53 Photochemical Reactions in Natural Product Synthesis availability and because the cyclopropane ring in the adduct is readily hydro- genated to the required dimethyl derivative. Subsequent oxidation gave the cyclopentenone (85) which rearranged on hydrolysis with mild base to relieve strain in the fused cyclobutane ring. The ring expanded product (85) was used in an approach to the synthesis of methyl isomarasmate (87).s0 The usefulness of these cycloaddition reactions is further clearly demon- strated by the synthesis of the steroid skeleton (89) in good yield from the p-diketone (88) and cyclopentene.6s 0 (92) (93) (94) 6o P.de Mayo D. Helmlinger R. B. Yates and L. Westfelt Abstracts International Sym- posium on ‘Synthetic Methods and Rearrangements in Alicyclic Chemistry’ Oxford July 1969 The Chemical Society London p. 32. 54 The addition of allene to the a/?-unsaturated ketone (90) has been used in the synthesis of the diterpenic alkaloids atisineB1 and veatchine,Ba via the inter- mediate (91). Similarly vinylogous amides also add to allene and this has been made the basis of an approach to the lycopodium alkaloidsBs For example reaction of the amide (92) with allene gives the cyclobutene (93) further con- verted into the known degradation product annatonine (94).s4 5 Photolysis of Carbonyl Groups General trends in the reactivity of carbonyl groups have been well established.os Saturated ketones react principally either by a-cleavage (e.g.95 to 96) the Norrish Type I process or by y-hydrogen abstraction (e.g. 97 to 98) often followed by further fragmentation as in the Norrish Type I1 process (98 to 99). Both reactions are believed to take place via an excited rz -+ ?T* triplet state.B6 B (95) hv ~ (97) 1 a R. W. Guthrie 2. Valenta and K. Wiesner Tetrahedron Letters 1966,4645. K. Wiesner S. Uyeo A. Philipp and 2. Valenta Tetrahedron Letters 1968 6279. K. Wiesner V. Musil and K. J. Wiesner Tetrahedron Letters 1968 5643.64 K. Wiesner I. Jirkovsky M. Fishman and C. A. J. Williams Tetrahedron Letters 1967 1523. *t5 J. S. Swenton J. Chem. Educ. 1969,46,217. 66 R. Srinivasan Adv. Photochem. 1963,1,83. 55 Photochemical Reactions in Natural Product Synthesis A. Cyclic Ketones.-These very often do not possess an available hydrogen atom and therefore a-cleavage is often favoured. The diradical (e.g. 96) cau collapse to either an aldehydo-olefin (100) or a keten (101).67 In the presence of oxygen an olefin-carboxylic acid can result formed by addition of oxygen to the excited carbonyl group (e.g. 95 to 102). In this way Quinkert has been able to synthesise nyctanthic acid (105) from b-amyrone (103) and roburic acid (106) from a-amyrone (104).68 (103 R1 = H Ra = Me) (104 Ra = Me Ra = H) (105 R1 = H Ra =Me) (106 R1 = Me Ro = H) Loss of carbon monoxide from the diradical formed during a-cleavage (e.g.96) only occurs in solution if the resulting diradical is stabilised.gB Thus the sugar pyranosidulose (107) collapses to the diradical (108) by extrusion of carbon monoxide since both the radicals are stabilised by a-oxygen substituents; recombination leads to the furanoid pentoses (109).70 Suitably a-substituted cyclic ketones which possess an available y-hydrogen atom also collapse by the Norrish Type I1 process.71 Thus colupulone (110) Po ohle > M e y - I I \ I \ I \ O x 0 O x 0 O x 0 67 G. Quinkert Angew. Chem. Internal. Edn. 1962,1 166. 6Q (a) G. Quinkert Pure Appl. Chem. 1964,9 607; (b) M. P. Cava and D. Mangold Tetru- he&on Letters 1964 1751; (c) K. Mislow and A.J. Gordon J. Amer. Chem. SOC. 1963 85 3521. ?O(u) P. M. Collins Chem. Comm. 1968 403; (6) For a review on the photochemistry of carbohydrates see G. 0. Phillips Adv. Carbohydrate Chem. 1963,18,9. 71 J. E. Gano Tetrahedron Letters 1969 2549. G. Quinkert and H.-G. Heine Tetrahedron Letters 1963 1659. 56 Sarnrnes which is very sterically crowded about the ring carbonyl function gives a good yield of 4-deoxycohumulone (1 1 l).7a Similar reactions of quinones are well documented. Jz v Y AT HO B. Conjugated Cyclic Ketones.-In the absence of a suitable addend so that cycloaddition reactions (see above) are not observed conjugated cyclic ketones tend to react so that the absorbing chromophore is lost.'* A simple case is the rearrangement of verbenone (1 12) to the unconjugated chrysanthenone (1 13).76 Similarly umbelhlone (1 14) rearranges to the phenol thymol (1 15).76 More complex are the transformations of the cross-conjugated ketones such as a-santonin (116) which has long been known to be sensitive towards light.?? The mechanisms of the photochemical transformations of santonin have been W.M. Fernandez Chem. Comm. 1967 1212. 73 CJ J. E. Baldwin and J. E. Brown Chem. Comm. 1969 167. K. Schaffner Adv. Photochem. 1966,4 81. 75 J. J. Hunt and 0. H. Whitham Proc. Chem. SOC. 1959 160. 76 J. W. Wheeler and R. H. Eastman J. Amer. Chem. SOC. 1959,81,236. 77 Kahler Arch. Pharm. 1830,34 318. 57 Photochemical React ions in Natural Product Synthesis 0 Jq-*oqy*0 0 0 0 0 (116 R = H) (121 R = OAC) ’0 A (120,R = H) / (122 R = OAC) / / O a 0 ( 1 23) OAc ‘ the subject of much study.’* They are best rationalised in terms of Zimmerman’s explanati~n.~~ An initial n + w* transition produces a diradical (117) which isomerises to (118) followed by collapse to a dipolar species (119).In aqueous acetic acid the anion is protonated and the resulting carbonium ion rearranges to isophotosantonic acid lactone (120) which has a guaianolide skeleton. This rearrangement has been shown to be generalso and consequently it has been used in the synthesis of several perhydroazulenes of the guaianolide series. For example 8-epi-artemisin acetate (121) on photolysis afTorded 8-epi-isophoto- artemisic acid lactone acetate (122). Dehydration and reduction followed by (a) P. 3. Kropp ‘Organic Photochemistry’ ed. 0. L. Chapman Marcel Dekker Inc. New York 1967 vol.1 p. 1; (b) D. H. R. Barton P. de Mayo and M. Shafiq Proc. Chem. Soc. 1957,205; (c) D. H. R. Barton P. de Mayo and M. Shafiq J . Chem. SOC. 1958,140; (d)D. Arigoni H. Bosshard H. Bruderer G. Buchi 0. Jeger and L. J. Krebaum Helv. Chim. Ada 1957,40 1732; (e) 0. L. Chapman and L. F. Englert J. Amer. Chem. SOC. 1963,85 3028; ( f ) M. H. Fisch and J. H. Richards J. Amer. Chem. SOC. 1963 85 3029. (a) H. E. Zimmerman Pure Appl. Chem. 1964 9 493; (b) H. E. Zimmerman and D. I. Schuster J. Amer. Chem. SOC. 1962 84,4527. D. H. R. Barton J. E. D. Levisalles and J. T. Pinhey J. Chem. SOC. 1962 3472. 58 Sammes chromous chloride reduction of the lactone gave an isomeric lactone 1 l-epi-de- oxygeigerin which with mild base was isomerised to deoxygeigerin (1 23). Oxidation with lead tetra-acetate eventually produced geigerin acetate (1 24).*l Related syntheses include an approach to aromadendrene (127),8a via the dienone (125) and its isophoto-derivative (126). Also synthesised in this manner have been cx-bulnesene (128)s3 and desacetoxymatricarine (129).84 Q eq 0 In an approach to the spirovetivane sesquiterpenoids Marshall has also used the rearrangement of the dienone (130).s5 In aprotic solvents the dipolar inter- mediate (e.g. 119) immediately collapses to the lumi-isomer (131) since no protons are available to quench the anion. Compound (131) was readily trans- formed into the spiro-compound (1 32) itself eventually converted into vetivone (133). C. Vinyl Esters.-Upon photolysis vinyl esters (e.g. 134) are often cleaved to give two radical species which can recombine via resonance forms to give a 81 D.H. R. Barton J. T. Pinhey and R. J. Wells J . Chem. SOC. 1964,2518. 82 J. Streith and A. Blind Bull. SOC. chim. France 1968,2133. 83 E. Piers and K. F. Cheng Chem. Comm. 1969 562. 84 E. H. White S. Eguchi and J. N. Marx Tetrahedron 1969 25 2099. J. A. Marshall and P. C. Johnson Chem. Comm. 1968 391. 59 Photochemical Reactions in Natural Product Synthesis p-diketone (135).86 Phenolic esters are similarly cleaved by irradiation to give eventually ortho- or para-acylated phenols a photochemical type of Fries rearra~~gement.~’ This reaction can therefore be used when the ester bears P (134) ( 1 3 5 ) functional groups that are sensitive to Lewis acid catalysts. Use of this reaction has been made in a synthesis of racemic griseofulvin (138).88 The substituted aromatic ester (136) was rearranged by photolysis to the ketone (137) a pre- cursor of griseofulvin.* * M e 0 0 OMe Me0 M e 0 OH OH c1 (137) - M e 0 fJpo 6 Photosensitised Addition of Oxygen An important method of oxidation invoIves the photosensitised addition of oxygen to olefins.8s Although alternative mechanisms are available the sensitis- ing dye generally acts as a catalyst in the conversion of oxygen from its ground triplet state to either of its metastable singIet states (Figure 2). In most cases it is the lower excited state ldg that appears to be responsible for the autoxida- tion~.~O The chemistry of the upper excited state l&+ has not been well studied although it does appear to have a different reactivity from that of the lAg state.@l O6 CJ A.Yogev M. Gorodetsky and Y. Mazur J. Amer. Chem. SOC. 1964,86,5208 D. Bellus and P. Hrdlovic Chem. Rev. 1967 67 599. D. Taub C. H. KUO H. L. Slates and N. L. Wendler Tetrahedron 1963,19 1. *@ C. S. Foote Science 1968 162 963. so C. S. Foote S. Wexler R. Higgins and W. Ando J. Amer. Chern. SOC. 1968,90,975. O1 (a) D. R. Kearns R. A. Hollins A. U. Khan R. W. Chambers and P. Radlick J. Amer. Chem. SOC. 1967 89 5455; (b) D. R. Kearns R. A. Hollins A. U. Khan and P. Radlick J. Amer. Chem. SOC. 1967 89 5456. 60 State 1Cg+ Highest occupied orbitals -3- -4- Sammes Energy 37 kcal. - 22 kcal. ?-+ 0 (Ground state) 9- 4- Figure 2 Energy levels of molecular oxygen Two modes of addition to olefinic systems may be recognised. Isolated double bonds react by addition with allylic migration of the double bond in a highly specific manner (139) to Thus p-pinene (141) can be readily and specifically converted into myrtenol (142).93 Conjugated dienes prefer to react by normal Diels-Alder type of cycloaddition as in the conversion of a-terpenene (143) into the anthelmintic ascaridole (14Qg4 An interesting variant of the latter reaction lead to the synthesis of the powerful vesicant cantharidin (146) from the anhydride (145).96 iB; hv ~ '*& - reduction '& O2 ISens. A. Nickon and J. F. Bagli J . Amer. Cltem. SOC. 1961 83 1498; (b) A. Nickon and W. L. Mendelson J. Amer. Chem. Soc. 1963,85 1894; (c) cf. K. Alder and H. von Brachel Annulen 1962 651 141. st G. 0. Schenck H. Eggert and W. Denk Annalen 1953,584 176. K. Ziegler and G. 0. Schenck Naturwiss. 1944,32 157.s6 G. 0. Schenck and R. Wurtz Naturwiss. 1953,40,581. 61 Photochemical Reactions in Nutural Product Synthesis Similarities between photosensitised oxygenations and certain biological oxidations have often been noted,g6 for example in the biosynthesis of abscisin-I1 (147).D7 The diene precursor (148) was oxidised in vitvo with oxygen to give the cyclic peroxide (149) which gave abscisin after treatment with dilute b a ~ e . ~ ' * ~ * *o ____) hv Go ____) Reduction &o \ I 0 2 /Sens. 0 I I C0,H ' 0 ' 0 i) HBr (145) / ii) OH- ' 0 (147) (a) See refs. 90 and 92a; (6) W. Waters and E. McKeown J. Chem. SOC. (B) 1966 1040; (c) J. E. Baldwin H. H. Basson and H. Krauss Chem. Comm. 1968,984. 97 J. W. Cornforth B. V. Milborrow and G. Ryback Nature 1965 206 715. (a) M. Mousseron-Canet J.C. Mani J. L. Olive and J. P. Dalle Compt. rend. 1966 262 C 1397; (b) cf. R. LeMahieu M. Carson and R. W. Kierstead J. Org. Chem. 1968 33 3660. 62 Sammes Oxidation of quercetin tetramethyl ether (150) gave the depside (152) via the peroxide (151) by a process analogous with that occurring nat~rally.~@ Singlet oxygen is also often involved in the oxidation of fatty acids,loO carotenoids,lO1 and naturaIly occurring heterocyclic systems. 89 7 Intramolecular Hydrogen Abstraction Several very specific photoreactions are known which involve intramolecular chemical attack at a carbon atom some distance removed from a functional group. A short list with leading references is given in the Table. These reactions proceed by intramolecular formation of a carbon radical which can then be quenched by such reagents as nitric oxide iodine etc.For example nitrite esters (e.g. 153) react with preferential abstraction of a y-hydrogen atom via a six- membered transition state to give an oximino-alcohol (154).lo2 Many applica- NOH ss T. Matsuura H. Matsushima and H. Sakamoto J. Amer. Chem. SOC. 1967 89 6370. loo (a) H. R. Rawls and P. J. von Santen Tetrahedron Letters 1968,1675; (b) cf. S. Bergstrom Science 1967 157 382. l01 (a) M. Mousseron-Canet J. P. Dalle and J. C. Mani Photochem. and Photobiol. 1969 9,91; (6) S. Isoe S. B. Hyeon H. Ichikawa S. Katsumura and T. Sakan Tetrahedron Letters 1968 5561. 1°*D. H. R. Barton J. M. Beaton L. E. Gellcr and M. M. Pechet J. Arner. Chem. SOC. 1961,83,4076. 63 3 Photochemical Reactions in Natural Product Synthesis Table Photolytic intramolecular hydrogen abstraction reactions Functional group Alcohol Amine Amide Nitrile Carboxylic acid Derivative Princbal products Nitrite y-Oximino-alcohol Hypochlorite y-Chloro-alcohol Hypoiodite y-Iodo-alcohol or tetrahydrofuran N-Chloro- y-Chloro-amine or amine pyrrolidine N-Iodoamide y-Lactone N-Bromo-t- y-Lactone or butylamide imino-ether N-Nitroso- y-Oximino- acetamide acetamide N-Chloro-amide y-Chloro-amide Nitrile oxide 7 Acyl azide ) 'y- and &lactams J Sulphonamide N-Chloro- y- and s-chloro- sulphonamide sulphonamides a b d e f g h C 1 j k 1 1 Comments Reference a Alkoxy radical b formed RO.c d Hofmann-Loeffler- e Freytag reaction. Photolysed with acid present Via C-iodide f g h i l j mixtures if possible A. L. Nussbaum and C. H. Robinson Tetrahedron 1961 17 35.M. Akhtar and D. H. R. Barton J. Amer. Chem. SOC. 1961 83,2213. M. Akhtar Adv. Photochem. 1964. 2 263. K. Heusler and J. Kalvoda Angew. Chem. Znternat. Ed. 1964 3 525. M. Wolff Chem. Rev. 1963 63 55. D. H. R. Barton A. L. J. Beckwith and A. Goosen J. Chem. Soc. 1965 181. R. S. Neale N. L. Marcus and R. G. Schepers J . Amer. Chem. SOC. 1966 88 3051. Y. L. Chow and A. C. H. Lee Chem. and Znd. 1967 827. R. C. Petterson and A. Wambsgans J. Amer. Chern. SOC. 1964 86 1648. G. Just and W. Zehetner Tetrahedron Letters 1967 3389. J. W. ApSimon and 0. E. Edwards Canad. J. Chem. 1962,40 896. M. Okahara T. Ohashi and S. Kanai Tetrahedron Letters 1967 1629. tions of these reactions to the synthesis of natural products have been reported and only a few are presented here to illustrate their usefulness.The readily available corticosterone acetate was converted into its nitrite ester (155) before photolysis in benzene solution. Nitrous acid treatment of the derived oxime (156) gave the important hormone aldosterone acetate (157) in overall 15 % yield.lo3 N-Chloroamines are very sensitive t o lightlo4 and under acidic conditions the 108 D. H. R. Barton and J. M. Beaton J. Arner. Chem. Soc. 1961,83,4083. M. Wolff Chem. Rev. 1963,63,55. 64 Sammes 0’ d ’ 0 & OH r O A c OAc 0 chloro-mine (158) gives the 7-chloroammonium salt (159). Mild base treat- ment affords the pyrrolidine (1 60) the steroidal alkaloid dihydroconnessine.lo6 C1 (158) ( 1 59) ( 1 60) Io5 E. J. Corey and W. R. Hertler J. Amer. Cheni. Soc. 1960 82 1657. 65 Photochemical Reactions in Natural Product Synthesis Most intramolecular hydrogen abstraction reactions proceed via the favoured 6-membered transition state leading to 7-substituted products.A notable excep- tion is for the acyl nitrenes generated from either the acyl azide or nitrile oxide which prefer to abstract hydrogen from the 8-position.lo6 For example the acyI azide (161) affords mainly the 8-lactam (162) as well as a minor amount of y-lactams. The major product was converted into the phenol (163) a degrada- OMe OMe \ OH I (1 61) ( 1 62) (163) tion product of the alkaloid atisine.lo7 An explanation for the required larger transition state is that singlet nitrene is responsible which inserts in a concerted manner into the 8-carbon-hydrogen bond (e.g. 161A) with carbon to nitrogen H bond formation in the transition state that is again via a formally 6-membered intermediate.lo8 Isocyanates are also formed during this photolysis and again involve singlet excited species probably of the excited azide.loD Sensitised photolysis inhibits the rearrangement to isocyanate but also retards lactam format ion.l0 8 Photosensitive Protecting Groups Protecting groups employed in synthetic work are usually removed chemically for example by acid or base or by hydrogenolysis.llo Problems often arise when the substrate itself is also sensitive under the conditions required to regenerate the protected group.The use of protecting groups which may be Io6 (a) W. L. Meyer and A. S . Levington J. Org. Chem. 1963 28,2859; (&) R. F. C. Brown Austral. J. Chem. 1964 17 47. lo' J. W. ApSimon and 0. E. Edwards Canad.J. Chem. 1962,40 896. l08 I. Brown and 0. E. Edwards Canad. J. Chem. 1967,45,2599. 109 (a) W. Lwowski Angew. Chem. Internat. Edn. 1967 6 897; (&) G. T. Tissue S. Linke and W. Lwowski J . Amer. Chem. SOC. 1967,89 6303 6308. J. F. W. McOmie Adv. Org. Chem. 1963 3 191. 66 Sammes removed by irradiation avoiding the need for chemical treatment of the sub- strate is an attractive alternative. Two approaches to the design of such photosensitive protecting groups have been developed. In the first the internal redox reaction of substituted o-nitro- toluenes has been used.lll Brief irradiation of o-nitrodiphenylmethyl esters (e.g. 164) gives the corresponding o-nitrosohemiacetal (1 65) which collapses spon- taneously into o-nitrosobenzophenone (166) and the free acid.Amines can also be protected with this group. 01 I G O - OC'OK I'll In an alternative approach use is made of the greater reactivity of excited aromatic compounds compared to that of their ground states.l12 Thus m-nitro- phenyl esters are photosensitive and in protic solvents the acid is di~p1aced.l~~ Similarly 3,5-dimethoxybenzyl esters react with liberation of the protected g r 0 ~ p . l ~ ~ Related to the latter reaction is the observed photolytic decarboxylation of the 3,5-dimethoxybenzene derivative (167).llS This probably reacts via the 0- ll1 J. A. Barltrop P. J. Plant and P. Schofield Chem. Comm. 1966 822. For a recent summary see (a) E. Havinga and M. E. Kronenberg Pure Appl. Chem. 1968 16 137; (b) E. Havinga R. 0. de Jongh and M. E. Kronenberg Helv. Chim. Acfa 1967 50 2550.113 T. Wieland and C. Lamperstorfer Mukromol. Chem. 1966 31 1658. 114 J. W. Chamberlain J. Org. Chem. 1966,31 1658. 115 J. D. White and J. B. Bremner Abstracts 155th meeting of the American Chemical Society San Francisco 1968 170th abstract. 67 Photochemical Reactions in Natural Product Synthesis excited species (168) which eventually collapses to the ketone (169) used in a synthetic approach to mitorubrin.ll6 Other photosensitive protecting groups have been described including the benzyloxycarbonyl moiety,l17 desyl derivatives,lls and 2,4-dinitrobenzene- sulphenyl esters.lls It should be emphasised that many organic photochemical reactions have been reported which could be adapted for use in removing photo- sensitive protecting groups. An example is with the biphenylurethane (170) which is smoothly transformed by photolysis to phenanthridone (171) with liberation of the alcoho1.lZ0 116 G. Buchi J. D. White and G. N. Wogan J. Amer. Chem. SOC. 1965 87 3484. 11' J. A. Barltrop and P. Schofield J. Chem. SOC. 1965 4758. 118 J. C. Sheehan and R. M. Wilson J . Amer. Chem. SOC. 1964 86 5277. llS D. H. R. Barton Y. L. Chow A. Cox and G. W. Kirby J . Chem. SOC. 1965 3571. 120 N. C. Yang A. Shani and G. R. Lenz J . Amer. Chem. SOC. 1966,88,5369. 68

ISSN:0009-2681    

DOI:10.1039/QR9702400037

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

The Co-ordination of Ambidentate Ligands By A. H. Norbury and A. I. P. Sinha DEPARTMENT OF CHEMISTRY LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY LOUGHBOROUGH LEICESTERSHIRE 1 Introduction Early in the development of co-ordination chemistry Jorgensenl prepared two isomers of the molecular formula [Co(NH,),NOz]CIz which differed only in the mode of attachment of the NO,-group to the cobalt atom. He believed that the unstable red form contained the nitrito-(Co-ONO) and the stable yellow form the nitro-(Co-NO,) linkage. This view has been confirmed by recent spectroscopic studies.* Werner3 recognised that this was an example of a general type of isomerism which he termed ‘salt isomerism’. The almost equivalent terms ‘structural i~omerism’~ and ‘linkage isomerism’6 are currently more fashionable.Linkage isomers differ only in the actual atom (of a poly- atomic ligand) attached to the central metal ion all other attached groups the geometrical configurations and the conformations of the isomeric molecules being the same. Some common ligands known to form linkage isomers are CN- NOp- NCS- NCSe- and SO,,- the subject has been reviewed recentlye6 Kornblum’ wrote when describing a particular class of organic reactions ‘It is desirable to have a simple name for anions which possess two different reactive positions. Among such ions are NOz- CN- SCN- diazotate ions enolate ions and the anions obtained from a-hydroxypyridine nitroparaffins acid amides thio-amides etc. Since these anions undergo covalence formation at one or the other of two available positions the term ambident is proposed.’ The same term has been applied to inorganic chemistry especially in the sense that ambidentate ligands may form linkage isomers but also to describe those ligands which are potentially capable of co-ordinating through more than one site even though no linkage isomers are known in many cases one form of co-ordination is favoured by one metal while a different metal attracts the alternative donor site of the ligand.This approach results in the number and S M. Jorgensen Z . anorg. Chem. 1894,5 147. R. R. Penland T J. Lane and J V. Quagliano,/. Anter. Chem. Soc. 1956 78 887. * A. Werner Ber. 1907 40 765. ’ R. G. Wilkins and M. G. J. Williams ‘Modern Co-ordination Chemistry’ ed. J. Lewis and R. Wilkins Interscience New York 1960 p. 175. F Basolo and R. G. Pearson ‘Mechanisms of Inorganic Reactions’ John Wiley New York 2nd edn.1967 p. 13. R. T. M. Frascr ‘Werner Centennial’ Advances in Chemistry Series no. 62 American Chemical Society 1967 p. 296; J. L. Burmeister Co-ordinarion Chem. Rev. 1966 1 205; 1968,3 225; Ref. 5 p. 291. N. Komblum R. A. Smiley R. K. Blackwood and D. C. IWand J. Amer. Chem. SOC. 1955,17 6269. 69 The Co-ordination of Ambidentate Ligands variety of ligands falling under the heading ambidentate being very large. For example a diatomic group AB may form complexes with the following types of linkage M-AB M-BA M-A(B)-M M-B(A)-M or M-AB-M. The acetylacetonate anion is known to co-ordinate to platinum either through the y-carbon atom or through both oxygen atoms,8 and acetylacetone can co- ordinate through the C=C bond in addition.@ Cysteine may form chelates in any one of the three following ways This is not to suggest that all these structures are of equal probability but to establish that the more varied acceptor properties of metal ions relative to carbon* extend the number of ambidentate ligands considerably.Similarly ndonors with another co-ordination site e.g. allylamine alkylvinylketones and related compounds have been reviewed recently.lo However there are clear distinctions between these examples and the following definition is proposed to clarify the situation Ambidentate ligand - a ligand containing two or more different types of potential sigma donor sites only one of which sites is involved in co-ordination at any one time. This definition is very similar to Kornblum’s. It excludes bridging systems and flexidentate ligands having more than one co-ordination site on different atoms of the same element such as Sod2- or ethylenediamine without excluding such compounds as 4aminopyridine containing two chemically different nitrogen atoms.It restricts possible chelates to situations in which they act as unidentate ligands only. The subdivisions of this definition i.e. chelating ambi- dentate ligands and 0-v ambidentate ligands follow without needing further explanation. In practice it is found that one of the various possible modes of linkage of an ambidentate ligand is more common than the others although these alternative structures can be assumed under special circumstances. The chemistry of ambidentate ligands therefore presents interesting opportunities in synthetic inorganic chemistry and a study of the factors favouring different modes of co-ordination of the ambidentate ligands is of potential theoretical interest.In this Review we shall discuss the ambidentate behaviour or potential ambidentate behaviour of a number of ligands. Emphasis will be placed on sigma donors and examples of bridging behaviour or chelation will be included only when relevant to the main discussion. To facilitate the survey the following classification based on the complexity of the ligand molecules will be adopted *Aspects of ambidentate behaviour in organic chemistry have been discussed recently by Hunig Angew. Chem. Internat. Edn. 1964 3 548; Gompper ibid. 1964 3 560. (i) Diatomic ambidentate ligands. K. Kite and M. R. Truter J. Chem. SOC. (A) 1968,934. G. Allen J. Lewis R.G. Long and C. Oldham Nature 1964,202 580. lo J. A. McGinnety and M. J. Mays Ann. Reports (A) 1967,64,352; 1968,65,383. 70 Norbury and Sinha (ii) Polyatomic ligands in which the two different donor atoms are directly (iii) Polyatomic ligands in which the two different donor atoms are separated (iv) Miscellaneous ambidentate ligands. Ligands containing two different donor atoms separated by two or three non-donor atoms will not be considered generally as these are potential chelates. linked. by a non-donor atom. 2 Diatomic Ambidentate Ligands Considering C N 0 F P S C1 As Se Br Sb Te and I as the common donor atoms seventy-eight (i.e. the number of ways of choosing two from thir- teen) diatomic ligands are theoretically possible. However only the hypohalite and cyanide ions CO CS and NO are known as possible ligands although the fact that CS has no long-term chemical existence in the non-complexed state suggests that further diatomic ligands may be discovered.A. Cyanide Complexes.-The chemistry of the cyanide ion has been reviewed generally1' and with special reference to its ambidentate nature.llC Normally anionic complexes of the type [Mm (CN)n]m-n are obtained. There are however a number of well-substantiated examples in which the nitrogen is involved in co-ordination as well as the carbon [there do not seem to be any examples of M-C(N)-M bridging]. Thus a number of metal cyanide complexes form adducts with BF, in which a M-CEN-BF system exists.12 In a similar fashion the irradiation of a solution of M(CO)6 (M = Cr or W) and subsequent treatment with HCN gave (CO)5MNCH which isomerised under suitable con- ditions to give (C0)5MCNH.13 Such behaviour is reminiscent of polymeric cyanide complexes many of which have the face-centred-cubic lattice of Prussian Blue.This structure has three types of site for the metal ion two of which are octahedral holes surrounded by six carbon or nitrogen atoms respectively and the third type is a large interstitial hole.l* Table 1 shows whether or not metal ions are in one or the other of the octahedral holes in a number of these compounds (no attention is paid to whether or not the interstitial site is occupied) and is in good general agreement with predictions based on crystal field stabilisation energies.lS However the calculations predicted that KCr111-(CN)6 - FeII should be less stable than KFe11-(CN)6-Cr111 and since [Cr(CN)J3- is kinetic- ally rather inert there appeared to be the possibility of linkage isomerism.This proved to be the case and the addition of an iron(n) solution to a solution of K,Cr(CN) gave a brick-red precipitate which converts to the dark green ' l a B. M. Chadwick and A. G. Sharpe Adv. Inorg. Chem. Radiochem. 1966 8 84; b D. Britton Perspectives in Structural Chem. 1967,1 109; c D. F. Shriver Structure and Bonding 1966 1 32. l2 D. F. Shriver J. Amer. Chem. Soc. 1963 85 1405 and refs. in 1 l(c). l3 J. F. Guttenberger Chem. Ber. 1968 101,403. l4 H. B Weiser W. 0. Milligan and J. B. Bates J. Chem. Phys. 1942,46,99. l6 D. F. Shriver S. A. Shriver and S. E. Anderson Inorg. Chem. 1965 4 725. 71 The Co-ordination of Ambidentate Ligands Table 1 The structures of some Prussian Blue analogues* Metal in N hole Mnz + Niz + CuZ + Znz + Fe3 + Mnz + Fez + coz + Niz + cu2 + Zn2+ Ni2+ Zn2 + Cr3 + Mnz+ or Fe3+ C hole Fe3 + Fe3 + Fe3 + Fe3 + Fez+ Co3 + Co3 + Co3 + Co3 + Co3 + Co3 + Cr3+ Cr3+ Fe2+ Fez + Reference a a a a a a a a b a a a n a at C t *The compounds are not always accurately stoicheiometric and may range between the limiting compositions KMIIMIII (CN,)and MxI,M1II,(CN),,.In general the N holes contain high spin ions and the C holes low spin ions as is strikingly shown in K Mnxl,(CN) which has one of each.& ?See text. a D. F. Shriver S. A. Shriver and S. E. Anderson Inorg. Chent. 1965 4 725 and refs. therein; D. F. Shriver and D. B. Brown Inorg. Chem. 1969.8,42; C D. B. Brown and D. F. Shriver Inorg. Chem. 1969,s ,37; d A.M. Qureshi and A. G. Sharpe J.Inorg. Nuclear Chem. 1968,30,2269. isomer at 100". Physical measurements confirmed this to be a Crll*-C~N-Fell to FeI1-CrN-CrI1I isomerisation. It was also shown1 that Fe,[Mn(CN),] initially contained Fe-N and Mn-C bonds which rearrange to give a material containing Fe-C bonds and that a compound of approximate composition Co,[Cr(CN),] appeared to undergo isomerisation on heating in an inert atmos- phere to give a product containing C0II-C bonds; subsequent air oxidation led to the presence of CrIT1-N and CoIII-C bonds. Isocyano-compounds are not well established. They have been a subject for debate in the chemistry of the non-metals (with the exception of carbon) and have been postulated as intermediates in a number of reaction^.^^^^^^ However convincing evidence has been put forward to support the presence of small amounts (0.2 %) of Me,SiNC in the isomeric cyanide.ls Transition-metal isocyanides [Co(CN),NCI3- and [Cr(H20),NCI2+ have been detected during studies of the kinetics of a number of redox reactions of [Co(CN),I3- with l6 D.B. Brown and D F. Shriver Inorg. Chern. 1969 8 37. l7 J. S . Thayer Inorg. Chem. 1968 7 2599. M. R. Booth and S. G. Frankiss Chem. Comm. 1968 1347. 72 Norbury and Sinha various substituted c ~ b a l t ( m ) ~ ~ and chromium(m)20 species; the latter inter- mediate was also observed in exchange reactions involving [Cr(H20)J3 + and some aquo-cyano-complexes of chromium(m).21 The oxidation of [CO(NH~)~NCS]~+ to [Co(NH3),CNI2+ seems likely to involve the same type of intermediate but the similar conversion of [Pd L(NCS)]+ to [Pd L(CN)]+ (L = tetraethyldiethylene triamine) is believed to take place viu the re-entry of a free cyanide ion to the co-ordination sphere.22 The close similarity of these transient intermediates to complexes containing bridging cyanides is emphasised by the preparation of CO(NH~),-NC-CO(CN)~.~~ Recently Kuroda and Gentile2* have claimed the existence of discrete isocyano- complexes of the type [Co trien (NC)2]C104 on the basis of electronic and vibra- tional spectra.However the interpretation of the former has been ~hallenged,~ and BurmeisteF has questioned both the latter and the absence of any rationale for the formation and stability of these compounds although Espenson21 has estimated that the isomerisation energy for [(Cr(H2O),NCI2+ is small.In the absence of further evidence the existence of stable iscsyano-complexes must remain a matter for further experimental work. The co-ordination behaviour of the cyanide ion and the preponderance of metal-carbon bonds have been explainedz6 on the basis of the localisation of the highest filled molecular orbital with 0 symmetry on the carbon atom as shown by LCAO-MO-SCF calculations. Similar LCAO-MO calculations by Purce1Iz7 show that the nitrogen lone pair of electrons is more basic in the Lewis sense for HCN and CH,CN than for CN- a similar increase in basicity on co- ordination would account for the many bridged compounds of the type M-CN-M’. The formation of unstable isocyano intermediates is then a consequence of this bridging mode and the isolation of such compounds may depend largely on kinetic factors.B. Complexes Containing Carbon Monoxide.-Carbon monoxide normally acts as a unidentate ligand through the carbon atom. It is also known as a bridging group where unlike the isoelectronic cyanide ion it co-ordinates through the carbon only in either a ketonic or triply bridging fashion.28 Iso- carbonyl complexes have been postulated as intermediates in some decarbonyla- tion reactions for example:29 lo J. L. Burmeister and D. Sutherland Chem. Comm. 1965,175; J. Halpern and S. Nakamura J . Amer. Chem. SOC. 1965 87 3002; J. P. Birk and J. Halpern ibid. 1968,90 305. 2o J. H. Espenson and J. P. Birk J. Amer. Chem. SOC. 1965 87 3280; J. P. Birk and 3. H. Espenson ibid. 1968 90 1153. 21 J. P. Birk and J. H. Espenson J . Amer. Chem. SOC. 1968 90 2266.22 K. Schug B. Miniatas A. J. Sadowski T. Yano and K. Ueno Inorg. Chem. 1968,7 1669. 23 R. A. de Castello C. P. Mac-Coll N. B. Egen and A. Haim Inorg. Chem. 1969 8 699. 24 K. Kuroda and P. S. Gentile Inorg. Nuclear Chem. Letters 1967 3 151. 25 K. Konya H. Nishikawa and M. Shibita Inorg. Chem. 1968 7 1165. 26 D. F. Shiver and J. Posner J. Amer. Chem. SOC. 1966 88 1672. 27 K. F. Purcell J. Amer. Chem. SOC. 1967 89 247. 28 F. A. Cotton and G. Wilkinson ‘Advanced Inorganic Chemistry’ 2nd edn. John Wiley London 1966. J. Chatt B. L. Shaw and A. E. Field J. Chem. SOC. 1964 3466. 73 The Co-ordination of Ambidentate Ligands c El 1 Nyholm30 has suggested the possibility of co-ordination through the oxygen atom in AuCOCl but this has not been confirmed. Some 1 2 adducts between arene metal tricarbonyls and mercury@) halides may be formulated as com- pounds with the mercury(I1) halide co-ordinated to the oxygen of the carbonyl These few examples involving oxygen co-ordination in contrast to the very many carbon-bonded carbonyls may be explained in terms of the molecular orbital description of carbon The lone pair of electrons on the carbon atom is strongly directed away from the C-0 bond and hence most readily available for co-ordination while the greater electronegativity of the oxygen atom causes the electron cloud to be very tightly held by this atom and so less available for co-ordination.Factors favouring the ketonic linkage of carbon monoxide are not clear. In general a polymeric structure is favoured for a complex of the empirical formula M(CO) if n is such that the available orbitals on the metal atom are not fully occupied.However the polymerisation does not necessarily involve ketonic bridging; it can also be effected through metal-met a1 bonding. C. Complexes Containing the Thiocarbonyl Group.-The diatomic entity CS is not known to exist as a stable species. It is therefore remarkable that a few stable complexes containing the thiocarbonyl group as a ligand are known. [Rh(Ph,P),(CS)CI] is obtained by refluxing tris-(tripheny1phosphine)chloro- rhodium(1) with carbon disulphide in the presence of methanol when the CS group is abstracted to give the thiocarbonyl complex.33 On oxidation with chlorine the rhodium(m) complex [Rh(Ph,P),(CS)ClJ is obtained. The corre- sponding bromo-complexes have also been prepared.33 The ratio (15:l) of the C-0 and C-S stretching frequencies in the corresponding carbonyl and thiocarbonyl complexes is in agreement with that expected from a consideration of the masses of the two groups if the mode of linkage is assumed R.S . Nyholm Proc. Chem. SOC. 1961,273. 31 K. Edgar B. F. G. Johnson J. Lewis and S. B. Wild J. Chem. SOC. (A) 1968,2851. 32 C. A. Coulson ‘Valence’ 2nd edn. Oxford University Press 1961. 33 M. C. Baird and G. Wilkinson Chem. Comm. 1966,267; M. C . Baird G. Hartwell jun. and G. Wilkinson J. Chem. SOC. (A) 1967 2037. 74 Norbirry and Sinha to be similar in the two. The formation of a linear metal-carbon-sulphur bond has been confirmed by X-ray analysis of Rl~(Ph,P),(cs)Cl.~~ The corresponding iridium complex is also known and has been prepared from [IrCO(mCS2) (PPh3)3]+ which was isolated as the tetraphenylb~rate.~~ Such cationic species are believed to be involved in the formation of the thio- carbonyl complexes and the following reaction scheme has been suggested:35 Thiocarbonyl complexes of other metals have since been If the arguments advanced to explain the mode of co-ordination of CN- and CO are valid the similar calculation for CS suggests that thiocarbonyl complexes should be more stable than carbonyl~,~~ and that CS is more likely to form normal and iso-complexes.D. Complexes Containing Nitric Oxide.-Nitrosyl complexes have been reviewed recently.38 In all the known monomeric nitrosyl complexes the ligand is bonded through the nitrogen atom which is also involved in both double and triple bridging as in (7T'CgH5)3 Mn3(N0)4:38 7145H5 I 0 Nitric oxide is an odd-electron molecule and may fairly easily lose one electron to give the nitrosonium ion NO+.The extent to which this happens in complexes 34 J. C. de Boer D. Rodgers A. C. Skapski and P. G. H. Troughton Chem. Comm. 1966 756. 36 M. P. Yagupsky and G. Wilkinson J. Chem. SOC. (A) 1968 2813. 36 J. D. Gilbert M. C. Baird and G. Wilkinson J. Chem. SOC. (A) 1968 2198; L. Busetto and R. J. Angelici J. Amer. Chem. SOC. 1968,90 3283; E. Klumpp G. Bor and L. Marko J . Organometal Chem. 1968,11,207 37 W. G. Richards Trans. Faraday SOC. 1967 63 257. 38 B. F. G. Johnson and J. A. McCleverty Prog. Inorg. Chem. 1966,7 277; W. P. Griffiths Adv. Organometal Chem. 1968 7 211; M. J. Cleare Platinum Metals Rev. 1968 12 131. 39 R. C. Elder F. A. Cotton and R. A. Schunn J.Amer. Chem. SOC. 1967 89 3645. 75 The Co-ordination of Ambidentate Ligands is the subject of some speculation but it seems that a discussion of metal- nitrosyl bonding in terms of NO or NO+ is very much over-simplified. Some of this speculation will have been resolved by X-ray measurements which have shown that the black isomer of [Co(NH3),N0l2+ has a Co-N-0 moiety with a significantly greater N-0 distance (1.24 or 1.41 A)40asb than usual (1.1- 1.2 The structure of Ir(N0) (CO) (PPh,),Cl has been shown to have a ‘bent’ nitrosyl group (Ir-N-0 = 124°):41 the first time that this species has been unequivo- cally established. It was suggested that the bent structure arose because the nitrosyl group here accepts electrons from the metal. As with CN- and CO the lone pair of electrons is more available on one atom than the For NO nitrogen is the favoured atom and it is nitrogen through which metal-nitrosyl bonds are formed.whereas the red isomer is dimeric with a bridging hyponitrite n E. Hypohalito-complexes.-The strong oxidising properties of the hypohalous acids and their tendency to disproportionate probably account for the lack of information on the co-ordination of their ions. If any hypohalite complexes are prepared it is to be expected that they are of the form M-OX though M-XO complexes may perhaps occur for the less electronegative halogens. 3 Polyatomic Ambidentate Ligands with Adjacent Donor Atoms This class is represented by various ligands with nitrogen and oxygen sulphur and oxygen etc. as the donor atoms. These alternatives provide convenient sub- headings.A. Nitrogen versus Oxygen.-The nitrite ion is the most widely studied of these ligands. It can form nitro-(M-NO,) or nitrito-(-ONO) complexes and it also has a number of bridging (through 0 and N or 0) or chelating (through 0 and 0) modes. For those complexes where the nitrite ion is the only ligand it seems that nitro-complexes are formed predominantly as long as the central metal is co-ordinatively saturated as in the four-co-ordinate K2[Pt(N02)4].44 When this is not so as in [M(N0,)4]2- (M = Mn Co or Cd) chelating or bridging groups are formed to satisfy the hexaco-ordination of the In a series of octa- hedral complexes K,M’[M”(NO,),] where M’ = PbII or BaII and M” = FeII CoII NP or CdI the central metal was shown by spectroscopic methods to be surrounded by six nitro-group~.~~ Neutron diffraction confirmed this result for K,P~[CU(NO,),].~~ A mixed structure has been proposed for K,[Ni(NO,),- a D.A. Hall and A. A. Taggart J. Chem. SOC. 1965,1359; b D. H. Dale and D. C. Hodgkin ibid. 1965 1364. 41 D. J. Hodgson and J. A. Ibers Znorg. Chem. 1968,7,2345. 42 B. F. Hoskins F. D. Whillans D. H. Dale and D. C. Hodgkin Chem. Comm. 1969 69. 45 H. Brion C. Moser and M. Yamazaki J . Chem. Phys. 1959,30 673. 44 K. Nakamoto J. Fujita and H. Murata J. Amer. Chem. SOC. 1958,80,4817. 46 D. M. L. Goodgame and M. A. Hitchman J . Chem. SOC. (A) 1967 612. 46 H. Elliott B. J. Hathaway and R. C . Slade Znorg. Chem. 1966 5 669. 47 N. W. Isaacs and C. H. L. Kennard J. Chem. SOC. (A) 1969 386. 76 Norbury and Sinha (ONO),] and explained on the basis of the greater steric requirements of the nitro- than the nitrito-group:48 The nature of the other cation may have some bearing on the preferred co- ordination of the group and weak bridges may be involved.However the previous discussion oversimplifies the results since some nitrito-complexes have been reported,* as in [M(ON0)J2- (M = CuI1 or ZnII) K,[Cu(NO,),] has been but the spectra have been reinterpreted to show nitrito- 1 inkages. O The influence of steric effects is seen more clearly in some mixed ligand complexes NiLz(N02),. Nitro-complexes are formed when Lz is made up of four ammonia,5o or two eth~lenediamine,~~ N-alkylethylenediami~~e,~~ 2,2'- bipyridine,,l 2-aminomethylpiperidine51 or 2-aminomethylpyridine51 groups. If the number of substituents on the ligand is increased (e.g.to NN-dialkylethy- lenediamine,50 by adding a methyl group to the nitrogen or ring of the piperidine or pyridine ligands,,l or by replacing the planar bis bipyridine with the non- planar tetrakis pyridinesO system) then nitrito-complexes are formed. Un- substituted polymethylenediamines give nitro-complexe~.~~ Steric effects are further demonstrated by the fact that [Ni(vac-stieen) (NO,),],* having equatorial benzene rings is a nitro-c~mplex~~ while mi(rnesa-stieen) having axially directed rings and consequently exerting greater repulsion on the NO2- group contains the nitrit~-linkage.~O The nitrite group also shows a strong tendency to chelate and this has been observed in cobalt(i1) and nickel(I1) complexes of the form MLz(NO2) where E is a unidentate or & bidentate ligand.54 In view of the foregoing it is not surprising that examples can be found where the nitrito-nitro equilibrium lies strongly neither to the left nor right and where linkage isomerism has been found.Thus [Ni(NN-dimethylethylenediamine),- (ONO) 2J and [Ni(NN'-diethylethylenediamine) (ONO) ,] exist in nitro-nitrito equilibria in chlor~form,~~ and a number of compounds [M(NH3) (N02)ln+ *Stieen = 1,2-diphenylethylenediamine. 48 D. M. L. Goodgame and M. A. Hitchman Inorg. Chem. 1967 6 813. 49 R D. Gillard and G . Wilkinson J. Chem. SOC. 1963 5399. ao D. M. L. Goodgame and M. A. Hitchman Inorg. Chem. 1964,3 1389. 51 L. El-Sayed and R. 0. Ragsdale Inorg. Chem. 1967 6 1638. 62 A. Takeuchi K. Sato K. Sone S. Yamada and K. Yamasaki Znorg. Chim. Acta 1967 1 399. 53 D. M. L. Goodgame and M.A. Hitchman Znorg. Chem. 1966,5 1303. 64 D. M. L. Goodgame and M. A. Hitchman Znorg. Chem. 1965 4 721 ; L. EI-Sayed and R. 0. Ragsdale ibid. 1967 6 1644. 77 The Co-ordination of Ambidentate Ligands (M = CoIII RhlI1 I F and PtIV) are prepared from their nitrito-isomers formed initially as unstable intermediates.1~66 (See ref. 6 for complete list.) If these nitrito-intermediates are isolable as a consequence of kinetic control arising in less labile d6 systems chromium(rn) offers an interesting comparison since the known complexes are all nitrito and show no tendency to isomerise.6s In contrast to the rich and varied chemistry of the nitro-group little is known about other ligands having nitrogen and oxygen as their donor atoms. Co- ordination compounds of hydroxylamine with cobalt nickel palladium and platinum are known and their i.r.spectra indicate that the ligand is co-ordinated through the nitrogen atom.57 Vanadium(v) complexes with N-benzoyl-N- phenylhydroxylamine and its analogues have been reported but no structural information is chelation seems likely. B. Sulphur versus Oxygen.-The sulphite ion is known to form complexes with a number of elements.69 In many cases it acts as a unidentate ligand but bridging and chelating behaviour is also possible. In the former case bonding through sulphur appears to be preferred for example in complexes of cobalt (In) O * ir i dium(m) a palladium(n) 63 and plat inum(n). 64 TI ,[Cu( SO ,) ,] is believed to contain the oxygen-bonded sulphite group from its i.r. spectra although it was impossible to obtain an analytically pure sample:s6 the oxidation state of the copper [initially(@] is not specified and it is interesting to note that the compound Cu2S03CuS0,,2H20 has the sulphur atoms co-ordinated to a tetrahedral copper(1) while the oxygen co-ordinates to the same atom and to an octahedral copper(@ giving a three-dimensional network of linked octahedra and tetrahedra.66 Linkage isomerism in sulphite complexes was incorrectly6' quoteds8 as occurring after the addition of sulphur dioxide or sodium sulphite solutions to a solution of [CO(NH,),H,O]~+ at low temperature and pH.An unstable pink intermediate supposedly O-bonded [CO(NH,)~OSO,] + was reported as appearing. In fact only the stable yellow-brown S-bonded [CO(NH~)~SO,]+ is known.60 65 F. Basolo and G. S. Hammaker J. Amer. Chem.Soc. 1960 82 1001; F. BasoIo and G. S. Hammaker Inorg. Chem. 1962 1 1. 56 W. W. Fee C. S. Garner and J. N. MacB. Harrowfield Inorg. Chem. 1967,6 87. 67 Yu. Ya. Kharitonov M. A. Sarukhanov and I. B. Baranovskii Russ. J. Inorg. Chem. 1967,12,82; 1966,11 1359. 68 A. K. Majumdar B. C. Bhattacharyya and G Das J. Indian Chem. SOC. 1968 45 964. 69 S. E. Livingstone Quart. Rev. 1965 19 386. 6o H. Siebert Z. anorg. Chem. 1959,298 51. 62 A. V. Babaeva Yu Ya Kharitonov and Z. M. Novozhenyuk Russ. J. Inorg. Chem. 1961 6 115; I. B. Baronovskii and A. V. Babaeva ibid. 1968 13 721. 63 M. A. Spinnler and L. N. Becka J. Chem. SOC. (A) 1967,1194; M. V. Capparelli and L. N. Becka ibid. 1969 260. 64 A V. Babaeva Yu Ya Kharitonov and Z. M. Novozhenyuk Russ. J. Inorg. Chem. 1961 6 1159. 66 G. Newman and D.B. Powell Spectrochim. Acta 1963 19 213. 66 P. Kierkegaard and B. Nyberg Acta Chem. Scand. 1965,19,2189. 67 See Note 3 in J. L. Burmeister H. J. Gysling and J. C. Lim J. Amer. Chem. Soc. 1969 91,44. 68 D. R. Stranks quoted by R. T. M. Fraser ref. 6a. S. Baggio and L. N. Becka Chem. Comm. 1967 506. 78 Norbury and Sinha Co-ordination compounds containing sulphur dioxide were first prepared by G l e ~ ~ ~ by the action of haleacids on the sulphitoammines of ruthenium. Complexes of other platinum metals (except ~ s m i u m ) ~ ~ ~ ~ ~ iron,7o cobalt,7a manganese,7o ~nolybdenum,~~ and tungsten73 have been obtained recently. Sulphur dioxide generally acts as a unidentate ligand when it is co-ordinated through the sulphur atom. Vlcek and B a ~ o l o ~ ~ have reported a cobalt complex Co,(CN),,SO, in which the ligand is considered to act as a bidentate bridging group; the bridging is supposed to take place only through the sulphur atom.X-Ray analyses of some of the complexes containing unidentate sulphur dioxide have confirmed M-S bonding,74 although it has been suggested that in the compound IrCI(C0) (PPh,) (SO,) electrons are donated by the metal to an empty orbital on the sulphur (cf. the corresponding nitrosyl c~mpound).~~ Nucleophilic attack on SOz was also postulated as the first stage in the preparation of a rhodium(1) adduct.'l Sulphur dioxide will also react in a rather different fashion with some transi- tion-metal compounds especially organometallics with which it can undergo insertion reactions :7s Mn(CO),R + SO2 -+ Mn(CO) SOzR Wojcicki and his co-w~rkers~~ have established that a structure (I) involving M-S bonding analogous to a sulphone applies in substituted carbonyl com- plexes involving molybdenum manganese rhenium and iron.i M-S-R 0 I Sulphinates can also be prepared by the oxidative addition of sulphonyl chlorides to square planar iridium(1) compounds. (Ph,P),Ir(CO)CI + RSOzCI --+ (Ph,P),Ir(CO)CI,SO,R K. Gleu W. Breucl and W. Rehm 2. anorg. Chem. 1936,227,237. 70 L. Vaska and S. S. Bath J. Amer. Chem. SOC. 1966 88 1333; L. Vaska and L. Catone J. Amer. Chem. SOC. 1966,88,5324; J. J. Levison and S. D. Robinson Chem. Comm. 1967 199; Inorg. Nuclear Chem. Letters 1968 4 407. 71 R. Cramer J. Amer. Chem. SOC. 1967 89 5377. '* A. A. VlEek and F. Basolo Znorg. Chem. 1966 5 156. 73 C. G. Hull and M. H. B Stiddard J . Chem.SOC. (A) 1968,710. 74 L. H. Vogt jun. J. L. Katz and S. E. Wiberley Inorg. Chem. 1965,4,1157; S . J. La Place and J. A. Ibers ibid. 1966,5 405. 76 See for example in E. W. Abel and B. C. Grosse Organometallic Chem. Rev. 1967,2,443. 76 J. P. Bibler and A. Wojcicki J . Amer. Chem. SOC. 1964 86 5051; 1966 88 4862; Znorg. Chem. 1966,5 889; F. A. Hartman and A. Wojcicki J . Amer. Chem. SOC. 1966,88 844; 1967 89 2493; Inorg. Nuclear Chem. Letters 1966 2 303. 79 The Co-ordination of Ambidentate Ligands The structure of these compounds also involves M-S bonding and while many of the alkyl derivatives appear quite stable when R = p-tolyl heating to 110" leads to the evolution of SO2 and the formation of an Ir-C bond the reverse of the insertion reactions above.77 Cobalt compounds of this type have been prepared by both SO2 and oxidative addition.79 Main-group alkyls and aryls undergo insertion reactions and polymeric complexes with M-O(SR)O-M bridges have been proposed for aluminium 8o gallium,80 and lead.Mercury forms a similar type of polynier PhHg0,SPh which demon- strates linkage isomerism to give a monomeric Hg-S compound.82 Thus far reactions concerning the formation of sulphinates by indirect means have been considered but some workers have prepared sulyhinato-complexes of divalent transition metals by reaction of hydrated halide with sodium aryl sulphinate in X-Ray analysis shows that bis(to1uene-p-sulphinato)copper(n) tetra- hydrate contains monodentate 0-bonded sulpliinate groups,84 and it is thought that the compounds M(C6H5S02)2(H20)2 (MI1 = Mn Fe Co Ni Cu) have bridging or chelating structures with only oxygen co-ordinated.83 Some mixed ligand complexes of palladium(r1) show Pd-S bonding 85 An interesting example of sulphinate linkage isomerism which involves chelation and is therefore strictly outside the scope of this Review is:8e 0 2 (Ph3P)2 PtCZH4 + 0:j ___+ a ; r L 2 EtOH 7OoC I 0 77 J. P. Collman and W. R. Roper J . Amer. Chem. SOC. 1966,88 180. 78 K. S. Murray R. J. Cozens G. B. Deacon P. W. Felder and B. 0. West Inorg Nuclear Chem. Letters 1968,4 705. 7Q K. Yamamoto T. Shono and K. Shinra Nippon Kagaku Zasshi 1967,88,958 (Chem. Abs. 1968,68 92568~). 81 R. Gelius 2. anorg. Chem. 1967 349 22; Von F. Huber and F.-J. Padberg ibid. 1967 351 1. 82 G. B. Deacon and P. W. Felder J. Amer. Chem. Soc. 1968 90 493. 83 C. W. Dudley and C.Oldham Inorg. Chim. A d a 1968,2 199. 84 D. A. Langs and C. R. Hare Chem. Comm. 1967 853. 85B. Chiswell and L. M. Venanzi J. Chem. Suc. (A) 1966 1246; C. W. Dudley and C. Oldham Inorg. Chim. Ada 1969,3 3. 86 C. D. Cook and J. S. Jauhal J. Amer. Chem. SOC. 1968 90 1464. G. E. Coates and R. N. Mukherjee J . Chem. Soc. 1964 1295. 80 Norbury and Sinha Sulphoxides have been widely studied as ligands and their co-ordination behaviour reviewed. 87 Dimethyl sulphoxide co-ordinates through oxygen to the lanthanides and yttrium,88 and to the first-row transition element^,^^-^^ and through sulphur to platinum(@ palladium(~~),~~-~~ i r i d i ~ m ( m ) ~ ~ ~ and rhodi~m(rr),~~* there being X-ray confirmation of the i.r. diagnosis for both types of b ~ n d i n g . ~ l - ~ ~ A number of metal carbonyl compounds with dimethyl sulphoxide have been collated75 and form metal-oxygen Both S- and O-bonded ligands appear to be present in Pd(Me2S0)42+.94 A similar possibility exists in UC14,5DMS0 but adducts with other actinide tetrahalides are clearly O-bonded.95 The use of other sulphoxides-tetrametliylene Q6 alkyl 97 or pheny197~Q8-does not alter the pattern of co-ordination behaviour described above. The co-ordinating properties of these ligands have been compared with similar oxide ligandsg9 A few coniplexes of thionyl chloride have been pre- pared,loO but structural information is sparse. AlC1;SOCl is thought to have an A1-0 bond.lol C. Selenium versus Oxygen.-There is little information on adducts with selenium-oxygen ligands. Selenito-complexes of cobalt nickel and copper are known but their structures do not appear to have been studied.lo2 Dimethyl selenoxide forms O-bonded adducts with some transition-metal chlorideslo3 and with tin(Iv):lo4 it forms a series of O-bonded hexakis complexes with transi- 13' J.Gopalakrishnan and C. C. Patel J. Sci. Ind. Res. India 1968 27 475. BQ D. W. Meek D. K. Straub and R. S. Drago J. Amer. Chem. SOC. 1960 82 6013; F. A. Cotton and R. Francis J. Znorg. Nuclear Chem. 1961 17 62; F. Kutek Coll. Czech. Chem. Comm. 1968,33 1930; W. B. Moniz C. F. Poranski and D. L. Venezky U.S. Clearinghouse Fed. Sci. Tech. Inform. 1967 AD663552 (Chenz. Abs. 1968 69 15455a). F. A. Cotton and R. Francis J. Amer. Chem. SOC. 1960 82 2968; J. Selbin W. E. Bull and L. H. Holmes J. Inorg. Nuclear Chem. 1961 16 219. M. J. Bennett F.A. Cotton and D. L. Weaver Nature 1966 212 286; M. J. Bennett F. A. Cotton D. L. Weaver R. J. Williams and W. H. Watson Acta Cryst. 1967 23 788. 82 D. A. Langs C. R. Hare and R. G. Little Chem. Comm. 1967 1080. g3 a M. C. McPartlin and R. Mason Chem. Comm. 1967,545; b S . A. Johnson H. R. Hunt and H. M. Neumann Inorg. Chem. 1963 2 960. g4 B. B. Wayland and R. F. Schramm Chem. Comm. 1968 1465; Inorg. Chem. 1969,8,971. Q6 K. W. Bagnall D. Brown D. H. Holah and F. Lux J. Chem. SOC. (A) 1968,465. g6 R. Francis and F. A. Cotton J . Chem. SOC. 1961 2078; D. W. Meek W. E. Hatfield R. S. Drago and T. S. Piper Inorg. Chem. 1964 3 1637. 97 W. F. Currier and J. H. Weber Inorg. Chem. 1967,6,1539; W. Kitching and C. J. Moore Inorg. Nuclear Chem. Letters 1968,4 691. 88 S. K. Ramalingam and S.Soundararajan Bull. Chem. SOC. Japan 1968 41 106; P. W. N. M. Van Leeuwen Rec. Trav. chim. 1967 86 201. gg R. S. Drago and D. W. Meek J . Phys. Chem. 1961 65 1446. loo D. G. Karraker Inorg. Chem. 1964 3 1618; B. A. Voitovich E. V. Zvagol'skaya and N. Kh. Tumanova Izvest. Akad. Nauk S.S.R. Metally 1965 46 (Chem. Abs. 1966 64 15430a). lol D. A. Long and R. T. Bailey Trans. Faraday SOC. 1963,59,594. lo2 H. L. Riley J. Chem. Sac. 1928 2985; P. Ray and A. N. Ghosh J. Indian. Chem. SOC. 1936 13,494. lo3 K. A. Jensen and V. Krishnan Acta. Chem. Scand. 1967 21 1988. lo* T. Tanaka and T. Kamitani Inorg. Chim. Acta 1968,2 175. S. K. Ramalingam and S. Soundararajan J. Inorg. Nuclear Chem. 1967 29 1763. 81 The Co-ordination of Ambidentate Ligands tion-metal perchlorates. O6 Diphenylselenoxide forms 0- bonded adduct s with the chlorides of MnII CoII NiII &I1 HgII ZrIV and SnN.los D.Nitrogen versus Phosphorus.-The chemistry of amino-fluorophosphines has been reviewed.lo7 There is a tendency for oxidising species (CuCI,) to oxidise MezNPFz to the phosphorane Me2NPF2CI2; this ligand forms complexes with CuCl via the phosphorus.1os P-Bonded adducts are formed between various ligands of this type and a number of carbonyl complexes.107~100 Some X-ray studies have been made on nickel tetrakis piperidine-N-difluoro- phosphine.l1° Similarly some P-P bonds have been formed by reactions of (Me),-% (Me,VnP (n = 0-3) with PF,.ll1 E. Nitrogen versus Sulphur.-Tetrasulphur tetranitride is known to form adducts with Lewis a ~ i d ~ . 1 ~ ~ * ~ ~ ~ The molecular structure of two of these com- pounds S4N4 SbC15 and S4N4,BF shows that co-ordination takes place via one nitrogen atom.ll* In other situations the metal may be chelated by sulphur- nitrogen systems of varying sizes.113 Reactions of S4N4 with metal carbonyls lead to ‘thionitrosyls’ but such structural information as is available suggests that these also are ~he1ates.l~~ Thionitroso-complexes (RNS)Fe,(CO) have been reported but these are believed to involve a complicated Fe-N-S-Fe bridging structure.116 4 Polyatomic Ambidentate Ligands with One Central Atom Separating the Donor Atoms The presence of a central atom especially one of the second or subsequent rows means that the possibility of chelation is increased over the ligands of the previous section.The examples cited consist largely of cases where the ligand is unidentate.A. Nitrogen versus Sulphur.-The widely studied thiocyanate ion is considered first. It forms compounds with most of the elements in the periodic table and its mode of co-ordination may be determined conveniently using i.r. spectro- scopy providing the results are interpreted with caution. The three fundamental lo5 R. Paetzold and 0. Bochmann 2. Chem. 1968 8 308. loo R. Paetzold and P. Vordank 2. anorg. Chem. 1966,347 294. lo’ M. Murray and R. Schmutzler 2. Chem. 1968 8,241. lo8 K. Cohn and R. W. Parry Inorg. Chem. 1968,7,46. lo* C. G. Barlow J. F. Nixon and M. Webster J . Chem. SOC. (A) 1968,2216. B. Greenberg A. Amendola and R. Schmutzler Naturwiss. 1963 50 518. ll1 D. H. Brown K D. Crosbie G. W. Fraser and D. W. A. Sharp J. Chem. SOC. (A) 1969 551.lla K. J. Wynne and W. L. Jolly Inorg. Chem. 1967 6 107. llS A. J. Bannister and J. S. Padley J . Chem. SOC. (A) 1969 658. l14D. Neubauer and J. Weiss 2. anorg. Chem. 1960 303 28; H. G. B. Drew D. H. Templeton and A. Zalkin Inorg. Chem. 1967 6 1906. 116 M. Goehring and A. Debo 2. anorg. Chem. 1953 273 319; T. S. Piper J. Amer. Chem. SOC. 1958 80 30. 116 S. Otsuka T. Yoshida and A. Nakamura Inorg. Chem. 1968,7 1833. 82 Norbury and Sinha frequencies of thiocyanate group lie in the range v 2040-2080; v2 465-480; and v 780-860 cm-l for an N-bonded group whereas for an S-bonded group the ranges are vl 2080-2120 Y 410-470 and v3 690-720 cm-l the fre- quencies may be described approximately as C-N stretch N-C-S bend and C-S stretch re~pective1y.l~~ There are known however linkage isomers where v1 is greater for the S-bonded than for the N-bonded isomer.118-119 Furthermore the first overtone of v2 has an intensity comparable to v, and a band in the 880-800 cm-l region may be assigned to either 2v2 for an S-bonded or v3 for an N-bonded compound.120 By consideration of all three fundamentals one can often determine the mode of co-ordination from the positions of observed bands but in a complicated spectrum this is not always possible.In such circumstances it is preferable to measure the integrated intensity of v1 when values greater or less than the free ion value suggest N-bonded or S-bonded thiocyanate groups respectively :121 this procedure has been rationalised.122 Table 2 shows some thiocyanate complexes of the transition elements (mixed ligand complexes will be discussed shortly) and it is not surprising in view of the above that different conclusions have been reached by different authors on the structure of the rhenium(1v) complex.Table 2 shows that the mode of bonding varies from metal to metal and that using the Ahrland-Chatt-Davies classifica- t i ~ n ’ ~ ~ class ‘b’ metals form thiocyanato- and class ‘a’ isothiocyanato-complexes. This pattern is amplified by consideration of further compounds. The lanthanides (except CeIII PmIII TmIII and LuIII) form M(NCS)63-,124 and some mixed ligand isothi~cyanates.~~~ Scandium(& forms isothiocyanato-complexes in the presence of a variety of nitrogen and oxygen donors.12s Octaisothiocyanato- complexes are formed by t h o r i u r n ( ~ ) ~ ~ ~ and u r a n i u m ( ~ v ) ~ ~ ~ ~ ~ ~ and the uranyl group forms several isothiocyanato-complexes,128~129 Isothiocyanates also pre- dominate amongst the main group elements.130 Complexes of gallium(~r~)~~~ 117 ‘Spectroscopic Properties of Inorganic and Organometallic Compounds’ Chemical Society London 1968 vol.1 p. 196. lleH.-H. Schmidtke J. Amer. Chem. SOC. 1965 87 2522; Z. phys. Chem. (Frankfurt) 1965 45 305. llS A. H. Norbury and A. T. P. Sinha Znorg. Nuclear Chem. Letters 1968,4 617. lzoA. Sabatini and I. Bertini Inorg. Chem. 1965 4 1665. 121a S. Fronaeus and R. Larsson Acta. Chem. Scand. 1962 16 1447; b C. Pecile Znorg. Chem. 1966,5,210. lz2 R. Larsson and A. Miezis Acta Chem. Scand. 1969 23 37. lz3 S. Ahrland J. Chatt and N. R. Davies Quart. Rev. 1958 12 265. 12* J. L. Burmeister S. D. Patterson and E. A. Deardorff Znorg.Chim. Acta 1969 3 105. 12s F. A. Hart and F. P. Laming J. Znorg. Nuclear Chem. 1964,26 579; D. R. Cousins and F. A. Hart ibid, 1968 30 3009; A. M. Colub and Au Van Luong Russ J. Inorg. Chem. 1968,13 1737; 1969 14,47. 126 N. P. Crawford and G. A. Melson J. Chem. SOC. (A) 1969,427; 1049. lz7 I. E. Grey and P. W. Smith Aust. J. Chem. 1969 22 311. lee K. W. Bagnall D. Brown and R. Colton J. Chem. Sac. 1964 2527. 12s V. I. Belova Ya K. Syrkin and E. N. Traggeim Russ. J. Inorg. Chem. 1964,9 1441. 130 D. B. Sowerby J. Inorg. Nuclear Chem. 1961 22 205; J. S. Thayer and R. West Adv. Organometallic Chem. 1967 5 187; M. F. Lappert and H. Pyszora Adv. Znorg. Chem. Radiochem. 1966 9 165. S. J. Pate1 and D. G. Tuck Canad. J . Chem. 1969 47 229. 83 Table 2 Some thiocyanate compIexes of the transition elements 00 ...................................................................................b c d d c d c d c d i a c d e 2 Ti(NCS),3- v(NCs)e3- CI~NCS),~- I Mn(NCS)4a- Fe(NCS),a- CO(NCS)~~- Ni(NCS)42- CU(NCS),~- i Zn(NCS),2- Q ? 2 !3 is- s* % g h i j i b k c C C l i e i b P f f d Ni(NCS)64- 3 i MII(NCS),~- Fe(NCS),3- ................. ..................................................... z . . . . . . . . . . . . . . . . Zr(CNS),4- * Nb(NCS),’- i MO(NCS)63- Tc(NCS),- RIJ(NCS)~~- i Rh(SCN)e3- Pd(SCN)42- [(AgSCN)- i Cd(NCS),- i (SCN)]- i (SCN)22- i $ b Hf(CNS),4- * Ta(NCS),- iw(cNs) Re(Ncs)~~- os(NCs)e3- i Pt(SCN)42- Au(SCN),- Hg(SCN)42- % g i ............... i 3 g h i m n c C C C e e 0 C 8- Re(SCN),- Pt(scN)~~- 0 Re(sCN),’- ..................................................Tsothiocyanato - complexes M-NCS. Thiocyanato - complexes M-SCN. Structure unknown M-CNS. *In solution only. a W. Lenz H. L. Schlaefer and A. Ludi 2. anorg. Chem. 1969 365 55; b J. Lewis R. S. Nyholm and P. W. Smith J. Chem. SOC. 1961 4590; c H.-H. Schmidtke Ber. Bunsengese!lschaft physik. Chem. 1967 71 1138 and refs. therein; D. Forster and D. M. L. Goodgame Inorg. Chem. 1965,4 715; e K. A. Taylor T. V. Long and R. A. Plane J. Chem. Phys. 1967 47 138; f D. Forster and D. M. L. Goodgame J. Chem. SOC. 1965,268; B A. M. Golub and V. N. Sergunkin Russ. J. Inorg. Chem. 1966 11 419; T. M. Brown and G. F. Knox J. Amer. Chem. SOC. 1967 89 5296; f H. Boehland E. Tiede and E. Zenker J. Less Common Metals 1968,15,89; f G. F. Knox and T. M. Brown Inorg Chem. 1969 8 1401; k K.Schwochau and H. H. Pieper Inorg. Nuclear Chem. Letters 1968 4 71 1 ; 1 I. Lindquist and B. Strandberg Acra Cryst.. 1957 10 173; H. Funk and H. Bohland Z. anorg. Chem. 1963 324 168; n F. A. Cotton W. R. Robinson R. A. Walton and R. Whyman Inorg. Chem. 1967 6 929; 0 R. A. Bailey and S. L. Kozak Inorg. Chem. 1967 6 419; 2155. Borderline class ‘a’/class ‘b’ metals enclosed in ............. Class ‘a’ metals on the left (plus Zn). Class ‘b’ metals -bottom right. Norbury and Sinha and indium(111)l~~ form isothiocyanates although the cyclic trimers (Eta M S W 3 (M = Al Ga or In) have M-S-M bridges.133 Tin(@ forms N-bonded thio- cyan ate^.^^^ Table 3 shows the effect of other ligands on the preferred linkage of the thiocyanato-group to palladium(n) and cobalt(m). It is apparent that ligands with a tendency to back-bond with the metals i.e.to accept some electron density from the metal into empty orbitals encourage the formation of Pd-NCS and Co-SCN bonds respectively. Thus any explanation involving n-bonding for the behaviour of palladium135 (e.g. that the decreased electron density result- Table 3 Eflect ofL on the equilibrium L-M-SCN + L-M-NCS Pd (SCN),,- *Pd bipy(NCS)2 Pd (4,4'-Me,bipy) (NCS) Pd phen(SCN) Pd (S,NO,-phen) (NCS) (SCN) a c00\rH3),(Ncs)2 + h Co(NH3)4(NCS) 2+ h a Co en2 L (NCS)+ h a L = C1- or NCS- Co(tet a) (NCS),+ i b c Co(tet b) (NCS),+ i d *Co@H) L (NCS) k d L = NO2- various d pyridines and anilines co(cl sH3 ZN4) (Ncs)2+ j Pd (PPh3)2 (NCS) C *Co(CN)5(SCNy- 1 *Pd (AsPh,)Z(NCS)z b c Co bipy (SCN),+ m n Pd (SbPh3)2(SCN)2 c Co phen (SCN)2+ m Cobalamin-SCN 0 Pd dien (SCN) + e Co(DH) L (SCN) k Pd Et4 dien(NCS)+ SCN- L = C1- Br- SCN- H20 Pd Et dien(SCN)+ PBh4 g NH3 PPh e f *Stable linkage isomer.Abbreviations bipy = 2,2'bipyridyl; phen = o-phenanthroline; dien = diethylenetriamine; tet a and tet b = trans- and cis-hexamethyl-l,4,8,1l-tetra-azacyclotetradecane; CI,H3,N = hexamethyl-l,4,8,1 I-tetra-azacyclotetradeca-4,ll-diene; DH = dimethylglyoximato; cobalamin = Vit B,,a with H,O replaced. a A. Turco and C. Pecile. Nature 1961 191 66; F. Basolo J. L. Burmeister and A. J. PoC J. Amer. Chem. SOC. 1963 85 1700; C J. L. Burmeister and F. Basolo Inorg. Chem. 1964 3 1587; d I. Bertini and A. Sabatini Inorg. Chem.. 1966 5 1025; e W. H. Baddley F. Basolo and J. L. Burmeister Inorg. Chem. 1964 3 1202; f W. H. Baddley F. Basolo and K.Wiedenbaum J. Amer. Chem. SOC. 1966,88 1577; g J. L. Burmeister H. J. Gysling and J. C. Lim J. Amer. Chem. SOC. 1969 91 44; h M. M. Chamberlain and J. C. Bailar J. Amer. Chem. SOC. 1959 81 6412; * P. 0. Whimp and N. F. Curtis J. Chem. SOC. (A) 1966 867; N. Sadasivan J. A. Kernohan and J. F. Endicott Inorg. Chem. 1967 6 770; k A. H. Norbury and A. I. P. Sinha Inorg. Nuclear Chem. Letters 1968 4 617 and (with P. E. Shaw) unpublished results; J. L. Burmeister Inorg. Chem. 1964 3 919; I. Stotz W. K. Wilmarth and A. Haim ibid. 1968 7 1250; m N. Maki and S. Sakuraba Bull. Chem. Sac. Japan 1969 42 579; n A. H. Norbury and P. E. Shaw unpublished results; 0 D. C. Hodgkin Fortschr. Chent. org. Naturstoffe 1958 15 167. S. J. Patel D. B. Sowerby and D. 0. Tuck J. Chem. SOC. (A) 1967 1187.K. Dehnicke Angew. Chem. Internat. Edn. 1967 6 947. 134 B. R. Chamberlain and W. Moser J. Chem. SOC. (A) 1969 354. 85 The Co-ordination of Ambidentate Ligunds ing from back-bonding increases the effective positive charge on the metal or makes it ‘harder’,lt6 and favours the formation of the more ionic Pd-N bond)la6 is contradicted by the cobalt compounds. Similarly the concept of which accounts for the latter is not consistent with the changes in the palladium compounds. Rhodium(1) forms square planar isothiocyanato-complexes with n-acceptor type ligands in general agreement with the palladium whereas rhodium(m) is similar to cobalt(II1) and iridium(n1) in forming [M(NH,),NCS]+.ll* Further although the stable isomer of Mn(CO),SCN is S-bonded and this is maintained where only one CO group is substituted the replacement of two CO groups by weaker rr-bonding ligands or by anilines which cannot take part in rr-bonding results in the formation of isothio- cyan ate^.^^^ The behaviour of these manganese compounds is in contrast to that of palladium but is in general agreement with that of cobalt.It seems therefore that the co-ordination of the thiocyanate group towards certain metals may be modified by the presence of other ligands. This does not occur for every metal cf scandium and the lanthanide~,l~~-l~~ but if the metal belongs to class ‘b’ or shows characteristics intermediate between class ‘a’ and class ‘b’ then the effect of other ligands may be important in determining its be- haviour. The fact that neither [Cr(CO),NCS]- 140 nor (?T-C,H,)CT(NO)~NCS~~~ form thiocyanato-linkages whereas the similar compounds Mn(CO)5SCN,139 (n-C,H,)Fe(CO)2SCN,141 (n-C,H,)M(CO),SCN,141 and [M(C0)5SCN]-,140 (where M = Mo or W) all do perhaps is because chromium(o) is closer to class ‘a’ than the remainder and is too close to be modified by the five carbonyl groups.The precise nature of the ligand effect is not apparent although it must be electronic in origin. It acts in one way towards the group of compounds exempli- fied by palladium and in the opposite way to the cobalt compounds. It is not clear whether the existence of two groups of compounds relates to class ‘b’ and borderline cases to square planar and octahedral cases to d8 and ds cases or to some other description more fundamental than ‘the palladium and cobalt groups’. However it is not surprising to find borderline situations in which the factors just discussed appear to be in approximate balance.Under these circumstances linkage isomers may be f o ~ n d ~ ~ ~ ~ ~ or other effects may be recognised as tilting the equilibrium position one way or the other. Thus the differing steric require- ments of the M - S over the M - N- C - S linkage have been used to \ C \ N 135 A. Turco and C. Pecile Nature 1961 191 66. 136 R. G. Pearson J . Amer. Chem. SOC. 1963 85 3533. 13’ C. K. Jorgenson Inorg. Chem. 1964,3 1201. 13* M. A. Jennings and A. Wojcicki Inorg. Chem. 1967 6 1854. 139 a M. F. Farona and A. Wojcicki Znorg. Chem. 1965,4 857; b i965,4 1402. 140 A. Wojcicki and M. F. Farona J. Inorg. Nuclear Chem. 1964 26 2289. 141 T. E. Sloan and A. Wojcicki Inorg. Chem. 1968 7 1268.86 Norbury and Sinha explain the formation of [Pd Et,dienNCS]+ (see Table 3) and of cis- Mn(CO),L,SCN (where L = Ph,As or Ph,Sb) when the trans-isomer is an isothio~yanate.~~~b The nature of the solvent can change the mode of linkage in ~ o 1 ~ t i o n ~ ~ ~ ~ ~ ~ ~ - ~ as can the counter-ion The complexity and versatility of the thiocyanate group are well illustrated by the isolation and characterisation of the three possible linkage isomers in the system bisthiocyanato-tri-(2-pyridyl)- aminecopper(~~).~~~ In contrast there are few examples in which organic thiocyanates or isothio- cyanates act as ligands. Ethyl thiocyanate forms bis-adducts with TiC14 TiBr, and SnCI in which M-S bonds are believed to be Ethyl isothio- cyanate forms a 1 :1 adduct with TiC1 which is a liquid at room temperature.146 No structural assignments are made by the authors but the adduct shows an i.r.band at 1605 cm-l 146 which is similar to that observed in the compounds [Pt(Ph,P),(RNCS)] where R = methyl or ~heny1.l~' These latter compounds have been assigned the structure Ph3 P A similar structure is proposed for [Rh(Ph,P) (PhNCS),] with the extra phenyl isothiocyanate acting as a unidentate ligand through su1ph~r.l~~ It is thought that the analogous platinum compound with allyl isothiocyanate contains the allyl radical and the thiocyanate group co-ordinated separately. A similar process is suggested for the reaction of allyl isothiocyanate with palladium(~~) chloride but the ligand remains intact and co-ordinates to cobalt(n) and chromium(m) through nitrogen. l4 * There appears to be relatively little structural information concerning the co- ordination of thioamides.NN-Dimethyl thioacetamide co-ordinates to cobalt(@ through On the other hand there is a wealth of information on the co-ordination of thiourea. It frequently acts as a unidentate ligand when it co-ordinates through sulphur to palladium(@ plat inum(n) and several 3d elements.160 S-Bonded compounds [Re tu,X,] are k n o ~ n . ~ ~ ~ (tu = thiourea.) 142 A. H. Norbury P. E. Shaw and A. I. P. Sinha unpublished results. 143 F. Basolo W. H. Baddley and K. J. Wiedenbaum J . Amer. Chem. SOC. 1966 88 1576. 144 a J. L. Burmeister H. J. Gysling and J. C. Lim J . Amer. Chem. SOC. 1969,91,44; b D. F. Cutterman and H. B. Gray ibid. 1969,91 3105. 146 G. C. Kulasingam and W. R. McWhinnie J. Chem.SOC. (A) 1968 254. 146 S. C. Jain and R. Rivest Canad. J. Chem. 1965,43 787. 14' M. C. Baird and G. Wilkinson J. Chem. SOC. (A) 1967 875 148 A. Dutta-Ahmed Inorg. Nuclear Chem. Letters 1968 4 289. 140 S. K. Madan and D. Mueller J. Inorg. Nuclear Chem. 1966 28 177. 160 A. Yamaguchi R. B. Penland S. Mizushima T. J. Lane C. Curran and J. V. Quagliano J. Amer. Chem. SOC. 1958,80,527; D. C. Flint and M. Goodgame J. Chem. SOC. (A) 1966 744; D. M. Adams and J. B. Cornell ibid. 1967 884. 161 F. A. Cotton C. Oldham and R. A. Walton Inorg. Chem. 1967,6,214. 87 me Co-ordination of Ambidentate Ligands In the compounds M ~U~(NCS)~ (M = Mn Coy Ni or Cd) thiourea forms a bidentate bridge through the sulphur atom.152 The only example of a unidentate N-bonded thiourea (or thioamide) appears to be in the compound Ti t ~ ~ C l ~ The ligand thiocarbohydrazide (H2N-NH)2CS frequently acts as a chelate but as a unidentate ligand it co-ordinates through sulphur to ZnII CdII and HgII and to some 3d elements.ls4 Benzothiazole (11) has been shown to form complexes with iron(r~),l~~ cobalt(11),~~~-~ nickel(^^),^^^^^^^ copper(x~),~~~ and zinc(11)l~~ and with a number of different anions.Co-ordination occurs via the nitrogen atom,156~157 except when the ligand has a bridging r61e in which case both heteroatoms are invoIved.lss 2-Methylbenzothiazole behaves in a similar f a ~ h i 0 n . l ~ ~ Unidentate dithiocarbate complexes of rhodium and iron have been reported in which metal-sulphur bonds are formed.16@ Similar bonding occurs with some quadrivalent actinides.lG0 Dithio-oxamide chelates to TiBr and SnCl through the nitrogen atoms.lsl B.Nitrogen versus Oxygen.-The cyanate ion is known to form complexes with a wide number of metals although its co-ordination behaviour has not been studied as widely as that of the thiocyanate ion. Anionic isocyanato- complexes [Mm(NCO)n]m-n have been reported162 for M = MnII FeII FeIII CoII Nil1 CuII ZnII CdII PdII and SnIV. Varying the 0- and .n-donor properties of the Iigands in some complexes of the type [ML,(NCO),] where M = PdII or PtII does not alter the mode of co-ordination of the cyanate group.163 Iso- cyanato-complexes are generally formed in mixed ligand complexes of 162 M. Nardelli A. Braibanti and G. Fava Gazzeffa 1957 87 1209. lS4 G. R. Burns Inorg. Chem. 1968,7,277. lSs N. N. Y. Chon M. Goodgame and M. J. Weeks J.Chem. SOC. (A) 1968,2499. lSB E. J. Duff M. N. Hughes and K. J. Rutt J. Chem. SOC. (A) 1968,2354. 16' R. A. Ford G. Halkyard and A. E. Underhill Inorg. Nuclear Chem. Letters 1968,4 507. lS8 E. J. Duff M. N. Hughes and K. J. Rutt J. Chem. SOC. (A) 1969,2126. lSQ C. O'Connor J. D. Gilbert and G. Wilkinson J. Chem. SOC. (A) 1969 84. l60 J. P. Bibler and D. G. Karraker U.S. At. Energy Comm. DP-MS-66-82 (Chem. A h . 1968,68 8892s). 165 A. H. Norbury and A. I. P. Sinha Inorg. Nuclear Chem. Letters 1967 3 355; J. Chem. SOC. (A) 1968,1598. R. Rivest Canad. J . Chem. 1962 40 2234. S. C. Jain and R. Rivest J . Znorg. Nuclear Chem. 1967 29 2787. D. Forster and D. M. L. Goodgame J . Chem. SOC. 1965,262; 1268. 88 Norbury and Sinha nickel,166 palladium,167 and p l a t i n ~ m . ~ ~ ~ ~ ~ ~ It was originally suggested that bridging of the type M-NCO-M occurred in the octahedral complex [CO(~-NCC,H,N),(NCO),],~~~ but subsequent research established that the bridging was of the type M-N(C0)-M in this and a number of related complexes.16 N.m.r.evidence indicated that linkage isomerism of the cyanate group took place in solutions of [Pt(Et,P),(NCO)H] but only the isocyanate was isolated as a s01id.l~~ The oxygen atom is believed to be the donor atom to chromium(I1) in melts of CrC1 in KNCS but again no solid cyanates have been is01ated.l~~ The bis(cyclopentadienyl)titanium(lv) species forms a bis-cyanate complex which has been reported as N-b~ndedl~~ or O-b~ndedl~~ by different authors. (7rTT’C5H5)2 Ti(NC0)174 and (7r-C,H5) V(NCO),176 both form N-bonded com- plexes.If (n-C5H5)2Ti(OCN)2 is the correct formulation it joins the complex hexacyanates of rhenium(rv) rhenium(v) and rn~Iybden~m(m)~~~ as the only isolated O-bonded inorganic cyanates. A number of organic cyanates are known,177 but their co-ordination behaviour has yet to be investigated. There are no examples of unidentate co-ordination‘ of organic isocyanates although some instances have been cited of bridging in complexes by these rn01ecules.~~~~~~~ Amides have been extensively studied as ligands. Their uses as non-aqueous solvents have been reviewed.179 Primary amides co-ordinate through oxygen to a number of conventional Lewis acids,lSo to the later 3d elementsla1J82 and S. M. Nelson Proc. Chem. Soc. 1961 372. 166 H. C. A. King E. Koros. and S . M. Nelson Nature 1962 196 572. 166 S.M. Nelson and T. M. Shepperd Inorg. Chem. 1965 4 813. la7 W. Beck and W. P. Fehlhammer Angew. Chem. Znternat. Edn. 1967 6 169. 16* J. Nelson and S. M. Nelson J. Chem. SOC. (A) 1969 1597. 170 J. Powell and B. L. Shaw J. Chem. SOC. 1965 3879. 171 D. H. Kerridge and M. Mosley J. Chem. SOC. (A) 1967 1874. 172 R. S. P. Coutts and P. C. Wailes Aust. J . Chem. 1966 19 2069. 173 J. L. Burmeister E. A. Deardorff and C. E. Van Dyke Znorg. Chem. 1969,8 170. 174 R. S. P. Coutts and P. C. Wailes Znorg. Nuclear Chem. Letters 1967 3 1. 176 G. Doyle and R. S . Stuart Inorg. Chem. 1968,7,2479. 176 R. A. Bailey and S. L. Kozak J. Inorg. Nuclear Chem. 1969,31,689. 177 H. Hoyer Chem. Ber. 1961 94 1042; D. Martin Angew. Chem. Znternat. Edn. 1964,3 311; K. A. Jensen M. Due and A. Holm Acta Chem. Scand.1965 19,438; N. Groving and A. Holm ibid. 1965 19 443. 178 T. A. Manuel Znorg. Chem. 1964,3 1703; R. B. King and M. B. Bisnette Znorg. Chem. 1966 5 306. 179 R. C. Paul ‘New Pathways in Inorganic Chemistry’ ed. E. A. V. Ebsworth A. G. Maddock and A. G. Sharpe Cambridge University Press 1968 p. 233. 180 J. Archambault and R. Rivest Canad. J- Chem. 1958 36 1461; W. Gerrard M. F. Lappert H. Pyszora and J. W. Wallis J. Chem. SOC. 1960 2144; A. Clearfield and E. Malkiewich J. Inorg. Nuclear Chem. 1963,25 237; R. C. Paul B. R. Sreenathan and S . L. Chadha ibid. 1966 28 1225. lE1 C. R. Rollinson and R. C. White Znorg. Chem. 1962 1 281; W. E. Bull S. K. Madan and J. E. Willis ibid. 1963 2 303; S. K. Madan and H. H. Denk J. Inorg. Nuclear Chem. 1965,27 1049. R. S. Drago D. W. Meek M. D. Joesten and L.LaRoche Inorg. Chem. 1963 2 124; M. B. Welch R. S. Stephens and R. 0. Ragsdale Inorg. Chim. Acta 1968,2 367. J. C. Bailar jun. and H. Itatani J. Amer. Chem. SOC. 1967 89 1592. 89 The Co-ordination of Ambidentate Ligands to palladium(n) and platinum.1s3 The donor properties are modified on varying the amide substituentslS2 but there is no evidence to suggest that the mode of co-ordination is affected. Bridging may involve both N and 0,1E4 although there are no unequivocal examples to confirm this. Urea and substituted ureas act as unidentate ligands. Urea is N-bonded in its complexes with palladium(@ and plat in urn(^^)^^^ and in [Sn(~rea),ClJ~~~ but is 0-bonded in complexes with chromium(nr) iron(m) copper(@ zinc(u),ls6 and even in [Sn(~rea),Br~J.~~~ Substituted ureas also co-ordinate through oxygen.lE7 Benzoxazole (111) forms complexes with cobalt(Ir) nickel(@ copper(@ and zinc(@ which are N-bonded,188 but 2-methylbenzoxazole co-ordinates to the same metals through oxygen.189 C.Nitrogen versus Selenium.-Selenocyanate complexes have been reviewed.lsO The co-ordination behaviour of the selenocyanate ion is similar to that of the thiocyanate ion in that it normally co-ordinates with class ‘a’ metals through nitrogen and with class ‘b’ metals through s e l e n i ~ m . ~ ~ ~ ~ ~ Although some selenocyanato-vanadium(n1) complexes have recently been reported,lg2* in contradiction to earlier work,lD3 the mode of co-ordination appears to be less sensitive to the electronic influence of other ligands in the complex. Thus unlike the change from Pd-S to Pd-N bonding in [PdL,(SCN),] complexes *The existence of selenocyanato-vanadium(@ complexes was reported in Chem.Abs. 1969 70 102599q/192. The abstract is incorrect. The original paper confims that the compounds contain the N-bonded isoselenocyanato-grouping. lS3 M. Donati D. Morelli F. Conti and R. Ugo Chiniica e Industria 1968 50 231 (Chem. A h . 1968,68 101331t). 184 S. D. Hill and A. H. Norbury unpublished results. lS6 R. B. Penland S. Mizushima C. Curran and J. V. Quagliano J . Amer. Chem. SOC. 1957 79 1575. lS6 D. S. Bystrov T. N. Sumarokova and V. N. FiIiminov Oprics and Spectroscopy 1960 9 239. lS7 R. A. Bailey I. R. Feins and T. R. Peterson Cunad. J . Chzm. 1969 47 171. lS8 E. J. Duff and M. N. Hughes J. Chem. SOC. (A) 1968,2144. lS9 E. J Duff and M. N. Hughes J. Chem.SOC. (A) 1969,477. l o o A. Lodzinska Studia Societatis Scientarium Torunensis 1961 IIIB(4) 1 (Chem. Abs. l959,59,7148a); A. M. Golub and V. V. Skopenko Russ. Chem. Rev. 1965,34,901. H.-H. Schmidtke Ber. Bunsengesellschaft Phys. Chem. 1967 71 1138; R. M. Alasaniya V. V. Skopenko and G. V. Tsintsadze Tr. Griir. Politekh. Inst. 1967,7,21 (Chem. Abs. 1968 69 40772j); A. I. Brusilovets V. V. Skopenko and G. V. Tsintsadze Russ. J . Inorg. Chem. 1969,14,239. 19a V. V. Skopenko and E. 1. Ivanova Russ. J. Inorg. Chem. 1969,14 388. lgs J. L. Burmeister and L. E. Williams J. Inorg. Nuclear Chem. 1967 29 839. 90 Norbury and Sinha when L is changed from a a-bonding ligand to a n-bonding ligand in all the corresponding palladium-selenocyanate complexes except [Pd(n-Bu,P),(NCSe),] palladium-selenium bonds are formed.lQ4 Steric effects on the other hand can modify the mode of bonding and have been used to implement the synthesis of linkage isomers:144a the counterion can also be important as was noted for the thiocyanate There is little information on other ligands of this type (N-X-Se).Some NN'-disubstituted selenoureas act as unidentate ligands through selenium towards palladium(n) and platinum(m).lQ5 D. Sulphur versus Oxygen.-The thiosulphate ion like the sulphite ion co- ordinates most readily through sulphur to the 3d element^^^^-^ and to palladium(u) and plat in urn(^^)^^^^^^^ Chromate oxidation of the thiosulphate ion probably results in the formation of [03CrS203]2-.200 Chelation through sulphur and one oxygen has been confirmed by X-ray crystallography.201 Linkage isomerism has been suggested for the ion [CO(CN)~S,OJ~- on chemical evidence,202 but this is unconfirmed by any modern physical techniques.There is kinetic evidence for linkage isomerism in the complex ion [CO(NH,),S,O,]+.~~~ The thiophosphate ion sPo33- forms Co-S bonds in some ammine com- plexes of c ~ b a l t ( m ) . ~ ~ ~ 5 Misceheous Ambidentate Ligands There are some ligands which do not fit conveniently into any of the previous sections and yet which merit a brief mention. Thus sulphinamides MeS(O)NR% like dimethylsulphoxide co-ordinate to 3d elements through oxygen.20q The nitrosodicyanomethanide ion [ONC(CN),]- forms tetrakispyridine complexes with cobalt(n) nickel(@ and copper(@ through the nitrosyl nitrogen,2os whereas mi py,(ONO),] contains the nitrito-lh~kage.~~ When 2-(diphenyl-arsinomethyl) pyridine does act as a unidentate ligand towards cobalt(@ it is through nitrogen that it co-ordinates.206 1,4Thioxan forms S-bonded complexes with copper silver palladium 1*4 J.L. Burmeister and H. J. Gysling Inorg. Chim. Acta 1967 1 100. lgsT. Tarantelli and C. Furlani J. Chem. SOC. (A) 1968 1717; P. J. Hendra and Z. Jovic Spectrochim. Acta 1968 24 A 1713. lBaE. P. Berth R. B. Penland S. Mizushima C. Curran and J. V. QuagIiano J. Arncr. Chem. SOC. 1959,81 3818; C. D. Flint and M. Goodgame J. Chem. SOC. (A) 1967 1718. lQ7 J. Hidaka J. Fujita Y. Shimura and R. Tsuchida Bull. Chem. SOC. Japan 1959,32 1317. l 9 8 J. A. Costamagna and R. Levitus J. Inorg. Nuclear Chem. 1966,28,1116; A. V. Babaeva I. B. Baranovskii and Yu. Ya. Kharitonov Russ. J. Inorg.Chem. 1963 8 307. lQ9 J. B. Goddard and F. Basolo Inorg. Chem. 1968,7 936. 2OO I. Baldea and G. Niac Inorg. Chem. 1968,7 1232. 201 G. F. Gasparri A. Musatti and N. Mardelli Chem. Comm. 1966,602. 2or P. R. Ray and S. N. Maulik Z. anorg. Chem. 1931 199 353. 208 D. E. Peters and R. T. M. Fraser J. Amer. Chem. Soc. 1965,87,2758. 2os H. Koehler and B. Seifert 2. anorg. Chem. 1968,360 137. 306 €3. Uhlig and M. Schaefer Z. anorg. Chem. 1968,359 178. K. M. Nykerk D. P. Eyman and R. L. Smith Inorg. Chem. 1967,6,2262. 91 The Co-ordination of Ambiden fate Ligands platinum rhodium cadmium and and recent studies show that the same bonding applies with titanium(Iv) niobium(v) tantalum(v) tin(Iv) aluminium(In) and molybdenurn(~v).~~~ The ligand selenoxan bonds through the selenium atom in its complexes with titanium(Iv) tin(rv) and n i o b i u m ( ~ ) .~ ~ ~ In the complex BF3C4H80S however i.r. evidence indicates co-ordination of the ligand through the oxygen atom.20D Co-ordination through both oxygen and sulphur atoms has been suggested210 for the polymeric complex [TiC13C4H,0S]n. Similarly 2,6-dimethyl-4-thiopyrone (IV) forms S-bonded complexes with CuI As1 FeII CoII NiII PdII P P Hg” SbTI1 and Bi111.211 The corresponding pyrone co-ordinates to a number of Lewis acids through the keto-oxygen,212 as is the case when xanthone213 or 4-pyridone214 are the donors. Ant ipyrine (V) also co-ordinates through oxygen.215 II Ph The heterocyclic nitrogen is the preferred donor atom in some cyano- pyridines216 and in 4-amin0pyridine.~~~ Imidazole probably co-ordinates through the tertiary nitrogen also.21s 207 P.J. Hendra and D. B. Powell J. Chem. SOC. 1960 5105. 208 R. A. Walton Inorg. Chem. 1966,5 443; J . Chem. SOC. (A) 1967 1852. 2os K. A. Baker and G. W. A. Fowles J . Chem. SOC. (A) 1968 801. 210 G. W. A. Fowles R. A. Hoodless and R. A. Walton J. Chem. Soc. 1963 5873. 211 H. B. Gray E. Billig R. Hall and L. C. King J. Inorg. Nuclear Chem. 1962,24 1089. 212 D. Cook Canad. J . Chem. 1961 39 1184; 1963 41,505. 213 D. Cook Canad. J. Chem. 1963,41,522. 214 D. Cook Canad. J . Chem. 1963 41 515. 21s S. S. Krishnamurthy and S. Soundararajan J. Less-Common Metals 1967 13 619; A Ravi J. Gopalakrishnan and C. C. Patel Indian J. Chem. 1967,5,356; D. N. Sathyanarayana and C. C. Patel ibid. 1967 5 360. 21r(R. A. Walton J . Inorg. Nuclear Chem. 1966 28 2229; F.Farha and R. T. Iwamoto Inorg. Chem. 1965,4 844. 2l7 L. M. Vallarino W. E. Hill and J. V. Quagliano Inorg. Chem. 1965 4 1598. 218 W. J. Eilbeck F. Holmes and A. E. Underhill J. Chem. SOC. (A) 1967 757. 92 Norbury and Sinha 6 Conclusions Any conclusions that can be drawn from this survey are at best tentative since many of the ligands included have had their co-ordination behaviour towards only cobalt nickel and copper studied. This consequence of the recent interest in crystal field theory has as its corollary the fact that the co-ordination chemistry of the titanium and vanadium sub-groups has been somewhat neglected. How- ever some of the results pertaining to these two latter sub-groups suggest that their allocation to ‘class a’ should be changed and that in many aspects of their chemistry they should be classified as ‘bprderline’ (see Section 5-thioxan).The transition elements as a whole are therefore either ‘borderline’ or ‘class b’ and as originally stated,lZ3 their characters may change as their oxidation or magnetic states are altered or as P e a r ~ o n ~ ~ ~ later noted as the nature of the surrounding ligands is modified. If the above supposition is correct it is no longer useful to explain the behaviour of a particular ambidentate ligand dimethyl sulphoxide for example as arising from the preference of ‘class b’ metals for sulphur and of ‘class a’ for oxygen even though the description is apposite in this case. If other similar ligands are considered then S2032- S032- SO2 and RS0,- all prefer to co- ordinate through sulphur and only in the case of the sulphinate group is there any pronounced tendency to form hi-0 bonds (see sections 3B and 4D).Thus the preferred mode of co-ordination of an ambidentate ligand is deter- mined by the nature of the respective donor sites as well as that of the acceptor. This is particularly apparent when the ligands revielved in Section 2 (CO CN- NO+ and CS) are considered. Calculations suggest that in every case the mode of bonding can be correlated with the availability of a large correctly orientated pair of electrons on the bonding atom. The interesting change then in consider- ing the sulphur-oxygen ligands of the previous paragraph is that which takes place in the distribution of electrons within the S-0 bond in X,SO as X changes from 0- to Me. As a consequence of this change the differing ligands have vary- ing abilities to differentiate between the acceptor systems.There are no other comparable systems of ambidentate ligands in which the two donor atoms belong to the same periodic group. It is even less satisfactory to use the ‘a/b’ classification to account for the behaviour of a ligand containing donor atoms from different groups as well as different rows of the periodic table. However this is done for the archetypal ambidentate ligand the thiocyanate ion and as was seen in Table 2 done with remarkable success. The overriding importance of charge distribution is seen when the thiocyanate ion is compared with thioamides having the same N-C-S framework. The electron distribution in thioamides is indicated by the canonical forms 93 The Co-ordination of Ambidentate Ligands and co-ordination accordingly takes place through sulphur.When ligands containing donor atoms of the same row are considered the same effects are seen to predominate. NCO- with its electronic charge concen- trated on the nitrogen co-ordinates largely through nitrogen while amides with similar canonical forms to thioamides co-ordinate through oxygen. In the case of NO2- it is apparent that steric effects can stabilise nitrito-compounds which would normally be thermodynamically less stable than the nitro-com- pounds. Kinetic results show how the nitrito-group can readily isomerise by an intramolecular process. However it is not clear why nitro-complexes should be thus preferred. The few results available for ligands with the same donor atoms in different states of hybridisation indicate that co-ordination occurs more readily through an atom with its lonepair of electrons in an spa rather than an sp3 orbital but that the necessarily greater double-bonding associated with atoms in sp hybridisa- tion reduces or counteracts the greater extension of the sp hybrid orbital (see Section 5).The varied behaviour of the thiocyanate ion and the interest and activity in its study are consequences of its equitable electron distribution. It is therefore more likely to be sensitive to relatively small changes in the acceptor and is thus a useful probe into the electronic nature of the acceptor. It is anticipated that the most promising developments in the chemistry of ambidentate ligands will come from the search for other ligands with similarly equitable electron distributions for the study of borderline acceptors or alternatively in the search for ligands which are ambidentate towards a particular group of metals the titanium sub-group for example and which may therefore be used to probe the nature of these metals in their complexes.In either case ligands which can have their own character modified by changing substituents are to be preferred. 94

ISSN:0009-2681    

DOI:10.1039/QR9702400069

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Wave Functions for Small Molecules based on Linear Combinations of Atomic Orbitals By R. G. Clark* and E. Theal Stewart CHEMISTRY DEPARTMENT UNIVERSITY OF DUNDEE SCOTLAND 1 Atomic Orbitals in Molecular Wave Functions That molecules are made up of atoms is not in itself a good reason for construct- ing molecular wave functions from atomic wave functions. The LCAO (linear combinations of atomic orbitals) procedure has no fundamental wave-mechanical basis and it can thus be justified in a quantitative sense only by the evidence of a large number of uniformly successful calculations on a variety of molecules. It is only very recently that this evidence has become available. Because of the difficulty caused by the number and the complexity of the electron-repulsion integrals for all but the smallest molecules the accumulation of the required information has had to await the development of large-scale electronic computing techniques.Until the late nineteen-fifties most LCAO calculations were restricted to two widely different groups of molecules on the one hand there were strict and accurate calculations on a few exceedingly small molecules and on the other there were Huckel (or modified Hiickel) calculations on a great number of large w-orbital systems. The former were too unrepresentative to be generally informa- tive the latter too heavily loaded with experimental data. It is obvious from the Bibliography in Section 6 that great progress has been made during the last decade in bridging the gap between the two groups of mol- ecules. It is now known not only that LCAO calculations can give molecular energies correct to within ca.0.5 % but that reasonable estimates can be made of spectroscopic constants and various molecular properties dependent on elec- tronic charge distribution. Computations on strict wave-mechanical principles have been carried out on molecules having nearly seventy electrons without recourse to experimental data. The work is constantly being extended but the main conclusions are now clear so this seems an appropriate time to review the subject for chemists in general and to indicate both what has been achieved and what has not. Several earlier accounts have been given for specialist readers the latest and most comprehen- sive being by Krauss.l An admirably clear summary of the position in 1959 was provided by Allen and Karo,l and we take this as our starting point.*Present address Department of Computing Science University of Stirling Scotland. l M. Krauss 'Compendium of Ab Znitio Calculations of Molecular Energies and Properties' Technical Note 438 National Bureau of Standards Washington 1967. L. C. Allen and A. M. Karo Rev. Mod. Phys. 1960 32 275. 95 LCAO Wave Functions for Small Molecules 2 Molecular-orbital Wave Functions If as in the majority of cases considered in this Review N molecular orbitals xl x2 . . . XN are used to describe the ground state of a 2N-electron molecule there is only one way of distributing the spin factors u and p and the complete wave function is thus the single determinant3 where xi signifies xiu and xi signifies xtp. The determinant (1) is obtained by imposing the Pauli antisymmetry requirement on the much simpler Schrodinger function (2) Because it is formulated by ignoring electron-repulsion terms in the Hamilton- ian operator the simple product (2) cannot be an exact solution of the relevant Schrodinger energy equation however accurate the individual molecular orbitals may be; the same is true of the determinant (1).If all the molecular orbitals have their best possible forms (and not merely the best forms obtainable by varying a small number of parameters such as LCAO coefficients or orbital exponents) the wave function (1) is described as a Hartree-Fock or self-consistent-field wave function. The difference between the Hartree-Fock energy for an atom or molecule and the energy obtained from an exact wave function with the same Hamiltonian operator is known (conveniently but for reasons which transcend the basic postulates of wave mechanics) as the correlation energy.The correlation energy of a molecule is almost always larger than the sum of the correlation energies of the component atom^.^ The wave functions discussed in this Review though very often (and very improperly) describedas Hartree-Fock or self-consistent-field wave functions are in fact just LCAO wave functions in which each molecular orbital x is written as a linear combination of atomic orbitals $a $b . . . $m (3) The rn atomic orbitals in any calculation are known as the basis set. The greater the value of my the more nearly an LCAO wave function approaches the Hartree- Fock wave function. The atomic orbitals are assigned the usual spherical polar form3 # = ROO where the angular factors @ and @ are exactly the same as for the hydrogen atom Xi(1) Xa(3) zd4) X N ( N - 1) xN(N) X = ca#a -k Cb#b + - - + cm#m and R is given by R =p-le-C;t (4) C.A. Coulson and E. T. Stewart in ‘The Chemistry of Alkenes’ ed. S. Patai Interscience New York 1964 ch. 1. 4 R. K. Nesbet Adv. Chem. Phys. 1965,9 321. 96 Clark and Stewart (as in a Slater5 orbital). For each atom in each molecule the ‘orbital exponent’ 5 depends not only on the quantum numbers n and I but also (because molecules lack the characteristic spherical symmetry of atoms) on the quantum number rn. Optimising the molecular wave functions subject to the restrictions imposed by the forms (3) and (4) requires the minimisation of the electronic energy with respect to the coefficients c in the molecular orbitals (3) and the exponential parameters < in the atomic orbitals (4).The variation of the coefficients is straightforward in principle and is normally effected by a method of successive approximation devised by Roothaan;’ the variation process is always carried to completion. On the other hand optimisation with respect to all the atomic- orbital exponential parameters [ in a molecular-orbital wave function is a very tedious process which can only be carried out by trial and error; it is usually left incomplete or omitted altogether. The values of f used in a molecular-orbital wave function are often those appropriate to variational calculations on the free atoms or simply those specified by Slater’s Rules? As a half measure the members of a set of arbitrarily chosen orbital exponents may all be multiplied by the same scale factor.If the energy is minimised with respect to the scale factor the variation principle and the virial theorem can be satisfied simultaneously.8 As far as computation is concerned an LCAO wave function can usually be improved more efficiently by extending the basis set than by optimising the atomic orbital exponents. The larger the basis set the less the improvement that remains to be brought about by varying the atomic-orbital exponents. Of course the ‘chemical’ interpretation of a wave function is simplest when the basis set is as small as possible. A method of extending the basis set which is directly related to the choice of orbital exponents involves the use of Clementi’s ‘double-zeta’ atomic orbitals,@ in which the simple exponential factor exp(- f:r) in the radial function (4) is replaced by a linear combination of two exponential factors exp(- c1r) and exp(- &r).Optimum values of and la are known for atomic orbitals with 2 < 36. These same exponents are used in a molecular calculation but the linear- combination coefficients are included in the variation process applied to the coefficients in equation (3). If a molecular wave function based on a single configuration of molecular orbitals is insufficiently accurate for its purpose the spin-orbital product (2) must be replaced by a linear combination of spin-orbitals products i.e. the determi- nant (1) must be replaced by a linear combination of determinants Y = C l Y + C2Y2 + . . (5) 6 J. C. Slater ‘Quantum Theory of Atomic Structure’ McGraw-Hill New York vol.1 1960. J. C. Slater ‘Electronic Structure of Molecules’ McGraw-Hill New York 1963; W. N. Lipscomb Adv. Magn. Resonance 1966,2 137. 7 C. C. J. Roothaan Rev. Mod. Phys. (a) 1951,23,69; (b) 1960 32 179. P.-0. Lowdin J. Mol. Spectroscopy 1959 3 46; A. D. McLean J. Chem. Phys. 1964,40 2774. 0 E. Clementi J. Chern. Phys. 1964,40,1944; E. Clementi R. Matcha and A. Veillard. ibid. 1967,47 1865. 97 LCAO Wave Functions for Small Molecules Each spin-orbital product or determinant represents one configuration. Varia- tional techniques for determining the coefficients Cl C2 . . . have been described by Clementi.lo The purely mathematical process of building up a wave function by linear combination of single-configuration determinants is known (mis- leadingly but almost universally) as configuration 'interaction'.Even a singleconfiguration wave function may have the form (5) if there are several equivalent ways of assigning spin factors to the molecular orbitals each determinant corresponding to one assignment. This will be the case if (as in most excited states) there are unpaired molecular orbital^.^ In these circumstances the linear -combination coefficients are found by using the Schrodinger spin opera- tors; the variation principle is not required for determining the coefficients C, C, . . . in (9 but the form of (5) complicates the algebraic procedures76 used in determining the coefficients ca Cb . . . in (3). 3 Types of Basis Function Although this Review is concerned with the construction of molecular orbitals from atomic orbitals we must mention in passing other types of basis function which are at present of considerable importance.Even with the extensive computing resources now available the accurate evaluation of the three- and four-centre integrals which arise in LCAO calcula- tions on polyatomic molecules still takes a considerable time. These are electron- repulsion integrals in which the four atomic orbitals in the integrands are functions of co-ordinates having three or four different origins (at the various nuclei). The difficulty is greatest for non-linear molecules which accounts for the otherwise surprising number of unusual linear molecules listed in Section 6. The evaluation of individual three- or four-centre integrals can be greatly simplified by replacing atomic orbitals of the Slater type (4) by Gaussian func- tions,11,12 in which the exponents are - cr2 instead of - (r.Unfortunately to obtain wave functions of comparable accuracy requires about five times as many Gaussian functions as Slater functions; i.e. ca. 54 (= 625) times as many integrals have to be evaluated. Nevertheless Gaussian basis sets have been used with considerable success for quite large molecules (e.g. formyl fluoride,13 benzene,14 pyridine,15 naphthalene16) and even to calculate energy surfaces for the NH3-HCl reacti0n.l' 'Gaussian lobe' functions utilise the relative ease of evaluating multicentre integrals in another way. In this method (Allen12) angularly dependent atomic lo E. Clementi J. Chem. Phys. 1967,46,3842; A. Veillard and E. Clementi Theor. Chim. Acta 1967 7 133.l1 I. G. Csizmadia M. C. Harrison J. W. Moskowitz and B. T. Sutcliffe Theor. Chim. Acta 1966 6 191. 12 L. C. Allen Internat. J. Quantum Chem. 1967 1 S39. l3 I. 0. Csizmadia M. C. Harrison and B. T. Sutcliffe Theor. Chim. Actu 1966 6 217. l4 J. M. Schulman and J. W. Moskowitz J . G e m . Phys. 1965,43 3287. 15 E. Clementi J . Chem. Phys. 1967 46 473 1. 16 R. J. Buenker and S. D. Peyerimhoff Chem. Phys. Letters 1969 3 37. 1' E. Clementi J . Chem. Phys. 1967,46 3851; 1967 47,2323; E. Clementi and J. N. Gayles ibid. 1967 47 3837. 98 Clark and Stewart orbitals are simulated by suitable positioning of groups of Gaussian ‘lobes’ (purely radial Gaussian functions). The problem of evaluating multicentre integrals can be avoided altogether by the use of single-centre basis sets.18 For example a wave function for hydrogen fluoridels can be constructed from F orbitals only and a wave function for methaneao from C orbitals only.In a similar way a wave function for acetylenea1 can be constructed from orbitals centred on the two C atoms. As would be expected single-centre calculations require very large basis sets and they are of limited application. As procedures for the evaluation of multicentre integrals are continuing to improve,22 it seems likely that atomic orbitals will eventually displace Gaussian functions from routine molecular calculations. The recent publication by Stevenson and Lips~omb~~ of LCAO wave functions for ScH,NH and TiH3F is a notable step in this direction. 4 Carbon Monoxide In this Section we illustrate the information that may be obtained from LCAO calculations by reference to carbon monoxide one of the molecules which have been studied most intensively.A. Wave Functions.-The CO molecule has fourteen electrons so at least seven atomic orbitals must be combined to construct the seven molecular orbitals (or fourteen molecular spin-orbitals) required for the simplest type of ground-state wave function. The sevenatomic orbitals must obviouslyinclude Is 2s,2pZ (=2po) from each atom. It is impossible to choose any other 2p orbital without being committed to both 2pz and 2pU (which are degenerate) from each atom; so a minimum basis set comprises ten atomic orbitals. These give ten linearly inde- pendent molecular orbitals instead of the seven required for a wavefunction of type (1). The three molecular orbitals which are superfluous in the ground state (‘virtual) orbitals) can be used in excited states;7a but strictly a separate minimis- ation of all atomic-orbital coefficients should be carried out for each excited state.7b As shown in Section 6 minimum-basis-set calculations have been performed by using atomic-orbital exponents determined both by Slater’s Rules and by atomic variational calculations and by Sahni et aLa5 and H U O ~ ~ using atomic- orbital exponents varied in the molecular calculation.Ne~bet,~‘ H U O ~ ~ and l8 B. D. Joshi J . Chem. Phys. 1967,46 875. R. Moccia J. Chem. Phys. 1964 40 2164. 2o D. M. Bishop Mol. Phys. 1963 6 305. 21 J. R. Hoyland J. Chem. Phys. 1968,48 5736. 28 R. E. Christoffersen and K. Ruedenberg J. Chem. Phys. 1968 49 4285; L. S. Salmon F. W. Birss and K. Ruedenberg ibid.1968 49 4293; D. M. Silver and K. Ruedenberg ibid. 1968 49 4301 4306 (and references cited therein). 24 B. J. Ransil Rev. Mod. Phys. 1960 32 239 245. 26 R. C. Sahni C D. La Budde and B. C. Sawhney Trans. Faraduy SOC. 1966 62 1933. 26 W. M. Huo J. Chem. Phys. 1965,43 624. 27 R. K. Nesbet J . Chem. Phys. 1964 40 3619. P. E. Stevenson and W. N. Lipscomb J . Chem. Phys. 1969,50 3306. 99 LCAO Wave Functions for Small Molecules Yoshimine and McLean2* have carried out calculations with a variety of extended basis sets. Extension of the basis set has been found (as is generally the case) to be more effective than optimising orbital exponents2* or using configuration inter- action with a small basis set. With a minimum basis set a single-determinant function of type (1) gives 99.09% of the observed molecular energy and a 14- term function of type (5) gives only 0.04 % more.2B Single-determinant extended- basis-set calculations give ca.99.5 %. The molecular orbitals comprising wave function are reproduced in Table 1. They are of three symmetry types a m and nu. It is obvious that the la and 20 molecular orbitals are virtually unchanged atomic orbitals la NN Is0 and 20% ISC. This is because the two 1s orbitals are concentrated so much about their respective nuclei that they do not overlap appreciably either with each other or with the other atomic orbitals in the system. This is clear also from the orbital energies the la and 20 molecular orbital energies are not significantly different from the corresponding orbital energies in other molecules containing the same atoms or indeed from the 1s orbital energies in the free atoms.It will be noted that there is no lack of combination between 2s and 2pz orbitals in all the higher-energy (T orbitals; hybridisation is a feature of all properly optimised LCAO wave functions and is not restricted to particular electronic or geometrical configurations. The sharp quantum distinction between s andpa orbitals which depends upon the spherical symmetry of atoms is lost in molecules. The ratio between 2s and 2pz coefficients in Table 1 varies from one orbital to another; the fixed ratio which is found in simple valence-bond functions is not determined variationally and has no relevance in molecular-orbital wave functions . Wave functions similar to that in Table 1 have been discussed by Coulson and Stewart in more detail than can be given here.B. Dissociation Energy.-Because the correlation energy of a molecule almost always exceeds the sum of the correlation energies of its atoms Hartree-Fock calculations tend to underestimate dissociation energies sometimes very grossly. The same is usually true of single-configuration LCAO approximations to Hartree-Fock calculations as is shown for CO in Table 2 It happens in the results quoted that the better wave function gives the better dissociation energy but this is not a general feature. Extending a basis set may either increase or decrease a calculated dissociation energy depending on whether the improvement in the molecular energy is greater or less than the improvement in the sum of the atomic energies. It is very well knowns that one of the principal disadvantages of molecular- orbital wave functions is that they correspond to charged instead of neutral atomic dissociation products.It is possible to estimate the correlation energy of a molecule by assuming it to be approximately the same as that of the Hartree- a* M. Yoshimine and A. D. McLean Internat. J. Quantum Chem. 1967,1 S313. 2Q S. Fraga and B. J. Rand J. Chem. Phys. 1962,36 1127. 100 Table 1 Molecular-orbital wave function for the ground state of carbon monoxidea Configuration :* la22a230240217;221~~2502 Internuclear distance = 2.132 B? = 1.1282 A Molecular Coefficients of carbon orbitals Coeficients of oxygen orbitals: Orbital A A Orbital* f \ I \ energy§ 1s 2s 2PzlT 2Pz 2Pv Is 2s 2P4 2Px 2PY (Ht) 10 - 0.0002 0.0069 0.0063 - 0.9960 - 0.0202 - 0.0058 - 20,706 2a 0.9964 0.0171 0-0059 - 0.0002 - 0.0054 - 0.0007 - 11.353 3a - 0.1152 0.2401 0.1687 - 0.2148 0.7588 0.2232 - 1.499 40 0.1468 - 0.5383 - 0.0668 0.1263 0.6529 - 0.6350 - 0.732 SU - 0.1403 0.7579 - 0.5658 0.0022 0.0366 - 0.4379 - 0.481 60 - 0.0916 0.9694 1.2509 0.1197 - 1.1289 - 0.9415 0,932 IT 0-4686 0.7712 - 0.583 1% 0-4686 0.7712 - 0.583 27nr 0.9225 - 0.6897 0-261 2% 0.9225 - 0.6897 0.261 Molecular energy - 112.344 H (caIcuIated); - 113.377 H (observed).a Ref. 24. * The numerals preceding the symbols u and r are not quantum numbers but merely serial numbers indicating the order of orbital energies for each sym- metry type. The molecular orbitals 60 27rz and 2vrY ('virtual orbitals') are not relevant in the ground state. 7 The positive z axes point from the nuclei towards each other.f The orbital energy E associated with a particular spin-orbital consists of (i) the kinetic energy of the spin-orbital (ii) the potential energy of attraction between the spin-orbital and all the nuclei (iii) the potential energy of repulsion between the spin-orbital and all the other spin-orbitals in the system. For tl a formal definition see refs. 3 and 6. x c s 0 t 1 B (= Bohr) = 0.052917 nm; 1 H (= Hartree) = 4-3594 aJ (atomic units for infinite nuclear mass). $' Atomic-orbital exponents as specified by Slater's Rules. 9 s % w LCAO Wave Functions jor Small Molecules Fock ionic dissociation As the correlation energies of atoms and ions with 2 d 30 are ~ C I I O W ~ ~ ~ this provides a means of improving calculated molecular energies and hence calculated dissociation energies.32 Nesbet2' has estimated the correlation energy of C2+ + 02- to be 3.18 ev higher than that of C + 0. If a correction of this amount is added to the better of the two calculated dissociation energies (7.84 ev) quoted in Table 2 the adjusted value (11.02 ev) very closely approaches the observed dissociation energy (1 1.24 ev). Other methods of adjustment have been s u g g e ~ t e d . ~ ~ ~ ~ Table 2 Molecular properties of the ground state of carbon monoxide Molecular energy (H) Error in calculated molecular Dissociation energy (ev) Internuclear distance at energy minimum (B) Spectroscopic constants me (cm-l) UeXe (cm-l) Be (cm-l) a e (cm-l) energy (%I k (los dyne cm2) Dipole moment (D) Quadrupole coupling constant cmz) Electric field gradient at oxygen nucleus (atomic units) Calculated with minimum basis set - 112.344' 0.91 5-38' 2~182~ 2398~4~ 8~989~ 1.8419b 0.01 1 3 b - 0.730" - 0.15" Calculated with extended basis setd - 112.786 0.52 7.84 2.08 1 243 1 11.69 2.027 0.01 525 0.274 23.86 - 0.0214 - 0.679 Observede - 113.377 1 1 *242 2.1 32 21 69.8 13.295 1.9313 0.01 75 19.02 - 0.118 0.01 63 - 0.64 a Ref.24; b Ref. 25; C Ref. 49(a); Ref. 26; e Quoted in ref. 26. C. Spectroscopic Constants.-By performing an LCAO calculation for a range of internuclear distances it is possible to express the molecular energy as a function of internuclear distance and so to calculate spectroscopic constants. Table 2 shows that the results are by no means perfect as would be expected if only 3oR. K. Nesbet J. Chem. Phys. 1962,36 1518. *l E. Clementi and A.Veillard J. Chern. Phys. 1966 44 3050. aaE. Clementi J. Chem. Phys. 1963 38 2780; 1963 39 487. 33 K. Carlson and P. Skancke J. Chern. Phys. 1964,40,613; V. McKoy ibid. 1965,42,2232; F. Grimaldi ibid. 1965 43 S59. s4 C. Hollister and 0. Sinanoglu J. Amer. Chem. Soc. 1966 88 13. 102 Clark and Stewart because of the faulty estimation of dissociation energy. McLeana6 and Schwende- manS6 have discussed the various sources of error. A more subtle though not necessarily more accurate method of calculating spectroscopic constants involves the use of the Hellmann-Feynman theorema7 (which applies to genuine Hartree-Fock wave functionsa8 as well as to exact wave functions). D. Electronic Charge Distribution.-There is good reason to believe that the electronic charge distribution corresponding to a Hartree-Fock wave function should not differ significantly from the true d i s t r i b u t i ~ n .~ ~ ~ ~ ~ It may be hoped that the same is true of an LCAO wave function derived by applying the variation principle to a sufficiently large basis set (though it must be remembered that the quality of a wave function is not always measured very sensitively by its energy4I). To illustrate the effect of molecule formation on electronic charge distribution we show in the Figure how the charge distribution in the carbon monoxide molecule (as from an extended-basis-set wave functionzs) differs from that in a hypothetical system consisting of a carbon atom and an oxygen atom at the same internuclear separation. According to earlier views based on less adequate evidence than is now available the formation of a molecule was believed to be accompanied by an increase in electronic charge between the nuclei and a corresponding decrease beyond the nuclei.It is clear however from the Figure and from similar diagrams for many other diatomic (homonuclear and heteronuclear) that there is (i) an increase in electronic charge around the internuclear axis in the region between the nuclei (ii) a sharp decrease immedi- ately around each nucleus and (iii) an increase beyond each nucleus. E. Dipole Moment.-The fact that an enlargement of the basis set does not neces- sarily improve the agreement between calculated and experimental values of molecular properties (other than molecular energies) is shown strikingly in the case of the dipole moment of carbon monoxide (Table 2).With the minimum basis set the absolute value is much too large whereas with various extended sets26-28 the sign is wrong. It has taken a 200-term configuration-interaction calculationq4 to produce a value (- 0.17 D) which gives reasonable agreement between theory and experiment. It should be pointed out that the dipole moment of carbon monoxide is unusually small and depends very sensitively on inter- nuclear distance; for these reasons it would perhaps be wrong to attach too much 36 A. D. McLean J. Chem. Phys. 1964 40 243. 36 R. H. Schwendeman J. Chem. Phys. 1966,44,2115. 37 L. Salem J. Chem. Phys. 1963 38 1227; J. Goodisman ibid. 1963 39 2397; R. H. Schwendeman ibid. 1966,44 556; D. P. Chong Theor. Chim. Acta 1968 11 205. 38 R. E. Stanton J. Chem. Phys. 1962 36 1298.39 G. G. Hall Phil. Mag. 1961,6 249; A h . Quantum Chem. 1964 1 241. 40 J. Gerratt Ann. Reports(A) 1968 65 3. 41 J. Goodisman J. Chem. Phys. 1963 38 304. 43 R. F. W. Bader and A. K. Chandra Canad. J. Chem. 1968,46,953. 44 F. Grimaldi A. Lecourt and C. Moser Internat. J. Quantum Chem. 1967 1 S153. R. F. W. Bader and A. D. Bandrauk J. Chem. Phys. 1968,49 1653 1666. 103 LCAO Wave Functions for Small Molecules 4 0 / \ \ ‘. Figure Electron density diflerence map for carbon monoxide. The horizontal line represents the internuclear axis and its intersections with the vertical lines mark the positions of the carbon (left) and oxygen (right) nuclei. At each point A p = pco - ( Contours are plotted for I APT= 0,O.OOl. 0.01 0.1 e B - ~ . Unbroken lines dotted lines and doshed lines represent contours on which A is respectively zero positive and negative.In each zone I A p I decreases outwards front the internuclear axis. (All dorted lines and dashed lines form closed loops they are shown incomplete in regions where on the scale of the diagram they merge into the aa’jacent unbroken lines.) [Redrawn from a more detailed map by Bader and Bandrauk ref. 42.1 + PO). where p is the electron density. significance to the discrepancies associated with the single-configuration wave- functions. F. Electrical and Magnetic Properties.-Other quantities which have been calculated for the CO molecule include magnetic sus~eptibility,~~ magnetic shielding,4s rotational magnetic polari~abilities,~~ and quadrupole 45 M. Karplus and H. J. Kolker J. Chem. Phys. 1963,38 1263; J .R. de la Vega D. Ziobro and H. F. Hameka Physica 1967 37 265. 46 C. W. Kern and W. N . Lipscomb J. Chem. Phys. 1962 37 260. 47 J. R. de la Vega and H. F. Hameka J. Chem. Phys. 1967,47 1834. 48 M. Karplus and H. J. Kolker J. Chem. Phys. 1963 39 2011 ; J. M. O’Hare and R. P. Hurst ibid. 1967 46 2356; A. D. McLean and M. Yoshimine ibid. 1967 46 3682. 104 Clark and Stewart coupling constant.49 Several mefhods6O have been devised for calculating these quantities with LCAO wave functions. Lipscomb61 and Gerratt 40 have provided comprehensive reviews of the subject and have shown that reasonable agreement with experiment is usually obtained. As far as multipole moments are concerned both theoretical calculations and experimental determinations appear to be beset by considerable difficulties.62 G.Excited States.-From a basis set of m atomic orbitals m linearly independent and orthogonal molecular orbitals are obtained automatically when the electronic energy is minimised with respect to the atomic-orbital coefficients. A 2N-electron ground-state wave function in which all molecular orbitals are paired requires only the N molecular orbitals of lowest orbital energy and thus the (m - N ) higher-energy orbitals are superfluous (‘virtual’ orbitals). The simplest way of constructing wave functions for excited states is to replace one or more ground- state molecular orbitals by ‘virtual’ orbitals. This method has been used with a variety of basis sets by Lefebvre-Brion Moser and NesbeP3 to calculate ‘potential-energy’ curves spectroscopic constants dipole moments and dipole- moment derivatives for a number of low-lying excited states of CO.A more thorough-going application of the variation principle requires that the atomic-orbital coefficients should be optimised afresh for each excited state. This procedure which involves a surprising increase in technical difficulty has been used by H u o ~ ~ for CO (extended basis set) and by Sahni and S a ~ h n e y ~ ~ for CO+ (minimum basis set). Calculations on excited states always give less satisfactory agreement with experiment than do calculations on ground states. 5 Improved and Adjusted Wave Functions A. Configuration Interaction.-As pointed out in Section 4 single-determinant wave functions do not correspond to neutral dissociation products. In the case of certain molecules as Das Wahl and others660 57 have shown for Li, F, and NaF this difficulty can be overcome by adding to the wave function a second determin- ant based on another molecular configuration.The variation principle ensures of (a) J. W. Richardson Rev. Mod. Phys. 1960,32,461; (b) H. Lefebvre-Brion C. M. Moser M. Karplus Rev. Mod. Phys. 1960,32,455; D. F. Tuan S. T. Epstein and J. 0. Hirsch- R. K. Nesbet and M. Yamazaki J. Chem. Phys. 1963,38,2311. felder J. Chem. Phys. 1966,44 431. 61 W. N. Lipscomb Adv. Magn. Resonance 1966,2 137. 63 D. E. Stogryn and A. P. Stogryn MoZ. Phys. 1966,11 371. sB H. Lefebvre-Brion C. M. Moser and R. K. Nesbet J. MoZ. Spectroscopy 1964,13 418; R. K. Nesbet J. Chem. Phys. 1965,43,4403. 64 W. M. HUO J. Chem. Phys. 1966,45 1554. O6 R. C. Sahni and B. C. Sawhney Trans. Faraday SOC.1967 63 1. 66 G. Das and A. C. Wahl J. Chem. Phys. 1966,44,87; G. Das ibid. 1967,46,1568; G. Das and A. C. Wahl ibid. 1967 47 2934; B. Levy ibid. 1968 48 1994. 67 A. C. Wahl P. J. Bertoncini 0. Das and T. L. Gilbert Internat. J . Quantum Chem. 1967 1 S123. 105 LCAO Wave Functions for Small Molecules course that this reduces the calculated molecular energy. Calculated spectro- scopic quantities are also improved. The change is most striking in Fz for which the double-configuration wave function unlike the simple LCAO wave func- tion,68 does not give a negative dissociation energy. The nature of the double-configuration wave function is illustrated most simply by reference to Liz for which the single-determinant function is based on the configuration lagz lauz 2og2. If this is combined with a second determinant based on the configuration lag2 lou2 2au2 and the linear-combination coefficient (a function of internuclear distance) is determined by applying the variation principle the resultingwave function [of the type ( 5 ) in Section 21 becomes awavefunction for two neutral Li atoms (Is2 2s) as R 4 co .Exactly analogous wave functions have long been known for the hydrogen m01ecule.~ (In H2 a linear combination of ogz and uu2 gives the correct behaviour on dissociation; agz by itself does not.) Wahl et aL6’ have given a general survey of configuration-interaction wave functions for diatomic molecules. B. Open-shell Wave Functions.-Another way of improving single-configurat ion wave functions is to avoid the orbital pairing indicated in equations (1) and (2).General procedures of this type have been devised by L o ~ d i n ~ ~ Goddard,60 and Kaldor.61 C. Constrained Wave Functions.-In the integrals from which molecular properties are calculated the integrands vary considerably from one point to another in the co-ordinate space. The manner in which different regions of a molecule are weighted in the integration depends upon the nature of the Schrodinger operator representing the molecular property. For some properties the regions nearest the nuclei are the most important for others the regions furthest away. This is why even a very flexible wave function may if optimised with respect to the energy give disappointingly imprecise values for other quantities; whereas a poor wave function is almost certain to give poor results a good wave function (in the varia- tional sense) will not necessarily give good results.For this reason early success in the precise calculation of a wide range of properties seems unlikely to be achieved merely by making variational wave functions more and more complicated. Mukherji and Karplus,62 in an altogether different approach have argued that if quite a simple energy-optimised wave function is adjusted so as to give the observed numerical values for some molecular properties the adjusted wave function might be expected to give good values for other properties represented by integrals in which the same regions of the molecule are important. They re-varied some of the LCAO coeficients in Rand’s minimum-basis-set wave function2* for HF subject to the constraint that the 68 A. C. Wahl J.Chem. Phys. 1964,41,2600. 69 P.-0. Lowdin Phys. Rev. 1955 97 1509. W. A. Goddard Phys. Rev. 1967 157 73 81 ; J. Chem. Phys. 1968,48,450 5337. U. Kaldor J. Chem. Phys. 1968,48 835. eaA. Mukherji and M. Karplus J. Chem. Phys. 1962 38 44. 106 Clark and Stewart relevant integrals should give the experimental values for the dipole moment and the deuteron quadrupole coupling constant. This reduced the errors in the calculated diamagnetic and paramagnetic susceptibilities by more than half. The adjustment in the previously optimised LCAO coefficients necessarily raised the calculated energy but by a mere 0404%. Variants and extensions of the procedure followed by Mukherji and Karplus have been formulated by other authors.6a 6 Bibliography To demonstrate the scope and the extent of strict wave-mechanical calculations on LCAO wave functions we give in Tables 3-8 a list of the relevant publications in the period from 1960 to mid-1969 on systems of four or more electrons.It is clear that in addition to energy a very wide range of molecular properties can be calculated from LCAO wave functions (in some cases not very precisely as yet). We include in these Tables many calculations which have been superceded by others of greater complexity. This is partly for the reason given at the end of Section 5 but mainly because the effects of varying orbital exponents extending basis sets and using multiconfiguration wave functions can be judged only by consideration of a substantial collection of numerical examples. We have made this collection as complete as possible. It seems likely that in the next few years calculations will be carried out on polyatomic molecules much more complicated than those we list and that work on diatomic molecules will normally go beyond the single-configuration LCAO approximation.Y. Rasiel and D. R. Whitman J. Chem. Phys. 1965,42,2124; D. P. Chong and Y. Rasiel ibid. 1966,44 1819; W. B. Brown ibid. 1966.44,567; C. P. Yue and D. P. Chong Theor. Chim. Ada 1968,12,431; S. Fraga and F. W. Birss ibid. 1966,5,398; S. Fraga and G. Malli ibid. 1966,5 446. Notes on Tables 3-6. In the columns headed ‘Basis set’ the first letter denotes the size M = minimum; E = extended. S . . . . . determined arbitrarily e.g. by Slater’s Rules; A . . . . . optimised for the free atom; P . . . . . partially optimised for the molecule; M . . . .. completely optimised for the molecule. If the second letter is E the wave function is formulated in elliptical co-ordinates. The numbers in parentheses refer for each atom to the number of radially distinct orbitals of each symmetry type in the order (0 n a) the order of the atoms matching that in the chemical formula. For non-planar molecules only the total number of atomic orbitals on each nucleus is listed. The letter C following the parentheses indicates a ‘configuration-interaction’ calculation. In the case of FH for example MS(3,1)(1,0)C denotes a multiconfiguration wave function using the following minimum basis set of atomic orbitals with the exponents determined by Slater’s Rules The second letter if S A P or M refers to the atomic-orbital exponents F(u) IS 2 ~ 2pz (= 2 p ~ ) ; F(?T) 2pz 2py (not radially distinct); H(u) 1s; H(n) nil.107 LCAO Wave Functions for Small Molecules For a wave function in elliptical co-ordinates only one set of numbers is required to specify the orbital symmetry types. (Such wave functions do not come strictly within the scope of this Review but they are listed for comparative purposes.) If a calculation is carried out for more than one geometrical configuration (more than one set of bond lengths and bond angles) the number is listed under the heading ‘Additional calculations’. In the same column are noted calculations on excited states (e) and calculations on positive ions (i). Letters in parentheses in the Reference columns denote papers which are concerned with excited-state wave functions and do not give details of the corresponding ground-state wave functions.Such reference letters in parentheses are given quite arbitrarily together with the first entry for each molecule. In almost all cases the internuclear distances (R) for which the molecular energies (E) and the dipole moments ( p ) have been calculated are those determined experimentally. (To save space molecular dimensions are not given in Table 6.) The sign of the dipole moment is positive if the lighter nucleus is at the positive end. Table 3 Molecular energies (E) o j homonuclear diatomic molecules* Basis set MM(3,O) MM(3 ,O)C EP( 15,3)C EP(17,6)C M M (3 90) MM(3,O)C E~(16,8)C EE( 10,8)C MM(391) MM(3,l)C MS(3,U MM(3,1) MM(3,1) MM(3,l)C W 5 2 ) W 7 3 ) EM(20,6) MS(3,l) MM(3,l) MM(3,l)C EM( 18,lO) EP( 18,lO)C ES(6,3) - E (HI 14.842 14-852 14.899 14.903 29-058 29.105 29.220 49.145 75.224 75.319 108.574 108.634 108634 108-661 108.785 108.971 108.993 149.092 197.877 197.956 198.768 198.838 679.166 R (B) 5-05 1 5.05 1 5.25 5-07 3.78 3.78 4.5 3.0 2.3475 2.3475 2.0675 2.068 2-1 2.068 2-068 2.068 2.068 2-28 17 2.68 2.68 2.68 2.68 3-58 Additional calculations 8 10 1 O,e e,i 53,i e,i 5 16,i e,i 6 Reference a b d a b e f 44 b g(0-9) a h b i k g a b 1 m C i C *LCAO wave functions and potential-energy curves have been calculated for Hear Nen and Ar (T.L. Gilbert and A. C. Wahl J. Chem. Phys. 1967,47,3425). a B. J. Ransil Rev. Mod. Phys. 1960,32,239,245; b S . Fraga and B. J. Ransil J. Chem. Phys. 1962,36 1127; C G. Das and A. C. Wahl J. Chem. Phys. 1966,44 87; G . Das J. Chem. Phys. 1967 46 1568; e C. F. Bender and E.R. Davidson J. Chem. Phys. 1967 47 4972; f C. F. Bender and E. R. Davidson J. Chem. Phys. 1967,46,3313; 9 R. C. Sahni and E. J. de Lorenzo J. Chem. Phys. 1965 42 3612; h R. C. Sahni and B. C . Sawhney Internat. J. 108 Clark and Stewart Quantum Chem. 1967 1 251; * J. W. Richardson J. Chem. Phys. 1961 35 1829; 1 R. K. Nesbet J. Chem. Phys. 1964,40 3619; k P. E. Cade K. D. Sales and A. C. Wahl J. Chem. Phys. 1966,44 1973; 1 A. C. Wahl J. Chem. Phys. I964,41,2600;m D. B. Boyd and W. N. Lipscomb J. Chem. Phys. 1967,46,910; n R. K. Nesbet and P. F. Fougere J. Chem. Phys. 1966,44,285; G. Verhaegen ibid. 1968,49,4696; 0 G. Verhaegen W. G. Richards and C. M. Moser J. Chem. Phys. 1967 47 2595.; P R. K. Nesbet J. Chem. Phys. 1965,43,4403; 4 H. Lefebvre-Brion and C. M. Moser J . Chem.Phys. 1965 43 1394. Table 4 Molecular energies (E) and dipole moments (p) of diatomic hydrides Molecule Basis set LiH BeH+ BeH BH CH+ CH NH OH OH- FH NeH+ NaH MgH AlH SiH - E R p Additional Refer- (H) (B) (D) calculations ence 7.970 7.984 7.987 7.987 8.006 8.017 8.039 8.041 8.061 14.836 15.153 15.221 25.075 25.090 25.131 37-859 38,279 54-325 54.345 54,978 75.421 75.41 8 99.536 99.564 99.991 100-057 lOO.058 100.070 100.071 100.257 128628 3-01 5 3.01 5 3.02 3-015 3.015 3-02 3.2 2.99 3.01 5 2.68 2.538 2.538 2.329 2.329 2-336 2-34 2.124 1 *976 1 -976 1.961 1.8342 1.781 1.733 1-733 1.733 1-7328 1.7328 1.7328 1.7328 1 -733 1.83 - 5.92 - 5-57 - 6.035 - 6.002 16,i - 6.04 - 5.89 3 4 - 5.96 2 - 5.965 6 - 0.282 16,i - 0.07 e 1.58 1-53 1.733 16,i 1-57 16,i* 2.01 2.06 1.627 16,i* 1.78 16,i 3.353 17 1.44 1.3 2.009 3 1 *827 1.984 6,i 1.942 6,i 1 *934 1 -649 6 6 162.393 3.566 - 6-962 15,i 200.157 3.271 - 1.516 15,i 242.463 3.114 0.17 15,i 289.436 2.874 0-302 15,i* LCAO Wave Functions for Small Molecules Moleule Basis set -E (H) PH EM(12,6)(4,2) 341.293 SH EM(12,6)(4,2) 398,102 SH- EM(12,6)(4,2) 398.146 CIH EA(9,4)(3,1) 459.804 EM(12,6)(4,2) 460-1 10 EP(17,10)(6,3) 460.1 12 2.708 2.551 2.5 12 2.4085 2-4087 2.4087 p Additional (D) calculations 0.538 15,i* 0.861 15,i 3.546 15 1.387 3,i 1.197 15,i 1,215 Refer- ence * Electron affinities calculated by similar calculations on negative ions (P.E. Cade Proc. Phys. SOC. 1967 91 842). a B. J. Ransil Rev. Mod. Phys. 1963,32,239,245; S . Fraga and B. J. Ransil J. Chem. Phys. 1962,36,1127; C J. R. Hoyland J. Chem. Phys. 1967,47,1556; d P.E. Cade and W. M . Huo J. Chem. Phys. 1967,47,614; e P . Linder Theor. Chim. Acta 1966,5 336; f R. K. Nesbet and S. L. Kahalas J. Chem. Phys. 1963 39 529; 9 F. E. Harris and H. S. Taylor Physicu 1964 30 105; h D. D. Ebbing J. Chem. Phys. 1962,36,1361; C. F. Bender and E. R. Davidson J. Phys. Chem. 1966,70,2675; F. Jenc Coll. Czech. Chem. Comm. 1963,28,2064.; k A. C. H. Chan and E. R. Davidson J. Chem. Phys. 1968,49,727; P. E. Cade J. Chem. Phys. 1967 47 2390; R. K. Nesbet Rev. Mod. Phys. 1960,32,272; 0 E. Clementi J. Chem. Phys. 1962,36,33;p A. D. McLean and M. Yoshimine J. Chem. Phys. 1967,47 3256; Q C. F. Bender and E. R. Davidson J. Chem. Phys. 1967,47 360; r S. Peyerimhoff J. Chem. Phys. 1965,43,998; 8 P. E. Cade and W. M. Huo J. Chem. Phys. 1967,47 649; t R. K. Nesbet J.Chem. Phys. 1964,41 1W;U H. S. Taylor J. Chem. Phys. 1963 39 3382; C. F. Bender and E. R. Davidson ibid. 1968,49,4222; R. E. Brown and H. Shull Znternat. J. Quantum Chem. 1968,2 663; W. M. Huo J. Chem. Phys. 1968 49,1482; W C. F. Bender and E. R. Davidson J. Chem. Phys. 1968,49,4989; W. G. Richards and R. C. Wilson Trans. Faraday SOC. 1968 64 1729. R. K. Nesbet J. Chem. Phys. 1962,36 1518; Table 5 Molecular energies (E) and dbole moments (p) of heteronuclear diatomic molecules Molecule Basis set - E R p Additional Refer- A lkali-metal halides (HI (B) (I)) calculations ence LiF LiCl LiBr NaF NaCl NaBr KF KCI RbF MA(3,1)(3,1) MP(3,1)(3,1 )C EP(6,2)(9,4) EP(7,3)(11,6) EP(8,3)( 1497) EP(7,3)( 18,10,2) EP(13,6)(10,5) EP(13,6)(14,7) EP( 1 1,5)(18,10,2) EP(18,8)(9,5) EP( 1 5,7)( 1 2,6) EP(21,11,2)(8,4) 106.381 2.85 106.412 2-85 106.989 2.8877 106.992 2.8877 467.055 3.825 2579-89 4-0655 261.379 3.628 621 -457 4.4609 2734.29 4.728 698.685 4.1035 1058.76 5.039 3037.77 4.3653 3-43 6.297 13 6.3 7 7.256 9 10 - 8.367 9 9.101 8 7 5 a b d e f g h f i f f C 110 Molecule Basis set Group I1 compounds Be0 EP(5,2)(8,4) EP(6,2)(10,5) BeS ES(5,2)(13,6) EP(10,4)( 12,6) MgO EP( 14,7)( 12,6) CaO EP(l5,7)(12,6) SrO EP(20,10,2)(7,3) Group V compounds NF MM(3,1)(3,1) PN EP(14,7)(11,6) PO ES(6,3)(3,1) Transition-metal compounds ScO EP(8,4,2)(3,2,1) ScF EP(10,5,2)(4,3) TiN EP(8,4,2)(3,2) Ti0 EP(8,4,2)(3,2) vo EP(8,4,2)(3,2) W10,5,2)(4,3) 89.428 2.676 89.448 2.4377 7.29 89.454 2-4377 7.35 412.097 274-386 3,3052 9.18 751.559 3.4412 11.48 3206.23 3.6283 10-2 78.717 2.421 - 1.430 123.604 2.385 - 1.96 123.676 2.385 - 1.13 124.140 2.385 - 0.668 124.166 2.391 - 0.945 124.167 2.391 - 0.88 341.483 3.126 1.34 1 1 1.956 2.075 112.326 2.132 112.344 2-132 112.392 2.132 1 12.396 2- 132 112.759 2.132 1 12.786 2.1 32 112.789 2.132 91.927 2.18 435.330 2.9 363.852 2.854 - 0.592 - 0.730 0.0872 0.397 0-274 0.28 - 1.84 1.6 3-68 153.205 2.45 395.185 2.818 3.23 414,137 2.738 - 0.7 833.096 3.05 - 2.6 858.545 3.31 - 4-64 901.127 3.00 - 3-55 921.542 3.0618 - 2.863 922.498 2.91 - 5.93 1015.89 2.91 - 3.61 Clark and Stewart Additional Refer- calculations ence 4,e 10 9 8 7 7 e e 5 8 7 7 57,i 72 5 7 7 3 7 13,e,i 7 1 3,e e 2,e 3,e e 3 4 LCAO Wave Functions for Small Molecules B.J. Ransil Rev. Mod. Phys. 1960,32,239,245; S . Fraga and B. J. Rand J. Chem. Phys. 1962,36 1127; C A.D. McLean J. Chem. Phys. 1963,39,2653; M. Yoshimine and A. D. McLean Internat. J. Quantum Chem. 1967 1 S313; e R. L. Matcha J. Chem. Phys. 1967 47 4595; f A. D. McLean and M. Yoshimine IBM J. Research and Development 1968 12 206; * R. L. Matcha J. Chem. Phys. 1967 47 5295; h R. L. Matcha J. Chem. Phys. 1968 48,335; R. L. Matcha J. Chem. Phys. 1968,49,1264; f G. Verhaegen and W. G. Richards J. Chem. Phys. 1966,45,1828; k M. Yoshimine J. Chem. Phys. 1964,40,2970; 1 G. Verhaegen and W. G. Richards Proc. Phys. SOC. 1967,90 579; J. L. Masse and M. Btirlocher Helv. Chim. Acta. 1964,47 314;n R. K. Nesbet J. Chem. Phys. 1964 40 3619; OW. M. HUO J. Chem. Phys. 1965 43 624; p R. C. Sahni and B. C. Sawhney Trans. Faraday SOC. 1967 63 1; Q H. Brion and C. M. Moser J. Chem. Phys. 1960,32 1194; r R.C. Sahni C. D. La Budde and B. C. Sawhney Trans. Faraday SOC. 1966,62 1933; 8 R. Bonaccorsi C. Petron- golo E. Scrocco and J. Tomasi J. Chem. Phys. 1968 48 1500; t W. G. Richards Trans. Faraday SOC. 1967 63 257;u R. C. Sahni Trans. Faraday SOC. 1967,63 801; V D. B. Boyd and W. N. Lipscomb J. Chem. Phys. 1967,46,910; W K. D. Carlson E. Ludena and C. M. Moser J. Chem. Phys. 1965 43 2408; 2 K. D. Carlson and C. M. Moser J. Chem. Phys. 1967 46 3 5 ; Y K. D. Carlson C. R. Claydon and C. M. Moser J. Chem. Phys. 1967 46 4963; K. D. Carlson and R. K. Nesbet J. Chem. Phys. 1964,41,1051 ;aa K. D. Carlson and C. M. Moser J. Chem. Phys. 1966,44,3259; bb W. M. Huo K. F. Freed and W. Klemperer J. Chem. Phys. 1967,46,3556; CC W. G. Richards G. Verhaegen and C. M. Moser J. Chem. Phys. 1966,45 3226; dd G.Verhaegen W. G. Richards and C. M. Moser J. Chem. Phys. 1967 46 160; ee H. Lefebvre-Brion and C. M. Moser J. Mol. Spectroscopy 1965 15 21 1 ; ff P. Merryman C. M. Moser and R. K. Nesbet J. Chem. Phys. 1960,32,631; H. Lefebvre- Brion C. M. Moser and R. K. Nesbet ibid. 1960,33 931; 1961,34 1950; 1961,35 1702; J. Mol. Spectroscopy 1964 13 418; W. M. Huo J. Chem. Phys. 1966 45 1554; 88 K. D. Carlson and C. M. Moser J. Phys. Chem. 1963,67 2644. Table 6 Molecular energies (E) of polyatomic molecules Molecule Basis set - E (H) 26-338 26-352 36-907 52-678 52-715 100-730 38-904 40.1 14 40- 128 40-181 40.205 77.834 77.876 79.069 79.098 290.5 1 9 Additional Reference calculations a * b a b d & 13,e 4 a9.f 3 g 3 h i a(@?) 7,e,i i 2 a,k 2 I m Clark and Stewart Molecule Basis set 76.544 76.61 7 76.678 76.854 115.583 83.731 175.724 535.767 168.578 92.547 92.590 92.9 15 183.982 184.657 166.459 167-270 167.076 191.780 489-91 1 55 1 -825 1 13.088 113.165 150.844 186.843 187.076 187.723 508.492 5 10.3 3 1 1 13.450 1 13.427 Additional Reference calculations n a e,i 0 P 4 4 r S S S 12 t lf a S V P t S t S S S W X e Y n 4 S z aa bb,cc bb,dd S 113 LCAO Wave Functions for Small Molecules Molecule Basis set 56.005 56499 34 1 a309 162.542 162.705 163.224 183.757 202.901 203.108 203.174 203 -9 8 6 227.708 488.77 75.681 76.005 150.157 150.223 223.479 272.425 273.526 397-842 198.283 199.393 199.573 Additional Reference calculations a 2,i ee IT w t gg t 9 S W gg hh ii hh ii kk 11 8 mm 7 nn hh hh ii m W 00 21 S PP PP B-H distance treated as variational parameter.114 Clark and Stewart W. E. Palke and W. N. Lipscomb J. Amer. Chem. SOC. 1966,88,2384; W. E. Palke and W. N. Lipscomb J. Chem. Phys. 1966,45 3948; C R. A. Hegstrom W. E. Palke and W. N. Lipscomb J. Chem. Phys. 1967 46 920; W. E. Palke and W. N. Lipscomb J. Chem. Phys. 1966,45 3945;e J. M. Foster and S. F. Boys Rev. Mod. Phys. 1960,32 305;f J. Sinai J. Chem. Phys. 1963,39,1575; g R. M. Pitzer J. Chem. Phys. 1967,46,4871 ; h B. J. Womick J. Chem. Phys. 1964 40 2860; G. P. Arrighini C. Guidotti M. Maestro R. Moccia and 0. Salvetti J. Chem. Phys. 1968,49 2224; J U. Kaldor and I. Shavitt J. Chem. Phys. 1968,48 191; k R. M. Pitzer and W. N. Lipscomb J. Chem. Phys. 1963 39 1995; R. M. Pitzer J . Chem. Phys. 1967 47 965; m F. P. Boer and W. N. Lipscomb J. Chem. Phys. 1969,50,989;n A.D. McLean J. Chem. Phys. 1960,32,1595; 0 M. G. Griffith and L. Good- man J. Chem. Phys. 1967 47 4494; P A. D. McLean and M. Yoshimine IBM J. Research and Development 1968,12,206; Q M. D. Newton and W. N. Lipscomb J. Amer. Chem. SOC. 1967 89 4261; 'A. Veillard J. Chem. Phys. 1968 48 1994; 8 M. Yoshimine and A. D. McLean Internat. J. Quantum Chem. 1967 1 S313; R. Bonaccorsi C. Petrongolo E. Scrocco and J. Tomasi J. Chem. Phys. 1968 48 1500;UA. D. McLean J. Chem. Phys. 1962,37,627; v E. Clementi and A. D. McLean J. Chem. Phys. 1962,36,563; E. Clementi J. Chem. Phys. 1961 34 1468; 2 E. Clementi and A. D. McLean J. Chem. Phys. 1962,36 45; Y E. Clementi J. Amer. Chem. SOC. 1961 83 4501; A. D. McLean J. Chem. Phys. 1963 38 1347; a@ E. Clementi J. Chem. Phys. 1962 36 750; bb M. D.Newton and W. E. Palke J. Chem. Phys. 1966,45 2329; Cc J. M. Foster and S . F. Boys Rev. Mod. Phys. 1960 32,303; S . Aung R. M. Pitzer and S. I. Chan J. Chem. Phys. 1966,45,3457; dd P. L. Good- friend F. W. Birss and A. B. F. Duncan Rev. Mod. Phys. 1960 32 307; ee U. Kaldor and I. Shavitt J. Chem. Phys. l966,45,888;ffD. B. Boyd and W. N. Lipscomb J. Chem. Phys. 1967,46,910; gg E. Clementi and A. D. McLean J. Chem. Phys. 1963,39,323; hh C. Petron- golo E. Scrocco and J. Tomasi J. Chem. Phys. 1968,48,407; f( R. Bonaccorsi C. Petron- golo E. Scrocco and J. Tomasi J. Chem. Phys. 1968 48 1497; jj D. B. Boyd and W. N. Lipscomb J. Chem. Phys. 1968 48 4968; kk J. Andriessen Chem. Phys. Letters 1969 3 257; S. Aung R. M. Pitzer and S. I. Chan J . Chem. Phys. 1968,49,2071; mm U. Kaldor and I.Shavitt J. Chem. Phys. 1966,44 1823;nn W. E. Palke and R. M. Pitzer J. Chem. Phys. 1967 46 3948; ooE. Clementi and A. D. McLean J. Chem. Phys. 1962 36 745; *P P. E. Stevenson and W. N. Lipscomb J. Chem. Phys. 1969,50,3306; QQ T. H. Dunning and V. McKoy J. Chem. Phys. 1967,47 1735. Tables 7 and 8 on the following pages list papers in which various mole- cular properties are calculated from previously published wave functions. 115 Table 7 Literature references to calculations of various molecular quantities Diatomic molecules Molecule LiH BeH BH CH NH OH FH NaH,MgH,AIH,SH,SiH,PH ClH Liz Be2 B&2 c2 N2 F2 LiF Be0 BeF MgF BN BF co NO Electron density a-f c-f c-f b-f,k,n c CYg c 0 0 e,f,i-m e,Li i,i f-. e-kP e,f,h,k,ss e,f,i-l h d e-hP d-h P Magnetic Polar isabilit ies properties q-Y w,kk,ll u-x,z,aa bb U s-w,cc,dd dd kk 11 s-v,ee,fl kk,N V f l u,v,aa,ii t-v,ii,jj 11 kk,ll,mm b 0 2 8 Quadrupole Spectroscopic coupling constant quantities fb and/or dipole 3 9 moment nn,oo vv-xx 6' 3 ww x x $ ww ww aaa bbb ww ww UII,CCC a P.Politzer and R. E. Brown J. Chem. Phys. 1966,45,451; b R. F. W. Bader and W. H. Henneker J. Amer. Chem. SOC. 1966,88,280; C R. F. W. Bader I. Keaveny and P. E. Cade J. Chem. Phys. 1967,47,3381; d G. Doggett J. Chem. SOC. (A). 1969,229; e V. Magnasco and A. Pefico J. Chem. Phys. 1967,47,971 ; f B. J. Ransil and S. Fraga J. Chem. Phys. 1961,34,727; 8 E. Clementi and H. Clementi J. Chem. Phys. 1962,36,2824; R. F. W. Bader and A. D. Bandrank J. Chem. Phys. 1968,49 1653 1666; f A. C. Wahl Science 1966,151,961 ; f R. F. W. Bader W. H.Henneker ana P. E. Cade J. Chem. Phys. 1967 46 3341; k B. J. Ransil and J. J. Sinai J. Chem. Phys. 1967 46 4050; W. D. Lyon and J. 0. Hirschfelder J. Chem. Phys. 1967,46 1788; R. F. W. Bader and A. K. Chandra Canad. J. Chem. 1968,46,953; n C. W. Kern and M. Karplus J. Chem. Phys. 1964,40 1374; 196543,2926; O P. E. Cade R. F. W. Bader W. H. Henneker and I. Keaveny J. Chem. Phys. 1969,50,5313; P E. R. Davidson J. Chem. Phys. 1967,46,3320; Q M. Karplus and H. J. Kolker J. Chem. Phys. 1961,35,2235; C. W. Kern and W. N. Lipscomb Phys. Rev. Letters 1961,7,19; R . M. Stevens R. M. Piker. and W. N. Lipscomb J. Chem. Phys. 1963,38 550; J. R. de la Vega and H. F. Hameka ibid. 1964,40 1929; J. Gruninger and H. F. Hameka Chem. Phys. Letters 1967 1 14; G. P. Arrighini F. Grossi and M. Maestro Theor.Chim. Acta 1966 5 266; H. J. Kolker and M. Karplus J. Chem. Phys. 1964,41 1259; t M. Karplus and H. J. Kolker J. Chem. Phys. 1963. 38 1263; U C. W. Kern and W. N. Lipscomb J. Chem. Phys. 1962,37,260; 1) J. R. de la Vega and H. F. Hameka J. Chem. Phys. 1967.47 1834; J. R. de la Vega Y. Fang and H. F. Hameka Physica 1967,36,577; R. A. Hegstrom and W. N. Lipscomb J. Chem. Phys. 1967.46 1594; Y R. M. Stevens and W. N. Lipscomb J. Chem. Phys. 1964,40,2238; R. M. Stevens and W. N. Lipscomb J. Chem. Phys. 1965,42,3666; aa R. A. Hegstrom and W. N. Lipscomb J. Chem. Phys. 1966 45,2378; 1968,48,835; bb D. S. Bartow and J. W. Richardson J. Chem. Phys. 1965,42,4018; Cc T. P. Das and M. Karplus J. Chem. Phys. 1962,36 2275; R. P. Hurst M. Karplus and T. P. Das ibid. 1962.36,2786; H. Hamano H.Kim and H. F. Hameka Physica 1963,29,111; D. Zeroka and H. F. Hameka J. Chem. Phys. 1966,45,300; Y. Kato and A. Saika ibid. 1967,46,1975; dd R. M. Stevens and W. N. Lipscomb J. Chem. Phys. 1964 41,184; ee R. M. Stevens and W. N. Lipscomb J. Chem. Phys. 1965,42,4302; ff J. R. de la Vega and H. F. Hameka Physica 1967,35 313; H. Kim H. Hamano and H. F. Hameka Physica 1963,29 117; Y . I'Haya Znternat. J. Quantum Chem.. 1967,1,693; hh R. M. Stevens and W. N. Lipscomb J . Chem. Phys. 1964,41,3710; (6 J. R. de la Vega D. Ziobro and H. F. Hameka Physica 1967,37,265; R. M. Stevens and M. Karplus J. Chem. Phys. 1968,49,1094; kk M. Karplus and H. J. Kolker J. Chem. Phys. 1963,39,2011; 11 J. M. O'Hare and R. P. Hurst J. Chem. Phys. 1967,46,2356; mm A. D. McLean and M. Yoshimine J. Chem. Phys.1967,46,3682; 129 P. E. Cade and W. M. HUO J. Chem. Phys. 1966,45,1063; O 0 H. J. Kolker and M. Karplus J. Cliem. Phys. 1962,36,960; p p M. Dixon and J. A. S. Smith Trans. Faraday Sac. 1968,64 1; QQ J. W. Richardson Rev. Mod. Phys. 1960,32,461; rr S . L. Kahalas and R. K. Nesbet Phys. Rev. Letters 1961 6 549; 88 R. F. W. Bader and W. H. Henneker J. Amer. Chem. Soc.,1965 87,3063 ; tt B. J. Ransil J. Chem. Phys. 1959,30,1113 ; H. Lefebvre-Brion C. M. Moser R. K. Nesbet and M. Yamazaki ibid. 1963,38,23 11 ; uu S. H. Lin Theor. Chim. Acta 1967 8 1; v v B. J. Ransil and S . Fraga J. Chern. Phys. 1961 35 669; w S. R. La Paglia Theor. Chim. Acta 1967 8 185; zx J. Gerratt and I. M. Mills J. Chem. Phys. 1968,49 1730; w M. A. Marchetti and S. R. La Paglia J. Chem. Phys. 1968,48,434 zz S. R.La Paglia J. Mol. Spectroscopy 1967,24,302;aaa T. E. H. Walker and W. G. Richards Proc. Phys. Soc. 1967,92,285; bbb T. E. H. Walker and W. G. Richards J. Phys. (B) 1968,1,1061; Ccc H. Lefebvre-Brion and C. M. Moser J. Mol. Spectroscopy 1965,15,211; J. Chem. Phys. 1966,44,2951. ' $ 3 B w 2 w 43 .) LCAO Wave Functions for Small Molecules Table 8 Literature re ferenccs to calculations of various molecular quantities Polyatomic molecules Molecule Electron Magnetic density properties 0 P C S C C BPHI CHI CaH C2Hd HCN CaNa GIG co2 HCHO 0 sco NH3 Q N3- h N2O S H2O H202 HF2- h FCN m n CICN HCCCN OCN- SCN- n C2H4 a C. W. Kern and W. N. Lipscomb J. Chem. Phys. 1962 37 275; b J. L. Sinai J. Chem. Phys. 1964 40 3596; C G. P. Arrighini M. Maestro and R. Moccia Chem. Phys. Letters 1967,1,242; J.Chem. Phys. 1968,49,882; d T. Caves and M. Karplus J. Chem. Phys. 1966 45 1670; E. Clementi and H. Clementi J. Chem. Phys. 1962 36 2824; f A. D. McLean B. J. Ransil and R. S. Mulliken J. Chem. Phys. 1960,32 1873.17 C. Barbier and G. Berthier Internat. J. Quantum Chem. 1967 1 657; R. H. Pritchard and C. W. Kern J. Arner. Chem. Soc. 1969,91,1631; fi C. W. Kern and M. Karplus J. Chern. Phys. 1965,42,1062; { E. A. G. Armour and A. J. Stone Proc. Roy. SOC. 1967 A 302,25; 0. J. Sovers M. Karplus and C. W. Kern J. Chem. Phys. 1966,45 3895; k R. E. Wyatt and R. G. Pam J. Chem. Phys. 1965,43 S217; 1966,44 1529; 0. J. Sovers C. W. Kern R. M. Pitzer and M. Karplus ibid. 1968 49 2592; Z L. Burnelle Theor. Chim. Actu 1964 2 177; J. B. Moffatt and H. E. Popkie Internat. J. Quunturn Chem.1968,2 565. n R. Bonaccorsi E. Scrocco and J. Tomasi J. Chem. Phys. 1969 50 2940; 0 W. H. F. Flygare J. M. Pochan G. I. Kerbey T. Caves M. Karplus S. Aung R. M. Pitzer and S. I. Chan J. Chem. Phys. 1966,45 2793; P A. D. McLean and M. Yoshimine J. Chem. Phys. 1967 46 3682; 0 C. W. Kern J. Chem. Phys. 1967 46 4543; r M. P. Melrose and R. G. Parr Theor. Chim. Acta 1967 8 150; A. D. McLean and M. Yoshimine J. Chem. Phys. 1966,45 3676. k r r Quadrupole Internal rotation coupling or inversion constant barrier d h i i,n 118

ISSN:0009-2681    

DOI:10.1039/QR9702400095

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Developments in the Chemistry of Diazo-alkanes By G. W. Cowell" and A. Ledwith D O N N A N LABORATORIES UNIVERSITY OF LIVERPOOL Diazo-alkanes (R,CN,) have been useful intermediates in organic chemistry for over forty years and consequently many reactions of these molecules have been fully investigated.' It is now almost a decade since the last review2 was completed and in the meantime there has been a tremendous growth of interest in diazo-alkanes from both synthetic and theoretical (mechanistic) viewpoints.s Diazomethane (1) is the simplest diazo-alkane and is best represented as a resonance hybrid comprising linear structures with opposing dipoles + - - + - + - + CHP=N=N tf CH,-N=N tj CHZ-NEN * CHS-NZN (1) (1b) (Ic) (Id) Under appropriate conditions diazomethane will behave as an acid or a base as an electrophile or a nucleophile as a 1,3-dipole or as a carbene source.Much of the renewed activity in diazo-alkane chemistry derives from world- wide studies of the reactivity and structure of carbenes (RaC:)."*' The latter are now recognised as the most common intermediates in photolysis and thermolysis of diazo-alkanes and were characterised during the early nineteen fifties largely as a result of pioneering studies by Doering,E Skell,7 Hertzberg,s and their collaborators. Other major developments in diazo-alkane chemistry include cycloadditi~n,~ and catalysed alkylation homologation and polymerisation processes.1° *Present Address I.C.I. Mond Division The Heath Runcorn. 1 R. Huisgen Angew. Chem. 1955 67 439. * Brief surveys have appeared in two recent texts P. A.S. Smith 'Open Chain Nitrogen Compounds' Benjamin Inc. New York 1966 vol. 2 p. 211 ; C. G. Overberger J. P. Anselme and J. G. Lombardino 'Organic Compounds with Nitrogen-Nitrogen Bonds' Ronald Press New York 1966 p. 41. A. Ledwith 'The Chemistry of Carbenes' R.I.C. Lecture Series Monographs 1964 No. 5. 6 W. Kirmse 'Carbene Chemistry' Academic Press London 1964; J. Hine 'Divalent Carbon' Ronald Press Co. New York 1964. 6 W. VON E. Doering and A. K. Hoffman J. Amer. Chem. SOC.. 1954,76 6162; W. VON E. Doering and L. H. Knox ibid. 1956,78 4947. * G. Hertzberg and J. Shoosmith Nature 1959 183 1801; G. Hertzberg Proc. Roy. SOC. 1961 A 262 291. 0 R. Huisgen Angew. Chem. Internat. Edn. 1963 2 565 633. lo (a) C. E. H. Bawn and A. Ledwith Progr. Boron Chem. 1964 1 345; (b) E. Miiller H. Kessler and B.Zeeh Fortsch. Chem. Forsch. 1966 7 128. H. Zollinger 'Azo and Diazo Chemistry' Interscience London 1961. P. S. Skell and R. C. Woodworth J. Amer. Chem. SOC.. 1956,78,4496. 119 Developments in the Chemistry of Diazo-alkanes It is the purpose of this review article to survey major developments in diazo- alkane chemistry during the last fifteen years exclusive of the chemistry of carbenes which has already been discussed in a complementary review4 and in several other recent surveys.sB11s12 Familiarity with the general chemistry of diazo-alkanes will be assumed and photolysis and thermolysis of diazo-alkanes will be considered only as they reflect on the properties of the diazo-alkane. 1 Structure and Stability of Diazo-alkanes A linear planar structure for diazomethane was established by electron diffrac- tionI3 and microwave spectroscopic techniqucs,14 and the dipole moment (1.4 D) and bond lengths (> C---N - N) support the resonance hybrid formulation.Simple Huckel molecular orbital (HMO) calculati~ns~~ indicate a resonance energy of 0.6 /3 and predict values for the bond lengths and dipole moment in close agreement with experiment. A complete analysislG of the vibrational spectrum of gaseous and solid CH,N2 (and CDzN2) confirmed the linear planar structure with sp2 hybridised carbon but indicated that there must be a non-planar structure (with sp3 hybri- dised carbon) only a few kilocalories higher in energy than the equilibrium. It follows therefore that thermal reactions of diazomethane could involve a high-energy non-planar molecule similar in structure to the low-lying elec- tronicallyexci ted states.Two structural isomers of diazomethane are known diazirinel' (2) and isodiazomethane18 (3). 1.300 1.139 Diazirine (2) and its alkyl homologues are readily synthesised but are unreactive towards organic acids whereas the corresponding diazo-alkanes react readily to form esters. Lower homologues are explosively unstable as for the diazo- l1 A. Ledwith Ann. Reports (B) 1968 65 143. la D. Bethell Adv. Phys. Org. Chem. 1969 7 153; A. M. Trozzolo Accounts Chem. Res. 1968,1 329; G. L. Closs Topics Stereochem. 1968,2 193. la H. Boersch Monutsh. 1935 65 331. l6 A. J. Owen Tetrahedron 1961 14,237. l6 C. Bradley Moore and G. C. Pimentel J. Chem. Phys. 1964,40 329 340 1529. l7 E. Schmitz Angew. Chem. Internut. Edn. 1964,3,333; E.Schmitz Adv. Heterocyclic Chem. 1964,2 122; E. Schmitz and R. Ohne Tetrahedron Letters 1961,612; S . R. Paulsen Angew. Chem. 1960,72,781. 18 J. P. Anselme J. Chem. Educ. 1966,43,596; E. Muller and D. Ludsteck Chem. Ber. 1954 87,1887. A. P. Cox L. F. Thomas and J. Sheridan Nature 1958,181,1000. 120 Cowell and Ledwith alkanes and may be used as thermal and photochemical precursors to carbenes but less conveniently. Isodiazomethane (3) is formed by reaction of diazomethane with organometallic reagents particularly methyl-lithium via the diazomethyl anion (4) + - CH,Li KHBPO. - + CH2=N=N ~h [CH-N=N]-Li+ # HC=N=N-H OH- OH- (4) (3) Formation of isodiazomethane by acidic hydrolysis of (4) is apparently the result of a kinetically controlled reaction. Protonation occurs rapidly at the more nucleophilic nitrogen of (4) to give the thermodynamically less stable is0mer.t Isodiazomethane is the parent of dipolar compounds known as nitril- imines (5).+ - R1-C=N-N-R2 ( 5 ) Both diazirines and nitrilimines differ markedly from diazo-alkanes in their chemical and physical properties and will not be considered further. There is however considerable interest in the possibility of isomerisation of diazirines to diazo-alkanes before fragmentation in photolysis and therm01ysis.l~ Diazo-alkanes exhibit a strong asymmetric (> C") stretching mode in the i.r. centred between 4.7 and 4.9 pm the exact position of the band is charac- teristic of the number of substituents rather than of their electronic character.2o This asymmetric stretching mode also gives rise to a very strong first overtone centred around 2.4 p with molar extinction coefficients comparable to those for the corresponding visible absorption spectra.21 For the lower diazo-alkanes in particular the strong absorption in the near4.r.provides a very convenient probe for monitoring concentration without the risk of photochemical de- composition. N.m.r. studies of diazo-alkane~~~~~~ show that the proton attached to the diazo carbon atom is shielded (7 3.5-7) to an extent which depends markedly on the electronic nature of substituents. For example electron-withdrawing t Recent work has cast doubt on the accepted structure for isodiazomethane suggesting :C=N -NH as an alternative.18a M. J. Amrich and J. A. Bell J. Amer. Chem. Soc. 1964,86 292; H. M. Frey and 1. D. R. Stevens J. Chem.Soc. 1962,3865; C. G. Overberger and J. P. Anselme Tetrahedron Letters 1963 1405. zOP. Yates B. Shapiro N. Yoda and J. Fugger J. Amer. Chem. SOC. 1957 79 5756; A. Fottani C. Pecille and S. Ghersetti Tetrahedron 1960 11 285; W. D. Hormann and E. Fahr Annulen 1963,663,l; E. Fahr ibid. 1958,617,ll; E. Fahr H. Aman and A. Roedig ibid. 1964 675 59. E. Fahr and K. H. Keil Annulen 1963,663,4; A. Ledwith and E. C. Friedrich unpublished results. A. Ledwith and E. C. Friedrich J. Chem. Soc. 1964 504; D. F. Koster and A. Danti J . Chem. Phys. 1964,41 582. z3 F. Kaplan and G. K. Meloy J. Amer. Chem. SOC. 1966 88,950. E. Muller R. Beutler and B. Zeeh Annulen 1968 719 72. 121 Developments in the Chemistry of Diazo-alkanes - + substituents favour a resonance structure having a formal carbanion (> C-N2) whereas electron-releasing substituents favour a formal positive charge on carbon ( > C-N,).Diazocyclopentadiene (6) 9-diazofluoreneY 3-diazopropene and ar-diazocarbonyl compounds (7) are in the former category whereas diazo- ethane and p-methoxyphenyl diazomethane (8) are examples of the latter. + - Resonance in a-diazo-carbonyl derivatives (7) causes restricted rotation around the C-C giving rise to cis- and trans-isomers (7a b) + 0- 0- H RI-C-CH-N-N +-+ C=C y c = C / \ / \ \ 7" \ / 0 I I - + K' H R' N +EN cis- trans- For diazo-ketones (7) the barriers to rotation are in the range 15-18 kcal/mole with the cis-form (7a) predominating. On the other hand for diazoacetic esters (7 R1 = OR2) population of cis- and trans-forms is roughly the same and the barriers to rotation are 9-12 kcal/mole.Because of the prevalence of cis- conformers in diazo-ketones it was that the Wolff rearrangement,25 proceeding via a concerted mechanism could occur only from the cis-form (7a) in which the migrating group (R-) would be trans to the leaving group The thermal stability of diazo-alkanes depends markedly on the nature of substituents. Conjugating substituents increase stability irrespective of whether they are electron releasing or electron attracting. Diazomethane and diazo- + (-N,N). z4 C. Pecile R. Fottani and S. Ghersetti Tetrahedron 1964 20 823. 26W. E. Bachman and W. S. Struve Org. Reactions 1942 1 3 8 ; F. Weygand and H. J. Bestmann Angew. Chem. 1960,72 535. 1 22 Cowell and Ledwith ethane are gases under normal atmospheric conditions decomposing readily under the influence of rough surfaces.Dilute solutions in organic solvents are reasonably stable but explosions are quite common when the pure materials are handled. ChlorodiazomethaneZ8 (CICH =Np) appears to be even less stable than diazomethane decomposing to chlorocarbene at -20". On the other hand diazo-alkanes having carbonyl aryl nitrile or other conjugating sub- stituents are much more stable and may be handled conveniently as pure liquids or solids. Since the time of the last review,'t2 the variety of substituents in diazo-alkanes has increased substantially. Particularly important is a wide range of fluorinated and perfluoro-diazo-alkanes e.g. CF3CHN2,87 (CF3)2CN2,28 CF3COC(:N2)- CF3,2a MeC( :N2)CF,,30 and PhC( :N2)CF3.31 Fluorinated substituents confer added stability to the diazo-compound which may be conveniently generated by diazotisation of the appropriate fluorinated amine (see later).Diazo-alkanes having o r g a n ~ t i n ~ ~ mercury,33 silver,34 alkyl sulphide,g6 aryl and alkyl ~ulphone,~~ organopho~phorus,~~ nitro-,38 cy~lopropyl-,~~ and ~yano-~O groups have also been synthesised. Products from thermal decompositions of diazo-alkanes in aprotic media are usually mixtures of olefin and azine formed by reaction of a carbene fragment with the starting diazo-alkane p > C = C < + N 2 1 ROH >CH-OR 26 G. L. Closs and J. J. Coyle J. Amer. Chem. SOC. 1962 84 4350. 27 R. Fields and R. N. Haszeldine J . Chem. SOC. 1964 1881; B. L. Dyatkin and E. P. Mochalina Izvest. Akad. Nauk. SSSR Ser. khim. 1964 1225. Se D. M. Gale W. J. Middleton and C.G. Krespan J Amer. Chem. SOC. 1966 88 3617. e9 B. L. Dyatkin E. P. Mochalina Izvest. Akad. Nauk. SSSR Ser. khim. 1965 1035. 30 R. A. Shepard and P. L. Sciaraffa J. Org. Chem. 1966 31 964. 81 R. A. Shepard and S. E. Wentworth J . Org. Chem. 1967,32 3197. 32 M. F. Lappert and J. Lorberth Chem. Cumm. 1967 836. 33 A. N. Wright K. A. W. Kramer and G. Steel Nature 1963,199,903; P. Yates and F. X. Garneau Tetrahedron Letters 1967 71. 84 U. Schoellkopf and N. Rieber Angew. Chem. Internat. Edn. 1967 6 261. 86 U. Schoellkopf and U. Wiskott Annalen 1966,694,44. 36 J. Diekmann J. Org. Chem. 1963 28 2933; A. M. Leusen R. J. MuIder and J. Strating Rec. Trav. chim. 1967 86,225. 37 D. Seyferth P. Hilbert and R. S. Marmor J. Amer. Chem. Soc. 1967,89,481 I ; L. Homer H. Hoffman H. Ertel and G.Klahre Tetrahedron Letters 1961 9; N. Kreutzkamp E Schmidt-Samoa and A. K. Herberg Angew. Chem. 1965,77 1138. 38 U. Schoellkopf and P. Markush Tetrahedron Letters 1966 6199. 39 R. A. Moss and F. C. Shulman Chem. Comm. 1966,372 4o E. Ciganek J. Org. Chem. 1965,30,4198 123 Developments in the Chemistry of Dinzo-alkanes In the presence of hydroxylic additives the intermediate carbene may be trapped to yield the corresponding ether or alcohol. Detailed kinetic studies of the de- composition of PhCHN341 and Ph2CNao2 have been made and in addition to the major reactions shown above there is evidence for a minor process in which the appropriate aryl azine is formed directly by a bimolecular reaction of the aryl diazomethane e.g. Although catalytic decomposition of diazomethane is ready even at the surface of glass vessels a careful kinetic study of thermolysis in the gas phase established an activation energy of 34 kcal/rn~le.~~ This compares with an estimate of 43 kcal/mole for the bond dissociation energy of the C-N linkage,derived from electron impact data.44 The latter studies also yielded a value of 9-03 ev for the ionisation potential of diazomethane and an estimate of 71 kcal/mole for dHfo.A reliable estimate of the heat of formation of diazomethane is necessary for interpretation of excess energy contributions to various (vapour phase) carbene reactions with ~ l e f i n s ~ ~ and there has been considerable uncertainty as to the true value.43 However independent calc~lations~~ of the thermo- chemistry of the reaction by use of self-consistent field (SCF) energies yield values for AHO in very good agreement with experiment taking dHro (CH2N2) = 71 kcal/mole.Absorption spectra of diazo-alkanes are characterised by low intensity low energy transitions in the v i ~ i b l e ~ ~ ~ ~ ~ and more intense absorption in the near and far-u.~.~O Absorbance in the visible spectrum makes diazo-alkanes con- venient substrates for the photochemical production of carbenes 2 AraCNz + Ar,C=N-N=CAr2 + N H2 + CHzN2 __+ CH4 + N2 + - hv >C=N=N t >C + NB 3000 - 6000A Diazo-alkanes which have conjugating substituents show broad unresolved bands in the visible spectrum e.g. for Ph2CN2 in THF,60 Amax = 529 nm 41 D. Bethell and D. Whittaker J. Chem. SOC. (B) 1966,778. 49 D. Bethell D. Whittaker and J. D. Callister J. Chem. SOC. 1965,2466; D.Bethell A. R. Newall G. Stevens and D. Whittaker J . Chem. SOC. (B.) 1969 749; See also D. Bethell and R. D. Howard ibid. 1969,745; H. Reimlinger Chem. Ber. 1964,97,339,3503; G. Murgulescu and T. Oncescu J. Chim. Phys. 1961,58 508. 4* D. W. Setser and B. S. Rabinovitch Canad. J . Chem. 1962,40 1425. 44 G. S. Paulett and R. Ettinger J. Chem. Phys. 1963 39 825 3534; 0. S. Paulett and R. Ettinger. ibid. 1964 41 2557; J. A. Bell ibid. 1964 41 2556. 4bH. M. Frey Progr. Reaction Kinetics 1964 2 131; J. A. Bell Progr. Phys. Org. Chem. 1964,2 1. 46 L. C. Snyder and H. Basch J. Amer. Chem. SOC. 1969 91 2189. 47 J. N. Bradley G. W. Cowell and A. Ledwith J . Chem. SOC. 1964 353. 48 D. W. Adamson and J. Kenner J . Chem. Soc. 1937 1551 ; R. K. Brinton and D. H. Volman J. Chem. Phys. 1951,19 1394; F.W. Kirkbride and R. G. W. Norrish J. Chem. SOC. 1933 119; G. Kortum. Z. phys. Chem. 1941 B 50 361. 49 A. J. Merer Canad. J . Phys. 1964,42 1242. 60 A. Ledwith and L. Phillips J. Chern. Soc. 1965 5969. 1 24 Cowell and Ledwith emax = 94 1. mole-' cm-l. Diazomethane and the lower diazo-alkanes have visible spectra in the gas and in hexane ~olution,4~ which show consider- able fine structure e.g. for diazomethane in h e ~ a n e ~ ~ there are four maxima in the region 390460 nm with Emax in the range 6.0-10.01. mole-' cm-l. Formal assignment of the various transitions responsible for the fine structure has been attempted from both experimental o b ~ e r v a t i o n s ~ ~ ~ ~ ~ and theoretical calcula- tions.61 Although there is some disagreement as to the nature of the various excited ~ t a t e s ~ ~ @ * ~ ~ the fact that fine structure is apparent in indicates that the upper electronic state is binding and consequently may have an appreci- able lifetime.It follows therefore that photolytic reactions of diazomethane in solution cannot be assumed to involve carbene without additional justification. For example photolysis of diazomethane in 1,2-epoxypropane (9) yielded the expected carbene-insertion products (10) and (ll) with acetone as major product.62 Ji v MeCH-CH2 + CH2N2 -+ hlc.CH2CH-CH2 + Me CH-CH-Mc + Me2C0 \ / / / 0 0 \ / 19" 0 (10) 4% ( 1 1 ) 12% 2 8 % (9) Although diazomethane was the primary absorbing species it is clear that frag- mentation to carbene was not the only photochemical process apparently diazomethane photosensitises the isomerisation of 1 ,Zepoxypropane to acetone.Other examples of anomalous products resulting from photodecomposition of diazo-alkanes are the photosensitised autoxidation of cyclohexane to cyclo- hexanol and cyclohexanone in the presence of diazofluorene or diazodiphenyl- and the orbital-symmetry-forbidden (photo)cycloaddition of diazofluorene to norbornene and n~rbornadiene.~~~ It must be emphasised however that the energy content of the visible light absorbed by dia~o-alkanes~' is in excess of the expected bond dissociation energy of the C-N li~~kage,~~or the activation energy for thermal breakdo~vn.~~ Consequently in most cases fragmentation to carbene and nitrogen will follow absorption of a photon at these wavelengths. For the lower diazo-alkanes quantum yields for photochemical fragmentation appear to be unity but much lower quantum yields have been observed with diazo-alkanes possessing con- jugating ~ ~ b ~ t i t u e n t ~ .~ ~ ~ s1 R. Hoffman Tetrahedron 1966 22 539. 62 J. N. Bradley and A. Ledwith J . Chem. SOC. ( B ) 1967 96. 69 (a) G. A. Hamilton and J. R. Giacin J . Amer. Chem. SOC. 1966,88 1584; (b) N. Filipescu and J. R. DeMember Tetrahedron 1968 24 5181. (a) W. Kirmse and L. Horner Annulen 1959 625 34; (b) W. Jugelt and F. Pragst Tetra- hedron 1968,24,5123; Angew. Chem. Internal. Edn. 1968,7,290; ( c ) P. D. Bartlett andT. G. Traylor J . Amer. Chem. SOC. 1962 84 3408; ( d ) A. M. Reader P. S. Bailey and H. M. White J . Org. Chem. 1965 30 784. 125 Developments in the Chemistry of Diazo-alkanes Simple cation-radicals are formed by reversible one-electron oxidation (platinum anode) of diphenyl diazomethane and several 4-substituted deri- vatives,Wb i.e.For diphenyl diazomethane E+ = +0.95 v (S.C.E.) and the initially formed cation-radical induces a chain reaction yielding tetraphenylethylene as the main product. Complete oxidation with molecular oxygens4c or yieIds benzophenones by complex reaction mechanisms. 2 Synthesis of Diazo-alkanes The classical methods for preparation of diazo-alkanes involve treatment of a nitroso-compound of the general formula RCH,N(NO)X with a suitable base to yield the diazo-alkane RCHN,. Thus diazomethane is readily prepared by treating either N-nitroso-N-methylurethaness (12) or N-nitroso-N-methyl- ureass (1 3) with alkali. KOH CHZN + KHC03 + EtOH KOH - CHzN2 + KOCN + 2 HzO For the preparation of disubstituted diazo-alkanes the oxidation of a keto- hydrazone (14) is normally ~ ~ e d .~ ~ - ~ ~ asA. P. N. Franchimont Rec. Truv. chim. 1890 9 146. ssE. A. Werner J . Chem. SOC. 1919 1093. 67 T. Curtiss and H. Long J . prakt. Chem. 1891 44 544; See also ref. 116. 68 J. R. Dyer R. B. Randall jun. and H. M. Deutsch J . Org. Chem. 1964,29 3423. A. C. Day P. Raymond R. M. Southam and M. C. Whiting J. Chem. SOC. (C) 1966,467. go 0. M. Kaufman J. A. Smith G . C. von der Stouw and H. Shechter J. Amer. Chem. SOC. 1965 87,935. D. E. Applequist and H. Babad J. Org. Chem. 1962 27 288. raK. Nakagawa H. Ondue and K. Minami Chem. Comm. 1966 736. 68 D. H. R. Barton R. E. O’Brien and S. Sternhell J. Chem. SOC. 1962,470. 126 Cowell and Ledwith +HgO R\ CN2 R\ R' R' C=N-NH2 In the past ten years the efficiencies of the classical methods of preparation have been improved and several new intermediates for diazo-alkane preparation reported.Rundels* has described a variation of the 'Forster Reaction'66 for the prepara- tion of diazomethane in 7675% yields in which the sodium salt of formald- oxime is treated with chloramine. CH,=NONa + NH2Cl + NaCl + CH2N2 + H20 In its original form the Forster reactionsb involved reaction of an cc-oximino- ketone with chloramine to give a diazo-ketone. Meinwald et aLs6 have treated fluorenone oxime with chloramine to form diazofluorene so that it now appears as if the presence of a carbonyl function is irrelevant to the formation of a diazo-compound. Thus with suitable choice of reaction conditions a new simple route to diazo-alkanes is available.Two new stable crystalline intermediates for the preparation of diazomethane both easily made from readily available materials have been reported. The first N-nitro-3-(methylamino)sulpholane (15) formed by reaction of nitrous H2 C-CH-N-Me I I I H2C ,CH2 NO so2 acid with 3-(methylamino)sulpholane decomposes upon heating in aqueous base at 60" to give a 70% yield of dia~omethane.~' This intermediate has the advantage of being readily soluble in water a property not shown by N-methyl- N-nitrosotoluenep-sulphonamide p-CH3C6HaS02N(NO)CH, also a stable intermediate for the preparation of diazomethane.6s The other new intermediate NW-dinitroso-NN'-dimethyloxamide [C(O)N(NO)Me], prepared by nitros- ation of the readily available NN-dimethyloxamide (C(0)NHMe) 2 undergoes decomposition in basic media to give diazomethane in high yields.sQ Higher W.Rundel Angew. Chem. 1962,74,469. 6~ M. 0. Forster J . Chem. SOC. 1915 107 260. oa J. Meinwald P. G. Gassman and E. G. Miller J. Amer. Chem. SOC. 1959 81 4751. "'V. Horak and M. Prochazka Czech. 98007 1959; V. Horak and M. Prochazka Chem. and Ind. 1961 472. '* T. J. De Boer and H. J. Backer Org. Synth. 1956 36 16. H. K. Reimlinger Chem. Ber. 1961 94 2547. 5 127 Developments in the Chemistry of Diazo-alkanes homologues [C(O)N(NO)R] (R = Et Pr or Bu) are liquids but gave relatively good yields of the corresponding diazo-alkanes. For those cases where dry gaseous diazomethane is required Dessaux and Durand70 have described a low-temperature reaction system utilising the classical reaction of N-nitro-N-methylurea with potassium hydro~ide.~~ [The hazards of working with gaseous diazo-alkanes should always be remembered and suitable precautions taken (see Zollinger2).] Toluene-p-sulphonyl azide‘l (1 6) in tetrahydrofuran converts hydrazones of benzophenone 9-fluorenone acetophenone and benzil to the corresponding diazo-compounds.Reaction conditions in these cases are both mild and non- oxidising. The mechanism of the formation of diazo-alkanes from nitroso-compounds In earlier work Applequist and M~Greer’~ had proposed that reaction of has been investigated by several groups of workers. nitroso-ureas with base involved a displacement on the carbonyl groups 0 H - - R’ N=O R’ I I I I I I H O H R~-C-N-C-NH~ f OEt ___+ R1-C-N=N-0 + EtOC-NH RL I H -Hz 0 I R:CN2 4 R’-C-N=N-OH More recently Jones et aL73 have shown that the lithium-ethoxide-induced conversion of several nitroso-ureas to diazo-alkanes proceeds instead by addi- tion of ethoxide ion to the nitroso-group 70 0.Dessaux and M. Durand Bull. SOC. chim. France 1963 1,41. 71 W. Fischer and J. P. Anselme Tetrahedron Letters 1968 877. D. E. Applequist and D. E. McGreer J. Amer. Chem. SOC. 1960 82 1965. W. M. Jones D. L. Muck and T. K. Tonday jun. J . Amer. Chem. SOC. 1966 88 68; W. M. Jones and D. L. Muck ibid. 1966 88 3798. 128 Cowell and Ledwith EtO 0- \ / R N=O R N H i I I t / R-C-NN-C-NHZ + -0Et + R-C-N-C-N I II I I \ H O H O H R EtO OH I -HNCO \ / R-C-N=N-OH t R N -0Et I I R-C-N-C-NH- I II H O - H 2 y A -HNCO R-C-N=N-OEt 4 -OH- I I H On the other hand N-nitroso-N-alkylure t hanes and N-ni troso-N-alkylamides appear to undergo competitive reaction at the nitroso nitrogen and the carbonyl carbon.The competitive processes are sensitive to the alkyl group the group attached to the carbonyl carbon atom the solvent and the nature of the base. Both mechanisms have a common latter stage which has been investigated in detail by Depending upon the nature of the alkyl group formation of diazo-alkane or decomposition of the diazotic acid (17) to carbonium ion products occurs -NZ -OH- + R2 CHN=N-OH In order of decreasing product ratio k,:k (carbonium ion diazo-alkane) the alkyl systems (R) studied could be placed in the sequence 74 R A. Moss J. Org. Chem 1966,31,1082; See also H. Hart and J. L. Brewbaker J. Amer. Chem. SOC. 1969 91,716. 5+ 129 Developments in the Chemistry of Diazo-alkanes Thus primary alkyl groups occupy a central position while when R is secondary e.g.cyclohexylidene k is reduced and k increased by enhanced carbonium ion stability. Therefore k k increases relative to that for primary R so that no diazo-alkane formation is observed. When R is changed from primary alkyl to ally1 or benzyl k is sufficiently enhanced so that k k,is too small to permit observation of solvolysis of the diazotic acid derivative. By180 labelling Reimlinger et a2.76 have shown that formation of diazomethane from N-nitro-N-acetylglycine ethyl ester (18) follows a cyclic path Carboxylic acid chlorides and bromides upon treatment with excess of diazomethane can be converted almost without exception and in good yields into diazo-ketones :76 RCOCl + 2CH,N ._+ RCOCHN + MeCl + N This constitutes the first stage of the Arndt-Eistert synthesis of homologous 76 H.K. Reimlinger L. Skattebol and F. Billiou Chem. Ber. 1961,94,2429. 76 F. Arndt and J. Amende Chem. Ber. 1928,61 1122. 130 Coweil and Ledwith carboxylic acids.77 The last ten years has seen the development of two other methods of preparation of diazo-ketones which do not involve the use of diazo- methane or its homologues. In 1953 Doering and DePuy prepared diazocyclopentadiene (6) by the reaction of toluene-p-sulphonyl azide (1 6) with cyclopentadienyl-lithium.78 The use of toluene-p-sulphonyl azide has now been widely applied in the conversion of compounds R1COCH2R2 to diazo-ketones R1COCN2R2.7s An alkaline reaction medium is needed in all cases to get the best yields of diazo-compounds.Alterna- tively,sO acyl bromides RCOCH,Br are treated with hydrazine to form the hydrazones which are subsequently oxidised to diazo-ketones RCOCHN2 by manganese dioxide. Diazo-ketones RCOCN&H of the azibenzyl type were prepared analogously from desyl halides RCOCH(C,H,)X. Certain aliphatic diazo-compounds may be prepared by diazotisation of the appropriate amine provided that the amine possesses a strongly electron- withdrawing substituent on the or-carbon atom. Diazoacetic ester the first aliphatic diazo-compound known,SX was prepared in this manner by treatment of glycine ethyl ester hydrochloride with potassium nitrite HCI.NH2CH2C02Et + KN02 3 N2CHC02Et + KCI + 2H,O The trifluoromethyl group has an inductive effect similar to that of the ester group in aminoacetic ester and preparations of 2,2,2-trifluorodiazoethane2' and 2,2,3,3,4,4,4-heptafl~orodiazo-n-butane,~~ and 1,l ,l-trifluoro-2-diazo- propane30 by diazotisation of the corresponding amines have recently been reported.On the other hand bis-trifluoromethyl diazomethane and bis-perfluoro- ethyl diazomethane are more conveniently prepared by oxidation of the corre- sponding hydrazones with lead tetra-acetate.as Both compounds show remark- ableastability in the presence of acids. Diazotisation may be used similarly to prepare a range of diazocyclopentadienes81ar * which react as aryl diazonium salts rather than diazo-alkanes on account of the aromatic character of cyclopentadienyl anion (e.g. 6). 3 Cycloaddition Reactions of Diazo-alkanes Cycloadducts of diazo-alkanes have been known for a great many yearss2 but it was not until the early 1960s that the classification 1,3-dipolar cycloadditions3 became generally accepted.This followed a series of outstanding studies by 77 F. Arndt and B. Eistert Chem. Ber. 1935 68 200. 70 M. Regitz Tetrahedron Letters 1964 1403; M . Regitz and G. Heck Chem. Ber. 1964 97 1482; M. Rosenberger and P. Yates Tetraheciion Letters 1964 2285; M . Regitz and A. Liedhegener Chem. Ber. 1966 99 3128. S. Hauptmonn M. Kluge K. D. Seidig and H. Wilde Angew. Chem. 1965 4 688. T. Curtius Chem. Ber. 1883 16 2230. W. VON E. Doering and C. H. DePuy J. Amer. Chem. SOC. 1953,75,5955. 81 (a) 0. W. Webster J. Amer. Chem. Soc. 1966 88,4055; (b) D. J. Cram and R. D. Partos ibid. 1963,85 1273; P. L. Pauson and B. J. Williams J. Chem. Soc. 1961,4153. 82L.I. Smith Chem. Rev. 1938 23 193. 88 R. Huisgen Proc. Chem. SOC. 1961 357. 131 Developments in the Chemistry of Diazo-alkanes Huisgen and his collaborator^^^^^^^^ in which diazo-alkanes were shown to represent just one example of a wider class of 1,3-dipolar molecules abc which undergo 1,3-~ycloadditions and are described by zwitterionic octet structures e.g. + - - + a=- 6-4 a-b=c (b = -NR -0) + - Specific classes of molecular 1,3-dipoles include diazo-alkanes (R,C=N =N,) nitrile oxides (Ar-C EN-0) azides (Ar-N=N=N) nitrones (Ar-CH=N(Me)-O) and nitrile amines (Ar-C,N-N-Ar). + - + - f - f - 1,3-Dipolar cycloadditions exhibit common mechanistic features 84986 they are not markedly influenced as to rate or stereochemistry by solvent polarity; they show low enthalpies of activation (5-15 kcal/mole) and large negative entropies of activation (-25 to -45 e.u.); they produce five-membered cyclic compounds in which the stereochemistry of the reacting olefin (dipolaro- phile) is maintained; reaction rates are markedly increased by conjugation of the reacting site in the dipolarophile but reduced by the steric effect of all types of substituent.Study of cycloadditions has been stimulated enormously by current theories relating to conservation of orbital symmetry in concerted reactions.ll9 86s87Diazo- alkanes provide particularly useful substrates for kinetic studies of these pro- cesses with olefinic dipolarophiles. Thus diazomethane and methyl meth- acrylate88 give a high yield of the dl-pyrazoline (20) by what is now classified8@ as a 3 + 2 cycloaddition Me Me + - I I CH2 =N=N + CH2 =C-C02 Me --+ H2 C-C-CO2 Me I I Reactivity of diazo-alkanes in cycloaddition is markedly reduced by conjugating substituents but increased by alkyl groups reactivity falls in the sequenceg~ 88 84 R.Huisgen R. Grashey and J. Sauer in 'The Chemistry of Alkenes' ed. S. Patai Inter- science London 1964 p. 739 86 R. Hoffman and R. B. Woodward Accounts Chem. Res. 1968,1 17. R. Huisgen J. Org. Chem. 1968,33 2291. S . I. Miller Adv. Phys. Org. Chem. 1968 6 185; G. B. Gill Quart. Revs. 1968 22 338. Ledwith and D. Parry J . Chem. SOC. (B) 1966 1408. 132 Cowell and Ledwith MeCHN > CH,N > > PhzCNz > N,CHCO,Et indicating dominance of electronic effects. On the other hand ring strain or polarising influence of con- jugating substituents strongly promotes dipolarophile reactivity and all types of substituent exert a retarding steric effect.For reactions with both CH2NzSs~s0 and Ph2CN2,s1 dipolarophile activity falls in the sequence HC=CH CH=CH 1 I >> CH*=CH*C02Et > I I > CH2=d-C02 Et > MeCH=CHC02 Et oc,o,co COzEt CO2Et Me Apart from the obvious synthetic value of cycloadditions there has been con- siderable interest in the reaction mechanism. s~ 8 4 9 8 5 9 s29 s3 Basically the problem is to decide between a concerted or two-step mechanism i.e. d=e d- e d-e* A two-step mechanism involving polar intermediates had seemed unlikely because of the lack of any clearly defined dependence of reaction rate on solvent p~larity.~ However FirestonesZ has recently argued cogently in favour of a two-step mechanism involving biradical intermediates.Some years ago Huisgens proposed that 3 + 2 cycloadditions of diazo-alkanes occurred via a concerted process involving a cyclic transition state oriented in two planes e.g. (21) for diphenyl diazomethane. 8g R. Huisgen Angew. Chem. Znternat. Edn. 1968 7 321. go A. Ledwith and Yang Shih-Lin J. Chem. SOC. (B) 1967 83. g1 R. Huisgen H. Stangl H. J. Sturm and H. Wagenhofer Angew. Chem. 1961 73 170. 92 R. A. Firestone J. Org. Chem. 1968 33 2285. gs 0. E. Polansky and P. Schuster Tetrahedron Letters 1964 2019. 133 Developments in the Chemistry of Diazo-alkanes Woodward-Hoffmann rules for conservation of orbital symmetry86 now supply the theoretical basis and taken with most of the experimental work,86 provide overwhelming support for Huisgen’s earlier predictions.l1 It must be noted however that the nature of orientation [i.e. whether (a) or (b)]. 1 34 Co well and Ledwith is not adequately predicted by either concerted or biradical mechanisms. The only significant evidence for a two-step cycloaddition of a diazo-alkane proceed- ing via polar intermediates was obtained from reactions of p-methoxyphenyl diazomethane (22) with p-methoxystyrene (23). cis- and truns-3,5-bis(p-anisyl)- 1-pyrazolines (24) were formed in roughly equal + Ar-CH-7 H - I N N ,CH-Ar T + Ar-CH-CH Me6 -6>-CH -CH2 I I e---- - I NBNNcH-Ar N\ *,CH - cis- and trans- (24) ( 2 5 ) Formation of a cis-&substituted pyrazoline is without precedent in the reactions of aryl diazo-alkanes with styrenes. The conclusion must be that stabilisation of a dipolar intermediate (25) by the p-methoxy group permits rotation around the original styrene C=C bond to give roughly equal amounts of cis- and trans- pyrazolines on collapse of the dipolar species.A few recent examples showing the wide synthetic value of 3 + 2 cyclo- additions of diazo-alkanes are indicated below 94 C. G. Overberger N. Weinshenker and J. P. Anselme J. Amer. Chem. SOC. 1965,87,4119. 135 Developments in the Chemistry of Diazo-alkanes 11 Me3 Si-CH=CH2 + Ph2CN2 --+ Me3 Si-CH-CH I I NbN D h 2 Ph-CHZN-Ph + CHzN2 + Ph-CH-N-Ph I I (ref. 97) (ref. 98) (ref. 99) J. H. Atherton and R. Fields J. Chem. SOC. (C) 1968 1507. oa M. G. Barlow R. N. Hazseldine and W. D. Morton Chem. Comm. 1969,931. B7 G. Manecke and H. U. Schenck Tetrahedron Letters 1968 2061 ; See also I. Tabushi K.Takagi M. Okano and R. Oda Tetrahedron 1967,23,2621. *''I. A. D'Yakonov I. B. Repinskaya and G. V. Golodnikov Zhur. org. Khim. 1966 2 2256. P. K. Kadaba Tetrahedron 1966 22,2453. 136 Cowell and Ledwith The synthetic value of 3 + 2 cycloadditions is the greater because in most cases the pyrazoline products are thermally and photochemically unstable,1,2,100J01 permitting convenient generation of cyclopropanes or aikylated olefins / \ HC-C I + CH2 \ / CH 4 2 I R R The 3 + 2 cycloadducts of diazomethane and diazoethane with 2-methyl naphthaquinone (26) are activated sufficiently to undergo base-catalysed d e composition providing a novel synthetic route to the highly reactive quinone methides (27).lo2 Trapping of (27) by reaction with primary or secondary aromatic amines in air gives rise to the intensely coloured adducts (28).lo3 0 0 0 0 * OH The intense colours of (28) are due to charge-transfer transitions involving orbital overlap in the non-conjugated donor (amine) and acceptor (quinone) parts of the molecule and represent the most striking examples of this type of intramolecular interaction so far reported.lO* looT.U. Van Auken and K. L. Rinehart J. Amer. Chem. SOC. 1962 84 3736. lol J. Hamelin and R. Came Bull. SOC. chim. France 1968 2162 3000. loa F. M. Dean L. E. Houghton and R. B. Morton J. Chem. SOC. (0 1967 1980. lo* F. M. Dean L. E. Houghton and R. B. Morton J. Chem. SOC. (0,1968,2065. lO4 R. Carruthers F. M. Dean L. E. Houghton and A. Ledwith Chem. Comm. 1967,1206. 137 Developments in the Chemistry of Diazo-alkanes Alkenyl diazo-compounds may be used to form pyrazolines by reaction with dipolarophiles in the normal manner O7 but also undergo a slower intramolecular cycloaddition yielding p r y a z 0 1 e s .~ ~ ~ ~ ~ ~ ~ For 3-diazopropene (29) it was shown1OB that the initial adduct was a pyrazolenine (30) which underwent thermal and photochemical prototropy to give pyrazole (31). CH-CH - + A CH2-CH-CH-NEN 0 CH2-CH I II (hv) II I1 N ,cH I H I EH2-CH-CH-kN The reaction has been extendedlo7 to a series of aryl and alkyl substituted homologues of (29) giving good yields of the corresponding pyrazoles and from the very small rate-enhancing effect of 4substituents in Ar-CH=CH-CH=Nz (p = -0.40) the reaction was confirmed as an intramolecular concerted process. Interestingly formation of pyrazole (31) from 1,3-bisdiazopropane (32) does not involve prior formation of 3-diazo- propene (29) but probably occurs via the diazocarbene intermediatelo* (33) - + :CH-CH CH-N=N (30) d (3 1) -N2 ~ N2 CH-CH2 -CHNZ 4 Reactions of Diazo-alkanes with Free Radicals Free radical processes are fairly common in reactions of c a r b e n e ~ ~ ~ ~ ~ ~ but there are comparatively few reported examples of the reactions of diazo-alkanes with free r a d i ~ a l s .~ * b ~ ~ ~ ~ - ~ ~ ~ Most of these have been discussed in earlier reviews lo6 D. W. Adamson and J. W. Kenner J. Chem. Soc. 1935,286; C. D. Hurd and S. C. Lui J. Amer. Chem. SOC. 1935 57 2656. lMA. Ledwith and D. Parry J. Chem. SOC. (B) 1967 41 [for a related interconversion of diazoalkene and pyrazolenine see A. C. Day and M. C. Whiting J. Chem. SOC. (C) 1966 lo' J.L. Brewbaker and H. Hart J . Amer. Chem. SOC. 1969 91 71 1 lo8 H. Hart and J. L. Brewbaker J . Amer. Chem. SOC. 1969 91 706. loo W. Schlenk and C. Bornhardt Annulen 1912,394 183. ll1 W. H. Urry J. R. Eisner and J. W. Wilt J. Amer. Chem. SOC. 1957,79 918. 112 W. J. Middleton D. M. Gale and C. G. Krespan J. Amer. Chem. SOC. 1968 90 6813. lla E. Miiller A. Moosmayer and A. Rieker 2. Nuturforsch. 1963 18b 982. 114 E. Muller R. Renner and A. Rieker Tetrahedron Letters 1968 891. 17191. D. B. Denney and M. F. Newman J. Amer. Chem. SOC. 1967 89,4692. 138 Cowell and Ledwith and will not be considered here. It should be noted however that for all reac- tions of diazomethane with free radicals rearrangement or radical recombination is the preferred course. There is no evidence for a chain process leading to poly- methylene involving initiation by a known free radical.This point will become more relevant during discussion of polymerisation of diazo-alkanes (see later). 5 Reactions of Diazo-alkanes with Carbonyl Compounds Diazo-alkanes react thermally with aldehydes and ketones to give mixtures of homologous carbonyl compounds ( 3 9 (36) and epoxides (37),116-117 e.g. - 0 b R' COCH(R3)R2 I -N2 R'COR2 + R3CHN2 + R1-C-R2 I + R3CHN2 (35) + R1 CH( R3 )COR2 (38) + ~ l ~ ~ c - 0 \ / CHR3 (37) Epoxide formation is favoured by electron-withdrawing substituents in the carbonyl compound e.g. chloral118 (CC13CHO) and the reactions are catalysed by protic agents particularly alcohols. Alcohols also control the nature of reaction products reactions of aldehydes with diazomethane in dry ether give predominantly methyl ketones whereas increasing homologation of the aldehyde occurs as alcohols are added to the solvent.Most of the synthetic and mechanistic work was carried out before 1960 and has been extensively r e v i e ~ e d . ~ J # ~ ~ ~ Recently the long established view that both epoxide and homologous carbonyl compound arise from a common intermediate (38) (by ring closure or Wagner-Meerwein rearrangement respectively) has been questioned. From a detailed kinetic and product analysis of the dark reaction between diazomethane and acetone catalysed by n-butanol it was concluded11e that two reactions occur simultaneously one leads to formation of 12-epoxypropane (39) and the other gives a mixture of (39) and ethyl methyl ketone (40). It was suggested that the two competing processes are derived directly from the two important resonance structures for diazomethane i.e.116 C. D. Gutsche and D. Redmore 'Carbocyclic Ring Expansion Reactions' Academic Press New York 1968 p. 81. 116 C. D. Gutsche Org. Reactions 1954 8 364. 11' N. C. Hancox Roy. Australian Chem. Inst. J. and Proc. 1949 16 282. 11* R. E. Bowman A. Campbell and W. R. N. Williamson J. Chem. Soc. 1964 3846. ll@ J. N. Bradley G. W. Cowell and A. Ledwith J. Chem. SOC. 1964,4334. 139 6 Developments in the Chemistry of Diazo-alkanes + - 6+ s- CH,=N=N + Me,C-O - + s+ 6- CH2-NsN + Me2C=0 + Me,C- 0 I I MeCHz COMe Me C-0- -N2 I I +CH2 1 In acetone solvent reaction (B) was of a higher kinetic order in n-butanol than reaction (A) and the product was mainly homologous ketone.Consistent with these observations hydrogen bonding between alcohol promoter and carbonyl oxygen of acetone would be expected to favour process (B) and to minimise epoxide formation. More powerful co-ordination of the carbonyl oxygen function with Lewis Acids completely eliminates epoxide formationlo (see below). Although it is commonly assumed that reactions of carbonyl compounds with diazo-alkanes involve nucleophilic attack by the latter characterisation of a mechanistic dichotomy raises the possibility that primary (unstable) inter- mediates (41) and (42) might be formed by competing 3 + 2 cycloadditions. If this idea was based solely on the kinetic analysis of the n-butanol-diazo- methane-acetone reaction (i.e. purely a solvent effect) it should properly be regarded as entirely speculative.However independent worklao leads to essen- tially the same conclusions from a consideration of substituent effects in quite different substrates. Intramolecular reactions of the diazocarbonyl derivatives (43) yield mixtures of bicyclic ketone (44) and epoxide (43 in a ratio which depends markedly on substituents. From (43a) bicyclic ketone (Ma) is the major product the yield of epoxide increasing with increasing substitution by methyl lZo C. D. Gutsche and J. E. Bowers J. Org. Chem. 1967 32 1203; See also C. D. Gutsche and C. T. Chang J . Amer. Chem. SOC. 1962,84,2263. 140 Cowell and Ledwith groups so that for (43b) the product is almost exclusively epoxide (45b). Two distinct processes are shown to be involved similar to reactions (A) and (B) with mode of addition of the diazo unit controlled by conformational effects of the cyclohexanone system.120* E t 6 ' L C H 2 N C 0 M e R' I EtOH NO R' (45) + (44) R' Of much greater synthetic value is the homologation of ketones catalysed by Lewis Acids.lo This type of reaction was discovered independently by several groups of workers but has been developed largely by Muller and his collaborators.lo~ House Grubbs and Gannonlzl found that reactions between diazomethane and acyclic ketones in ether were strongly promoted by addition of one mole equivalent of boron trifluoride.Compared with alcohol-catalysed systems reaction times are much shorter (minutes rather than hours or days) yields are higher and most important formation of epoxide does not occur. Cyclic ketones gave ring homologation and for unsymmetrical acyclic derivatives migratory aptitudes fell in the order Ph- Me,C=CH- > Me- > Pr- > * Since this survey was completed the stereochemistry and mechanisms of related ring expansion of cyclopropanoneslzO~.b and steroidal ketoneslZoC by diazo-alkanes have recently been discussed in detail.lZo (a) N. J. Turro and R. B. Gagosian Chem. Comrn. 1969 949; (b) J. A. Marshall and J. J. Partridge J. Org. Chern. 1968 33 4090; (c) J. B. Jones and P. Price Chern. Curnrn. 1969 1478. H. 0. House E. J. Grubbs and W. F. Gannon J. Amer. Chem. Soc. 1960,82,4099. 141 Developments in the Chemistry of Diazo-alkanes Pri-w PhCH,-w But- closely similar to that found for pinacol rearrange- ment during deamination of corresponding l,l-disubstituted-2-aminoethanols. 1 ,ZUnsaturated ketones which normally react with diazomethane to give pyrazoline derivatives (47) gave only homologous ketones (48 49) in good yield e.g.for mesityl oxide (46) in ether at 0" CH,N - BF (47) 1 Me&=CH-CH2COMe + Me2C=CH-CO-CH2Me + NZ Similarly Johnston Neeman Birkeland and Fedoruklaa showed that certain steroid ketones yielded ring homologised products when treated with diazo- methane in methylene chloride in the presence of catalytic amounts of fluoro- boric acid. Miiller and his ~ ~ l l a b o r a t o r ~ ~ ~ ~ studied related homologation reactions of cyclic ketones (50). Using catalytic amounts of boron trifluoride in ether all the cyclic ketones from cyclohexanone to cyclotetradecanone were successfully homologised to the next highest ring ketone in good yield with an approxi- mately 2:l molar excess of diazomethane.The yield of homologous ketone (51) falls as the ring size increases and in addition there is a marked tendency for more than one methylene group to enter the ring especially with the larger ring ketones increasing the ring size by two or three carbon atoms via repetitive reaction on the successive homologues. Whilst many Friedel-Crafts halides such as boron trifluoride aluminium chloride zinc chloride titanium tetrachloride etc. could be used as catalysts for the homologation of cyclic ketones with diazomethane other boron com- pounds e.g. boron trichloride boron tribromide trialkylboranes and trialkyl borates were completely ineffective and served only to convert the diazomethane into polymethylene. Since the catalytic efficiency of the various halides parallels their effect on the U.V.absorption spectra of the ketones the following mechanism is indicated W. S. Johnson M. Neeman S. P. Birkeland and N. A. Fedoruk J. Amer. Chem. Soc. 1962 84 989; W. S. Johnson M. Neeman and S. P. Birkeland Tetrahehon Letters 1960 No. 5 1. E. Miiller B. Zeeh and R. Meischkeil Annulen 1964 677,47; E. Miiller and M. Bauer ibid. 1962 654 92; E. Miiller M. Bauer and W. Rundel 2. Naturforsch. 1960 15b 268; E. Miiller M. Bauer and W. Rundel Tetrahedron Letters 1960 No. 13 30; E. Miiller and R. Heischkeil ibid. 1964 2809. 142 Cowell and Ledwith I CH2N2 (50) + + Homologation of acyclic ketones presumably involves a similar mechanism and for or/i?-unsaturated ketones (52) a non-classical homoallylic cation (53) would ensure homologation on the ethylenic side of the carbonyl group i.e.+ R1COCH-CHR2 + BF3 ' R~-C-CH=CHR~ I (52) R' (53) - I BF3 Homologation reactions of cyclic ketones have been extended to include diazo- ethane whereas 60-80 % yields of the next higher a-methyl-substituted ring ketone can be obtained from diazoethane using aluminium chloride as catalyst boron trifluoride and other boron compounds are 6 Reactions of Diazo-alkanes with Carbonium Ions Certain stable organic cations may be conveniently homologised by reaction with diazomethane. Other diazo-alkanes react but give either lower yields or lar E. Miiller M. Bauer and W. Rundel Tetrahedron Letters 1961 136. 143 Developments in the Chemistry of Diazo-alkanes complex mixtures of products. The best example is the conversion of xanthylium perchlorate (54 X = 0) into dibenzo[b f]oxepinelZ5 (55 X = 0) in 60% yield + m -N2 + Similar conversion was effected with the corresponding thio-compound (54 X = S) but N-methyl acridinium iodide (54 X = -NCH,) gave mainly the iodomethyl homo1oguefZ5 (56 X = -NCH3) reflecting increased nucleo- philicity of I- over ClO,-.In theory homoallylic cations could be generated by reaction of a suitable allylic carbonium ion with diazomethane i.e. but there is only one reported examplela6 involving the transformation of (57) into 1,1,4,4-tetraphenylbutadiene (58) 126 H. W. Whitlock Tetrahedron Letters 1961 593. lZ6 H. W. Whitlock and M. R. Pesce Tetrahedron Letters 1964 743. 144 Co well and Ledwith + 4 -H+ Ph2 C=CHCH=CPh2 - Ph2 GCH-CHZ-CPh2 Triphenylmethyl cation reacts smoothly with dia~omethanel~~ forming triphenyl- ethylene and 1,2,3-triphenylpropene (59) by phenyl migration in the homo- logised intermediates + + P h P + CHzN2 ___+ Ph3C-CH2N2 -N2 Ph,C-CH2 Ph + "2 + PhCH2-C(Ph)-CII Ph +- N2 CH2 C(Ph)2 CH2 Ph Ph2 C=CHPh .c PhCH=C(Ph)-CH2 Ph (59) Tropylium ion reacts vigorously with diazomethane and diphenyl diazomcthane causing (catalytic) formation of ethylene and tetraphenylethylene respectively.128 Yields of olefin are quantitative and presumably result from the well-known129 fragment at ion of 2-cyclo hep t a trienylet hyl cations C7H7+ + R2C=CR2 + N2 12' H.W. Whitlock J. Amer. Chem. SOC. 1962 84 2807. lt8 A. Ledwith and A. C. White unpublished results. K. Conrow J . Amer. Chem. SOC. 1959 81 5461. 145 Developments in the Chemistry of Diazo-alkanes Diphenyl diazomethane forms tetraphenylethylene by a similar process involving triphenylmethyl cafion.l2? A related reaction of synthetic value is the formation of aziridinium salts (60) by reactions of diazomethane with protonated enamine~,'~~ e.g.Me. ,Me M P M N Me' 'Me N+ (2110 Me' 'Me "'xMe N+ CQO4 Me' 'Me 7 Diazo-alkanes in the Formation of Organometallic Compounds The reaction between diazo-alkanes and metal halides is a particularly useful synthetic route to carbon-functional organometallic e.g. RCHN2 4 2 X,-l MCHX b Xn-2M(CHX)2 etc. I R I MXn + RCHN2 R Metal halides are the most common reagents and good yields of halogenoalkyl derivative are obtained especially for the elements forming covalent bonds with carbon. A detailed kinetic studys0 of the reaction between diphenyl diazomethane and mercury(r1) chloride in tetrahydrofuran established that polar intermediates were involved as suggested initially by Huisgen.' However whereas diazo- methane reacts with mercury(r1) chloride to give ultimately Hg(CH,Cl), the corresponding reaction with diphenyl diazomethane involves the following steps lsoN.J. Leonard J. V. Paukstelis and L. E. Brady J . Org. Chem. 1964 29 3383; N. J. Leonard and K. Jann J. Amer. Chem. Soc. 1962,84,4806; N. J. Leonard and K. Jam ibid. 1960,82 6418. lS1 D. Seyferth Chem. Rev. 1955,55,1155. 146 Ph I I A quantitative yield of products (63) or (64) was obtained according to the initial molar ratio diazo-compound :HgCI 2 and both organometallics were rapidly hydrolysed by small amounts of water to give benzophenone and benzpinacolone respectively e.g.Corresponding reactions of diphenyl diazomethane with zinc chloride and zinc bromide are ~ i m i l a r ~ ~ ~ ~ ~ ~ ~ but the dipolar intermediates corresponding to (61) and (62) react also with diazo-compound to give benzophenone azine. Reactions of diazo-alkanes with zinc i ~ d i d e ~ ~ ~ ~ ~ b are of special importance because the initial product is thought to be related to the Simmons-Smith cyclopropane synthesis,186 as exemplified CIHgC(Ph),Cl + HzO -4 Hgo + Ph,C=O + 2HC1 Zn-Cu w + CH2I2 _____+ CH2 IZnCH21 4 2 Zn12 + CH2N2 D. Bethell and K. C. Brown Chem. Comm. 1967 1266. laS D. E. Applequist and H. Babad J. Org. Chem. 1962 27 288. Is'(,) G. Wittig and K. Schwarzenbach Angew. Chem. 1959 71 652; Annulen 1961 650 1 ; G. Wittig and F.Winder ibid. 1962,656 18; (6) S. H . Goh L. E. Closs and G. L. Closs J. Org. Chem. 1969,34,25. H. E. Simmons and R. D. Smith J. Amer. Chem. Soc. 1959 81 4256; E. P. Blanchard and H. E. Simmons ibid. 1964,86,1337; H. E. Simmons E. P. Blanchard and R. D. Smith ibid. 1964 86 133. 147 Developments in the Chemistry of Diazo-alkanes Cyclopropane formation also occurs when diazomethane reacts with olefins in the presence of dialkylaluminium halides (65) but in this case the intermediate y-halogenopropyl organometallic (66) may be isolated at low temperatures and shown to generate cyclopropane:136 CH -CH2 RzA!2X + \'/ CH In contrast trialkylaluminium derivatives (65 X = alkyl) or dialkylaluminium hydrides (65 X = H) yield stable homologous products except in the presence of strong donor molecules (e.g.tetrahydrofuran) when polymethylene is the sole product. The latter is formed exclusively under all conditions when dialkylaluminium fluoride or alkoxide derivatives are (i.e. 65 X = -F -OR2). Organoboron compounds react with diazo-alkanes in a manner very similar to that of the corresponding aluminium derivatives. Starting materials are more readily accessible and the reactions are of synthetic value and pertinent to the mechanism of polymerisation of diazo-alkanes (discussed separately). Some years ago the gas-phase reaction between diazomethane and boron trifluoride was shown to give F2BCH2F providing the first example of methyl- enation of a boron Many boron compounds were known to catalyse polymerisation of diazomethane in solution and consequently the methylenation reaction was as the propagation step for boron- catalysed polymerisations.In particular it was that alkyl boron derivatives would undergo homologation i.e. CHa Na CH2 N2 R-B< - RCH2B< - - RCH,CH,B < -Nz -N2 etc . Davies and his ~o-workers~~~ substantiated this suggestion and synthesised previously unavailable neopentyl boron compounds by treating the corre- sponding t-butyl derivative with diazomethane. In addition n-butyl boronic anhydride was shown to react with diazomethane forming a mixture of organo- boron compounds which after oxidation and hydrolysis produced all the H. Hoberg Annalen 1962,656 1; H. Hoberg Angew. Chem. 1961,73 114. lS7 H. Hoberg Annulen 1966,695 I ; H. Hobcrg Angew. Chem. Internat. Edn. 1965,4 1088. lS8 J. Goubeau and K. H. Rohwedder Annalen 1957 604 168.lseC. E. H. Bawn A. Ledwith and P. Matthies J . Polymer Sci. 1959 34 93. 140 A. G. Davies D. G. Hare 0. R. Khan and J. Sikora J . Chem. SOC. 1963 4461; Proc. G e m . SOC. 1961. 172. 148 Cowell and Ledwith +% normal alcohols diazomethane to -A% Ph-B < -k i/ f up to c8. Similarly triphenylborane reacts with diphenyl give triphenylmethanol as a major product :141 Ph I OH- Ph2CN2 ____+ Ph-C-B< Ph3C@H H202 -N 2 I Ph I Very recently the alkylenation of organoboron compounds has been extended to provide a useful synthetic route to ketones esters and nit rile^.^^^*^^^ Alkyl boron compounds are readily available as in situ intermediates following hydro- boronation of alkenes.lJ4 Immediate reaction with dia~o-ketones,'~~ diazoacetic ester,143 or cyan~diazomethanel~~ gives after alkaline oxidation very good yields of alkylated products as indicated OH=H2O2 I RCH2 CH2 CH-B < RCH2-CH2CHB< I CN I O H k f i 2 I OH'H202 RCH2CH2CH2C02Et RCH2CH2CH2CN RCH 2CH2CH2COMe Most workers assume that alkylenation of aluminium,137 b o r ~ n ~ ~ silicon,1451146 and tin147 compounds involves primary co-ordination of diazo- alkane carbon with a vacant p-orbital in the Lewis Acid as illustrated above for reaction of diphenyl diazomethane with mercury(@ ramifications are discussed below in connection with polymerisation.8 Polymerisation of Diazo-alkanes Catalysed by Boron Compounds As outlined above many inorganic compounds react with diazo-alkanes to form organometsllic derivatives. Compounds of b o r ~ n ~ ~ ~ v ~ ~ ~ aluminium,137 and silicon145 may be active catalysts for polymerisations or may be alkylenated according to solvent and substituents on the metal.Boron compounds have been most widely used for the former purpose and there is every reason for lP1 J. E. Lettler and B. G. Ramsey Proc. Chem. SOC. 1961 117. 14* J. Hooz and S. Lincke J. Amer. Chem. SOC. 1968 90 5936; J. Hooz and D. M. G u m Chem. Comm. 1969 139; J. Amer. Chem. SOC. 1969,91 6195. lP3 J. Hooz and S. Lincke J . Amer. Chem. SOC. 1968,90 6891. lP4 H. C. Brown 'Hydroboronation' Benjamin Inc. New York 1962. lQ5 R. A. Shaw J. Chem. SOC. 1957 2831. lP6 K. A. W. Kramer and A. N. Wright J. Chem. SOC. 1963 3604; Chem. Ber. 1963 96 1877. lP7 M. Lesbre and R. Buisson Bull. SOC. chim. France 1957 1204. 149 Developments in the Chemistry of Diozo-alkanes assuming that mechanisms of reactions involving the other elements would be closely related.Polyalkylidenes [-(CH)n-] are synthesised almost exclusively by catalysed I R polymerisation of the appropriate dia~o-alkane.*~* The linear homopolymers of diazomethane and copolymers with other diazo-alkanes provide excellent models14B for commercially important linear and branched polyethylenes and for anticipated (but not yet realised) homopolymers of 1,Zdisubstituted ethylenes. Polyethylidenelso [-(CH)n-] and polybenzylidenels* [-(CH)n-] are particularly I Ph I Me interesting in this respect (c.J the non-homopolymerisability of cis- and frans- but-2-ene and stilbenes). Boron halides boron alkyls and boric esters have been used as catalysts but boron trifluoride and trialkyl borates give best The various mech- anisms proposed for polymerisation of diazo-alkanes catalysed by boron compounds have been r e ~ i e w e d ~ ~ ~ ~ ~ ~ ~ but recent makes it probable that more than one process must be operative depending on the nature of the catalyst.Successive methylenation is an obvious possibility e.g. - + BX? + CH2N2 X~B-CH~NZ - X3B CH2X + N2 (68) (69) 14*C. E. H. Bawn and A. Ledwith 'Encyclopedia of Polymer Science and Technology' Wiley Interscience New York 1969 10 337. 140 M. J. Richardson P. J. Flory and J. B. Jackson Polymer 1963 4 221. lSo G. D. Buckley L. H. Cross and N. H. Ray J. Chem. SOC. 1950,2714. lS1 C. E. H. Bawn A. Ledwith and P. Matthies J. Polymer Sci. 1958 33 21. lS2 S. W. Kantor and R. C. Osthoff J. Amer. Chem. Soc. 1953 75 931. lS3 G. D. Buckley and N. H. Ray J.Chern. SOC. 1952,3701. 150 Cowell and Ledwith Polyalkylenated and compounds have been characterised supporting such a scheme but for reactions with silicon tetrachloride the products are either polymethylene or mixtures of mono- bis- tris- and tetrakis- chloromethyl silicon derivatives.145 Further confirmation of the polymethylena- tion mechanism was obtained by polymerisation of diazomethane with triphenyl- borane.141 Treatment of the polymer with alkaline hydrogen peroxide gave benzyl alcohol and polymethylene containing a monosubstituted benzene ring compelling evidence for the formation of molecules such as (67-69 X = Ph-). Similar results were obtained with tris a-naphthyl boron as c a t a l y ~ t . ~ ~ ~ ~ Clearly the repetitive methylenation mechanism is appropriate in some circum- stances.By such a reaction the average molecular weight of polyalkylidene chains would be given by the ratio 3(RCHN2):BX, but for catalysis by excess quantities of BFs lS6 BR, 140 or B2H6,156 diazomethane is completelypolymerised to high molecular weight products with only partial utilisation of the catalyst. Further Davies and his co-workers have that polymerisation of diazomethane may be intercepted by addition of a nucleophilic reagent AH (e.g. amine water or alcohol) forming CH,A. It was therefore that three reactions (C,D,E) may occur simultaneously i.e. - + -N X2B-CH2-N2 X2BCH2CH2X etc. (C) CH2N2 X2 BCH2 X I CH2 X X3B-CH2CH2N2+ CH2N2 X,B+CH,N X2€kH2N2+- 4 2 ' CH2 N2 I X 14. + X3 BCH2 CH2 CH2 N2 efc. (D) ."I + - X3B-CH2AH ____+ X3B + CH3A (E) Polymerisation of diazomethane is a very rapid reaction and consequently detailed kinetic investigation is experimentally difficult.Nevertheless Davies et al.lao suggest that reaction (D) is the dominant polymerisation mechanism under most reaction conditions although minor variations to allow propagation by ion-pairs such as X,BCH2(CH2)nCH2N2+ BX4- could not be excluded. Concurrent methylenation of added nucleophilic reagents is readily explained 154 M. G. Krakovyak E. V. Anufrieva and S. S. Skorokhodov Vysokomol. Soedineniya 1966 8 1681. lS6 G. H. Dorion S. E. Polchlopek and E. H. Sheers Angew. Chem. 1964,76,495. A. Ledwith Ph.D. Thesis Liverpool 1957. 151 Developments in the Chemistry of Diazo-alkanes by a mechanism such as (E) although it is now evident that this is a special case of a useful alkylation reaction for which alternative mechanisms have been proposed (see later).is significant in discussion of boron trifluoride catalysed polymerisation of diazo-alkanes. Decomposition of 2-phenyl-2-methyl diazopro- pane (70) catalysed by protic and Lewis Acids yields a mixture of alkene products consequent on methyl or phenyl migration A related + Me I + - I Me BF3 b Ph-C-CH d Ph-C-CH-BF3 I I Ph-C-CHN2 Me I \- Me BF Me Me \ Ye Ph Ph Ph / \ BF Me -BF + C-CH \ CH2 ,C-CH2Me \\ + ,C=CH-Me 4 In strictly anhydrous conditions boron trifluoride gave a product composition different from that given by protic acids but consistent with indiscriminate phenyl or methyl migration. Other Lewis Acids gave product mixtures in between the boron trifluoride-protic acid extremes and addition of small amounts of water or alcohols to the boron trifluoride system produced a mixture identical with that from protic acids.The latter react with diazo-alkanes to give products arising from the corresponding diazonium and carbonium ions (see later section). It must be concluded therefore that boron trifluoride catalysed decomposition does not involve the corresponding carbonium ion. Formation of a boron trifluoride-carbene adduct (71) was suggested and its demonstrated rearrangement to (72) or (73) involving charge separation helps to overcome 157 H. Philip M. K. Lowery and J. Havel Terrahedron Letters 1967 5049. 152 Cowell and Ledwith previous to long standinglsa polymerisation mechanisms such as (D) above. Polar intermediates have long been assumed for polymerisation of diazo-alkanes largely because of the very high rates of reaction and the in- effectiveness of conventional radical Similar criteria have been used to support the assumption that polar intermediates were dominant in reactions of oxygen with organoboron corn pound^,^^^^^^^ but very recent work demonstrates rather that free radical intermediates are important.1so Free radical reactions of boron compounds now appear to be much more generaP than had been supposed and it is at least a possibility that boron-catalysed polymerisation of diazomethane might involve some kind of boron-complexed radical species.9 Reactions of Diazo-alkanes Catalysed by Copper Salts Catalysis by copper metal cuprous and cupric salts is frequently utilised to facilitate reactions of diazo-alkanes including p o 1 y m e r i ~ a t i o n ~ ~ ~ ~ ~ ~ ~ and (apparent) formation of c a r b e n e ~ .~ ~ ~ Interest in the latter possibility has been widespread although the most comprehensive study is that of E. Miiller and his collaborators.lOb Cupric salts are immediately reduced by diazomethane and consequently cuprous compounds are the most convenient catalysts. Presumably comer metal functions via surface impurities. Benzene reacts readily with diazomethanelg4 in the presence of cuprous halides to form cycloheptatriene (74) in high yield The reaction is general for aromatic systems substituted benzenes giving a mixture of the corresponding substituted cycloheptatrienes,' O b i.e. R = alkyl halogen or alkoxy 16* A. G. Davies 'Organic Peroxides' Butterworths London 1961. lS9 A. G. Davies Progr.Boron Chem. 1964 1 265. la0 A. G. Davies and B. P. Roberts J. Chem. Soc. (B) 1969 3 11 ; 1967 17. 1969 911; P. G. Allies and P. B. Brindley Chem. and Ind. 1967 319; 1968 1439. 163 J. Feitzin A. J. Restaino and R. B. Mesrobian J . Amer. Chem. SOC. 1955 77 206. 164 E. Miiller and H. Fricke Annalen 1963 661 38; E. Miiller H. Kessler H. Fricke and W. Kiedaisch ibid. 1964 675 63. A. 0. Davies and B. P. Roberts Chem. Comm. 1969 699; K. U. Ingold Chem. Comm. C. E. H. Bawn and T. B. Rhodes Trans. Faraday SOC. 1954,50 934. 153 Developments in the Chemistry of Diazo-alkanes CH2N2 Cu Br Condensed aromatics also give mixtures of products,16S e.g. for anthracene;lUs (-N2) &+m+ 80% 11% 9% Related reactions of cyclic and acyclic olefins produce cyclopropanes (75) in good yield :134,167-170 R' \c/R2 CuC1 or CuBr .4 2 A survey of the scope of copper-catalysed homologations has been published' Ob and in all cases the products resemble those expected from reaction of carbene (CH,:) with the same substrate. Particularly important is the reaction of diazo- methane with allylic compounds studied by Kirmse and his collaborators.171a-C Thus cis- and trans-isomers of (76) react to give the corresponding cyclopropanes with complete retention of config~ration,~~~~ i.e. lB6 W. E. VON Doering and M. S. Goldstein Tetrahedron 1959 5 53; E. Muller H. Fricke and H. Kessler Tetrahedron Letters 1964 1525; E. Muller H. Kessler and H. Suhr ibid. 1965,423; C. R. Ganellin ibid. 1964,2919. lBB E. Muller and H. Kessler Annalen 1966 692 58. 167 M. F. Dull and P. 0. Abend J. Amer. Chenr.Sue. 1959 81 2588. 168 W. K. Roth and J. Konig Annalen 1965,688,28. Kessler Tetrahedron Letters 1968 3037. 170 W. Roth Annalen 1964,671 10. 171 (a) W. Kirmse and M. Kapps Angew. Chem. Internat. Edn. 1965,4 691 ; (b) W. Kirmse M. Kapps and R. B. Hager Chem. Ber. 1966,99,2855; (c) W. Kirmse and M. Kapps ibid. 1968 101 994; W. Kirmse and H. Arold ibid. 1968 101 1008. E. Miiller H. Fricke and W. Rundel Z. Nuturfursch 1960 15b 753; E. Muller and H. 154 Cowetl and Ledwith Simple ally1 halides (77) give mixtures of cyclopropane (78) and 4-halogeno- but-1-enes (79) depending on the nature of R X and the solvent i.e. R I R CH2=C-CH2X I "E:,"2 + &CH2X + CH2=C-CH2CH2X w 2 1 (77) (78) (79) Cyclopropanes (78) are the main product when X = C1 but 4-halogenobut-I- enes (79) predominate when X = Br.By means of deuterium labelling in methyl derivatives it was demonstrated that formation of (79) involves a complete allylic rearrangement e.g. R2 R' R' R' I C I t + R2C$%H2 -+ XCH2CH-C=CH2 I CH2N2 R2 CH-C-CH2 X CUY ( 4 2 ) 1 /.A S]H2 I (CAY) In contrast the corresponding reaction of carbene produced by photolysis of diazomethane gives the 4-halogenobut-1-ene derivative by direct insertion of CH2 into the C-Cl bond.171 Furthermore copper-catalysed reactions of diazo- alkanes are normally free of products resulting from insertion of carbenes into C-H bonds so typical of free carbene Free carbenes therefore do not play a role in copper-catalysed reactions of diazo-alkanes and two yossibIe reaction paths (F and G) are indicated bel~w,'~b 155 Developments in the Chemistry of Diazo-alkanes - + -N2 - + X-CU + CH2Nz __+ XCU-CH~N~ XCU-CH~ ___+ CUCH~X I I (80) I (81) Reaction (F) would be anticipated for reactions with aromatic molecules because of resonance stabilisation of the dipolar intermediates (83) e.g.By analogy with intermediates in the Simmons-Smith cyclopropane copper-catalysed reactions of diazomethane with olefins probably involve intermediates such as (82) formed via either reaction (F) or (G). A copper-carbene complex first suggested by Yate~"~ to explain copper- catalysed reactions of diazoacetic ester with olefins has recently been by detailed kinetic studies of similar reactions with the further con- clusion that cyclopropane formation involves a copper-carbene-olefin complex such as (82). It is perhaps worth recalling that a methylcarbene-platinum complex was originally proposed17* as a structure for the platinum(II)-ethylene adduct (Zeise's salt) and recently stable transition metal-carbene complexes have been ~haracterised.~~~ Diazo-alkanes may also form a stable complex with transition metals via the nitrogen atoms in favourable Synthesis of unstable copper@ alkyls (81) was first to explain copper-catalysed polymerisation of diazo-alkanes.The latter has been known for many yearslSo and represents perhaps the most convenient method for lla P. Yates J. Amer. Chem. SOC. 1952 74 5376. 17$ W. R. Moser J. Amer. Chem. SOC. 1969 91 1135 1141. 17* J. Chatt Research 1951 4 180. 171 E. 0. Fischer and A. Riedel Chem. Ber. 1968,101 151 ; 0. S. Mills and A. D. Redhouse J. Chem. SOC. (A) 1968 642. 17* P. E. Baikie and 0.S. Mills Chem. Cornrn. 1967 1228; M. M. Bagga P. E. Baikie 0. S. Mills and P. L. Pauson ibid. 1967 1106. l" C. E. H. Bawn A. Ledwith and J. A. Whittleston Angew. Chem. 1960 72 115. 156 Cowell and Ledwith producing homopolymers of alkyl-substituted diazomethanes although poly- methylene may also be obtained in this way.18zp163 A significant conclusion emerging from the results of several groups of ~orkerslOb~l~~ is that for reactions of diazomethane copper salts active for formation of cyclopropanes (e.g. CuCl CuBr) are poorly effective as catalysts for polymethylene formation. Conversely copper salts of organic acids (e.g. copper(n) stearate) which are particularly useful for polymerisation are almost completely inactive in cyclo- propane formation. Although copper(n) salts are used for convenience in polymerisation of diazo-alkanes the corresponding copper(1) salts constitute the active catalysts.lT7 The precise mechanism for polymerisation is even less clear than for catalysis by boron compounds but analogous possibilities exist e.g.repetitive insertion growth of a copper(1) alkyl (81) or a rapidly growing cationic chain from (80) (cf. reactions C D). Protic additives have lesser effects on the copper-catalysed polymerisations which argues against a propagating ionic species. Indeed for the polymerisation of diazoethane in tetrahydrofuran catalysed by CuI-amine a d d ~ c t s ~ ~ ~ the polymer yield and molecular weight are unaffected by massive amounts of water. It seems likely therefore that copper- catalysed polymerisation of diazo-alkanes involves a propagating 'free radical' suitably stabilised as a copper(1) complex e.g.(RCH,CH,.)CuX (solvent)=. Many other transition-metal compounds have been used to polymerise diazo- alkane^,^^^^^'^ the most successful (and enigmatic) being nickelo~ene.~~~ Metallic gold surfaces and colloidal gold180s181 have the added advantage of inducing formation of stereoregular182 polyethylidene ; mechanisms for these reactions are not known but it is probable that they involve intermediates similar to those discussed above for catalysis by copper compounds. 10 Reactions of Diazo-alkanes with Protic Acids The basic character of diazo-alkanes derives mainly from the resonance structure R,C-N_N. Thus with a protic acid HB reaction occurs primarily at the nucleophilic carbon atom yielding an alkyl diazonium salt which rapidly de- composes to give the corresponding highly reactive carbonium ion + - H+B- R2C=N2 ___+ R2CHN2+B- b R2CH+B- "2 R2CHNu R2CHB R2CHB1 + H+ 178 C.E. H. Bawn and A. Ledwith Chem. and Ind. 1957 1180; A. Ledwith and A. C. White unpublished results. lso A. G. Nasini and L. Trossarelli J. Polymer Sci. Part C Polymer Syntposia 1965 3 378; A. G. Nasini L. Trossarelli and G. Saini Makromol. Chem. 1961,U-46 5 5 0 ; A. G. Nasini G. Saini L. Trossarelli and E. Campi J. Polymer Sci. 1960 48 435; G . Saini and A. G. Nasini Atti. Accad. Sci. Torino 1955-56 90 586. lal A. Ledwith Chem. and Ind. 1956 1310. la2 C. E. H. Bawn and A. Ledwith Quart. Rev. 1962,26 363. H. Werner and J. H. Richards J. Amer. Chem. SOC. 1968,90,4976. 157 Developments iti the Chemistry of Dinzo-alkanes Reactivity of a series of diazo-alkanes towards a particular acid depends upon the nature of the groups (R) and their effect upon the basic character of the a-carbon of the diazo-alkane.Thus substituting aryl groups for the hydrogen atoms in diazomethane decreases reactivity as will carbonyl and alkoxycarbonyl groups. Nevertheless rapid and quantitative esterification of organic acids by diazo-alkanes is a very common reaction and is frequently used for estimation of the latter. A recent variation of the reaction of diazo-alkanes with strong acids provides a convenient spectrophotometric procedure for assay purposes.' 83 The diazo-alkane is allowed to react with the pyridinium perchlorate (84) to give a quantitative yield of a quinonoid dye (85) absorbing in the region 5190- 5880 A W (84) (Ar = 4-nitrophenyl) Ar-CH N-CH2 R More generally pyridines quinolines and isoquinolines are readily converted into corresponding N-methyl quaternary salts by reaction with diazomethane in the presence of fluoroboric acid.ls4 Deamination of aliphatic primary aminesIs5 and protonation of the corre- sponding d i a z o - a l k a n e ~ ~ ~ ~ ~ * ~ ~ have long been known to give rise to very similar reaction products presumably via the common diazonium ion.Very recently Friedman and his co-workers*8s have made an elegant study of the fate of the isobutyl cation (88) formed both by deamination of 2-methyl l-propylamine (86) and by acid-catalysed decomposition of 2-methyl-l-diazopropane (87) in common solvent systems. Isobutyl cation (88) gives mainly hydrocarbon products (89-93) after formation from either (86) or (87) 18* R.Preussemann H. Hengy H. Druckrey Annalen 1965,684 57. M J. H. Ridd Quart. Rev. 1961 15 418. lE6 L. Friedman and J. H. Baylen J. Amer. Chem. SOC. 1969 91 1790; L. Friedman A. T. Jurewicz and J. H. Bayless ibid. 1969 91 1795; L. Friedman and A. T. Jurewicz ibid. 1969 91 1800 1803 1808. R. Daniels and C. 0. Kormenoy J. Org. Chem. 1962 27 1860. 158 Cowell and Ledwith + Me CHCI-I D-+>+\+\ +w (89) (90) (91 1 (92) (93) When the diazo precursor (94) was thermally decomposed in the presence of stoicheiometric quantities of HOAc and DOAc in aprotic solvents the product composition was identical with that from reaction of (86) and its -NDz analogue with octyl or amyl nitrites. By careful analysis of the deuterium content of the hydrocarbon products from these and related reactions it was shown that if the alkyl group is primary diazo-alkanes are intermediates in thermal de- composition of nitrosoamides in poorly solvating media.The extent of diazo- alkane formation diminishes with an increase in solvating power of the medium so that in protic media such as aqueous acetic acid there is no evidence for its intermediacy at all. Under all the conditions of reaction products arise from the diazonium species formed either by protonation of the intermediate diazo- alkane or directly from precursor nitrosoamine or ni trosoamide i.e. + - Me2 CH-CH2 -N=N-OAc 7 2 Me2CH-CH2N,0Ac MegH-CH=N2 products 159 Developnients in the Chemistry of Diazo-alkanes Product composition varies with solvent reflecting probable solvation charac- teristics of the diazonium ion pairs ( 9 3 and related reactions of s-alkylamine derivatives proceed without formation of the appropriate diazo-alkane probably because of the very short lifetime of secondary diazonium ions.lS6 There is only limited evidence' 86a regarding lifetimes of s-alkyl diazonium ions (R2CHN2+) but it is well known that substitution in the opposite sense i.e.with electron-withdrawing groups enhances the lifetime of the diazonium unit. Stable alkyl diazonium ions are formed when there are strongly electron withdrawing and conjugating substit~ents.~~~ The simplest alkyl diazonium ion to be characterised is 2,2,2-trifluoroethyl diazonium ion188 (CF3CH2N2+) formed by dissolving CF3CHN2 in FS03H at -60" (half life ca. 1 hr). A most interesting application of the reactivity of diazo-alkanes towards protic acids is the formation of 'hot' methyl halides (96) by gas phase reactions of diazomethane with hydrogen chloride hydrogen bromide and hydrogen iodide.x CH2N2 + HX -N [.H3X] D2 b DCH2X + HD collisional deactivation I CH3 X The reactions are homogeneous quantitative extremely rapid and involve polar transition states although it is not clear whether these should be linear or cyclic.189 Overall the reactions are exothermic by approximately dHfO for diazomethane and consequently the methyl halide is generated in a high-energy vibrationally-excited state reflected in deuterium exchange between nascent [CH,X]* and D2 or CD diluents. Experiments of this type are important as test reactions for theories of unimolecular decomposition and energy transfer between vibrational and translational states.One of the most important reactions of diazomethane (methylation) depends on its ability to react with a weakly acidic hydrogen atom in enols lactams thiolactams e t ~ . ~ ~ ~ e ~ ~ ~ The reaction is especially useful for sensitive compounds because of high yields and simplicity of reaction products facilitating work-up procedures. If tautomeric or potentially tautomeric compounds (97) are treated with diazomethane two reactive centres are available and the methylated 186 (a) H. Maskill R. M. Southam and M. C. Whiting Chem. Comm. 1965 496. Reimlinger Angew. Chem. Internat. Edn. 1963 2 482. 18@ S. H. Bauer D. Marshall and T. Baer J . Amer. Chem. SOC. 1965 87 5514; J. C. Hassler and D. W. Setser ibid.1965 87 3793. K. Bott Angew. Chem. Internat. Edn. 1964 3 804 Tetrahedron 1966 22 1251; H. J. R. Mohrig and K. Keegstra J . Amer. Chem. SOC. 1967 89 5492. 160 CoweN and Ledwith products are frequently different from those which are obtained by other methods e.g. Y=R-X-H + CH2N2 + (97) - Y=R-X - Y-R=X + MeN * Y=RXMe i- MeY-R=X Collapse of the methyl diazonium @-pair (98) gives kinetically controlled products rather than the thermodynamically more stable methyl derivatives obtained with other methylating agents. There are many examples117 of pro- nounced solvent effects on product composition from reactions of diazomethane with tautomeric systems and there has been much discussion of relative Bronsted acidity and of electrostatic factors in controlling product d i ~ t r i b u t i o n .~ ~ ~ ~ From present-day knowledge of polar intermediates it would appear that the mechanism and product distributions are a consequence of solvation and dis- sociation equilibria of ion-pairs,l 92 together with relative nucleophilicities and steric effects of ambident anions.lg3 However more detailed kinetic work is needed for a complete understanding of these factors in controlling methylation. Two recent studies are pertinent. Hammond and Williamslg4 reinvestigated the reactions between diazomethane and acetylacetone in diethyl ether. In agreement with results of earlier workers the main product was shown to be the enol ether (99). However there was a small but significant yield of 3-methyl acetylacetone (loo) suggesting involve- ment of the symmetrical ion pair (101) as common intermediate.Addition of toluene-p-sulphonic acid to diazomethane in ether caused rapid formation of methyl toluene-p-sulphonate and polymethylene but in the presence of acetylacetone concomitant alkylation occurred yielding (99) and (100). Formation of higher alkyl ethers was not observed even when poly- methylene was formed,194 and hence the ion-pairs (101) must be of the ‘intimate’ or ‘contact’ type. 92 lgO F. Amdt B. Eistert R. Gompper and W. Walter Chem. Ber. 1961 94 2125. lgl R. Gompper Adv. Heterocyclic Chem. 1964 2 245. lg2 S. Winstein B. Appel R. Baker and A. Diaz Chem. SOC. Special Publ. No. 19 1965 p. 109; M. Szwarc Accounts Chem. Res. 1969 2 87. lQ3 J. 0. Edwards and R. G. Pearson J. Amer. Chem. SOC. 1966 84 16; N. Kornblum Trans. N. Y. Acad. Sci. 1966 29 1.lu4 G. S. Hammond and R. M. Williams J. Org. Chem. 1962 27 3775. 161 Developments in the Chemistry of Diazo-alkanes Kornblum and Coffeylg6 made a thorough reinvestigation of the alkylation of 2-pyridone (102) with diazomethane. Contrary to the widely accepted view that this reaction yields exclusively oxygen methylation (103) it has now been shown that nitrogen methylation (104) predominates in the solvent systems MeOH-Et,O and CH,CI,-Et,O. 4- MeN2 + CH2N2 ... *.= Me Me Me QMe H H OMe I Me I Me<=CHCOMe Me-CO-CH-COMe With diazomethane the product ratio N-alky1ate:O-alkylate was 1.7 whereas with diazoethane the value was 0.36 indicating that the oxygen end of the 2- pyridone anion was less sterically hindered and hence more readily attacked by ethyl diazonium ion. In recent years a substantial effort has been devoted to elucidation of the kinetics and mechanism of acid-cataIysed reactions of more stable diazo-alkanes particularly diphenyl diazomethane and diazoacetic ester in both protic and aprotic media.The topic has been extensively reviewed by More O'Ferra111g6 and will therefore be treated only briefly. Pioneering studies by Roberts and his ~ ~ l l a b o r a t o r ~ ~ ~ ~ in the early nineteen fifties had established that decomposition of diphenyl diazomethane catalysed by strong acids involved rate-determining proton transfer to the a carbon atom. N. Kornblum and a. P. Coffey J. Org. Chem. 1966 31 3447. R. A. More O'Ferrall Adv. Phys. Org. Chem. 1967 5 331. le7 J. D. Roberts and W. Watanabe J. Amer. Chem. SOC. 1950 72 4869; J. D. Roberts W. Watanabe and R.E. McMahon ibid. 1951 73 760 2521; J. D. Roberts and C. M. Regan ibid. 1952 74 3695. 1 62 Cowell arid Ledwith Subsequent loss of nitrogen yields benzhydryl cation which reacts with the diazo precursor to give tetraphenylethylene or with protic solvent (ROH) to give benzhydrol or benzhydryl ethers. Essentially similar steps were observed for catalysis by weak acids except that a major product was the benzhydryl ester of the weak acid. Significantly however salts of the weak acid had no effect on the product ratio ester ether. Later work showed that this product ratio was also insensitive to changes in reaction tem- perat~u-e,~~~ and moderate changes in the reactivity of the catalysing acid.lDD These experimental observations which imply that the ester is not formed from dissociated anions of the acid and that the product-partitioning occurs via steps of low activation energy may be rationalised in terms of competing ion-pair return and reaction.lsa~lOO Direct proof of the intervention of ion-pairs was obtained by Diaz and Winsteinaoo from the reaction of diphenyl diazomethane with l *O-labelled p-nitrobenzoic acid in ethanol.Competition between ion-pair + b Ph2CH Slow b Ph2CHNz + -N2 Ph2CN2 + H+ H+ + Ph2C=CPh2 Ph2CHOR + H+ return dissociation and ethanolysis of *O-labelled benzhydryl p-nitrobenzoate had previously been established by Goering and Levy201 by comparison of rates of 180-scrambling and acid production. Using similar techniques Diaz and Winstein200 showed that within probable experimental error the same ion-pair (or spectrum of ion-pairs) was involved in decomposition of the diazo-compound + PhzCNz + ArCOOH ___+ Ph2CHN;O2CAr + - Ph2CHO-C-Ar Ph2 CH O--C-Ar _I Ph2 C€I+ + ArCO II 1 6 6 ;I l80 ]Et0H (Ar- = p-nitrophenyl Ph2 CHOEt + ArCOOH 1Q8 K.Bowden A. Buckley N. B. Chapman and J. Shorter J. Chem. SOC. 1964 3380. lB0 R. A. More O'Ferrall Wo Kong Kwok and S. I. Miller J. Amer. Chem. SOC. 1964 86 5553. 2oo A. F. Diaz and S. Winstein J. Amer. Chem. Soc. 1966 88 1318. H. L. Goering and J. F. Levy J. Amer. Chem. SOC. 1962,84 3853. 163 Developments in the Chemistry of Diazo-alkanes This work also confirms the previous assumption that nitrogen evolution from a secondary diazonium benzoate ion-pair is very much faster than its dissociation. General acid catalysis via rate-determining proton transfer is now widely accepted as the primary step in acid-catalysed decomposition of diaryl diazo- methanes in protic and the reactions have found extensive use in studies of polar and steric effects with particular reference to linear free energy relationships.202 For aprotic solvents reaction characteristics are generally similar,199~203-206 with occasional complications arising when the ester formed is itself solvolytically unstable.For example the reaction between diphenyl diazomethane and toluene-p-sulphonic acid in ether solvents gives quantitative yields of the highly reactive benzhydryl toluene-p-sulphonate (109 Ph2CNz + HOTos _j Ph,CHOTos + N2 (105) and constitutes the most convenient synthetic route to this reactive The same ester is formed in acetonitrile solvent but the increased ionising and dissociating power of this solvent drastically shortens its half life.2o3 Conse- quently kinetic studies of the toluene-p-sulphonic acid-catalysed decomposition of diphenyl diazomethane in a~etonitrile~~~ are complicated by rate-limiting ionisation of the rapidly formed ester (105).In contrast to the diaryl diazomethanes reactions of diazoacetic ester (N,CHCO,Et) involve specific acid catalysis,208 with pre-equilibrium proton transfer between acid and diazoester forming ethyl glycollate (106) in water and the corresponding ethyl ether in ethanol. EtOCOCHN2 + Hi- p EtOCOCH,N,+ -+ 4 2 EtOCOCH2+ j H2O I H2O + EtOCOCH20H + H+ (106) + (or EtOCOCHz OH2 ) Evidence for specific acid catalysis includes solvent kinetic isotope effect D,O/H,O = 2.9 at 25" and the fact that deuterium exchange of the alpha 2ozA.Buckley N. B. Chapman and J. Shorter J. Chem. SOC. (B) 1968 195; A. Buckley N. B. Chapman M. R. J. Dack J. Shorter and H. M. Wall ibid. 1968,631 ; N. B. Chapman J. R. Lee and J. Shorter ibid. 1969 769; see also ref. 54b. 203 D. Bethel1 and J. D. Callister J. Chem. SOC. 1963 3801 3808. 204 D. Bethel1 and R. D. Howard J. Chem. SOC. (B) 1968,430; Chem. Comm. 1966,94. zo5N. B. Chapman A. Eshaw J. Shorter and K. J. Toye Tetrahedron Letters 1965 1049. 206 F. Klages K. Bott P. Hegenberg and H. A. Jung Chem. Bet-. 1965 98 3765. 207 A. Ledwith and D. J. Morris J. Chem. SOC. 1964 508. 208 R. P. Bell 'Acid Base Catalysis' Oxford Univ. Press London 1941 p. 100; J. D. Roberts C. M. Reagen and I. Allen J. Amer. Chem. SOC. 1952 74 3679. 164 Cowed and Ledwith hydrogen atom is more rapid than the overall rate of hydrolysis.209 Again in contrast to the reactions of diphenyl diazomethane addition of other nucleo- philes (e.g.chloride ion) permits trapping of the reaction intermediate and it is clear from the kinetic data that loss of nitrogen from the diazonium ion requires assistance from solvent or added nucleophile210 ( s N 2 process). Similar effects have been observed in acid-catalysed decomposition of ct-diazo-ketones211 and ct-diazo-sulphones.212 It should be observed that diazomethane and the lower diazo-alkanes undergo acid and base catalysed deuterium exchange with D20 more rapidly than esterification or decomy~sition.~~~ 11 Lewis-acid-catalysed Alkylation of Alcohols and Amines Normally the -OH and -NH groups in alcohols and amines are not sufficiently acidic to react with diazo-alkanes.The reactions may however be catalysed by Lewis Acids such as aluminium alkoxides boron trifluoride aluminium chloride and by fluoroboric acid,1° e.g. BF3 R20H + R1CHN2 +R20CH2R1 + N2. For alkylation of alcohols the most effective catalysts are boron t r i f l ~ o r i d e ~ ~ ~ - - and fluoroboric acid218 in ether and methylene chloride solvents respectively. A wide range of primary secondary and tertiary alcohols have been converted into corresponding alkyl ethers in this way; yields are good for methylation with diazomethane but are seldom higher than 50 % for higher diazo-alkanes.'O For catalysis by fluoroboric acid in methylene chloride,218 relative rates of methylation (isomeric butanols) were in the order primary :secondary :tertiary = 2.2 1.3 1.0 whereas for boron trifluoride in ether216 (isomeric pentanols) the corresponding reactivities were 1.7 1-55 1 indicating great similarities in the two processes and low selectivity by the reagents.Primary and secondary amines are alkylated in a similar manner,217 except that in this case catalysis by fluoroboric acid corresponds with the use of pre- formed amine salt (107) as the reactant 209 P. Gross H. Steiner and F. Krauss Trans. Faraday SOC. 1938 34 351. 210 W. J. Albery and R. P. Bell Trans. Faraday SOC. 1961 57 1942; W. J. Albery J E. C. Hutchins R. M. Hyde and R. H. Johnson J. Chem. SOC. (B) 1968,219. 211 J. B. F. N. Engberts N. F. Bosch and B. Zwanenburg Rec. Trav. chim. 1966 85 1068; H. E. Baumgarten and C. H. Anderson J. Amer. Chem. SOC.1961 83 399; H. Dahn and H. Gold Helv. Chim. Acta 1963 46 983; H. Dahn A. Donzel A. Merbach and H. Gold ibid. 1963 46 994; D. M. Jordan Din. Abs. 1966 26 3633; S. Aziz and J. G. Tillett Tetrahedron Letters 1968 2321; J. Chem. SOC. (B) 1968 1302. 212 B. Zwanenburg and J. B. F. N. Ehgberts Rec. Trav. chirn. 1965 84 165; Tetrahedron 1968,24 1737. 213K. J. van der Merwe P. S. Steyn and S. H. Eggers Tetrahedron Letters 1964 3923; W. Kirmse and H. A. Rinkler Annalen 1967 707 57; A. Bhati J. Chem. SOC. 1963 729. 214 E. Miiller M. Bauer and W. Rundel 2. Naturforsch 1959 14b 209. 21s E. Miiller and W. Rundel Angew. Chem. 1958,70 105. 216 E. Miiller R. Meischkeil and M. Bauer Annalen 1964 677 55. 217E. Miiller W. Rundel and H. Huber-Emden Angew. Chem. 1957 69 614; E. Miiller and H. Huber-Emden Annalen 1961,649,70; E.Miiller H. Huber-Emden and W. Rundel ibid. 1959 623 34. 218 M. Neeman M. C. Caserio J. D. Roberts and W. S. Johnson Tetrahedron 1959 6 36; J. Amer. Chem. SOC. 1958 80 2584. 165 Developments in the Chemistry of Diazo-alkanes BF3 R'NHz + R'CHN2 __+ R2NH-CHaR' + N2 H + +/ (107) \ R22NH2BF4- + R'CHNZ * R22N BF4- CH2R1 Several mechanisms have been proposed and it may be that distinct processes operate according to the particular catalyst. Fluoroboric acid would logically be expected to function by primary protonation of the diazo-alkane yielding the corresponding diazonium ion (108) as discussed in the previous section i.e. ROH HBFI + CH2N2 __+ CHaNa+BFI- + ROCH3 + HBF4 (108) -N2 Miiller and his c o l l a b o r a t o r ~ ~ ~ ~ ~ ~ ~ ~ have made extensive use of alkylations catalysed by boron trifluoride and suggest that the reaction mechanism is similar to that originally proposed by Meerwein and Hintz2l9 for catalysis by aluminium alkoxides viz + CH2N2 R-O-Me + ROH + BF3 ,A R-0-H -I 4 2 - I BF BF3 ROMe + BF3 + f RzNH f BF3 & R2-NH CHzN2 b R2NMe c5 R2NMe + BF3 ,I 4 2 -I BF3 BF3 A third has already been discussed (reaction E p.151) in connec- tion with the mechanism of boron trifluoride catalysed polymerisation of diazo- alkanes. Irrespective of the precise reaction mechanism for alkylation it is bound to have a bearing on the related boron trifluoride catalysed polymerisa- tions (and homologation of ketones) and in fact low yields of alkylated products invariably imply significant concurrent polymerisation of the diazo-alkane.12 Reactions of Diazo-alkanes with Phosphines It has been known for many yearsZ2O that diazo-alkanes react with phosphines forming phosphazines (109) 21e H. Meerwein and 0. Hintz Annalen 1930 484 1 . 230 H. Staudinger and J. Meyer Helv. Chim. Acra. 1919 2 619 635; H. Staudinger and G. Luscher ibid. 1922,5 75. 166 Cowell and Ledwith I - + R ' 2 C-N=N-PR2 3 However in the presence of copper(1) chloride similar reactions with Ph,P lead to corresponding phosphorus ylides (110) and the process may be used as a one-step synthesis of olefins from ketones,221 i.e. RI2C=CRJ2 + Ph3P0 Product olefins were obtained in ca. 30% yields the remaining diazo-alkane forming phosphazine. 13 Health Hazards in Use of Diazo-alkanes The explosive nature of pure samples of diazo-alkanes has already been referred to and is widely recognised.Pharmacological effects have also been noted in previous surveys.2s62p116 In recent years however there has been a growing recognition that diazomethane and its nitrosourethane precursor are active carcinogenic materials.222 It appears that carcinoma of the lungs and stomach may result from inhalation of the vapours of the lower diazo-alkanes and their volatile precursors. Whilst there is yet no published evidence for carcinogenic activity in the wider range of diazo-alkane precursors or in the more stable diazo-alkanes it may be that this results from lack of experimentation rather than lack of The authors thank Dr. D. Bethel1 for many helpful suggestions and discussions. 221 G. Wittig and M. Schlosser Tetrahedron 1962 18 1023. a22H.Marquardt F. K. Zimrnermann and R. Schwaier Naturwiss 1963 50 625; R. Schoental and P. W. Magee Brit. J. Cancer 1962 16 92; I. J. MiPahi and P. Emmelot Cancer Res. 1962,22,339; R. Schoental Nature 1961,192,670; 1960,188,420; R. Schoental Acta. Unio. Intern. Contra Cancrum 1963 19 680. 238 C. E Searle Chem. in Britain 1970 6 5. 1 67

ISSN:0009-2681    

DOI:10.1039/QR9702400119

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

INDEXES Volume 20 1966 Contains the cumulative indexes for the volumes 1 to 20 The indexes in this issue cover volumes 21 22 23 and 24 INDEXES Volume 20 1966 Contains the cumulative indexes for the volumes 1 to 20 The indexes in this issue cover volumes 21 22 23 and 24 Index INDEX OF AUTHORS Abel E. W. 23 325; Abraham E. P. 21 231 Anbar M. 22,579 Ashby E. C. 21,259 Bamford C. H. 23,271 Barefield E. K. 22,457 Barnes A. J. 23,392 Bartlett P. D. 24,473 Beech G. 23,410 Berkoff C. E. 23,372 Blackburn E. V. 23,482 Blandamer M. J. 24 Bottomley F. 24,617 Boyd D. R. 22,95 Bransden B. H. 21,474 Brett N. H. 24,185 Brocklehurst B. 22 147 Bruce J. M. 21,405 Buchanan J. G. St. C. Buckingham A. D. 21 Buckingham J. 23,37 Buncel E. 22,123 Burgess J. 22,276 Busch D. H. 22,457 Bushby R. J.24,585 Cadogan J. I. G. 22 Chambers D. B. 22,31 Cherry R. J. 22,160 Christian S. D. 24,20 Clark R. G. 24,95 Clive D. 22,435 Cookson R. C. 22,423 Cornforth J. W. 23 125 Cowell G. W. 24,119 Cox R. A. 22,499 Dale B. W. 22,527 Davidson R. S. 21 249 Dickens P. G. 21,30 Drake J. E. 24,263 Eschenmoser A. 24,366 Evans U. R. 21,29 Fensham P. J. 21,507 Fox M. F. 24,565 Gash B. W. 24,20 Geddes R. 23 57 Gill G. B. 22 338 24,498 1 69 23 522 195 222 Glockling F. 22,317 van Gorkom M. 22,14 Hall G. E. 22,14 Hallam H. E. 23,392 Hill J. 23 18 Hopkinson S. M. 23,98 Horspool W. M. 23 Howe A. T. 21,507 Hughes M. N. 22,l Jefferson A. 22,391 Jones J. H. 22,302 Jones W. J. 23,73 Keenan A. G. 23,430 Kerr J. A. 22,549 Klingsberg E. 23,537 Lambert J. D. 21,67 Lederer E. 23,453 Ledwith A. 24,119 Lee A. G. 24,310 Lee J.B. 21,429 Light J. R. C. 22,317 Lloyd A. C. 22,549 Luckhurst G. R. 22 Luz Z. 21,458 MacKenzie K. J. D. 24 McAuley A. 23,18 McCaffery A. J. 23,552 McDonald T. R. 24 McFarlane W. 23,187 McKervey M. A. 22,95 McLennan D. J. 21 Milne G. W. A. 22 75 Muetterties E. L. 21 Miiller E. W. 23,177 Nelson S. M. 22,457 Norbury A. H. 24,69 Norris A. R. 22,123 Nyholm R. 24,1 O’Connor C. 24,553 Orr B. J. 21,195 Parker W. 21,331 Pelletier S. W. 21,525 Penzer G. R. 21,43 Pethrick R. A. 23,301 Pietra F. 23,504 204 179 185 238 490 109 Pollock J. M. 24,601 Pope M. T. 22,527 Radda G. K. 21,43 Rahman R. 24,208 Ramage R. 21,331 Riddell F. G. 21,364 Riddle C. 24,263 Rigby M. 24,416 Roberts J. S. 21,331 Robinson D. L. 21,314 Ruff I. 22,199 Russell K. E. 22,123 Safe S. 24,208 Salmond W. G. 22,253 Sammes P.G. 24,37 Schatz P. N. 23,552 Scheinmann F. 22 391 Sharp J. H. 24,185 Sheldrick B. 24,454 Shorter J. 24,433 Siegmund R. F. 23,430 Silver B. L. 21,458 Singer K. 24,238 Sinha A. T. P. 24,69 Sklarz B. 21,3 Sternhell S. 23,236 Stewart E. T. 24,95 Stone F. G. A. 23,325; Symons M. C. R. 22 Taha A. A. 24,20 Taylor A. 24,208 Theobald D. W. 21 Thompson C. 22,45 Timmons C. J. 23 482 Uff B. C. 21,429 Waley S. G. 21,379 Walker D. C. 21,79 Weatherston J. 21,287 Wehry E. L. 21,213 Weiss F. 24,278 Whittingham M. S. 22 Williams I. 23 1 Williams R. J. P. 24,331 Winstein S. 23,141 Woodgate P. D. 23,522 Wright C . M. 21 109 Wyn-Jones E. 23 301 24,498 276 314 30 641 Index INDEX OF TITLES Acetylenes base catalysed isomeri- sation of 24 585 Alkaloids the chemistry of the C20- di terpene 22 525 C-Alkylation some problems con- concerning biological reactions and phytosterol biosynthesis 23,453 Amide hydrolysis acidic and basic 24 553 Amino-acid and peptide derivatives mass spectra of 22 302 Amlidentate ligands the co-ordination of 24,69 Ammonium perchlorate thermal de- composition of 23,430 Aqueous salt solutions structure and properties of 24 169 Aromatic nitro-compounds inter- action of with bases 22 123 Aromatic substitution reactions mechanisms for nucleophilic and pho tonucleophilic 23 504 Arthropod defensive substances the chemistry of 21 287 Arylhydrazones the chemistry of 23 37 Biogenesis sesquiterpene 21 331 Biological pigments semiconduction and photoconduction of 22 160 Biosynthesis some problems con- cerning biological C-Alkylation re- actions and phytosterol 23 453 Biosynthesis starch 23 57 Carbanion mechanism of olefin- forming elimination 21,490 Carbonyls the chemistry of transition metal structural considerations 23 325 CENTENARY LECTURE.Field-ion micro- scopy and the electronic structure of metal surfaces 23 177 CENTENARY LECTURE. Mechanisms of cyclo addition 24,473 CENTENARY LECTURE. Nonclassical ions and homoaromaticity 23 141 CENTENARY LECTURE. Roads to corrins 24 366 Cephalosporin C Group 21 231 Chemistry of arthropod defensive substance 21 287 Asymmetric synthesis 22,95 Claisen rearrangement molecular re- arrangements related to the 22 391 Clemmensen reduction of difunctional ketones 23 522 Computers in chemical analysis appli- cation of amino-acid analysis and sequence determination 24 454 Conformational analysis heterocyclic 21 364 Co-ordination number molecular polyhedra of high 21 109 Crystals liquid as solvents in nuclear magnetic resonance 22 179 Decomposition reactions of radicals 22,549 Diazo-alkanes developments in the chemistry of 24 119 Diffusion in ionic solids 24 601 Diterpene alkaloids the chemistry of the Go 21 525 Electron the hydrated 21,79 Electronic structure field-ion micro- scopy and the of metal surfaces 23 177 Electrons reactions of hydrated with inorganic compounds 22 579 Electron-transfer theory of thermal reactions in solution 22 199 Electron spin resonance chemical applications of oxygen-17 nuclear and 21,458 Electron spin resonance of the triplet state 22,45 Electronic properties of binary com- pounds of the first-row transition metals 21 507 Electrophilic oxygen organic reactions involving 21,429 Elementary particles 21,474 Elements nuclear spin-spin coupling between directly bound 23 187 Energy transfer vibration-vibration in gaseous collisions 21 67 Enzyme action mechanism of 21,379 Enzyme mechanisms exploration of 23 125 Enzymes mechanism of act ion and specificity of proteolytic 23 1 Faraday effect 23 552 642 Index Field-ion microscopy and the elec- tronic structure of metal surfaces 23 177 Free radicals hydrogen abstraction in the liquid phase by 21,249 Glycosides the chemistry and bio- chemistry of phenolic 23,98 Grignard reagents.Compositions and mechanisms of reaction 21 259 Heterocyclic conformational analysis 21 364 Hormones the chemistry and bio- chemistry of insect 23 372 Hydrated electrons reactions of with inorganic compounds 22,579 Hydrazine the reactions of transition- metal complexes with 24 617 Hydrides of silicon and germanium with elements of groups V and VI volatile compounds of the 24 263 Hydrogen abstraction in the liquid phase by free radicals 21 249 Hydrous layer silicates and their related hydroxides the thermal decomposition of 24 185 Inert gases the reactions of ions and excited atoms of the 22 147 Infrared studies of matrix-isolated species 23,392 Inorganic compounds reactions of hydrated electrons with 22 579 Insect hormones the chemistry and biochemistry of 23 372 Internal rotation the determination of the energies associated with 23 301 Interproton correlation of spin-spin coupling constants with structure 23,236 Ions nonclassical and homoaroma- ticity 23 141 Ions and excited atoms of the inert gases the reactions of 22 147 Ion-solvent and ion-ion interactions by magnetic resonance techniques study of 22 276 Iron cobalt and nickel complexes having anomalous magnetic mo- ments 22,457 Isoalloxazines (Flavins) the chemistry and biological function of 21 43 Isopoly-vanadates -niobates and -tantalates 22 527 Hyponi tri tes 22 1 Ketones the Clemmensen reduction of difunct ional 23 522 Lasers 23,73 Light-induced reactions of quinones 21,405 Liquid phase hydrogen abstraction in the by free radicals 21,249 Macromolecular structure and pro- perties of ribonucleic acids 22,499 Magnetic moments iron cobalt and nickel complexes having 22 457 Magnetic resonance techniques study of ion solvent and ion-ion inter- actions by 22 276 Mass spectra of amino-acid and pep t ide derivatives 22 302 Mass spectra of organometallic com- pounds 22 317 Mass spectroscopy application of Matrix-isolated species infrared stu- dies of 23 392 Metal complex formation some recent studies in the thermodynamics of 23,410 Metal-ion kinetics and mechanism of complex formation in solution 23 18 Molecular complexes of water in organic solvents and in the vapour phase 24,20 Molecular hyperpolarisabilities 21 195 Molecular rearrangements related to the Claisen rearrangement 22 391 Natural product synthesis photo- chemical reactions in 24 37 Nuclear and electronic spin resonance chemical applications of oxygen-1 7 21 458 Nuclear magnetic resonance equi- valence of nuclei in high resolution 22 14 Nuclear magnetic resonance liquid crystals as solvents in 22 179 Nuclear spin-spin coupling between directly bound elements 23 187 Nucleophilic and photonucleophilic aromatic substitution mechanisms for 23 504 Niobates isopoly-vanadates and -tantalates 22,527 high resolution 22,75 643 Index Olefin-forming elimination carbanion mechanism of 21,493 Organic chemistry of periodates 21 3 Organic reactions involving electro- philic oxygen 21,429 Organo-metallic compounds mass spectra of 23,317 Organothallium chemistry 24 310 Oxygen organic reactions involving electrophilic 21,429 Oxygen-17 nuclear and electron spin resonance chemical applications of 21,458 PEDLER LECTURE.Exploration of en- zyme mechanisms by asymmetric labelling 23 125 Periodates organic chemistry of 21 3 Phenolic glycosides the chemistry and biochemistry of 23,98 Phosphorus reagents reduction of nitro- and nitroso-compounds by tervalent 22,222 Photochemical behaviour of transition- metal complexes 21,213 Photochemical reactions in natural Photochemistry of some allylic com- pounds 22,423 Photocyclisation of stilbene analogues 23,482 Phytosterol biosynthesis some pro- blems concerning biological C- alkylation reactions and 23 453 Polar steric and resonance effects in organic reactions the separation of linear free energy relationships Polymerisation solid-phase addition 23,271 Polysulphides the stereochemistry of 24,208 1 ,2-QuinonesY synthetic; synthesis and thermal reactions 23,204 Quinones light-induced reactions of 21,213 Radicals decomposition reactions of 22,549 Reduction of nitro- and nitroso- compounds by tervalent phosphorus reagents 22,222 RNA macromolecular structure and properties of 22,499 Rusting the mechanism of 21 29 product synthesis 24,37 by the use of 24,433 Silicon and germanium with elements of groups V and VI volatile com- pounds of the hydrides of 24 263 Simple inorganic anions in solution the photolysis of 24,565 Simple liquids the study of by computer simulation 24,238 Solid-phase addition polymerisation 23 271 Solvents liquid crystals as in nuclear magnetic resonance 22 179 Stilbene analogues the photocyclisa- tion of 23,482 Sulphur heterocycles valence shell expansion in 22,253 Tantalates isopoly-vandates 40- tates and 22 527 Tetracyclines chemistry of 22 435 Theory of thermal electron-transfer reactions in solution 22 199 Thermal decomposition of ammonium perchlorate 23,430 Thermodynamics some recent studies in the of metal complex formation 23,410 Thiothiophthene no-bond resonance compounds 23 537 TJLDEN LECTURE.Biochemistry of sodium potassium magnesium and calcium 24 331 TJLDEN LECTURE. Photochemistry of some allylic compounds 22 423 Transition-metal carbonyls the chem- istry of; structural considerations 23 325 Transi tion-met a1 carbonyls the chemistry of synthesis and reacti- vity 24,498 Transition-metal complexes photo- chemical behaviour of 21 213 Transition-metal complexes some Transition metals electronic pro- perties of binary compounds of the first-row 21 507 Trimethylenemethane and related a$,-disubstituted isobutenes 24 278 Tungsten bronzes and related com- pounds 22 30 Vibration-vibration energy transfer in gaseous collisions 21,67 Starch biosynthesis 23 57 Synthesis asymmetric 22,95 perfluoro ligands of 2491 644 Index Valence-shell expansion in sulphur Wave functions for small molecules heterocycles 22 253 based on linear combination of Vanadates iso-poly- -niobates and atomic orbitals 24 95 dantalates 22 527 Woodward-Hoffmann orbital sym- Van der Waal’s fluid a renaissance metry rules to concerted organic 24 416 reactions application of 22 338 645

ISSN:0009-2681    

DOI:10.1039/QR9702400639

出版商:RSC    

年代:1970

数据来源: RSC