1975 1607Photochemistry of Bromobiphenyls : Steric Effects and Electron TransferBy Nigel J. Bunce, Stephen Safe, and L. Octavio Ruzo, Chemistry Department, University of Guelph, Guelph,Ontario N1 G 2W1, CanadaEvidence is presented that photolytic fission of the C-Br bonds of bromobiphenyls is preceded by electron transfer.The enhanced rate of photodecomposition of 2-bromobiphenyls is shown to result from the release of steric strainin the excited state.THE photochemistry of aryl halides is under activeinvestigation, not least because of the importance ofsome of these compounds as environmental po1lutants.lAryl chlorides, which have been studied most extensively,afford mainly products explicable in terms of inter-mediate aryl radicals 2 probably derived from the tripletexcited state of the halide.However, it seems unlikelythat simple homolytic C-C1 fission [equation (i)] occurs,3Ar-X _jl Are + X-since there is usually a large energy defect between thetriplet excitation energy and typical C-C1 bond dis-sociation energie~.~ The possibility of electron transferhas therefore been invoked 394 [equation (ii)]. Triplet(ii) D + 3ArCl + Do+ + Ar. + C1-energies are largely independent of the identity of thehalogen (Table 1) so that the energy defect decreases inthe order chloride bromide iodide.TABLE 1Triplet energies and bond dissociation energies (B.D.E.)of some aryl halides aTriplet energiesSubstituent B.D.E.b Phenyl Naphthyl BiphenylyiH 440 350 6 255 e 275 ec1 365 d 250 f 275 9Br 305 250 f 280 gI 255 245 f 275 gAll energies in kJ mol-l.b S. W. Benson, ‘ The Founda-tions of Chemical Kinetics,’ McGraw-Hill, New York, 1960,p. 670. e G. N. Lewis and M. Kasha, J . Amer. Chem. Sac.,1944, 66, 2100. d Monomeric halogenobenzenes reported non-phosphorescent (E. C. Lim and S. K. Chakrabart!, Mol. Phys.,1967, 13, 293). 6 J. G. Calvert and J. N. Pitts, Photochem-istry,’ Wiley, New York, 1966, p. 297. f Average value for a-and @-isomers from ref. e and L. G. Thompson and s. E. Webber,J . Phys. Chem., 1972, 76, 221. Data are for 4-substitutedcompounds (ref. 13) ; other isomers appear similar.In the case of aryl iodides, where the preparativeaspects of the photochemistry have been thoroughlys t ~ d i e d , ~ fragmentation may well occur by the simplefission corresponding to (i).Much less is known about0. Hutzinger, S. Safe, and V. Zitko, ‘The Chemistry ofPCB’s,’ CRC Press, Cleveland, 1974; L. Fishbein, J . Chromatog.,1972,68, 345; see also citations in ref. 2.R. J. Majer and J. P. Simmons, Adv. Photochem., 1964, 2,137; J. T. Pinhey and R. D. G. Rigby, Tetrahedron Letters, 1969,1267.8 L. 0. Ruzo, N. J. Bunce, and S. Safe, Canad. J . Chem., 1975,53,688.M. Ohashi, K. Tsujimoto, and K. Seki, J.C.S. Chem. Comm.,1973, 384; B. Stevens, Adu. Photochem., 1971, 8, 161.R. K. Sharma and N. Kharasch, Angew. Chem. Internut.Edn., 1968, 7 , 36.the photochemistry of aryl bromides,6 though thesubject promises to increase in importance with thegrowing use of polybromobiphenyls as plasticisers andflame retardants.’ It is with the photoreactivity ofsome simple bromobiphenyls that this report is con-cerned.RESULTS AND DISCUSSIONDirect PhotoZysis.-Direct irradiation of a series ofbromobiphenyls in cyclohexane at ca.300 nm results inreductive debromination as the major reaction (Table 2).TABLE 2Direct photolysis of brominated biphenyls 4%3-Bromo 2.13 x 0.004 (1.0) 5Substrate Concn. (M) 4 r Conversion2-Bromo 7.85 x 0.046 (11.5) 164-Brorno 1.65 x 0.015 (3.8) 252,5-Dibromo 7.31 x 0.041 (10.3) 162,2’-Dibromo 3.95 x 0.034 (8.4) 234,4’-Dibromo 2.23 x loe2 0.022 (5.4) 27a At 300 nm; solvent cyclohexane. By ferrioxalateactinometry. Numbers in parentheses are relative to meta-isomer. Corrected where necessary for incomplete lightabsorption a t 300 nm.Examination of the involatile residue shows dimerisationto quaterphenyls to be occurring also.The products arethus analogous to those found in other aryl halidesystems 1-5 and explicable in terms of intermediate arylradicals. The reactions appear to proceed from tripletexcited states, as might be expected for heavy-atomcompounds, in that rates are retarded by a factor of twoin air-saturated as compared with degassed solutions,while quenching, albeit inefficient,8 is observed in thepresence of typical triplet quenchers.Isomers containing ortho-bromine atoms are photo-lysed significantly more efficiently than the others, andthis suggests a steric acceleration such as was soughtunsuccessfully in the iodobiphenyl ~ e r i e s , ~ and recentlyobserved in a series of tetrachlorobiphenyls.lO We nowpresent evidence to show that this acceleration is indeeda steric phenomenon.Wagner l1 has inferred experimentally that the tripletT. Matsuura and K.Omura, Bull. Chem. SOC. Japan, 1966,39, 944; M. G. Kuz’min, Y. A. Mikleev, and L. N. Guseva,Doklady Akad. Nuuk S.S.R., 1967, 176, 368; C. Phrkhny andY. J. Lee, Tetrahedron Letters, 1974, 116.E.g. Technical Bulletins, Michigan Chemical Corp., Chicago.8 L. 0. Ruzo and N. J. Bunce, Tetrahedron Letters. 1975,511.9 N. Kharasch and T. G. Alston, unpublished observations,10 L. 0. Ruzo, M. J. Zakik, and R. D. Schuetz, J . Amer. Chem.11 P. J. Wagner, J . Amer. Chem. SOC., 1967, 89, 2820.cited in ref.5.SOC., 1974, 96, 3809J.C.S. Perkin Iexcited state of biphenyl is planar, unlike the groundstate which, in solution, is believed to be twisted aboutthe inter-ring bond. This is consistent with the simpleMO picture of biphenyl where the inter-ring x-bond orderis greater in the excited than the ground state,l2 so thatthere should be more destabilization of the excitedstate upon twisting. The greater reactivity of ortho-substituted bromobiphenyls could thus be explained interms of relief of steric strain upon ejection of thebromine substituent, allowing a planar biphenylyl radicalto be attained. Four experimental facts support thisconcept, as follows. (i) Photolysis of 2,5-dibromo-biphenyl affords almost exclusively 3-bromobiphenyl,i.e.only the ortho-bromine atom is lost. (ii) The decom-position of 2,2'-dibromobiphenyl is accelerated less thanthat of 2-bromobiphenyl, even though the former ismore severely hindered (see U.V. data in Table 3 and thearguments below). However, loss of a single bromineatom from the dibromo-compound leaves a monobromo-biphenylyl radical which is still subject to steric con-straint. (iii) The i.r. C-Br stretching bands of thebromobiphenyls (Table 3) show that the C-Br bond inTABLE 3Spectroscopic properties of brominated biphenylsMassspectrumIntensity u.v.aL r I.r.b (M - Br)+/Compound l,,/nm E~~ egOOnm v+&m-l M+2-Bromo 236sh 4600 2 466 0.933-Bromo 264 18 100 71 445 0.744-Bromo 266 21 600 86 472 0.642,2'-Dibromo 228sh 3 400 0.4 448 2.624,4'-Dibromo 265 24000 140 496 0.222,6-Dibromo 238sh 4 700 22 439,4490 Ethanolic solution; Cary 118C spectrophotometer.e Varian-MAT CH 7 spectro- b Beckman IR 12 instrument.meter operated at 70 eV ionizing voltage.o-bromobiphenyls in the ground electronic state is notunusually weak.The stretching frequencies parallel thecalculated (zero order HMO, planar model) C-Br x-bondorders; thus the ortho rate increase suggests a stericeffect in the excited state rather than an intrinsicweakness of ortho C-Br bonds. (iv) Similar phosphor-escence spectra are observed l3 for all three monobromo-biphenyl isomers, although Er for the ortho-isomer maybe a little higher. Phosphorescence lifetimes aremarkedly different, that of the ortho-compound beingappreciably shorter. A similar reduction in the tripletstate lifetime as estimated by the quenching method hasbeen observed in ortho-substituted isomers in the tetra-chlorobiphenyl series.1° This accords with Wagner's l1observations on the allowedness of So f+ TI transitionsin biphenyl where the two states have different equili-brium geometries.For 2-bromobiphenyl, if both Soand Tl are twisted, the Tl + So transition becomesmore allowed and the Tl lifetime is reduced. A corollary12 H. H. JafE ant M. Orchin, ' Theory and Interpretation ofUltraviolet Spectra, Wiley, New York, 1964, ch. 12.15 C. M. O'Donnell, K. F. Harbaugh, R. P. Fisher, and J. D.Winefordner, Analyt. Chem., 1973, 45, 609.14 H.Suzuki, ' E!ectronic Absorption Spectra and Geometry ofOrganic Molecules, Academic Press, New York, 1967, ch. 12.is that if the excited state of 2-bromobiphenyl wereequally as long lived as its isomers', the expected en-hancement of the efficiency of photodecomposition wouldbe even greater than that reported in Table 2.The angle of twisting in the orth-substituted biphenylswas determined by using a simplification of Suzuki'smethod.14 Zero-order HMO calculations were carriedout making the following assumptions: (i) that thecalculated gap (AE) between the highest occupied andlowest vacant MOs of the ground state biphenyl wasproportional to the spectroscopic transition energy ;(ii) that the same HMO parameters were applicable forthe Br substituents regardless of location; (iii) that theinter-ring bond integral varied as cos20, where 0 is theangle between the planes of the rings.Unlike Suzuki,we took no account of any lengthening of the inter-ringbond upon twisting.Based on literature spectroscopic data l4 for crystalline(0 = O0),l6 solution (0 unknown), and vapour (0 = 43"),17plots of AE us. v,, and AE us. 8 indicate an interplanarangle of 20" for biphenyl in solution (see Table 4); theTABLE 4Calculated transition energies for biphenyl derivativesassuming various geometriesInterplanarangle (") AE 0Biphenyl0 1.46010 1.47320 1.61630 1.68046 1.7122-Bromobiphenyl0 1.46220 1.60730 1.67345 1.70360 1.84090 1.946Interplanarangle (") AEa2,2'-Dibromobiphen yl0 1.46020 1.60030 1.66545 1.69560 1.83090 1.9463-Bromobiphen yl0 1.46820 1.6124-Bromobiphenyl0 1.44720 1.6020 1.4374,4'-DibromobiphenylIn p units.test results are thus in satisfactory agreement with thoseof Suzuki.A calibration plot of vm= vs.AE for some referencecompounds was then made. The choice of referencecompounds in studies of this kind is always a problem,and is compounded in biphenyl systems by the fact thatit cannot be assumed that the non-ortho-substitutedcompounds are necessarily planar. We chose bromo-benzene, biphenyl (ZOO), and 4,4'-dibromobiphenyl asreference models, The last compound was assumed tobe planar in view of the documented18 influence ofconjugating substituents in negating small steric effectsin the biphenyl series.Data points for 3- and 4-bromo-biphenyl, calculated both for the planar situation and for15 A. Streitweiser, ' Molecular Orbital Theory for OrganicChemists,' Wiley, New York, 1961, p. 135.16 A. Hargreaves and S. H. Rizvi, Acta Cryst., 1962, 15, 366.1 7 0. Bastiansen, Acta Chem. Scand., 1949,8,409.18 D. W. Sheiwood and M. Calvin, J. Amer. Chem. SOL, 1942,64, 13501975 16096 = 20" are also placed on the graph (see Figure), andthe deviation from the curve of the planar values arguesation. In the presence of triethylamine, reaction (iii)leads to an aryl radical which subsequently abstracts3ArBr + Et,N Et3N*+ + Are + Br- (iii) strongly for the inability of a single bromo-substitientto overcome the steric effect in biphenyl.From thebenzene); 2o it is smaller'than the 75" observed byelectron diffraction,21 but this is to be expected byI I I I I I I38 40 -42 LL 46 481 o - ~ v mclx / cm-1Calculated energy gap (AE) us. spectroscopic transition energy forbromobiphenyls and reference compounds ; open circles,planar model; closed circles, 20" interplanar angleanalogy with biphenyl, where the interplanar angle issmaller in solution than in the vapour phase.Triethylamine-assisted PhotoZyses.-The effect of tri-ethylamine is to enhance both the rate of photodecom-position of the bromobiphenyls and the proportion of thephotoreduction product (Table 5). Photolyses of arylTABLE 5Direct us. triethylamine-assisted photolyses of bromo-biphenyls a%LightConcn. abs.at % %2-Bromo 0.176 55 0.056 (14.0) 50 444-Bromo 0.168 99 0.010 (2.5) 9 442,2'-Dibromo 0.172 16 0.027 (6.8) 25 60Substrate (M) 300nm + r b Conversion ArHcDirect photolysis3-Bromo 0.172 99 0.004 (1.0) 4 41With Et,N ( 2 . 9 ~ )2-Bromo 0.176 55 0.104 (26.0) 93 963-Bromo 0.172 99 0.008 (2.0) 8 864-Bromo 0.168 ' ~ 9 9 0.011 (2.8) 10 982,2'-Dibromo 0.172 15 0.060 (16.0) 56 74halides in the presence of bases have generally beenconsidered to proceed by way of electron transfer.4~6922The present results parallel those of our previous studyof the chloronaphthalenes and permit a similar explan-*d See Table 2. 8 Chemical yield of photoreduction product.la G. H. Beaven and D. M. Hall, J . C h m .Soc., 1966, 4637.*O A. C. Littlejohn and J. W. Smith, J. Chem. Soc., 1954, 2652.21 0. Bastiansen, Acta Chem. Scand., 1950,4, 926.Ar'H + 3ArBr + Ar'H*+ + Are + Br- (v)where Ar'H may be a second molecule of the bromo-biphenyl or a product aromatic compound. Some ofthe resulting radical cations may now attack other arylresidues in a kind of Friedel-Crafts substitution, thusaffording quaterphenyls in this system. Support forthis proposal comes from the work of Ronlkn et aZ.,23who prepared quaterphenyls by anodic oxidation ofbiphenyls.Just as in the direct photolyses, the ortho-substitutedcompounds are the most reactive in the presence ofamine. Once again, calculations on a planar model ofbromobiphenylyl radical anions give no expectation ofan abnormally weak ortho C-Br bond, so that a stericeffect must again be operating.Consistent with this,the inter-ring x-bond order is higher in the radical anionthan in the neutral molecule, as it is in the excited ascompared with the ground state (Table 6). Thus theTABLE 6Selected calculated bond orders in bromobiphenyls 02,2'- 4,4'-2-Bromo 3-Bromo 4-Bromo Dibromo DibromoC-C Bond ordersGround state 0.338 0.337 0.338 0.338 0.338Excited state 0.594 0.693 0.691 0.694 0.689Radical cation 0.467 0.464 0.463 0.467 0.461Radical anion 0.465 0.466 0.466 0.465 0.466Ground state 0.111 0.109 0.110 0.111 0.110Excited state 0.139 0.116 0.151 0.133 0.147Radical cation 0.160 0.119 0.172 0.144 0.168Radical anion 0.100 0.106 0.079 0.100 0.079C-Br bond ordersa Assuming planar model.finding that an electron-transfer mechanism is operatingand hence that bromine is lost from the radical anion inno way vitiates the steric arguments developed above ;indeed it can be seen that the C-Br x-bond orders in theradical anion are such as to suggest that $ma-substitutedbromobiphenyls would be the most reactive.A similarinter-ring bond order change is also apparent in com-paring the neutral molecule with the radical cation, andit is interesting that the mass spectra of o-bromo-biphenyls show a greater extent of bromine ejection than22 J. A. Barltrop and R. J. Owers, Chem. Comm., 1970, 1462;J. A. Baltrop, Pure A$@. Chem., 1973, 33, 179.23 A. RonlAn, K. Bechgaard, and V. D. Parker, Acta Chenz.Scand., 1973, 27, 23751610 J.C.S.Perkin Ithe meta- or @am-isomers. Such an effectobserved previously in the chloro-analogues.2*has beenEXPERIMENTALPhotolytic and analytical procedures have been describedin detail elsewhere.2 The photolyses were carried out induplicate, and reported values are averages, based upon a tleast duplicate analyses of each reaction mixture. Thebiphenyl derivatives were commercial products, purified byappropriate combinations of recrystallisation, distillation,and chromatography until free of impurities (g.l.c.), exceptfor 2,5-dibromobiphenyl. This was prepared by diazotising2,5-dibromoaniline with pentyl nitrite in the presence of anexcess of benzene,25 and obtained in 49% yield as an oilafter column chromatography and distillation (Found : M+,311.8972. Cl,H87sBr81Brequires M , 311.8972).Spectroscopic properties of the compounds studied aregiven in Table 3; photolysis products and quantum yieldsare reported in Tables 2 and 5. HMO parameters were inall cases those recommended by Streitweiser ; l5 selectedresults are presented in Tables 4 and 6 and in the Figure.We thank the National Research Council of Canada forcontinued financial support, and the University of Albertafor the high resolution mass spectrum.[5/360 Received. 20th February, 1976124 S. Safe and 0. Hutzinger, J.C.S. Chem. Comm., 1971, 446;J.C.S. Perkin I , 1972, 686.J. I. G. Cadogan, J . Chem. SOL, 1962, 4257
展开▼
