ISSN:0069-3030    

DOI:10.1039/OC97774FX001

出版商:RSC    

年代:1977

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC97774BX003

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

11 12 M. B. Hursthouse and S. Neidle MULTAN procedure. It seems that these methods have the attraction of relative simplicity and do actually solve difficult structures.' Of especial interest to the non-theoretical crystallographer is an analysis of a number of 'failures' of crystal- structure analysis;' it is apparent that exercise of a non 'black-box' mentality in the application of the direct methods computer programs available can often pay dividends in the solution of such structures. Some even small structures are it seems inherently impossible to solve by simple approaches. A procedure to over- come these problems has been suggested,' which 'should be able routinely to solve structures with 100 atoms' 2 Electron Density Distribution More papers have described the determination of electron density distributions in molecules.X-N maps for sulphamic acid using X-ray and ncutron data collected at 78 K show strong bond populations in all bonds with peaks in the S-N and S-0 bonds similar to peaks in bonds between first row atoms only." Broad peaks for the lone pairs on oxygen atoms suggest a situation intermediate between sp and sp2 hybridization and the absence of any build-up of electron density in the H.-O bonds supports the idea of an electrostatic model. For the molecule of ['H12]-2,5- dimethylhex-3-yne-2,5-diol the deformation density maxima are 0.58 and 0.36 e A-3 for the triple and single C-C bonds but only 0.09 for the C-0 single bonds." In the molecule of tetraphenylbutatriene the central C=C bond length is 1.260A in contrast to the outer bonds whose lengths are 1.348 A.12aThe defor- mation densities in the butadiene skeleton show that the .rr-lobes in adjacent C=C bonds are mutually perpendicular.The deformation densities in allene carboxyl and amide groups have also been determined for a complex between allenedicar- boxylic acid and acetamide.126 In the molecule of sym -dibenzo-cyclo-octa-13-diene-3,7-diyne (l),the electron density around the centre of the triple bond is asymmetric with a sIight accumulation of charge inside the ring. l3 3 Molecular Conformations The incorporation of potential energy calculations into structural studies is also attracting more and more attention. One of the main problems in this kind of work is the reliability and suitability of the interatomic potential functions used in the calculations and attempts are being made to determine these experimentally.An example of this approach is the calculation of such functions via an analysis of ' P. Main Actu Cryst. 1978 A34 31. L. Lessinger Acta Cryst. 1976 A32 538. M. M. Woolfson Acta Cryst. 1977 A33 219. lo J. W. Bats P. Coppens. and T. F. Koetzle Actu Cryst. 1977 B33 37. '' R. B. Helmhold and A. Vos. Acta Cryst. 1977 A33 456. (a)Z. Berkovitch-Yellin and L. Leiserowitz Acta Cryst. 1977 B33 3657; (b)Z. Berkovitch-Yellin,L. Leiserowitz and F. Nader ibid. p. 3670. l3 R. Destro T. Pilati and M. Simonetta. Acfa Crysf.. 1977 B33 447. Physical Methods-Part (ii) X-Ray Crystallography crystal lattice vibrational frequencies in some hydrocarbons.l4 Applications of potential energy calculations have been described for the system l-ethyl-l-methyl- 4-piperidinium perchlorate where deformations of some interbond angles in the cation have been correlated with particular van der Waals' and for the adrenaline In this latter case the conformation of the molecule found in some structures does not correspond to a minimum in the free-molecule con- formational energy map and it has been shown that the lattice energy compensates for the loss in conformation energy. Structure analyses of some polyphenyls have also raised some interesting conformational points. At room temperature the crystal structure of p-terphenyl apparently contains only one molecule per asym- metric unit in which some of the atoms show very large thermal motion.On cooling two of the unit cell edges double up to give a superstructure where the end rings of the molecule are stabilized into one of the two potential wells associated with each C-C single bond.I7" Similar effects are found for p-quaterphenyl"' and biphenyl. 17' A topic in many ways related to the study of molecular conformation is that of the geometry of strained molecules particularly those containing ring systems. Classic examples of strained aromatic molecules are the cyclophanes and studies of meta-I' and para-cyclophane derivatives have been described. In the centrosym- metric quadruple-layered cyclophane (2),19the two outer benzene rings are boat- shaped and the inner rings adopt a novel twist shape.(21 In the crowded fulvalene l-methyl-2-(9'-fluorenylidene)-1,2-dihydropyridine,*' the twist about the linkage bond is 35.7' indicating a significant contribution from the form (3). Other examples of strained-ring molecules which show bond lengthening and/or angle strain are the A-C'-C stereoisomer of dodecahydro- trypticene,*' the triterpene oxide companulin (4),22 and 6,9-diaza-5,10-l4 T. L. Starr and D. E. Williams Acta Cryst. 1977 A33 771. Is W. Fedeli F. Mama E. Giglio C. Quagliata and N. Scarcelli Actu Cryst. 1976 B32 878. 16 J. Caillet P. Claveni and B. Pullmann Acru Crysf.,1976 B32 2740. l7 (a) J. L. Baudoir Y. Delugeard and H. Caillean Acta Cryst. 1976 B32 150; (6)Y.Delugeard J. Desucher and J. L. Baudoir ibid..p. 702;(c) G. P. Charbonneau and Y. Delugeard ibid.,p. 1420. 18 Y. Kai N. Yasuoka and N. Kasai Actu Cryst. 1971 B33 754. H Mizuno K. Nishiguchi T. Toyoda T. Otsubo S. Misurni and N. Morimoto Acta Crysr. 1977 B33 329. 2o H. L. Arnmon Acru Cryst. 1976 B32 2693. 21 A. Albinati J. Briickner and G. Allegra Actu Crysf. 1977.B33 229. '' F. Mo Acra Cryst. 1977.B33 641. M. B. Hursthouse and S. Neidle dioxotricyclo[7,3,0,0'.6]dodecane (5) a molecule designed to contain non-planar amide In the overcrowded but planar tetramethylcytoxine molecule (6) Me ,Me Yd the steric strain arising out of the retention of the sp2 geometry for the dimethyl- amino nitrogen atom and the Me...Me clash is taken up by the spreading of the angles at N-4 C-4 and C-5.24 In our last Report we mentioned three papers which described the correlation of crystal structure of some carboxylic acids with their reaction with amines and ammonia and another example of this has been given.In the structures of (S)-(+)-2,2-diphenylcyclopropanecarboxylicacid and (R)-( +)-2,2-diphenyl-1-methylcyclopropanecarboxylicacid the crystal packing particularly the closed environment of the polar groups explains the observed ditropic attack by NH3 gas on single In a similar study26 the crystal structure analyses of dimethyl rneso-(2R,3S,4R,5S)-and rneso-(2R,3R,4S,5S)- a,a'-dimethyl-P,P'-dibromoadipate have assisted in the interpretation of the reac- tion of single crystals of the two compounds with gaseous NH3and amines to yield the corresponding diesters of (E,E)-and (E,Z)-butadiene by double dehy- drobromination.In both cases the reaction is topochemically controlled in that the configurations of the products correlate directly with the conformation of the starting molecules in their crystals. Other examples of this very interesting kind of study are the solid-state dehydration of 2-hydroxy-2-(P-benzoy~-~-phenyl-hydrazy1)indane- 1,3-dione [(7a) +(7b)],*' where compound (7a) also appears to be the first example of the type of intermediate found in reactions of carbonyl 23 S. E. Ealick and R. Vander Helm Acru Crysf..1977 B33 76. 24 J. K. Dattaguysta W. Saenger K. Bolewska and I. Kulakowska Act0 Cryst. 1977 B33 85. 25 C. C. Chiang C.-T. Lin A. H.-J. Wand D. Y. Cuttin and I. C. Paul.J. Amer. Chem. Soc. 1977 99 6303. 26 D. Rabinovich and Z. Shakked Acru Cryst. 1977 B33 809. 27 S. A. Puckett I. C. Paul and D. Y. Curtin J. Amer. Chem. Soc. 1976.98 787 2371. Physical Methods-Part (ii)X-Ray Crystallography compounds with arylhydrazines and related compounds and the conversion in the solid state of the yellow form of 2-(4’-methoxypheny1)- 1,4-benzoquinone into the red form.28 In this latter reaction the process begins at a limited number of nucleation sites in the crystal and spreads by migration in well-defined fronts and the colours of the two forms correlate with the different types of packing adopted particularly in relation to intermolecular .rr-interactions. Peptide-containing antibiotics continue to be of major structural interest partly on account of their interesting conformational and hydrogen-bonding properties.[Phe4 Va16]antamanide is a synthetic analogue of the cyclic decapeptide antamanide that possesses biological activity in respect of ion selectivity. It appears to have a novel type of (5 -+ 1)NH...O=C hydrogen bond.29 An X-ray study of a degradation product has finally confirmed the structure of the complex antibiotic vancomycin (8);30 suggestions have been made for a possible binding mode to peptides terminating in the residues D-alanyl-D-alanine a crucial process in its biological action. An a-helical conformation has been observed31 for the amide units in the crystal structure of N-acetylactinobolin. 28 G. R. Desirajin I. C. Paul and D. Y. Curtin J.Amer. Chem. Soc. 1977 99. 1594. 29 I. L. Karle,J. Amer. Chem. SOC.,1977,99 5152. 30 G. M. Sheldrick P. G. Jones 0. Kennard D. H. Williams and G. A. Smith Nufure 1978,271 223 31 R. B. Von Dreele Actu Cryst. 1976 B32 2852. 16 M. B. Hursthouse and S. Neidle The linear tetrapyrrole bilirub has a compact conformation in the solid state which is stabilized by intramolecular hydrogen bonds,’ a feature shown by both indepen- dent molecules in the asymmetric unit. This structure is not dissimilar to that of a biladiene analogue (9)32-both have the two planar pyrromethane groups hinged together at right angles. Me Me Me Me Me (9) The remarkable crystal structure of anhydrous cholesterol contains eight independent molecules in the asymmetric unit,33 a total of 224 non-hydrogen atoms.Although the skeletal frameworks are all the same there are considerable variations in the tail conformations. The packing of molecules in the crystal lattice differs from that in cholesterol m~nohydrate,~~ which has a bilayer-type structure with the rings almost parallel to one another. An X-ray analysis of 3B-acetoxy-17aa-(2-acetoxyethoxy)- 17a 17ao- dimethyl -D-homoandrost-5 -en -176-01 (10) has disproved the notion of isolating rotamers that have restricted rotation about the steroidal C-17-C-20 bond.35 The structure of prostaglandin FZa(11) has two OCH,CH ,OAc OH (10) (1 1) independent molecules in the asymmetric unit both have the side-chains aligned roughly A model for the biological recognition by prostaglandin recep- tors has been suggested on the basis of this structure which explains much of the binding data.The structures of a number of nucleic acid constituents have been reported some of which are notable for their broader biological significance and interest. A new pattern of hydrogen-bonding has been observed in the structure of 5-nitro-6-rnethylura~il,~~ which has adjacent bases perpendicular to one another in contrast 32 G. Struckmeier U. Thewalt and J. Engel J.C.S. Chem. Comm. 1976 963 ” H. S. Shieh L. G. Hoard and C. E. Nordman Nature 1977,267 287. 34 B. M. Craven Narure 1976,260 727. 35 Y. Osawa T. Makino and C. M. Weeks J.C.S. Chem. Comm. 1976,990. ” D. A. Langs M. Erman and G. T. DeTitta Science 1977 197 1003. ’’ R.Parthasarathy and T. Srikrishnan Acta Cryst. 1977 B33 1749. Physical Methods-Part (ii) X-Ray Crystallography to the normal parallel arrangements. The structures of the hypermodified nucleosides t6A and g6A (the former occurs in the anticodon loop of tRNA) have conformational alterations in this loop that depend on the codon being read. The interactions of drugs with nucleic acids have been investigated via the structures of several model systems such as ethidium-dinu~leoside~~*~~ and 9-aminoacridine-dinucleoside41complexes and an extrapolation from these models to the polymers has been attempted.42 The crystal of a proflavine-dinucleoside phosphate (CpG) has however cast some doubt on the general validity of these conclusions; in particular the conformational changes induced in the miniature double helix can be related to even small alterations in the base pair geome try.44 38 R.Parthasarathy J. M. Ohrt and G. B. Chheda Biochemistry 1977 16,4999. 39 C.C.Tsai S. C. Jain and H. M. Sobell J. Mol. Biob 1977 144 301. 40 S. C.Jain C. C. Tsai and H. M. Sobell I.Mol. Biol.. 1977 144 317. T. D.Sakore S. C. Jain C. C. Tsai and H. M. Sobell Proc. Nut. Acad. Sci. U.S.A.,1977,74 188. 41 42 H. M.Sobell C. C. Tsai S. C. Jain and S. G. Gilbert J. Mol. Biol. 1977 114,'333. 43 S. Neidle A. Achari G. L. Taylor H. M. Berman H. L. Carrell J. P. Glusker and W. C. Stallings Nature 1977 269 304. 44 H.M.Berman S. Neidle and R. K. Stodola Proc. Nut. Acad. Sci. U.S.A. 1978,75 828.

ISSN:0069-3030    

DOI:10.1039/OC9777400011

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (iii) Ultraviolet and Visible Spectroscopy of Bio-organic Molecules By P.-S. SONG Department of Chemistry Texas Tech University Lubbock Texas 79409 U.S.A. 1 Introduction Although recent developments in various spectroscopic properties of organic molecules have been reviewed in these Reports there has not been a specific review of the literature on the U.V. and visible spectroscopy of bio-organic mole- cules in recent years. The present Report attempts to review selected literature in this field. In reviewing the literature luminescence spectroscopy of biomolecules will be largely excluded unless it is directly relevant to the discussion of the u .v.-visible spectroscopy of molecules under consideration. Furthermore inter- molecular interactions (e.g.chlorophyll aggregates excitons complexes etc.) and effects of proteins on u.v.-visible spectra of chromophores (e.g. retinal in rhodop- sin) will not be discussed. 2 Cyclic Polyenes Porphyrins.-Porphyrins including metalloporphyrins can be treated as cyclic conjugated polyenes. Chlorophylls (chl) and bacteriochlorophylls (Bchl) are struc- turally and spectroscopically derived from the chlorin and bacteriochlorin chromophores respectively. The absorption spectra of these compounds are characterized by visible bands in the 500-800 nm region and near-u.v. bands (Soret) in the region of 350450nm. The latter are considerably more intense (E =1-3 x lo5,f=1-2) than the former (E = 1-8 x lo4,f = 0.01-0.3) par-ticularly in symmetrical porphyrins.This spectral shape contrasts with the well known spectral feature (i.e.intense visible band with weak or no near-u.v. bands) of linear polyenes such as carotenoids. The genealogy of theoretical models for explaining the visible (and u.v.) spectral characteristics of porphyrins is as follows Free-electron perimeter model of Simpson (1949) and Kuhn (1949) 1 Huckel molecular orbital (MO) model of Longuet-Higgins Rector and Platt (1950) 1 Four-orbital configuration interaction (CI) model of Gouterman (1959) 18 Ph ysica 1 Methods-Part (iii) Spectroscopy of Bio-organ ic Molecules 19 1 SCF MO CI Pariser-Parr-Pople model of Weiss Kobayashi and Gouterman (1965) 1 All-valence-electron MO model of Schaffer and Gouterman (1972) 1 Ab initio SCF CI model of Christoffersen and his co-workers (1977)' In the free-electron perimeter model porphyrin of D4hsymmetry can be viewed as an 18~-electron cyclic polyene with an approximately circular field around the perimeter.The highest occupied (orbital angular momentum quantum number q = f4) and lowest empty orbital (q = f5) levels are doubly degenerate. Between these degenerate levels four excited electron configurations are possible account- ing for T -+ T* transitions to doubly degenerate B,, (from Aq = f1 configura-tions) and QX. (from Aq = *9 configurations) states. These transitions are assigned to the Soret and visible bands respectively. The degenerate Q and B bands split into Q, Q and B, By,respectively as the symmetry of the metalloporphyrin-like system descends from D4h to DZh(e.g.free-base porphyrin) and C2,(e.g.chlorin). The splits between Q and Q and B and By are about 3000cm-' and 250cm-' respectively as predicted from semi- empirical SCF MO CI calculations including electron correlation.2 Previous MO calculations and ab initio CI' tended to overestimate the B,-By split. In addition to the four-state (Q,, and BX.,)absorption spectrum one prominent vibronic satellite appears for each Q 0-0 and Q 0-0 origin thus producing the well known visible bands I(C?, 0-0) II(Qx 1-0) III(Q,,,0-0) and IV(Q, 1-0) for free-base porphyrins. Four similar bands (I-IV) appear in the spectra of chlorins and bacteriochlorins except that Bands I and I11 are designated as Qy(O-0) and Q (0-0) respectively in accordance with the polarization axes.Qualitatively speaking Q,. and B,, bands of porphyrins may be compared with La.b(from Aq = f5 configurations) and Ba,6(from Aq = f1 configurations) bands of naph- thalene and other polyacenes respectively. Although the above model is qualitatively satisfactory in describing the major spectral features of Soret and visible bands in porphyrins the weak- and near-u.v. (Nx,,,L,,, M,,, erc.) bands n -+ v* transitions and effects of metal ions are not at all represented. The assignment of bands I1 and IV as vibronic satellites of the Q,. transitions is by no means Alternatively band IV may be assigned to a third 7r + T* transition (e.g. a band at 485 nm with E = 3.4 X lo4 for meso- tetraphenylporphin and at 495 nm for octaethylporphin in the vapour phase).6 The SCF MO CI theory also predicts such a transition,' but ab initio CI theory (see ' (a)J.D. Petke G. M. Maggiora L. L. Shipman and R. E. Christoffersen J. Mof. Spectroscopy. in press; (b)D. Spangler G. M. Maggiora L. L. Shipman and R. E. Christoffersen J. Amer. Chem. SOC. 1977,99,7470,7478. K. Tomono and K. Nishimoto Bull. Chem. SOC.Japan. 1976,49 1179. R. Plus and M. Lutz Spectroscopy Letters 1974 7,73. J. Bohandy B. F. Kim and C. K. Jen J. Mol. Spectroscopy 1973 43 199. A. H. Corwin A. B. Chivois R. W. Poor D. G. Whitten and E. W. Baker J. Amer. Chem. SOC. 1968 90 6577. J. B. Kim J. J. Leonard and F. R. Longo J. Amer. Chem. SOC., 1974,94 3986. ' A. J. McHugh M.Gouterman and C. Weiss jun. Theor. Chim.Acta 1972 24 346. 20 P.-S.Song below) does not support this. Furthermore this assignment is not likely in view of the low-temperature Shpol'skii spectroscopic analysis of the vibronic origin for this band.' The ab initio CI calculations' predict that one electronic transition (to the doubly degenerate 2lE state) in Mg2'-porphyrin accounts for the major Soret intensity while the higher-lying states and several forbidden 7r -+T* transitions make rela- tively minor contributions to the Soret band at 387 nm. The N band at ca. 327 nm is identified as arising from T -+ T* transition to another doubly degenerzte 4lE state. In free-base porphyrin five 7r -+ T* transitions (to 2'B2, 21B3U, llBlu 31Bzu and are predicted to contribute to the Soret band intensity with a maximum at 385 nm.The N band at 315 nm may consist of two allowed transitions 41B3 and 41B2,. Absorption bands of porphyrins in the vacuum-u.v. region are apparently diffuse.' No evidence for Rydberg states was revealed in the vacuum-u.v. spectra of tetraphenylporphin and its metalloporphyrin derivatives." The vapour-phase absorption bands of porphins from 640-210 nm have been assignedxo Q, Qy,B N L,and M(Q B N and L for Mg2'-porphin). The n -+T* transitions in porphyrins including chlorins can originate from the pyridine-like nitrogen n -orbitals and from formyl or aza substituents at the cyclo- pentanone ring and methine bridge positions. A different definition of n -+ T* assignment for porphyrins and chlorins has also been described." A PPP MO variant taking into account n-bonding orbital electrons predicts an n + T*tran-sition of free-base porphin at 316 nm.I2 Similarly the extended HMO method predicts an n -+ T* transition in the near u.v.' in contrast to the CNDO/S predic-tion at 269 nm for porphins and chlorin~.~~ There is indirect evidence that the Soret band broadening in tetra-azaporphin and phthalocyanine is attributable to the underlying n T* state which is localized at the methine bridge.14 Linear dichroism of free-base and Mn2'-porphyrins shows all transitions in the range 350-900nm to be in-plane polarized," in contrast to an earlier report which showed a negative dichroism at ca.400 nm in tetraphenylporphin. It now appears however that dichroism at 400nm is definitely negativeI6 and that it represents either n -+ T* or a vibronic component of T -+ T* origin.Various studies do not generally support the n +T* assignment for the visible bands I and 111 which has been made by some workers. It is clear that information concerning the location and intensity of n +T*bands in porphyrins (and chlorophylls see below) is either lacking or only tentative. However it is quite likely that 'n T* state(s) lie in the vicinity of the Soret and near U.V.region resulting in band broadening of the T +7r* transitions." Ab initio CI calculation also predicts an n -+ T* (1'B1,) transition under the Soret band.' A. T. Grudyushko K. N. Solov'ev and A. S. Starukhin Optika i Spectroskopiya 1976,40,267.B. H. Schechtman and W. E. Spicer J Mol. Spectroscopy 1970 33.28. lo L. Edwards D. H. Dolphin M. Gouterman and A. E. Adler J. Mol. Spectroscopy 1970,35,90; ibid. 1971 38 16. S. G. Boxer G. L. Closs and J. J. Katz J. Amer. Chem. SOC.,1974,96 7058. l2 M. Sundbom Acta Chem. Scand. 1968,22 1317. l3 G. M. Maggiora and L. J. Weimann Chem. Phys. Letters 1973 22 297. 14 M. Gouterman in 'Excited States of Matter' ed. C. W. Shoppee Texas Tech. University Press Lubbock Texas 1973 p. 63. R. Gale R. D. Peacock and B. Samori Chem. Phys. Letters 1976 37,430. l6 B. Norden and A. Davidson Chem. Phys. Letters 1976 37,433. K. N. Solov'ev A. T. Gradushko and M. P. Tsvirko J. Luminescence 1976,14 365. Physical Methods-Part (iii) Spectroscopy of Bio-organic Molecules 21 The visible spectrum of porphin with its characteristic four-band (I-IV) system changes to the typical D4h-porphyrin spectrum of two-band (aand /3 bands) upon metal substitution,I8 as indicated earlier.The effects of different metal substitu- tions on the porphyrin absorption spectrum are partly attributed to perturbations on orbital energy levels of bl (or a2 in symmetry notation) cl(e,) and c2(eg) molecular orbitals which are substantially localized on the central N atoms. Effects of different metal ions on the D4h porphyrin spectrum are generally not drastic with some exceptions such as Mn2'- Fe2'- and Fe3'-porphyrins. Band I (Qx,0-0) usually shifts to the red with increasing atomic number of the central metal ion but spectral shifts of Q tA and B tA bands in both direction and magnitude are determined by ligand (Lkporphyrin interactions in L-Me-porphyrin-type complexes (for example see ref.5). Also the Bx-By band split by fifth and sixth ligands (e.g. cytochome c) could be larger than the Qx-Qysplit in contrast to the case of free-base porphyrin. In general high-spin metalloporphyrins show stronger charge-transfer (CT) bands than low-spin complexes as has been found for Fe2'-tetraphenylpor- phyrin." CT bands arising from the occupied porphyrin MO (azuin particular) to the empty metal d-orbitals (e,) may occur throughout the near-u.v.-near-i.r. region depending on the strength of ligand field and the nature of ligands. For example porphyrin + Fe3' CT bands have been assigned for Fe3'-myoglobin using polarized absorption spectroscopy and the weak band at 695 nm in Fe3+-cyto- chrome c (low spin) was found to be out-of-plane polarized consistent with the ligand (azu)-+metal (d,z; al,) CT transition,*' whereas ligand (a2,or ulu)-+ metal (d ; e,) CT transitions are predicted to occur in the near-u.v.region (320400 nm) in Cr3+-tetraphenylporphyrinin chloroform.21 The d -+ d transitions are generally so weak that their resolution is usually masked by the strong porphyrin bands. Metalloporphyrins of d3,d4,d6 and ds configurations show d -+ d transitions of low frequency in the near-i.r. or i.r. region (e.g. Cr3"-tetraphenylporphyrin21),as can be predicted from porphyrin being a weak-field ligand. The d + d transitions for various metalloporphyrin camplexes have been assigned but the assignment of these transitions are often not unam- biguous owing to CT and porphyrin bands which are more intense.It is well known that the visible band Q +A of porphyrins with D4, symmetry can be described as a planar oscillator as a consequence of the two-fold degeneracy for Qx,ystates. Thus the degree of fluorescence polarization with respect to the visible absorption in porphyrins is independent of excitation wavelengths within the visible band and is in the neighbourhood of f a value characteristic of a planar oscillator.22 The fluorescence polarization degree increases substantially as the porphyrin symmetry is lowered from D4,,to D2,,or Cz0 as in free-base porphins and chlorins respectively.The SztSo transition (Q in free-base porphyrins and 0 in hydrogenated porphyrins) is then polarized perpendicular to the S1tSo absorption or fluorescence polarization axis yielding a negative degree of M. Gouterrnan L. K. McCaffery and M. D. Rowe J.C.S. Dalron 1972 596. l9 H. Kobayashi and Y. Yanagawa Bull. Chem. SOC.,Japan 1972,45,450. 2o W. A. Eaton and R. M. Hochstrasser J. Chem. Phys. 1967,46 2533; ibid. 1968,49,985. M. Gouterman L. K. Hanson G. E. Khalil and W. R. Leenstra J. Chem. Phys. 1975,62 2343. 22 G. D. Yegorova V. A. Mashenkov K. N. Solov'ev and N. A. Yushkevich Biofizika 1973,18,40. 22 P.-S. Song fluorescence polarization. However fluorescence polarization does not yield absolute orientations of these transition moment dipoles.The lowest transition 0 tA in free-base porphins is theoretically predicted to be polarized along the NH-NH axis (x) and the Q +A transition is perpen- dicular to the former. This is consistent with the effects of substitution along the x-and y-axes on the hyperchromicity of the Q (band I) and 0,(band 111) bands respectively and with the polarized reflectance spectroscopy of free-base tetra- phenylporphin single and the polarized quasi-line absorption spectrum of Zn2’-porphin in triphenylene.24”5 Vibronic coupling between electronic states (e.g. Q state with higher electronic states) can be invoked to explain mixed polarization characteristics of the porphyrin absorption bands. According to the vibronic theory the Q 1-0 band gains its intensity from the y-component of the Soret transitions through bl vibration-induced mixing whereas the al* vibiation mixes the x-component of the Soret band with the Q 1-0 band.26 High-resolution vibrational analysis of the absorp- tion spe~tra,~’-~’ site-selection fluorescence spectroscopy,28 and resonance Raman ~pectroscopy~~ provide powerful tools to carry out vibronic analysis of the visible and Soret bands of metalloporphyrins.Fluorescence polarization degree varies over the Soret band of porphins (e.g. octaethylporphin) suggesting that there are two more or less perpendicularly polarized transitions to B and Bystates,” but other electronic transitions which are not readily resolved by the fluorescence polarization technique are also likely to contribute to the Soret band as mentioned earlier.Ch1orophylls.-Chls and Bchls show absorption spectra similar to those of chlorin and bacteriochlorin respectively. Chl-a shows the characteristic four visible bands system [band I at 660 nm= 0 (0-0); I1 at 615 nm = Q (1-0); 111 at 575 nm= Q (0-0); IV at ca. 530 nm = Q (1-0)] and the Soret band (429- and 410 nm) with absorbance peak ratios of 1.0:0.16 0.1 :0.06 1.31:0.87 in ether. The oscillator strength f,of the Q (0-0) averaged from absorption spectra in different solvents is 0.151.30 Attempts have been made to assign the u..v. bands of chls in the 200410 nm regi~n,~’,~~ but they remain to be established. The Q +A band of Bchl-a occurs at 769 nm and the Soret bands at 392 and 357 nm with absorbance peak ratios of 1:0.55 :0.8 in ether.Protochl-a chl-a and Bchl-a assume approximately circular triangular and rectangular v-electron fields respectively. Thus the Q tA band shows a pro- gressive red shift whereas the Q +A band shows a relatively small blue shift. The Qy-Q split is also expected to increase in going from protochl-a to Bchl-a. 23 B. G. Anex and R. S. Umans J. Arner. Chem. SOC.,1964,86 5026. 24 B. F. Kim J. Bohandy and C. K. Jen Specrrochirn. Acta 1974,30A 2031. 2s B. F. Kim and J. Bohandy J. Mol. Spectroscopy 1977,65,90. 26 Y.J. Aronowitz and M. Gouterman J. Mol. Spectroscopy 1977,64 267. 27 J. A. Shelnutt D. C. O’Shea N. Y. Yu. L. D. Cheng and R. H. Felton J. Chern. Phys. 1977,66,3387. 28 J. Funfschilling and D. F. Williams Photochem. and Photobiol.1975 22 151; ibid. 1977 26 109. 29 T. G.Spiro Accounts Chem. Res. 1974,7 339. 30 L. L. Shipman Photochem. and Photobiol. 1977 26 287. 31 C. Weiss jun. J. Mol. Spectroscopy 1972. 44 37. 32 P.-S. Song T. A. Moore and M. Sun in ‘The Chemistry of Plant Pigments’ ed. C. 0.Chichester Academic Press New York,1972 pp. 33-74. Physical Methods-Part (iii) Spectroscopy of Bio-organic Molecules 23 Configuration analysis of wavefunctions for chl-a and Bchl-a suggests that the Q states of these molecules are very similar as the peripheral C=C (3b-4b) bond in chl-a is not locally excited to any appreciable extent. The isocyclic ring C=O group is also unimportant in substantially perturbing the Q +A transition of chlorin and bacteriochlorin macrocycles.Furthermore the vinyl substituent does not significantly contribute to the Q state functions of chls. These analyses are therefore consistent with the use of the four-otbital model of porphyrins for chls in that the low-energy electronic states Q,, and BX7,can be described by two degenerate MOs of the macrocylic ring.33 It is possible that a shallow hump at 325 nm in the absorption spectrum of chl-a is an n + T* band. Since cyclopentanone shows the n + n* band in this general region this possibility is energetically feasible. However the intensity at 325 nm is too high to be attributed. to an n + T* band of chi-a although perturbations generated by nearby states can enhance its intensity considerably. It should also be noted that in deuterioporphyrins N eA (n+ r*)transitions with E S lo4occur in the same region (320-350 nm).15N N.m.r. studies” suggest that the lowest n + T* transition is localized in ring IV of chl-a and pheophytin-a but the suggestion that the n + n* transition might occur near the Q band with relatively strong intensity has not been directly confirmed. The absorption band maximum (I,Q, 0-0) of chl-a is polarized (Po> 0.4) parallel to the fluorescence emission oscillator and its polarization approximately corresponds to the y-axis according to the PPP MO calc~lation.~~ Band I1 (Q, 1-0) at 615 nm is made up of two components one at ca. 632 nm with Po-0 and another at 613 nm with Po-0.28. These bands may be assigned to Q (1-0) and Q (2-0) respectively. If this assignment is correct the 575 nm band with Po -0.03 is most likely assigned to Q (0-0) (band 111).M.c.d. results are consistent with the assignment here.35 Pheophytin-a chlorin and porphins at 77 K also show negative polarization for this band. The Q tA transition is thus polarized (approximately along the x-axis) nearly perpendicular to the 0 tA tran~ition.~~ However Shipman et assign the 635 nm component (or ca. 632 nm34) of chl-a in ethanol at 77 K to the Q,(O-0) on the basis of spectral analogies with Bchl-a and an apparent shift of the room-temperature absorption band 111(Q, 0-0) to ca. 635 nm at 77 K. Although this is possibly due to the effect of an anisotropic polarizability of the medium with increased refractive index at 77 K preferentially red-shifting the Q (0-0) band the Q (1-0) assignment for this band with a negative polariza- tion degree is more likely on the basis of the following observation.High-resolution fluorescence excitation and polarization spectra of chl-a in ether at 15 K revealed a well-resolved peak at 632 nm.37 If this peak were Q,(O-0) one would not see its mirror image in the fluorescence spectrum. However the 723 cm-’ vibrational mode of the 632 nm excitation band shows its mirror image (729 cm-’) in the fluorescence emission spectrum at 15 K. 33 P.-S.Song C. A. Chin I. Yamazaki and H. Baba Znternat. J. Quantum Chem. Quantum Biol. Symp. 1977,4 305; ibid. 1976,3,89. 34 P. Koka and P.-S. Song Biochim. Biophys. Acta 1977 495 220. ” C. Houssier and K. Sauer J. Amer. Chem. SOC.,1970,92 779.36 L. L. Shipman T. M. Cotton J. R. Norris and J. J. Katz J. Amer. Chem. SOC. 1976,98 8222. ” W. W. Mantulin and P.-S.Song unpublished results. 24 P.-S.Song The minimum polarization at 435 nm in chl-a is attributable to the B +A transition in agreement with theoretical predictions and m.c.d. Absorp-tion and polarization characteristics of protochl-a chl-6 and pheophytins are qualitively similar to those of chl-a. In particular chl-b shows a fluorescence excitation polarization spectrum closely resembling that of chl-a in ether at 77 K except that the Q (0-0) band shows a lower Po value (0.35) than that of ~h1-a.~~ The large red shift of Q relative to that of chl-a is accompanied by increase in the Q,-Q gap which contributes to improved resolution of fluorescence polarization in Bchl-a.Thus Po for Q (0-0) of Bchl-a is 0.4 in ethanol at 77 K whereas Po for Q (0-0) at 625 nm is 0.28.39 The above discussion is based on the simplifying assumption that each of the resolved vibronic bands Q (0-0) and (1-0) Q (0-0) and (1-0) contains no strong contribution due to other overlapping vibronic transitions. However low-temperature site-selection spectroscopy of chl-a and -b in which the fluorescence spectra are resolved into a number of vibronic components28 clearly indicates complexity in interpreting the polarization spectra of chlorophyls and porphyrins. Corrins and Tetrapyrro1es.-Corrins show two visible bands designated as cy -and p-bands along with the intense near-u.v. y-band (==Soret) and moderately intense U.V.bands. The a-and @-bands are the 0-0 and 1-0 components of the first allowed electronic transition. Kuhn (1959) was the first to describe the corrin spectrum in terms of the free electron model. The four-orbital model equivalent to the Gouterman model for porphyrins was proposed by Day4’ within the framework of the HMO and PPP MO approximations. The latter was also applied to corrins by other^.^',^^ The HMO method was found to be inadequate for describing the spectral intensities of corrins. On the basis of these results it is now well established that the visible and near-u.v. band intensities in corrins are mainly of the T +T*type. This conclusion is generally valid for corrinoids such as vitamin BI2 since ‘free-base’ corrins (e.g.descobalt-B12) show the characteristic visible and near-u.v. spectral intensities. The T +T* assignment for the entire u.v.-visible spectrum of free-base corrins is clearly confirmed by the polarized phos-phorescence excitation spectrum of descobalt-B12 which shows Po< 0.1 for all visible and U.V. peaks with respect to the out-of-plane polarized phosphorescence of 372 T* The low-temperature absorption spectrum of descobalt-BI2 in ethanol shows well resolved peaks at 525 493 465(sh) 435(sh) 395 375 358(sh) 326 314 300(sh) 285 280(sh) 268 260(sh) and 238nm with peak ratios of 1 0.76 0.36 0.12 0.16 0.17 0.12 2.09 1.04 0.64 0.48 0.54 1.39 0.76 0.43. Although calculated oscillator strengths are invariably greater than experimental f values the f ratios produced by the PPP SCF MO CI calculation are 1(Q,):0.07(Qx):6.8 (near U.V.&) which agrees with the obser ed ratios of 1:0.07:6.5 in descobalt B12. In analogy to the porphyrin notation we lave assigned 38 C. A. Chin Ph.D. Thesis Texas Tech University Lubbock Texas 1975. 39 T. G. Ebrey and R. K. Clayton Photochem. and Photobiol. 1969,10 109. ‘O P. Day Theor. Chim. Acta 1967,7 328. 41 H. Johansen and L.L. Ingraham J. Theor. Biol. 1969.23 191. 42 P. 0.Offenhartz B. H. Offenhartz and M. M. Fung J. Amer. Chem. SOC.,1970,92 2966. 43 R. D. Fugate C. A. Chin and P.-S.Song Biochim. Biophys. Actu 1976,421 1. Physical Methods-Part (iii)Spectroscopyof Bio-organicMolecules 25 the 525 nm peak and its vibrational satellites at 493,465 and 435 nm to Q 0-0,l-0 2-0 and 3-0 respectively while the 395,375 and 358 nm peaks are designated as Q 0-0 1-0 and 2-0 respectively.The subscripts x and y correspond to the polarization axes.43 The 362 nm maximum may then be designated as a R equivalent. These designations are consistent with the relative polarization data discussed below. The lowest-energy transition Q, arises mainly from the 7(HOMO) -+ 8(LEMO) configuration while Q and B transitions are contributed to mainly by 6 -D 8 and 7 +9 configuration The fourth (S4tSo)and fifth (SseSo)transitions are mainly due to 7 +10 and 6 +9 configurations respectively. The latter is polarized nearly parallel to the Q,-axis in descobalt BI2,whereas the S4transition is polarized between the x-and y-axes.43 The predicted polarization axes for Q and Q bands are the y-and x-axis respectively.The fluorescence polarization spec- trum of desc0ba1t-B~~ shows that Q and B transitions are polarized perpendicular to the Q reminiscent of the chl-a Although the linear dichroism of vitamin BI2in stretched poly(viny1 alcohol) film is too small to deduce the absolute polarization axes of the visible and near-u.v. bands,4s the negative dichroism at the near-u.v. band is indicative of the perpendicular polarization of the B transition with respect to the Q polarization direction. Both ~.d.~~*~~~~~ and m.~.d.~' spectra also suggest this polarization picture. The absolute polarizations of the absorption spectrum of a single-crystal Ni2'-corrin (nirrin) indicate that the visible band is polarized along the y-axis and the near-u.v.band is oppositely polari~ed.~' Thus the polarized reflection spectroscopy result is consistent with the PPP prediction^.^^ Biliverdin an open tetrapyrrole bile pigment derived from haems shows an absorption spectrum similar to those of porphyrins with the visible band maximum at 670 nm (E -1.1x lo4) and a near U.V. band at 376 nm (E -3.8 x lo4) in water (pH 11.8) at room temperat~re.~~ The absorption bands are not resolved. The low-temperature absorption spectrum of biliverdin in ethanol at 77 K resolves shoulders at 660 nm (f = 0.14) and 435 nm (f-0.09) along with maxima at 707 nm (f = 0.19) and 380 nm (f = 1.06)?' These spectral characteristics are reminiscent of the spectra of porphyrins.For descriptive purposes we will use the porphyrin notations Qx., for the visible and near-u.v. ('Soret') bands of bili~erdin.~' Thus the long-wavelength band at 707 nm is designated as Q which is predominantly due to the 16(HOMO)+ 17 (LEMO) configuration. The second band at 660 nm resolved in the low-temperature ethanol glass is largely contributed by the 14 -+ 17 configuration while the strong near-u.v. transition arises from the 16+18 configuration is predicted to occur between the Q and near-u.v. regions. It is generally agreed that the conformation of biliverdin is neither fully linear nor fully c~clic.~~*~' The most likely conformation is probably 'semi-circular' since 44 A. J. Thompson J. Amer. Chem. SOC.,1969 91 2780. 45 R. Eckert and H.Kuhn 2.Elektrochem. 1960,64 356. ''R. Bonnett D. M.Godfrey V. B. Math P. M. Scopes and R. N. Thomas J.C.S. Perkin I 1973 252 47 B. Briat and C. Djerassi Nature 1968 217 918. 48 B. G. Anex and G. J. Eckhardt Abstracts Symposium on Molecular Structure and Spectroscopy Columbus Ohio 1966. 49 G. Blauer and G. Wagniere J. Amer. Chem. SOC. 1975 97 1949; ibid. 1976,98 7806. Q. Chae and P.-S. Song J. Amer. Chem. SOC.,1975,97,4176. P.-S.Song the oscillator strength ratio fvisible/fnear u.v. is inconsistent with either extreme conformation. It should be noted that this ratio is a sensitive measure of the chromophore conformation with the maximum ratio that for linear polyenes and the minimum ratio that for cyclic polyenes (see above). In addition the fluorescence excitation polarization spectrum of biliverdin yields a value of ca.SO"for the angle between the polarization axes of the visible and near-u.v. bands consistent with a semi-circular c~nformation.~' The spectroscopic assignments and conformational analysis of other phycobilin chromophores are now in progress using the methods described for biliverdin (J. Jung and P.-S. Song to be published). The absolute polarization directions of the visible and near-u.v. absorption bands are not known. However approximate polarization directions for these bands have been deduced from the linear dichroic data which show negative and positive dichroism for the 675 and 370nm bands respectively of biliverdin in stretched PVA film." By assigning the PVA stretching axis to the long axis of the semi- circular conformation of biliverdin the polarization of the visible band at 675 nm has been determined to be along the A-C ring axis while the 380nm band is polarized perpendicular to the former.It should be noted that a number of simplifying assumptions are involved in these deductions although the relative polarizations of these two bands are consistent with the polarized fluorescence excitation spectrum mentioned earlier. Further refinements in the data analysis of linear dichroic spectra and a polarized single-crystal spectroscopic study are needed for definitive assignments of the polarization axes. 3 Linear Polyenes Simple Po1yenes.-In contrast to cyclic polyenes linear polyenes show strong absorption at the first absorption band with considerably weaker absorption at shorter wavelengths.Until recently the electronic spectra of simple linear poly- enes of conjugated C=C bonds (N =2-6 where N is the number of conjugated C=C bonds) have been 'satisfactorily' described in terms of free-electron and simple MO theories. For a polyene within the 'four-orbital' framework (e.g.butadiene) the electronic transitions arise from electron configurations corresponding to the change in orbital quantum number An = 1 for N(HOM0) +(N+l)(LEMO) and An = 2 for (N-1)+(N+ 1) and N+ (N+2) configurations. The degeneracy of the latter excitations (c.f.Coulson-Rushbrooke theorem) is lifted via configuration inter- actions between them. It is well known that the first transition 'B t'A (or 'Bt'A in Platt notation) is strongly electric-dipole allowed with its polarization direction along the long molecular axis.Both oscillator strength [f -0.74 for all-trans-butadiene -3.4 for all-trans-@-carotene] and A max increase with con- jugation. The latter converges to an asymptotic value of 610 nm for 'infinitely' long polyenes as a result of C-C and C=C bond-order alternations which restrict the electron motion in a one-dimensional periodic potential and/or v-electron cor- relation effects." The 'A tA and 'A +A transitions are symmetry-forbidden but the former can be readily observable even in trans-polyenes and it becomes strongly A. Ovchinnikov I. I. Ukrainskii and G. V. Kventsel Sou. Phys. Uspekhi,1973 15 575. Physical Methods-Part (iii) Spectroscopy of Bio-organic Molecules 27 allowed in cis-polyenes ('cis peak') due to the loss of the centre of symmetry.The doubly forbidden (g +g,-+-) 'A +-A transition is mainly the result of a doubly excited configuration. In contrast to the 'B,state which is ionic in charac- ter the 'A excited state is covalent with a high degree of double-bond character for the C-C single bond relative to the 'B or A ground state. The long held traditional assignment of polyene spectra described above has been questioned on both experimental and theoretical grounds in recent years.52 The new assignment puts the 'A; excited state below the strongly allowed 'B state The lowering of the 'A state arises from configuration interactions among singly and multiply excited ~onfigurations.~~*~~ The quantum mechanical prediction for the location of 'A is by no means definitive at the present state of compu-tation since both ab initio and semi-empirical MO-CI (singly plus doubly excited configurations included) used involve several approximations that may affect the accuracy of the predicted location of the 'A; state; some of these approximations include neglect of the 0-core and its redistribution restriction of basis set etc.It is also possible that the order of the 'A state reverses upon inclusion of triplyexcited configurations in the CI calculations. Nonetheless the new order for the 'A state is strongly supported on both theoretical and experimental grounds as mentioned below. The gas-phase study of cis and trans-hexa-l,3,5-triene showed no detectable absorption on the red edge of the main band and that results of PPP-type cal- culations including only singly excited configurations matched more closely the experimental U.V.spectra than the singly +doubly excited configuration interaction calculations (see Gavin and Rice in refs. 52 and 55). Karplus et al? subsequently argued that the failure to detect the 'A absorption band in the gas-phase spectrum of the hexatriene is not inconsistent with the original PPP (single+double CI) prediction for the low-lying 'A state since the transition to 'A occurs too close to the allowed 'B band and it is symmetry-f~rbidden.~~ Apparently such a 'masking' is not a problem for diphenylpolyenes at low temperature where the weak absorption has been seen.52 The low-lying 'A state is consistent with the valence-bond picture of polyenes in contrast to the free-electron and simple LCAO MO methods.The valence-bond picture indicates that the 'A,-state is lowered in energy as the conjugation of polyenes is extended and that the 'A +-A transition may occur below the 'B tA tran~ition.~~ The assignment of 'A to the lowest excited IT T* state has a profound impli- cation in re-interpreting spectroscopic and photochemical properties of linear polyenes. The former includes several anomalies (e.g. almost no overlap between absorption and emission bands) which had been largely explained on the basis of Franck-Condon arguments (e.g. distortion of the polyene molecule by a low- frequency mode).In addition to the near 'non-overlap' of the 'B,+-A absorp-tion and the fluorescence bands the spectroscopic anomalies of polyenes include 52 (a) B. Hudson and B. Kohler Ann. Rev. Phys. Chem. 1974 25 437; see original references therein (1972-1974); (6)R. L. Christensen and B. E. Kohler J. Phys. Chem. 1976,80,2197. '' R. J. Buenker and J. L. Whitten J. Chem. Phys. 1968,49 5381. 54 K. Schulten and M. Karplus Chem. Phys. Letters 1972 14 305. " M. Karplus R. M. Gavin jun. and S. A. Rice J. Chem. Phys.. 1975.63 5507. 28 P.-S.Song (a) anomalously long T~ and (b) apparent differential solvent shifts of the main 'B absorption and fluorescence bands. Thus the former shifts to the red as the solvent polarizability (n -l)/(n +2) increases but the fluorescence maximum of diphenyloctatetraene shifts to the red only ~lightly.'~ As a result the Stokes shift ranges from 2000 to 6000 cm-'.In general polyenes show a large Stokes shift of 3000-7000~m-'.~~ Along with such a large Stokes shift there is an apparent separation between the 'B t-A 0-0 and the first observed line of the fluores- cence. This separation is understandable in mixed-crystal and Shpol'skii quasi-line spectra of diphenyloctatetraene at 1.84.2 K.52 A series of new quasi-lines can be observed between the red edge of the absorption band and the blue edge of fluorescence under high-resolution conditions. Such weak quasi-lines are not attributable to the 'B tA tran~ition.'~ These new absorption lines are rich in low-frequency vibrations (<500 cm-').The weak sharp absorption shows an apparent 0-0 frequency at 452.24 nm for diphenyloctatetraene in a biphenyl host coincident with the first emission line of the fluorescence spectrum. The weak absorption and fluorescence show an approximate mirror image. In contrast to the strong 'B eA absorption band the weak band cf =0.05f0.02) is relatively insensitive to the solvent matrix. On the basis of this and other less direct evidence the weak absorption has been assigned to the 'A tA transition consistent with the single +double CI PPP calculations. It is concluded that this new assignment is generally valid for all polyenes particularly for diphenylpolyenes of 3-6 conjugated C=C bonds decapentaene carboxylic acid 2,10-dimethylundecapentaene parinaric acid," and retinyl polyenes (see Furthermore it is predicted that the 'A state is lowered in energy with increasing conjugation in contrast to the allowed 'B state which converges to the asymptotic level.The 'A tA absorption and its fluorescence spectrum of 2 lo-dimethylun- decapentaene in n-nonane matrices at 4.2 K have been recorded at high resolution revealing several predominant low frequency non-totally symmetric vibrations (see above). These vibrations apparently induce the forbidden 'A tA transition in agreement with the Herzberg-Teller vibronic theory which predicts the mixing of the closely lying 'A and 'B The fluorescence polarization at the weak edge of the absorption in diphenyl- octatetraene remains high (>0.4) indicating that the transition moment is parallel to the emission and 'B transition moment^.'^ Earlier this was used as an argument against the 'A assignment^.'^ However the polarization data can be accommodated in terms of the Herzberg-Teller-type vibronic mixing of the two states 'A and 'B,.The vibronic mixing is expected to decrease with the extent of conjugation as the gap between the two states increases resulting in a quadratic decrease in the oscillator strength of the former transiti~n.~~'~' In an attempt to obtain an independent evidence for the 'A state assignment in polyene electron scattering (ES) spectroscopy has been applied to hexatriene. A shoulder at 282nm in the ES spectrum increases its intensity as the momentum transfer is raised.This is taken as evidence for the 'A assignment of the lowest 56 M. Kovner Acta Physicochim. U.R.S.S. 1944 19 385. " L. A. Sklar B. S. Hudson M. Peterson and J Diamond Biochemistry 1977,16 813. '* R. L. Christensen and B. E. Kohler J. Chem. Phys. 1975,63 1837. 59 T. A. Moore and P.-S. Song Chem. Phys. Letters 1973 19 128. 6o J. B. Birks and D. J. S. Birch Chem. Phys. Letters 1975 31 608. Physical Methods -Part (iii) Spectroscopy ofBio- organ ic Molecules 29 singlet excited state in polyenes.61 Several attempts to resolve the 'A state of polyenes have also been made without conclusive evidence for or against the assignment by using two-photon absorption for which the 'A tA transition is allowed. In 1,4-diphenylbutadiene it is possible that the two-photon absorption occurs at ca.500cm-' below the 'B state at 347.2nm.62 The 'A state in diphenylhexatriene and diphenyloctatetraene has also been located at 386 and 442.5 nm respectively.62 However two-photon absorption with the thermal blooming technique did not reveal the 'A state for hexa-1 3,5-triene and more recent studies of electron-impact and multiphoton ionization spectra of hexatriene did not provide conclusive evidence for the low-lying 'A state in the 270-300 nm region.63 Nonetheless the failure to observe such a low-lying state in hexatriene does not necessarily entail rejection of the new state ordering in polyenes since the two-photon absorption cross-section could be too small to be measured and/or masked by nearby intense two-photon absorption bands (e.g.Rydberg band). Another line of evidence for the low-lying 'A state has been explored using c.d. and m.c.d. A weak absorption at the red edge of the main absorption band of cycloheptatriene shows an m.c.d. sign opposite to that of the latter. This weak band has been assigned to the 'Ai-type band.52 However m.c.d. has not yielded unambiguous conclusions in several polyene systems including those complexed at inducible optical active sites on proteins. The induced c.d. spectra of a-and P-parinaric acid octadeca-9,11,13,15-tetraenoicacidtbovine serum albumin complexes showed a negative c.d. band at the long-wavelength edge and a positive c.d. band at hma,.64In addition no mirror image between the main absorption and fluorescence spectra was observed.These results are consistent with the 'A assignment for the weak absorption. However interpretation of the induced c.d. spectra is complicated by the exciton contribution arising from inter-chromophore interaction^.^^ It appears that the 'A assignment as the lowest 7r +7r* transition in polyenes is an attractive proposal which is able to accommodate characteristic anomalies (see above) of polyene spectra. There still seem to be some spectroscopic features not readily resolved at present however. These include the apparent absence of the 'A; band intensity in unsubstituted hexatriene in the gas phase uncertainties as to accurate values of the 'A +A oscillator strength and the location of the 'A state in longer polyenes with six or more conjugated C=C bonds.Apparently the end aryl group substitution can affect the relative ordering of 'A and 'B It is concluded that 'A lies below '8,for all orientations of the phenyl group in diphenylhexatriene and diphenyloctatetraene.60 Accepting the 'A; nature for the weak red edge absorption of polyenes (e.g. diphenyloctatetraene) and assuming the mirror image between the 'A absorption and fluroescence which overlap just slightly one would then expect the intensity ratio of the absorption at A,, to that 61 D. E. Post W. M. Hetherington and B. Hudson J. Chem. Phys. 1976,64 4020. (a)R. L. Swofford and W. M. McClain J. Chern. Phys. 1973 59,5740; (b)G. R. Holtom and W. M. McClain Chem. Phys. Letters 1976 44 436. (a)A. J. Towarowski and D.S. Kliger Chem. Phys. Lerfers 1977 50 36; (6) W. M. Flicker 0.A. Mosher and A. Kupperman ibid. 1977,45,492;(c)D. H. Parker S. J. Sheng and M. A. El-Sayed J. Chem. Phys. 1976,65 5534. (a)L. A. Sklar B. S. Hudson and R. D. Simoni Proc Nut. Acad. Sci. U.S.A.,1975,72,1649;(b)L. A. Sklar B. S. Hudson and R. D. Simoni. Biochemistry 1977 16 5100. 30 P.-S.Song of 'A 0-0 to be roughly comparable to that of the fluorescence at AF,max and 'A 0-0. The long-wavelength absorption and fluorescence of -apo-8'-carotenal and astacene in the solid state have been assigned to the 'A However these are likely to be due to impurities3* or reaction products. Retinyl Po1yenes.-Retinyl polyenes such as retinol and retinal show broad struc- tureless absorption spectra at room temperature and at 77 K.The main absorption band 'B +A, A,, == 330-335 nm,f= 1.36 of all-trans-retinol is due to the 'B,-type (or 'B in Platt notation) transition. (Hereafter we will retain the CZhor free-electron notation for genealogical correlation with the previous section.) The cis band 'A; tA (or 'C +A) occurs at 240-250 nm. and its intensity is substantially enhanced in cis-isomers (f =0.1-0.2). The 'B transition is most likely polarized along the long molecular axis while the former is polarized nearly perpendicular to the latter (-55" from the fluorescence and 65" from PPP-CI calculation). Because of the lack of symmetry and distortion of the planarity at the C-6-C-7 bond extensive configuration mixing of various excited states is expected.Except for the above two states 'B and two 'A states of retinol that may lie in the 210-290 nm region have not been resolved. One of the 'A states has recently been assigned as the lowest T,T* state (352 nm; i.e. 1600 cm-' lower than 'I?,) in all-trans-retinol on the basis of two-photon absorp- tion.66 This assignment is consistent with the earlier assignment and with spec- troscopic anomalies of retinol which are shared by other polyenes (see aboue) including 2,1O-dimethylundecapentaene,axerophtene anhydro-vitamin A and all-tr~ns-retinal.~~~~~ The 'A level in all-trans-retinol is ca. 1600 cm-' below the 'B state as mentioned above. This gap is less than in 2,lO-dimethylundecapen- taene (3250 cm-' for the latter).67 The absorption bands of retinyl polyenes containing p -ionone are usually diffuse and structureless even at low temperature.The diffuseness is expected from vibronic interactions between close-lying states but the torsional potential of the C-6-C-7 bond in the ground and excited states appears to play the most important Thus it is noted that anhydrovitamin A at 77 K67 and all-trans-retinol bound specifically to P-lactoglobulin (unpublished results) show structured absorp- tion spectra possibly as the result of a fixed torsional angle about the C-6-C-7 bond. Retinals exhibit absorption spectra similar to that of retinol with A,, at ca. 360-380 nm (f= 1) which is due to the 'B tA genealogy. The 'B transition is nearly maximally polarized with respect to the fluorescence oscillator under pho- toselection conditions at 77 K.69 This transition is most likely polarized along the (a) K.Mandal and T. N. Misra Bull. Chem. Soc. Japan 1976,49 198,975; (6)B. Mallik K. M. Jain K. Mandal and T. N. Misra Indian J. Pure Appl. Phys. 1975 13 699. 66 (a)R. R. Birge J. A. Bennett B. M. Pierce and T. M. Thomas J. Amer. Chem. Soc. 1978 in press; (6) R. R. Birge K. Schulten and M. Karplus Chem. Phys. Letters 1975,31,451. 67 (a)R. L. Christensen and B. E. Kohler Photochem. and Phoroobiof.,1973 18 293; (6) ibid. 1974 19 401. B. Honig A. Warshel and M. Karplus Accourps Chem. Res. 1975 8 92 and references therein. 69 (a)P.-S. Song Q. Chae M. Fujita and H. Baba J. Amer. Chem. Soc. 1976,98,819;(b)T. A. Moore Ph.D. Thesis Texas Tech.University Lubbock Texas 1975;(c) T. A. Moore and P. S. Song Nature (NewBiol.) 1973 242 30. Physical Methods-Part (iii) Spectroscopy of Bio-organic Molecules long molecular axis and shorter-wavelength transitions are expected to be polarized at some angle to the former on the basis of the declining degrees of fluorescence polarization at A <320 nm. The PPP calculation predicts an angle of 52" between 'B and 'A; (cis peak) polarization axes.69 The lowest excited state is suggested to be of 'A; type in all-trans-retinal and 1l-cis-l2-s-tran~-retinal.~~~~~ Although this suggestion is consistent with the spec- tral anomalies (long & large Stokes shift etc.) shared by other polyenes and retinol its frequency and intensity are not accurately known at the present partly owing to the lack of highly resolved absorption and fluorescence spectra.making it difficult Furthermore the fluorescence of retinal is A,,-dependent,67*69*70 to utilize fluorescence as a probe for the 'A state. Retinal (e.g. all-trans-isomer) does not fluoresce in dry non-polar solvents but the H-bonded fraction of retinal in H-bonding solvents is apparently responsible for the fluorescence. This accounts for the A,,-dependence of fluorescence in the ~olvent.~' The lowest excited singlet state of all-trans-retinal in dry hydrocarbon solvents7' and of 1l-~is-retinal~~ is then assigned to the 'n77r* state. The *A,tA transition in retinal is no longer symmetry-forbidden. For example the PPP calculation of f values ranges from 0.16 for the planar 11-cis-s- trans-retinal ('A; =S1) to 0.22 for the planar 11-cis-s-cis-retinal ('A; = S2).The calculated f values for other retinal isomers range from 0.176 for 13-cis- and 0.288 for 1l-cis-12-s-(150")-trans-retinal.The half-bandwidth of the main absorption band of the trans- and cis-isomers remains approximately the Thus the predicted strong absorption due to the 'A transition is not readily apparent in the low-temperature absorption spectra. Without identifying the symmetry ('A; us. '&) we have argued for the lowest singlet state to be a non-discrete T,T*state admixed with a nearby n,T* state on the basis of the observation that retinal carbonyl-cation binding enhances the fluorescence quantum yield in a manner analogous to the H-bonding effect noted above.69 However '7ry7r*-n,7r* perturbation is apparently not responsible for the band broadness since the torsional potential about the C-6-C-7 bond plays an important role in the spectral diffuseness in retinals.68 Cation binding to the all-trans-retinal carbonyl results in less than a 3 nm red shift.69 However the H-bonded retinal-phenol complex shows a substantial spec- tral shift (A,, =385 nm in 3-methylpentane and 420 nm in 3-methylpentane- 1mM-phenol; A F.max =520 and 600 nm re~pectively).~~ Furthermore a significant overlap between the main absorption and fluorescence uncharacteristic of other polyenes is observed.Thus it is possible that the strong H-bonding has either lowered the '8,state below the 'A and 'n,7r* states or introduced a charge- transfer character into the lowest singlet state.The 77 K absorption spectra of retinals show structured band(s) over the 260-320 nm region which have been assigned to an n +7r* transition.'* The intensity of this band is enhanced for cis-isomers. However other assignments (second B, A, a;T*,etc.) for this region cannot be ruled out. The cis peak (of 'A 'O R. S. Becker K. Inuzuka J. King and D. E. Balke J. Amer. Chem. Sm.,1971,93 43. 71 (a)T. Takemura P. K. Das G. Hug and R. S. Becker J. Amer. Chem. Suc. 98 7099; (6) R. S. Becker G. Hug P. K. Das A. M. Schaffer T. Takemura N. Yamamoto and W. Waddell J. Phys. Chem. 1976,80,2265. 72 R. R. Birge M. J. Sullivan and B. E. Kohler J. Amer. Chem. Suc. 1976.98 358.32 P.-S.Song genealogy) is located at ca. 250 nm and is strongly enhanced in 11-cis-retinal at 77 K.72 The PPP double-CI calculation of 11-cis-retinal predicts two additional 'A states (3'A and 4'A or 'B in 11-cis-12-s-cis) and the 'A; state.66 Thus the structured absorption band at the 260-320 nm region (A,, =280 nm)73 may be assigned to one or both of these 'A transitions. From the foregoing review it is clear that the assignments of various bands other than the main absorption band (B,)are still tentative. The spectroscopy of retinal Schiff's bases is even less characterized than that of retinals although general features of the absorption spectra of Schiff's bases at room termperature and 77 K are similar to those of corresponding retinal~.~~ Recently a unusually large red shift of protonated retinal Schiff's base has been observed at 77K showing absorption maximum at ca.540 nm. It is suggested that the red shift results from a lowering of the transition energy of the protonated Schiff's base as a result of the preferential lowering of the excited singlet state due to its interaction with the hydrogen halide solvent cage. Carotenoids.-Carotenoid aldehydes containing one /3 -ionone end are called P-apo-carotenals of which retinal (five C=C bonds) may be regarded as the shortest carotenaI with A,, -360 nm. The main band maximum shifts to the red with increasing conjugation (e.g. 414 nm for P-apo-l2'-carotenal with seven C=C bonds and 508 nm for torularhodinaldehyde or 3' 4'-dehydro-@,t,b-caroten-l7'-al with thirteen C=C bonds).This shift is usually accompanied by enhanced vibra- tional resolution in the visible absorption band. This trend is also observed with carotenoids with two p-ionone ends (e.g. P-carotene). The vibrational structure is even more prominent at 77 K.74*75 Thus the torsional potential around the C-6-C-7 s-bond is no longer the dominant factor for the spectral diffuseness as the spectral diffuseness decreases with the number of C=C bonds in carotenoid~.~~ The total bandwidth (SF/crn-') is empirically given by SY = A + 64000N X 6/n* where n N and S(=0.244*0.021 cm-') are the number of double bonds the number of p-ionone rings and the reduction in the effective number of double bonds per p-ionone ring contributing to the spectral width respectively.Thus the second term gives the width contribution due to the torsional potential around the C-6-C-7 s-bond while the first term A = 549k 14 cm-' is of all contributions to the bandwidth from factors other than the distribution of torsional angles. The main absorption band 'B (f -3) is probably polarized along the long axis of the carotenoid conjugation in p-carotene and related carotenoids. The polarized absorption and reflectance spectra of all-trans -p -carotene single crystals have been measured. The 'B band is polarized at an angle of between 0 and 40" to the long molecular axis.77 Linear dichroic results of carotenoids in stretched 73 (a)A. M. Schaffer W. H. Waddell and R. S. Becker J. Amer. Chem. SOC. 1974,96,2063; (b)W.H. Waddell A. M. Schaffer and R. S. Becker ibid. 1977 99 8456. 74 (a)P.4. Song and T. A. Moore Photochem. and Photobiol. 1974,19,435; (b)Q. Chae P.3. Song J. E. Johansen and S. Liaaen-Jensen J. Amer. Chem. SOC. 1977,99 5609; (c)P. Koka and P.-S. Song Biochim. Biophys. Acta 1977,495 220. 7s B. Ke F. Imsgard H. Kj~sen,and S. Liaaen-Jensen Biochim. Biophys. Acta 1970 210 139. 76 R. Hemley and B. E. Kohler Biophys. J. 1978 in press. 77 (a)D. Chapman R. J. Cherry and A. Morrison Proc. Roy. SOC.1967 A301,173; (b) L. J. Parkhurst and B. G. Anex J. Chem. Phys. 1966,45.862. Physical Methods-Part (iii) Spectroscopy ofBio-organic Molecules polyethylene films and in liquid crystals are interpretable in terms of the long molecular axis polarization of the main band.74*78 The main absorption polarization in cis-carotenoids is also expected to be along the long molecular axis.According to the new a~signment,~~'~~ the lowest singlet state in carotenoids is also of the A symmetry. While the frequencies of the transitions to the allowed excited singlet state 'B, can be fitted to the equation i;lcm-' = 16500+64 100/n where n is the effective number of double bonds in carotenoids and Y reaches an assymptotic value for long carotenoids it is predicted that the lowest singlet ('A;) state continues to be lowered in energy with n. Unfortunately carotenoids are non-fluorescent thus making it difficult to confirm this prediction directly. At present there are no theoretical (e.g. dduble CI) or experimental data on the exact location of 'A in carotenoids.Not surprisingly the PPP singly excited CI predicts the 'A state to be at an energy above 'A:. The linear dichroism remains constant over the main absorption band and its red edge in cis-carotenal Since the oscillator strength of the 'A band is not expected to be negligible as is the case for the dichroic ratio measurements may have detected unique polarization characteristics. However the failure to resolve different dichroic ratios does not necessarily rule out the presence of a 'A band. Thus the weak absorption at A >A,, observed for cis-carotenals at 77 K may be attributable to either a 'A; or another T +T* transition strongly perturbed by the carbonyl group or by exciton interactions (e.g.see ref. 69) since carotenols do not show such long-wavelength ab~orption.~~ character is also a An n -* r* transition strongly admixed with ~,r* po~sibility.~~ Thus no definite assignment for the weak long-wavelength band in carotenals can be made at present. The nature of the weak absorption bands at wavelengths shorter than A,, in carotenoids is not well established except for the 'cis peak' (e.g. 337nm for 15 15'-cis-6-carotene in hexane at room remperature; 350 nm at 77 K) which becomes strongly allowed in cis-carotenoids. The cis peak can be safely assigned to the 'A; state (lC+ 'A in Platt notation) consistent with the PPP MO prediction.32778 The polarization of the cis-band has been deduced from linear dichroic spec- troscopy of /3 -carotene and several other car~tenals,~~*~~.~~ and its polarization is perpendicular to the long molecular axis.The 375 nm band in 13-cis-rhodopinal at 77 K is highly structured analogous to the structured band at ca. 280nm of retina171 and renierapurpurin-20-a1 (A =376 nm and 320 nm both having some structure and essentially identical dichroic ratios). It is not certain which of these bands is of the 'A genealogy; the former is tentatively identified as the 'C('AZ) transition. The shorter-wavelength band (lDt'A) is then assignable to the second 'B,+-A; transition. Carotenoids with chiral centres usually yield three distinct c.d. bands of alternating signs in the region of 200-300 nm. It is clear that several weakly absorbing transitions occur in U.V.spectra of carotenoids and more detailed studies are needed to delineate the number and identity of these tran- sitions in carotenoids. In contrast to carotenals spectra of the Schiff's bases of carotenals and carotenones lack any vibrational structure. ''T. A. Moore and P.-S. Song J. Mol. Spectroscopy 1974,52 209 216 224 and references therein. 79 (a)V. R. Salares R. Mendelsohn P. R. Carex and H. J. Bernstein J. Phys. Chern.,1976,80 1137; (b) V. R. Salares N. M. Young H. J. Bernstein and P. R. Carey Biochemistry 1977 16,4751. 34 P.-S. Song The wavelength maxima of near-u.v. (including cis-peak) and U.V. bands arising from excited configurations involving higher empty MOs in carotenoids can be predicted by Dales's rule which states that A (x = 2nd 3rd transition erc.) lies very close to the A ,, (the main long-wavelength band) of a polyene with n/x where n is the number of double bonds.This empirical rule is useful but it fails in longer and substituted carotenoids particularly at low temperature. Although several PPP-type calculations have been made the spectroscopic characterization of shorter-wavelength bands of carotenoids remains to be carried out as mentioned above. As it stands now the state ordering in a typical carotenoid (e.g. all-rrans-P-carotene) is 'B,< 'A < 2'B, with the location of the .symmetry-forbidden 'A; state yet to be spectroscopically resolved. However the validity of the low-lying 'A state for organic vapour-absorbed solid samples of P -apo-8'-carotenal and other p01yenes~~ is questionable.The recent theoretical calculations of absorption and fluorescence spectra of P-carotene suggest that the 'B state relaxes prior to emission by changing bond alternation along the relaxation co-ordinate in the excited state. This implies that the relaxed 'B state is the lowest and emitting state.80 However the fluorescence spectrum used is probably in error since pure p-carotene does not fluoresce. 4 Heterocyclic Compounds Indoles and Tryptophan.-The T -+ n* transitions 'Lb and 'La are not well resolved in indoles owing to considerable overlap and vibronic mixing between these two The U.V. absorption in the region of 260-275 nm is assigned to the 'Latransition although the 'Lb intensity (maximum at 287- 290 nm) contributes to the overall extinction and spectral shape in this region.It has been possible to resolve the 'Laand 'Lb bands of indole and tryptophan from fluorescence excitation polarization (P) measurements in propylene glycol at -58 "C by monitoring the long-wavelength fluorescence where the 'La-+ 'A emission predominates (cf. indole shows dual fluorescence due to 'Lb-+ 'A and 'La-+ The long-wavelength edge at 305-310 nm with P -0.4 is assigned to the 'Lae'A transition in agreement with previous studies. The 'Lbt'A 0-0 maximum in indole is located at 289.5 nm where the fluorescence polarization degree is minimal (P= O.l) with its vibrational satellites at 286 (0+ 538.3 cm-') and 282.5 nm (0+717.6 or 735.7 ~m-').*~ The P value then reaches a maximum (P== 0.3) at 260 nm.Both 'L and 'Lb contribute more or less equally to the absorption at A 6250 nm. Tryptophan shows essentially similar polarization characteristics. The 'Lb A (0-0) is resolved at 291 nm whereas the 'LaA,, is at 267 nm. The angle 9 between 'La and 'Lb transition moments can be calculated from the PPP MO method and it ranges from 54" with CI to 78" without CL8' If we assign the long-wavelength edge of the absorption to the 'La(0-0) transition which possesses a maximum P with respect to the 'L + 'A emission an estimate of 9 can T. Kakitani and H. Kakitani J. Phys. Soc. Japan 1977.42 1287. '' (a)P.-S. Song and W. E. Kurtin J. Amer. Chem. Soc. 1969 91 4892; (b)M. Sun and P.-S.Song Photochem. and Photobiol. 1977 25 3. B. Valeur and G.Weber Photochem. and Photobiol. 1977 25 441. E. H. Strickland. J. Horwitz. and C. Billups Biochemistry 1970 9 4914. Physical Methods-Part (iii) Spectroscopy ofBio-organic Molecules 35 be anywhere between 45 and 90° depending on the assumptions made concerning the linearity of the oscillators and the resolution of 'Laand 'Lbintensities in the excitation and emission spectra. The lower value results if the 290 nm absorption with minimum P with respect to the 'La+'A fluorescence is assumed to be exclusively due to 'Lb whereas the higher value results if both 'La and 'Lb contribute equally to this absorption. The actual composition of extinction at this peak is probably somewhere between these assumptions. According to a polarized single-crystal absorption the 'La and 'Lb transition moments are oriented at -38" and 54" to the long molecular axis respectively.On the other hand the PPP CI method" yields the 'Lapolarization in agreement with the above determination (polarization along N-1 -C-4 axis) but the 'Lbtransition is nearly short-axis polarized. Nucleic Acid Bases.-In spite of the obvious simplicity the genealogy of U.V. bands of purine and pyrimidine in terms of the benzene notation ('Lb,'La, and or 'BZu, 'Blu,and has been heuristically useful. Thus the U.V. bands of purine in trimethyl phosphate (TMP) at 265 nm (E =6.9 X lo3),240 nm (E == 3 X lo3),and 200 (E == 18.1x 103)-188(& -21.1 x lo3)nm are correlated to 1Lb(B2u), 'La(Blu) and lBa,b(Elu)bands respectively and the U.V. bands of pyrimidine in methyl- cyclohexane at 242 nm (& =2 x lo3),210 nm (E = 1X lo3),and 190 nm (E =6 x lo3) are also correlated respectively.According to this genealogical correlation for example the U.V. bands of 9-ethyl guanine in TMP at 275 nm (E ~9.4~ lo3) 256 nm (E = 15.4x lo3) and 203(~ == 20 X 103)-190(~ =27.4 X lo3)nm are cor- related to Lb(BZu),La(Blu),and Ba.b (Elu),respectively and similar correlations are made to the absorption bands of cytosine at 277 nm (E ~7.5 x lo3) 237 nm (E == 3.5 x lo3) and 204 (E = 11.9x 103)-185 (E =12.2x lo3)nm. In addition polarization directions of these bands can be correlated with those of benzene. For example the 260 nm band (lLb)of 1-methyl-thymine is polarized along the axis intersecting N-1-C-2 and C-4-C-5 bonds (inclined by 11*2" toward N-1).86 With the above description of the gross features of the U.V.spectra of bases more recent studies will be selectively reviewed in the following sections. Purine Bases Analysis of polarized reflection spectra of the(1UU) face of purine crystals yields the transition moment directions of both T +T* and n 3T* transition^.^' The first band at 294 nm (f=00.0057 in the crystal 0.003-0.005 in solution) is out-of-plane polarized and is due to an n +T* transition which probably is highly localized at the N-3 position of purine.85 The second band at 263 nm ('Lb),is in-plane polarized at i-48" from the C-4-C-5 axis (toward C-6). The weak band resolved at 250 nm in the crystal is out-of-plane polarized and is assigned to the second n -+T* transition which is probably localized at N-1.The strong absorption at 200 nm is probably due to Bb,as it is polarized nearly parallel to the 263 nm band but the identity of the out-of-plane polarized 190 nm band remains to be established. An n +T* assignment for this band is unlikely owing to 84 Y. Yarnarnoto and J. Tanaka Bull. Chem. SOC.,Japan 1972,45 1362. '' (a)L. B. Clark and I. Tinoco jun. J. Amer. Chem. SOC., 1965.87 11; (b)W. Hug and I. Tinoco jun. ibid. 1973 95 2803; (c) W. Hug and I. Tinoco jun. ibid. 1974,96 665. 86 R. F. Stewart and N. Davidson J. Chem. Phys. 1963,39 255. " (a)H. H. Chen and L. B. Clark J. Chem. Phys. 1969,51,1862;(b)L. B. Clark J. Arner. Chem. Soc. 1977,99,3934. 36 P.-S. Song its relatively strong intensity (E == 6 X lo3).The experimental polarization direc- tions reported here are in good agreement with PPP-type results but agreement is less satisfactory in purine bases containing additional functional groups such as C=O or NH2. The n +n* transitions in purine bases have been calculated by the CNDO-CI meth~d.~'.~~ Agreement with available experimental n -+ n-* transition energies is not sufficiently quantitative but it is possible to use theoretical data in aiding the resolution of weak n +n* bands in purines. The lowest n + n* transition is predicted at 256 nm for purine 228 nm for adenine and 239 nm for guanine-N-1 which are theoretically above the lowest n -+ n* (lLb)transition in the respective bases.85 The calculations based on the same CNDO method predict an n +n-* transition to be the lowest in adenine thymine and cytosine but not in guanine.88 At least for purine there is no doubt that the lowest singlet excited state is an n,n* type as the high-resolution spectra of purine in crystal and in inert-gas matrices at 4-20 K exhibit vibrational progression characteristic of an n +n-* transition (318.5nm in low-temperature matrix).89 The assigned wavelengths of the lowest n -+ n* transitions in adenine and guanine are 280 and 264 nm respectively by means of polarized absorption and reflectance spectra of crystals solvent effects and c.d.spectroscopy. Poly(dA) shows a positive c.d. peak at 280 nm which can be assigned to an n +n* transition of adenine.90 Similarly the 270 nm absorption in deoxyadenosine is assigned to an n +T* transition.However the identification of an n +n* transition at 264 nm is not definitive since this region is significantly masked by n -+ n* transitions. The SCrRPA method predicts the second n +n* at 259nm however. It is reasonable to conclude that the lowest singlet transition is of an n -+n-* type in adenine.91 The lowest n +T* transition appears to be strongly localized on N-3.85 The location of the lowest n +n-* transition in guanine remains to be resolved. Recent polarized single-crystal reflectance data suggest an n -+ n-* transition at 300.3 nm for 9-ethylguanine at 77 K.*'However the possibility of an exciton and matrix-shifted 'BZu(0-0) band at 77 K cannot be ruled out. The long-wavelength band (A = 274 nm) of single crystals of 9-methyl-adenine 1-methylthymine (AT dimer) is due to the lowest n-+n-* transition of the former.Its polarization is along the short molecular axis C-5-C-6) while the weaker band with A,, -255 nm is long-axis polarized.86 According to the com- monly used convention the transition moment directions (8) for purines are measured as positive towards C-6 (clockwise) with respect to the C-4-C-5 axis. The first n +n* transition moment of adenine is then at an angle of -12* 3". Thus the two n +n* transitions are polarized at least 45" from each other. This conclusion is further confirmed by the dichroic analysis of the spectra of adenine and 9-met hyladenine .91 88 N. V. Zheltovskii and V. I. Danilov Biofizika 1974 19 784.89 (a)J. J. Smith Photochem. and Photobiol. 1976,23 365;(b)M.J. Robey and I. G. Ross ibid. 1975 21 363. 90 (a)C.A. Bush and H. A. Scheraga Biopolymers 1969,7 395; (6)C.A.Bush J. Amer. Chem. SOC. 1973 95,214. 9' (a)A. F. Fucaloro and L. S. Forster J. Amer. Chem. SOC. 1971,93,6443;(b) Spectrochim. Acta 1974 30A 883. Physical Methods-Part (iii) Spectroscopy of Bio-organic Molecules 37 The 260 nm band of adenine in solution is correlated to 'B2u(Lb) but it is apparently composed of two electronic transitions (La+ Lb) as predicted by theoretical treatments and spectroscopic measurements such as m.c.d. The linear dichroism along the short-wavelength tail of the 260 nm band decreases indicating the presence of a transition corresponding to the second T +T* transition (La) which is polarized at some angle to the Lb transition moment direction.The difficulty with the Laassignment is that its f value appears to be weaker than that of the Lb band in contrast to theory and to what is expected from correlation with the benzene and naphthalene spectra. 9-Ethylguanine in TMP shows absorption bands at 275 nm (shoulder E =9.4X lo3 Lb or B2,) 25.6 nm (E = 15.4X lo3,Laor Blu),and 203 (E =2OX 1O3b19O (E =27.4 x lo3,Ba.6 or El,)nm. In aqueous solution pH 6.8 is the 256 nm band of 9-ethylguanine shifts to 249 nm.87 Based on combined information from polarized specular reflectance measurements on single crystals of 9-ethylguanine and from fluorescence polarization data the first band (B2,)is either polarized nearly short-axis or close to the N-1-N-3 direction (i.e.44*5" or -14*5" with the N-3-C-6 direction) while the second band (Blu)is polarized close to the C-2-C-8 axis (i.e. 115f10" or 95 * 10" with the N-3-C-6 dire~tion).~~ Newer results from polarized single-crystal reflection spectra of 9-ethylguanine yield polarization angles of -4" or +35" for B2 and -75" for Blu,87in general agreement with the above results. Polarization directions for guanine HCl 2H20 are also a~ailable.~' Although PPP generates reasonable r +r transition energies (e.g. 277 nm for B2 and 246 nm for Blu),the calculated polarization directions are not satisfactory as their directions are reversed relative to the observed directions. Both experiments based on polarized reflectance c.d.m.c.d. and fluorescence polarization and theories confirm that the first two r -+ T* transitions in guanine and guanosine are polarized nearly perpendicular to each other. Pyrimidine Bases. Uracil in TMP shows 'B2,(Lb)at 258 nm (E ~7.8 X lO',f= 0.175) and 'El (Ba.6)at 203 (E == 8.2 x lo3)-181 (E = 11.8X lo3) nm. The 'n,v* state probably is at higher energy (250 nm)8' than 'B2,. The c.d. of uridine in water (pH 7) resolves a band at 240 nm with negative ellipticity which may be assigned to 1 93 B1,,in agreement with some theoretical treatments (e.g.B1,at 224 nm by PPP but 195 nm by CNDO-CI or 210 nm estimated by analogy with the 237 nm band of cytosine8'). The 258 nm band of uracil may be of composite nature as revealed by (a)the electric field effectg4 and (b) the polarized single-crystal reflectance spectra of its model 6-azauracil which shows resolved bands at 256 and 278nm in addition to the weak band.95 The latter appears to correspond to the weak absorption at ca.230 nm in solution. 1,3-Dimethyluracil and thymines show simiIar absorption spectra. However the nature of the weak band hidden under the strong absorption band is yet to be elucidated. Several attempts using c.d. solvent perturbation and other spectroscopic methods have not been successful in resolving n +T* transitions in uracil. 92 (a)P. R. Callis B. Fanconi and W. T. Simpson J. Amer. Chem. SOC.,1971 93,6679; (b)P. R. Callis and W. T. Simpson ibid. 1970,92,3593. 93 K. K. Cheong Y. C. Fu R. K. Robins and H. Eyring J.Phys. Chem. 1969,73,4219. 94 K. Seibold and H. Labhart Biopolymers 1971 10 2063. 95 J. N. Brown L. M. Trefonas. A. F. Fucaloro and B. G. Anex J. Amer. Chem. SOC.,1974,96 1597. 38 P.-S. Song However the polarized single-crystal absorption spectrum of 1-methyluracil suggests a weak n +T* transition (E = 247) at 264 nm which is polarized perpen- dicular to the molecular plane; the first T -+ T* transition (&) in the crystal is at 275.5 nm96 The longest-wavelength band in 1-methyluracil and 1-methylthymine is polarized almost parallel to the N-1-C-4 direction (0" or 7" and 19" respectively) with respect to the N-1-C-4 direction with clockwise rotation denoted as +). Semi-empirical MO calculation^^^ are in satisfactory agreement with the experi- mental directions.Another polarized absorption study reports a polarization angle of -38* 14" for the main band at 260 nm for thymine itself and the second absorption band at 220 nm is polarized along the short molecular axis (-86°).98 The angle of 48" between these two transition moment directions is substantially lower than the value for uracil and thymine estimated from the linear dichroic data.91 Cytosine in TMP shows A,, at -277 nm ('BZu,E -7.5 x lo3) a shoulder at E 237 nm ('B1,,== 3.5 x lo3) and 204 nm (E = 11.9x 103)-185 nm (E = 12.2 x lo3,'El,,). The first two T -B T* transitions are polarized nearly parallel accord- ing to the polarized reflectance study on cytosine," cytosine monohydrate and l-methylcyt~sine.~~ The 'BZuband is polarized at an angle of 14* lo and 'Bluis polarized at -5 * 3°.96 Although agreement between observed and MO-calculated polarization angles is good (within -10") for thymine and uracil this is not the case with cytosine.However CNDO-CI seems to yield better angles.85 From polarized absorption measurements on oriented films of polycytidylic acid an n -B T* transition was located at 278 nm.99 Such an n +T* transition may be strongly localized on N-3.89 However both CNDO-CI (calc. n -+ T* at 253 nm) and polarized single crystal ab~orption~~ studies fail to resolve an n -+ T* at A longer than the first T* +T* band. Flavins.-Flavins (e.g. riboflavin) show four major absorption bands at 445 nm (E = 12x lo3) 360 nm (E == 10x lo3) 270 nm (E =32 x lo3) and 230 nm.All four bands are attributable to the n+~*-type transition although other types of transition particularly n -+T* may contribute to some of these band intensities. The PPP calculations on the isoalloxazine nucleus support the T +T* nature of the major absorption bands in flavins. loo Calculated transition energies and relative oscillator strengths of the flavin nucleus 7,8-dimethylisoalloxazine are in agreement with observed absorption spectra of flavins."' The PPP theory predicts a weak T +T* transition (S3)at ca. 300 nm where a fiuorescence polarization minimum is also recorded indicating a separate transition in this region."' M.c.d. provides indirect evidence for this transition as the m.c.d. 96 (a)W. Eaton and T. P. Lewis J. Chem. Phys.1970,53 2164; (b)T.P.Lewis and W. Eaton J.Amer. Chem. SOC. 1971,93 2054. 97 J. S. Kwiatkowski and B. Pullman Adv. Heterocyclic Chem. 1975 18 199; for theoretical results see references therein. 98 M. Tanaka and J. Tanaka Bull. Chern. SOC. Japan 1971,44,672,938. 99 A. Rich and M. Kasha J. Amer. Chem. SOC.,1960 82 6197. loo (a)J. L. Fox S. P. Laberge K. Nishimoto and L. S. Forster Biochim. Biophys. Acta 1967 136 544; (b)K."ishimoto Bull. Chem. SOC. Japan 1967,40 2493. lo' (b)P.4. Song Internat. J. Quantum Chem. (a)P.-S. Song Ann. New York Acad. Sci. 1969,158,410; 1969 3,,303; (c)P.-S. Song T. A. Moore and W. E. Kurtin 2.Nuturforsch. 1972,B27,1011 Physical Methods-Part (iii) Spectroscopy of Bio-organic Molecules maximum is at A <A,, =370 nm for riboflavin.lo2 C.d.lo3 and linear dichroic'04 studies support this possibility.The polarized single-crystal absorption of FMN in flavodoxin indicates that both S1and Sz bands are polarized along the long molecular axis."' Defining the polarization angle 8 as the clockwise angle measured from the long axis (N-10- N-1 direction) the Sland S2 bands at 450 and 370 nm are then polarized at 16" and -4" respectively making At? of 20' for the angle between the two transition moments. The Kronig-Kramers transformation of specular reflection spectra of crystalline bis(l0-methylisoalloxazine)copper(II) perchlorate tetrahydrate yields the corresponding absorption spectra. lo6 Again S1(f =0.16) and Sz (f =0.15) transitions are found to be polarized along the long molecular axis with 8 = 26" and lo respectively and A8 of 25".The rotation of the two transition moments by 5-10' clockwise relative to those of the polarized single-crystal absorption of FMN may be attributed to substituent or crystal environmental effects. Considering this agreement between the two sets of polarization data is excellent. Thus earlier theoretical transition moment directions calculated from PPP (S1 along C-8-N-3 and S2 along C-7-C-2 with A8 = 25-30" depending on several parameters in the calculation)lol are experimentally confirmed. The polarization of both transitions is along the C-7-C-2 axis according to MIND0/3.1°7 Both fluorescence polarizationlO'*'Os and linear dichroi~m~~~ data are qualitatively consistent with the polarization directions from the single-crystal reflection and absorption studies and the PPP prediction.Polarization directions of S1and S2 bands are not significantly altered by substituting N-5 with carbon as in deazaflavin yielding A8 = 25" (from fluorescence polarization) and 27" (from PPP108). Higher-energy T -+ T* transitions of flavins in the 200-300 nm region are not well understood. It is likeIy that the U.V. band at 270 nm is composed of more than one 7r -+ T* transition. Thus specular reflectance spectroscopy of 10-methyl-isoalloxazine reveals bands at 284 nm (f = 0.2) 257 nm (f = 0.39) and 221 nm (f= 0.1l) whereas the solution spectrum shows only two peaks at 269 nm (f = 0.55) and 225 nm (f=0.11). Their polarization angles are 3' -19' and 42" respectively.Four possible n -+ T* transitions localized at N-1 C-2=0 C-4=0 and N-5 are not spectrally resolved as they are masked under the intense T -+ n* bands. It is likely that the lowest n -+ n* transition is localized at N-5 and that its low-intensity band is hidden under the second 7r -+ T* band at 370 nrn."' C.d. and specular reflection spectra of flavins do not reveal the location of an n -+ T*band indicating that such a transition if present is probably too weak (E <500) to be detectable. The n -* T* transition assigned to the solvent-sensitive shoulders of the 450 and 370 nm bands by various authors are not experimentally compatible and its loca- tion remains obscure. However it appears most likely that the lowest n -+ T* '02 (a)D.E. Edmondson and G. Tollin Biochemistry 1971 10 113; (6)G. Tollin ibid. 1968 7 1720. Io3 H. Harders S. Forster W. Voelter and A. Bacher Biochemistry 1974 13 3360. '04 J. Siodmiak and D. Frackowiak Photochem. and Photobiol. 1972 16 173. W. A. Eaton J. Hofrichter M. W. Makinen R. D. Anderson and M. L. Ludwig Biochemistry 1975 14 2146. M. W. Yu C. J. Fritchie A. F. Fucaloro and B. G. Anex J. Amer. Chem. SOC.,1976,98 6496. '" M. F. Teitell S. H. Suck and J. L. Fox J. Amer. Chem. Soc. 1978 in press. (a)M. Sun T. A. Moore and P.-S. Song J. Amer. Chem. SOC. 1972.94 1730; (6) M. Sun and P.-S. Song Biochemistry 1973 12 4663. 40 P.-s. Song transition occurs near the 370 nm band as dimethylalloxazines the tautomers of dimethylisoalloxazines with the S1tSo A,, at -380-390 nm are considerably less fluorescent than the latter suggesting that an n,r* state lies only slightly above the fluorescent r,v* state."' In this argument it is assumed that the tautomerism does not affect n +r* energy as much as it does r -+ r* energy.The fluorescence polarization degree in the red region of the first absorption band of alloxazine and lumichromes is >0.4.The angle between the two near-u.v. absorption bands (378 and 318 nm) of alloxazine is estimated to be about 30" from the fluorescence polarization data. This angle is somewhat larger than in iso-alloxazines. PPP also predicts a larger angle (430).lo8 The predominant configuration for the 580 nm band of flavin semiquinone arises from the same orbital pairs as in oxidized flavins i.e.HOMOjLEMO (half-However this band cannot be unambiguously identified as the S1 transition which may arise from an orbital promotion other than HOMO +LEMO.log Both experimental and theoretical investigations are called for before satisfactory spectroscopic assignments including polarizations can be developed for flavin semiquinone. Work derived from this laboratory has been supported by the Robert A. Welch Foundation (D-182). I also wish to thank Professors R. Birge R. Christofferson J. L. Fox and B. Kohler for making their preprints available. lo9 B. Grabe Acta Chem. Scand. 1974,28 363.

ISSN:0069-3030    

DOI:10.1039/OC9777400018

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

41 G. Klopman and P.Andreozzi dilute aqueous solutions.2 Although the relationship between the model system [formamide+(H20)5molecules (1) representing an aqueous formamide solution and (formamide)5 molecules (2) representing pure liquid formamide J and the liquid I I II '733 state was crude some valuable structural information was obtained. A comparison of the interaction energies of the linear formamide tetramer with that of the mixed linear-cyclic tetramers indicated that linear chains are more stable than mixed linear-cyclic systems for the pure liquid. Calculations also showed that the inter- action energy of H20(5)with the CH proton is very small so that a totally hydrated formamide molecule might be sufficiently accurately represented by considering only four HzO molecules in the first hydration shell.In some cases attempts are being made to include many molecules of solvent in the calculation of solute-solvent supersystems. For this purpose Monte Carlo techniques have proven to be particularly useful. As they are usually applied to calculations on liquids Monte Carlo methods are used to generate a series of configurations and orientations of the solute-solvent aggregate which are then weighted appropriately to calculate average thermodynamic properties of the J. F. Hinton and R.D. Harpool J. Amer. Chem. Soc. 1977.99 349. Theoreticai Chemistry system. As Clementi3 points out these statistical properties are important factors to be considered for a realistic description of an ensemble of interacting molecules.. In one Monte Carlo study Owicki and Scheraga investigated the structure and properties of liquid water4 and dilute aqueous methane’ in the isothermal-isobaric ensemble at 298 K and atmospheric pressure. The method provided reasonable estimates of the internal energy heat capacity and mean molar volume for each system. Energy probability distribution functions were used to help describe the hydrogen-bonding interactions in the liquids. It was found that the hydrogen- bonding environment in water is better described as a continuum of bent or stretched bonds rather than relatively discrete sets of bonded and non-bonded interactions. Interactions between water molecules in the solvation shell of methane are more stable and more sharply distributed than those in the bulk liquid.The size of this effect diminishes with increasing distance from methane supporting the view that non-polar solutes increase the degree of hydrogen-bond-ing or ‘structure’ in their hydration shell. Clementi6 also used a Monte Carlo technique to study the structure of a number of alkali-metal halides surrounded by a cluster of 200 water molecules. Co-ordination numbers and hydration-shell radii for the cations and anions were computed and information about the cluster size and shape was also determined. Another quantum mechanical model which has been used quite extensively to study solute-solvent interactions is based on the development of pair or higher- order potentials. The basic idea is to perform thorough ab irzitio calculations for small systems in which only one or two solvent molecules are explicitly included.An analytical many-body potential is then fitted to the data. Assuming that the pair potentials are transferable one can then include many solvent molecules interac- ting with the solute. This procedure is much easier to use than straightforward quantum mechanical calculations of a supermolecule with hundreds of solvent molecules. One excellent example of this approach to the treatment of solvation effects was a series of papers by Clementi and co-workers. In an introductory paper Clementi3 outlines a theoretical and computational model used to describe complex chemical systems of interacting species. His methodology operationally links both quantum chemistry and statistical thermodynamics and is subsequently tested by considering the interaction of water with various biomolecules.In the first two papers of the series SCF-LCAO-MO computations were presented for 21amino-acids’ and the four bases of DNA’ interacting with one molecule of water at a large number of positions and orientations around each biomolecule. The 1690 computed total energies for the HzO-amino complexes and the 368 computed total energies for the H20-base complexes were then fitted with an analytical potential of the form E =1 1 (-Aib/r +B;b/ry + C;b/rii)+E(biomolecule)+E(water) i j#i where i and j designate two atoms one in the biomolecule and the second in the E. Clementi Bull. SOC.chim. belges 1976,85,969.‘J. C. Owicki and H. A. Scheraga J Amer. Chem. SOC., 1977,99,7403. J. C. Owicki and H. A. Scheraga J. Amer. Chem. SOC., 1977.99 7413. E. Clementi R. Barsotti J. Fromm and R. 0.Watts Theor. Chim. Acta 1976 43 101. ’E. Clementi F. Cavallone and R. Scordamaglia J. Amer. Chem. SOC.,1977 99 5531. R. Scordamaglia F. Cavallone and E. Clementi J. Amer. Chem. SOC., 1977 99 5545. G. Klopman and P.Andreozzi water molecule a is an index that distinguishes the electronic environment of an atom in the biomolecules b is an index that distinguishes between either a hydro- gen or the oxygen atom in the HzOmolecule A B and C are fitting constants and E(biomolecu1e) and E(water) are the total energies of the biomolecule and of the water molecule respectively at infinite separation from the other.Contour energy maps can then be used to illustrate the structural organization of water around the various biomolecules. Some of these are shown in Figure 1. In 15 12 9 6 3 0 leu ala 0 i-. 0 3 .9 12 15 0 3 6 9. 12 15 0 3 6 9. I2 I5 0 3 6 .9 12 15 adenine guanine cytosine thymine Figure 1 Structural organization of water around various biomolecules (Reproduced by permission from Bull. SOC. chim. belges 1976,85 969) the concluding paper of the series Bolis and Clementig investigate the reliability and transferability of their library of pair potentials using phenylalanine as a test case. They computed the interaction with a molecule of water at 75 different positions relative to phenylalanine in the SCF-LCAO-MO approximation.The interaction energies for the complex using the fitted pair potentials displayed a reasonable agreement with those computed in the ab initio framework with an error of about 1kcal mol-'. Attempts have also been made to simulate the solvent molecules of solute- solvent clusters by point charges or point dipoles. One successful attempt in this area was the fractional-charge model developed by Noell and Morokuma. lo This model consists of placing fractional point charges at the location of solvent atomic centres. Neither electrons nor atomic basic functions are explicitly associated with such centres. If desired however a specified number of solvent molecules may be G. Bolis and E.Clementi J. Amer. Chern.Soc. 1977,99,5550. J. 0.Noell and K. Morokuma J. Phys. Chern. 1976,80,2675. Theoretical Chemistry included explicitly in the usual ab initio framework. One may consider this as a solute core to which additional solvent molecules may be added as fractionally charged centres. The fractional charges chosen are those which reproduce the calculated or experimental dipole moment of the solvent molecule. Extensive testing of the model for the hydration of Li' and F-was performed to assess the applicability of the method. As expected it was found that the inter- action energies between the ions and their solvent hosts were nearly independent of whether the first-shell water molecules are explicitly included. The fractional- charge model was also applied to the electronic structure of H3N-BH3 in the crystal state.In this case H3N-BH3 in the centre of a unit cell acted as a solute and ten nearest neighbours as the solvent systems. Finally the method was used to study changes in the nature of the complex H3N-HF associated with hydration. Here it was found that addition of the first hydration shell has a significant effect on the general characteristic of the potential energy surface for the complex. The mini- mum on the potential energy surface has shifted to a longer H-F distance and shorter N-H distance. In addition one has a levelling of the potential energy surface and a conformation representative of a proton-transferred-type structure (NH,'-.-F-) now has an energy ca. 5 kcal mol-' above the minimum (compared with 27 kcal mol-') in the gas phase.Despite the reported success of the above method it must be noted that the model lacks the proper short-range repulsive behaviour owing to the fact that orbitals are not explicitly associated with fractionally charged solvent molecules. The method does not include short-range exchange repulsion which prevents the meaningful optimization of solvent conformations. Furthermore although the method accurately depicts the electrostatic interaction between solute and solvent and the polarization of solute by soIvent it is not capable of representing charge- transfer or exchange interactions nor of assessing the polarization of solvent by solute. A related method which has been used to study interactions in solution considers the solvent molecules as point dipoles.Ab initio calculations are then performed on the solute in the presence of dipolar fields. Simons," for example reported on some initial attempts af developing such a model for anion-solvent interactions and the hydration energy of solvated electrons. The dipole representation of water molecules was a rather poor approximation for the study of anion-solvent interactions. Simons's fixed finite dipole model also was not capable of providing an accurate description for the properties of the hydrated electron. A somewhat different approach for studying solute-solvent interactions is based on modifying the hamiltonian of the system to include the solvent field effect. One recent example involved the use of CND0/2 with a modified shell repulsion potential for calculation of the properties of alkali halide salts in water.I2 The shell interaction potential was approximated by a simple function containing several empirically chosen parameters characteristic of each atom.These parameters were selected to obtain the correct dependence of the total energy on the interatomic distance for certain simple systems. The formation energies of the hydrates of alkali-metal ions and halide ions calculated in this manner agreed with experimen- " J. Simons Internat. J. Quantum Chem. 1977 11,971. l2 A. V. Bandura N. P. Novoselov and R. A. Evaresto Theor. and Exp. Chem. 1977,12,463. G. Klopman and P. Andreozti tal values within limits of 1-3 kcalmol-' whereas with the normal CND0/2 procedure they were 1.5-3 times larger than the experimental ones.Electrostatic molecular potential^'^ have often been used to represent the potential around molecules. The resulting diagrams allow one to visualize the potential wells where solvent molecules are likely to be found. It has now been shown that to generate such potential maps it is not necessary to determine accurate molecular wave function^.'^ Indeed a model where the molecule is composed of completely localized electron pairs is often sufficient to generate very accurate potential maps. This model was recently extended by Bonaccorsi Scrocco and Tomasi15 to include the polarization of the solute by the solvent. These authors derived an analytical expression of the polarization term in the interaction energy between a molecule and a point-like charge.The molecule is partitioned into a number of groups defined with respect to each bond and each lone pair and suitable group polarizabilities are associated with the groups. The polarization energy is cal- culated with similar procedures on the same basis set as the electrostatic term permitting one to compare numerical values of similar accuracy. The method was used to study the approach of a point charge along two opposite directions of the molecular axis of HCN and along the nitrogen lone-pair axis for several amines. Some attempts have also been made to develop quantum mechanical models for dipole-dipole and van der Waals interactions in molecular systems. Gavezzotti and Simonetta16 developed a model to account for intermolecular co-operation and dipole-dipole interactions during molecular rearrangements in organic crystals.They found that the simplest way to study the energetic changes accompanying the reorientational motion of crystals was to consider a molecular cluster made up of a finite number of molecules and to calculate the pairwise interactions of each molecule with all others in the cluster. A given rearrangement was visualized as the displacement of one or more internal co-ordinates of one fundamental molecule. Then one or more other molecules in the cluster were allowed appropriate motions with respect to which the cluster energy was minimized. The model was success- fully applied to the study of the role of intermolecular co-operation in lowering rotational barriers of some naphthalene and benzene derivatives.For van der Waals systems a valence-bond-type method has been used to calculate the long-range interaction potentials between molecule^.^^ Calculations showed that the anisotropic electrostatic interactions even for compounds with no net dipole were of the same order of magnitude as the dispersion terms. The anisotropic forces are believed to have important effects on some crystal properties. Solvation of Monoatomic Ions.-A great number of quantum mechanical studies have been made to investigate the solvation of various cations and anions in water and other solvents. One extensive Monte Carlo study by Clement? dealt with the hydration of alkali-metal ions in clusters of up to 200 water molecules.The study produced valuable information on the cluster size and shape around the ions and l3 R. Bonaccorsi E. Scrocco and J. Tomasi J. Chem. Phys. 1970,52 5270. l4 R. Bonaccorsi E. Scrocco and J. Tomasi J. Amer. Chem. SOC.,1977,99,4546. R. Bonaccorsi E. Scrocco and J. Tomasi Theor. Chim. Acta 1977,43,63. l6 A. Gaveuotti and M. Simonetta Actu Cryst. 1976 A32 997. P. E. S. Wormer F. Mulder and A. Van Der Avoird Internat. J. Quantum Chem. 1977 11,959. Theoretical Chemistry 47 gave reasonable estimates of the co-ordination numbers and hydration-shell radii of alkali-metal cations. Using the supermolecule technique Pullman'' also studied the solvation and fixation of the metal cations of Groups I and 11.She found that the bond energies for the cations interacting with H20 increased in the following order Be2+> Mg2' >Ca2' >Li' >Na' >IS'. This order was rationalized on the basis of two observations first the electrostatic attraction for dications should be larger than for monocations and secondly within each group of metals the exchange repulsion term favours the ion of smaller ionic radius. The same order of bond energies was determined in another non-empirical investigation of these cations interacting with HF and NH3 as ~olvents.'~ On going from HF to H20 to NH3 the bond energy of the ion-solvent complex increased. These results were also explained in terms of the electronegativity and effective size of the F 0 and N atoms involved in the donor-acceptor interactions.Ab initio calculations2' for the solvation of A13+ and Cu2' failed to reproduce the experimental hydration numbers and hydration enthalpies when solvation shells beyond the first were neglected. However a model of A13+ with six water mole- cules in the first hydration shell and twelve in the second shell yielded a computed hydration energy of 1041 kcal mol-' in reasonable agreement with the experimen- tal enthalpy of 1116 kcal mol-'. This model also accounts for the SN1mechanism proposed for the exchange of water molecules between the hydration sphere of A13' and the bulk solvent. Provided that the exchange is considered to take place between the first and second hydration shells the activation energy for the S,l process (1)is 21 kcal mol-' compared with 38 kcal mol-' for the SN2process (2).SN' [A1(H20)6]3' [A1(H20)5(H202S)]3+ SN2 [A1(H20) (H2O2')I3' __* [A1(H2O)7I3' (2) In general quantum mechanical studies on the solvation of anions are inherently more difficult than those for cations owing to the size and polarizability of the anions. We previously mentioned some difficulties encountered in the calculations of anion-solvent systems using point dipoles to represent the ~olvent.~ One suc- cessful anion-solvent study was a recent CND0/2 investigation of the optimal co-ordination number (n)of the aquo-complexes of Br- and I-. It was found that the optimal co-ordination occurs when these ions are surrounded by six water molecules.21 Solvation of Organic Cations.-A number of attempts have been made to investi- gate the influence of a solvent on the relative stability of organic cations.For example McManus and Worley2* studied halonium ion-carbocation equilibria (3) using MIND0/3 to evaluate the importance of carbocation solvation. All of the cyclic chloronium ions which they studied were 10-30 kcal mol-' more stable than A. Pullman Bull. SOC.chim. belges. 1976 85 963. '' V. M. Pinchuk Y. A. Kruglyak and M. D. Dolgushin Theor. and Exp. Chem. 1977,12 116. 2o H. Veillard J. Amer. Chem. SOC.,1977,99 7194. 2' V. B. Volkov and D. A. Zhogolev Chem. Phys. Letters 1977 49 591. 22 S. P. McManus and S. D. Worley Tetrahedron Letters 1977 555. G.Klopman and P.Andreozti a their open chloroalkyl carbocation isomers suggesting that chlorine is quite effective in internally solvating the carbocations in the absence of solvent.MIND0/3 predicted that a tertiary carbocation is more than 10 kcal mol-’ less stable than its isomeric chloronium ion. Since the experimental value is known to be between 0 and 2kcal mol-’ in solution in S02ClF they concluded that the solvent must supply a substantial amount of stabilization to the open carbocation. J~rgensen~~ reported MIND0/3 and perturbation theory calculations for complexes of carbocations with an HCI molecule to provide insight into the relative extent of specific solvation at carbocation centers. He found that specific solvation at positively charged carbons becomes less favourable with increasing charge delocalization.For example the relatively localized homocubyl cation (3) was better stabilized24 by the solvated leaving group than the more delocalized pyra- midal ion (4) and the bishomoaromatic ion (5),having the relative energies 0.0,3.2 and 6.6 kcal mol-’ respectively. Interestingly enough the gas-phase energies are in a distinctly different order (4)< (5)< (3). Thus it is concluded that the relative energies of isomeric carbonium ions may vary appreciably from solution to the gas phase and caution must be exercised in using relative energies of carbonium ions in the gas phase as a gauge for the relative energies of intermediates in solution. (4) Since Jorgensen’s model considered explicitly only interactions with one solvent molecule it was believed that introducing additional solvent molecules could eliminate the discrepancy between the solvation energy of the delocalized and localized isomers.To probe this issue more thoroughly Jorgensen and Munr~e~~ studied the influence of increasing solvation on the relative energies of bisected and bridged ethyl cations. Their MIND0/3 results revealed that the solvation energy difference for these cations showed no sign of being reduced even after addition of up to five solvent molecules supporting their view that additional solvation will not return the energetic order to that for the isolated ions in the gas phase. Another study of interactions between solvent and organic ions involved an investigation of the nature of solvation and hydrogen-bonding of pyridinium ions in H20 at the STO-3G Relative solvation enthalpies for the transfer of a series of pyridinium ions from the gas phase to water were calculated by combining 23 W.L. Jorgensen J. Amer. Chem. SOC.,1977,99 280. 24 W. L. Jorgensen J. Amer. Chem. Soc. 1977,99,4272. 25 W. L. Jorgensen and J. E. Munroe Tetrahedron Letters 1977 581. 26 E. M. Arnett B. Chawla L. Bell M. Taagepera W. J. Hehre and R. W. Taft J. Amer. Chem. SOC. 1977,99,5729. Theoretical Chemistry relative heats of ionization in the gas phase and in water with relative heats of solvation of the corresponding neutral pyridines. Both the hydrogen bonds from water to the pyridines (6) and from the pyridinium ions to water (7) were investigated.As expected the hydrogen bond between the pyridinium ions and water was very strong and accounted for most of the difference between the gas-phase basicities and those in water. Theoretical calculations have also been used to study the influence of solvent on the isomer distribution of electrophilic reactions. For example Rayez and Dannenberg” carried out INDO calculations for orrho- meta- and para-pro- tonated toluene in the gas phase and in the presence of one or two trifluoroacetic acid solvent molecules. The calculated percentage of ortho meta- and para-products was found to be in agreement with experiment. The paralortho ratio significantly decreased upon solvation whereas the para/meta ratio remained large supporting the idea that relative cationic solvation might be quite important in consideration of normal organic reactivities.Association of Monoatomic ions with Organic Molecules.-The stability of various conformers of a molecule can be affected by co-ordination with cations. For example CND0/2 calculations have shown that the non-planar conformers of but-l-ene,*’ which are less stable in the free state than the planar cis form have larger stabilization energies when co-ordinated with Na+ ions. These changes are believed to be particularly important in biochemical molecules where the presence of a cation may favour a structure susceptible to producing specific enzymatic action. It is thus not surprising to find that a number of such studies have found their way into a theoretical investigation.Pullman and co-workers have written an interesting series of papers on the binding of cations to biomolecules. Ab inirio computations were performed on the binding of the Na’ ion to the purine and pyrimidine bases of the nucleic acids.2g The bases uracil (8) cytosine (9),guanine (lo) and adenine (11)are shown along -32.9 0 ~.:10“ 0 @’ -28.7 H (8) (9) 27 J. C. Rayez and J. J. Dannenberg Tetrahedron Letters 1977 671. 28 V. W.Meiler D. Deininger and D. Michel 2.phys. Chem. (Leipzig) 1977 258 139 29 D. Perahia A. Pullman and B. Pullman Theor.Chim. Acta 1977 43 207. G. Klopman and P. Andreozzi -48.2 0 0 -14.6 -24.0 with the favoured binding site of the cation. The calculated binding energies in kcalmol-' were -32.9 -51.73 -53.89 and -26.35 for (8) (9) (lo) and (11) respectively.These computations indicate two principal differences between these interactions. First for bases containing both oxygen and nitrogen cation binding at an individual oxygen binding site is favoured over binding at the nitrogen atom. This inversion in the intrinsic binding ability of Na' compared with H' is due to the exchange repulsive component of the binding energy which for the same distance is larger for nitrogen than for oxygen. Secondly we note the appearance in cytosine and guanine of bridged positions between a nitrogen and a carbonyl oxygen as the preferred binding sites. In the electrostatic potential energy maps for free cytosine and guanine the nitrogen and oxygen atoms were associated with separate minima and the bridge position was less favourable.However the equilibrium distance for a proton is smaller than for Na+ so that the proton can move into the potential minima close to the heteroatoms. In a second paper of the series Pullman et ~1.~'reported ab initio computations on the binding of alkali-metal and alkaline-earth cations to the phosphate group to determine the effects of binding on the conformational properties of the phos- phodiester Iinkage and the polar head of phospholipids. Using an extended STO- 3G basis set they studied the interactions of Na+ Mg" K" and Ca2' with the dimethyl phosphate anion [(12) as the mode1 for the phosphodiester linkage] at a bridged (B) and an external (E) site with three fundamental conformations.In all cases strongest binding corresponded to the B-site the two bivalent cations bind- ing more strongly than the univalent ones. Fixation of the cations at the B-site did not influence the order of conformational preference with respect to the torsion about the P-Oe,,, bonds existing in the free molecule. However binding of cations to the external E-site was able to produce substantial modification in the order of conformational stabilities. Thus depending on the site of binding cations may or may not perturb the intrinsic conformational preference of the phos- phodiester linkage. The authors also studied the interaction of Na' at the B- and E-sites of ethanolamine phosphate (EP) (13) in an attempt to investigate the effects of cation binding on the conformational properties of phospholipids.The intrinsically pre- ferred conformation for the free EP was a highly folded structure associated with (a4, as)= (270°,30"). The most stable form of the Na',EP adduct was substantially 30 B. Pullman N. Gresh H. Berthod and A. Pullman Theor. Chim. Actu 1977 44 151. TheoreticaI Chemistry H different however with (a4, as)= (-30° 1SOO). Thus fixation of the cation at B had the effect of extending the structure of the polar head with respect to a,. On the other hand considering Na’ binding with the E-site the most stable conformation is very nearly the same as that for free EP. Again it was found that the site of binding may have a profound influence on the conformation of a ligand.3 Theoretical Aspects of Hydrogen Bonding Studies on its Origin and Nature.-Among the various factors involved in molecu- lar association hydrogen-bonding is probably the most important. It is thus not surprising to find that the study of its origin and nature continues to generate considerable theoretical interest. Various energy-decomposition schemes have been used in the past to describe the factors responsible for its occurrence. Most of them attempt to separate the total interaction energy into electrostatic exchange and polarization energies. Morokuma et al.31now propose a new energy-decom- position scheme where the influence of charge transfer is evaluated as well. The procedure involves the manipulation of a matrix generated by the Hartree-Fock molecular orbitals of the isolated molecules.In their article the authors describe the specific steps for modifying the Gaussian 70 package for the calculation of these new energy components. This energy-decomposition analysis was applied to the study of (H20)2 (HFL H3N...HF and other complexes in which the proton donor is HF H20 NH3 or CH and the proton acceptor is HF H20 or NH3.32 The essential energy components in hydrogen-bonding include electrostatic polarization exchange repulsion charge transfer and their coupling. For ‘normal’ hydrogen bonds in which the proton acceptor is F 0,or N and the proton donor is a polar bond F-H 0-H or N-H the hydrogen bond is strongly electrostatic in nature with a small but significant contribution from charge transfer.Although charge transfer plays only a minor role in strong more electrostatic hydrogen bonds such as H3N- .HF it is essential for the stabilization of weaker bonds such as H3N-..HCH3 and H20.* .HCH3. During hydrogen-bond formation the electron density on the hydrogen-bonding proton decreases owing to exchange repulsion and polarization while its concentration on the acceptor atom is principally due to charge transfer and polarization. Charge-redistribution effects in the non-interacting parts of the donor and acceptor are controlled by the polarization term and generally follow an alternating charge pattern. The energy components of some lithium complexes K. Kitaura and K. Morokuma Infernat.J. Quantum Chem. 1977,10 325. 32 H. Urneyama and K.Morokuma J. Amer. Chem. SOC.,1977.99 1316. G. Klopman and P. Andreozzi (LiFX and (LiH)* were compared with the hydrogen-bonded (HF)2 complex. Based on these comparative findings it appears that hydrogen-bonding is a unique type of association which always involves a short polar and strong H-X bond as the proton donor. Since the polarity is moderate the electrostatic energy is not too great and exchange repulsion prevents the hydrogen from approaching too closely to the proton acceptor. Because of all these effects hydrogen bonds are always intermediate to weak interactions. .~~ A similar conclusion was reached by Dolgushin et ~ 1 These authors investi- gated the nature of the hydrogen bond by a comparative analysis of the interactions of Li+ F- OH- and H20 with H20.In agreement with Morokuma’s results they found the exchange terms to be repulsive but largely offset by the larger elec- trostatic attraction. In addition they found that the total charge of the proton- donor or proton-acceptor system is the critical point for hydrogen-bond formation. If the donor system is negatively charged or if the acceptor system is positively charged no hydrogen bond results. If on the other hand the acceptor system is negatively charged a strong hydrogen bond can exist. .~~ Clark et ~ 1 investigated the electronic reorganizations accompanying core and valence-shell ionization in some simple hydrogen-bonded systems including H,N-.H20 and (H20)3, (H20)*,HF-.H20 using a 4-3 1G and double-zeta basis set.The relaxation energies for a given core hole were found to increase on going from monomer to dimer irrespective of whether the binding energy for the core level increased or decreased. Core holes in the dimers caused substantial changes in the hydrogen-bond energies compared with the neutral systems. Structural Studies.-A number of studies were aimed at investigating the role of intramolecular and intermolecular hydrogen-bonding in determining the stereo- .~~ chemistry of molecular systems. For example Newton et ~ 1 attempted to determine the role of intramolecular factors in the planarity of the carbon-oxygen framework in the a-hydroxycarbonyl moiety of many a-hydroxy-acids and carboxylates. STO 4-31G calculations on three a-hydroxycarbonyl systems glyco- lic acid (14) the glycolate anion (15) and glycoaldehyde (16) gave equilibrium (14) (15) (16) conformations corresponding to internally hydrogen-bonded structures.Rotations of the a-hydroxy-group were calculated to cost much less energy than distortions which destroyed the planarity of the carbon-oxygen framework. These results were consistent with the crystal-state structures of the molecules where inter- molecular hydrogen-bonding takes precedence over internal hydrogen-bonding. Although the relative stabilities of these structures may be partly explained in 33 M. D. Dolgushin and V. M. Pinchuk Theor. Chim. Acta 1977 45 157. 34 D. T. Clark and B. J. Crornarty Theor. Chim. Ada 1977 44,181. 35 M. D. Newton and G. A. Jeffrey J. Amer. Chem.Soc. 1977,99,2413. TheoreticaI Chemistry terms of intramolecular hydrogen bonds analysis of the charge distribution indicated that the short-range 0-He. -0effects are smaller than those generally found in intermolecular hydrogen-bonding. Furthermore no weakening of the 0-H bond of the a-hydroxy-group was found when the acid and the aldehyde were compared with non-hydrogen-bonded conformations. In a related study Vishveshwara and Pople36 investigated the stereochemistry of the a-amino counterparts of glycolic acid (14) and glycoaldehyde (16). Using a 4-31G basis set they calculated the equilibrium geometry of glycine (17) and a-aminoacetaldehyde (18)as well as the effect of rotation of the a-amino-group on the relative stabilities of these compounds.The most stable conformations cor- responded to structures in which the amine group forms two bifurcated hydrogen bonds with the carbonyl oxygen. The rotational potential energy surfaces of the acid and the aldehyde also showed two other stable conformations. For glycine this corresponded to a structure with a hydrogen bond between the hydroxyl proton and nitrogen (19) lying only 2.2 kcal mol-' above (17). For a-aminoacetaldehyde the second local minimum lay only 1.4kcal mol-' above (18) and was stabilized by a strong interaction between the aldehyde proton and the nitrogen lone-pair electrons (20). (17) (18) (19) (20) Quantum mechanical methods were also used to study the effects of hydrogen-bonding on peptide bonds. In one such st~dy,~' the peptide unit was modelled by truns-N-methylacetamide (NMA) (2l) which was allowed to interact with various hydrogen-bonding species such as water formamide imidazole CH3NH3' and HCOO-.PRDDO calculations indicated that all of the species made non-planar deformations of the peptide unit more difficult. The effects of these species on the flexibility and electronic charge distribution of NMA can be understood by recog- nizing that hydrogen bonds to either the carbonyl or the amine group of the peptide linkage will stabilize (22) more than (21). Partial and full hydration of the peptide H H f .H H-C p H-\ +/ C=N (21) (22) unit were also studied. It was found that many of the effects of a full hydration shell could be simulated by the interaction of only two water molecules.36 S. Vishveshwara and J. Pople J. Amer. Chem. Soc. 1977,98 2422. 37 S. Scheiner and C. W. Kern J. Amer. Chem. SOC.,1977,99,7042. G. Klopman and P.Andreozzi In a somewhat related work molecular orbital calculations were used to study the hydrolysis of formamide as a model for the hydrolysis of peptides by carboxy- peptidase A.38A proton donor H30+was positioned near the nitrogen atom of the model substrate. Nucleophilic attack by a water dimer a model for the basic form of Glu-270 led to hydrolysis of the peptide (23). A Li cation positioned proximate to the carbonyl oxygen was seen to facilitate hydrolysis. The electrophile first polarizes the carbonyl bond of the substrate reducing the negative charge on the oxygen and making the carbon more susceptible to nucleophilic attack.Li' and [Be(OH)(NH3)2]+,believed to be good models for Zn2' and its Iigands were found to be more effective at catalysing the hydrolysis than were various hydrogen- bonding species like NH4+. Theoretical Aspects of Proton Transfer.-A number of theoretical studies have been related to the question of proton transfer in hydrogen-bonded systems. Iwata and M~rokuma~~ examined the ground and various excited states of the formic acid monomer'and dimer (24) with an STO-3G basis set. The two-configuration elec- 01...H 4 -04 / \ Hl--Cl C2-H3 \ / 02-HZ...O3 (24) tron hole potential method was used for the calculation of the excited state of the dimers. The potential energy curves for the symmetrical simultaneous movement of two bridging protons for all of the states were studied and it was found that proton transfer in the ground state was the one with the lowest barrier.The authors did not find an excited state with a low barrier for the hydrogen exchange. However they note that since the transitions being considered ti-r* and T-T* are not' directly involved in weakening the 0-2-H-2 bond or strengthening the H-2-a.0-3 bond the barrier should not change drastically upon excitation. As a result states which should have low barriers for proton exchange are those involv- ing an excitation from the 0-2-H-2 u-orbital or to the 0-2-H-2 u*-orbital. The potential energy surface for the proton transfer in the H2S dimer4' was also calculated.Despite the limited basis set used the calculated energy of 38 S. Scheiner and W. N. Lipscomb J. Amer. Chem. SOC.,1977,99 3466. 39 S. Iwata and K. Morokuma Theor. Chim. Acta 1977,44323. 40 K. Pecul Theor. Chim. Acta 1977.44 77. Theoretical Chemistry 55 207 kcal mol-' was in fairly good agreement with the experimental value of 151-199kcal mol-' for the energy of proton transfer. On the basis of their results the authors suggest that the H2S dimer can be stable at any orientation of the monomers. The behaviour of the hydrogen-bonded proton in malonaldehyde [skeleton as in (25)] continues to stir the interest of theoreticians and its dynamics have been dH0 I I AH I H (25) studied by determining the reaction co-ordinates from the gradients of potential obtained with a CNDO/2 The motion along the reaction co-ordinate was calculated considering all vibrational motions perpendicular to the reaction co-ordinate.It was found that a rapid transformation occurs between the two asymmetric conformers through a low potential barrier. One of the interesting features of this work was the finding that the distribution of the hydrogen-bonded proton on the reaction co-ordinate varied with the increase of temperature. The mean position of the proton was symmetric in the low-temperature range and became asymmetrically oriented only at high temperature. The existence of an out-of-plane vibrational motion with a negative force constant led the investigators to suggest that the motion of the proton proceeds along an ellipse-like orbit.The possibility of tunnelling in the proton-exchange reactions between methyl- oxonium ion and methyl alcohol methyl alcohol and methoxide ion hydronium ion and water and water and hydroxide ion was investigated by STO 4-31Gcal-culations on the proton transfer in the above The calculated tunnelling frequencies were about two orders of magnitude larger than the experimental values for the four systems considered. Small perturbations such as the rotation of methyl groups were found to destroy the symmetry of the profiles and were believed to be responsible for the discrepancies between calculated and experi- mental results. In a recent paper attempts were made to study proton-transfer mechanisms in the water dimer and trimer coupled to an envir~nment.~~ The environment was simulated by reaction fields together with uniform external electric fields.The reaction fields were varied with respect to the direction of the dipo!e moment of the supermolecule and g the solute-surrounding coupling tensor was used to gauge the reaction field strength. For negative g values the proton potential curve dimer has only one minimum. For positive g values however a second minimum is apparent in the region corresponding to the ion-pair structure H,O'...OH-as g increases in value (Figure 2). A potential barrier against the proton displacement from H30+toward OH-can thus be calculated but was seen to depend on the magnitude of g. 41 S. Kato K. Kato.and K. Fukui J. Amer. Chem. SOC.,1977,99,684. 42 J. H.Busch and J. R. de la Vega J. Amer. Chem. SOC.,1977,99,2397. O3 0.Tapia and E. Poulain Internat. J. Quantum Chem. 1977,11,473. G. Klopman and P. Andreozzi Figure 2 Proton potential curves for the water dimer model as a function of the orientation and reaction field strength (Reproducedby permission from Internat. J. Quantum Chem. 1977 11,473) 4 Other Molecular Associations Quantum mechanical calculations have been used to investigate van der Waals charge-transfer and ionic associations. For example Lochmann et aE.44used the PCILO method to analyse the individual energy contributions responsible for the stabilization of van der Waals hydrogen-bonding and charge-transfer complexes. The nonane dimer representing a van der Waals type interaction was found to be stabilized by -0.85 kcal mol-' while the water dimer and a Li'-cis-but-2-ene complex were stabilized by -4.15 and -3.60 kcal mol-' respectively.The zeroth- order energy involving an electrostatic interaction was destabilizing for all systems. Stabilization of the nonane dimer was seen to arise from inter-bond correlation as well as delocalization but the delocalization contribution was significantly more important in the other two complexes considered. In a second paper Lochmann calculated intermolecular interactions again using the supermolecule approach within the PCILO method but this time using fixed polaritie~.~~ By employing fixed 44 R. Lochmann and T. Weller InternaF. J.Quantum Chem. 1976 10,909. 45 R.Lochmann Internat. J. Quantum Chem.. 1977 11,293. Theoretical Chemistry 57 polarities determined by minimization of the polarization energy of the isolated subsystems of the complex for all distances between them his PCILO calculations were without any iteration cycle. Both the stabilization energies and equilibrium distances for the nonane dimer water dimer and a benzene-tetracyanoethylene complex were in good agreement with the calculated values using optimized wave- functions. A more sophisticated analysis of non-covalent interactions was presented by K~llman,~~ using a STO 4-31G basis set and the Morokuma energy-decomposition scheme. The interaction energies and equilibrium structures for a wide variety of intermolecular complexes were presented including van der Waals molecules hydrogen-bonded complexes charge-transfer complexes radical complexes and three-body interactions.It was found that the electrostatic energy and electrostatic potential are the most useful guides to qualitative and semi-quantitative predictions of the energies of non-covalent interactions. The electrostatic energy also correctly predicts the directionality of many inter.actions including (C12)2 (HF), (HClh F--.H20 and C1-. -H20. Consequently it was suggested that a general equation can be used to calculate the interaction energy for many non-covalent interactions involving Lewis acids and bases. The procedure was found to be successful in a host of test cases including (C12)2 C02.-.H20 S02-.NH3 Li'. -OH2 F--H20 NH4+.-.F- and F-. -HF. In a related paper Kollman et al.47 calculated the proton affinities Li' affinities and hydrogen-bond affinities for some simple bases. As before they found that the electrostatic potential is a good guide to predicting the relative H+ HF and Li' affinity for basic sites. However the order of the basicities of various compounds varied significantly depending on the nature of the acid and on the substituent on the basic site. Specifically the methyl substituent effect was found to be quite different for fluoro and amine bases. In the case of fluoro bases electrostatic and charge- redistribution effects reinforce each other; in amine bases they oppose each other. STO-3G and 4-31G calculations have been used to determine the relative proton affinities of pyridine and the diazine~.~~ The experimentally observed order (pyridine > 1,2-diazine> 1,3-diazine> 1,4-diazine) was reproduced only at the 4-31G level.In the protonated ions increasing stability correlated with increasing an orbital energy of the base. This contrasts with the behaviour of the hydrogen- bonded complexes where increasing stability correlated with increasing p-character of the nitrogen lone-pair orbital. A series of electron donor-acceptor complexes of halogens was studied by Umeyama et al.49 Based on the energy components essential for binding the complexes were qualitatively classified as weak electrostatic-charge-transfer complexes intermediate electrostatic complexes weak electrostatic complexes and very weak dispersion-charge-transfer complexes.N-Methyl substituent effects were studied in a number of donor-NH2R systems. It was found that methyl substitution in H2N-ClF resulted in a small destabilization due to cancellation of 46 P. Kollman J. Amer. Chem. SOC.,1977,99 4875. '' P. Kollman and S. Rothenberg J. Amer. Chem. SOC., 1977,99 1333. J. E. Del Bene J. Amer. Chem. SOC.,1977,99 3617. 49 H. Umeyama K. Morokuma and S. Yamabe J. Amer. Chem. SOC.,1977,99,330. G. Klopman and P. Andreozzi exchange repulsion and charge-transfer terms (see Table). This contrasted with a small stabilization effect for H3N-BH3 due to polarization-exchange repulsion stabilization and large stabilization for H3N-..H’ which was found to result from a polarization effect. Table Comparisonof interaction energy components and N-methyl substituent effects between various complexes at equilibrium geometry in kcal mol-’ HSN-CIF H3N-BH3 H3N-H’ RIA 2.717 1.705 1.02 Total interaction energy Electrostatic Exchange repulsion Polarization Charge transfer Coupling AE ES EX PL CT MIX -8.23 (0.29) -1 1.18 (0.32) 7.41 (0.53) -1.05 (0.02) -3.59 (-0.57) 0.19 (-0.01) -44.7 (-0.8) -92.9 (-1.2) 86.9 (4.4) -17.2 (-5.0) -27.1 (-1.4) 5.6 (2.4) -221.9 (-8.5) -99.8 (3.3) -27.4 (-12.8) -88.3 (-3.4) -6.5 (4.4) 0.0(0.0) The numbers in parentheses are the difference between the value for the CH3H2N complex and that for the H3N complex. A negative number indicates that the CH3N2N complex is more stable and oice uersa.LaGrange and co-workerssO studied a similar series of charge-transfer complexes with a 4-31G basis set. For the complexes studied H3N-C12 HZO-C12 and HF-CI2 it was found that the intermolecular distance is smaller than the sum of the van der Waals radii. It was also found that the energy of formation of the complexes diminished regularly as the eIectronegativity of the donor atom increased. Finally SCF studies have been carried out on a novel class of molecular complexes involving neutral alkali-metal and halogen atoms with dipolar molecules in particular interactions between Li Na or F with NH3 H20 HF PH3 H2S or HC1.” The minimum-energy structures for the alkali-metal atom-dipolar mole- cule interactions are M-BH, where the negatively charged end of the hydride approaches the neutral atom.There is transfer of charge from the hydrides to the metal and this leads to a rather substantial dipole moment for the complexes. It was found that the interaction energies of a lithium atom with different hydrides decrease in the order NH3 >H20>HF> PH3>H2S> HCl. The interaction of Na with the hydrides follows the same relative order as the Li complexes but the interaction energies are smaller. J. LaGrange G. Leroy and G. Louterman-Leloup Bull. SOC.chim. belges. 1976,86 241. ’’ M. Trenary H. F. Shaefer and P. Kollman J. Amer. Chem. Sac. 1977,99 3885.

ISSN:0069-3030    

DOI:10.1039/OC9777400041

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i)Pericyclic Reactions By R. J. BUSHBY Department of Organic Chemistry The University Leeds LS2 9JT A new two-volume work on pericyclic reactions has been published’ which contains a number of useful articles particularly those where a fresh view of the literature has been obtained by dividing according to substrate type (carbanions carbonium ions etc.) rather than the more normal division by reaction type. 1 Cycloadditions and Cycloreversions More energy surface calculations on cycloaddition reactions have been reported2 and in an interesting paper Houk3 has compared the results of semi-empirical and of ab initio MO methods. For the Diels-Alder reaction and the addition of 1,3-dipoles to olefins he concludes that most NDO semi-empirical methods favour an unsymmetrical (biradical) mode of addition whereas ab initio methods favour a symmetrical (concerted) transition state (TS) because neglect of overlap even in the MIND0 method results in the filled-orbital-filled-orbital repulsions being under-estimated.In particular these NDO methods under-estimate the repulsive interaction between the two highest occupied molecular orbitals which for an allowed cycloaddition contributes more to the destabilization of the unsymmetrical TS than to the symmetrical TS. Although this provides a simple explanation of why the results obtained by the two methods differ it does not enable one to judge which is closer to the truth as the effect of correlation on the balance between symmetric and unsymmetric TS remains largely a matter for conjecture.However the sym- metric TS seems the more likely as Salem’s work on the Diels-Alder reaction had previously shown that a parallelism exists between the results of STO-3G and 4-31G +3 x 3CI calc~lations.~ As far as the experimental evidence on [2+4] cycloaddition reactions is con- cerned Firestone has produced another lively article5 which contends that this points towards biradical intermediates but most authors seem to favour the more balanced arguments forwarded by Huisgen in favour of the concerted mechanism.6 ‘Pericyclic Reactions’ ed. A. P. Marchand and R. E. Lehr Academic Press New York 1977 Vols. I and 11. (a) L. A. Burke and G. Leroy Theor. Chim. Acta 1977 44 219; cf. ibid. 1975 40 313; (b)M.V. Basilevsky A. G. Shamov and V.A. Tikhomirov I. Amer. Chem. SOC.,1977,99 1369. P. Caramella K. N. Houk,and L. N. Domelsmith J. Amer. Chem. Suc. 1977,99,4511. R. E. Townsend G. Ramunni G. Segal W. J. Hehre and L. Salem J. Amer. Chem. Soc. 1976 98 2190. ’ R. A. Firestone Tetrahedron 1977,33 3009. ‘R. Huisgen J. Org. Chem. 1976,41,403. 59 R. J. Bushby There can be no doubt however that suitable substitution can divert most of these reactions into a two-step pathway and also that intermediates are involved in many thermal 12+21 cycloaddition reactions. In an interesting discussion of the nature of these intermediates and of the problems involved in their interception7 it has once again been stressed that an over-rigid distinction cannot be drawn between the biradical (1)and the zwitterion (2).However the reaction of tetracyanoethylene with enol ethers has been taken as a model of one involving an intermediate of high zwitterionic type.76 a-b-a-b + (1) (2) Most treatments of orientation in cycloaddition reactions have been in terms of PMO theory' and Trong Anh and co-workers have pointed out the advantages of their simple approach to the problem.8u*b This concentrates on selecting the site at which bonding is first likely to occur by looking for the most important HOMO- LUMO two -cenPre interaction (rather than the four-centre interaction normally employed) which then dictates the course of the reaction. This method produces reliable predictions in most cases even the troublesome case of acrolein dimeriza- tion," and the results are furthermore independent of the degree of concertedness of the reaction.As well as the more traditional cr-p correlations,' localization" and FMO" approaches have also proved their value in rationalizing relative reactivities. Hence the energy barriers for the cycloaddition of maleic anhydride to various aromatic systems may be correlated with the corresponding changes in resonance energy." On the other hand as predicted by FMO theory values of logk for the Diels-Alder reaction of various l-aryl-4-benzylidene-5-pyrazolones""(3) Ph Ar (3) correlate well with their measured reduction potentials and the rates of addition of diazomethane to double bonds correlate with experimental estimates of the energy difference E(HOM0 diazomethane)- E(LUM0 dipolarophile.l16 When '(a)R.Huisgen Accounts Chem. Res. 1977 10 199; (6) ibid. p. 117. (a)0.Eisenstein J. M. Lefour N. Trong Anh and R. F. Hudson Tetrahedron 1977 33 523; (b)C. Minot and N. Trong Anh ibid. p. 533; (c) V. Bachler and F. Mark ibid. p. 2857; (d)V. Bachler and F. Mark Theor. Chim. Acta 1976 43 121. C. K. Bradsher T. G. Wallis I. J. Westerman and N. A. Porter J. Amer. Chem. SOC.,1977,99,2588;S. Ito and I. Saito Tetrahedron Letters 1977 1203. lo W. C. Herndon J.C.S. Chem. Comm. 1977 817; cfi R. D. Brown J. Chem. SOC. 1950,691 2730. '' (a)G. Desimoni P. P. Righetti E. Selva G. Tacconi V. Riganti and M. Specchiarello Tetrahedron 1977,33 2829; (b)J. Geittner R. Huisgen and R. Sustmann Tetrahedron Letters 1977,881;(c)ibid.p. 877. Reaction Mechanisms-Part (ii) Pericyclic Reactions 61 however CNDO calculated energies are employed more than just the simple FMO interactions have to be included.”“ Further kinetic evidence has been provided for a twisted quasi-zwitterionic TS in the thermal retro-[2 +21 reactions12 of cyclobutanones and among photochemical [2+21 reactions a particularly neat example has been reported by Paq~ette’~ in the conversion of compounds (4) into (5) via the caged intermediate (6). Me Me Me Studies of the Diels-Alder reaction have once again emphasized that factors besides secondary orbital interactions particularly steric factors can influence endolexo ratios14 and X-ray crystallography has confirmed that the adduct formed between 2-cyano- 1,3-diphenylallyl anion and trans-stdbene also has the expected geometry (7) for a concerted [2 +4] suprafacial addition process.1s Among the related [2 +41 addition reactions of 1,3-dipoles Huisgen has reported details of work on azomethine imines16 and Padwa has continued his studies of nitrillium ylides.” He has suggested”” that the observation that ylide (8; R = H CF3 or aryl) gives adduct (9) whereas the corresponding ylide in which R is alkyl gives adduct (lo) can be rationalized by consideration of the geometry of the intermediate.” Hence the intermediate with R=H or aryl is thought to be linear R RYR ph+rph 12 H.M. Frey and R. A. Smith J.C.S. Perkin II 1977 752; ibid. p. 2082. 13 L. A.Paquette T. G. Wallis K. Hirotsu and J. Clardy J. Amer. Chem. Soc. 1977,99 2815. 14 T. Sasaki K. Kanematsu K. Iizuka and N. Izumichi Tetrahedron 1976,32,2879;D. W. Jones J.C.S. Perkin I 1977 980; K. Seguchi A. Sera and K. Maruyama Bull. Chem. SOC.Japan 1976 49 3558. 15 W. T. Ford and G. F. Luteri J. Amer. Chem. SOC.,1977,99 5330. 16 R. Huisgen and A. Eckell Chem. Ber. 1977,110,522,540,559,571;R. Huisgen M. V. George A. S. Kende and A. Eckell ibid. p. 578. 17 (a) A. Padwa P. H. J. Carlsen and A. Ku J. Amer. Chem. SOC.,1977 99 2798; (b) A. Padwa and P. H. J. Carlsen ibid. p. 1514; A. Padwa and N. Kamigata ibid.,p. 1871. P. Caramella R. W. Gandour J. A. Hall C. S. Deville and K. N. Houk J. Amer. Gem. Soc. 1977,99 18 385. R.J. Bushby and to behave as a 1,3-dipole but the intermediate with R=alkyl is thought to be bent with high resultant carbene character. An independent synthesis and study of the reactions of compound (11)l9 has shown that of the two adducts formed between tropone and cyclopentadiene (12) and (13) the first probably arises by initial formation of (11) followed by a Cope (11) (12) (13) rearrangement but that the second cannot be formed from this intermediate. It is probably the result of a concerted [4 +61 cycloaddition. This completes an interes- ting analogy between this reaction and the reaction of cycloheptatriene and 2,5-dimethyl-3,4-diphenylcyclopentadienone.20 Whereas it has now been proved that the reported [2+6] adduct between N-ethoxyazepine and diethyl azodicarboxylate is in fact a normal Diels-Alder product2'" the formation of an analogous adduct (14) with nitrosobenzene has been confirmed by X-ray a,:'ph NCO2Et (14) The reaction of electron-deficient acetylenic dienophiles with the pentalenes (1 5) (X =H CHO CN or C02Me) has been shown to involve attack on the electron- rich ring to give the [2+8] adducts (16) whereas cyclopentadiene attacks the electron-poor ring to give the [2+4] adducts (17).22 It is also interesting to note (15) (16) (17) that although 8-phenylheptafulvene seems to give with tetracyanoethylene the expected [2+8] adduct (18) the 8,8-diphenyl compound gives a [2+4] adduct l9 M.Franck-Neumann and D. Martina Tetrahedron Letters 1977 2293. 2o K.N. Houk and R. B. Woodward J. Amer. Chem. SOC.,1970,92,4143. '' (a)W. S. Murphy and K. P. Raman J.C.S. Perkin I 1977,1824; (6)W. S. Murphy K. P. Raman and B. Hathaway ibid. 2521. 22 M. Suda and K. Hafner Tetrahedron Letters 1977,2449,2453; see also B. Kitschke and H. J. Lindner ibid. p. 25 11. Reaction Mechanisms-Part (ii) Pericyclic Reactions (19).23 In this case presumably steric rather than electronic factors are responsible for the change in selectivity. A remarkable formal [2+8] adduct (20) whose structure has been confirmed by X-ray crystallography is also formed in the reaction of 4-phenyl-1,2,4-triazoline-3,5-dione with ~ctalene.~~ Ph I (18) (19) (20) Aspects of the elimination of nitrogent5 and of sulphur dioxide26 have been reviewed and a detailed study of the elimination of nitrogen from optically active cis- and trans-3-ethyl-5-methylpyra~olines~~ has shown that the results for the two isomers differ so that no universal stereochemical course can be assigned to these reactions.It has also been shown that elimination of nitrogen is enormously accelerated by the introduction of a trimethylsilyl group in the 4-po~ition.~~ This has been rationalized in terms of concerted nitrogen loss and silyl migration (21). Hammett LFER studies of the elimination of sulphur dioxide from the thiiren dioxides (22) have provided evidence for a stepwise process29 and it has been shown that elimination of sulphur monoxide from the cis- and trans-thiiran oxide (23) proceeds with 95% retention of stereo~hemistry,~’ in contrast to earlier results for related systems.306 2 SigmatropicReactions STO-3G and 4-31G ab initio MO calculations on the 1,3-sigmatropic rearrange- ment of vinyl alcohol suggest that the hydrogen follows an antarafacial route but that the energy barrier to this is high (calculated as 85 kcal m~l-’).~’ This apparent 23 K.Komatsu M. Fujimori and K. Okamoto Tetrahedron 1977 33 2791. 24 E. Vogel H.-V. Runzheimer F. Hogrefe B. Baasner and J. Lex Angew. Chem. Internat. Edn. 1977 16 871. ’’ H. Meier and K.-P. Zeller Angew. Chem. Internat. Edn. 1977 16 835. 26 W. L. Mock,in Vol. 11of ref. 1. 27 T. C. Clarke L. A. Wendling and R. G. Bergman J. Amer. Chem. SOC.,1977,99,2740. 28 R. F. Cunico and H. M. Lee J. Amer. Chem.Soc. 1977,99,7613. 29 J. C. Philips and 0.Morales J.C.S. Chem. Comm. 1977 713. 30 (a) W. G. L. Aalbersberg and K. P. C. Vollhardt J. Amer. Chem. Soc. 1977 99 2792; (6) G. E. Hartzell and J. N. Paige J. Org. Chem. 1967 32 459; K. Kondo M. Matsumoto and A. Negishi Tetrahedron Letters 1972 2131. 31 W. J. Bouma D. Poppinger and L. Radom J. Amer. Chem. SOC.,1977,99,6443. R. J. Bushby resistance to unimolecular tautomerism has been advanced as an explanation of the stability of this species in the vapour phase. In the liquid form tautomerism is rapid but here bimolecular mechanisms are far more likely. In contrast semi-empirical MO calculations on other 1,3-sigmatropic shifts predict a suprafacial route prob- ably with high biradical/zwitterionic character,32 and experimental evidence in support of this latter prediction has been 1,5-Sigmatropic shifts in indenyl systems continue to attract attention,34 and for the rearrangement of the vinyl-substituted indenes (24) it has been shown that a good LFER exists between the rate of the sigmatropic shift and that for the corresponding attack of morpholine on CH2=CHX.34" This emphasizes the importance of the secondary orbital interaction between the HOMO(indeny1) and LUMO(migrating vinyl group).A similar secondary orbital interaction (25) has been used to explain the high rate of rearrangement of the spirononatriene (26) relative to its isomer (27) (rate ratio > 1000 l).35Studies of conformationally constrained indenyl suggest that the migrating vinyl group prefers an em orientation as in compound (28) rather than an endo orientation as in compound (29).An exu orientation avoids the unfavourable secondary orbital interactions (marked *) found in the endo TS (30). LUMO ethylene CH=CHX HOMO pentadienyl W. W. Schoeller J. Amer. Chem. SOC.,1977 99 5919; J. P. Grima F. Chopfin and G.Xaufmann J. Organometallic Chem. 1977,124 315; T. Minato S. Inagaki H. Fujimoto and K. Fukui Bull. Chem. SOC.Japan 1977,50 1651. and J. E. Baldwin S. E. Branz and J. A. Walker J. Org. Chem. 1977 42 4142d W. Kirmse and H.-R. Murawski J.C.S. Chem. Comm. 1977 122. (a)D. J. Field and D. W. Jones J.C.S. Chem. Comm. 1977,688;(b)D.W. Jones and G. Kneen J.C.S. Perkin I 1977 1313; W. A. Pettit and J. W. Wilson J.Amer. Chem. SOC.,1977 99 6372; K. K. de Fonseka C. Manning J. J. McCullough and A. J. Yarwood ibid.,p. 8257. M. F. Semmelhack H. N. Weller and J. S. Foos J. Amer. Chem. SOC.,1977,99,292. Reaction Mechanisms-Part (ii) Pericyclic Reactions Schmid and co-~orkers~~ have provided impressive evidence for potential 2,5 -biradical character (31) in the TS of the Cope rearrangement by studying the quantitative effect of radical-stabilizing substituents at these positions uiz. (32; X I X (31) (32) R=Me X=H CN or C02Me). They point out however that this biradical mechanism is unable to explain all of the phenomena associated with these re- arrangements and suggest that they are normally concerted and only diverted into the biradical pathway when such 2,5-substituents are present.Similar studies have been reported by Dewar3’ who however no favours this flexible mechanism hypothesis but proposes a 2,5-biradicaloid even in the rearrangement of hexa-1,5-diene. His MIND0/3 calculation^^^ suggest that this is a true inter- mediate and that it prefers a chair conformation. The balance between boat and chair TS (or intermediate!) in other 3,3-sigmatropic rearrangements has been investigated by several groups.39 For 3,3’-dimethyl-3,3‘-bi~yclopropenyl~~~ the chair-like TS (33) is preferred over the boat-like TS (34) by 4.3 kcal mol-’ and a chair-like TS also seems to be preferred in the thermal (35a) and acid-catalysed (35b) amino-Claisen rearrangement^.^^^ These latter reactions have been studied in some and appear to involve a concerted pericyclic mechanism.The large acceleration brought about by protonation of the nitrogen is attributed to charge delocalization in the reaction (36). In contrast the Claisen rearrangement (35) a;X=NH b;X=NHz+ (3 ) implicated in t,,e reaction of allylic alcohols with cyclic orthoe~ters~~~ may proceed either predominantly through a chair-like TS or a boat-like TS depending largely on steric factors and the two TS’s also seem to be finely balanced in the rearrangements of ortho-dienones such as compound (38).39d In this particular 36 (a)R. Wehrli H. Schmid D. Bellus and H.-J. Hansen Helu. Chim. Actu 1977 60 1325; full English summary Chimia (Swirl.),1976,30,416. 37 (a)M. J. S. Dewar and L. E. Wade J.Amer. Chem. Soc. 1977,99,4417;(6) cf. J. Amer. Chem. Soc. 1973 95 290. M. J. S. Dewar G. P. Ford M. L. McKee H. S. Rzepa and L. E. Wade J. Amer. Chem. Soc. 1977,99 5069. 39 (a)J. H. Davis K. J. Shea and R. G. Bergman J. Amer. Chem. Soc. 1977,99,1499; (b)S. Jolidon and H.-J. Hansen Helv. Chim. Actu 1977 60 978; (c) R. J. Cave B. Lythgoe D. A. Metcalfe and I. Waterhouse J.C.S. Perkin I 1977 1218; (d)A. Wunderli T. Winkler and H.-J. Hansen Helv. Chim. Actu 1977,60 2436. R. J. Bushby case the threo-form rearranges mainly via a chair-like TS (39) whereas the erythro-form prefers a boat-like TS (40). (37) (38) (39) (40) Several studies have been reported which bear on the possible involvement of radical pairs in the sigmatropic rearrangements of anionic and zwitterionic Hence the 2,3-sigmatropic rearrangement of the anion (41) normally proceeds with inversion of the ally1 group but as the temperature of the reaction is raised there is some scrambling of the isotopic label (marked *) pointing to a dissociation-recombination mechanism.40a Studies of pentadienyl-N-acyl- ammonioamidates (42)"Ob show that the proportions of the 1,2- and 5,2-rear- rangement products (43) and (44) are characteristic only of the substituents R1,R2 and R3and not the group X and hence these rearrangements are formulated as involving the radical intermediate (45) whereas the proportion of the 3,2-re- arrangement product (46) is a function of the nature of X and this product must be formed at least in part by a different mechanism possibly via the concerted TS (47).Studies of the Wittig rearrangement of the optically active ether (48)40c show similar degrees of racemization in the products of both 1,2- and 1,4-shifts in contrast to the corresponding Stevens rearrangement where the product of the Me i-NCOX Me,N-NCOX R:;+yyy 3 Me2N-NCOX 4 R2 R3 R2 R3 R2 R3 (42) (43) (44) Me,N-"COX Me,?-yNCOX R' R' / R2 R3 R2 R3 R2 R3 40 (a)E. Grovenstein and A. B. Cottingham J. Amer. Chem. SOC. 1977 99 1881; (b) K. Chan-trapromma W. D. Ollis and I. 0. Sutherland J.C.S. Chem. Comm. 1977 97; (c) H. Felkin and C. Frajerman Tetrahedron Letters 1977,3485; (d)E. F. Jenny and J. Druey Angew. Chem. Internat. Edn. 1962 1 155. Reaction Mechanisms-Part (ii) Pericyclic Reactions 1,4-shift is the more highly racemized of the (Scheme).An attractive simple explanation of this observation is based on the assumption that the anion (48) adopts a cis configuration and the anion (50) an all-trans configuration; then the radical pair (49) is presumably generated in the configuration shown in which the a-phenylethyl radical has an equal distance to travel to form either product whereas the radical pair (51) is generated in an extended configuration where it hzs much further to travel in order to produce the 1,4-rearrangement product with consequently greater chances for racemization. Ph P -H -++Me + p0-CLPh 0-Ph Me / Me -30% racemization -30% racemization Ph Me Me + PhL N M % NMez -0% racemization -50% racemization Scheme Various mechanisms have been demonstrated for dyotropic rearrangements of silicon compounds all of which appear to involve some degree of nucleophilic attack on silicon.41 Hence the rearrangement of the allyl system (52) is thought to involve initial co-ordination between the oxygen and the silicon followed by migration of the allyl group as shown (53),41" and the fluoride-ion-catalysed rear- rangement of the same compound to proceed via initial displacement of the trimethylsilyl group to give the intermediate (54).41b Similarly the rearrangement of the ester (55) is formulated as involving the intermediate (56).41c 41 (a)M.T. Reetz Chem. Ber. 1977 110 965; (b) M. T. Reetz and N. Greif ibid. p. 2958; (c) M. T. Reetz and N.Greif Angew. Chem. Infernat.Edn. 1977 16 712. R. J. Bushby Me Me I +- Ph2C- SiMe,I o+c,o I I R R (55) (56) 3 Ene Reactions Detailed studies of the ene reaction of terminal alkenes with maleic anhydride have been interpreted in terms of a concerted mechanism and a preferred ex0 TS,42 and the stereochemistry of the dimer formed from the tricycloundecene (57) has also been advanced as evidence for a concerted Lewis-acid catalysis of the ene reaction of chloral allows the reaction to be performed under mild synthetically useful conditions and added ferric chloride has also been shown to reverse the preferred stereochemistry in the reaction with a-~inene.~~ This can be understood in terms of the simple steric model illustrated in formulae (58) and (59) I H (57) (58) .(59) in which it is assumed that the reactions are concerted and that in the ferric chloride-chloral complex the trichloromethyl and ferric chloride groupings are trans to each other. 4 Electrocyclic Reactions Several theoretical studies of electrocyclic reactions have been reported45 and calculations of the energy surface for the opening of cyclopropylidenes to allenes have revealed a complex combination of disrotatory and conrotatory motions and provided an explanation of how chirality is retained.45a 42 F. R. Benn J. Dwyer and I. Chappell J.C.S. Perkin 11,1977 533. 43 R. Greenhouse W. T. Borden T. Ravindranathan K. Hirotsu and J. Clardy J. Amer. Chem. Soc. 1977,99,6955. 44 G. B.Gill and B. Wallace J.C.S. Chem. Comm. 1977 380 382. 45 (a) P. W. Dillon and G. R. Underwood J. Amer. Chem. Soc. 1977 99 2435; (b) N. L. Bauld and J. Cessac ibid. p. 23; Y.Jean and A. Devaquet ibid. p. 1949; M. J. S. Dewar G. P. Ford and H. S. Rzepa J.C.S. Chem. Comm. 1977,728; R. Cimiraglia,M. Persico and J. Tomasi J. Phys. Chem. 1977 81,1876. Reaction Mechanisms-Part (ii) Pericyclic Reactions The ring-opening reactions of Dewar-ben~ene"~ have and of bi~yclopentene~~ been shown to proceed in a straightforward manner even though formally disallowed. In the case of 5 -methylbicyclopentene Bald~in"~ has provided evi- dence that products (60) and (61) arise by a 'hot molecule' rearrangement of the initial 1,4-bond cleavage product as a result of which he claims that the contro- versy which has surrounded these rearrangements may be finally 'retired'.The electrocyclic ring-opening reactions of ep~xides"~ have been reviewed48a and trapping of the resultant carbonyl ylides has shown that as expected the thermal reaction proceeds mainly in a ~onrotatory~~~*~ and the photochemical reaction mainly in a disrotatory manner.48d7e In both cases however some cross- over products are obtained and these seem to arise not only by isomerization of the intermediate but also by a competing ring opening in the disallowed sense. In the case of vinyl-substituted sy~terns,~~~'~ such as compound (62),48' the initial product (63) of conrotatory ring opening can be trapped with maleic anhydride or may undergo a disrotatory closure to form a five-membered ring (64).It is interesting to note that n.m.r. observation of the reduction of the cyclopropyl bromide (65) with lithium in THF at -60 "Cshows initial formation of the 2,E-anion (66)49 which then more slowly isomerizes to the equilibrium Z,E/E,E Ph Ph mixture. This is consistent with the expected disrotatory opening of an inter- mediate cyclopropyl radical.49b Attempts to probe the stereochemistry of the 46 M. J. Goldstein and R. S. Leight J. Amer. Chem. Soc. 1977,99 8112. " G. D. Andrews and J. E. Baldwin J. Amer. Chem. SOC.,1977,99,4853. (a)R. Huisgen Angew. Chem. Internal. Edn. 1977 16 572; (6) V. Markowski and R. Huisgen J.C.S. Chem. Comm. 1977,439,440; (c) M. S. Medimagh and J. Chuche Tetrahedron Letters 1977,793;(d) W.Eberbach and U. Trostman ibid. p. 3569; (e)V. Markowski and R. Huisgen ibid. 1976 p. 4643. 49 (a) G. Boche and D. R. Schneider Tetrahedron Lerters 1976 3657; (b) cf. M. Newcomb and W. T. Ford J. Amer. Chem. SOC.,1974,96 2968; and S. Sustmann and C. Ruchardt Chem. Ber. 1975.108 3043. R. J. Bushby related pentadienyl anion-cyclopentenyl anion interconver~ion~~ have been largely frustrated by uncertainties which surround the geometry of the reacting or by isomerization of the reaction produ~t.”~ Thermal cyclization of the 12welectron system (67) gives the expected product of conrotatory closure (68).’l This cannot however be claimed as a great success for the predictive power of the orbital symmetry rules as the reaction is reversed photochemically ! (a)D.H. Hunter S. K. Sim and P. R. Steiner Canad. J. Chem. 1977 55 1229; (b)C. W. Schoppee and G. N. Henderson J.C.S. Perkin I 1977 1028. H. Sauter B. Gallenkamp. and H. Prinzbach Chern.Ber. 1977,110 1382.

ISSN:0069-3030    

DOI:10.1039/OC9777400059

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii)Polar Reactions By H.R. HUDSON Department of Chemistry The Polytechnic of North London Holloway Road London N7 8DB 1 Introduction Nucleophilic substitution at saturated carbon particularly in solvolysis reactions continues to account for a substantial proportion of the research publications coming under this heading. The first section of this year’s Report deals with some recent studies on structural effects in this area whilst a critical account of medium effects on the rate and mechanism of solvolysis reactions is given elsewhere.’ Also included below are sections on nucleophilic substitution at unsaturated carbon in compounds containing carbon-carbon (vinylic) or carbon-nitrogen double bonds carbocations addition and elimination.Carbanions reactions of carbonyl compounds and other relevant topics are deferred to make way for a new section on ion-molecule reactions in the gas phase in so far as these throw light on the mechanisms of polar organic reactions. Full coverage of all aspects of organic reaction mechanisms is given in an annual review of this subject.* 2 Nucleophilic Substitution at Saturated Carbon The tool of increasing electron demand which was introduced to study partic- ipation and the non-classical carbonium-ion pr~blem,~ has become a powerful means for the investigation of solvolytic processes. The method involves the attachment of a series of substituted aryl groups to the potential carbocation centre and a determination of the p+ value of the system as a measure of its sensitivity to electronic influences.In the solvolysis of aryldialkylcarbinyl p-nitrobenzoates (1) in 80% aqueous a~etone,~ the similarity in p+ values for t-cumyl 3-methyl-2-buty1 and 3-pentyl derivatives indicates that the stabilizing effect of the alkyl groups (Me Et and Pi) must be nearly the same in each case. High negative values for the cyclopropyl and cyclobutyl members of the 1-aryl- 1-cycloalkyl p-nitrobenzoate series (2) have been attributed to the effect of I-strain which destabilizes the carbocation and results in T. W. Bentley and P.von R. Schleyer Ado. Phys. Org. Chem. 1977,14 1. ‘Organic Reaction Mechanisms 1977’ ed. A. R. Butler and M. J. Perkins Wiley London 1978. H. C. Brown ‘The Non-Classical Ion Problem’ Plenum New York 1977.H. C. Brown M. Ravindranathan E. N. Peters C. G. Rao and M. M. Rho J. Amer. Chem. Soc. 1977 99,5373. 71 H. R. Hudson R' I Ar-C-X (dH2)n-l C(Ar)X I R2 (1) X = p-N02C6H4C02-; Ar =p-XC6H4 (X = MeO Me H or CF3)or 3,5-(CF3)2C6H3 an increased electron demand on the aryl system. Cyclohexyl also gives rise to a relatively high negative p+ value which has been attributed to the resistance of this conformationally stable system to the introduction of an sp2 centre. Unambiguous evidence for participation of carbon has been obtained by the use of a similar procedure in the solvolysis of a series of 9-aryl-9-pentacyclo-[4,3,0,02*4 03,' O5.']nonyl p-nitrobenzoates (3).' The p+ value for this system (-2.05) is comparable to that observed in the solvolysis of 7-norbornenyl deriva- tives in which n-participation is significant.Comparison of solvolysis rates with those for the corresponding 7-norbornyl derivatives (4) shows also that with increasing electron demand at the cationic centre the rates of solvolysis of the pentacyclic derivatives increase markedly relative to those for the 7-norbornyl compounds. Solvolysis of the parent substrate (3; R =H) is 10'o-1012 times faster than for the related 7-norbornyl derivative. The results which are also supported by low 9-Me/9-H and 9-Ph/9-Me rate ratios in the pentacyclic series are inter- preted in terms of (To)-participation by the remote cyclopropane bond leading to the trishomocyclopropenyl cation (5). X =p-NO2C6H4CO2-;R =p-XC6H4(X = MeO H or CF3) or 3r5-(CF3)2C6H3 Absence of significant a-participation has been demonstrated in the solvolyses of tertiary 2-aryl-2-norbornyl (6) and 2-aryl-2-camphenilyl p-nitrobenzoates (7).6 In X =p-NO2C6H4Co2-;Ar =p-XC6H4 (X =MeO Me H or CF3)or 3,5-(CF3)&H3 these series the effect of deactivating the aromatic ring and thereby increasing electron demand at the cationic centre should be to increase the exolendo rate ratios if cr-participation is an important factor.Essentially constant exolendo rate H. C. Brown and M. Ravindranathan J. Amer. Chem. SOC. 1977 99 299. H. C. Brown K. Takeuchi and M. Ravindranathan J. Amer. Chem. SOC.,1977,99 2684. Reaction Mechanisms-Part (ii) Polar Reactions ratios are however obtained within each series.The results imply that the high exolendo rate ratios observed in the tertiary 2-norbornyl systems are due mainly to decreased rates of reaction in the sterically hindered endo direction of the norbor- nane structure; and by extrapolation a similar conclusion has been drawn for the corresponding secondary systems. A fundamental difference in the behaviour of tertiary and secondary systems has been reported in the solvolysis of 5-norbornen-exo -2-yl substrates.’ Whereas the secondary ester (8) undergoes assisted ionization during acetolysis to yield the symmetrical norbornenyl-nortricyclyl cation (9) and thence racemic products (10) and (11) (Scheme l) solvolysis of optically active 1,2-dimethyl-5-norbornen-exo -2-yl p -nitrobenzoate (12) in 90% aqueous acetone yields 2-methylene- l-methyl-5-norbornene (16) (with 62% retention of configuration) 1,2-dimethy1-5-norbornen-exo-2-o1 (15) (with 15% retention of configuration) and racemic 1,6-dimethyl-3-nortricyclanol(17)(Scheme 2).The results which are also similar for methanolysis are consistent with the initial formation of a sym-metrical tertiary cation (13). /(10) (93%) HOAc* &Gis (9) (11) (7%) Scheme 1 /1 1 (X= p-N02C6H4C02-) Scheme 2 ’H.L. Goering and C.-S. Chang I. Amer. Chem. Soc.,1977,99,1547. H.R. Hudson Evidence for C-C 0-delocalization in simple secondary carbocations has been obtained in the acetolysis of some N-alkyl-N-nitroso-acetamides which give small yields of primary acetates.' For example the cyclohexyl derivative gives cyclohexyl acetate and cyclopentylmethyl acetate in the ratio 99.05 :0.95.Rearrangement of 1-ethylpropyl to 2-methylbutyl (0.3%)and of s-butyl to isobutyl (0.2%) similarly occurs. It has been argued that the equilibrium coefficient between a secondary and a primary cation (70-80 kJ mol-' apart in energy) will be ca. 10l2 and would require a physically unrealistic difference of lo9or more in the reaction rates of two assumed classical intermediates to account for the product ratios observed. A non-classical ion or protonated cyclopropane (19) generated only when the R-C and C-A bonds of the substrate (18) are anti-parallel has been proposed to account for these strongly endothermal secondary to primary rearrangements (Scheme 3) and it has been suggested that C-C 0-delocalization in simple carbocations may be more widespread than has frequently been implied.RZ-CHz R,-CH R'-CH2 \ -* CH,,\C -\C-A ,,' \ I H 'Ri H (18) (19) Scheme 3 Evidence has been obtained for the involvement of protonated cyclopropane intermediates in the trifluoroacetolysis of n-[ 1-l4C]buty1 mercuric perchlorate at 50 or 72 0C.9 Under these conditions the 14C label appears at all positions of the major solvolysis product (2-butyl trifluoroacetate) at positions 1 and 2 of the n-butyl ester and is divided in a 50 50 ratio between C-1 and the rest of the molecule in the small amount of isobutyl ester (ca. 2%) which is formed. The results are consistent with 8-14% of the products being formed from a series of equilibrating CH y3 (743 I CH .c.H2 / +*.\ **:y ,' t I\ H,C-CH H,C--CH3 H3c'--m (20) (21) (22) m=14~ CH3 I C.M. Cooper P. J. Jenner N. Perry H. Storesund J. Russell-King and M. C. Whiting J.C.S. Chem. Comm. 1977,668. C. C. Lee and R. Reichle J. Org. Chem. 1977,42. 2058. Reaction Mechanisms-Part (ii) Polar Reactions 75 edge- or corner-protonated cyclopropanes [(20)-(291. At 35 OC only successive 1,2-hydride shifts appear to be involved (Scheme 4). CH3CH2CH214&H2 + CH3CH26H14CH3$ CH36HCH214CH3 Scheme 4 The description of secondary solvolysis in terms of competing neighbouring- group (ka)and nucleophilic solvent-assisted (k,)processes is now well established [equation (l)].'"In limiting or unassisted solvolyses the rate constant is represen-kl= k,+ k (1) ted by k, and there has been much debate on the possible importance of such a process in secondary systems.l1 Recent studies on the solvolysis of cyclo-octyl tosylate have now shown that the solvolysis of a secondary substrate by the k process may not be unusual even in a rather nucleophilic solvent and where there are no obvious barriers to back-side approach by the nucleophile provided that relief of ground-state strain is sufficient to provide a competitive pathway.12 The kinetic evidence is based on comparisons with adamantyl tosylate in aqueous ethanol or aqueous 2,2,2-trifluoroethanol and is supported by theoretical cal- culations on the ionization of methylcyclo-octane (Scheme 5) (the methyl group Scheme 5 being considered sterically similar to a leaving group such as chloride or p-nitrobenzoate).The calculations indicate a value of -3.06 kcal mol-' for the S-strain whereas other acyclic or monocyclic substrates show positive 6-strain values. In another context new light has been thrown on the importance of steric acceleration in solvolytic reactions by a determination of the X-ray crystal struc- ture of tris(t-buty1)methyl p-nitr~benzoate.'~ Both B-strain which results from crowding of the bulky t-butyl groups and F-strain which results from steric interference between the t-butyl groups and the p-nitrobenzoate moiety are clearly demonstrated by distortions of bond lengths and bond angles. The relief of both types of strain as the transition state is formed must therefore be assumed to have significant effects on reaction rate.3 Nucleophilic Substitution at sp2 Carbon Vinylic Substitution.-In spite of considerable work in recent years on vinyl cations as reaction intermediates l4 a number of fundamental questions related to the lo €3. Capon and S. P. McManus 'Neighbouring Group Participation' Vol. 1 Plenum New York 1976. J. R. Pritt and M. C. Whiting J.C.S. Perkin II. 1975 1458 and references cited therein. J. M. Harris D. L. Mount M. R. Smith and S. P. McManus J. Amer. Chem. SOC. 1977,99 1283. l3 P. T. Cheng S. C. Nyburg C. Thankachan and T. T. Tidwell Angew. Chem. Internat. Edn. 1977,16 654. I4 Z. Rappoport Accounts Chem. Res. 1976 9 265; M.Hanack ibid. p. 364; Ann. Reports (B),1975 72.71. H. R. Hudson extent of bond-breaking in the transition state and to substituent effects have remained unanswered. Recent studies on the solvolysis of ring-substituted -styryl trifluoromethanesulphonates (triflates) (26) and their dideuterio-analogues (27) HK=C(Ar)OS02CF3 D2C =C(Ar)OS02CF3 (26) (27) Ar = XC6H4 (X =H P-C~, m-Cl p-CF3 or P-NO~) have gone some way towards elucidating these factor~.~’ For both series excellent Hammett plots were obtained with p = -4.1 which is indicative of considerable charge development in the transition state even with the strongly deactivating p-nitro-substituent present. A P-deuterium isotope effect of kH/kD = 1.45 for the parent compounds agrees well with earlier results.Taken with other literature data a full range of kH/kD values from 1.21 for p-methoxy to 1.71 for p-nitro is obtained and shows a linear correlation with substituent u+values. The results indicate that rotation of the aryl group to deconjugate with the C=C double bond and to conjugate with the nascent empty p-orbital occurs early and must be complete or nearly so at the transition state leading to the vinyl cation. Novel rearrangements of the vinyl cation have been reported in the solvolyses of the spiro-triflate (28) and the primary vinyl triflate (30).16 At 130°C in aqueous ethanol or aqueous trifluoroethanol buffered with pyridine the spiro substrate gives a yield of 90-98% of diene (29). It has been concluded that migration of the adjacent cyclohexyl bond to the vinyl cation centre occurs probably in a concerted fashion with anchimeric assistance (Scheme 6).Cycloheptanone is the principal product obtained under similar conditions from the primary vinyl substrate and it has been suggested that in this case concerted ionization and bond migration acruss the vinyl group occurs to give the cyclic vinyl cation (31) as the first intermediate (Scheme 7). The rearrangement suggests that a ‘bent’ secondary vinyl cation of this type is more stable than the corresponding linear primary vinyl cation. (30) (31) Scheme 7 *’P. J. Strang R. J. Hargrove and T. E. Dueber J.C.S. Perkin IZ 1977 1486. l6 P. J. Strang and T. E. Dueber Tetrahedron Letters 1977 563. Reaction Mechanisms-Part (ii) Polar Reactions 77 acetolysis of triaryl[2- ‘3C]vinyl bromides (Scheme 8) in the presence of silver shown that the relative extents of isotopic scrambling which occur during the acetolysis of triaryl[2-13C]vinyl bromides (Scheme 8) in the presence of silver Ar213C=C(Ar)Br HoAc+ Ar213C=C(Ar)OAc+A~O’~c(Ar)=cArz Scheme 8 acetate are in the approximate ratio 1 :2 :3 for the phenyl p-tolyl and p-anisyl derivatives respectively.l7 The differences are much smaller than would be expec- ted if migratory aptitude was the predominant factor in determining the extent of rearrangement and it has been suggested that the stability of the initial cation and the effect of the particular aryl group on the electrophilic character of the migration terminus are also important.Higher percentages of rearrangement occur in trifluoroacetic acid. Kinetic solvent isotope effect studies show that an addition- elimination sequence can occur although it becomes less significant if a salt such as silver or sodium acetate is added. The acid-catalysed hydrolysis of vinyl sulphides has been shown to proceed by a mechanism analogous to that of vinyl ethers.l* Slow proton transfer to the C=C double bond gives a sulphur-stabilized carbenium ion which reacts rapidly with water to form products (Scheme 9). Scheme 9 An interesting controversy has arisen over the stereochemistry of nucleophilic substitution of (E)-and (Z)-3-chloro-2-phenylpropenonitriles[(32)and (33);X= Cl].” Whilst retention of configuration is confirmed in reactions of these isomers Ph X Ph H \/ \c=c / NC/c=c\H /\ NC x (€)-isomer (2)-isomer (32) (331 with weak bases such as morpholine piperidine and phenol-triethylamine it is claimed that 1:1 mixtures of the corresponding (E)-and (2)-vinyl ethers are obtained in reactions with strong bases such as sodium ethoxide in ethanol or sodium phenoxide in THF.20 Such a result is explicable by initial retention followed by a number of ‘racemization’ mechanisms,’l and could possibly be due to the use of higher concentrations of reactants in the later work.Of more fundamen- tal importance is the view2’ that the previous configurational assignments for (E)-and (Z)-3-ethoxy-2-phenylpropenonitriles[(32) and (33); X=OEt]” should be C. C. Lee A. J.Paine and E. C. F. KO Canad.J. Chem. 1977,55 2310. R. A. McClelland Canad.J. Chem. 1977 55 548. 19 2. Rappoport and A. Topol J.C.S. Perkin IZ 1975 982. 2o G. Le Guillanton and M. Cariou J.C.S. Perkin 11 1977 997. 2’ Z.Rappoport J.C.S. Perkin IZ,1977 1000. H. R. Hudson reversed. This would imply that an unprecedented inversion of configuration had occurred in the substitution reaction with ethoxide and the result should be viewed with caution at this stage. Nucleophilic Attack on C=N Double Bonds.-Nucleophilic substitutionsof diaryl-imidoyl chlorides (34;X = C1) by secondary amines in benzene or acetonitrile show complex mechanistic behaviour. In benzene both first- and second-order terms in [amine] are observed and the Hammett plots show minima which are indicative of two competitive processes having opposite electronic demands.22 The results have been ascribed to a combination of an SN2(ion-pair) process for electron-donating substituents (Scheme 10) and a nucleophilic addition-elimination process (AdN-E) for electron-attracting substituents (Scheme 11).Superimposed on these schemes are less important third-order amine-catalysed routes in which the amine may serve as a base electrophile or bifunctional catalyst. In acetonitrile the amine-catalysed route disappears and it has been suggested that the AdN-E process occurs together with reaction either ria ion-pairs or free nitrilium ions. X IAr-C=N-Ar’ NR2I I (34) R2NH Scheme 11 The less reactive imidoyl cyanides (34;X =CN) undergo nucleophilic substitu- tion by amines in acetonitrile or by alkoxides in alcohols only by an addition- elimination sequence.23 It has been suggested that a common initial step involves nucleophilic attack on the imidoyl cyanide by the amine alkoxide ion or alcohol and that the subsequent expulsion of the leaving group may be uncatalysed or amine-catalysed in acetonitrile or solvent-assisted in the alcohol.4 Carbocations Preparation and Rearrangements in Super-acid Media.-The first direct obser- vation of a long-lived cyclopropyl cation has been made by dissolving the highly strained 11-methyl-1 l-bromotricyclo[4,4,1,01~6]undecane(35) in S02CIF at -60°C and adding the solution slowly to SbF5 in SO2C1F at -120°C (Scheme 12).24 The ‘H n.m.r. spectrum at -90 “C reveals substantial deshielding of the 22 R.Ta-Shma and Z. Rappoport J. Amer. Chem. SOC.,1977,99,1845. 23 R. Ta-Shma and Z. Rappoport J.C.S.Perkin ZZ 1977,659. 24 G.A. Olah G. Laing D. B. Ledlie and M. G.Costopoulos J. Amer. Chem. SOC.,1977,99 4196. Reaction Mechanisms-Part (ii) Polar Reactions Scheme 12 methyl and methylene protons which is indicative of a discrete carbocation struc- ture. The I3C n.m.r. spectrum shows that the ion is unsymmetrical and is best described as a ‘bent’ cyclopropane cation (36). Deshielding at C-1 C-6 and C-11 suggests that the positive charge is delocalized by interaction of the C-1-C-6 u-bond with the p-orbital at C-1I. Some new examples of substituted pyramidal dications have been prepared by dissolution of the precursors (37) and (38) in HFS03-SbF5 (1 :1) in S02ClF at -60°C.25 Those containing two ethyl or two isopropyl groups are found to have these substituents in the basal positions and their formation is rationalized as shown (Scheme 13).HS03F-SbF5 in HO R R=Et or Pr’ (37) Scheme 13 25 C. Giordano R. F. Heldeweg and H. Hogeveen J. Amer. Chem. SOC.,1977,99,5181. H. R. Hudson Ring closure of ally1 to cyclopropylcarbinyl has been systematically studied by treatment of 4-methylpent-1-en-3-01 (39; R' = Me R2 = H) 2,4-dimethylpent-1-en-3-01 (39; R'= R2= Me) and pent-1-en-3-01 (39; R' = R2= H) with FS03H- SbFs in S02C1F at -78 or -1200C.26 The corresponding cyclopropylcarbinyl cations (41) which are thought to be formed via the homoallylic ions (40) rear- range further at higher temperatures to the more highly substituted allylic s ructures (42) (Scheme 14).The general order of thermodynamic stability ffr ions in which R' = R2= H is (RCH=CH-CH2)+ < cyclo-C3H5C+R2< (RCH-CH-CHR)' and the stabilizing effect of a cyclopropyl group on a carbenium centre has been estimated as 11-17 kcal mol-' larger than that of a vinyl group. R2 I R' H R2 R' C R' R2 Ill I A\ I I H3C-C-C-C=CH2 + H3C-C-C CH2 + H3C-?-CH2-C=CH2 It II H OH HH (39) Scheme 14 A study of rearrangements and equilibria in the ions formed under stable ion conditions from P-arylethyl chlorides and their side-chain-substituted derivatives shows that u-bridged ethylenearenium (phenonium) ions are formed only from 2-chloroethylbenzene and its ring-substituted derivative^.^' The ethylene-benzenium ion (43) is formed quantitatively in HF-SbF5 at -60°C and subsequently rearranges at higher temperatures via the highly unstable 2-phenyl- ethyl cation (44) to give the benzylic ion (45) (Scheme 15).Substitution of methyl Scheme 15 groups at C or CBleads upon ionization to rearranged benzylic and/or equili- brating /3 -phenylethyl cations but the experimental determination of the energy difference between these two types is possible only in the case of the ions derived 26 H. Mayr and G. A. Olah J. Amer. Chern. SOC.,1977,99 510. 2' G. A. Olah R.J. Spear and D. A. Forsyth J. Amer. Chem. SOC.,1977,99 2615. Reaction Mechanisms-Part (ii) Polar Reactions from l-(p-methoxyphenyl)-2-chloropropane(Scheme 16).The relatively small energy difference (8-10 kcal mol-') has been interpreted in terms of stabilization of the @-phenylethyl cation by wbridging (46),although the concentration of this species at equilibrium is thought to be exceedingly low. CH2CHCICH CH -CIi -CH CH2-,CH-CH3 CH 2-,CH -CH I I .' Q' OCH OCH OCH (46) Scheme 16 The proton-decoupled and proton-coupled 13Cn.m.r. spectra of all C3to Cs alkyl cations show the methyl substituent effects to be constant and additive.28 They can therefore be used to estimate 13Cchemical shifts of static alkyl cations as well as of symmetrical or unsymmetrical equilibrating carbocations. There is little or no contribution from hydrogen-or methyl-bridged structures in equilibrating t-cations and the experimental data allow calculations to be made of AG AH and AS for the differences between them.Application of the additive substituent effect method to equilibrating secondary carbocations e.g. s-butyl shows significant deviations of the estimated from the experimental values and indicates that there is a contribution from partial hydrogen-bridging. Other Aspects.-A critical examination of the use of 13C chemical shifts for establishing electron densities in carbocations has been made on the basis of solvolysis rate studies for the p-nitrobenzoates shown [(47)-(49)J.29 The large (X =P-NO~C~H~CO~-) Relative kf 1.O 103 lo2.' 13Cshift/p.p.m (from TMS) 329.2 254.4 280.6 (*80% aqueous acetone 25 "C) rate enhancements observed with phenyl or cyclopropyl substituents on the a-carbon atom appear to result from major electron supply from the ring system rather than from the relief of B-strain.Nevertheless the relative rates show no correlation with the 13Cchemical shifts for the corresponding carbocations and it has been questioned whether such shifts can be used in their present state of understanding to prove unequivocally the structures of ions such as 2-norbornyl or 2-bicyclo[ 2,1,1] hexyl. A 'bona fide free methyl cation' has been generated in the liquid phase by allowing tritiated methane to undergo @-decayin solution in benzene and toluene *' G. A. Olah and D. J. Donovan J. Amer. Chem. SOC.,1977,99,5026. z9 H.C. Brown and E. N.Peters J. Amer. Chem. Soc. 1977,99,1712. H. R. Hudson [equation (2)].30 Alkylation of the aromatic ring ensues. This novel approach appears to provide a means for observing the intrinsic reactivity in solution or even CT P-decay) CT;+ 3He+P-(2) in the solid state of a simple carbocation completely unperturbed by the effects of solvent or counterion etc. 5 @-Elimination The range of mechanisms available for /3 -elimination has been discussed previously [cf.Ann. Reports (B),1976 73 61; 1975 72 761. Substrates with good leaving groups attached to a s-or t-carbon atom and having no activation of the proton on CB react with weak bases which are also strong nucleophiles by a mechanism designated E2C. The precise nature of the ‘loose’ transition state involved is still a topic for active discussion which centres on whether there is a covalent ‘SN2-like,’ component to the interaction of the base with C,[(50) or (51)] or whether such interaction is electrostatic in nature (52).What is said to be compelling evidence ._ (50) (5 1) (52) against the extensive development of positive charge on C has been obtained by comparison of the secondary deuterium isotope effects associated with the P’C-H and rC-H bonds of cyclohexyl tosylate in its elimination reaction with tetra- butylammonium hydroxide in acetone.31 The similarity in magnitude of these effects (kSe-d/k,,-d= 0.98) is taken to indicate a substantial degree of formation of a double bond in the transition state. It has however been pointed that 1,3-diaxial interactions between the y-hydrogens and the leaving group will be relieved as the transition state is formed whatever the mechanism and that the changes in associated vibrational frequencies will induce an isotope effect at C of the type observed.An examination of the elimination (E2) and substitution (SN2) reactions of cyclohexyl tosylate with triphenylphosphine shows that this neutral weak base which is also a good carbon nucleophile is a poor reagent for elimina- tion compared to anionic weak bases which promote the E2C process (PhS- OAc- C1- PhO- Br- etc) or compared to strong neutral bases such as tri- ethylamine or 1,5-diazabicyclo[4,3,O]non-5-ene(DBN) whose reactions are more E2H-like in character.32 The results suggest that whilst interaction between an anionic base and C is important in the E2C transition state this interaction is primarily electrostatic.The methoxide-induced eliminations of l,l-diaryl-2,2-dichloroethanes[(p-XC6H4)2CHCHC12] and their 0-deuteriated analogues provide what is claimed to 30 F. Cacace and P. Giacomello J. Amer. Chem. SOC.,1977,99 5477. 31 D. Cook J. Org. Chem. 1976,41 2173. 32 D. J. McLennan J.C.S. Perkin IZ 1977 293. Reaction Mechanisms-Part (ii)Polar Reactions be the first example of mechanistic changeover from R (for X = H or C1) to ElcB (for X=NOz) as a result of changing the substituent in the @-bound aromatic ring.33 Similar conclusions have been reached as a result of studying chlorine isotope effects in the same system (X = MeO C1 H or NO,) and it has been shown that the C-Cl bond must be very nearly intact in the E2 transition [cf.Ann.Reports(B) 1976 73 63 for a preliminary communication]. The influence of changes in substituents on C on the mechanism followed is exemplified in the reactions of methanolic methoxide with 2,2-di-(p-nitrophenyl)-l 1,l-trifluoro-ethane 2,2-di-(p-nitrophenyl)-l-fluoroethane and their P-deuteriated ana-logue~.~~ The presence of three fluorine atoms on C leads to a clear example of reversible formation of the carbanion intermediate i.e. the (ElcB)R mechanism is in operation. In contrast the analogous trichloro-substrate has been shown to undergo elimination via the irreversible carbanion mechanism designated (E~cB)~.~~ With only one fluorine (i.e.the leaving group) on C, the reaction follows the E2 mechanism with a carbanion-like transition state.An interesting example of alkene formation with hydride ion as the leaving group is provided by the elimination reactions of organolithium and organomagnesium compounds in the presence of hydride acceptors e.g. tetraphenylcarbonium tetrafluoroborate tricyclohexylborane or tri-s-butylborane [equation (3)].37 M = Li or Mg;A = Ph4C+BF, (c~CIO-C~H~~)~B, or (s-C~H~)~B Preferential formation of the Hofmann product has been attributed to steric influences. 6 Electrophilic Addition Two possible mechanisms have generally been considered to be available for the acid-catalysed hydration of olefins. One involves the rate-determining protonation of carbon (the AsE2mechanism) (Scheme 17) whilst the alternative route which fast + H30+ slow )+--tH(+H20)) H O ~ + HH+ Scheme 17 reflects the dependence of reaction rate on acidity function rather than pH involves the initial formation of a .n-complex which subsequently undergoes rate-deter- mining conversion into the solvated carbocation (Scheme 18).A review of the literature data on olefin hydration together with numerous new experimental results now provides evidence that all olefin hydrations proceed by the AsE2 33 D. J. McLennan J.C.S. Perkin 11 1977 1753. 34 A. Grout D. J. McLennan and I. H. Spackman J.C.S. Perkin 11 1977 1758. 35 J. Kurzawa and K. T. Leffek Cunud. J. Chem. 1977,5S 1696. 36 D.J. McLennan and R. J. Wong J.C.S.Perkin 11 1974 1373. 37 M. T.Reetz and W. Stephan Angew. Chem. Internat. Edn. 1977,16,44. H. R. Hudson Scheme 18 process.38 This conclusion is based on the extremely good correlation [equation (4)] which is obtained by plotting the second-order constants for olefin hydration in aqueous acid at 25 "Cagainst the sum of the relevant up' parameters for the alkyl substituents attached to the carbocation centre of the intermediate R'R2R3C+. For a range of ninety-six olefins including 1,l-and 1,2-disubstituted alkenes 2-substituted buta- 1,3-dienes substituted styrenes vinyl esters N-vinylacetamide and 2-bromopropene a correlation coefficient of 0.938 is obtained with p = -10.5 and C = -8.92. A comparison of ethylene with p-nitrostyrene at Ho-7.37 shows that ethylene is the less reactive as predicted by equation (4) and indicates that ethylene itself protonates through an AsE2 transition state to give the open ethyl cation.This quite remarkable conclusion is at variance with many currently held views on the role of carbocation intermediates in nucleophilic solvents. Indeed it has so far proved impossible to detect a free ethyl cation in super-acid media let alone in water and it seems likely that the interpretation of this result will come under careful scrutiny. Two sets of workers have now concluded that contrary to previous reports solvent effects do not cause significant changes in the relative rates of bromination of alkenes and alkynes although the absolute rates may change con~iderably.~~~~~ Structural effects are however significant in determining the rate-constant ratio kC=C/kCrC,which is ca.lo3 for styrene and phenylacetylene but ca. 1.0 for cinnamic acid (or its 4-nitro-derivative) and the corresponding acetylene in a range of hydroxylic solvents.39 The role of structural effects is greatly enhanced in chlorinated hydrocarbon solvents in which the reaction is also seen to be second- order in bromine.41 The importance of electrophilic solvent assistance to the leaving group (Br-) is clear but there is a difference of opinion as to whether the cationic intermediate is subject to specific nucleophilic ~olvation,~' or simply to a medium It has been proposed that the accepted mechanism of olefin bromination (Scheme 19) is also valid for acetylene^.^' Scheme 19 7 Ion-Molecule Reactions in the Gaseous Phase Increasing attention has been paid in recent years to the 'intrinsic' (ie.solvent-free) properties of basic processes such as proton transfer nucleophilic substitution V.J. Nowlan and T. T. Tidwell Accounts Chem. Rex 1977,10 252 and references cited therein. 39 G. H. Schrnid A. Modro and K. Yates J. Org. Chem. 1977,42 2021. M. F. Ruasse and J.-E. Dubois J. Org. Chem. 1977,42 2689. " A. Modro G. H. Schrnid and K. Yates J. Org. Chem. 1977.42 3673. Reaction Mechanisms-Part (ii) Polar Reactions 85 carbocation formation etc. as revealed by studies of ion-molecule reactions in the gaseous phase. Investigations have been made mainly with techniques such as pulsed ion cyclotron resonance (ICR) mass ~pectrometry,~~ pulsed electron beam high-pressure mass ~pectrometry,~~ or that photoionization mass spe~trometry,~~ involving the use of flowing afterglow apparatus.45 Apart from the inherent inter- est of gas-phase reactions the results can throw considerable light on the nature of solvation processes and can enable a clearer insight to be obtained into many aspects of solution chemistry.Proton Transfer.-Earlier work has shown that the presence of alkyl substituents in a molecule can increase both acidity and basicity in the gaseous phase an effect said to be due primarily to polarization of the alkyl group by the nearby ionic Distant alkyl groups however appear to destabilize a negative charge so that gas-phase acidities tend to be decreased by their influence in phenols carboxylic acids and acetylene^.^^ Gas-phase Acidities.The order of acidities for a series of alkanethiols has now been like that for to be exactly the reverse in the gaseous phase to that in aqueous solution i.e. for the gas-phase reaction shown [equation (5) (X = 0 RX-+ BU'XH -+ BU'X-+ RXH (5 1 (X = 0 or S) or S)]AG* increases for various R groups in the order Me<Et<Pr" (given for X=S only) <Pr'<Bu'. Whereas the reversal of acidity order for alcohols in dimethyl sulphoxide has been shown to result primarily from the effect of the size of the alkyl substituent on the heat of solution of the alkoxide anion,49 a similar analysis for alcohols in water cannot be made as only the free energies of ionization are available.A consideration of the effect of alkyl groups on the relative values of enthalpy and entropy of ionization for alkanethiols in water shows however that the main factor causing the inversion of order in this case is the entropy of ionizati~n.~~ It has further been concluded that the main factor controlling the relative gas-phase acidities of alkanethiols hydrogen sulphide and the alcohols is the electron affinity of the corresponding RS' or RO' radicals bond energies being nearly constant in each series. Water is anomalous The intrinsic acidities of some fifty substituted phenols and benzoic acids have been determined from equilibrium constants and AGO values for their gas-phase proton-transfer reactions with a range of standard aliphatic acids of known acidities [equation (6)]." The results show that phenol and acetic acid have similar acid 42 J.L. Beauchamp Ann. Rev. Phys. Chem. 1971,22,527. O3 A. J. Cunningham J. D. Payzant and P. Kebarle J. Amer. Chem. SOC.,1972 94 7627. 44 A. D. Williamson P. Le Breton and J. L. Beauchamp J. Amer. Chem. SOC.,1976,98 2705; P. R. Le Breton A. D. Williamson J. L. Beauchamp and W. T. Huntress J. Chem. Phys. 1975,62,1623; M. S. Foster A. D. Williamson and J. L. Beauchamp Znterngt. J. Muss Spectrometry Zon Phys. 1974,15,429. 45 D. K. Bohme R. S. Hemsworth J. W. Rundle and H. 1. Schiff J. Chem. Phys 1973,58,3504. 4d J. I. Brauman and L. K. Blair I. Amer. Chem. Soc. 1968 90,6561; 1970,92 5986. 47 (a)R. T. McIver jun. and J. S.Miller J. Amer. Chem. Sac. 1974,% 4323; (b)R. T. McIver jun. and J. H. Silvers ibid. 1973,95 8462; (c)K. Hfraoka R. Yamdagni and P. Kebarle ibid. 1973,95,6833. O8 J. E. Bartmess and R. T. McIver jun. J. Amer. Chem. Soc. 1977,99,4163. 49 E. M. Arnett L. E. Small R. T. McIver jun. and J. S. Miller J. Amer. Chem. Soc. 1974,96 5638. T. B. McMahon and P. Kebarle J. Amer. Chem. SOC. 1977,99,2222. H. R. Hudson A,H+A + Ar+A,H (6) strengths in the gaseous phase whereas benzoic acid is stronger by ca. 8.7 kcal mol-’. Solvation must account for the differences in water in which acetic and benzoic acids have similar strengths and are stronger than phenol by nearly 6pKa units. It can be visualized that the acetate anion with its relatively small methyl group and with its negative charge localized on the two oxygen atoms will be more strongly solvated than will the phenoxide anion which is larger and which has the charge delocalized over the aromatic ring.Ionization of acetic acid in water is therefore relatively more favourable. The relative acid strengths of benzoic acid in the gaseous phase and in aqueous solution have been interpreted in terms of contributions from charge-separated canonical forms for which solvation is more favourable than for the uncharged species and which are more important in the case of the acid than the anion. Substituent effects for meta -and para -substituted benzoic acids in the gaseous phase are revealed through linear plots of free-energy changes for the gas-phase reaction (7) against Taft’s go or Brown’s (+ values the former giving a slightly XCdH4C02-+PhC02H + XC6H4CO2H+PhCO2-(7) better correlation.o-and p-Hydroxybenzoic acids have anomalously high gas- phase acidities but it can be shown that in the gaseous phase the phenolic proton is in each case the more acidic and that the compounds are better regarded as substituted phenols in this context. Enhanced acidity of the phenolic proton can be attributed to the -M effect of the ortho- or para-carboxy-group. On the other hand the carboxy-group itself is made less acidic by the +M effect of a hydroxy- group in the ortho- or para-position. The strength of the ortho-hydroxy-acid is further increased by stabilization of the anion through internal hydrogen-bonding between 0-and the C02H group.A good linear correlation is obtained between the gas-phase and aqueous solu- tion acidities of meta- and para-substituted phenols as measured by AG* under these two conditions for the same proton-transfer reaction [equation (S)]. It is XC6H40-+ PhOH 4 XC6H4OH +PhO-(8) noteworthy that AGeis 6.8 times larger in the gas phase than in aqueous solution for phenols and 10.6 times larger for the acids. Although attenuations of substit-uent effects from gaseous phase to solution are not uncommon those for the phenols and benzoic acids in water are unusually large and are attributed to hydrogen-bonding in this solvent in which substituent effects are largely reflected in the entropy term. In keeping with a previous study of gas-phase acidities and solution acidities in dimethyl s~lphoxide,~~ the attenuation of substituent effects in phenols in this medium is small (factor of ca 2) and is contained in the enthalpy term.Gas-phase Basicities. It has been shown that the observed strengths of amines in the gaseous phase are consistent with a simple electrostatic An alternative ” F. G. Bordwell J. E. Bartmess G.E. Drucker Z. Margolis and W. G. Matthews J. Amer. Chem. Soc. 1975,97,3226. ’* D. H. Aue H. M. Webb and M. T. Bowers J. Amer. Chem. Soc. 1976,98 311,318. Reaction Mechanisms-Part (ii)Polar Reactions 87 approach53 shows that an excellent linear correlation is obtained by plotting AGe or AH0 values for the gas-phase proton-transfer reaction [equation (9)] against RCH2NH2+CH36H3 + RCH&H3 +CH3NH2 (9) new ‘intrinsic’ substituent constants uI,obtained from gas-phase ionization-potential data and polarizability rn~dels.’~ Two interpretations have been Either (a) the observed effects of alkyl substituents on gas-phase base strengths and the oxparameters are both determined by polarizability effects and involve no internal inductive effects or (b) the q parameters are a measure of internal inductive effects of alkyl groups but the observed effects of the alkyl substituents are a combination of polarizability and internal inductive effects both having very nearly the same structural dependencies on chain length and branch- ing.The second interpretation is probably correct in view of recent comparisons of the quantitative effects of alkyl groups on gas-phase acidities of alcohols with those of gas-phase basicities of amines.” Nucleophilic Substitution.-Studies of gas-phase SN2reactions [equation (lo)] have revealed a wide variety of reaction In certain cases intermediate X-+RY + Y-+RX (10) adducts between anions and alkyl halides have been obtained (e.g.C1CH3Br-) with stabilities ranging from 8.6 to 14.4kcal mol-’.’’ The possibility of detecting the neutral product RX has also been demonstrated and it has been shown that the interaction of chloride ion with either cis-or trans-4-bromocyclohexan-1-01pro-ceeds with preponderant inversion of configuration.’’ Reaction rates and efficien- cies (i.e.the fraction in each case of collisions resulting in reaction) for a wide range of systems (X=OH C1 Br F CN MeO MeS or Bu‘O; RY=MeCI MeBr CF3C02Me PhOMe MeOMe Me3CCH2Cl Me3CC02Et MeSH or MeC02H) lead to the conclusion that the reaction is best described in terms of a ‘double-well’ potential [curve (a) Figure 11 the energy barrier in many cases being lower than the energy of the reactants.59 The increase in the barrier in solution results solely from the differential solvation of the reactants and the transition state the more localized charge on the reactant anion making this species better solvated.In a polar aprotic solvent such as dimethyl sulphoxide the barrier is raised less than in a protic solvent which affords the possibility of specific hydrogen-bonding (Figure 1). The intrinsic nucleophilicities of anions in the gaseous phase follow the reverse order of the polarizabilities (e.g.MeO->MeS-and F->C1->Br-) a result which may be due to the stronger interaction which will occur between the more 53 R. W. Taft and L. S. Levitt J. Org. Chem. 1977 42 916. 54 L. S. Levitt and H. F. Wilding Progr. Phys. Org. Chem. 1976,12 119. 55 R. W. Taft in ‘Proton Transfer Reactions’ ed. E. F. Caldin and V. Gold Chapman and Hall London 1974 Ch. 2 p. 66. 56 J. 1. Brauman W. N. Olmstead and C. A. Lieder J.Amer. Chem. Soc. 1974,% 4030; K. Tanaka G. I. Mackay J. D. Payzant and D. K. Bohme Canad. J. Chem. 1976 54 1634 and references cited therein. 57 R. C. Dougherty J. Dalton and J. D. Roberts Org. Mass Spectrometry 1974 8,77; R. C. Dougherty and J. D. Roberts ibid.,p.81; R. C. Dougherty ibid. p. 85. 58 C. A. Lieder and J. I..Brauman J. Amer. Chem. SOC.,1974,96,4028; Internat. J. Mass Spectrometry Ion Phys. 1975 16 307. 59 W. N. Olmstead and J. I. Brauman J. Amer. Chem. SOC.,1977.99,4219. H. R. Hudson X-*RY X-R-Y -* RX + Y' REACTION COORDINATE Figure 1 Representative diagrams of the reaction co -ordinates for a nucleophilic displacement reaction (a) in the gaseous phase and (b) in dipolar aprotic and (c) in protic solvents (Reproduced by permission from J. Amer. Chem. SOC.,1977,99,4219) concentrated charge of the smaller anions and the carbon centre. The parallel between polarizability and nucleophilicity of anions in protic solvents is thus seen to be purely an artefact of solvation effects; indeed the order may be reversed in aprotic solvents.Carbocations.-Stabilities. Heterolytic bond-dissociation energies D(R'-Br-) have been determined for a range of alkyl carbenium ions acyl cations and cyclic halonium ions by examining the equilibria shown [equation (1l)].""A combina-Rf+R2X$[RIXR2+]*$R1X+R; (11) tion of the gas-phase data with heats of solution in HS03F-SbF5 gives via appro-priate chemical cycles the relative enthalpies of solvation which have been shown to be related to ion size the smaller ions being better solvated. Relative stabilities (measured as bromide affinities) for the cyclic bromonium ions are the same in the gaseous phase as in solution. In both cases stability increases with ring size and in the three-membered rings with methyl substitution.The range of stabilities is however attenuated in solution. The adamantyl cation is found to be more stable than t-butyl in the gaseous phase indicating that the strain energy due to non- planarity of the adamantyl system is smaller than the stabilization afforded by interaction of the charge with the hydrocarbon framework. Condensation Reactions. A number of gas-phase condensation reactions of carbenium ions of possible relevance to prebiotic synthesis have been shown to be 6o R. H. Staley R. D. Wieting and J. L. Beauchamp J. Amer. Chem. Soc. 1977,99 5964. Reaction Mechanisms-Part (ii) Polar Reactions analogous to the corresponding reactions in solution. With water the isopropyl and t-butyl cations yield protonated alcohols which can undergo further hydration (Scheme 20) or elimination to yield alkene.61 Other n-donors (MeOH NH3 + H20 C4H; -%[C4H9(OH2)]' %[C4H9(OH2h] __* [C4H9(OH2)3]' + etc.Scheme 20 MeNH2 etc.)probably condense in a similar fashion.61 The gas-phase equivalent of the Koch-Haaf synthesis has likewise been shown to occur with carbenium ions generated in methane containing small amounts of carbon monoxide and water (Scheme 21) the species produced being identified as the protonated acid.62 R+ CO-~R~O H20c RCOOH; Scheme 21 Formation of formic acid was also noticed but this underwent decarbonylatior, possibly as shown (Scheme 22). n Scheme 22 Ester Cleavage.-An interesting divergence between behaviour in the gaseous phase and in solution is seen in the mechanism of ester cleavage by alkoxide ion.Whereas the BAc2 mechanism is almost universal in base-catalysed hydrolyses in aqueous media the gas-phase reaction of deuteriomethoxide ion with methyl trifluoroacetate or methyl benzoate yields the carboxylate anion as the only detectable product [equation (12)].63 A possible mechanism involves attack on the CD30-+RC02CH3 + RC02-+CD30CH3 (12) ester alkyl group via an SN2transition state (54). In protic solvents on the other hand a tetrahedral intermediate (55) is more likely to be favoured by stabilization through hydrogen-bonding to the carbonyl oxygen atom. 0 0- It 8-R-C-0 8--CH3 --OCD3 R-C-OCH~ IOCD3 (54) (55) 61 K. Hiraoka and P. Kebarle J. Amer. Chem. SOC.,1977,99,360.62 K. Hiraoka and P. Kebarle J. Amer. Chem. SOC.,1977,99 366. M. Comisarow Canad. J. Chem. 1977 55 171.

ISSN:0069-3030    

DOI:10.1039/OC9777400071

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Electron Spin Resonance Spectroscopy and Free Radical Reactions By A. T. BULLOCK Department of Chemistry University of Aberdeen Old Aberdeen Scotland A89 2UE Of several review articles and books published during the year attention is especi- ally drawn to a book devoted to applications of e.s.r. to polymer research’ and an admirable albeit wholly theoretical article on theories of chemically induced dynamic electron polarization (CIDEP).* Wan and Elliott3 have briefly reviewed CIDEP mechanisms and more importantly have given illustrations of how CIDEP and CIDNP may be used together to study photochemical reactions. E.s.r. studies of triplet states have been reviewed4 with attention drawn to cyclic 47r-electron systems CHz and environmental effects.Especial emphasis was put onto ground state triplets. Finally of relevance to the general field of this report although not specifically about e.s.r. studies is a review of free-radical rearrangements in telomerization. 1 Kinetics and Mechanism Much of the work described in this section deals with radicals having the unpaired electron centred largely on atoms other than carbon. There have been detailed studies of the generation and subsequent reactions of alkyloxy and aryloxy radicals. A possible general route to alkyloxy radicals has been described which involves the photolysis (A = 365 nm) of alkoxy-vanadium(v) chelates.6 The primary step is OR (1) (2) where Q is the 8-quinolyloxo ligand and R represents a series of nineteen alkyl groups.The production of paramagnetic compound (2) containing Vrv was ’ B. Rhby and J. F. Rabek ‘PolymersjProperties and Applications. Vol. 1. E.S.R. Spectroscopy in Polymer Research’ Springer-Verlag Berlin Heidelberg and New York 1977. ’P.W. Atkins and G. T. Evans ‘Advances in Chemical Physics’ XXXV p. 1 el. seq. ed. I. Prigogine and S. A. Rice. J. Wiley and Sons New York. London Sydney and Toronto 1976. J. K. S. Wan and A. J. Elliot Accounts Chem. Res. 1977,10 161. E Wasserman and R. S. Hutton Accounts Chem. Res. 1977,10,27. ’R. Kh. Freidlina and A. B. Terent’ev Accounts Chem. Res. 1977 10 9. S. M. Aliwi and C. H. Bamford J.C.S. Faraday I 1977.73 776. 90 Reaction Mechanisms-Part (iii) Electron Spin Resonance demonstrated directly by e.s.r.whilst comparison of the quantum yields for photo- initiated free radical polymerization of methyl methacrylate in the presence of (1) showed that each molecule of chelate decomposing gave rise to one initiating radical. Subsequent reactions of the alkyloxy radicals were studied by spin-trap- ping experiments using phenyl t-butyl nitrone (PBN) at various concentrations. In the case of primary alkyloxy radicals the e.s.r. spectrum depended on [PBN]. For example for R = Bun a simple spectrum was obtained at [PBN] = 0.008 M which was attributable to the adduct from the isomerized hydroxyalkyl radical (4). PrnCH20. 3 Pr"CH0H (2) (3 (4) At high [PBN] (ca. 0.3 M) both (3) and (4)were trapped. Most secondary aIkyloxy radicals gave the same spectra at high and low concentrations of PBN.The coupling constants as-Hand aNof the resultant nitroxides were characteristic of alkyl adducts. Clearly p -scission occurs according to the general equation R(R)CHO* + R'CHO+.R' (31 However p-scission was found to be less important for R=Pr' cyclopentyl and cyclohexyl where the behaviour was similar to that found for the primary alkyloxy radicals i.e. complex spectra at high [PBN] with and uN suggesting alkyloxy and hydroxyalkyl adducts. Only two derivatives with t-alkyl groups were studied namely R = But and t-pentyl. Both gave simple spectra at high and low [PBN] with splittings characteristic of alkyl adducts. Again this was thought to be a consequence of /3 -scission viz. Me3C0. -+ CH3+ Me2C0 (44 EtMe2CO-+ C2H5+ Me2C0 (4b) Product analyses confirmed this the yield of Me2C0 being 60% for reaction (4a) and 100% for (4b).The reactions of secondary alkyloxy radicals have also been studied in a flow system.' They were produced by Ti"' reduction of secondary alkyl hydroperoxides and cyclic hydroperoxides. No RO-radicals were observed directly but some were trapped by the aci-anion of nitromethane and characterized by the e.s.r. parameters of the ROCH2NP2 adducts. In all cases carbon-centred radicals were observed to be derived from the parent alkyloxy radicals and were explained in terms of the following four reaction pathways. (i) 1,2-hydrogen shift [cf.equation (2) for n-alkyloxy radicals] 1.2-H R'R~CHO--CR'R~OH H20 (ii) 1,5-hydrogen shift 1,s-H e.g.Me(CH&CH(O.)Me -CH2(CH2)2CHMeOH (6) ' B. C. Gilbert R.G. G. Holmes and R. 0.C. Norman J.Chem. Research 1977 (S)l; (M)Ol01. A. T.Bullock (iiia) C-C fragmentation e.g. (oIo*z .CH20(CH2)20CH0 (7) 0 (iiib) C-Me fragmentation FC-Me e.g. MeCH20CH(O*)Me-*CH3 (iv) C-0 fragmentation (iFC-0 e.g. Me2CHOC(0.)Me,-Me2eOH (9) (ii)l.2-H The authors suggest that the 1,2-H shift [equation (5)J is not intramolecular but probably involves the solvent.8 It was concluded that alkyloxy radicals from secondary hydroperoxides show an ease of reaction in the order 1S-H>FC- > 1,2-H>FCkMe.This contrasts with the order for primary alkyloxy radicals uir. 1,2-H "1,5-H>Fc-c. Aryloxy radicals have been obtained by photorearrangement of nitro-compound^.^ Photochemical rearrangements of nitro-groups are often postulated to proceed uia the nitrite" and it is claimed' that the detection of aryloxy radicals during the photolysis of several hindered nitro-compounds is strong evidence for the intermediate formation of nitrite groups.As an example photolysis of 6-nitro-benzo[a]pyrene (5) gave the spectrum of the aryloxy radical (6) [Scheme (l)].The radical (6) had previously been prepared by H-atom abstraction from benzo[aIpyren-6-01.l1 NO2 (5)=ArN02 Scheme 1 Other oxygen-centred radicals have been studied notably the decay kinetics of cumylperoxy radicals. l2 Previous e.s.r. determinations of the decay kinetics of peroxy radicals have given first second and intermediate orders.The authors point out that in most previous reports the radicals were produced photolytically B. C. Gilbert R. G. G. Holmes H. A. H. Laue and R. 0.C. Norman J.C.S. Perkin II 1976 1047. Y. Ioki J.C.S. PerkinII 1977 1240. lo 0.L. Chapman D. C. Heckert J. W. Reasoner and S. P. Thackaberry J. Amer. Chem. Soc. 1966,88 5550. C. Nagata M. Inomata M. Kodama and Y. Tagashira Gann 1968,59 289. '2 S. Fukuzumi and Y. Ono,J.C.S. Perkin II 1977,622. Reaction Mechanisms-Part (iii) Electron Spin Resonance and suggest that the measured kinetics may have been complicated by the presence of other products of the photolysis. To avoid this the cumylperoxy radicals were produced by flowing a solution (CCl, benzene and cumene solvents) of cumyl hydroperoxide (0.744.70 M) through a tube packed with 20-30 mesh manganese dioxide or cobalt oxide supported on silica.On stopping the flow decay of the cumylperoxy radicals was found to be strictly first-order (kt=0.15* 0.015 s-' at 300 K) irrespective of the initial hydroperoxide and cumylperoxy concentrations and of the nature of the solvent and catalyst. The authors s~bsequently'~ confirmed the first-order decay by measuring the radical concen- trations during the steady-state decomposition of cumene hydroperoxide with Pb02 the latter being held in suspension in the cavity. The decomposition was found to be a radical chain-reaction. The hydroperoxide formed a complex on the surface of Pb02 the complex subsequently dissociated to the peroxy radical which then desorbed into solution.From the kinetic studies it was found necessary to postulate the formation of a 2 1 complex between the hydroperoxide and the peroxy radical. Two pieces of spectral data supported this namely the g-value and the observed line-width both varied monotonically with the initial concentration of cumene hydroperoxide. The kinetic g-value and linewidth results were all accommodated by K =0.50 l2 moF2 for the equilibrium RO2. +2R02H $ (RO2* .* 2RO2H) (10) The same authors have used similar techniques i.e. the heterogeneously catalysed production of radicals to study the liquid-phase autoxidation of cumene with Pb02,14 the determination of cross-propagation rate coefficients in the autoxidation of hydrocarbon^,'^ and the mechanism of the formation of the p-benzosemiquinone anion over manganese dioxide.16 In the autoxidation of cumene (with Pb02) the kinetics were studied over the temperature range 291-393 K.' E.s.r.was used to monitor the concentration of the chain-propagating cumylperoxy radical and the rate of oxygen consumption was measured under the same conditions. Both the radical concentration and the rate of oxygen consumption were constant with time and independent of the catalyst weight liquid volume ratio. Experimentally it was found that [RO2*]= k,[RH][Pb02]* (11) and -d[02]/dr =kb[RH]2[Pb02]0 (12) These results together with the product distribution were shown to be consistent with the following mechanism. Ri Initiation R02H+Pb02 +RO2.k14 R02-+RH -Propagation R. +R02H l3 S. Fukuzumi and Y. Ono J.C.S. Perkin ZI 1977,625. l4 S. Fukuzumi and Y. Ono J.C.S. Perkin ZZ 1977,784. S. Fukuzumi and Y. Ono J. Phys. Chem. 1977,81,1895. l6 Y. Ono T. Matsumura and S. Fukuzumi J.C.S. Perkin II 1977 1421. kl8 ROy d PhCOMe+MeO. fast MeO. +R02. dstable products The value of 2kr7was found to be 5.0~ lo51 mo1-ls-l (291K) in agreement with that found ea~1ier.l~ The following Arrhenius parameters were found loglo(k14/l mol-' s-l) = 5.3-3o/e (20a) and loglo(k17/1 rnoi-' s-') = 10.9-30/e (20b) where 0 = 2.303 RT kJ mol-'. The Pb02-catalysed decomposition of t-butyl hydroperoxide in the presence of hydrocarbons has been studied by the same method." It has been shown that simultaneous measurements of [Bu'Oz*] (by e.s.r.) and the rate of oxygen genera- tion lead to a direct determination of the rate coefficients for cross-propagation reactions (e.g.hydrogen abstraction from hydrocarbons by Bu'02-) and for self- reaction of Bu'Oz*. Rate coefficients for termination chain transfer and cross termination involving Bu'02* were also determined by e.s.r. It was suggested that the technique provides a method for determining absolute rate coefficients for reactions involving Bu'Oz* at various temperatures that is more accurate and simple than the classic hydroperoxide method.17 Before leaving this technique we note that the mechanism of the formation of the p-benzosemiquinone anion over manganese dioxide has been studied.l6 The kinetic results could be fitted to the equation d[SAlldt =k(b[HQ10-[SAl)(c[Mn0~10-[SAI) (21) where k,b and c are constants [Mn02Jo is the Mn02 weight to liquid volume ratio and [Ha] and [SA] are respectively the initial concentrations of hydroquinone and semiquinone anion radical. It was proposed that the active surface species Mn02* abstracts a hydrogen atom from hydroquinone to give the neutral radical which then desorbs and is converted into the semiquinone anion radical. The number of surface species of Mn02* was determined to be 7.7 x 10l8m-, in good agreement with previous values obtained for the oxidations of l,l-diphenyl-2-picrylhydraz-ine18 and cinnamyl alc~hol.'~ l7 J. A. Howard W. A. Schwalm and K. U. Ingold Adu. Chem. Ser.1968 No.75 p. 6. A. T. T. Oei and J. L. Garnett J. Catalysis 1970 19 176. l9 D. Dollimore and K. H. Tonge J. Chem. SOC.(B),1967,1380. Reaction Mechanisms-Part (iii)Electron Spin Resonance 95 The reactions of esters and anhydrides in a Ti"'/HzO2 flow system have been studied in some detail2' following an earlier observation that vinyl acetate in this flow system gave mainly the spectrum of the methyl radicaL2' It has now been found that the alkyl radicals are formed by the route indicated in equations (22)-(24) uiz. RCOzR' + HO2-+ RC020-+ R'OH (22) RC020H+Ti"' -+ RC02*+ OH-+Ti'" (23) The first step is perhydrolysis followed by one-electron reduction of the resultant peroxo-acid. Decarboxylation of the acyloxy radical RC02* then gives the alkyl radical.In some cases the spin trap CH2:NOz- enabled the intermediate acyloxy radicals to be detected in addition to the alkyl radicals. From the relative concen- trations of the spin adducts RC02CH2N02'- and RCH2N02'- it was found that the rates of decarboxylation decrease in the series shown in Scheme 2. A lower limit co; Scheme 2 for the rate of decarboxylation of CH3C02- was estimated to be 2 X lo7s-l whilst an upper limit for PhCO2. is 2.5 x lo5s-l. Decarboxylation reactions have also been observed in the adduct radicals formed by the reaction of photochemically produced acyl radicals with the fumarate dianion.22 The acyl adducts (7) under-went decarboxylation to give the 1-carboxy-3-oxyallyl radical dianions (8) [Scheme 31. For R=Me and Et the e.s.r.spectra of (7) and (8) were observed simul- taneously. Plots of loglo{[RC(O-)=CHCHO2-]/[RCOCH(CO~-)CHCO,-I) us 1/T showed that the activation energies for the decarboxylation step were equal for R = Me and Et to within experimental error and had the value 40 f2 kJ mol-'. The acyl radicals were readily identifiable since hyperfine coupling to protons bonded to 2o B. C. Gilbert R. G. G. Holmes P. D. R. Marshall and R. 0.C. Norman J. Chem. Research 1977 (S)172; (M)1949. D. J. Edge B. C. Gilbert R. 0.C. Norman and P. R. West J. Chem. SOC.(B) 1971 189. 22 S. Steenken and M. Lyda J. Phys. Chem. 1977.81,2201. 96 A. T. Bullock C4 and Cs (C and C of the acyl precursor) of the ally1 radical (8)could be measured. In contrast to these exampleszo*z2 of ready decarboxylations a study of the radical zwitterions formed by the one-electron oxidation of a series of m'ethoxylated benzoic acids by TI2+ Agzt and SO,-has shown that the presence of even one methoxy- group is sufficient to stabilize the radicals with respect to decarb~xylation.~~ The radical zwitterions could also be produced from the addition of *OHto mono- di- and tri-methoxylated benzoic The adducts reacted with H' (k = 10'-lo91mo1-ls-l) to give the zwitterions via elimination of water from the protonated adduct.There have been several reports of sulphur-centred radicals Thus a series of aromatic and aliphatic sulphinyl radicals have been observed and ~haracterized.~' The aromatic sulphinyls were produced by one or other of the three routes shown in Scheme 4,where the -OH radicals were generated in a conventional Ti1I1/H2O2 OH 0 *OH I -H+ // ArSSAr ArSSAr _r Ar-S -ArS .OH H202 -OH //O ArSH -ArS.+ArSOH +ArS 0 hv // ArS(0)CI -+ ArS + C1. Scheme 4 flow system. The aromatic radicals were all 71-radicals with extensive delocaliza- tion on to the aromatic ring. The aliphatic sulphinyl radicals were generated by photolysis of sulphinyl chlorides at low temperatures (ca. -100 "C) and were characterized by g ~2.011 and a'-* in the range 0.8-1.1 mT. Particular attention was paid to MeSO for which g = 2.0100 a(3H)= 1.15 mT and AH the peak-to- peak derivative linewidth was 1.10 mT. The authors suggest that a previous reportz6 of the spectrum of MeSO obtained by the photolysis of t-butyl-methanesulphenate refers to some other sulphur radical [a(3H)= 0.657 mT g = 2.00965 AH = 0.06 mT].Previous failures to detect MeSO and EtSO were attri- buted to efficient spin-rotation relaxation for which Tl q/T. Sulphinyl radicals have also been observed in a study of the photochemical decomposition of organic sulphites and reactions of the sulphites with alkyloxy radical^.^' Dialkyl sulphites (R1R2CH0)zS=0 reacted with Bu'O. in two ways. Addition at the sulphur atom gave sulphuranyloxy radicals (R'RzCHO)zS(0)OB~' while hydrogen abstraction also occurred giving the carbon-centred radical ~CR'RzOS(0)OCHR'Rz. This then fragmented to give alkoxysulphinyl radicals R'R2CHOS0. Photolysis of the dialkyl sulphites in the absence of di-t-butyl peroxide resulted in fission of the S-0 bond to give alkoxysulphinyl and alkyloxy (-0CHR'R') radicals; the latter 23 P.O'Neill S. Steenken and D. Schulte-Frohlinde J. Phys. Chem. 1977,81,26. 24 P. O'Neill S. Steenken and D. Schulte-Frohlinde J. Phys. Chem. 1977 81 31. 25 B. C. Gilbert C. M. Kirk R. 0.C. Norman and H. A. H. Laue J.C.S.Perkin 11 1977,497. '' T. Kawamura P. J. Krusic and J. K. Kochi Tetrahedron Letters 1972 4075. 27 B. C. Gilbert C. M. Kirk and R. 0.C. Norman J. Chem. Research 1977 (S)173; (M)1974. Reaction Mechanisms-Part (iii) Electron Spin Resonance 97 subsequently rearranged to *CR'R20H. The spectrum of HOSO was also obser- ved in this work and possible schemes for its generation were given. The structure and formation of some dialkoxysulphuranyl radicals (10) have been described.28 Two routes seemed to involve sulphenates (9) namely the photolysis of disulphides R'SSR' and thioethers R'SR' in the presence of peroxides.These are shown in Scheme 5. Support for the intermediacy of sulphenates in routes (a) and (b) of Scheme 5 comes from the fact that sulphenates OR2 (a) R'SSR'+*OR~ 4 R'-S / -SR' OR2 .OR2 / CSR (9) -R'-$ R'-S-OR2 (b) R'SR'+-OR2 -+ R'-$-R1 I OR2 Scheme 5 themselves give dialkoxysulphuranyl radicals on photolysis. Evidence for the loss of OR'in Scheme 5(b) was provided by the result for theitan from which the ring-opened radical *CH2CH2CH2SOBu' was detected. The third route to dialk- oxysulphuranyl radicals was shown to be the direct addition of alkyl radicals to dialkylsulphoxylates (R0)2S.Except for R' = CF3 (g = 2,0079) all g values were in the range 2.0090-2.0096. Radicals of the type RCH2 (OBU')~ show spectra consistent with structure (11) whilst Ph 9(OBu'X is essentially a w-radical of structure (12). Deuterium labelling experiments together with e.s.r. have shown that the addition of MeO- radicals to (Me0)2S to give the trimethoxysulphuranyl radical (MeO)& has a high degree of stereosele~tivity.~~ The incoming radical was found to take up an apical site in the adduct. Furthermore neither intra- nor inter-molecular ligand exchange took place to an appreciable extent during the average lifetime of the sulphuranyl radicals. Of radicals carrying a significant spin density on nitrogen atoms nitroxides continue to attract attention.The syntheses e.s.r. spectra and stabilities of two 28 W. B. Gara B. P. Roberts B. C. Gilbert C. M. Kirk and R.0.C.Norman J. Chem. Research 1977 (S)152; (M)1748. 29 J. W. Cooper and B. P. Roberts J.C.S. Chem. Comm. 1977 228. A. T. Bullock 3-oxy- 1,3-diazacyclohexene- 1-oxide radicals [( 13a) (1 3b)l have been de~cribed.~' They were found to be significantly more stable than comparable a-unsubstituted nitr~xides.~' Thus (13a) decomposed with first-order kinetics with k = 6x s-' (298 K). The authors discounted the possibility of a monomer-dimer equilibrium which would show first-order kinetics for decomposition involving either slow bimolecular disproportionation of monomer or unimolecular de-composition of dimer and suggest the possibility of H-atom transfer from C-4 to 0-1 producing a reactive carbon atom.From the coupling constant of uH-4 (1.09 mT) the appropriate dihedral angle was estimated to be 10"in (13a). This is I 0-(13) a; R=H b; R=Me consistent with H-4 having a pseudo-axial conformation two of the three methyl groups being pseudo-equatorial. Kinetic studies have also been carried out on the nitroxides (14) (15) and (16) produced by photolysis of Et3SiH solutions of 2- 3- N(i))OSiEt3 (J/N'"'"" Q oN(0)OSiEt3 (14) (15) (16) and 4-nitropyridines in sit^.^^ Again first-order decays were observed but temperature-jump experiments clearly showed that dimer lay in the reaction pathway.Equation (25) shows the mechanism although a clear choice between decay of monomer and decay of dimer could not be made Products t 2ArN(b)OR $ Dimer -B Products (25) A literature misassignment has been corrected. The e.s.r. signal obtained on the reaction of pentyl nitrite with aniline in benzene had been attributed to the u-radical P~N-NOW.~~ However it has now been that this reaction and the reaction of pentyl nitrite with 1,3-diphenyl-triazine give signals consisting of overlapping spectra. The species responsible were shown to be diphenylnitroxide and phenylpentyloxynitroxide.The reactions between trifluoronitrosomethane and some 1,3-diketones have been shown to generate the nitroxide radicals CF3N(0)CH(COR')COR2 and their tautomers CF,N(O)C(COR') C(OH)R2 30 S.N. Ghriofa R.Daray and M. Conlon J.C.S. Perkin I 1977,651. 31 D. F. Bowman T. Gillan and K. U. Ingold J. Amer. Chem. Soc. 1971,93,6555. 32 L. Lunazzi G. Placucci and N. Ronchi J.C.S. Perkin 11,1977 1132. " A. F. Levit,and I. P. Gragerov Zhur. org. Khim. 1969 5 31. 3A J. I. G. Cadogan R. G. M. Landells R.M. Paton J. T. Sharp and R. U. Weber J. Chem. Research 1977 (S)108. Reaction Mechanisms-Part (iii) Electron Spin Resonance 99 together with the iminoxy radicals (R*CO)(R'CO)C Under similar rezction conditions however diethyl 2-methyl-3-oxosuccinate afforded only the nitroxide CF,N(b)CMe(C02Et)COC02Et.A reaction scheme was proposed which involved hydroxylamine anions CF3N(6)CH(COR2)COR' as intermediates. A series of iminyl and triazenyl radicals have been derived by radical attack on some organic a~ides.~~ Bu'O- radicals were found to react with a series of primary and secondary alkyl azides to give spectra characteristic of the iminyl radicals (g = 2.0028-2.0029 aN=0.95-0.98 mT) uiz.Bu'O. +HC(R')(R2)N-&~N Bu'OH +N2+R'(R2)C=N-(26) The intermediate a-azidoalkyl radicals were not observed. In contrast Et3Si* and Ph3Si. added to primary alkyl azides to give either R2-N=N-fiSiR (17) or R2(R:Si)N-N=N-(18). From a consideration of g-values (2.0010-2.0012) and the three values of aN(1.76-2.0 mT 0.34-0.40 mT and 0.12-0.19 mT) it was argued that (1 8) was the probable structure i.e. the 3,3-disubstituted triazenyl radical. The chain process involved in the decomposition of diphenyldiazomethane induced by copper perchlorate has been studied kinetically and product analyses have teen carried out.The e.s.r. spectra observed suggested that the radical cations Ph2CN2 and Ph2CN NCPh2 are key intermediates in the chain rea~tion.~' Following earlier studies of the u*-T* orbital crossover in a series of fluorinated benzene radical anions,38 the isotropic spectra of several fluorinated pyridine anions have been observed in an adamantane matrix.39 Very large couplings to I9F in penta- and 2,3,4,6-tetrafluoropyridineanions together with small values of a for the more lightly fluorinated species indicate the former to be u-radicals whilst the latter are .rr-radicals. The u*-T* orbital crossover was rationalized in terms of the stabilization of u* and destabilization of T* orbitals due to the inductive effects by back donation from fluorine.Its relevance to the present report lies in the suggestion by the authors that the availability of low-lying u* states should be carefully considered when assessing the chemistry of polyfluoro-aromatics. This is especially true for their reactions with nucleophiles. Phosphorus-centred radicals are represented by studies of some reactions of phosphate radicals4' and of the stereochemical non-rigidity and relative ligand apicophilicities of some phosphoranyl radical^.^**^^ The phosphate radical P642- and its protonated forms HPO4- and H2Pb4 were prepared in situ by photolysis of hu potassium peroxodiphosphate (PzOa4- -2P642-).40 While the phosphate radi- cals themselves were not detected by e.s.r.their adducts to fumaric and maleic acids and to the aci-anion of nitromethane were observed. In general the reac- tions were similar to the related species Sb4- i.e. adduct radicals were obtained 35 B. L. Booth D. J. Edge R. N. Haszeldine and R. G. G. Holrnes J.C.S.Perkin 11,1977 7. 36 J. W. Cooper B. P. Roberts and J. N. Winter J.C.S. Chem. Comm. 1977,320. 37 D. Bethell K. L. Handoo S. A. Fairhurst and L. H. Sutcliffe J.C.S. Chem. Comm. 1977 326. 38 M. B. Yim and D. E. Wood J. Amer. Chem. SOC.,1976,98,2053. 39 M. B. Yim S. DiGregorio and D. E. Wood J. Amer. Chem. SOC.,1977,99,4260. 40 P. Maruthamuthu and H. Taniguchi J. Phys. Chem.. 1977,81 1934. 41 J. W. Cooper M. J. Parrott and B.P. Roberts J.C.S. Perkin 11 1977 730. 42 R. W. Dennis I. H. Elson B. P. Roberts and R. C. Dobbie J.C.S. Perkin 11 1977 889. 100 A. T. Bullock with unsaturated substrates hydroxyalkyl radicals from aliphatic alcohols and inorganic radicals such as to3-from HC03- and P032-from HP032-. However they differed in their reactions with aliphatic and aromatic carboxylic acids. At neutral pH phosphate radicals caused hydrogen abstraction from saturated alipha- tic mono- and dicarboxylic acids giving rise to a-carbon radicals whereas SO,-gave mainly radicals produced by decarboxylation. On the other hand phthalic acid gave a substituted phenyl radical on reaction with the peroxodisulphate system but did not do so with the phosphate radical. The authors concluded that HP0,- is a milder oxidant than the structurally related So4’-radical.A series of phosphoranyl radicals have been found to exhibit linewidth effects which were interpreted in terms of intramolecular ligand exchange at the phos- phorus atom,41 In general it was concluded that the following order of ligand apicophilicity holds F CI RCO2 >RC(O)NR OCN > RO R2N >H > R Other line-shape changes have been observed in some fluoroalkoxy-and fluoroalkyl-phosphoranyl radicals.42 It was concluded that the apicophilicity of -CF3 was less than that of -C1 but usually greater than ROO.Additions of trifluoromethyl radicals to trialkyl phosphites was found to be rever~ible~~ just as an earlier study had demonstrated the reversibility of the addition of methyl radicals.43 Radical addition to tervalent phosphorus when followed by p-scission of an exis- ting ligand has been noted to be the free-radical equivalent of the Arbuzov rea~rangernent.~ Me2N* and phenyl radicals were generated photolytically and allowed to react with a series of cisltrans isomeric five- and six-membered ring phosphites.The reactions were found to be nearly stereospecific and the authors concluded that for the phosphoranyl intermediates permutational isomerization of the Berry or turnstile mechanisms could not compete kinetically with the product- forming p -scission In the field of carbon-centred radicals a flow system has been designed to test for the intermediacy of free radicals in the currently accepted mechanistic scheme for the reaction between benzylic halides and aromatic radical anions.,’ The accepted scheme is RX+NaCloHs + R.-* R-+ products (27) The bis(3,5-di-t-butylphenyl)methylradical was observed in the reaction between its bromide precursor and sodium naphthalenide. The generation of phenyl radi- cals and their subsequent abstraction reactions with a series of aliphatic substrates have been The abstracted atoms were H Br and I. Competitive experiments confirmed the nucleophilicity of the phenyl radical and it has been suggested that the transition state contains a significant contribution from polar structures such as Ph’. -H. . -CHXMe. Radical addition to alkynes and the subsequent intramolecular reactions of the resultant vinyl radicals have been J.W. Cooper and B. P. Roberts J.C.S. Perkin ZZ 1976 808. 44 W. G. Bentrude W. Del Alley N. A. Johnson M. Murakami K. Nishikida and H-W. Tan J. Amer. Chem. Soc. 1977,99,4383. 45 K. Schreimer H. Oehling H. E. Ziegler and I. Angres J. Amer. Chem. SOC.,1977 99 2638. 46 B. Ashworth B. C. Gilbert and R. 0.C. Norman J. Chem. Research 1977 (S)94; (M)1101. Reaction Mechanisms-Part (iii) Electron Spin Resonance 101 de~cribed.~’The vinyl radicals were not detected but were clearly implicated. Intermolecular addition and intramolecular abstraction of the vinyl radicals gave rise to the observed radicals. In the detailed schemes deduced several examples of 1,5-hydrogen atom shifts were observed including an oxygen-to-carbon example. In view of the wide range of reported values for the rate coefficient for mutual termination of t-butyl .radicals the kinetics of this reaction have been studied carefully in isobutane and cyclopentane over the temperature range 170-330 K.48 The radicals were generated by several photolytic routes using a rotating sector- digital signal averaging technique. The results were that 2k = 1.1X 10” 1mol-’ s-’ (298 K) with E = 4.3 kJ mol-’ in both solvents. These are close to recent gas-phase measurements at the same temperature (2k = 9 x lo91 mol-’ s-’)~’ and to those for termination in a series of alkanes” and were well fitted to the Smoluchowski equation suitably modified by the microviscosity theory.” The concordance of the results from several different methods of radical production indicated that the observed rate coefficients were not significantly affected by geminate recombination in the solvent cage.It was pointed that recent gas- and liquid-phase results cast serious doubt on the validity of the currently accepted thermochemistry for alkyl ~adicals.’~ In addition to examples cited earlier there have been several interesting reports on spin-trapping studies. The traps used most widely remain 2-methyl-2-nitroso- propane (MNP) and phenyl-t-butyl nitrone (PBN). There have been two reports on spin-trapping in the y-radiolysis of alcohol^.^^*^^ High concentrations of MNP were used to detect the intermediate methyl radicals in the radiolysis of Bu‘OH MeCH(OH)Me and MeCH20H and the yields of nitroxide adducts were in the same order as the yields of methane.53 However no adduct was observed in the case of MeOH which suggests either that the methyl radical is not a precursor of methane in this case or that it has excess energy and reacts with solvent rather than with the spin trap.PBN was the trap of choice in a study of the y-radiolysis of fluorinated In general both hydrogen-atom and radical adducts were observed. A particularly important observation was that in mixtures of (CF3),CHOH and Me,CHOH the ratio [H adduct]/[radical adduct] increased by more than one hundred-fold as the composition varied from the fully protiated to the pure fluorinated alcohol. This reflected a change in loss pathways for hydrogen atoms especially hydrogen-abstraction reactions in the protiated case.Both MNP and PBN have been used in a study of the thermolysis of aryldiazo alkyl ethers.” When carried out in aromatic solvents the reaction yields biaryls and is thought to involve aryl radicals as intermediates. The reaction is summarized by the equation Ar’-N=N-0-R+Ar2H 4 Ar’Ar2+N2+ROH (28) 47 W. T. Dixon J. Foxall G. H. Williams D. J. Edge B. C. Gilbert H. Kazarians-Moghaddam and R. 0. C. Norman J.C.S. Perkin IZ 1977,827. 48 J. E. Bennett and R. Summers J.C.S. Perkin 11 1977 1504. 49 D. A. Parkes and C. P. Quinn J.C.S. Furuduy I 1976,73 1952. H. Schuh and H. Fischer Internat. J. Chem. Kinetics 1976,8 341. ” A. Spernol and K. Wirtz 2.Nuturforsch. 1953,88,522. 52 R. Hiatt and S. W. Benson Internat. J. Chem. Kinetics 1973,5 385.53 F.P.Sargent and E. M. Gardy J. Phys. Chem. 1977,81 1215. 54 A.C.Ling and L. Kevan J. Phys. Chem. 1977,81 605. ” R.M. Paton and R. U. Weber J.C.S. Chem. Comm. 1977,769. 102 A. T. Bullock Product analyses of isomer ratios the observation of CIDNP and the spin-trapping experiments all confirmed the intermediacy of aryl radicals in this reaction. The phenyl radical has been trapped both by MNP and PBN in the radiolysis of benzene.56 Previous evidence for its presence as an intermediate had been rather indirect. The apparatus and method for trapping radicals produced in a silent electric discharge in the gas phase has been de~cribed.~~" One surprising feature was that trapping seemed to be quite selective only one or two radicals being observed in any one experiment.For example gas chromatographic analysis of the products from 2-methylpropane showed no less than 'seventeen products. However only the 2-methylpropyl radical was trapped in observable quantities. A general review of spin trapping of photolytically-produced radicals in the gas phase has also appeared.57b The kinetics of spin trapping continue to receive attenti~n.~**~~ A combination of pulse radiolysis and spin-trapping data was used to determine the rate of addition of MeO- to MNP for which a rate coefficient of 1.3X 10*1mol-*~-~ (-45 "C) was A more extensive report is concerned with the measure- ment of rate coefficients for spin-trapping primary alkyl radicals with MNP and PBN.59 The method made use of the fact that the 5-hexenyl radical (19) isomerizes to cyclopentylmethyl (20) at a rate k which is reliably known.The spin adducts of the two isomers were distinguished by labelling (19) at the starred position with 13C. Hyperfine coupling to this carbon was only detectable in the adduct of (19) with both traps. The same rate coefficient for trapping kT was assumed for both isomers and was calculated from kT = k,[ 19T.]/[T][20T*] where [19T-] represents the concentration of the adduct of (19) with the trap T and [20T*]is the concentration of the adduct of (20) with T. At 40°C in benzene kT(PBN)= 1.34x lo51 mol-' s-' and k,(MNP) =90.2 X lo51mol-' s-l. The authors suggested extensions of the technique. Other spin-trapping studies include investigations of hydrogen atom abstraction from polystyrene by Bu'O.radicals,60 micellar catalysis of radical reactions,61 and superoxide-alkyl halide reactions.62 56 F. P. Sargent and E. M. Gardy J. Chem. Phys. 1977,67 1793. ''(a)D. B. Hibbert A. J. B. Robertson and M. J. Perkins J.C.S. Faraday I 1977.73 1499; (6)E. G. Janzen Creat. Detect. Excited State 1976,4 83. 58 F. P. Sargent J. Phys. Chem. 1977,81 89. 59 P. Schmid and K. U. Ingold J. Amer. Chem. Soc.,1977,99 6434. 6o N. Ohto E. Niki and Y. Kamiya J.C.S. Perkin ZZ 1977 1416. 61 D. P. Bakalik and J. K. Thomas J. Phys. Chem. 1977,81 1905. 62 M. V. Meritt and R. A. Johnson J. Amer. Chem. Soc.,1977,99,3713. Reaction Mechanisms-Part (iii) Electron Spin Resonance 2 Chemically Induced Dynamic Electron Polarization There have been two reports of S-T, polarization in CIDEP.63*64 Both involve the use of time-resolved e.s.r.typically 2 ps after a radiolysis pulse. In the first,63 -CH2C02H was produced in H20/H2S04 solutions of sodium acetate the other important species being He. At pH 1.3 the low-field line of the triplet from *CH2C02H was in enhanced emission the central line in emission and the high- field line in enhanced absorption (but considerably less intense than the other two lines). However in D20/H2S04 solution an essentially 'normal' CIDEP spectrum was obtained i.e. low-field enhanced emission unpolarized central line and the high-field line in enhanced absorption more intense than the low-field line. The other radical present was Do. These results were explained in the following way.Substantial hyperfine coupling in H*was responsible for making S-T- polariza-tion feasible for radical pairs involving H* atoms. This coupling splits the T+,and T-l levels while Tois unaffected in first order. Thus not only is the S-T-l energy i gap substantially reduced but differentiation of the T,,l levels allows S-Tkl polarization pathways to contribute to the usual S-To polarization. Since Do has a much smaller hyperfine coupling (7.754 mT) than has H* (50.66 mT) there was no appreciable S-T- mixing observed from radical pairs involving Do. Similar mixing was observed by the same authors in an investigation of CIDEP in the pulse radiolysis of aqueous solutions of micelle~.~~ N20-saturated solutions of several anionic and cationic surfactants were studied.Below the critical micelle concen- tration (CMC) the radical pair S-To mixing was dominant in all cases. This mechanism still obtained above the CMC since micelle formation is a dynamic equilibrium but emission and enhanced absorption were observed with emission in substantial excess. Especially noteworthy was the emission from central lines in the spectra (radicals were of the general type R'CH2CHCH2R2). This behaviour noted only above the CMC was ascribed to S-T- mixing consequent upon restricted diffusion in the aggregate phase. CIDEP has been observed in radicals (21) and (22) when solutions of maleimide and the N-ethyl and N-methyl derivatives in various alcohols AH were examined using a pulse photolysis-signal averaging technique.65 The observed spectra showed contributions from both radical pair and triplet mechanisms.The results were accommodated by the mechanism given in equations (30a-e). hv ISC M(S0) M*(S) M*(T) (304 63 A. D. Trifunac and D. J. Nelson J. Amer. Chem. SOC.,1977,99 289. 64 A. D. Trifunac and D. J. Nelson Chem. Phys. Letters 1977,46,346. " P. B. Ayscough T. H. English G. Lambert and A. J. Elliot J.C.S. Furuduy I 1977,73 1302. 104 A. T. Bullock M*(T)+AH -+ MH+A (30b) A+M(s~)__* MA MA,MH -products MA MH +M(So) -polymer (30c) (304 (30e) Evidence that the triplet state was involved in the H-abstraction step (30b) was inferred from the initial polarization of the MH radicals. The counter-radical A reacted within microseconds adding to the double bond of M(So) as evidenced by the transfer of its polarization to MA.Kinetic measurements suggested that MA and MH have second-order termination rate coefficients > lo91mol-' s-l in alco- holic solvents at room temperature and may also undergo further addition to maleimide [reaction (30d)J. Finally a nitrogen laser coupled to a time-resolved e.s.r. spectrometer has been used to study the sensitized and unsensitized photoreductions of biacetyl with triethylamine.66 It was found that the amine quenched both singlet and triplet biacetyl. The reaction of the triplet resulted in efficient radical production but was not rapid enough to produce significant electron polarization. The sensitizer used was benzophenone.In this case polarized spectra of biacetyl radical anions were observed and shown to arise in the following manner. Rapid reaction of the triplet benzophenone with the amine gave rise to polarization in the primary radical MeeHNEt2. This polarization was then transferred via reaction with ground state biacetyl to the secondary radical anions. Estimates of the initial polarization of both primary radicals MeCHNEt and Ph2COH showed these to be equal thus providing the first quantitative evidence for this requirement of the triplet mechanism. K. A. McLaughlan R. C. Sealy and J. M. Wittman J.C.S. Faraday ZZ,1977,73,926.

ISSN:0069-3030    

DOI:10.1039/OC9777400090

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By S. A. MATLIN Department of Chemistry The City University St. John Street London ECl V 4PB 1 Arynes It has been a quiet year for aryne chemistry. The action of dialkylamines on u-and p-difluorobenzenes gives rise to fluorobenzyne intermediates from which isomeric mixtures of dialkylaminofluorobenzenes are formed.' Nucleophilic attack on the aryne triple bond by a side chain carbanion a to a nitrile group is the basis of a new synthesis of benzoheterocycles,* as illustrated by the high-yield formation of N-methylisoindole (Scheme 1). EcMe a KH2-liq.NH; N-Me / CN Scheme 1 The formation of biphenylenes substituted on one ring has been accomplished by the gas phase pyrolysis of mixtures of precursors of appropriate ben~ynes.~ The mediocre yields are offset by the simplicity of this method.The cyclobuta[c]benzyne (1)has been generated from the corresponding dihalide and undergoes the expected trapping reaction with f~ran.~ Compound (1)does not appear to form a dimer in marked contrast to the behaviour of its isomer (2) reported last year.5 This presumably reflects the lower stability of (1) compared with (2). (1) (2) Competition between nucleophilic attack (route a) and [4 + 21 and [2 + 2Jcyclo-addition pathways (routes b and c respectively) is seen in the reactions of allylic Grignard reagents with benzyne (Scheme 2).6 ' M. F. Moreau-Hochu and P. Caubere Tetrahedron 1977,33 955. B. Jaques and R. G. Wallace Tetrahedron 1977,33,581. A.Martineau and D. C. DeJongh Canad. J. Chem. 1977,SS 34. R. L. Hillard and K. P. C. Vollhardt Angew. Chem. Internat. Edn. 1977 16 399. R. L. Hillard and K. P. C. Vollhardt J. Amer. Chem. SOC.,1976,98 3579; S. A. Matlin Ann. Reports (B) 1976,73,85. J. G. Duboudin B. Jousseaume and M. Pinet J.C.S. Chem. Comm,. 1977,454. 105 106 S. A. Matlin I R R Scheme 2 1-Naphthyne reacts with the amine (3)by a [4+ 21 route with subsequent loss of diethylamine giving phenanthrene. Other arynes behave analogously.' (3) The initial adduct of 2-vinylnaphthalene and benzyne undergoes an unusual concerted ene reaction' with a further molecule of benzyne (Scheme 3; R = H or D). Scheme 3 2 Nitrenes In contrast to previous ab initio studies new calculations (generalized valence bond and configuration interaction) predict the ground state of aminonitrene (H2NN) to be a singlet lying about 15 kcal mol-' below the triplet state.' S.Tanirnoto. Tetrahedron Letters 1977 2903. Y. Ittah I. Shahak J. Blum and J. Klein. Synthesis 1977 678. J. H. Davis and W. A. Goddard J. Amer. Chem. SOC.,1977,99,7111. Arynes Carbenes Nitrenes and Related Species The low-temperature addition of the nitrenes formed by the oxidation of N- aminophthalimide (4) and N-aminobenzoxazolone (5) to olefins conjugated to G;-NH? ao>o N 0 I NH (4) (5) ester vinyl or aryl functions is highly stereoselective." For example phthali- midonitrene (Z-N:) adds to methyl acrylate at -30 "C affording exclusively the cis-aziridine (6) which largely inverts to the more stable trans-isomer (7) on Z (6) (7) warming.This strong preference for formation of the cis-isomer appears to be due to a secondary orbital interaction between the ?r electrons of the group conjugated to the olefin and those of the carbonyl group in the N-acyl-N-nitrene. When phthalimidonitrene is generated in the presence of 0-diketones addition of the nitrene to the enol double bond is followed by opening of the azirane ring to give an amide (Scheme 4)'' Z Scheme 4 Arylnitrenes are known to equilibrate wifh the corresponding 7-azanor- caradienes which can be trapped by a variety of nucleophiles as azepines.12 The photolysis of CY -azido-a y-dienoic esters (8)leads to pyrrole f~rmation'~ via the azirines (9).Contrary to earlier reports the thermolysis of 2-penta- 1,3-dienyl- 2H-azirines (10) also affords pyrroles and not azepines.I4 However azepine formation has been cleverly induced in one instance." The azirine (11; R=H) cvclises to a pyrrole via S C-H insertion by the nitrene (12; R = H) and the corresponding methyl-substituted case (1 1; R = Me) gives a lo R. S. Atkinson and J. R. Malpass J.C.S. Perkin I 1977 2242. " H. Person K. Luanglath and A. Foucaud Tetrahedron Letters 1977 221. '* M. Masaki K. Fukui and J. Kita Bull. SOC.chim. France 1977 50 2013; B. Nay E. F. V. Scriven H. Suschitzky and Z. U. Khan Synthesis 1977,757;S. E. Carroll B. Nay E. F. V. Scriven H. Suschitzky and D. R. Thomas Tetrahedron Letters 1977 3175; S.E. Carroll B. Nay E. F. V. Scriven and H. Suschitzky ibid.. p. 943. l3 H. Hemetsberger I. Spira and W. Schonfelder J. Chem. Res. (S) 1977 247. ld K. Isomura T. Tanaka and H. Taniguchi Chem. Letters 1977 397. l5 K. Isomura H. Taguchi T. Tanaka and H. Taniguchi Chem. Letters 1977,401. 108 S. A. Matlin R' R' \ hv \ C=CH-CH=C-C02R3 -C=CH-CH-C-CO~R' R2' I N3 -'OoC R2' \/ (8) (9) RZ 1-20"C R' 0COZR3 -RZ I R'9C0,R' H I H OT&C(-J2Me C0,Me OTcK N (11) (12) pyridine. With a phenyl group S to the nitrene (12; R =Ph) attack on the aromatic ring affords an azepine. A new method of generating acylnitrenes involves the pyrolysis of N-acyl-S,S-diphenylsulphimides.'6 The singlet state of acylnitrenes is stabilized by the use of dichloromethane as solvent leading to enhancement of yields of C-H insertion products in both interm~lecular'~and intramolecular18 reactions.The latter generally proceed by attack at the y S or E C-H bonds and are useful in remote functionalization. l9 o-Methylbenzenesulphonylnitrenesgreatly prefer intermolecular reactions to intramolecular insertion into the side chain C-H bonds presumably because of the geometry of the sulphonylnitrene. In the case of the sulphonylazide (13) the intramolecular cyclization product (14) results exclusively from attack at the more accessible secondary hydrogen rather than the more reactive tertiary one.2o The ww SO2N.3 SOZ-N \ H (13) (14) N. Furakawa M. Fukumura T. Nishio and S.Oae J.C.S. Perkin I 1977 96. l7 H. Takeuchi N. Murata Y. Nakagawa T. Tsuchida and K. Koyama J.C.S. Perkin II 1977 80. l8 W. Lwowski and S. Linke Annalen 1977 8. l9 P. F. Alewood M. Benu J. Wong and A. J. Jones Canad. J. Chem. 1977,55,2510;M. R. Czamy B. W. Benson and T. A. Spencer J. Org. Chem. 1977,42 556. R. A. Abrarnovitch R. Chellathurai,W. D. Holcomb I. T. McMaster and D. P. Vanderpool J. Org. Chem. 1977,42,2920. Arynes Carbenes Nitrenes and Related Species 109 biaryl ketone derivative (15) cyclizes to a seven-membered ring sultam (16) on thermolysis.21 There is no evidence for the formation of spiro intermediates or azepines on thermolysis of biarylsulphonylazides,22 which contrasts with the be haviour of u-(arylt hio)arylni trenes .23 0 0 S02-N \ Evidence in favour of an exclusively singlet pathway for the formation of carbazole from 2-nitrenobiphenyl has been pre~ented.~~ Nitrenes derived from the amines (4) and (5) will react with allylic sulphides to form ylides which undergo 2,3-sigmatropic rearrangements (Scheme 5).25 At Ar Z Ar Z ‘R2 Scheme 5 Acylnitrenes have been reported to abstract oxygen atoms from N-nitrosoamines26 and alkoxycarbonylnitrenes will abstract oxygen from DMSO.” In the latter case the resulting nitrosoformate has been trapped by cycloaddition with thebaine.Insertion of alkoxycarbonylnitrene into the C-H bonds of trans -1,2-dichloro-cyclohexane takes place largely at the 4-position suggesting a controlling inter- action of the nitrene with a halogen atom.28 The reactions of azides and azirines in the presence of transition metal catalysts appear to involve metal-nitrene complexes.29 A number of such complexes are now becoming well ~haracterized.~’ 21 R.A. Abramovitch and D. P. Vanderpool J.C.S. Chem. Comm. 1977 18. 22 R. A. Abramovitch T. Chellathurai I. T. McMaster T. Takaya C. I. Azogu and D. P. Vanderpool J. Org. Chem. 1977 42 2914. 23 I. M. McRobbie 0.Meth-Cohn and H. Suschitzky. J. Chem. Res. (S) 1977 17. 24 J. M. Lindley I. M. McRobbie. and 0.Meth-Cohn J.C.S. Perkin I 1977 2194. ” R. S. Atkinson and S. B. Awad J.C.S. Perkin I 1977 346. 26 K. Nishiyama and J.-P. Anselme J. Org. Chem. 1977,42 2636. 27 G. W. Kirby J. W. M. Mackinnon and R. P. Sharma Tetrahedron Letters 1977,215.28 P. A. Tardella and L. Pellacani Tetrahedron Letters 1977,4451. 29 H. Alper J. E.Prickett and S. Wollowitz J. Amer. Chem. SOC.,1977 99 4330; I. Yamamoto H. Tokanou H. Uemura and H. Gotoh J.C.S. Perkin I 1977 1241. 30 0.R. Chambers M. E. Harman D. S. Rycroft D. W. A. Sharp and J. M. Winfield J. Chern. Res. (S), 1977 150; J. Chatt and J. R. Dilworth J. Indian Chem. Soc. 1977 54 13; F. Basolo ibid. p. 7. 110 S. A. Marlin 3 Carbenes A number of reviews containing references to carbene chemistry have been pub- lished.31 Uncertainty still remains concefning the triplet-singlet (3B1-'A1) energy separa- tion in methylene. A recent spectroscopic measurement of 19.5f0.7 kcal mol-' much higher than previous experimental values has provoked two new state-of- the-art configuration interaction Both place the energy separation in the region of 11kcal mol-' and it seems unlikely that the true value will turn out to be significantly greater than this.M~Bride~~ has emphasized the structural importance of bent bonds in carbenes. For diphenylcarbene INDO calculations predict the triplet to be lower than the singlet by 4 kcal mol-' with the two states interconverting rapidly.34 In CHBr the triplet is calculated to be only 1kcal mol-' below the singlet and in other halo- genocarbenes (CHCl CHF CCl, CF,) the singlet becomes more stable than the triplet by progressively larger amounts.35 There is now good evidence36 for homoallylic conjunction between the carbene and the cyclopropane orbitals in the bridged systems (17) and (18).Generation.-Halogen exchange37 in the dihalogenocarbenes derived from CHBrC12 and CHBr2C1 under phase transfer catalysis (PTC) conditions leads to adducts of all three possible carbenes CBr2 CBrCl and CC1,. This can be avoided3* by the use of dibenzo[l8]crown[6] as the catalyst which gives exclusively CBrCl adducts from CHBr2Cl. The use of solid or immobilized phase transfer catalysts for halogenocarbene generation is receiving increasing attention.39 When tertiary amines are used as catalysts selectivity in the product distributions upon addition of dichlorocarbene to polyenes suggests the intermediate formation of nitrogen ylide~.~' In the PTC addition of dihalogenocarbenes to allylic alcohols " M. D. Roth Accounts Chem.Res. 1977 10 85; W. M. Jones ibid. p. 353; S. Braslavsky and J. Heicklen Chem. Rev. 1977 77,473; R. A. Firestone Tetrahedron 1977 33 3009. 32 R. R. Lucchese and H. F. Schaefer J. Amer. Chem. Soc. 1977 99 6765; B. 0. Roos and P. M. Siegbahn ibid.,p. 7716. 33 J. M. McBride J. Amer. Chem. Soc. 1977 99 6760. 34 J. Metcalfe and E. A. Halevi J.C.S. Perkin IZ 1977 634. 35 C. W. Bauschlicher jun. H. F. Schaefer and P. S. Bagus J. Amer. Chem. Soc. 1977 99,7106. '' P. K. Freeman T. A. Harding R.S. Raghavan and D. G. Kuper J. Org. Chem. 1977 42 3882; K. Okumura and S.-I. Murahashi Tetrahedron Letters 1977 3281. 37 E. V. Dehmlow M. Lissel and J. Heider Tetrahedron 1977 33 363. 38 M. Fedorynski Synthesis 1977,783. 39 E. Chiellini and R. Solaro J.C.S.Chem. Comm. 1977,231; S. L. Regen J. Org. Chem. 1977,42 875; S. Julia and A. Ginebreda Synthesis 1977 682. 40 Y. Kimura K. Isagawa and Y. Otsuji Chem. Letters 1977,951. For another example of nitrogen ylide formation see K. Berg-Nielsen Acta Chem. Scand. 1977 B31 224. Arynes Carbenes Nitrenes and Related Species there is no evidence for any intermediate interaction of the carbene with the OH function.41 CFCl and CFBr have been generated by reaction of the corresponding fluorotri- halogenomethanes with butyl-lithium but attempts to produce monofluorocarbene were not SUCC~SS~U~.~~ Carbenes are formed by the metallation of cycloalkane epoxides with lithium dialkylamides and show a highly temperature-dependent behaviour low tempera- tures favouring transannular C-H insertion Reactions.-The reactivities of a wide range of electrophilic carbenes in cyclo- propenation of olefins have been correlated quantitatively with inductive and conjugative effects of the carbene substituents.An exceptional case is that of ethoxycarbonylcarbene for which a strong preference for C-H insertion over cyclopropanation leads to deviation between the observed and predicted reac- ti~ity.~~ Substituent effects in nucleophilic carbenes have also been The ratio of 1;2 to 1,4 addition in the reactions of dihalogenocarbenes with norbornadiene is highly dependent on the carbene substituents. Perturbation theory supports the contention that the dominant electronic effect which augments the relative rate of 1,4 addition is the reduction in electrophilic character of the carbene p orbital.46 The nucleophilic dimethoxycarbene also adds 1,4 to cyclic diene~.~~ Further studies of ketocarbene-oxirene equilibration on the pathway to Wol.ff rearrangement have been published.Zeller48 finds 13-16'/0 oxiren participation in the formation of keten on photolysis of the diazo-aldehyde (19). In the case of the isomeric '3C-labelled diazoketones (20) and (21) a marked difference in the extent of oxiren participation from the two precursors suggests a strong displacement of the equilibrium (Scheme 6) in favour of carbene (22). (22) Scheme 6 41 K. Kleveland L. Skattebol and L. V. Sydnes Acta Chem. Scund. 1977 B31,463. 42 D. J. Burton and J. L. Hahnfeld I. Org. Chem. 1977.42 828.43 R. K. Boeckman jun. Tetrahedron Leffers 1977,4281. 44 R. A. Moss,C. B. Mallon and C.-T. Ho J. Amer. Chem. SOC.,1977,99,4105. 4s R. W. Hoffmann B. Hagenbruch and D. M. Smith Chem. Ber. 1977,110.23; H. Dun S. Frolich and M.Kausch Tetrahedron Letters 1977 1767. 46 Y. Jean Tetrahedron Letters 1977,2689. 47 W. Lilienblum and R. W. Hoffmann Chem. Ber. 1977,110 3405. 48 K.-P. Zeller Angew. Chem. Internat. Edn. 1977 16 781; Tetrahedron Letters 1977 707. 112 S. A. Matlin There has been further confirmation that ring contraction of normal size rings by Wolff rearrangement does not involve oxiren parti~ipation.~~ Thiirens are also attracting considerable attention.” A new route to these intermediates involves thermolysis of bis-amine disulphides in the presence of acetylenes the isomeric thionocarbenes being trapped as 1,3 dipoles by cyclo- additions (Scheme 7).In some cases synthetically useful yields of thiophens are obtained.” RIC=CR2 1RIC=CR2 R!QR2 R’ R:QR2 R* R’ R1 Scheme 7 Cyclopropylidenes may collapse to alleness2 or insert into neighbouring C-H The latter reaction is characteristic of tetrasubstituted cyclopropylidenes and generally follows a pattern in which insertion takes place into the CY C-H bond of the group geminal to the bulkiest substituent. For example the carbene (23; R’ = Pr’ R2 = Me) yields the bicyclobutane (24). However the aryl derivatives (23; R’ =Me or Pr’ R2= Ar) undergo insertion in the opposite dire~tion,~~ affording the bicyclobutanes (25).I (23) (24) (25) Two pathways can be envisaged for the rearrangement of vinylcyclopropylidenes to cyclopentadienes (Scheme 8). The results of a labelling study (o=”C) are consistent with operation of path A in agreement with earlier work.’’ 49 U. Timm K.-P. Zeller and H. Meier Tetrahedron 1977 33,453. L. Benati P. C. Montevecchi and G. Zanardi J. Org. Chem. 1977,42 577; H. Buhl B. Seitz and H. Meier Tetrahedron,1977 33 449; T. Wooldridge and T. D. Roberts Tetrahedron Letters 1977 2643. ” F. M. Benitez and J. R. Grunwell Tetrahedron Letters 1977 3413. 52 Y. Okude T. Hiyama and H. Nozaki Tetrahedron Letters 1977 3829. ” R. M. Cory and F. R. McLaren J.C.S. Chem. Comm. 1977,587. s4 T. Shono I. Nishiguchi T. Komamura and K. Fujita Tetrahedron Letters 1977,4327.” K. H. Holm and L. Skattebol Tetrahedron Letters 1977 2347. Arynes Carbenes Nitrenes and Related Species b Me Li Br Br Jfl Scheme 8 There is continuing interest in the cyclopropylidene-vinylmethylene rearrange-n~ent.~~ Generalized valence bond and MIND0/3 calculations predict” a methyl- ene-like triplet ground state for the parent vinylcarbene. The irradiation of the sulphinyl-3H-pyrazole (26) leads to nitrogen extrusion and formation of a sulphinyl cyclopropene which is evidently capable of ring opening to a vinylcarbene at room temperature. Thus trapping with furan or cyclopentadiene in a dark reaction after photolysis is complete affords the cyclopropane (27; X = 0 or CH2)rather than a Diels-Alder adduct of the cyclopr~pene.~~ A (26) (27) The vinylcarbene formed by irradiation of the 3-diazo-pyrazole (28) undergoes C-H insertion reactions and also rearrangement via the vinylnitrene (29) giving azirine (30) and its decomposition N RH Ph 1 Ph‘ II Ph (29) (30) s6 J.A. Pincock and A. A. Moutsokapas Canad. J. Chem. 1977,55 979; M. Vincens A. Dussauge and M. Vidal Tetrahedron 1977 33 2937; A. Padwa R. Loza and D. Getman Tetrahedron Letters 1977 2847. s7 J. H. Davis W. A. Goddard and R. G. Bergman J. Amer. Chem. SOC.,1977 99 2427; J. A. Pincock and R. J. Boyd Canad. J. Chem. 1977,55,2482. M. Franck-Neumann and J.-J. Lohmann Angew. Chem. Infernat. Edn. 1977 16 323. 59 W. L. Magee and H. Shechter. J. Amer. Chem. SOC.,1977,99 633. 114 S.A. Mutlin The photochemistry of benzocyclopropenes (31)has been examined6' and the formation of products accounted for by competition between ring contraction ?f the vinylcarbene (32) to fulvalene and trapping reactions of the biradical (33). R' R' It has now been shown that thermal reversion of the fulvalene to the phenyl- carbene is possible and consequently the results of all previous attempts to study the mechanism of thermolysis of benzocyclopropenes by labelling studies must be treated with caution.61 The intervention of phenylcarbene-cycloheptatrienylidene interconversions results in the formation of indane derivatives (35; M=C or Si) from the p-substituted phenylcarbene precursors (34;M = C or Si).62 The sila-indane (35; M = Si) is also formed63 in the gas-phase pyrolysis of phenyltrimethylsilyldiazo-methane (36) via the carbene (37).Na' Me3M+H=N-N-\ -Ts (34) Me Me : (35); (36) (37) A new entry into the diphenylcarbene-phenylcycloheptatrienylidene involves the pyrolysis of the acetate (38) and similarly results in the formation of fluorene. The participation of bicycloheptatrienes such as (39) in these interconversions offers a further possible entry into the Thus dehydrohalogenation of (40) affords a mixture of the isomeric ethers (41)and (42) 6o H. Durr and A.-J. Ahr Tetrahedron Letters 1977 1991. 61 C. Wentrup E. Wentrup-Byrne and P. Muller J.C.S. Chem. Comm. 1977 211. 62 A. Sekiguchi and W. Ando Bull. Chem. SOC.Japan 1977,50 3067. 63 W. Ando A. Sekiguchi and A.J. Rothschild J. Amer. Chem. SOC.,1977 99 6995. 64 R. W. Hoffrnann R. Schuttler and I. H. Loof Chem. Ber. 1977,110,3410. 65 W. E. Billups and L. E. Reed Tetruhedron Letters 1977,2239. Arynes Carbenes Nitrenes and Related Species Ph YOAC 11 11 OBu' (42) The first synthesis of a benzothiete (44; X =S) involves photochemical Wolff rearrangement of the diazoketone (43; X = S). Interestingly the migration ability in (43; X = S 0 or NR) decreases from S to 0 to nitrogen functions the latter being unable to rearrange66 and whereas vapour-phase copyrolysis of (43; X = CO) with alcohols yields 2-carboalkoxybenzocyclobutenones (44; X = CO),irradiation in methanol gives the homophthalate (45).67 66 E. Voigt and 13. Meier Chem. Ber.1977 110 2242. h7 R.J. Spangler J. H. Kim,and M. P. Cava J. Org. aern. 1977,42 1697. 116 S. A. Matiin Whereas carbenes normally insert into the 0-H bonds of alcoholic solvents the irradiation of aryldiazomethanes in low-temperature alcoholic matrices results in insertion into the C-H bond a to the OH group. This reaction appears to follow an abstraction-recombination mechanism from the triplet carbene under condi- tions where the mobility of the latter is restricted.68 Intramolecular insertion of carbenes into C-H bonds continues to be applied to the synthesis of bridged polycyclic systems.69 In the case of (46) dehydro-halogenation results in insertion into the C-H bond (Y to nitrogen but the initial product undergoes a base-catatysed eliminati~n,~' yielding the styryl derivative (47).H CI CH ==CHPh fj !*+Qfi +$ CH,Ph CHPh I CH ,CH Ph I H (46) (47) The directing influences of methyl substituents on the course of C-H insertion reactions of carbenes obtained from 7,7-dibromonorcarane derivatives have been examined in detail.7' Depending on its relative orientation a methyl group may have a buttressing effect on the cyclopropylidene ring which facilitates ring closure or it may inductively have a direct activating influence on the adjacent C-H bond. In the carbene (48) the distance for intramolecular C-H insertion to give 2,4-methanoadamantane appears to be too great and only 1,2-shifts to give the olefins (49) and (50) are 1,2-H shifts to carbenic centres are known in general to show a strong pref- erence for migration of the hydrogen atom into the empty p orbital of the cz~rbene~~ and theoretical justification for this has been provided.The poor stereoselectivity in axial versus equatorial H-migration in the cyclohexylidene (51) has therefore H (51) H. Tornioka and Y. Izawa J. Amer. Chem. SOC.,1977,99,6128. " D. Farcasiu H. Bohm and P. von R. Schleyer J. Org. Chem. 1977,42,96. 70 R. F. Boswell and R. G. Bass J. Org. Chem. 1977 42 2342. 7' L. A. Paquette and R. T. Taylor J. Amer. Chem. Soc. 1977,99,5708. 72 T. Sasaki S. Eguchi and Y. Hirako J. Org. Chem. 1977,42,2981. 73 E. P. Kyba and C. W. Hudson J. Org. Chem. 1977.42 1935. 117 Arynes Carbenes Nitrenes and Related Species caused some concern.A theoretical reinvestigation by MIND0/3 and MNDO methods supports the argument that migration of either hydrogen can proceed through a transition state in which the H atom is aligned with the empty p ~rbital.’~ Further work on this problem is clearly required but it may well be that stereoselectivities previously observed with bridged systems reflect the influence of torsional interactions rather than the stereoelectronic demand of the migration itself.75 Reversal of the reaction by which an olefin is formed by 1,Z-migration to a carbene is relatively uncommon. The formation of the carbene (53) from the bridgehead olefin (52) has been authenticated by trapping reactions and is attri- buted to release of o train.'^ (52) (53) Direct photolysis of cycloalkenes such as (54) also gives rise to carbene inter- mediate~.~’ (54) Ylide formation and rearrangement in the reactions of carbenes with divalent sulphur compounds has been reviewed.78 An interesting intermolecular example is the rearrangement” of the ylide formed on decomposition of the penicillin-derived diazoketone (55).1 74 E. P. Kyba J. Amer. Chem. SOC. 1977,99,8330. 75 P. K. Freeman T. A. Hardy J. R. Balyeat and L. D. Wescott jun. J. Org. Chem. 1977,42 3356. 76 T. H. Chan and D. Massuda J. Amer. Chem. SOC. 1977,99,936. 77 Y. Inoue S. Takamuku and H. Sakurai J.C.S. Perkin I? 1977 1635. 78 W. Ando Accounts Chem. Res. 1977 10 179. See also C. Huynh V. Ratovelomanana and S. Julia Bull. SOC. chim. France 1977 710. 7q I. Ernest Tetrahedron 1977 33 547.118 S. A. Math Methylenecarbenes can be formed by the treatment of vinyl triflates (56; R3=H) with strong base.80 New considerably milder procedures involve" the fluoride-ion initiated decomposition of silylvinyl triflates (56;R3=SiMe3) and room temperature thermal decomposition of tosylazo-alkenes (57). Isopropylidenecarbene formed by K0Bu'-induced elimination from the triflate (56; R'= R2=Me; R3= H) behaves as an electrophilic singlet in cyclopropanation reactions.82 Rgso2cF3 Rx=Nso2Ar R R2 H (56) (57) The 14C-labelled adamantylacetylene (58) shows 25% scrambling of the label to the adjacent alkyne carbon atom on flash vacuum pyrolysis owing to reversible formation of the methylenecarbene (59) by migration of either the H or adamantyl groups.83 (58) (59a) (59b) Dimethylvinylidenecarbene best generated in the free states4 by phase transfer catalysed elimination from the bromide (60) undergoes mainly addition reactionss5 with the double bonds of unsaturated heterocycles such as 2,5-dihydrofuran.'O P. J. Stang and M. G.Mangum J. Amer. Chem. SOC.,1977,99 2597. " P.J. Stang and D. P. Fox J. Org. Chem. 1977,42 1667. 82 P.J. Stang J. R. Marsden M. G. Mangum and D. P. Fox J. Org. Chem. 1977 42,1802. For a theoretical study see J. H. Davis W. A. Goddard and L. B. Harding J. Amer. Chem. SOC.,1977,99 2919. R3 R. F. C. Brown,F. W. Eastwood and G. P. Jackman Austral. J. Chem. 1977,30 1757. T.B.Patrick and D. J. Schmidt J. Org. Chem. 1977,42 3354. " S.Landor V. Rogers. and H. R. Sood Tetrahedron 1977,33 73.

ISSN:0069-3030    

DOI:10.1039/OC9777400105

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

6 Organometallic Chemistry Part (i) The Transition Elements By A. STEWART D. J. THOMPSON and M. V. TWIGG 1.C.l Corporate Laboratory P.O. Box 1 I The Heath Runcorn Cheshire WA7 4QE 1 Introduction This year we have not included the subsection ‘Synthesis of N-Heterocyclic Compounds’ in order to avoid duplication with the section concerned with hetero- cyclic chemistry. However some advances in this area are reported where appro- pria t e. A number of relevant reviews have been published including one concerned with transition metal clusters.’ Others deal with cyclometalation reactions,2 olefin metathe~is,~ transition metal catalysed cyclizations of acetylene^,^ carbon monox- ide insertion reactions,’ olefin insertion reactions,6 and the use of transition metal derivatives in organic synthesis.’ Amongst new books is the first volume of a series concerned with the application of transition metal organometallic compounds in organic synthesis,’ and the second volume of the well known book by Wender and ~ino.~ 2 Metal-catalysed Hydrogenation Continued progress has been made in improving the selectivity of metal-catalysed reductions and more specifically asymmetric synthesis.The majority of the publications on asymmetric hydrogenation relate to homogeneous rhodium- catalysed reactions and reflect the continuing search for new and more specific chiral ligands. One example for rhodium is the diphosphinite (+) (lS,2S)-mns-1,2-bis(diphenylphosphinoxy)cycIopentane (1) which is conformationally far more rigid than its cyclohexane analogue (2).” In the reduction of a-ethylstyrene the optical yield 60% using (1) was much higher than that found with (2) or (-)-DIOP [(1S,2S)-2,3-0-isopropylidene-2,3 -di hydroxy- 1,4-bis(diphenylp hosp hino)- butane] and the highest so far attained for this substrate by homogeneous catalysts.’ A. K. Smith and J. M. Basset J. Mol. Catalysis 1977 2 229. * M. I. Bruce Angew. Chem. Internal. Edn. 1977,16,73. J. J. Rooney and A. Stewart in ‘Catalysis’ Vol. 1 Specialist Periodical Reports The Chemical Society ed. C. Kemball 1977 Chapter 8. K. P. C. Vollhardt Accounts Chem. Res. 1977,10 1. F. Calderazzo Angew. Chem. Internal. Edn. 1977,16 299. ‘G. Henrici-Olivk and S. Olivt Topics in Current Chem. 1976,67 107. L. Hegedus J.Organometallic Chem. 1977 143 309. ‘Transition Metal Organometallics in Organic Synthesis’ Vol. 1 ed. H. Alper Academic Press 1976. ’‘Organic Synthesis via Metal Carbonyls’ Vol. 2 by I. Wender and P. Pino Wiley-Interscience 1977. lo T. Hayashi M. Tanaka and I. Ogata Tetrahedron Letters 1977,295. 119 A. Stewart D. J. Thompson,and M. V. Twigg However stereoselectivity was lower for unsaturated carboxylic acids but comparable for their esters indicating that (1)is a ligand unique for the asymmetric hydrogenation of olefinic substrates bearing no polar substituents. A new class of ligand has been reported whose chirality is the result of atro-pisomerism and not to an asymmetric centre on phosphorus or carbon." The described example is (- )- l,l-bi-2-naphthylbis(diphenylphosphinite)(3) and two 3 OPPh2 PPh \/ (31 equivalents of this with one of [Rh(cycl~octene)~Cl]~ produced a catalyst system which in the hydrogenation of unsaturated acids and esters gave better optical yields at lower temperatures.In comparison to catalysts derived from ligands with chirality at phosphorus or carbon this system was less active but gave similar optical yields. The preparation of (2S,3S)-bis(diphenylphosphino)butane S,S-chiraphos (4) and its use as a ligand for a Rh' complex have produced remarkably high optical Me Me H->4H PhzP PPh (4) yields in the hydrogenation of a-N-acylaminoacrylic acids to N-acylaminoacids at ambient temperature and hydrogen pressure. ** X-ray evidence indicated that (5) is (51 the preferred conformation of the chelate ring with equatorial methyl groups.With all nine substrates hydrogenated the @)-amino acid derivatives were produced R. H. Grubbs and R. A. DeVries Tetrahedron Letters 1977 1879. l2 M. D. Fryzuk and B. Bosnich J. Arner. Chem. SOC., 1977,99,6262. Organometallic Chemistry-Part (i) The Transition Elements and two of these leucine and phenylalanine were optically pure. The optical yield was sensitive to both the N-acyl and p-vinylic substituents and also to the solvent employed. The authors suggest that the rigidity of the chelated diphosphine with its dissymmetrically orientated phenyl groups is the major source of interaction with the achiral substrates examined and according to their rationalization in the design of (4),they also prepared (R )-1,2-bis(diphenylphosphine)propane,(R)-prophos SO that the naturally-occurring amino acids would result.This in fact was the case and optical yields similar to those obtained with (4)were found. Several ligands (6) derived from L-hydroxylproline have been investigated in Rh-catalysed reductions of certain achiral substrates.l3 Using neutral Rh' complexes with (6b) esters of pyruvic acid were converted to the corresponding lactates with higher optical yields than with (-)-DIOP.13" In the case of (6b) and propyl pyruvate an enantiomeric excess of about 76% was found. When dry aprotic solvents (benzene or THF) were employed optical yields were higher than when methanol was used. Compounds (6b) (6c) and (6d) have been used for synthesis of (R)-and (S)-N-benzyloxycarbonylalaninefrom their olefinic deriva- the (S)-compound being formed when Et3N was present in the reaction.The highest optical yields 59% (R) and 21% (S) were found with (6d) and it was suggested that the N-substituents of PPM (6a) play an important role. In the hydrogenation of substituted cinnamic acids with (6a) and (6b) Et3N had a remark- able effect on optical yields with (6b) but not with 6a),13= indicating some novel interaction which serves to produce 83-9 1% enantiomeric excess in favourable cases. A new lipophilized bisphosphine-rhodium complex allows greater solubility in aliphatic hydrocarbons and increases lipophilic interaction with substrates. 13d Here the new ligand (6e) developed from PPM was applied in the hydrogenation of ethyl and n-butyl pyruvates with optical yields of the (R)-configuration of the product as high as 67.3% in the ethyl ester case in cyclohexane solvent.Application of the same catalyst system to the reduction of olefinic double bonds resulted in complete conversions but low optical yields,'3d certainly lower than systems involv- [ >CH,Ph2 N R (6) a; (PPM) R =H-b; (BPPM) R =t-Bu02C-c; (APPM) R =MeCO-d; (PPPM) R =(Me)3C-CO-e;(CPPM) R =cholesteryloxycarbonyl C27H4502C- A set of phosphines chiral at both phosphorus and carbon have been synthesized though only two of them (S)p-and (R),-menthylmethylphenylphosphine were purified sufficiently well for use as ligands in the reduction of olefinic bonds in a,p-unsaturated carboxylic acids.l4 Reduction of (E)-p-methyl cinnamic acid gave l3 (a)I. Ojima T. Kogure and K. Achiwa J.C.S. Chern. Cornrn. 1977,428; (b)K. Achiwa Chem. Lerters 1977 777; (c)K. Achiwa J. Amer. Chem. SOC.,1976 98 8265; (d) K. Achiwa Tetrahedron Letters 1977,373.5. l4 C. Fisher and H. S. Mosher Tetrahedron Letters 1977 2487. A. Stewart D. J. Thompson andM. V. Twigg the (R) product in 67.1% optical yield. The opposite configuration of the product was formed with the other phosphine. Further work is in progress to improve the purity of the phosphine ligands. Discovery of the catalyst precursor [Rh(COD)L]'BF,- where L = (R,R)-1,2-ethanediylbis(o-methoxyphenyl)phenylphosphine,has allowed Knowles and co-workers to operate at higher temperatures and pressures and as a result they have found great versatility with their ~ystem.'~ Excellent selectivity (about 90% enantiomeric excess) was found in reduction of the a-enol ester (Z)-ethyl-2- acetyloxy-3-phenyl-2-propenoate,the first outstanding result with a nonamide substrate and optical yields of up to 96% were obtained in the hydrogenation of (2)-a-acylaminocinnamic acids but only up to 47% for (E)isomers at much lower rates.A catalyst picture was presented on the basis of an X-ray structure deter- mination and reasons for the discrimination between olefin configurations were discussed. Bulky (2)-adamantyl or bornyl-a -acetamidocinnamate inhibited the reduction of the (2)methyl ester catalysed by a neutral Rh-DIOP complex and underwent (2)to (E)isomerization.'6" Neither in the presence or absence of inhibitors did the methyl ester undergo appreciable isomerization.Further over a range of a-acylamino unsaturated substrates with the grouping NHCOR optical yields of the reaction products decreased as the steric bulk of this group increased from R =Me to R = adamantyl.16b The a-formamido analogue (R = H) although having an even smaller amide group than in the corresponding a-acetamide showed a decrease in optical yield. With a trifluoroacetamido group a reversal in chirality of the major product was observed. The use of chiral ruthenium complexes has been extended to asymmetric induction in catalytic hydrogenation of the olefinic bonds in a,p-unsaturated mono- and di-carboxylic acids using precursor cluster complexes.l7 With reaction -)-DIOPI3 temperatures of 90-120 "C H4R~4(C0)8[( )-DIOPI2 and Ru~(CO)~~[( gave optical yields which varied with substrate and were improved by the presence of Et3N but for (E)-a-methylcinnamic acid the former complex produced up to 68% enantiomeric excess a figure reached by lowering hydrogen pressure. 17" Lower optical yields were found with the corresponding esters and also with substrates without carboxylic groups even at lower reaction temperatures. The authors suggested that since both cluster catalyst precursors gave products with the same predominant configuration and comparable optical purity both complexes may have led to the same catalytic intermediates and that a free carboxylate group in the substrate is required for a substantial asymmetric bias.Regioselectivity and asymmetric induction in catalytic hydrogenation of a$-unsaturated dicarboxylic acids citraconic and mesaconic acids using H4Ru4(C0)8[( -)-DIOP]* gives in addition to (-)(S)-methylsuccinic acid a mixture of y-lactones in ratios which depend on the substrate and the reaction temperat~re.'~~ l5 8. D. Vineyard W. S. Knowles M. J. Sabacky G. L. Bachman and D. J. Weinkauff J. Amer. Chem. SOC.,1977,99 5946. l6 (a)R. Glaser and J. Blumenfeld Tetrahedron Letrers 1977 2525; (b)R. Glaser and S. Geresh ibid.,p. 2527. (a)C. Botteghi S. Gladiali M. Bianchi U. Matteoli P. Frediani P. G. Vergamini and E. Benedetti J. Orgunometullic Chern. 1977 140 221; (b) M.Bianchi F. Piacenti P. Frediani U. Matteoli C. Botteghi S. Gladiali and E. Benedetti J. Orgunometullic Chem. 1977 141 107. Organometallic Chemistry-Part (i) The Transition Elements 123 Two new catalyst systems for reduction of aromatics have appeared. One is the [Rh(q5-CsMes)Cl2I2 complex operating under homogeneous conditions with added base (Et3N) which is required as a cocatalyst.18a All cis isomers are the chief products and some hydrogenolysis of functional groups is observed. The other catalyst system employs salicylaldehyde complexes of Co Ni and Cu though there are doubts about the homogeneity of these systems.lsb When lithium aluminium hydride is used as reductant the catalysts are more active. Rhodium catalysts have been developed for hydrogenation of ketones.l9 The complexes RhC1(CsH12)PPh3 and Rh2H2C12(C8H12)(PPh3)2 in the presence of strong alkali promoted the reduction of several ketones although the dirhodium species gave the best results.19o Pretreatment with sodium borohydride resulted in higher and more reproducible hydrogenation rates. The authors suggested the existence of a hydroxo complex as an active intermediate. An alkaline medium was again required for the reduction of ketones at room temperature and at atmos- pheric pressure catalysed by complexes of the type [Rh(2,2'-bipyridine) (diene)]'PF6-which also act as hydrogenolysis catalysts for molecular oxygen.lg* Consequently hydrogenation activity was not destroyed even if molecular oxygen was present in large amounts.Notable points are that selectivity was observed for reduction of carbon-oxygen double bonds even in the presence of olefinic bonds and the system was also active in the selective reduction of dienes and 2-alkynes to alkenes. In non-co-ordinating solvents such as benzene (in the presence of Et3N) and dichloromethane (with or without Et3N) the complex [Rh(C0D)(PPh3),]PF6 was found to be active in the hydrogenation of ketones as well as alkenes and alkynes to alkanes.''= In contrast the authors found that [Rh(OCOPh)(COD) (PPh,)] and [Rh(COD)(PPh3)2py]PF6 were highly selective in the reduction of 1-alkynes to 1-alkenes if Et3N and benzoic acid were present. Ruthenium and iridium complexes have been used as selective catalysts. A report has appeared showing that RuC~*(PP~~)~ is a highly selective catalyst for hydrogenation of cyclododeca-175,9-trieneand other polyenes to monoenes in the presence of Et3N and for reduction of the olefinic bond in a diene with a terminal and internal double bond.20a The same catalyst was found to be effective in the hydrogenation of aldehydes (both aliphatic and aromatic) but not ketones to alcohols at temperatures of 50-80°C and 10 atmospheres of hydrogen pres-sure.20b Nitro groups were unaffected.An iridium complex ITH~(PP~~)~ in the presence of acetic acid was also selective for the reduction of aldehydes and not ketones.21 A new low-valent cobalt complex H3Co[P(O-Pri),I3 has the notable property of being a soluble selective catalyst for the hydrogenation of @-unsaturated ketones and amides to the saturated ketones and amides.22 Unsaturated aldehydes were not reduced under similar conditions.Catalyst activity was greatly increased (a)M. J. Russell C. White and P. M. Maitlis J.C.S. Chem. Comm. 1977 427; (b)P. Patnaik and S. Sarkar Tetrahedron Letters 1977 253 1. l9 (a) M. Gargano P. Giannoccaro and M. Rossi J. Organometallic Chem. 1977 129 239; (6) G. Zassinovich G. Mestroni and A. Camus J. Mol. Cat. 1977,2,63; G. Mestroni G. Zassinovich and A. Camus J. Urganometal[ic Chem. 1977 140 63; (c)R. H. Crabtree A. Gautier G. Giordano and T. Khan J. Organometallic Chem. 1977 141 113. 2o (a)J. Tsuji and H. Suzuki Chem. Letters 1977 1083; (6)ibid. p. 1085. '' w. Strohmeier and H. Steigerwald J. Organometallic Chem.1977,129 C43. 22 M. C. Rakowski and E. L. Muetterties J. Amer. Chem. SOC..1977 99 739. 124 A. Stewart D. J. Thompson and M. V. Twigg without loss in selectivity by reacting at 70°C. An example of the use of this catalyst is in the reaction of benzalacetone resulting in a 50% conversion to benzylacetone after 1 day. Alkynes and conjugated dienes in mixtures with alkenes are selectively hydro- genated to alkenes without significant reduction of alkene to alkane using hetero- geneous catalysts with Ni Pd or Pt intercalated in gra~hite.’~ The comparison of several homogeneous and heterogeneous olefin hydro- genation catalysts has been made for the metals Rh Co Ni and Pd.24 Phosphine complexes were compared with metal chlorides and bromides supported on phos-phine-modified silica carriers and the heterogenized systems were found to be 2 to 4 orders of magnitude more active than their homogeneous counterparts for the reduction of cyclohexene in THF.Selectivity in the reduction of ap-unsaturated aldehydes can be influenced to give preferential hydrogenation of the carbon-oxygen double bonds.25 With rhodium halide catalyst systems the presence of carbon monoxide and highly basic tertiary amines such as Et,N and N-methylpyrrolidine significantly increased not only catalytic activity but selectivity to cinnamyl alcohol in the reduction of cinnamaldehyde under the 0x0 reaction conditions giving no hydroformylation products; up to 85% selectivity to the unsaturated alcohol was achieved with the best system examined Rh2C12(C0)4 (7).Prereduction of RhC13 3H20 with carbon monoxide suggested that reduction of the trichloride occurred during hydro- genation because activity after this treatment was such that lower reaction temperatures could be employed. Addition of triphenylphosphine to the systems examined resulted in alteration of the selectivity to one of olefin hydrogenation with no reduction of aldehyde groups. Immobilization of (7) was achieved on a cross-linked chloromethylated polystyrene which was functionalized with pyr- rolidine. Rh oc-1R \‘.TO C C 0 0 3 Isomerization Some interesting cases of transition metal catalysed rearrangements and iso- merizations have appeared. Several reactions of which equations (1)and (2) are examples have been reported where double-bond migrations are catalysed by RhCI3,3H2O allowing otherwise 0 0 difficult or impossible exocyclic-endocyclic isomerizations to occur under mild conditions and in good yields.26 Compound (8) readily attainable by isomerization [equation (2)] was previously obtainable only by a three-step synthesis.The iso-23 Ventron Corp. Canad. P. 1,000,306 1976 (Chem.Ah. 1974.80 145 363). 24 K. Kochloefl W. Liebelt. and H. Kndzinger J.C.S. Chem. Comm. 1977 510. 25 T. Mizoroki K. Seki S. Meguro and A. Ozaki Bull. Chem. SOC.Japan 1977 50 2148. ‘‘ J. Andrieux D. H. R. Barton and H. Patin J.C.S. Perkin I,1977 359. Organometallic Chemistry -Part (i) The Transition Elements merization where the migrating double bond moves into conjugation with the aromatic nucleus [equation (3)] was also found to be catalysed by the same metal ---* I 1 (2) \ / am Me Me / \ Me Me (8 ) i3) OR’ &OR2 &OR2 OR’ (9) chloride and the trans isomer predominated in the product in contrast to that found on isomerization of (9) by strong bases.On this evidence even simple transition metal compounds must still have great potential as catalysts in the field of organic synthesis. The application of an iridium catalyst IrCl(CO)(PPhl)2 to migrations of exocy-clic double bonds in some cycloalkanones has been de~cribed.~’ Isoaromatization and disproportionation reactions were also observed with the substrates examined. Nickel and rhodium complexes have been shown to catalyse the rearrangement of ally1 but-3-enoate to hepta-2,6-dienoic (10)or hepta-3,6-dienoic acid (11) in the presence of phosphine or phosphites.’* The isomerization can be driven towards either the 2,6- or the 3,6-isomers through selective hydrogen abstraction.With nickel and tri-o-tolyl phosphite the ratio of (10) to (11) was highest at 14:1 and with triethyl phosphite lowest at 0.8 :1. In anisole solvent stereoselectivity was observed the main product being the truns-2,6-isomer. With Rh(PPh3)3Cl in chloroform the rearrangement gave pre-dominantly the 3,6-isomer. Intermediates such as (1 2) and (1 3) were proposed. -(13) HaYnsfer CH2=CHCH2CH2CH=CHCO*H (10) ’’Z. Aizenshtat M. Hausmann Y. Pickholtz D. Tal and J. Blum J. Org. Chem.1977,42,2386. 28 G.P. Chuisoli G. Salerno and F. Dallatomasina J.C.S. Chem. Comm. 1977 793. 126 A. Stewart D. J. Thompson,and M. V.Twigg Palladium(I1) catalysis of the formation of 3-phenyl indoles from 2,2-diphenyl-2H-azirines has been rep~rted,’~ although as yet the mechanism is not clear. The use of PdC12(PhCN)2at 30°C and then washing with aqueous ammonia afforded the indole product the formation of which normally required thermal rearrange-ment at elevated temperatures. 4 Dimerization Oligomerization and Polymerization Much of the work published in these areas has contributed further to the under-standing of the effects of varying ligands and substrates on the selectivities to certain The palladium-catalysed formation of a 1,4-disilacyclohexa-1,s-diene from a 1-silacyclopropene has been noted3’ and the use of the nitrosyl rhodium complex [Rh(NO)(NCMe),][BF,] as a catalyst for alkene isomerization oligomerization and polymerization has been Stereospecific poly-merization of 1,3-butadiene to truns-l,4-polybutadiene occurred with this rhodium system whereas isoprene gave the oligomers from tetramers up to decamers.Of interest to the organic chemist is the continued work of Vollhardt et ul. in cobalt-catalysed syntheses of polycyclic ring systems.34 The elegant synthesis of the steroid nucleus in 71% yield [equation (4)]34Qillustrates the potential of the general reaction of co-oligomerization of ao-diynes with a monoalkyne equation (S) and the possibility of a wide variety of final structures.X in the triply unsaturated compound (14) can be varied.34bFor X = N the reaction gives annelated pyridines in yields of up to 80%.34c SiMe, I (4) Ill + -+ I SiMe C=CH R I I AR I CECH X n = 3,4 5; R = SiMe3,C02Me,Ph; X = CR,N 29 K. Isomura K. Uto. and H. Taniguchi J.C.S. Chem. Comm. 1977 664. 30 See for example in dimerization and co-oligomerization (a)P. Heimbach A. Roloff and H. Schenk-luhn Angew. Chem. Internat. Edn. 1977 16 252; (b)P. Heimbach B. Hugelin E. F. Nabbefeld D. Reinehr A. Roloff and E. Troxler ibid. 253; (c) K. Kaneda M. Terasawa T. Imanaka and S. Teranishi Tetrahedron Letters 1977 2957; (d)G. Giacomelli A. M. Caporusso and L. Lardicci J.C.S. Perkin I 1977 1333; (e) H. Suzuki K. Itoh Y. Ishii K. Simon and J.A. Ibers J. Amer. Chem. SOC. 1976,98 8494; (f)S. Yoshikawa J. Kiji and J. Furakawa Makromol. Chem. 1977,178 1077; (g)J. Ficini J. d’Angelo and S. Falou Tetrahedron Letters 1977 1645; (h)G. Henrici-OlivC and S. Olivi Transition Met. Chem. 1976 1 109; (i) H. T. Dieck and H. Bruder J.C.S. Chem. Comm. 1977 24. 31 See for example in polymerization C. Carlini R. Nocci and F. Ciardelli J. Polym. Sci. Polymer Chem. 1977 15 767. 32 M. Ishikawa T. Fuchikami and M. Kumada J.C.S. Chem. Comm. 1977,352. 33 N. G. Connelly P. T. Draggett and M. Green J. Orgunometallic Chem. 1977 140 C10. 34 (a)R. L. Funk and K. P. C. Vollhardt J. Amer. Chem. Soc. 1977 99 5483; (b)R. L. Hillard and K. P. C. Vollhardt ibid.,4058; (c)A. Naiman and K. P. C. Vollhardt Angew. Chem. Internut.Edn. 1977 16 708. Organometallic Chemistry-Part (i) The Transition Elements 5 Carbonylation Several interesting reactions involving cobalt carbonyl intermediates have appeared this year. The reaction of cyclic olefins and ethers with hydrosilanes and carbon monoxide catalysed by CO~(CO)~ Cyclic olefins which has been rep~rted.~' were expected to give product (15) by direct analogy with hydroformylation in fact gave the enol silyl ethers (16) in good yield (50-74Y0).~'" Under similar condi- tions cyclic ethers reacted to give the silyl protected hydroxyaldehyde (17) in about OSiEt2Me n C02(CO) / (CHA I I +CO+HSiMeEt2 --+ (CHSC=C 0 II c n +CO+HSiEt2Me CO~ICO)~ ____+ Et2MeSiO(CH2),C//O \ 'H n =2 3 4 (17) 50% yield.356 The intermediate [CO(CO)~]- which was generated by catalytic disproportionation of [CO~(CO)~~] or [Co,(CO),] using free or complexed halide ions reacted with the dihalide (18) to give 2-indanone (19) in 80% yield.36 (18) (19) The use of phase transfer catalysis in carbonylation has been extended to the catalytic carbonylation of benzyl halides using Co,(CO),/CO/NaOH in the presence of benzyltriethylammonium chloride as the phase transfer ~atalyst.~' Yields appear to be good only for benzyl bromides.Carbonylation via iron carbonyl complexes continues to generate interesting reactions. Two groups have reported the synthesis of lactones from dienes via epoxidation followed by carbonylation (Scheme l).38Whereas iron and cobalt catalysts give the a,@-unsaturated lactone (20) rhodium catalysts give the fly-unsaturated lactone (21).3s (a)Y. Seki A. Hidaka S. Murai and N. Sonoda Angew. Chem. Infernut. Edn. 1977,16 174; (6) Y. Seki S. Murai I. Yamamoto and N. Sonoda ibid. p. 789. 36 P. S. Braterman B. S. Walker and T. H. Robertson J.C.S. Chem. Comm. 1977,651. 37 (a) H. Alper and H. D. Abbayes J. Orgunometullic Chem. 1977,134 Cll; (6) L. Cassar and M. Foa ibid. p. C15. (a)R. Aumann and H. Ring Angew. Chem. Internat. Edn. 1977,16,50; (6) G.D. Annis and S. V. Ley J.C.S. Chem. Comm. 1977,581. A. Stewart D.J. Thompson andM. V. Twigg diene -+ R' R' Rh' R5 R R4 R53 R600 Scheme 3 A new synthesis of a-diketones from aldehydes and alkyl halides using Fe(CO)5 has been reported.39 The aldehyde protected as the ethylenedithioacetal is reac- ted with butyl lithium and Fe(CO)5 to generate the acyltetracarbonylferrate (22) which then reacts with the alkyl halide to give the a-diketone (23) in overall yield of around 60%.Reaction of the organotetracarbonylferrate (24) with Michael-type acceptors (25) gives the expected product (26) in high yield (ca. 900/,).40 Z \ II [R'Fe(CO),]@+ C=C-Z + R'CO*C-C-I%(C0)3 / II (24) (25) iH+ 2=C02Et COR2 CN II R'CO-C-C-Z II (26) The direct formylation and acylation of pyridine has been achieved using Fe(CO)S/PhLi. Depending on the work-up conditions a variety of products can be obtained (Scheme 2)." 6 Reaction of Co-ordinated Ligands Whereas a lot of work has been done on the reactions of diene tricarbonyl-iron complexes much less work has been done on the olefin tetracarbonyl-iron complexes.A report4* has appeared this year however on the nucleophilic attack 39 M. Yamashita and R. Suernitsu J.C.S. Chem. Cornrn. 1977 691. 40 M. P. Cooke jun. and R. M. Parlman J. Amer. Chern. SOC.,1977 99 5222. 41 C. S. Giam and K. Ueno J. Amer. Chern. Soc. 1977.99 3166. 42 B. W. Roberts and J. Wong J.C.S. Chem. Comm. 1977,20. Organometallic Chemistry-Part (i) The Transition Elements Scheme 2 on tetracarbonyliron complexes which give after oxidative work up products of type (27) in good yield. HZC=CHR' +[R2C(C02R3)2]-+ (R302C)2CR2CH2CH2R' I Fe(C0)4 (27) An unusual reaction of a cyclopentadienyl ligand occurs in the reaction of thiobenzophenones (28) with dicarbonylcyclopentadienyliron anion.43 The two react together to give the fulvene (29) in yields of up to 82%.This cleavage of a cyclopentadienyl ligand followed by desulphurization gives a mild and potentially useful route to fulvenes. DR Ra -0. c=s +[@F,,c*,.]-Q R (28) (29) R The use of [~'-C5H5Fe(CO)2](Fp) complexes in organic chemistry continues this year with the report of a new synthesis of ,El-la~tarns.~~ Nucleophilic addition of benzylamine to the complex (30) gives the intermediate (31) which is oxidized at -78°C with chlorine to give the ,El-lactam (32) in ca. 34% overall yield. The reaction is stereospecific e.g. trans-2-butene (30; R' R3=Me; R2=H) gave only cis-3,4-dimethylazelidinone(32; R' R3=Me; R2=H).The reaction can also be applied to the synthesis of fused-ring p -1actams. (30) PhCH2' (31) Fp =[T~-C~H~F~(CO)~] 43 H. Alper and H. N. Paik J.C.S. Chem. Comm. 1977 126. 44 P. K. Wong,M. Madhavarao D. F. Marten andM. Rosenblum J. Amer. Chem. SOC.,1977 99,2823. A. Stewart D. J. Thompson and M. V. Twigg Carbanion attack on v-anisole and v-toluene chromium tricarbonyl complexes (33) gives after oxidative work-up rnetu-substituted aromatics as the major pro- With the v-anisole complex the metu-substituted product is obtained with greater than 90% selectivity. The ?r-toluene complex gives mainly the metu- substituted product but with some ortho-substituted product. (33) Intramolecular carbanion attack on arene chromium tricarbonyl complexes followed by oxidative work up leads to the formation of the bicyclic product (34) in good yield [(89'/0) for n = 3].46 By varying the conditions (long reaction time acid work up) spiro compounds e.g.(35) can be obtained in high yield. The use of carbene-chromium complexes in organic synthesis is slowly increas- ing. Reaction of the carbene complex (36) with alkynes occurs in a stereoselective way to give the substituted a-naphthol chromium tricarbonyl complex (37) which can be readily oxidized to the corresponding 1,4-naphthoq~inone.~' The lithium enolate of cyclopentanone reacts with the vinyl-carbene complex (38) to give the complex (39).48 Oxidative work up gives (40) whereas (41) is obtained by treatment of (39) with pyridine.The cobaltacyclopentanone complex (42) reacts with isocyanates to give 2-0x0- 1,2,-dihydropyridines (43) in about 70% yield.49 With unsymmetrical complexes e.g. (42; R' R3=C02Me; R3 R4=C6H5) reaction proceeds regiospecifically to afford only one product (43; R' R3=COzMe; R3,R4=C6&). 45 M. F. Semmelhack and G. Clark J. Amer. Chem. Soc. 1977,99 1675. 46 M. F. Semmelhack Y.Thebtaranonth and L. Keller J. Amer. Chem. Soc. 1977 99 959. 47 K. H. Dotz and R. Dietz Chem. Ber. 1977,110 1555. 48 C. P. Casey and W. R. Brunsvold Inorg. Chem. 1977,16 391. 49 P. Hong and H. Yamazaki Synthesis 1977 SO. Organometallic Chemistry-Part (i) The Transition Elements 0-0 (CO)5Cr5 OMe (39) (38) 82% 0 0 (42) (43) There have been a number of reportsSo this year on the reactions of the organo- manganese complexes (44) which are prepared by reaction of organolithium or organomagnesium compounds with Mn12.These complexes react with a variety of acid chlorides to give the corresponding ketone (Scheme 3).50aThis reaction is very selective no alcoholic bi-products being formed. The ketones formed in this reaction do react further but at a very much slower rate. Aldehydes react much faster than ketones and at low temperatures the organomanganese reagent will selectively attack the aldehyde group even in the presence of an unprotected ketone (Scheme 3).50b The manganese complex (44) reacts with ethyl chloro- formate to the corresponding alkylated ethyl ester (Scheme 3).'OC CIC02Et R1CO.Cl RC02Et RMnI RCO-R' 80% 90% (44) 78% MeCO*(CH2)3CHO I MeCO.(CH&CHOH.R Scheme 3 Organozirconium complexes (45) which are produced by hydrozirconation of olefins or acetylenes using [v5-C5H5Zr(H)Cl] react with aluminium chloride to generate the corresponding organoaluminium dichloride (45)'l i.e.a 50 (a)G. Cahiez D. Bernard and J. F. Norrnant Synthesis 1977 130; (b)G. Cahiez and J. F. Norrnant Tetrahedron Letters 1977 3383; (c) G. Cahiez and J. F. Normant Bull. SOC.Chim. France 1977 570. D. B. Carr and J. Schwartz J. Amer. Chem. SOC.,1977 99 638. 132 A. Stewart D. J. Thompson and M. V. Twigg transmetallation reaction from zirconium to aluminium. The reaction proceeds well only for primary saturated alkyl and alkenyl aluminium dichlorides and the products (46) can be used as mild alkylating agents.[v5-C~Hs]2Zr(R)C1 +[RAIC12], +AlC13 [q5-C5H5]2ZrC12 0°C (45) (46) The alkenylzirconium complex (47) can be coupled with aryl halides in the presence of a catalytic amount of Ni(PPh3)4.52 Yields are generally excellent and the stereochemistry of the product is >98%E. The reaction can tolerate certain functional groups in particular oxy-functional groups that are incompatible with hydroaluminations but it does not work so well for internal alkynes. c1’ (47) 7 Ofefin Metathesis Interest in olefin metathesis continues unabated with the number of papers pub- lished in 1977 being similar to that during the previous year. Some twenty papers were presented at an ‘International Symposium on Metathesis’ held in Noordwij- kerhout Holland.53 Like last year more work was orientated towards mechanistic aspects than to direct application in synthetic organic chemistry.A further exten- sive review has a~peared.~ Mechanistic Studies.-Good evidence for the formation of carbenoid species in metathesis systems comes from the formation of methane and ethylene on mixing [WCI6] with Sn(CH,), or [MO(PP~~)~(NO)~C~~] with A12(CH3)3C13 two typical metathesis catalysts. Production of methane is attributed to metal-carbene forma- tion and ethylene from its dimerization the latter being a possible termination step in the chain mechanism CH,-M‘ CH,-M L,M -L,M-CH3 ___+ LnM=CH2+CH4 2L,M=CH2 -+ CHZ=CH2 When deca-2,8-diene was added propene was the initial product.This and pro- ducts from deuteriated reagents are in accord with the proposed mechanism.54 The results of an extensive examinatior~~~ of product ratio and kinetics of meta- thesis of cyclic and acyclic olefins are also in agreement with the metal carbene chain mechanism as are results of studies on the stereochemistry of metathesis. ” E. Negishi and D. E. Van Horn J. Amer. Chem. SOC.,1977,99 3168. 53 For details see Rec. Trav. Chim. 1977 96 M1-M144. 54 R. H. Grubbs and C. R. Hoppin J.C.S. Chem. Comm. 1977,634. ” T. J. Katz and J. McGinnis J. Amer. Chem. SOC.,1977 99 1903. Organometallic Chemistry -Part (i) The Transition Elements 133 Stereospecificity of the metathesis of cis- and trans-pent-2-ene has been consi- dered in terms of steric interactions in the metallocyclobutane intermediate by two groups of ~orkers.~~.~~ The use of [W(CO),CPh,] as initiator for the metathesis of cis-pent-2-ene leads to much higher yields of cis-products than when the more conventional tungsten catalyst systems containing Lewis acids are employed.It is suggested that low stereoselectivity may result from the Lewis acid facilitating carbon-metal bond cleavage in the metallocyclobutane and formation of a metal- lopropyl cation which would allow bond r~tation.’~ Gassman and have been able to generate significant concentrations of ‘M=CHR’ carbene species in the presence of ‘M=CH2’ by conducting the meta- thesis of a terminal olefin in the presence of ethylcyclopropane.These authors attribute rapid degenerate metathesis of terminal olefins to efficient highly selec- tive capture of ‘M=CHR’ intermediates to which ‘M-CHR’ is a major resonance contributor. The now generally accepted chain mechanism accounts for almost all experi- mental observations but Mango59 has pointed out that given the known free energies of olefins and cyclopropanes the absence of the latter as an olefin meta- thesis product appears to be in conflict with this mechanism. It may be expected that this point will be debated further during 1978. Applications.-The main reason for the limited application of catalytic olefin metathesis in organic synthesis is due to the strong inhibiting effect of many common functional groups.It is therefore of interest that olefins with an amino- group that normally do not undergo metathesis do so when the donor properties of the amine are destroyed by quaternization.60 A convenient heterogeneous catalyst system of rhenium heptoxide on alumina promoted by a small amount of tetramethyl tin is a catalyst for metathesis of methyl penta-4-enoate in CCl,. After an hour the clean conversion of reactant to the diester (48) was 5 1% .61 Similar catalysts have also been used62 for the metathesis 2CH2=CH(CH2)2C02Me$ Me02C(CH2)2CH=CH(CH2)2C02Me+ CH2=CH2 of cis-1-chloro-octadec-9-ene with trans -dec-5-ene which after five hours afforded a mixture of cis- and trans-14-chlorotetradec-5-enein 70% yield. Under similar conditions allylchloride and vinyl chloride failed to react with hex-1-ene.The metathesis of a series of w -0lefinic esters catalysed by WCI6/Sn(CH3) gave the thermodynamic mixture of the cis- and trans-diesters (49). As well as the expected ethylene the chloroester (50) was formed in low yield by the addition of R02C(CH2) CH=CH(CH2),COZR CH3CHCI(CH2),CO2R (49) (50) s6 J. L. Bilhou J. M. Basset R. Mutin and W. F. Graydon J. Amer. Chem. SOC.,1977,99,4083. 57 T. J. Katz and W. H. Hersh Tetrahedron Letters 1977 585. 58 P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC.,1977,99 622. 59 F. G. Mango J. Amer. Chem. SOC.,1977,99,6117. ‘’J. P. Laval A. Lattes R. Mutin and J. M. Basset J.C.S. Chem. Comm. 1977 502. 61 E. Verkuijlen F. Kapteijn J. C. Mol and C. Boelhouwer J.C.S. Chem. Comm. 1977 198.62 R. Nakamura and E. Echigoya Chem. Letters 1977 1227. A. Stewart D. J. Thompson andM. V. Twigg HC1 (derived from the solvent chlorobenzene) across the double bond of the react ant ole fin. 63 It is of considerable interest that complex (51; M=Rh or Ir) which is reminiscent of the proposed metallocyclobutane metathesis intermediate is a powerful catalyst for 1,1,3,3-tetramethyldisiloxane disproportionation into dimethylsiloxane and higher siloxane 01igomers.~~ Here strong Si-0 bonds are broken and reformed under unusually mild conditions and this reaction may provide a route to novel silicone polymers. Ph3P\h/CO !%(Me),\o Ph3P/I\/ H Si(Me) 8 Use of Metal Cluster Complexes in Catalysis The synthesis and characterization of new metal cluster complexes continues and their possible application as catalysts for organic reactions is an area of growing interest.' A number of studies have been concerned with the catalysis of reactions of small molecules such as the methanation of carbon monoxide which are of industrial importance and are at present catalysed by conventional heterogeneous supported metal catalysts.It is possible that mechanistic information for reactions catalysed by metal clusters may lead to an improved understanding of reactions on metal surfaces. Following their work on the slow homogeneous methanation of carbon monox- ide catalysed by [0s3(Cc?),,] or [Ir4(CO)12] in toluene Muetterties and co-workers that [Ir4(CO)12] in molten NaC1,2A1Cl3 converts carbon monoxide and hydrogen to methane and ethane with only minor quantities of other hydrocarbons.At 180"C this relatively fast reaction is homogeneous but unlike conventional Fischer-Tropsch syntheses has the potential of product selectivity. It is appropriate to note a curious reaction of (52) {formed from [ZT(C~H~)~C~,] and AlH'Buz} ,H -AIBu (CsHd2Zr -H \/C1 'H -A1Bu2 (52) which in benzene absorbs two equivalents of carbon monoxide under ambient conditions to give a complex mixture of aluminium alkyls that does not contain simple aluminium alkoxide. On acid hydrolysis linear alcohols in a ratio compar- able to that of a Fischer-Tropsch hydrocarbon synthesis are obtained. The mechanism of this process is thought to be complex involving reduction of carbon monoxide co-ordinated to zirconium insertion and transmetallation of organo-group from zirconium to aluminium.66 " R.Baker and M. J. Crimmin Tetrahedron Letrers 1977,441. 64 J. Greene and M. D. Curtis J. Amer. Chem. SOC.,1977,99,5176. " G. C. Demitras and E. L. Muetterties J. Amer. Chem. Soc. 1977,99 2796. ''L. I. Shoer and J. Schwartz J. Amer. Chem. SOC.,1977,99,5831. Organometallic Chemistry -Part (i) The Transition Elements It is not surprising that (53) and (54) are active catalysts for the hydroformylation of pent- 1-ene and ~ent-2-ene.‘~ During the reaction cluster dissociation does not appear to take place and reaction conditions are milder than those used for the more usual CO~(CO)~ catalyst. These easily prepared clusters are air stable in the solid and in solution and offer the advantage that they can be moderately selective for terminal products.Moreover it is possible to recover the clusters in high yield after reaction. (c0)2c0-=‘CO(CO), ’-’P /fkp0 (CO),Co -R-Co(CO) o=c \\// \C&O) (CO),CO-QFPC 0(co’2 R (53) (54) Carbon monoxide inhibits the isomerization of pent-1-ene and cis-pent-2-ene and [H4R~4(C0)12], catalysed by [H2R~4(C0)13] and the results of labelling experiments suggest the presence of 0-alkyl intermediates. With [HRU3(CO&,Hg] as catalyst a further isomerization path is operative that appears to involve a r-ally1 intermediate.68 The cluster [Ni4{CNC(CH3)3}7] is readily prepared from t-butylisocyanide and [Ni(COD),] but has the disadvantage of being markedly air-sensitive.It is however an active catalyst for a variety of reactions.69 Acetylene is smoothly trimerized to benzene although the corresponding reaction of dialkylacetylenes is slow. Oli-gomerization of butadiene takes place stereoselectively to cyclo-octa- 1,5 -diene in the presence of this cluster which maintains high activity over an extended period. While inactive for the hydrogenation of olefins terminal and internal acetylenes are cleanly hydrogenated to the corresponding cis-olefin without alkane formation. However cluster decomposition products do catalyse olefin hydrogenation. An investigation7’ of the homogeneous methanation of carbon monoxide is the presence of [Ti(C5H5)2(CO)2] showed that the reaction is stoicheiometric and not catalytic in titanium.Methane is produced under an atmosphere of pure hydrogen and a blue solution of the first ‘M~(CSH~)~’ cluster [Ti6(C5HS),O8] is obtained. ‘’I R. C. Ryan C. U. Pittman jun. and J. P. O’Connor J. Amer. Chem. SOC., 1977,99 1986. ‘* G. A. Vaglio D. Osella and M. Valle TransitionMet. Chem. 1977 2 94. 69 M. G. Thomas W. R. Pretzer B. F. Beier F. J. Hirsekorn and E. L. Muetterties J. Amer. Chem. SOC. 1977,99,743. ’O J. C. Huffman J. G. Stone W. C. Krusell and K. G. Caulton. J. Amer. Chem. Soc. 1977,99 5829.

ISSN:0069-3030    

DOI:10.1039/OC9777400119

出版商:RSC    

年代:1977

数据来源: RSC