4 Reaction Mechanisms Part (iii) Free-radical reactions By R. A. JACKSON School of Chemistry and Molecular Sciences University of Sussex Brighton BN 1 9QJ 1 General Using the radial buffer technique to obtain values for the equilibrium constants for the reaction of an alkyl iodide with a different alkyl radical Castelhano and Griller have derived values for for alkyl radicals in solution for Me* Eta Pr'. and But-values are 144 (standard) 117 80 and 39 kJ mol-' corresponding to bond dissociation energies of -418 402 393 kJ mol-' for primary secondary and tertiary C-H bonds respectively.' Studies on the relative importance of polar and resonance effects in abstraction of hydrogen from substituted toluenes are often complicated by competing ring attack.Pryor's group have shown that for (nucleophilic) t-butyl radicals no addition to the benzene ring takes place,' whereas more electrophilic radicals (e.g. p-NO&H,*) add to the ring rather than abstract benzylic hydrogen. For reaction (l) a dependence of log k on Hammett's CT was found with p = +0.49. An e.s.r. study of the same reaction gave similar res~lts,~ with p = +0.59. But*+H-CH2C6H4X -b Bu'-H + 'CH2C6H4X (1) Reviews on laser flash photolysis of 1,4-biradi~als,~ on radical anion fragmenta- tion during SRNlreaction^,^ and on phosphoranyl radicals6 have appeared. Tedder7 has discussed the factors which determine the reactivity and regioselectivity of free-radical substitution and addition reactions and proposes some 'rules' which govern these reactions.2 Structural Studies Low-temperature e.s.r. studies of n-propyl isobutyl and neopentyl radicals indicate that couplings due to y-hydrogens vary markedly with the C-C-C-H dihedral angle and the results are interpreted in terms of positive values of uH for a dihedral angle of 180"and negative values of uHfor the 60" conformation.' A. L. Castelhano and D. Griller J. Am. Chem. SOC.,1982,104,3655. W. A. Pryor F. Y. Tang R.H. Tang and D. F. Church J. Am. Chem. SOC. 1982,104,2885. ' H. R.Dutsch and H. Fischer Inf.J. Chem. Kinef. 1982,14 195. J. C. Scaiano Acc. Chem. Res. 1982 15 252. R. A. Rossi Acc. Chem. Res. 1982,15 164. W. G. Bentrude Acc. Chem. Res. 1982,15,117. J. M. Tedder Angew. Chem. Znt. Ed. Engl. 1982,21,401. K. U. Ingold and J.C. Walton J. Am. Chem. Soc. 1982,104,616. R. A. Jackson There has been considerable interest in small-ring radicals. Theory predicts that the trimethylcyclopropenyl radical (1) will deviate from D3,,symmetry because of the Jahn-Teller effect. E.s.r. ENDOR and ELDOR experiments on a yirradiated matrix of trimethylcyclopropenium fluoroborate in aqueous LiCl supports structure (l),with a(3H) = 12.5 G and a(6H) = 3.5 G; a barrier of 15-29 kJ mo1-l for interconversion between valence isomers was estimated.' Compound (1)has also Me H Me A M Me A M e Dq (1) (2) (3) been prepared in fluid solution by reaction of the cyclopropene with t-butoxyl radicals:" at 113 K splittings of 11.9 G (3H) and 3.0 G (6H) were observed in good agreement with the matrix results whereas at 240 K equilibrium between the three equivalent isomers gives aH = 6.0 G (9H).The corresponding dimethylcyc- lopropenyl radical (2) has aH= 37.09 G (lH) 3.25 G (6H) and does not readily interconvert to its isomers. The large value of aHfor the single hydrogen in (2) implies a positive sign and a structure with the C-H bond bent much further out of the plane of the cyclopropene ring than is the case for the corresponding bond in the cyclopropyl radical. The spiropentyl radical (3) has been obtained by hydrogen abstraction from spiropentane:" no rearrangement or ring opening takes place up to 270 K. Cycloalkylmethyl radicals with C4 and C rings adopt the bisected conformation (4) preferentially but for larger rings (C6 C8 Cll) the eclipsed conformation (5) is favoured.Steric effects are thought to dominate except for cyclobutylmethyl where hyperconjugation involving C and the two C,-C bonds is thought to be important." Cyclopropylacyl radicals along with oxa- and aza-analogues favour the cis-and trans-conformations such as (6) and (7).13 H I (4) n = 3,4 or 5 (5) n = 6,8,or 11 Substituents in cyclopentadienyl radicals break the degeneracy of the qSand qA orbitals with one node electron-releasing substituents raise the qslevel relative to qA,whereas electron-withdrawing substituents have the opposite effect. From the e.s.r. spectra of the l-substituted radicals it was concluded that electron release G. L. Closs W. T. Evanochko and J. R. Norris J. Am. Chem. SOC.,1982,104,350.lo R. Sutcliffe D. A. Lindsay D. Griller J. C. Walton and K. U. Ingold J. Am. Chem. SOC.,1982 104 4674. l1 A. J. Kennedy J. C. Walton and K. U. Ingold I. Chem. SOC.,Perkin Truns. 2 1982 751. M.L.Kemball J. C. Walton and K. U. Ingold J. Chem. SOC.,Perkin Truns. 2 1982 1017. l3 A.G. Davies and R. Sutcliffe J. Chem. SOC.,Perkin Trans. 2 1982 1483. Reaction Mechanisms -Part (iii)Free-radical Reactions 71 to the 7~ system falls in the sequences Me > Et -Pr > H and Me3C >> Me3Ge > H > Me,Sn > Me3Si > C13Si.14 Studies of the 2-adamantyl radical in fluid solution indicate a nearly planar structure but 2-trimethylsiloxy-Zadamantyl appears to be pyramidal at the radical site with strong correlation between the pyramidal inversion and rotation of the trimethylsilyl group.:5 The radicals Me2C-CN and Me$-C02Me form complexes with trimethyl- aluminium and are formulated as involving a donor bond from the nitrile nitrogen and the carbonyl oxygen atom respectively to the aluminium atom and with aAI= 1.7 and 0.5 G respectively.16 3 Formation Destruction and Radical Stability 1,2-Diphenylethane has been thermolysed in the gas phase and in solution in tetralin and dodecahydrotriphenylene (DHTP). The gas phase results give D(PhCH2-CH2Ph) = 265.7 kJ mol-' in reasonable agreement with recent values for AH of the benzyl radical. Decomposition in solution is slower by a factor of 2-3 since k(gas)> kctetrali,,> /qDHTP) corresponding to a decrease in rate with increasing viscosity it is suggested that the cage effect is' responsible for the difference between the gas phase and solution rates.17 Very low-pressure pyrolysis of pent-2-yne gives AH for the 3-methylpropargyl radical CH3C~CCH2* of 294.1 kJ mol-' corresponding to a stabilization energy of 44 f 10 kJ mol-' for this radical." The stabilization energy of the aminopropynyl radical H2N-CH-C=CH has been estimated from the temperature dependence of the hyperfine coupling by the two NH2 hydrogen atoms in the e.s.r.~pectrum'~ to be 107 kJ mol-'. The thermochemistry of other Group IV elements maintains its interest. A study of the reaction of iodine with phenylsilane" is consistent with a chain reaction; the I* + C6H5SiH3 + C6HSSiH2*+ HI (2) activation energy for reaction (2) gives D(C6H5SiH2-H) = 374 kJ mol-'.Com-parison with other Si-H dissociation energies suggests that the 'silabenzyl' stabiliz- ation energy is only about 7 kJmol-' in accord with much qualitative data. D(H,Ge-H) has been derived from analogous studies on Ge& as 346 kJ mol-'. This is very similar to the value of 340 kJ mol-' established earlier for Me3GeH and parallels the results for the analogous silicon compounds. Although methyl groups appear to have little influence on the X3Si-H or X3Ge-H bond dissociation energies for both X3Si-SiX3 and X3Ge-GeX3 the central bond is stronger by about 30 kJ mol-' for X = Me compared with X = H indicating that the methyl l4 A. G. Davies E. Lusztyk and J. Lusztyk J. Chem. Soc. Perkin Trans.2 1982 729; A. G. Davies J. P. Goddard E. Lusztyk and J. Lusztyk ibid. p. 737; P. J. Barker A. G. Davies R. Henriquez and J.-Y. Nedelec ibid. p. 745. Is M. Kira M. Watanabe. M. Ichinose and H. Sakurai J. Am. Chem. SOC.,1982,104,3762. 16 S. Brumby J Chem. SOC.,Chem. Commun. 1982,677. l7 S. E. Stein D. A. Robaugh A. D. Alfieri and R. E. Miller J. Am. Chem. Soc. 1982,104,6567. T. T. Nguyen and K. D. King Int. J. Chem. Kinet. 1982,14,613. D. Griller D. C. Nonhebel and J. C. Walton J. Chem. Soc. Chem. Commun. 1982 1059. *' M. Barber A. M. Doncaster and R. Walsh Int. J. Chem. Kinet. 1982 14. 669. R. A. Jackson groups are acting as bond strengtheners in these compounds.21 Shock-tube studies on the decomposition of tetramethylgermane in the presence of excess toluene give D(Me,Ge-Me) = 332 f 8 kJ mol-’ falling between the values for Me4Si (higher) and Me4Sn (lower).22 ?-Irradiation of polycrystalline alkanes with 11or more carbon atoms gives e.s.r.spectra at 77 K which differ markedly depending on whether the alkane has an even or an odd number of carbon atoms. For the even alkaqes the spectra can be simulatFd by assuming a mixture of penultimate (RCH2CHCH3) and internal (RCH2CHCH2R) radicals but for odd alkanes chain end radicals (RCH2CH2-) appear to be important too indicating that molecular packing and alignment in the crystal affects the radio~hemistry.~~ The kinetics of decomposition of PhCH(CH,)Co(drngH),OH in aqueous methanol indicates parallel reactions of the parent molecule and its conjugate acid (faster).It is suggested that both homolysis of the C-Co bond (-72%) and &elimination (-28%) to PhCH=CH2 and the Co-H compound take place.24 The rates of cleavage of the t-butyl radical from t-BuRR’COH (R and R’ are bridged-ring and non-cyclic tertiary groups) have been measured and compared with molecular mechanics calculations indicating that the radical strain energies in the bridgehead systems are slightly greater than in the corresponding alkanes but considerably less than those of the related carbo~ations.~’ Aryl radicals are produced efficiently from the tetrazene (8) and from the hexazadine (9)both thermally and photolytically. Aromatic compounds are attacked to give compounds substituted in the nucleus and propan-2-01 is oxidized to ace tone.26 COMe Ip-CIC6H4-N=N-N-NHCOMe COMe Ip-CIC6H4-N=N-N-N-N=N-c6H4cl-~ICOMe (8) (9) A new technique Laser-Powered Homogeneous Pyrolysis has been described. An infra-red laser transfers energy to an inert gas which transfers its energy to the substrate. Wall reactions tend to be unimportant and the need to define a reaction temperature is avoided by the use of a standard (e.g. t-butyl acetate) whose Arrhenius parameters for decomposition are known. It is suggested that 2,2’-azoisopropane decomposes via a concerted [reaction (3)] rather than a stepwise process in contrast to some previous findings although the alternative that decomposition involves a rate-determining trans to cis isomerization of the azo compound cannot entirely be ruled Me2CH-N=N-CHMe2 + 2Me2CH.+ N2 (3) 21 Y.Fujimoto Ching-Shih Chen Z. Szeleczky D. DiTuJlio and C. J. Sih J. Am. Chem. SOC.,1982 104,4718. 22 J. Dzarnoski M. A. Ring and H. E. O’Neal Znt. J. Chem. Kinet. 1982,14,279. 23 K. Toriyama M. Iwasaki and M. Fukaya J. Chem. SOC.,Chem. Commun. 1982 1293. 2* H. B. Gjerde and J. H. Espenson Organomefulfics,1982,1,435. ” J. S. Lomas and J.-E. Dubois J. Org. Chem. 1982 47,4505. ’‘ D. Mackay and D. D. McIntyre Can. J. Chem. 1982,60,990. ” D. F. McMillen. K. E. Lewis G. P. Smith and D. M. Golden J. Phys. Chem. 1982 86 709. Reaction Mechanisms -Part (iii) Free-radical Reactions Secondary deuterium kinetic isotope effects indicate that in 4-ethylidene-l- pyrazoline (lo) the C-N bond anti to the methyl groups breaks first to give an intermediate radical (1 1).Deuterium-substitution studies show that the product- determining step involves intramolecular displacement of nitrogen [reaction (4)] rather than the intermediacy of a trimethylenemethane diradical(12).*’ Trimethyl- enemethane-type radicals are however in reactions of the 5-alky- lidenebicyclo[2.l.0]pentanes (13) [e.g. reaction (5)] in which thermolysis breaks the bridging C-C bond to give the diradical (14). (13) (14) It is possible that the spiro-conjugated tetraradical (16) is formed on sensitized photolysis of the azo-compound (15). The products indicate involvement of one of two unprecedented processes either the formation of the tetraradical (16) or the frontside attack of a radical on a carbon-carbon bond [reaction (6b)I.” DD I1 ii hv ___) Sens.?? U ’* R. J. Crawford and Moon Ho Chang Tetrahedron 1982,38,837. 29 M. Rule J. A. Mondo and J. A. Berson J. Am. Chem. Soc. 1982 104.2209; M. R. Mazur and J. A. Berson ibid. p. 2217. ’O L. McEIwee-White and D. A. Dougherty J. Am. Chem. SOC.,1982,104,4722. 74 R. A.Jackson CIDNP studies at pressures in the range 0-200 MPa have been carried out on the thermolysis of acetyl benzoyl per~xide.~' High pressure retards the peroxide cleavage and increases the ratio of cage to escape product. AVS for the homolysis is 4 cm3 mol-' and that for the difference in formation of escape to cage products is 8 cm3 mol-'. At moderate viscosities the disproportionation :combination ratio for the cage reactions of t-butyl radicals generated by photolysis of 2,2'-azo- isobutane is similar to that for encounter pairs from di-t-butyl ketone photolysis but at viscosities of 12-808 cP the orientation of the cage t-butyl radicals favours a higher proportion of omb bin at ion.^^ 4 Radical Transfer Absolute rate constants have been measured by laser flash photolysis for the reactions of t-butoxyl radicals and some ketone triplets with several organ~silanes.~~ Triethylsilane n-C5HllSiH3 and C6H5SiH3 react with t-butoxyl at similar rates (within a factor of 2 at 300 K) the main reaction being the abstraction of hydrogen attached to silicon.Trichlorosilane reacts seven times more rapidly than triethyl- silane. Triethoxysilane reacts virtually exclusively by abstraction from the CH2 groups.A similar study on the reactions of t-butoxyl radicals with acyclic and cyclic ethers shows that abstraction of hydrogen from a C-H bond a to the oxygen is enhanced when there is a small (-30") dihedral angle with a v-type orbital on the oxygen. Highest rates were observed for 2-methyl-1,3-dioxolane and THF.34 Arrhenius parameters for the abstraction of Si-H from trimethylsilane by t-butoxyl radicals in the gas phase have been by a competition method to be lo8.' M-' s-' and 15.5 kJ mol-'; by consideration of reactions of t-butoxyl radicals with nine hydrogen donors it is suggested that logA (per hydrogen) = 8.4 f 0.5 M-'s-' and E/kJ mol-' = 0.42 AH + 36.4 (k2.9). Nitrogen dioxide reacts at high concentrations with alkenes by (reversible) addi- tion but at low concentrations (below 10 000 p.p.m.) hydrogen abstraction predomi- nates; the nitrous acid formed is likely to be harmful in biological This reaction parallels the addition/substitution reactions found for bromine and alkenes.Alkanes react with chlorosulphonyl isocyanate to give alkyl chlorides as the main organic product~.~' Selectivity studies indicate that the abstracting radical is prob- ably *NCO rather than *S02NC0 =SO,Cl or C1.. An ab initio approach to frontier orbital theory has been used to illuminate the different positions of attack of methyl radicals and chlorine atoms on propionic acid. 'Hydrogen atomic sphere charges' are calculated by integrating q2within the limit of a sphere (radius 0.5 A) round the H atom concerned both for the HOMO and the LUMO.For the electrophilic chlorine atom interaction of the 31 E. M. Schulman A. E. Merbach and W. J. le Noble J. Org. Chem. 1982,47,431. '* D. D. Tanner and P. M. Rahimi J. Am. Chem. Soc. 1982,104,225. 33 C. Chatgilialoglu J. C. Scaiano and K. U.Ingold Organometaflics 1982 1,466. 34 V. Malatesta and J. C. Scaiano J. Org. Chem. 1982,47 1455. " Chan Ryan Park Se Ahn Song Yong Em Lee and Kwang Yul Choo J. Am. Chem. Soc. 1982,104 6445. W. A. Pryor J. W. Lightsey and D. F. Church J. Am. Chem. SOC.,1982,104,6685. 37 M. W. Mosher J. Org Chem. 1982,47,1875. Reaction Mechanisms -Part (iii) Free-radical Reactions SOMO is mainly with the HOMO of propionic acid where the charge is greater at C-3 whereas for methyl the SOMO-LOMO interaction is more important with attack taking place at To explain the temperature-independent k,/k = 2.9 for abstraction of hydro- gen from allylbenzene by t-butoxyl radicals along with an inverse secondary deuterium isotope effect at both ends of the double bond it is postulated that the bent transition state (17) is involved with an unusual bridging effect of the t-butoxyl oxygen atom.39 (17) From studies of chlorination of 1-substituted butanes in various solvents it appears that solvents are of three types.Fluorocarbons act as inert solvents selectivity is approximately the same as in the gas phase. Selectivity in benzene and carbon disulphide is greater than in the gas phase probably due to a rr-complex and the formation of the adduct radical CS2Cl*.For the neat compound or solutions in polarizable solvents such as CC14 selectivity is less than in the gas phase this is attributed to stabilization of the polar transition state causing a levelling of sele~tivities.~' Absolute rate constants for abstraction of halogen from 21 organic halides by triethylsilyl radicals have been obtained by the laser flash photolysis method.41 The high log (A/M-'s-') factors (>lo) for CC14 C6H5CH2Br and C,H,I are attributed at least partially to extensive polar contributions to the transition state. For complete electron transfer the gain of two rotational degrees of freedom in the transition state would enhance the pre-exponential factor by ca.lo2.Iodine transfer between two aryl groups may be direct or may involve an Ar-I-Ar' intermediate evidence for the latter route has been obtained by studies of the reduction of diaryliodinium salts to give this intermediate.42 It is suggested that the abstraction of bromine from N-bromosuccinimide can follow two routes. Less reactive radicals such as bromine atoms may react via an out-of-plane transition state (18)to give a rr-succinimidyl radical (no ring opening) whereas more reactive radicals (e.g. primary alkyl radicals) react by an in-plane transition state (19) to give the cr-succinimidyl radical which can undergo ring opening.43 SH2 reactions at saturated carbon involving the stable cobalt (11) cobaloxime as the leaving group continue to a.ttract interest.Kinetic rtsults have been obtained for the reactions with CH3CHOC2H5 and (CH3)2COH radicals,44 and in R. J. Elliott and W. G. Richards J. Chem. SOC.,Perkin Trans. 2 1982 943. 39 H. Kwart M. Brechbiel W. Miles and L. D. Kwart I. Org. Chem. 1982,47,4524. 40 A. Potter J. M. Tedder. and J. C. Walton I. Chem. SOC.,Perkin Trans. 2 1982 143; A. Potter and J. M. Tedder ibid.,p. 1689. 41 C. Chatgilialoglu K. U. Ingold. and J. C. Scaiano J. Am. Chem. SOC.,1982,104.5123. 42 D.D.Tanner D. W. Reed and B. P. Setiloane J. Am. Chem. SOC.,1982,104,3917. 43 R.L.Tlumak and P. S. Skell I. Am. Chem. Soc. 1982,104,7267. 44 R. C.McHatton J. H. Espenson and A. Bakac J. Am. Chem. Soc. 1982,104,3531. R. A. Jackson *X 0 1 *S +X-Br 4,+ X-Br intramolecular sH2 reactions cyclopentane and sulpholane rings have been closed in this way.45 "F atoms react with tetramethyltin to give CH3"F.The &2 reaction (7)is postulated to occur with an efficiency of -& of the hydrogen-abstraction F-+ (CH3)4Sn -+ F-CH3 + (CH3)3Sn* (7) 5 Addition to Multiple Bonds and Homolytic Aromatic Substitution When positive muons are passed into alkenes adduct radicals are observed. Muonium (p+e-) behaves as a light isotope of hydrogen (mass = iH) and adds to the C=C bond in alkenes with a regioselectivity similar to that of hydrogen atom^.^' Addition and allylic hydrogen abstraction are competitive processes for t-butoxyl radicals with acyclic and monocyclic alkenes but for norbornene and norbor- nadiene addition is the only mode of A factors for attack on alkenes are 'normal' but activation energies are low compared with hydrogen abstraction from comparable saturated compounds.Isopropylperoxyl radicals react with 2,3- dimethylbut-2-ene according to equation (8),yielding the epoxide (20) as the only alkene-derived 0 /\ Me2CHO0,+ Me2C=CMe2 +Me2C-Me2 + Me2CH0. (8) (20) Stoicheiometric hydrogenation of alkenes by HCo(CO) can be achieved by a process formulated by Scheme 1.The deuterium isotope effect varies widely from 0.43 to 2.0 this is explained as due to the differing extent of hydrogen transfer to the alkene in the transition state.so ArRC=CR'R2 Arc-CR'R2H M dArC-CR'R2H+M* I I R R ArC-CR'R2H +M.MH ArCH-CR'R2H + M2 I I R R Scheme 1 45 P.Bougeard A. Bury C. J. Cooksey M. D. Johnson,J. M. Hungerford and G. M. Lampman J Am. Chem. SOC.,1982,104,5230. M. Kikuchi J. A. Cramer R. S. Iyer J. P. Frank and F. S.Rowland. J. Phys. Chem. 1982 86,2677. 47 E. Roduner W. Strub P. Burkhard J. Hochmann P. W. Percival H. Fischer M. Ramos and B. C. Webster Chem. Phys. 1982,67,275. 48 P. C. Wong,D. Griller and J. C. Scaiano,J. Am. Chem. Soc. 1982 104,5106. 49 M. I. Sway and D. J. Waddington J. Chem. SOC.,Perkin Trans. 2 1982,999. T. E. Nalesnik J. H Freudenberger and M. Orchin I. Mol. Caral. 1982,16.43. Reaction Mechanisms -Part (iii) Free-radical Reactions The ring-closure reaction (9) gave only moderate yields when carried out with tributyltin hydride but with a polymer-supported tin hydride the yields were excellent and the polymeric reagent could be regenerated for further use.51 Vinyl radicals generated by photolysis of the iodide in the presence of tributyltin hydride undergo ring closure reactions [e.g.reaction (lo)].The 5/6and 6/7ring-size preference is similar to that found for alkyl radicals. These cyclizations may be useful in synthesis of compounds with double bonds in known Above 150"C,the addition of trichlorosilyl radicals to alkenes becomes revers- ible,53 leading for example to isomerization of cis-but-2-ene to the cisltrans mixture and a value of D(CI,Si-C,,) = 328 f 8 kJ mol-'. For addition of trialkylsilanes across a C=C bond low yields are partially caused by telomerization competing with the transfer step. Good yields of adduct can be obtained by use of reaction conditions that assure a large excess of the silane throughout the reaction.54 In contrast to the cyclization of o-alkenyl radicals o-alkenyldimethylgermyl radicals derived from (21,n = 2-4) react predominantly by attack at the terminal CH2 group probably because of the geometric constraints caused by the longer Ge-C bond." n/Me Me2HGe(CH2),CH=CH2 -% (CH2),,+2 Ge (11) 'Me (21) Alkyl radicals react less rapidly with alkynes than with alkenes.It is suggested that an early transition state is involved and the LUMO in an alkyne is higher in energy than the LUMO of an alkene.56 s1 Y. Ueno K. Chino M. Watanabe 0.Moriya and M. Okawara I. Am. Chem. Soc. 1982 104 5564. '* G. Stork and N. H. Baine J Am.Chem. Soc. 1982,104,2321. 53 T. Dohmaru and Y. Nagata J. Phys. Chem. 1982 86 4522; J. Chem. Soc.. Faraday Trans. 1 1982 78 1141. 54 N. M. K. El-Durini and R. A. Jackson J. Organomet. Chem. 1982,232 117. 55 K. Mochida and K. Asami J. Organomet. Chem. 1982,232 13. " B. Giese and S. Lachhein Angew. Chem. Int. Ed. Engl.. 1982 21,768. R. A. Jackson Although hydroxyl radicals react with benzene by addition at room temperature at 290°C hydrogen abstraction takes place. In the absence of air biphenyl i!j produced but if air is present phenol is produced uia the PhOO. radical." Absolute rate constants have been determined for the addition of triethylsilyl radicals to twenty carbonyl compounds. Rates varied by more than 6 orders of magnitude from duroquinone (most reactive) to methyl acetate.'* Radical rear- R R R R I I rangements of type (12)have been studied by e.s.r.spectroscopy. It is suggested that the rearrangements do not go uia ring closure (to 23)followed by ring opening on account of the high A factors observed (-1013 s-l) and the fact that for (23 R = cyclopropyl) ring opening of the cyclopropyl group is observed rather than carbon-oxygen fission to give (22,R = cyclopropyl). It is suggested that a loose transition state approximating to (24)is inv01ved.'~ Triethylaluminium reacts with 3,6-di-t-butyl-1,2-benzoquinone in toluene to give an e.s.r. signal attributed to (25).In ether species (26)is observed indicating that polar solvents can break the chelation.60 Trimethysilyl radicals add to alkyl isocyan- Y (25) (26) ate to give imidoyl radicals (27).Ease of addition falls in the series R = Me > Et > Pr' > But suggesting a steric effect with the Me3% group closer to the N-alkyl group in the transition state than in the product.61 Me3Si.+ R-N=C=O -B R-N=C-OSiMe3 (13) (27) 57 P. Mulder and R. Louw,Tetrahedron Lett. 1982.23 2605. '13 C. Chatgilialoglu K. U. Ingold. and J. C. Scaiano J. Am. Chem. SOC., 1982 104 5 119. 59 L. R. C. Barclay D. Griller and K. U. Ingold J. Am. Chem. Soc. 1982 104,4399. 60 A. G. Davies Z. Florjanczyk,E. Lusztyk and J. Lusztyk J. Organomet. Chem. 1982,229 215. " J. A. Baban M. D. Cook,and B. P. Roberts J. Chem. Soc. Perkin Trans. 2 1982 1247. 79 Reaction Mechanisms -Part (iii)Free-radical Reactions Aromatic nitroso compounds can be used as spin traps in non-polar media but are relatively insoluble in polar solvents such as water.Compounds (28)-(30) have been suggested as water-soluble spin traps.62 0 N/p N+ N+O I 6 Fragmentation and Rearrangements Reduction of 0-thiocarbonyl derivatives of secondary alcohols in the presence of tributyltin hydride takes place in good yield at 100 "C but primary alcohol deriva- tives require much higher temperatures. However primary alcohol derivatives with P-oxygen substituents are reduced at lower temperatures the reaction involves the fragmentation (14) and it is suggested that these reactions are promoted by stabilization of the product radical Re by P-oxygen sub~tituents.~~ S-SnBu3 -+ R'-C + R* FnBu3 R1-C BS -B R'-C ./' \ 0-R \O-R \O Studies of pivaloxy radicals (ButCO2-) suggest that they decarboxylate more slowly than do acetoxy radicals in spite of the greater stability of the t-butyl radical produced. A possible interpretation is an early transition state with little C-C bond stretching and the increase in 0-C-0 bond angle causing greater steric inhibition of reaction for the pivaloxy radicals.64 Decomposition of t-butyl peroxide in the presence of NO leads to values of log (Als-') = 14.6 and E = 66.5 kJ mol-' for the fragmentation of the t-butoxyl radical [equation (15)].65 Bu'O. -B MezCO + Me-(15) In alcohols arenediazonium ions undergo competing processes an ionic reaction with the solvent to give the aryl alkyl ether and free-radical decomposition to give the arene.In the presence of P-cyclodextrin the arene only is formed even in the presence of oxygen and it is suggested that the radical species are surrounded by the cyclodextrin preventing approach by oxygen.66 " J. K. Brown P. J. Coldrick and E. J. Forbes J. Chem. SOC.,Chem. Commun. 1982 770; H. Kaur and M. J. Perkins Can. J. Chem. 1982 60 1587; R. Konaka and S. Sakata Chem. Lett. 1982,411. 63 D. H. R. Barton W. Hartwig and W. B. Motherwell J. Chem. SOC.,Chem. Commun. 1982,447. 64 D. D. May and P.S.Skell J. Am. Chem. SOC.,1982,104,4500. 65 L. Batt and G. N. Robinson Znt. J. Chem. Kinet. 1982 14 1053. 66 S. P. Breukelman G. D. Meakins and M. D. Tirel J. Chem. SOC.,Chem. Commun.1982,800. R. A. Jackson Studies of the mercury-photosensitized decomposition of hexamethyldisilane give Arrhenius parameters for reactions (16) and (17).67 Log (A16/S-l) = 12.3 the low value being appropriate for a unimolecular reaction with a tight transition state. For reaction (17) the A factor is normal [log (AI7/s-') = 15.01 and from E17 = 171 kJmol-' an estimate for the n bond energy of MezSi=CHz = 188 f 20 kJ mol-' is obtained slightly higher than a previous estimate of 163 f 20 kJ mol-'. Me3SiSiMezCHz*-+ Me3SiCHzSiMez* (16) Me3SiCHzSiMez*-+ Me3Si*+ MezSi=CHz (17) Although the decarboxylation of diacyl peroxides with a secondary chiral centre occurs with retention of configuration for peroxide (31) with a primary chiral centre racemization takes place during decarboxylation suggesting that a radical cage process is involved.68 CF3CONHCH(CO~Et)CHD-CO-O-O-CO-C~I&-CI-~ (31) Chloro-substituted radicals such as (33) and (34) formed radiolytically in aqueous solution undergo a loss of chloride ion which is rapid compared with (32) and with the SN1/E1 reaction of t-butyl chloride itself.The increase in rate caused either by a or p methylation of (32) suggests that a polar SN1-like transition state is &Cl C1 (32) (33) (34) 7 Electron Transfer The involvement at least to a partial extent of free radicals is increasingly being postulated in processes formerly thought to be ionic or molecular. For Diels-Alder reactions of anthracene with tetracyanoethylene transient charge-transfer absorp- tion was observed suggesting an intermediate donor-acceptor complex.7o In reac-tions of R3SnNa compounds with alkyl halides to give tetra-alkyltin compounds competing sN2 and electron transfer can take place.71 Radical involvement in the reaction is shown by a cyclization of the intermediate radicals (when suitable alkenyl halides are used) and by trapping with dicyclohexylphosphine.Experiments with (+)-2-bromo-octane show predominant inversion of stereochemistry which taken 67 I. M. T. Davison P. Potzinger and B. Reimann Ber. Bunsenges. Phys. Chem. 1982,86,13. 68 S.J. Field and D. W. Young J. Chem. SOC.,Perkin Trans. l. 1982 591. 69 G.Koltzenburg G. Behrens and D. Schulte-Frohlinde J. Am. Chem. SOC.,1982,104,7311. 'O S.Fukuzumi and J. K. Kochi Tetrahedron 1982,38 1035. E. C. Ashby and R. DePriest J. Am. Chem. SOC.,1982,104,6144. Reaction Mechanisms -Part (iii)Free-radical Reactions with the evidence for predominant radical character of the reaction indicates a mechanism for the reaction corresponding to (18). In reactions of alkyl halides with ketones in the presence of lithium an organolithium intermediate is often formed but for halides such as 1-bromoadamantane there is competing radical reaction in which the anion radical (R-X)-resulting from reaction of the alkyl halide with lithium reacts directly with the ketone without forming an intermediate organometallic Lithium isopropoxide acts as a good reducing agent for aromatic ketones. Radical intermedi- ates which are not the free ketyls are detected;73 it is suggested that reaction takes place according to equation (19).Ph2C=O + LiOPr' $ Ph2C=0 3 (Ph2C=O)-(LiOPri)+ + Ph2COLi + MeCOMe I LiOPr' H (19) The reaction of Ph2As- and Ph2Sb- with haloaromatic compounds gives products involving scrambling of the aryl groups which indicates that the addition of Ph2As- or Ph2Sb- to Are is rever~ible.~~ Ph2P- and Ph2As- ions also react with 1-bromoadamantane to give good yields of substitution products; the SRNl mechan-ism is thought to be Primary and secondary alkylmercury halides react with the secondary nitro-alkane salts by a light-initiated process formulated as the SRNl sequence (20)-(23).76 initiation RHgX + Rl2C=N0 2 RHg-+ X-+ R12CN02 (20) RHg.+ R-+ Hgi[RR12CN0,]'+ RHgX + RR'2CN02 + RHg-+ X-propagation Re + R12C=N02-B [RR12CN02] ' (21) (23) (22) Finally it is suggested that the 'a-effect' -enhancement of nucleophilicity by a non-bonding pair of electrons on an atom a to the nucleophilic centre -is partially due to development of some diradical character in the transition state.-To the 74-7 " G. Molle and P. Bauer J. Am. Chem. SOC.,1982 104,3481. 73 E. C. Ashby A. B. Goel and J. N. Argyropoulos Tetrahedron Lett. 1982,23,2273. 74 R.A. Alonso and R. A. Rossi J. Org. Chem.. 1982 47.77. 7s R. A. Rossi S. M. Palacios and A. N. Santiago J. Org. Chem. 1982,41,4654. 76 G. A. Russell J. Hershberger and K. Owens J. Organomet. Chem. 1982,225,43. a2 R. A. Jackson extent that electron transfer from the nucleophile to the electrophile takes place in the transition state stabilization by the lone pair will take place (35),as in hydrazyl nitroxide and other radical^.^' Note.Readers interested in a computer-readable version of the references in this Section for their own data systems are invited to contact the Reporter. 77 S. Hoz J. Org. Chem. 1982,47,3545.