4 Reaction Mechanisms Part (iii) Free-Radical Reactions By A. G. DAVIES Department of Chemistry Urliversity College London 20 Gordon Street London WClH OAJ 1 General A useful introductory text on organic free radicals has been published.’ Other general reviews include articles on the mechanisms of free-radical reactions,2 anchimerically assisted bond homoly~is,~ vibrational spectroscopy of free radical^,^ fluorine-containing free radicals,’ directive effects in gas-phase radical addition reactions homolytic aromatic ipso-substitution reactions,’ and the capto-dative stabilization of radicals carrying both an electron-acceptor and an electron-donor group.’ An interesting development in technique is the application of e.s.r. spectroscopy to the direct observation of the radicals that are formed during the pyrolysis of hydrocarbons.’ The compounds in benzene solution are passed slowly through the cavity at temperatures up to 566 “C and pressures up to 140 kg cm-2 usually well above their critical temperatures but the high density of the fluid quenches the angular momentum of molecular rotation and sharp well-resolved spectra are observed.As yet the technique has been applied mainly to radicals of the benzyl type. 2 Carbon-centred Radicals Electron spin resonance spectroscopy continues to be the technique that is used most widely to investigate the structures of free radicals. In an sp2-hybridized alkyl radical (1)the a-carbon atom is in the nodal plane of the p-orbital containing the unpaired electron and hence the hyperfine coupling to this carbon is low (about 40 G).On the other hand an sp3-hybridized radical (2) has a finite s-electron density at the carbon nucleus and the hyperfine coupling is large. The temperature dependence of 4°C) can further be used to determine the out-of-plane bending frequency of the radical. D. C. Nonhebel J. M. Tedder and J. C. Walton ‘Radicals’ Cambridge University Press 1979. C. Ruchardt Uspekhi Khim. 1978,47,2014. M. T.Reetz Angew. Chem. Internat. Edn. 1978 17 594. R. E.Hester Adv. Infrared Raman Spectrosc. 1978,4,1. ‘Fluorine-containing Free Radicals’ ed. J. W. Root A.C.S. Symposium Series 1978,Vol. 66. J. M. Tedder and J. C. Walton Adv. Phys. Org. Chem. 1978,16,51. J. C. Traynham Chem.Rev. 1979,79 323. H.G.Viehe R. Merknyi L. Stella and Z. Janousek Angew. Chem. Internat. Edn. 1979,18,917. R.Livingston H.Zeldes. and M. S. Conradi J. Amer. Chem. SOC.,1979,101,4312. A. G.Davies In the ethyl" and isopropyl radicals," the value of a("C,) increases monotoni- cally with temperature. These radicals have a planar structure at the minimum of the potential-energy curve and the out-of-plane bending frequencies which are derived (541 and 380 cm-' respectively) are in good agreement with the values obtained from i.r. spectroscopy.'* In contrast the t-butyl radical [a("C,) ca. 45 GI appears to be most stable when it is bent by 11.5" from planarity. The trineopentylmethyl radical13 [a(13Ca) ca. 40.5 GI shows the lowest value of a ("CC,) of any t-alkyl radical and is presumably forced nearer to planarity by steric repulsion between the neopentyl groups Below -85 "C the spectrum shows that the hydrogen atoms on the P-carbon atoms become non-equivalent as the neopentyl groups are frozen into specific conformations.Steric repulsion between pairs of these radicals also confers on them a high persistence in solution in benzene at 20 "C their half-life is 11minutes. Steric effects are also the basis of the persistence of (RS),C' radi~a1s.l~ Hexakis(trifluoromethylthiy1)ethanedissociates at room temperat~re.'~ The C-C bond length is 1.7 nm (compared with 1.54 nm in ethane) and the bond dissociation energy is only 13.7 kJ mol-l. The value of a(13C,) in the radical (3)is 40.09 G confirming that the radical is approximately planar [cf.(l)]; in contrast the radical (MeO),C' has a("C) = 154 G,showing that it is pyramidal [cf.(2)]. (CF3S)3C-C(SCF3)3 $ 2(CF3S)3C. (31 The value of a(H,) in an alkyl group can similarly give evidence regarding the conformation about the C -Cp bond. Hyperconjugative coupling will be largest when the P-C-H bond eclipses the semi-occupied p-orbital i.e. (4) and least when it is in the nodal plane as shown in (5). H Br (4) (5) (6) By this type of argument the P-bromoalkyl radical Me2CCH2Br about which there has been some controversy has been shown to have the preferred con-formation (6).16 Other spectra that can be observed when isobutyl bromide in an adamantane matrix is subjected to X-radiolysis apparently result from the inter- action of the radicals Me2CHCH2' and Me3C' with Br-.16*17 lo D.Griller P. R. Marriott and K. F. Preston J. Chem. Phys. 1979 71 3703. l1 D. Griller and K. F. Preston J. Amer. Chem. SOC.,1979 101 1875. l2 J. Pacansky D. E. Home G. P. Gardini and J. Bargon J. Phys. Chem. 1977,81,2149. l3 K. Schluetter and A. Berndt Tetrahedron Letters 1979 929. l4 R. Schecker U. Henkel and D. Seeback Chem. Ber. 1977,110,2880. A. Haas K. Schlosser and S. Steenken J. Amer. Chem. SOC., 1979,101,6282. l6 M. C. R. Symons and I. G. Smith J.C.S. Perkin ZZ,1979 1362. l7 R. V. Lloyd D. E. Wood andM. T. Rogers J. Amer. Chem. SOC., 1974.96.7130. Reaction Mechanisms -Part (iii) Free-Radical Reactions The first well-defined e.s.r. spectrum of an alkane radical cation (7) has been reported from the y-irradiation of 2,2,3,3,-tetramethylbutanein the presence of a carbon tetrahalide as an electron scavenger at 77 K.18 It is suggested that the two Me3C groups which are held together by a one-electron bond are approximately planar (7); six C-H bonds lie parallel with the singly occupied 0-orbital and by hyperconjugation show a large hyperfine coupling constant of 32 G and the other twelve hydrogen atoms show a=4.5 G.In the absence of the carbon tetrahalide electron return gives the alkane in an excited state and fragmentation occurs. The first long-lived mono-olefin radical cation (8)has also been ob~erved,'~ from the electrolytic oxidation of adamantylideneadamantane in an e.s.r. cell. It is thought to owe its persistence to the fact that the p-carbon atoms are all located at bridgeheads from which loss of a proton is forbidden by Bredt's rule.E.s.r. spectroscopy has also been used to show the existence of two isomeric forms of the pentadienyl radical that are not interconvertible on the time-scale of the technique. Photolysis of di-t-butyl peroxide in the presence of penta- 1,4-diene above -60 "C shows the presence of only one species which has been identified as the (Em-isomer (9) but at -120 "C the predominant radical that is formed is the (EZ)-isomer (1o).",~~ -.. Bu'O. .<. and/or &./,.-..--.- (9) (10) One of the more satisfying aspects of Huckel MO theory is its success in interpreting quantitatively the e.s.r. spectra that are observed when a substituent interacts with the 7-electron system of an annulene radical.It is inevitable that most studies should have been carried out on the benzene radical anions (C6H5R7) but the cyclopentadienyl radicals (C5H4K) would have certain advantages because the 7-electron system is concentrated over fewer nuclei (so that the observed effects are often larger) and because the radicals are uncharged and the unpaired electron resides in a bonding rather than an antibonding orbital. There has previously been no satisfactory route to the cyclopentadienyl radicals because the substituted cyclopentadienes C5H5R react with alkoxyl radicals by addition rather than hydrogen abstraction (except when R is a trialkylsilyl group)." M. C. R. Symons and I.G.Smith J. Chem. Res. (S),1979,382; J. T. Wang and F. Williams Chem. Phys. Letters 1979,67 202; M. C. R. Symons ibid. 1980,69 198. l9 S. F. Nelson and C. R. Kessel J. Amer. Chem. SOC.,1979 101 2503. 2o D. Griller K. U. Ingold and J. C. Walton J. Amer. Chem. Soc. 1979,101 758. R. Sustman and H. Schmidt Chem. Ber. 1979,112 1440. 22 M. Kira M. Watanabe and H. Sakurai J. Amer. Chem. Soc. 1977,99,7780. A. G. Davies The general routes to the radicals C5H4R'shown in Scheme 1 have now been developed and the e.s.r. spectra can be analysed to identify the substituent effects of the groups R.23s24 R R r R-I (R =Hor D) (R = H or alkyl) (R= H alkyl or R3Si) Scheme 1 The forms of the degenerate symmetrical ($s) and antisymmetrical (t+hA) wave-functions for the cyclopentadienyl radical are shown in (11) and (12).These are equally populated by the unpaired electron in the CsH5*radical resulting in an equal spin density at each carbon centre and an e.s.r. spectrum which consists of a binomial sextet; a(5H) =5.98 G. If the ring carries an electron-releasing substituent at position 1 the MO will be destabilized because of the high electron density at C-1 but $A will be unaffected because it has a node passing through this position. Two electrons will therefore enter the t+hA orbital and the unpaired electron will occupy the t,bS orbital. The value of a(Hj)will be given by c; (5 x 5.98) G where cjis the Huckel coefficient at position j; this predicts that u(H2,5)= 1.1G and a(H3,4)=7.8 G. Conversely an electron-attracting substituent should stabilize CLs which will be doubly occupied leaving the unpaired electron in JIA so that a(H2,5) = 10.8 G and a (H3,4)=4.1 G.When R is Me Et Pr' or But the e.s.r. spectrum that is observed shows that the unpaired electron is wholly in the $s orbital; for example for C5H4Me*radical a(H2,5) = 1.10 G and a(H3,4) =7.73 G at 24 "C. This is compatible with the accepted behaviour of alkyl groups acting as electron-releasing substituents. On the other hand the spectrum of C5H,.,SiMe3* shows that the Me3Si group weakly attracts electrons leaving the unpaired electron principally (but not wholly) in the t+hA orbital. 23 A. G. Davies and M.-W. Tse J.C.S. Chem. Comm. 1978 353; P. J. Barker A. G. Davies and J. D. Fisher ibid.1979 587. 24 M. Kira. M. Watanabe and H. Sakurai Chem. Letters 1979 973. Reaction Mechanisms -Part (iii) Free-Radical Reactions Penta-alkyldisilyl substituents R3SiR2Si however act as weakly electron-releasing groups in the cyclopentadienyl radical whereas in the benzene radical anion they act as electron-attracting substituents. The spectrum of the C5H4D* radical shows that the +A MO is higher in energy than $rS. This is ascribed not to a Coulombic effect but to the difference in the vibrational frequencies of the C-H and C-D bonds acting upon the resonance integral of the t,hs M0,24,25or upon the various Jahn-Teller-distorted isomers of the The above correlations between structure stability and e.s.r. spectra all relate to carbon-centred 7-radicals (1).Relatively little is known about such relationships for a-radicals partly because the e.s.r.spectra of two of the most important classes of these radicals namely the vinyl and aryl radicals are difficult to observe in fluid solution and the spectra of most of the small number of acyl a-radicals (13)that have been observed in the past have shown broad lines with no resolvable str~cture.~' The intramolecular interactions and the conformations which they impose are similar in the acyl radicals and their parent aldehydes. Further the n.m.r. couplings 3J(CH-CHO) in the aldehydes (14) and the e.s.r. couplings a(HB) in the acyl radicals (15) me similar functions of the dihedral angle 8 being largest when 8 = 0" [p-C-H bond trans to the (+-C(0)-H bond or singly occupied a-orbital] and least when 8 is 90-270".When R is a small alkyl group (Cl-C5) the most stable conformation of the aldehydes RCH2CH0 and of the acyl radicals RCH2C0 is that in which the R-C bond eclipses the carbonyl group [8 in (14) and (15) is 120 or 240'1. The n.m.r. coupling constant 3J(CH-CHO) is small (<1.8 Hz) and the e.s.r. coupling is unresolvable accounting for the broad signals that have been observed previously. If R is larger than C5 8 is reduced probably because of steric hindrance between R and the carbonyl group; the n.m.r. coupling increases (to ca. 2 Hz) and the e.s.r. coupling becomes resolvable (2-3 G). In the extreme di-t-butylacetaldehyde which is most stable in the conformation (16) in which the P-C-H bond is trans to the aldehydic C-H bond shows a large n.m.r.coupling 3J(CH-CHO) = 6 Hz and the cor- responding acyl radical with a similar conformation (8 = 0) shows a(H,) =ca. 11G.28 In 2,3-unsaturated aldehydes interaction between the 7-system of the carbonyl group and that of the double bond stabilizes the molecule in the s-trans con-formation and 3J(CH-CHO) = 6 Hz. The corresponding unsaturated acyl radicals adopt the same conformation (17) in which 8 = 60"and a(H,)= 19 G. H H 25 P. J. Barker and A. G. Davies J.C.S. Chem. Comm. 1979,815. 26 T. Clark J. Chandrasekhar and P. von R. Schleyer J.C.S. Chem. Comm. 1980,266. '' H. Paul and H. Fischer Helv. Chim. Actu 1973 56 1575. 2a A. G. Davies and R. Sutcliffe,J.C.S. Chem. Comm. 1979,473; J.C.S.Perkin ZZ 1980 819. A. G.Dauie9 Similarly the interaction of the r-system of the carbonyl group with the Walsh .rr-orbitals of the cyclopropyl ring restricts rotation about the C,-CB bond in cyclopropylcarbaldehyde but now the two conformations have approximately equal stability. Below -9O"C the aldehyde reacts with t-butoxyl radicals to show the spectra of the two radicals (18) in which 8 = 0" and a(H,) = 18.2 G and (19),where 8= 180",and a (HB)= 0.5 G. Above about -90 "C rotation about the C -Cp bond is rapid on the e.s.r. time-scale and a single time-averaged spectrum is observed; a(H,) = ca. 9 G. Simulation of the spectra over the region of intermediate exchange rates gives a barrier to rotation of 17.5 kJmol-' close to the value of 18.4* 1.7 kJ mol-' that has been obtained by microwave spectroscopy for the parent aldehyde." Similar considerations may be expected to apply to the a-imidoyl radicals (21).Previously their spectra had been observed only from the radicals obtained from the corresponding imines (20; R2=alkyl).30 A wider variety of imidoyl radicals (21a) H have now been prepared by the addition of radicals X*[e.g. ButO* Me3SiO* (Et0)2P0 Mesa and Me&] to alkyl isocyanides (R2= Me Bun But Me3Si et~.).~~ When X is Bu'O or Bu'S the imidoyl radicals undergo cleavage within X to give the radicals But- and OCNR' or SCNR' as appropriate but when X is MeS Et3Si or Ph the N-R2 bond breaks to give R2and XC=N. An unusual case of a radical existing in two distinct electronic configurations has been detected in the reaction of t-butoxyl radicals with the phosphorus(II1) iso- cyanate (22)32 (the proposed ?r and a-N configurations of the succinimidyl radical are a second example33).Two e.s.r. spectra are observed one being ascribed to the conventional trigonal-bipyramidal phosphoranyl radical (23) and the other to its isomerization product (24) where the electron occupies a a-orbital on the isocyanate ?? -0 NLV (23) 29 P. M. Blum A. G. Davies and R. Sutcliffe J.C.S. Chem. Comm. 1979 217. 30 W. C. Danen and C. T. West J. Amer. Chem. SOC.,1973,95,6872. 31 P. M. Blum and B. P. Roberts J.C.S. Perkin 11 1978 1313. 32 J. A. Baban and B. P. Roberts J.C.S. Chem. Comm. 1979 537. 33 See Ann. Reports (B), 1978,75,74. 79 Reaction Mechanisms -Part (iii) Free-Radical Reactions ligand.The large value of the I3Cahyperfine coupling (136.1 G) and the low g value (2.0009) are characteristic of acyl radicals. 3 Sulphur-centred Radicals The e.s.r. spectra of the RS*radicals and the RO-radicals (which were discussed in last year's cannot be detected unless a specific electronic interaction breaks the degeneracy of the doubly occupied and singly occupied orbitals on the heteroatoms. In the ROO-radicals this is accomplished by interaction with an electron pair on the P-oxygen atom. A similar interaction occurs in the RSS. radicals and the spectrum of Bu'SS' (g=2.025 AH, = 4 G) has now been detected from the photolysis of t-butylthiosulphenyl chloride Bu'SSCl. At -86 "C the rate constant for its second-order decay (2 x lo81mol-' s-') is about lo8times greater than that of the Bu'OO.The degeneracy can also be broken by nitrogen and a variety of dialkylaminothiyl radicals for example Et,NS. [a(N) = 10.7 G a(4H) = 6.1 G g = 2.01561 have recently been identified.36 On the other hand the persistent radicals that are observed when tetrasulphur tetranitride is photolysed in the presence of alkenes are not the sulphur-centred radicals (25) as was originally belie~ed,~' but are instead nitrogen-centred radicals [e.g. (26); a(N)= 13.11 G a(2H)= 3.49 G ~(2~~s) = 2.89 G g = 2.0064].38 Again the degeneracy can be broken by interaction with the r-electron system of an aromatic ring and the arylthiyl radicals ArS* have been detected when the products of the photolysis of diary1 disulphides were condensed on a cold finger at 77 K.39 Alkylthiyl radicals can react with alkenes either by addition to the double bond or by abstraction of allylic hydrogen.In the reaction between methylthiyl radicals and cyclopentene the balance between these two processes is strongly temperature- dependent. At -100 "C only the spectrum of the adduct (27) is observed; at -60 "C only the cyclopentenyl radical (28) is detect~d.~' Sylphides also fairly readily lose an electron to reagents such as HOS,~~ NO',42 Bu'OH,~~ or NH3t,44or electrochemically,43 to give the radical cations R2St.45These 34 See Ann. Reports (B) 1978 75 76. 35 J. E. Bennett and G. Brunton J.C.S. Chem. Comm. 1979,62.36 W. C. Danen and D. D. Newkirk J. Amer. Chem. SOC.,1976,98,516; B. Maillard and K. U. Ingold ibid. p. 520; J. A. Baban and B. P. Roberts J.C.S. Perkin 11,1978 678. 37 S. A. Fairhurst W. R. McIlwaine and L. H. Sutcliffe J. Mugn. Resonance 1979 35 121. 38 S. Rolfe D. Griller K. U. Ingold and L. H. Sutcliffe J. Org. Chem. 1979,44 3515. 39 W. Moerke A. Jezierski and H. Singer Z. Chem. 1979,19 147. 40 L. Lunazzi G. Placucci and L. Grossi J.C.S. Chem. Comm. 1979 533. 41 K.-D. Asmus D. Bannemann C.-H. Fischer and D. Weltwisch J. Amer. Chem. SOC.,1979,101,5322. 42 T. L. Wolford and P. B. Rousch J. Amer. Chem. Soc. 1978,100,6416. 43 W. B. Gara J. R. M. Giles and B. P. Roberts J.C.S. Perkin II 1979 1444. 44 B. C. Gilbert and P. R. Marriott J.C.S. Perkin 11,1979 1425.45 Reviewed by K.-D. Asrnus Accounts Chem. Res. 1979,12,436. A. G.Davies -100 MeS-+ 0 X60 OC 0+ MeSH (28) may be stabilized +sterically or electronically as the monomers; e.g. Bu',St (Me2N),S" and PhSMe. Otherwise they react (inter- or intra-molecularly) with a second sulphur centre giving the dimer cation radicals (R2SSR2)" with an equill- brium constant of 103-104 1 mol-' [e.g. Me,SSMe,'; a(12H) =6.3 GI. The unpaired electron is probably contained in a a*-orbital with the optical absorption due to a a+ a* transition (29). R R R 11; oso oso -0saPSa 'R I 'h R R (29) The sulphuranyl radicals 'SX3,e.g. CF3SSMe; and Me3SiOSMe; which occur as intermediates in SH2reactions at sulphur probably also have the unpaired electron confined in a orbital.^^'^^ A variety of sulphur-containing radicals are known in which the sulphur carries oxygen ligands put the unpaired electron appears to be centred mainly on sulphur (e.g.R2NS0 RSO, and RSO).47p48 The structures of the arenesulphonyl radicals ArS02 have been re-investigated. These are a-radicals with the unpaired electron in the plane of the benzene ring and (like the benzoyl radical) the hyperfine coupling constants are in the sequence a (H-rn)>a (H-o) a(H-p) and not o >p >rn as had previously been 4 Nitrogen-centred Radicals The detection of R2N' radicals by e.s.r. spectroscopy is not beset with the problems associated with orbital degeneracy that are encountered with RO' and RS' radicals and the variety of nitrogen-centred radicals which have been studied is increasing rapidly.The reactions often involve addition of a radical to a multiple bond to nitrogen. Thus aminyl radicals e.g. (30) have been prepared by the addition of alkyl radicals to imine~~~ and hydrazyl radicals e.g. (31) by addition to azodicarboxy- lates." Azo-compounds which carry an electronegative substituent X in the a-position similarly undergo addition by trialkylmetallic radicals (R3M) but this is 46 J. R. M. Giles and B. P. Roberts J.C.S. Chem. Comm. 1978,623. 47 C. Chatgilialoglu B. C. Gilbert C. M. Kirk and R. 0.C. Norman J.C.S. Perkin IZ 1979 1084; B. C. Gilbert and P. R. Marriott ibid.,p. 1425. 48 C. Chatgilialoglu B. C. Gilbert and R. 0.C. Norman J.C.S. Perkin 11,1979,770.49 B. P. Roberts and J. N. Winter J.C.S. Chem. Comm. 1978,960. 50 B. P. Roberts and J. N. Winter TetrahedronLetters 1979 3575. Reaction Mechanisms -Part (iii) Free-Radical Reactions But Me3CN=CH2 + R' + Me3CNCH2R EtO,CN=NCO,Et + But. + Et02CI!J-NC02Et (30) (31) followed by elimination of R3MX,then fl -scission of the resulting nitrogen-centred radical to give an azine (M = R3Si or R3Sn;X = C1 PhS or PhO) as shown in Scheme 2.5i R3M RM ClMe2CN=NCMe2CI 3C1Me2Ck-NCMe2CI .1 -CI. Me2C=NN=CMe2 t-Me2C=NNCMe2Cl Scheme 2 Photolysis of the azines gives the iminyl radicals (32),which can also be formed by the photolysis of di-t-butyl peroxide in the presence of primary or secondary alkyl azides. The iminyl radicals recombine to form the azines at diffusion-controlled rates.52 Bu'O' + H-CR2-NN2 + Bu'OH + N2 + R2C=N' + R2C=NN=CR2 (32) h" Iminyl radicals can also be prepared by addition to a nitrile.The 4-cyanobutyl radical (33)undergoes irreversible ring closure with a rate constant of 4.5 x lo2s-' at 259 K,52but photolysis of cyclobutanone azine (34) shows only the spectrum of the ring-opened radical. A reversible ring-closure of this type has been proposed to account for the 1,4-homolytic transfer of a cyanide group,53 as shown in Scheme 3 and an intermolecular addition provides the basis for the peroxide-initiated cyana- tion of hydrocarbons by methyl cyanoformate (Scheme 4).54 CSN No C-N I II 1 HOCCH,CH,kMe -Me -* HO&H,CH,CMe, I 1 Me Me "*OMe Me Scheme 3 C.Grugel and W. P.Neumann Annalen 1979,1675. B. P. Roberts and J. N. Winter J.C.S. Perkin ZZ 1979 1353. D. W. Watt J. Amer. Chem. SOC.,1976,98,271. D. D. Tanner and P. M. Rahimi J. Org. Chem. 1979,441674. A. G. Davies Bu'O' + RH + Bu'OH + R' R' + MeOCOCEN + MeOCOC-N' + MeOeO + RCrN R (>70°/o ) Scheme 4 Under the same conditions 2,4-dimethylpentane gave an azacyclopentene (35)by a sequence of reactions that includes the cycloaddition of an alkyl radical to the nitrogen of an imine. MeOCOC=NH J MeOCOC =N MeOCOc-NH (35) For use in chemical synthesis iminyl radicals can conveniently be generated by the oxidation of imino-oxyacetic acids with persulphate or thermolysis of the cor- responding t-butyl peroxy-esters e.g.as shown in Scheme 5." Ph2C=NOCH2C02H -CH20 ih2C=NO&12 +Ph2C=N' -+ (Ph2C=N)2 Ph2C=NOCH2C03But Y Scheme 5 If the structure of the arylimino moiety is varied these reactions can provide useful routes to a variety of compounds such as phenanthrolines quinolines pyridines pyrimidines and tetra lone^.^^ Examples are shown in Scheme 6. If hexabutylditin is photolysed in the presence of a 3-alkyl-3-bromodiazine the bromine is abstracted to give a new family of diazirinyl radicals (36).56Correlation of the results from 13Clabelling of the ring atom with INDO calculations shows that these are n-radicals in which a("C) has a negative sign and arises principally by spin polarization from nitrogen through the C-N bonds. The radical is therefore best represented by the valence-bond structures (37a) and (37b) with (38) making no significant contribution.-This structure is analogous to that of the aziridinyl (CR2CR2N) diaziridinyl -(CR2NRN) and oxaziridinyl (WN')<adicals but differs from that of the tri-t- butylcyclopropenyl radical (BU'C=CBU'CBU'') which has a 0-configuration. In contrast if a mixture of a bis(trimethylmeta1)mercurycompound (Me3M)zHg (M = Si or Ge) and a 3-alkyl-3-chlorodiazine is thermolysed or photolysed the R3M " A. R. Forrester M. Gill D. J. Meyer J. S. Sadd and R. H. Thompson J.C.S. Perkin I,1979,606; A. R. Forrester M. Gill J. S. Sadd and R. H. Thompson ibid. p. 612; A. R. Forrester M. Gill and R. H. Thompson ibid. pp. 616,621; A. R. Forrester M. Gill R.J. Napier and R. H. Thompson ibid.,p. 632; A. R. Forrester M. Gill C. J. Meyer and R. H. Thompson ibid. p. 637. 56 Y. Maeda and K. U. Ingold J. Amer. Chem. SOC.,1979,101,837. Reaction Mechanisms -Part (iii)Free-Radical Reactions /\ /\ C=NOCH $02H C=N* / Me/ Me (60%) Me NOCH,CO,H d)-0-m J 0 NH Scheme 6 radical is added at nitrogen to give a 2-metallated diaziridinyl radical (39).57When M = Si the two nitrogen atoms are non-equivalent but when M = Ge they are rendered equivalent presumably by a rapid 1,2-migration of Me3Ge. These di- aziridinyl radicals may then react further to give N-metallated 1,2,3,5-tetrazinyl radicals (40) which are in equilibrium with their corresponding dimers. Trialkylsilyl radicals similarly add to the nitrogen multiple-bond system of a variety of organic azides to give either 1,3- or 3,3-triazenyl radicals.52 The available evidence suggests that these are the 3,3-triazenyls (41) in which the unpaired electron is mainly associated with the central nitrogen in a 0-orbital that is in the N-N-N plane.R$i' + R2N3 + R:Si(R2)N-N=N' or R2N=N-NSiR (41) s7 C. Grugel and W. P.Neumann Annulen 1979,870. 84 A. G.Davies 5 Transition-MetalCompounds The ability of transition metals with univariant valence to act as good homolytic leaving groups gives rise to a variety of direct (R=R)or conjugated (R#R') bimolecular homolytic displacement processes at organic ligands on the metal. When these reactions are coupled into a chain process with a second step which regenerates the radical reagent they may be useful in organic synthesis.x' + R-M~ -+ XR'+ 'M~-' Stage (1) X-Y + OMN-' -+ X'+ Y-MN Stage (2) x-Y + R-M~-+ XR'+ Y-M~ Overall result There is some precedent for these processes in the chemistry of the Main-Group metals such as tin. Most of the work on transition-metal derivatives has been carried out with cobalt(II1) compounds but there are also examples of similar reactions which involve ligands on other metals; e.g. rhodium iridium and iron. Alkyl-cobalt complexes react with reagents X-Y such as Cl3C-C1 (NC)Cl,C-Cl (Et02C)2CH-Br and ArS02-Br by a conjugative displacement (SH2'or SH2y);e.g.,58*59 (42) yields (43) and allenyl-cobalt complexes react similarly to give propargyl derivatives; e.g.,57 (44)provides (45).Butenyl compounds e.g. (46) undergo ring-closure giving a useful route to cyclo-propylmethyl derivatives,60 and the reactions with benzylcobalt compounds e.g. (47) appear to provide examples of the elusive process of bimolecular homolytic substitution at saturated carbon.6l c13c'+ Co"'(dmgH);?L + C13C+" + [C~"(dmgH)~Ll (43) (dmgH = dimethylglyoximato) [Coi1(dmgH)2L] (44) (45) C13C' + -Co11'(dmgH)2(py)-+ 'I3'-+ [Co"(dmgH)2(~~ )I (46) C13C' + [ArCH2Co"'(dmgH)(imid)] + ArCH2CC13+ [Co"(dmgH)~(imid)] (47) (imid = imidazole) A. Bury C. J. Cooksey T. Funabiki B. D. Gupta and M. D. Johnson J.C.S. Perkin ZI 1979 1050. 59 A. E. Crease B. D. Gupta M. D. Johnson A. Bialkowska K. N. V. Duong and A. Gaudemar,J.C.S.Perkin I,1979 261 1. 6o A. Bury M. R. Ashcroft and M. D. Johnson J. Amer. Chem. SOC.,1978,100,3217. 61 B. D. Gupta T. Funabiki and M. D. Johnson,J.C.S. Chem. Comm. 1977,653.