4 Free-radical Reactions’ By A. R. FORRESTER Department of Chemistry University of Aberdeen Aberdeen AB9 2UE ELECTRON spin resonance measurements2 have confirmed previous predictions that the benzoyl radical is a a-radical the unpaired electron occupying a hybrid orbital situated orthogonal to the n-system (cc the isoelectronic PhCN’ which is a n-radical3 and PhNO?). The observation that the major splitting in its spectrum (1.6G) is due to the meta-protons is complementary to the findings of an n.m.r. study4 of stable ‘twisted’ n-radicals. In view of the above result 00 Recent books and reviews dealing with aspects of free-radical chemistry include E. S. Huyser ‘Free Radical Chain Reactions’ Wiley New York 1970; E. G Rozant- sev ‘Free Nitroxyl Radicals’ Plenum New York 1970; ‘Methods in Free Radical Chemistry’ Vols.1 and 2 ed. E. S. Huyser M. Dekker New York 1969 these Vols. contain chapters on Free Radical Study by Electron Paramagnetic Resonance by L. Kevan Free-Radicals and Photochemical Reactions by D. C. Neckers Free Radical Chlorination of Organic Molecules by M. L. Poutsma Thiyl Radicals by R. M. Kellogg and Free Radical Brominations by W. A. Thaler; ‘Electrolytic Reduc- tive Coupling Synthetic and Mechanistic Aspects’ M. M. Baizer and J. P. Petrovich Prog. Phys. Org. Chem. 1970 7 189; The Application of Radiation Chemistry to Mechanistic Studies in Organic Chemistry E. J. Fendler and J. H. Fendler ibid. 1970 p. 229; The Study of Free Radicals and their Reactions at Low Temperature using a Rotating Cryostat J.E. Bennet B. Mile A. Thomas and B. Ward Ado. Phys. Org. Chem. 1970 8 1;Synthetic Uses of Free Radicals G. B. Gill in ‘Modern Reactions in Organic Synthesis’ ed. C. J. Timmons Van Nostrand London 1970; Stereoselectivity in Free Radicals of the Norbornyl Type P. D. Bartlett G. N. Fickes F. C. Haupt and R. Helgeson Accounts Chem. Res. 1970 3 177; Problems and Possibilities of the Free Radical Addition of Thiols to Unsaturated Compounds K. Griesbaum Angew. Chem. Internat. Edn. 1970,9,273; Interactions between Atoms and Radicals in the Liquid Phase E. T. Denisov Russ. Chem. Rev. 1970 39 31; Reactions of Epoxy Compounds by a Radical Mechanism A. P. Meleshevich ibid. 1970 39 213; The Present State of the Theory of the Oxidation of Cyclo-Olefins A.A. Syrov and U. K. Tsyskovskii ibid. 1970 39 373; Preparative Aspects of the Radiation Chemistry of Organic Compounds I. V. Vereshchinskii ibid. 1970 39 405. P. J. Krusic and T. A. Rettig J. Amer. Chem. Soc. 1970,92 722. ’ A. Carrington and P. F. Todd Moi. Phys. 1963 6 161. A. Calder A. R. Forrester J. W. Emsley G. R. Luckhurst and R. A. Storey Mol. Phys. 1970 18 481. A. R. Forrester the difference between structures such as (1) and (2) should be fully realised. These were formulated5 (wrongly as canonical forms) for the precursor of the cyclic disulphide (3) which arose during photolytic decomposition of 2-phenyl- 1,3-benzoxathian-4-one. In the absence of e.s.r. measurements it is of interest to speculate whether the analogous benzimidoyl radicals (4) formed by treatment of the corresponding benzaldimine with di-isopropylperoxydicarbonate are c-or .n-radicak6 These transitory intermediates fragmented spontaneously ArCH=N-R' + R0.j ArC=N-R' -P ArCN + R1 (4) to nitriles and alkyl radicals.Lack of a 'clean' source of aroyl radicals has greatly hampered examination of their chemistry most precursors of such radicals (PhCHO PhCON=NCOPh) reacting readily with the radicals they produce. In this respect thermolysis and photolysis of benzoylformyl chloride (the iodide was not examined) was disappointing in view of the encouraging results ob- tained using pyruvyl chloride as a source of acetyl radical^.^ Photolysis of phenyl benzoate in the presence of tributyltin hydride which is considered to produce benzoyl radicals by initial S,2 reaction of the tributyltin radical on the ester [equation (l)],also offers little advantage since other features complicate the reaction and a complex mixture of products results.8 Bu,Sn.+ PhOCOPh -+ Bu,SnOPh + PheO (1) + Evidence that acyl radicals are relatively nucleophilic species (RCO is more stable than RCO) has been obtained by acylating hetero-aromatic bases in acid solution with aldehydes in the presence of peroxide and ferrous ions;' acetyla- tion of quinoline in this way for example gave a mixture of 2-and 4-acetyl- quinolines. Formamido" (CONH,) and alkyl radicals similarly formed also exhibited nucleophilic behaviour on reaction with these bases in acid solution. Enhancement of the n.m.r.signal of the formyl proton of benzaldehyde during photolysis has been attributed12 to the generation of a radical pair (5) which may either disproportionate to benzaldehyde or couple to give benzoin. No such radical pairs are thought to participate in the photochemical isomerisation of o-phthalaldehyde to phthalide (8). Instead a chain process involving the isomeric propagating radicals (6) and (7) has been rn00ted.l~ The rate at which A. 0. Pedersen S.-0. Lawesson P. D. Klemmensen and J. Kolc Tetrahedron 1970 26 1157. H.Ohta and K. Tokumaru Chem. Comm. 1970 1601. ' D. D. Tanner and N. C. Das J. Org. Chem. 1970,35 3972. L. E. Khoo and H. H. Lee Tetrahedron 1970 26 4261. T. Carrona G. P. Gardini and F. Minisci Chem. Comm. 1969 201; G.P. Gardini and F. Minisci J. Chem. SOC. (C),1970 929. lo F. Minisci G. P. Gardini R. Galli and F. Bertini Tetrahedron Letters 1970 15. F. Minisci R. Galli V. Malatesta and T. Carrona Tetrahedron 1970 26,4083. l2 M. Cocivera and A. M. Trozzolo J. Amer. Chem. SOC.,1970,92 1774. l3 D. A. Harrison R. N. Schwartz and J. Kagan J. Amer. Chem. SOC.,1970 92 5793. Free-radical Reactions phenacetyl radicals decarbonylate (k = lo8s-') has been deduced by photo- lysing a benzyl ketone in the presence of a stable nitroxide and measuring the ratio of >N-0-alkyl to >N-0-acyl product f~rmed.'~ 0 0 0 II II 0 OH PhC- *CHPh II I ' CH Formation of products with polarised nuclear spins on thermolysis of t-amine N-oxides' (Meisenheimer rearrangement) and sulphonium ylidesI6 (Stevens rearrangement) has led to radical-pair mechanisms being advanced for these re- arrangement~.'.~ However in both cases diffusion of the radical pair must occur to some extent since 'scrambled products' were obtained on thermolysis of mixtures of structurally related N-oxides18 and ylides16 (see also ref.19). By comparison emission signals obtained from a mixture of picoline N-oxide and acetic anhydride at 90°C were shown2' not to arise from the principal product (10) and hence have no bearing on the mechanism of the rearrangement (9)--* (10). Also rearrangement of benzyl toluene-p-sulphenate to benzyl p-tolyl sulphoxide was considered not to proceed by way of radicals in spite of the initial emission signal obtained from the methylene protons of the sulphoxide.2 The theory22 developed last year to account for chemically induced dynamic nuclear polarisation (CIDNP) has been extended to systems containing several nuclear spins and it is now possible to predict the form of CIDNP spectra by considering transitions to and from those nuclear spin states of the product which are correctly polarised with respect to the electron spin of the radical l4 W.K. Robbins and R. H. Eastman J. Amer. Chem. SOC.,1970,92,6076,6077. G. Ostermann and U. Schollkopf Annalen 1970,737 170; A. R. Lepley P. M. Cook and G. F. Willard J. Amer. Chem. SOC.,1970 92 1101. J. E. Baldwin W. F. Erickson R. E. Hackler and R. M. Scott Chem. Comm. 1970 576 U. Schollkopf J. Schossig and G. Ostermann Annalen 1970 737 158.'-For a review see U. Schollkopf Angew. Chem. Internat. Edn. 1970 9 763. '' N. Castagnoli J. C. Craig A. P. Milikian and S. K. Roy Tetrahedron 1970,26 4319. G. F. Hennion and M. J. Shoemaker J. Amer. Chem. SOC.,1970,92 1769. 'O H. Iwamura M. Iwamura T. Nishida and I. Miura Tetrahedron Letters 1970 31 17. 21 J. Jacobus Chem. Comm. 1970 709. 22 G. L. Closs J. Amer. Chem. SOC.,1969 91 4552; G. L. Closs and A. D. Trifunac ibid. 1970 92 2186. A. R. Forrester precursor.23 Application of the theory to predict "F CIDNP spectra is awaited.24 The harmful effect which an excess of nitrite ions has on the stability of dia- zonium salts has been attributed2' to the formation of the adduct (11) which decomposes rapidly to phenyl radicals and nitrogen.Using dimethyl sulphoxide as solvent this reaction may be utilised for the preparation of unsymmetrical biaryls and indeed appears to have advantages over the Gomberg and Ullmann + Ph.+ Nz + PhN=N-O* PhSO,N=N-SMe, I 0- (12) reactions. The identity of the species which abstracts hydrogen from the inter- mediate cyclohexadienyl radical is uncertain NO2 [equation (2)] N203 or PhN=N -0-[equation (311 being likely candidates (cf. decomposition of N-nitrosoacetanilide26 and P-phenylazoxyto~ylate~~). Photolysis of aryl-lithium ditrifluoroacetates2' (recommended for synthetic work) and N-phenylsulphonyl- dimethyl sulph~xime~~ (12) are other new methods of generating phenyl radicals. In the latter case the phenyl radicals so-formed are apparently more electrophilic that those produced thermally from benzoyl peroxide values for FK of 1.95 and 1.35 respectively being obtained on competitive phenylation of mixtures of toluene and benzene by these methods.Although methylbenzenediazo sulphone on heating in non-polar solvents decomposed mainly by the expected homolytic route to phenyl radicals and sulphur dioxide3* (cJ ref. 31) the corres- ponding benzyl sulphone (13) did not.32 Instead it gave mainly benzaldehyde phenyl hydrazone almost certainly by a radical-chain process which is not yet fully understood and clearly requires further study. However the alternative 23 G. A. Ward and J. C. W. Chien Chem. Phys. Letters 1970 6 245. 24 J. W. Rakshys Chem. Comm. 1970 578. 25 M.Kobayashi H. Minato N. Kobori and E. Yamada Bull. Chem. SOC.Japan 1970 43 1131. 26 M. J. Perkins Ann. Reports (B),1969 66 169; ibid. 1968 65 181 ; C. Ruchardt and C. C. Tan Chem. Ber. 1970,103 1774. 27 L. A. Neiman V. S. Smolyakov Yu. S. Nekrasov and M. M. Shemyakin Tetrahedron 1970 26 4963. 28 E. C. Taylor F. Kienzle and A. McKillop J. Amer. Chem. SOC.,1970 92 6088. 2q R. A. Abramovitch and T. Takaya Chem. Comm.. 1969 1369. 30 J. L. Kice and R. S. Gabrielsen J. Org. Chem. 1970 35 1004. M. Kobayashi H. Minato and N. Kobori Bull. Chem. SOC.Japan 1970 43 219. 32 J. L. Kice and R. S. Gabrielsen J. Org. Chem. 1970 35 1010. Free-radical Reactions 181 schemes [equations (4) and (5)] postulated to account for the product are in- teresting conjectures.Kinetic measurements on a number of N-aroyl-N'-aryldi-imides has confirmed that production of aryl radicals by alcoholysis of PhCH,SO,NNPh + PhCH2-+ SO2 + PhCH,N=NPh PhCHy I lt CHzPh PhCH=NNHPh (4) PhCH,SO,N=NPh Tu3) PhCHSO,N=NPh + PhCHNNPh -+PhCHN=NPh (5) 'S6 this source is initially a heterolytic process yielding an aryl di-imide which after oxidation homolytically fragments to aryl radicals [equation (6)].33 ArN=NCOAr' + ROH + Ar'COOR + ArN=NH + Arm + N (6) Similarity in the rates of production of arene (via Ar.) and aromatic ether (via Ar') in the well-known but not well-understood reaction of diazonium salts with methanol has led to the proposal that a common intermediate participates in both routes (an ill-defined activated ArN species).34 Similar propagation + steps have been suggested35 for formation of arene both in the above reaction and in the thermal decomposition of N-nitrosoacetanilide in ether [MeCHOEt replaces CH,OH in (7)].In the latter case the participation of the propagating .CH20H + ArN2+ + Ar-+ N + +CH20H Ar.+ CH,OH -P ArH +.CH,OH (7) radical MecHOEt was confirmed indirectly by e.s.r. measurements. Reduction of diazonium salts to arenes may also be effected by stannyl or silyl hydride~.~~ The relatively low ratio of 2- to 3-phenylthiophen (72 :38) obtained on pheny- lating thiophen with phenylazotriphenylmethane compared with those (ca. 93 :7) obtained using other sources of phenyl radicals has been shown37 to be due to interception of the precursor (14; X = S) of 2-phenylthiophen by the triphenylmethyl radical thus producing a pair of stereoisomeric dihydrothio- phenes (15; X = S).The corresponding dihydrofurans (15; X = 0) were obtained from furan and phenylazotriphenylmethane. Furan which is homo- lytically arylated exclusively in the 2-position is 11.5 times more reactive than ph>o + ph,C. -jPhaPh HX HXH (14) (15) 33 T. Carty and J. M. Nicholson Tetrahedron Letters 1970 4155. 34 T. J. Broxton J. F. Bunnett and C. H. Paik Chem. Comm. 1970 1363. 35 J. I. G. Cadogan R. M. Paton and C. Thomson Chem. Comm. 1970 229; D. F. DeTar and T. Kosuge J. Amer. Chem. SOC.,1958 80 6072. '' J. Nakayama M. Yoshida and 0. Simamura Tetrahedron 1970 26 4609. 37 C. M. Camaggi R. Leardini M. Tiecco and A.Tundo J. Chem. SOC.(B) 1969 1251; ibid. 1970 1683. 182 A. R. Forrester benzene and 4.4 times more reactive than thiophen to phenyl radicals.38 By comparison the relative reactivities of benzene thiophen and pyridine to phenyl radicals generated from nitrobenzene at 600 "Care in the ratio 1:5 :2.3 respec- tively. Considerable differences in the behaviour of 2- 3- and 4-thiazolyl radicals towards mono-substituted benzenes have been noted. Surprisingly the 4thiazolyl radical seems to be more electrophilic than its 2-isomer which like the 2-thienyl radical is more electrophilic than ~henyl.~' Relative reactivities towards phenyl radicals of the ring positions of benzo-thia~oles,~' dimethylpyridines$2 na~hthalene,~~?~~ 2-rnethyl-na~hthalene:~ polyfluoroarene~,~~ have been evaluated.For the last-men- and a~obenzene~~ tioned the activating effect ofthe phenylazo-group is comparable with that of the nitro-group. Related studies of homolytic aroyloxylation of a number of mono-substituted benzenes have confirmed that para-substituents affect the polarity of aryloxyl radicals to a greater extent than they do aryl radicals.47 Benzoyloxy- lation of a series of mono-substituted arenes using benzoyl peroxide-iodine has been shown to be a free-radical process although isomer distributions do infer that the attacking species is larger than a simple benzoyloxyl radical.48 A considerable amount of information has accrued recently on polyfluoro- phenyl radical^.^^.^' These radicals are clearly more electrophilic than phenyl but like phenyl give good yields of biaryl when generated in benzene.However reaction of pentafluorophenyl with hexafluorobenzene is complex and is un- satisfactory for the preparation of decafluorobiphenyl. Doubt has now been cast on the claim that o-iodophenyl radicals yield benzyne by spontaneous elimination of atomic iodine by the discovery that thermal decomposition of p-iodophenylazotriphenylmethane gives products derived exclusively from triphenylmethyl and o-iodophenyl radicals. ' This result lends support to the argument that production of arynes by photolysis of o-di-iodoarenes entails concerted loss of two iodine atoms.50 38 L. Benati N. La Barba M. Tiecco and A. Tundo J. Chem. SOC. (B) 1969 1253; L. Benati M.Tiecco A. Tundo and F. Taddei ibid. 1970 1443. 39 E. K. Fields and S. Meyerson J. Org. Chem. 1970 35 62 67. 40 A. L. Lee D. Mackay and E. L. Manery Canad. J. Chem. 1970,48,3554; G. Vernin R. Jauffred H. J.-M. Dou and J. Metzger J. Chem. SOC. (B) 1970 1678. 41 G. Vernin H. J.-M. Dou G. Loridan and J. Metzger Bull. SOC. chim. France 1970 2705. 42 J. M. Bonnier J. Court and M. Gelus Bull. SOC. chim. France 1970. 139; J. M. Bonnier and J. Court ibid. 1970 142. 43 N. Kobori M. Kobayashi and H. Minato Bull. Chem. SOC.Japan 1970 43 223. 44 J. M. Bonnier and J. Rinaudo Bull. Sac. chim. France 1970 146. " P. H. Oldham G. H. Williams and B. A. Wilson J. Chem. SOC.(B) 1970 1346. 46 J. Miller D. B. Paul L. Y.Wong and A. G. Kelso J. Cherii. Soc. (B) 1970 62.47 M. Kurz and M. M. Pellegrini J. Org. Chem. 1970 35 990. 48 P. Kovacic C. G. Reid and M. J. Brittain J. Org. Chem.. 1970 35 2152. 49 P. H. Oldham and G. H. Williams J. Chem. SOC.(C) 1970 1260; J. Burdon J. G. Campbell and J. C. Tatlow ibid. 1969 822. so J. M. Birchall R. N. Haszeldine and J. G. Speight J. Chem. SOC.(C) 1970 2187; J. P. N. Brewer I. F. Eckhard H. Heaney M. G. Johnson B. A. Marples and T. J. Ward ibid. 1970 2569. " G. W. Clark and J. A. Kampmeier Chem. Comm. 1970 996. Free-radical React ions The c~nclusions~~ of a study of the products of thermal decomposition of a number of diacyl peroxides labelled with l80and/or optically active in the main endorsed earlier views that ester formation proceeded by both heterolytic (involving carboxy inversion) and homolytic (radical-pair coupling) paths simultaneously the extent to which each contributed depending inter alia on the structure of the peroxide.However some of the limits expressed in this and earlier work may have to be amended in the light of the assertion by Goldstein and Judson53 that [3,3]-and/or [1,3]-sigmatropic shifts (Scheme 1)play an important r61e in the oxygen scrambling of peroxidic substances and hence that such scrambling should not be taken as a measure of the extent of radical-pair re- combination. An explanation different from that expressed above for the mech- anism of decomposition of diacyl peroxides has been conceived by Walling and his co-~orkers.~~ They prefer a scheme in which all products arise via the same transition state which passes to a transient ‘intimate radical-ion pair’ (16); this in turn may yield either polar or radical products depending on such factors as the structures of the peroxide and solvent.Other phenomena such as anchimetrically-assisted and molecule-induced homolysis were explained R I o/CQ-o I o\c4* I R R I R I o/c,,-;-.0 I .\c,O I R Scheme 1 52 T. Kashiwaga S. Kozuka and S. Oae Tetrahedron 1970 26 3619. ” M. J. Goldstein and H. A. Judson J. Amer. Chem. Soc. 1970,92 41 19 4120. 54 C. Walling H. P. Waits J. Milovanovic and C. G. Pappiaonnou J. Amer. Chem. SOC.,1970 92 4927. A. R. Forrester using the same model. The general lack of agreement among workers in this area is further exemplified by Leffler and Zepp's'' rationalisation of the forma- tion of 4-naphthyloxy-l-naphthoicacid (18 ; Ar = l-naphthyl) on thermal decomposition of bis(1-naphthoy1)peroxide.To them this product is the result of cage recombination of a geminate radical pair with rotation of one of the partner radicals yielding (17 ; Ar = l-naphthyl) and hence the acid (18 ; Ar = l-naphthyl). Other routes to (18 ;Ar = l-naphthyl) which did not involve H ArCOO Arcoo (1 7) (18) caged radicals were considered to be no more likely than that given above. In contrast with the above discord there appears to be general agreement56*57 that the products obtained by photolysis of diacyl peroxides are mainly derived from radicals reacting inside or outside the solvent cage.Thus by assuming for the hexenyl radicals (19) produced by photolysis of the corresponding peroxide that the rates of coupling in and diffusion from a solvent cage of pentane were fast relative to that of intramolecular cyclisation to (20) (k = lo' s-l at 25 "C),an estimate (35 %) of the radicals which coupled within the solvent cage was made by measuring the relative yields of dimers with and without a pentane ring.57 Complementary e.s.r. studies revealed that at -90 "Conly the spectrum of the hexenyl radicals was observed and above -35 "C only that of the cyclo- pentylmethyl radicals. From these observations it was concluded that only those radicals which escaped from the solvent cage were observed by e.s.r.Alkyl radicals generated by photolysis of peresters in cyclo-propane at < -40 "C 55 J. E. Leffler and R. G. Zepp J. Amer. Chem. SOC., 1970 92 3713. 56 T. Kashiwaga K. Fujimori S. Kozuka and S. Oae Tetrahedron 1970 26 3639. ST R. A. Sheldon and J. K. Kochi J. Amer. Chem. SOC., 1970,92,4395. Free-radical Reactions 185 were also detectable but the geminate alkoxyl radicals whose ability to abstract hydrogen atoms makes their lifetime in bulk solution extremely short were not.58 By comparing the amounts of ether and alkene produced in these photolyses a measure of the ratio kdisproportionation for a caged pair of alkyl-alkoxyl :krearrangement radicals was obtained. These values were about twice as large as the correspond- ing ones for alkyl-alkyl radical pairs.Photolysis of t-butyl percyclopropylacetate was a particularly interesting case giving six times as much of the ether (21) as (22). Assuming that krearrangement was lo8s-' the rate of combination of the Wf-O-OBu' 0 + ">/\OBu+ OBu+ radical-pair (Bu'O. .-a)was estimated to be approximately lo9 s-' (this figure is in fact kcombinationx [Bu'O.]). Ring opening" of the analogous radicals (23) can occur either with C-0 fission to give alcohols or with C-C fission to give vinyl ethers via the radicals (24) and (25) respectively depending on the nature of the substituents R' and R2. A novel series of acyl carbamoyl peroxides which thermally decompose in three ways (Scheme 2) has been prepared; one of these provides a new route to aryl nitrenes.60 00 ArN + CO + PhC0,H II II ArNHCO-OCPh + ArNH.+ C02 + PhCOO. ArNH+ + CO + PhC02-Scheme 2 58 R. A. Sheldon and J. K. Kochi J. Amer. Chem. SOC.,1970,92 5175. 59 E. L. Stogryn and M.A. Gianni Tetrahedron Letters 1970 3025. 6o R. Okazaki and 0.Simamura Chem. Comm. 1969 1308. 186 A. R.Forrester Unlike di-t-butyl peroxide di-t-amyl peroxide when boiled in the absence of solvent gave very little of the expected oxiranes (26; R' = Et R2 = H and R' = R2 = Me). The principal product was acetone formed by fragmentation of the t-amyloxyl radical (note that elimination of ethyl is preferred to elimination of methyl).61 The obvious cyclic mechanism [equation (S)] to account for the considerable quantity of hydrogen together with products from sec-BuO- Me MeCH2C-0.+ Me,CO + Et-I :v:e I Me (26) on thermolysis of di-sec-butyl peroxide does not seem to explain fully all the available facts such as the failure of the peroxide to generate hydrogen on photolysis or on thermolysis in the gas phase.62 A large number of hydro-peroxides and amino-peroxides e.g. (27; R = H and But) and (28) have been prepared by treatment of mixtures of carbonyl compounds and hydrogen peroxide or t-butyl hydroperoxide with ammonia and their thermal and base catalysed decompositions have been extensively studied.63 Interestingly thermolysis of the peroxide (28) gave among other products caprolactam (29) L J possibly as indicated. An unusual product formed by heating or oxidising t- butyl hydroperoxide in the presence of tetracyclone has been ascribed64 the ring-expanded lactone structure (31).This Baeyer-Villiger type of oxidation 61 E. S. Huyser and K. J. Jankauskas J. Org. Chem. 1970,35 3196. 62 R. Hiatt and S. Szilagyl Canad. J. Chem. 1970 48 615. 63 E. G. E. Hawkins J. Chem. SOC.(C),1969 2663 2671 2678 2682 2686 2691. 64 N. L. Weinberg Canad. J. Chem. 1970 48 1533. Free-radical Reactions 187 product was considered to arise via the initial adduct (30) by the sequence of reactions shown. Bu'O 0 0 (30) Ph 1 c- 0 Bu' (31) The first in a series of three volumes which deals in depth with the chemistry of peroxidic substances is now available.6s Thermal decomposition of optically active azo-compounds has been further studied and results obtained last year66 which showed that randomisation of orientation of the radical pairs competed effectively with radical recombination within the cage have been c~nfirmed.~~ Size and shape of the participating radi- cals however would seem to be all important factors in such competing processes.68 The observation6' of a CIDNP emission signal during thermal decomposition of triarylpentazadienes (32) has been attributed to the inter- mediate production of an arylazo-triazenyl radical pair (33) which recombines before the arylazo-radical can lose nitrogen.This result has bearing on previous Ar N=N-N -N=NAr3 -Ar'N=N-NArZ ArZN=N-N-N=NAr3 I I Ar2 -Ar3N=N. Ar' 1 arguments7' in favour of cage recombination of arylazo-radicals formed by thermolysis of unsymmetrical azo-compounds which decompose with initial homolysis of only one bond e.g.p-nitrophenylazotriphenylmethane.The dependence of the rate constants for decomposition of free-radical initi- ators on the viscosity of the medium has been used as a criterion for classifying initiators into 'one bond' (those which decompose by initial cleavage of only one '' 'Organic Peroxides' Vol. 1 ed. D. Swern Wiley New York 1970. " e.g. K. R. Kopecky and T. Gillan Canad. J. Chem. 1969 47 2371. 67 F. D. Greene M. A. Berwick and J. C. Stowell J. Amer. Chem. SOC.,1970 92 867. '8 C. G.Overberger and D. A. Labianca J. Org. Chem. 1970 35 1762. 69 J. Hollaender and W. P. Neumann Angew. Chem. Internat.Edn. 1970,9 804. 70 W. A. Pryor and K. Smith J. Amer. Chem. SOC.,1967 89 1741. A. R. Forrester bond) and 'multi-bond' (e.g. symmetrical azo-compounds) ~ategories.~ For most initiators (but not for some peracetates) results obtained by this method agree with previously obtained data. The strong preference of cumyl radical pairs (and indeed for other aralkyl radicals in which there is substantial electron delocalisation) to couple rather than disproportionate apparently for no good reason has been uncovered again during an investigation of the decomposition of a-cumylazocyclohexane.72 Other work on azo-compounds has been con- cerned with their photosensitised decomposition7 and pressure dependence of the decomposition of di- t- bu t yl h yponi trite (But 0-N=N -0Bu').The ease of formation of the sulphate radical-ion by reduction of potassium persulphate with titanium(Ir1) in a flow system has allowed its reactions with a wide variety of organic substrates to be monitored by e.s.r. spectro~copy.~~ Using this technique it has been shown that SO,' will abstract hydrogen (from saturated compounds such as alcohols) or an electron [e.g. from acetate or nitr~alkane-anions~~ (cc ref. 77)] and will add to unsaturated (alkenes) and aromatic (arene di- and tri-carboxylic acids) compounds. In many respects the behaviour of this relatively electrophilic species is similar to that of the hydroxyl radical its rate of hydrolysis to which in aqueous solution increases with increasing pH. Determinati~n~~ of the products obtained on treatment of mainly phenyl- and phenoxy-acetic acids with the -OH and SO,' generating reagents [H202-Tirn and S2O8-Ti1I' respectively] supported the addition- fragmentation schemes [e.g.equation (911 formulated to account for the detection of benzyl and phenoxymethyl radicals in the parallel e.s.r.experiments. With the sulphate radical-anion however the formation of a o-bonded adduct such (34) as (34) is not mandatory electron transfer followed by fragmentation of the ensuing radical-cation being a feasible alternative. o-Phenylpheno~ymethy1~~ and o-phenylbenzamido8' radicals formed by thermolysis of persulphate in the presence of the corresponding acids and amides respectively cyclised efficiently onto the neighbouring ring to give benzopyrans and phenanthridones " W.A. Pryor and K. Smith J. Amer. Chem. SOC.,1970,92 5402. 72 R. C. Neuman and E. S. Alhadeff J. Org. Chem. 1970,92,3401. 73 P. S. Engel and P. D. Bartlett J. Amer. Chem. SOC.,1970 92 5883. " R. C. Neuman and R. J. Bussey J. Amer. Chem. SOC.,1970,92,2440. 75 R. 0.C. Norman P. M. Storey and P. R. West J. Chem. SOC.(B) 1970 1087. 76 D. J. Edge R.0. C. Norman and P. M. Storey J. Chem. SOC.(B) 1970 1096. '' A. H. Pagano and H. Shechter J. Org. Chem. 1970,35,295. 78 R. 0. C. Norman and P. M. Storey J. Chem. SOC.(B) 1970 1099. 79 P. S. Dewar A. R. Forrester and R. H. Thomson Chem. Comm. 1970 850. P. M. Brown P. S. Dewar A. R. Forrester A. S. Ingram and R. H. Thomson Chem. Comm. 1970 849. Free-radical Reactions 189 in good yield.Highly resolved e.s.r. spectra of phenoxyalkyl radicals have been observed" by irradiating anisoles dissolved in di-t-butyl peroxide at -35 "C in the cavity of the spectrometer. The hydrogens of the methoxyl group of anisole are in fact extremely reactive towards t-butoxyl radicals being only slightly less so than those of toluene.82 Isopropyl phenyl ether which would be expected to be even more reactive than anisole in such reactions gave mainly phenol and isomeric isopropylphenyl isopropyl ethers (37) but significantly no dimeric or trimeric products derived from phenoxyl radicals.83 This observa- tion led to the suggestion that phenol and not phenoxyl was eliminated from the adduct (39 the resulting cumyl radical (36) abstracting hydrogen to give the product.Full corroborative evidence for such a sequence of events has still to be presented. Me (35) (36) (37) The key step in the oxidative decarboxylation of aliphatic acids with persul- phate in the presence of silver@) ions is the initial oxidation by the persulphate of silver(1) to silver(Ir) the latter being the effective oxidant of the acids.84 The alk yl radicals formed on decarboxylation suffer further oxidation to carbonium Ag' + S2082-+ Ag" + SO,; + SO4'-SO,; + Ag' * Ag" + SO,2-Ag" + RCOzH -P R. + COz + H+ + Ag' ions by SO4- S,082- or silver(@ and products derived from both radicals and ions are obtained. The synergistic effect of copper(@ ions on this reaction has been ascribed to the facility with which they oxidise alkyl radicals to carbonium ions.Related studies of the oxidative decarboxylation of acids using mangan- ese(II1) gave broadly similar results.85 However because of the low oxidation potential of the Cu"-Cu' system the ability of Cu" to oxidise alkyl radicals by a simple electron transfer process has been questioned. Evidence has been adduced favouring simultaneous reaction of copper(1r) with the unpaired electron and the 8-hydrogen atom of alkyl radicals to form alkenes.86 In addition oxidation of the cyclohexenylbutyl radical (38) to alkene by copper(I1) in non- polar solvents is considered to proceed without participation of the corresponding " A. Hudson and K. D. J. Root J. Chem. SOC.(B) 1970,656. H. Sakurai A. Hosomi and M.Kumada J. Org. Chem. 1970,35,993. 83 A. Ohno and N. Kito Bull. Chem. SOC.Japan 1970 1272. 84 J. M. Anderson and J. K. Kochi J. Amer. Chem. SOC.,1970,92 1651 ;J. Org. Chem. 1970 35 986. " J. M. Anderson and J. K. Kochi J. Amer. Chem. Soc. 1970,92 2450. 86 K. Torssell Arkiu Kemi 1970 31 401. A. R. Forrester carbonium ion (which would rapidly cyclise to 1,9- and 9,10-octalins) but rather by way of an organo-copper intermediate (39) which undergoes concerted decomposition as shown.87 In view of the above results the conclusion that ,‘cu -0 -c’ A~O Me (39) 2-phenylcyclopropane carboxylic acids are oxidatively decarboxylated by lead tetra-acetate in the presence of copper(1r) to carbonium-ions (via the corres- ponding radicals) which ring open and are then acetoxylated in a non-stereo- specific manner requires further consideration.88 Photolysis of lead(1v) carboxylates in carbon tetrachloride solution has been recommended for the transformation of carboxylic acids into chlorides with one less carbon atom.89 Thermolysis of iodine triacylates (formed in situ by ozonisation of acid anhydrides in the presence of iodine) with mercuric oxide gave carbon dioxide and esters in good yield.” This conversion which is a close relation of the Simonini reaction has been assigned the radical-chain mechanism given here in abbreviated form.(RCO2)3I -+ Re + (RC02)zI + I. R. + (RC02)3I + RCOOR + (RCO2)2I --+ (RCOZ),I R.+ COZ + RCO,I + Re+ C02 + I. R-+ 1. RI The structural requirements for intramolecular hydrogen abstraction by alkoxyl radicals have been probed by oxidising the medium-sized ring alcohols cyclodecanolgl and 4-and 5-phenylcyclo-octanols92with lead tetra-acetate.D. L. Struble A. L. J. Beckwith and G. E. Gream Tetrahedron Letters 1970 4795. 88 T. Shono I. Nisuiguchi and R. Oda Tetrahedron Letters 1970 373. 89 V. Franzen and R. Edens Annalen 1970 735 47. 90 G. B. Bachman G. F. Kite S. Tuccarbasu and G. M. Tullman J. Org. Chem. 1970 35,3167. 91 M. L. J. Mihailovic V. Andrejevic M. Jakovljevic D. Jeremid A. Stojiljkovic and R. E. Partch Chem. Comm. 1970 854. 92 A. C. Cope M. A. McKervey N. M. Weinshenker and R. B. Kinnel J. Org. Chem. 1970 35 2918. Free-radical Reactions With cyclodecanol little of the expected 1,4-bridged ether was obtained the main etherial products being the trans-epoxycyclodecane (42) and diasterio- isomeric 7-oxabicyclononanes (44).Experiments with deuteriated cyclodecanols supported a scheme for formation of the former in which a second 1,5-hydrogen (or hydride) shift (40)+(41) precedes oxidation and ring closure. For the latter product p-scission followed by a 1,5-hydrogen shift and recombination seems a likely route to the precursor (43) of the bicyclononanes. The high yield of the ether (45) (72 %) formed from trans-5-phenylcyclo-octanolwith lead Ph B tetra-acetate is noteworthy since a seven-membered transition state is required in the hydrogen transfer step. Fisch et aLg3has a similar experience with the bicyclic alcohol (46) conversion into oxoadamantane (47) proceeding in -86 % yield.However neither of the adamantyloxyl radicals (48)and (49) (formed by thermolysis of their hypohalites) appears to possess the geometrical require- ments for intramolecular hydrogen transfer since both fragmented to give the halo-ketone as final product.94 Oxidation of alkylarenes and alkyl aryl ethers both electrochemically and with inorganic reagents continues to attract attention. With cobalt(111) acetate in acetic acid toluene gave principally benzyl acetate formation of which via the initially-formed radical-cation (SO) (detected by e.s.r.) is easily envisaged [equation In the presence of chloride ions which accelerate the overall rate of oxidation the course of reaction was partly diverted and nuclear halo- genated products were also formedg6 [equation (1 l)].By comparison man- 93 M. Fisch s. Smallcombe J. C. Gramain M. A. McKervey and J. E. Anderson J. Org. Chem. 1970 35 1886. 94 R. M. Black and G. B. Gill Chem. Comm. 1970 972. 95 P. J. Andrulis and M. J. S. Dewar J. Amer. Chem. SOC.,1966 88 5483. 96 E. I. Heiba K. M. Dessau and W. J. Koehl J. Amer. Chem. SOC.,1969 91 6830. A. R. Forrester ganese(m) acetate gave little side-chain acetoxylated product unless the toluene had an electron-donating para-substituent (MeO) to lower its ionisation poten- tia1.97 Although bromide ions accelerated the latter oxidations they did not X alter the course of the reaction. Formation of ring and side-chain acetoxylated and ring-chlorinated products on oxidation of dimethoxybenzenes with lead tetra-acetate in the presence of chloride ions can be explained in a similar way.98 In a study of the anodic oxidation of toluene an additional mechanistic possibility was proposed to account for the production of benzyl acetate." A radical generated by Kolbe electrolysis of the electrolyte may abstract a hydrogen from toluene yielding a benzyl radical which after anodic oxidation to a car- bonium ion is solvolysed.Attempted synthesis' O0 of o-tolualdehyde from o-xylene by oxidation with cerium(rv) in dilute nitric acid revealed the existence of two independent oxidation routes one of which gave the aldehyde via the alcohol and the other gave 2-methylbenzyl nitrate which resisted hydrolysis to the alcohol.Detailed mechanistic studies are required to resolve this apparent dichotomy. Close similarities in the modes by which chromate and permanganate oxidise 9CH to ?C-OH have been established by examining the kinetic and stereo- chemical aspects of the oxidation of optically active y-phenylvaleric acid to the lactone (50)with permanganate in alkaline solution. lo' The essential features 0 II Me ,0-C I P~I~~\CH,-CH2 97 J. R. Gilmore and J. M. Mellor Chem. Comm. 1970 507; E. I. Heiba K. M. Dessau and W. J. Koehl J. Amer. Chem. SOC.,1969 91 138. 98 R.0.C. Norman and C. B. Thomas J. Chem. SOC.(B) 1970,421. 99 S. D. Ross M. Finkelstein and R.C. Petersen J. Org. Chem. 1970 35 781. loo L. A. Dust and E. W.Gill J. Chem. SOC.(0,1970 1630. lo' J. I. Brauman and A. J. Pandell J. Amer. Chem. SOC.,1970,92 329. Free-radical Reactions of the mechanism are slow abstraction of the tertiary hydrogen by permanganate with formation of a caged radical pair which combines before the stereochemical integrity of the carbon radical is completely lost. Hydrolysis of the manganese(v) ester so-formed produces the hydroxy-acid and hence the lactone. 9C-H + Mn0,-+ [>C.MnO,H-] -+ SC-OMnO,H-1 >C-OH + MnV The efficiency with which alkyl halides are reduced to alkanes by a chromium(u)-ethylene diamine reagent in aqueous dimethylformamide has been attributed to the intermediate production of an alkyl chromium(1Ir)- ethylene diamine species [equations (12) and (1311 which rapidly undergoes acid-catalysed hydrolysis giving alkane.'02 Since reduction of a-bromoethyl benzene with chromous sulphate in aqueous dimethylformamide gave mainly RX + CrIlenz+ *R.+ CrIIIen,Xz+ (12) R.+ Cr"en,'+ -+ RCr"'en,'+ (13) equal quantities and meso-and dl-2,3-diphenylb~tane,'~~ and since the second step [equation (13)] is thought to be rapid when R = alkyl (estimated k = 4x lo71 mol-s-') it would seem that the complexing agent and/or the structure of the alkyl group are important factors in determining the course of the above reduction.In a selective review such as this it is inevitable although unfortunate that several areas of free radical chemistry in which there is current activity e.g. addition to alkenes and halogenation have been largely ignored.It is to be hoped that subsequent reports will strive to restore the balance. lo' J. K. Kochi and J. W. Powers J. Amer. Chem. SOC.,1970 92 137. '03 W. G. Brown and D. E. McClure J. Org. Chem. 1970,35 2036.