18 Aromatic Compounds By J. W. BARTON School of Chemistry University of Bristol Cantock's Close Bristol BS8 1 TS 1 General Once again the use of the term 'aromatic' has been criticized' and the suggestion made that systems be described as Huckel counter-Huckel or in the case of larger annulenes as non-Hiickel according to their ground-state stability. Other terms are suggested to cover structural features and chemical reactivity. The 'scrambling' processes which take place after impact ionization of benzene have received further study,2 as has the structure of the abundant C7H7+ ion formed from toluene and the higher alkyl benzene^.^ A marked difference between the decomposing (C7H,NH2)+ ions from benzylamine and p-aminoto- luene is rep~rted.~ In the gas-phase radiolysis of toluene about one third of the C7H7+ions produced arise uia a symmetrical intermediate.the rest via benzyl cations. Radiolysis of ethylbenzene in the presence of dimethylamine gave NN-dimethylbenzylamine from unrearranged benzyl cations and no NN-dimethyltropylamine. Owing to steric interactions there is considerable twisting in 1,8-di-t-butyl- naphthalenes the t-butyl groups lying on opposite sides of the mean plane of the naphthalene ring. The barrier to flipping from this conformation to its mirror- image is greater than 24 kcal mol- ',but the barrier to rotation around a t-butyl- naphthalene bond is estimated to be only 6.5 kcal mol-1.6 Contrary to an observation discussed in last year's Rep~rt,~ it is unlikely that the energy barrier to ring inversion of any tetraphenylene derivative can be lower than that for cyclo-octatetraene (-15 kcal mol- '1 owing to the considerable non-bonded interactions of the benzene-ring hydrogens in the former.8a,e Although an n.m.r.' D. Lloyd and D. R. Marshall Angew. Chem. Internat. Edn. 1972 11,404. J. H. Beynon R. M. Caprioli W. 0.Perry and W. E. Baitinger J. Amer. Chem. Soc. 1972,94 6828. T. Ast J. H. Beynon and R. G. Cooks J. Amer. Chem. Soc. 1972,94 1834. A. P. Bruins N. M. Nibbering and T. J. de Boer Tetrahedron Letters 1972 1109. S. Takamuku N. Sagi K. Nagaoka and H. Sakurai J. Amer. Chem. Soc. 1972 94 6218. 'J. E. Anderson R. W. Franck and W. L. Mandella J. Amer. Chem. Soc. 1972,94,4608. ' H. Heaney Ann. Reports (B) 1971,68 516.(a) G. H. Senkler D. Gust P. X. Riccobono and K. Mislow J. Amer. Chem. Soc. 1972 94 8626; (6) G. W. Buchanan Tetrahedron Letters 1972 665; (c) A. Rosdal and J. Sandstrom ihid. p. 4187; (6) D. Gust G. H. Senkler and K. Mislow J.C.S. Chem. Comm. 1972 1345; (e)C. J. Finder D. Chung and N. L. Allinger Tetrahedron Letters 1972 4677. 563 564 J. W.Barton method has indicated a slightly lower barrier in (1) than in (2)," further n.m.r. studies on the 2-substituted tetraphenylenes [3;R = CH(OH)Me]'" and (3 ; R = COCHMe2)8' have given estimated lower limits of 21 and 26 kcal mol-' and the barrier to racemization of tetraphenylene-2-carboxylicacid (3; R = C02H) is found to be at least 45 kcal mol- 1.8d Certain 2,6-disubstituted aryl alkyl I I ketones show temperature-dependent n.m.r.spectra which are accountable for in terms ofrestricted rotation about the Ar-CO bond;'barriers to rotation lie in the range 9-15 kcal mol-'. The preferred conformations of mesityl 1- and 2-naphthyl ketones have been determined." Assignment of the left-handed (M) configuration to (-)-hexahelicene by a chemical method' agrees with the findings of a recent X-ray crystallographic study details of which have now appeared.12 The synthesis of (+)-(P)-pen- tahelicene from optically pure ( -)-(S)-2,2'-bis(bromomethyl)-1,l'-binaphthyl has been reported ;I3 it appears likely that all (+)-helicenes belong to the (P)and all (-)-helicenes to the (M)series. Mechanistic studies of the asymmetric syn- thesis of [6]-,[7]- [8]- and [9]-helicenes using circularly polarized light in- dicate that the optical activity induced is due to selective reactions of enantiomeric conformations of the cis-1,2-diarylethylene precursors ;I4 a recent study of the n.m.r.spectra of 1,2-diarylethylenes supports this view. ' The use of specific deuterium labelling combined with I3C n.m.r. spectroscopy has been used to study the course of photocyclization of 1,2-diarylethylenes to helicenes. Optically active 6,6'-diethyl-2,2-dimethylbiphenyl is photoracemized to an extent of 17% in 2 h loss of activity also occurring by arrangement involving benzvalene intermediates. In contrast this biphenyl is exceptionally stable to thermal racemization having a half-life of 283 h at 345"C.l7 The biphenyl (4) N.Nakamura and M. Oki Bull. Chem. SOC.Japan 1972,452565. lo C. L. Cheng G. L. D. Ritchie P. H. Gore and M. Jehangir J.C.S.Perkin If 1972,1432. l1 J. Tribout R. H. Martin M. Doyle and H. Wynberg TetrahedronLetters 1972,2839. D. A. Lightner D. T. Hefelfinger T. W. Powers G. W. Frank and K. N. Trueblood J. Amer. Chem. SOC.,1972 94 3492. l3 H. J. Bestmann and W. Both Angew. Chem. Internat. Edn. 1972 11 296. l4 W. J. Bernstein M. Calvin and 0. Buchardt J. Amer. Chem. SOC.,1972,94 494. ' R. H. Martin N. Defay H. P. Figeys K. LC Van J. J. Ruelle and J. J. Schurter Heh. Chim. Acta 1972 55 2241. l6 R. H. Martin and J. J. Schurter Tetrahedron 1972 28 1749. H. E. Zimmermann and D. S. Cramrine J. Amer. Chem. SOC.,1972 94 498. Aromatic Compounds Me Ph \/ CONHCH NO CONHCH /\ Me Ph (4) and its diastereomer which possess an axis of pseudoasymmetry have been synthesized.Homo- Spiro- and Bicyclo-aromaticity.-There is no 'scrambling' of the deuterium label amongst the ring carbon atoms in solutions of the homotropylium ion (5);thus the circumambulatory rearrangement observable with the bicyclo- [3,1,0]hex-3-enyl cation (6) does not take place in this ~ystem.'~ Confirmation that the primary product in the chlorination of cyclo-octatetraene is the endo-8-chlorohomotropylium ion (7)20 has been afforded by its isolation as the hexa- chloroantimonate ; further kinetic studies concerned with this reaction are (5) (6) (7) reported.2 When 1-methoxycyclo-octatetraene is dissolved in fluorosulphonic acid at -78 "C it protonates to give the ion (8) which at higher temperatures rearranges to the phenylmethylmethoxycarbonium ion (9) and further to ,Me H 9' \/H I; acetophenone and methyl fluorosulphonate.2 * The dibenzohomotropylium ion (10)has been generated and its reactions with nucleophiles have been studied.23 " G.Helmchem and V.Prelog Helu. Chim. Acta 1972 55 2599. l9 J. A. Berson and J. A. Jenkins J. Arner. Chem. SOC.,1972 94 8907. 2o I. 0.Sutherland Ann. Reports (B) 1967 64 296. 21 R. Huisgen and J. Gasteiger Angew. Chem. Internat. Edn. 1972,11 1104; Tetrahedron Letters 1972 3661 3665. 22 M. S. Brookhart and M. A. M. Atwater Tetrahedron Letters 1972 4399. 23 R. F. Childs M. A. Brown F. A. L. Anet and S. Winstein J.Amer. Chern. SOC. 1972,94,2175. 566 J. W.Barton The syn and anti forms of 4,5-benzo-2,3 :6,7-bishomotropone undergo reversible protonation to give the hydroxycations which are stable below -20 “C. From their n.m.r. spectra it is concluded that there is bishomoaromatic stabilization of the syn cation (1 1) but not of the anti.24 (10) (1 1) In a discussion of the topology of aromatic-type systems it has been suggested that certain spirocyles with continuous ribbons of p-orbitals in each ring may show special stabilization or ~piroaromaticity,~ examples being the ions (12) and (13) and biradical (14). In the first study of such systems it is found that spiroaromatic stabilization of the anion (I 5) is not a significant factor in its rate of formation.26 (15) The question of bicycloaromatic stabilization has been re-examined and the concepts have been tested by study of the ions derived from bicyclo[3,2,2]nona- 2,6,8-triene.Theanion (16) is a stable delocalized ion which undergoes a degenerate ‘~crambling’.~’ (16) Benzene Isomers and Benzene Oxides.-Reviews have appeared dealing with valence bond isomers of aromatic systems28 and with rearrangements and interconversions of (CH) Silver-ion-catalysed valence isomerization of 1,l’-dimethylbicyclopropenyl(17) gives the Dewar-benzenes (18) and (19) together with 0-and p-xylene. From the 24 H. A. Carver and R. F. Childs J. Amer. Chem. Soc. 1972 94 6201. 25 M. J. Goldstein and R. Hoffmann J. Amer. Chem. SOC.,1971 93 6193.26 M. F. Semmelhack R. J. DeFranco Z. Margolin and J. Stock J. Amer. Chem. Soc. 1972,94 2116. ‘’ J. B. Grutzner and S. Winstein J. Amer. Chem. SOC.,1972 94 2200. ’* E. E. van Tamelen Accounts Chem. Res. 1972,5 186. 29 L. T. Scott and M. Jones Chem. Rec. 1972 72 181. Aromatic Compounds Me Me Me vv Me (17) absence of isomer (20)amongst the products it is concluded that the reaction cannot involve a prismane intermediate.3 Photolysis or low-pressure thermolysis of the cyclopropene anhydride (21) gives a similar result in that 1,2,3,5- and 1,2,4,5-tetraphenylbenzenes are formed but not the 1,2,3,4-isomer. It is suggested that the intermediate anhydride (22) fragments to the benzvalene (23) which Ph pco-0 404Ph PhOPh PhDPh Ph Ph Ph Ph Ph Ph (21) (22) (23) rearranges to give the observed prod~cts.~' Several studies of the reactions of hexamethyl-Dewar-benzene have been reported,32 together with one study of the kinetics and thermodynamics of interconversions in the series hexakis(trifluoro- methyl)-benzene,-Dewar-benzene,-benzvalene and -prismane ;3 the 'downhill' reactions all require large activation energies (-40 kcal mol- ').Whereas the dichloro-Dewar-benzene derivative (24; X = Y = C1) is more stable than the parent (24; X = Y = H) the less symmetrical monochloro (24; X = C1 Y = H) and monofluoro (24; X = F. Y = H)derivatives rearrange to the corresponding X Y (24) benzenes some 10 and 40 times faster than the parent.34 cis-Hexa-1,5-diyn-3-ene undergoes a thermal degenerate rearrangement in which p-benzyne must be an intermediate.3s Trapping experiments e.g.the rearrangement in carbon tetra- chloride which gives only 1,4-dichlorobenzene suggest that the biradical form 30 W. H. de Wolf J. W. von Straten and F. Bickelhaupt Tetruhedron Letters 1972 3509. 31 E. B. Hoyt E. J. Reineberg P. Goodman P. Vaughan and V. Georgian Tetrahedron Letters 1972 1579. 32 L. Paquette R. J. Haluska M. R. Short L. K. Read and J. Clardy J. Amer. Chem. SOC.,1972 94 529; G. R.Crow and J. Redly Tetrahedron Letters 1972 3129 3133; L. Paquette S. A. Lang S. K. Porter and J. Clardy Tetrahedron Letters 1972 3 137 ; H. Hogeveen and R. W. Kwant Tetrahedron Letters 1972 3197. 33 D. M. Lemal and L. H. Dunlap J. Arner. Chem. SOC., 1972 94 6564.34 R. Breslow J. Napieralski and A. H. Schmidt J. Amer. Chem. SOC.,1972 94 5906. 35 R. H. Jones and R. G. Bergmann. J. Amer. Chem. Soc. 1972 94 661. 568 J. W.Barton (25c) may best represent the structure. The mechanism of aromatization of arene oxides36 and concomitant alkyl migrations37 have been further studied. syn-Benzene dioxide (26)38and the syn- and anti-forms (27) and (28) of benzene tri~xide~~ have been synthesized; there is also a report of the isolation of a benzene dioxide derivative (29),"' which probably has the anti-configuration 0 wo Ou 0 as a fungal metabolite. The dioxide (26) interconverts rapidly with 1,4-dioxocin (30) at temperatures above 50 "C the equilibrium favouring the latter (5 :95 at 60°C).The dioxocin (30) a lox-electron system is shown by its spectra and chemical behaviour to be olefinic in character. The syn-trioxide (27) undergoes an irreversible thermal isomerization to cis,cis,cis-trioxacyclonona-1,4,7-triene.39a 2 Benzene and Derivatives Current problems concerning the structure of the transition states and inter- mediates in heterolytic aromatic substitution have been discussed."' Recent reviews include accounts of the reactions of nucleophiles with aryl halides,42 36 G. J. Kasperek and T. C. Bruice J. Amer. Chem. Suc. 1972,94 198; G. J. Kasperek T. C. Bruice H. Yagi and D. M. Jerina J.C.S. Chem. Comm. 1972 784. 3' E. A. Fehnel J. Amer. Chem. Suc. 1972,94,3961; G.J. Kasperek T. C. Bruice H. Yagi and N. Kaubisch J. Amer.Chem. SOC.,1972 94 7876. 38 E. Vogel H. J. Altenbach and D. Cremer Angew. Chem. internat. Edn. 1972 11 935 931. 39 (a) E. Vogel H. J. Altenbach and C. D. Sommerfeld Angew. Chem. internat. Edn. 1972 11,939; (b) R. Schwesinger and H. Prinzbach ibid. p. 942; fc) C. H. Foster and G. A. Berchtold J.-Amer. Chem. SOC.,1972 94 7939. 40 D. B. Borders P. Shu and J. E. Lancaster J. Amer. Chem. SOC.,1972 94 2540. 4' P. Rys P. Skrabal and H. Zollinger Angew. Chem. Internat. Edn. 1972 11 874. 42 J. F. Bunnett Accounts. Chem. Res. 1972 5 139. Aromatic Compounds 569 the use of organocopper compounds in new nitrene-induced aro- matic rearrangement^,^^ and the oxidation of primary aromatic amine~.~~ Measurements of equilibrium ion-pair acidities for some polyfluorobenzenes have led to a pK value (per hydrogen) of 43.0 0.2 for benzene.46 Very rapid and in some cases selective deuterium exchange occurs when aromatics are treated with perdeuteriobenzene in the presence of metal halide catalysts.47 Comparison of the rates of protodetritiation of 1,3,5-triphenylbenzene with those of biphenyl indicate that the angular hydrogens in the former may show slight steric hindrance towards acid-catalysed hydrogen exchange.48 The first spectroscopic study of the C6H,+ (benzenium) ion in solution has been reported.49 Sigma-complexes corresponding to Wheland intermediates are isolable in the protonation alkylation and halogenation of certain 1,3,5-tris- For example the addition of bromine to 1,3,5-(dialky1arnino)ben~enes.~~ tripyrrolidinobenzene at -60 "C gives the dark red complex (31) which is stable for some time at ambient temperature but is deprotonated readily by strong bases.Good nucleophiles and reducing agents cleave off a bromine cation from the complex tertiary amines give the biaryl (32) probably via radical inter- mediates and carboxylate ions give some of the quinone iminium salt (33). I RR (33) 43 J. F. Normant Synthesis 1972 63. 44 J. I. G. Cadogan Accounts. Chern. Res. 1972,5 303. 45 M. Hedayatullah Bull. Soc. chim. France 1972 2957. 46 A. Streitweiser P. J. Scannon and H. M. Niemeyer J. Amer. Chem. SOC.,1972. 94 7937. " J. L. Garnett M. A. Long R. F. W. Vining and T. Mole J.C.S.Chern. Comm. 1972 1 172;J. Arner. Chern. SOC.,1972,94 591 3.48 H. V. Ansell R. B. Clegg and R. Taylor J.C.S. Perkin II 1972 766. 49 G. A. Olah R. H. Schlosberg R. D. Porter Y. K. Mo D. P. Kelly and G. D. Mateescu J. Arner. Chern. Soc. 1972,94 2034. 50 P. Mensel and F. Effenberger Angew. Chern. Internal. Edn. 1972 11 61 922. 570 J. W.Barton Electrophilic attack at positions occupied by groups other than hydrogen has received further attention. Rate studies are reported for dehalogenation’ la and des~lphonation~ by diazonium ions and the influence of ‘electrofugal’ Ib leaving groups5 has been discussed. The reaction of p-trimethylsilyltoluene with nitric acid in acetic anhydride to give p-nitrotoluene takes place predom- inantly by initial nitrosodesilylation ;5 nitrosodeiodination is thought to be involved in the conversion of 4-iodoanisole into 2-iodo-4-nitroanisole by nitric acid.54 The ratio of 3-nitro- to 4-nitro-o-xylene formed varies from -0.5 to -1.5 when o-xylene is nitrated in sulphuric acid solutions varying in strength from 50 to 70%.This is due to the behaviour of the 1,2-dimethyl-l-nitrocyclo-hexadienyl cation (34) formed in competition with the other two Wheland inter- mediates (35) and (36). At higher acid strengths the ion (34) rearranges to (35)’ leading to 3-nitro-o-xylene but not to (36) and thus the observed ratio of products is altered.55 Several electrophilic substitutions of preparative importance have been reported. Mixed trifluoromethanesulphonic-carboxylic acid anhydrides are highly active acylating agents which react directly with arenes to give ketones ; a catalytic system of trifluoromethanesulphonic acid together with an acid chloride is also effe~tive.~~ Arenes are formylated by reaction with hexamethyl- enetetramine in trifluoroacetic acid,’ and thiocarboxylate groups may be introduced using ethyl chlorothioformate in the presence of a Friedel-Crafts catalyst.Full details of an extensive study of thallium-catalysed bromination have appeared.59 Photolysis of arylthallium bistrifluoroacetates obtained by direct thallation of arenes in aqueous potassium thiocyanate gives aryl thio- cyanates in moderate yield ;60 copper cyanides in pyridine or acetonitrile convert arylthallium(m) salts into cyanides.6 Alkali-metal tetrafluorocobaltates are found to be useful for the polyfluorination of arenes;62 fluorination of the 51 P.B. Fischer and H. Zollinger (a)Helc. Chirn. Acfa 1972 55 2139; (b)ibid. p. 2146. 52 C. L. Perrin J. Org. Chem. 1971 36 420. 53 C. Eaborn Z. S. Salih and D. R. M. Walton J.C.S. Perkin II 1972 172. 54 A. R. Butler and A. P. Sanderson J.C.S. Perkin II 1972 989. 55 P. C. Myhre J. Amer. Chem. SOC.,1972 94 7921. 56 F. Effenberger and G. Epple Angew. Chem. Internat. Edn. 1972 11 299 300. 57 W. E. Smith J. Org. Chem. 1972 37 3972. 5M G. A.Olah and P. Schilling Annalen 1972 761 77. ” A. McKillop D. Bromley and E. C. Taylor f.Org. Chem. 1972,37 88. E. C. Taylor F. Kienzle and A. McKillop Synthesis 1972 38. 61 S. Uemura Y. Ikeda and K. Ichikawa Tetrahedron 1972 28 3025. ‘’ A. J. Edwards R.G. Plevey I. J. Sallomi and J. C. Tatlow J.C.S. Chem. Comm. 1972 1028. Aromatic Compounds aromatic ring of griseofulvin has been achieved under very mild conditions using trifluorofluoroxymethane.63 Following earlier observation^,^^ the reaction of aromatics with lead tetrakistrifluoroacetate has been shown to proceed by an electrophilic substitution mechanism ;65a the trifluoroacetoxy-compounds pro-duced are easily hydrolysed to Concerning nucleophilic substitutions various studies of rates of formation and stabilities of Meisenheimer complexes of di- and tri-nitro-benzenes66 and -naphthalenes6' have been reported. In the first recorded nucleophilic photo- substitution of hydrocarbons it is found that cyanide ion attacks biphenyl naphthalene and azulene at the positions where they are attacked by electro- philes.6 Cerium(1v) oxidation of the cyanocyclohexadienyl complex (37) I Mn(COh (37) prepared by reaction of the cationic complex (PhH)Mn(CO),+ with cyanide ion gives benzonitrile (80%)in what amounts to an indirect nucleophilic substitu- ti~n.~~ Arylnitrenium ions (38) generated by the silver-ion-assisted solvolysis + (38) of N-chloroanilines undergo nucleophilic attack at the 0-and p-positions.' In methanol simple electron-rich N-chloroanilines give mainly anisidines whereas with anilines possessing electron-withdrawing substituents nuclear chlorination is the main reaction." D. H. R. Barton R. H. Hesse L. Ogunkoya N. D. Westcott and M. M. Pechet '' J.C.S.Perkin I 1972 2889. R. E. Partch J. Amer. Chem. SOC.,1967 89 3662. (a) J. R. Kalman J. T. Pinhey and S. Sternhell Tetrahedron Leirers 1972 5369; (6) J. R. Campbell J. R. Kalman J. T. Pinhey and S. Sternhell ibid. p. 1763. 66 E. J. Fendler J. H. Fendler N. L. Arthur and C. E. Griffin J. Urg. Chem. 1972 37 812; E. J. Fendler and J. W. Larsen ibid.p. 2608; M. R. Crampton and H. A. Khan J.C.S. Perkin II 1972 1173 1178 2281 ; M. R. Crampton M. A. El Ghariani and H. A. Khan Tetrahedron 1972 28 3299; F. Terrier F. Millot and R. Schaal J.C.S. Perkin II 1972 1 192. " E. J. Fendler and J. H. Fendler J.C.S. Perkin II 1972 1403. " J. A. Vink C. M. Lok J. Cornelisse and E. Havinga J.C.S. Chem. Comm. 1972 71 1. 69 P. J. C. Walker and R.J. Mawby J.C.S. Chem. Comm. 1972 330. 70 P. Gassman G. A. Campbell and R. C. Frederick J. Amer. Chem. SOC.,1972 94 3884 3891. 572 J. W.Barton Phenol and NN-diethylaniline both react with chloroform under U.V. irradiation in methanolic solution to give the corresponding 0-and p-substituted benzalde- hydes. It is suggested that the reaction involves attack of an electron from the excited substrate on chloroform to give chloride ion and a dichloromethyl radical then coupling of the latter with a phenoxyl radical or its ~ounterpart.~’ Separate competing pathways have been postulated for the ring-expansion and ring-contraction reactions of phenylcarbene ; however when 3C-labelled phenylcarbene (39)is rearranged at 770 “C the I3C label is found to be uniformly distributed over all the carbon atoms of the resulting fulveneallene (40).72This result indicates that ‘scrambling’ takes place prior to the formation of a pre- fulvene-type intermediate (41) and is consistent with a ‘pre-equilibrium’ in which 11 phenylcarbene interconverts rapidly with the bicyclic intermediate (42) and with cycloheptatrienylidene (43) which can undergo hydrogen shifts.The rearrange- ment and insertion reactions of 2-methylphenylcarbenes have also been studied. Treatment of N-chloroaniline with n-butyl-lithium at -100 “C gives the nitren- oid (44) which eliminates lithium chloride on warming and gives products derived from ~henylnitrene.’~ Several photoadditions of olefins to benzene have been reported. The additions with cis-and trans-1,2-dichloroethylenes give P-chlorostyrenes as the main ” K.Hirao and 0.Yonemitsu J.C.S. Chem. Comm. 1972 812. l2 W. D. Crow and M. N. Paddon-Row J. Amer. Chem. SOC.,1972,94,4747. 73 G.G. Vander-Stouw A. R. Kraska and H. Schecter J. Amer. Chem. Soc. 1972 94 1655. l4 C. A. Wilkie and D. R. Dimmel J. Amer. Chem. Sor. 1972 94 8600. Aromatic Compounds (44) products not 1,8-dichloro-octatetraenesas previously rep~rted.~ With allene the 1,3-and 1,4-adducts (45) and (46) are formed the latter predominating;76 cyclonona- 1,2-diene behaves similarly. The primary products from the sensitized reaction of benzene with vinylene carbonate are the endo-and exo-1,2-cyclo- adducts of which the 4x0-isomer (47) can be isolated.These then undergo rearrangement to the 1,4-cycloadduct (48) retroaddition dimerization or addition of a further molecule of reagent.77 Molecular Rearrangements.-The migration of trimethylsilyl groups attached to aromatic rings is shown to be acid-catalysed. At 150 “C in the presence of trifluoroacetic acid 1,2-bistrimethylsilylbenzene rearranges rapidly to an equilibrium mixture of the 1,3-and 1,4-isomers the former predominating. A mechanism involving ring protonation isomerization of the cation then deproto- nation is suggested (Scheme l).78 SiMe SiMe SiMe SiMe SiMe -3 etc. Scheme 1 The results of a recent study of the Fischer-Hepp rearrangement of N-methyl- N-nitrosoaniline in hydrochloric acid do not accord with the accepted mechanism involving C-nitrosation by nitrosyl chloride but suggest that rearrangement 75 D.Bryce-Smith B. E. Foulger and A. Gilbert J.C.S. Chem. Comm. 1972 769. 76 D. Bryce-Smith B. E. Foulger and A. Gilbert J.C.S. Chem. Comm. 1972 664. ” P. Lechten and G. Hesse Annalen 1972,754 1 ;H. D. Scharf and R. Klar Chem. Ber. 1972 105 575. ’* D. Seyferth and D. L. White J. Organometailic Chem. 1972 34 119; J. Amer. Chem. SOC.,1972 94 3132. 574 J. W.Barton occurs intramolecularly by a unimolecular reaction of the protonated nitro- ~amine.'~ There is evidence that the benzidine rearrangement proceeds via the arenium ion (49)and may be regarded as an intramolecular alkylation reaction (Scheme 2)." The corresponding rearrangement of tetraphenylhydrazine (49) 1 Scheme 2 probably proceeds through radical cation intermediates.' Ally1 phenyl ethers undergo an ortho-Claisen rearrangement on dissolution in trifluoroacetic acid at room temperature.A transition state akin to (50) is envisaged ; a methyl or HH methoxy substituent gives approximately the same rate enhancement whether present in a rneta-or para-position.82 Gas-phase pyrolysis of phenyl propargyl ether at 460 "C gives P-indanone (26 %) together with benzocyclobutene (31%).83 The mode of formation of the latter is yet unknown but the former is thought to arise by a series of four thermally-allowed six-electron concerted reactions starting with a Claisen-type rearrangement (Scheme 3). Biary1s.-Aromatics react with tellurium tetrachloride in the presence of Lewis catalysts to give bis(ary1)tellurium dichlorides.These are easily reduced to diaryltellurides and on heating with degassed Raney nickel give the corresponding '' T. D. B. Morgan and D. L. H. Williams J.C.S. Perkin II 1972 74. *O G. A. Olah K. Dunne and D. P. Kelly J. Amer. Chem. SOC.,1972 94 7438; D. V. Banthorpe Tetrahedron Letters 1972 2707. 81 U. Svanholm K. Bechgaard 0. Hammerich and V. D. Parker Tetrahedron Letters 1972 3675. U. Svanholm and V. D. Parker J.C.S. Chem. Comm. 1972 645. 83 W. S. Trahanowsky and P. W. Mullen J. Amer. Chem. SOC.,1972,94 591 1. Aromatic Compounds DOta% s=c=O Scheme 3 biaryls in good yield.84 Arylboranes formed by treating Grignard reagents with diborane give biaryls on reaction with alkaline silver nitrate.85 In aprotic solvents allyl(prop- 1-yn-3-y1)ammonium salts (51) undergo a base-catalysed concerted 2,3-sigmatropic rearrangement to the amines (52) which cyclize to R' I //C-C?Ye R2-C N-CH,-C-C I+ H I Me x-R',RZ = H Me or Ph.R'/c=cH2 \ R2 & R2-CH CH-C-C (53) I NMe (52) the unsymmetrical biaryls (53) on heating.86 The formation of biaryls by photo- lysis of triaryl phosphates is also reported but the reaction appears to be limited in scope.M7 The bridged oxonium salt (54) has been obtained by heating Me BF (54) 84 J. Bergman Tetrahedron 1972,28 3323. S. W. Breuer and F. A. Broster Tetrahedron Letters 1972 2194. 86 R. W. Jemison T. Laird and W. D. Ollis J.C.S. Chem.Comm. 1972 556. R. A. Finnegan and J. A. Matson J. Amer. Chem. Soc. 1972 94,4780. 576 J. W.Barton 2-methoxybiphenyl-2’-diazonium fluoroborate in benzene ; it is an extremely powerful Meerwein-type methylating agent. 88 Arynes and Aryne Precursors.-The reaction of toluene with benzyne is less selective than that with tetrafluorobenzyne. In addition to the two [2 + 41 cycloadducts considerable amounts of 2-benzylbiphenyl are formed probably Scheme 4 via consecutive ‘ene’ reactions (Scheme 4) together with traces of diphenyl-methane a C-H insertion product.89 Generation of the arynecarbanion (55) from N-methyl-2-chlorobenzylaminoacetonitrileis followed by ring closure and elimination of cyanide ion to give 2-methylisoindole (56)directly in high yield.” CH,-NXHCN I-The aprotic diazotization of thioanthranilic acid affords the benzothiadiazine (57) rather than benzenediazonium-2-thiocarboxylate.Thermal decomposition of (57) with anthracene as a benzyne acceptor gives low yields of triptycene.” Diazotization of tetrafluoroanthranilic acid with nitrosylsulphuric acid in acetic acid proceeds with replacement of fluorine to give the very stable diazonium N I H N-N=N-N-Ts (57) F Li + (59) salt (58).Addition of an ethereal solution of toluene-p-sulphonyl azide to a solu- tion of the monoanion of 1-aminobenzotriazole in tetrahydrofuran gives a yellow precipitate assumed to be the tetrazene salt (59) which decomposes rapidly forming benzyne nitrogen and lithium toluene-p-sulphonamidate.92 A. J. Copson H. Heaney A. A. Logun and P. P. Sharma J.C.S. Chem. Comm. 1972 315. 89 J. M. Brinkley and L. Friedman Tetrahedron Letters 1972,4141 90 B. Jaques and R. G. Wallace J.C.S. Chem. Comm. 1972 397. 91 A. T. Fanning G. R. Bickford and T. D. Roberts J. Amer. Chem. SOC.,1972.94,8505. 92 M. Keating M. E. Peek C. W. Rees and R. C. Storr J.C.S. Perkin I 1972 1315. Aromatic Compounds 577 Quinone Methides Dimethides and Related Compounds.-p-Quinone methides have been prepared by the silver oxide oxidation of p-alkylphenols in carbon tetra~hloride.~~ When salicyl alcohols are oxidized with sodium periodate the spiro-epoxycyclohexa-2,4-dienones(60) are formed.94 These are stable if a bulky alkyl or halogen substituent is present in the nucleus otherwise they under- go Diels-Alder dimerization.On photolysis they give the corresponding sali- cylaldehydes in high yield. The synthesis of tetraquinocyclobutane was mentioned in last year’s Report ;95 it is now found that oxidative coupling of the acetylenic phenol (61) gives the bright orange diquinocyclobutene (62).96The so-called stilbene quinone (63) readily undergoes acid-catalysed hydration to the diphenyl- methanealdehyde (64).” OH R R (61) R = Bu‘ 0 OH RfJR RQR CH I CHCHO CH R 0.R OR OH 0 (64) (63) R = Bu‘ Diels-Alder additions of o-quinone dimethide generated by thermal re-arrangement of the Dewar isomer (65) are >98 % stereo~pecific.~~ Intra-molecular additions to o-quinodimethides discussed in last year’s Report.93 L. K. Dyall and S. Winstein J. Amer. Chem. SOC.,1972,94 2196. 94 H. D. Becker T. Bremhott and E. Adler Tetrahedron Letters 1972 4205. 95 H. Heaney Ann. Reports (B) 1971 68 521. 96 S. Hanff and A. Rieker Tetrahedron Letters 1972 1451. 97 L. Taimr and J. PospiSil Tetrahedron Letters 1972 4279. 98 N. L. Bauld F. R. Farr and C. S. Chang Tetrahedron Letters 1972 2443. 578 J. W.Barton have now been carried out photochemically ;99 cyclizations are also possible where the dienophilic function is carbonyl 'OOa,b azomethine or nitrile loob as with 1-acylbenzocyclobutenes which give chromenes (Scheme 5). Ooa Dianions Scheme 5 of ad-diphenyl-o-quinone dimethides are formed by the reaction of potassium metal with cis-and trans- 1,2-diphenylbenzocyclobutenesin methyltetrahydro- furan at -78 "C.From the products found on subsequent treatment with di- chlorodimethylsilane it is concluded that the ion formed from the cis-compound has the (E,Z) configuration (66) and that from the trans-isomer the (E,E) con-figuration (67) i.e. that the ring-opening reactions are conrotatory." ' Diels-Alder addition of tetrachlorocyclopropene to the dimethide from trans-1,2-diphenylbenzocyclobutenehas been used as the basis of a synthesis of the cyclo- propa[h]naphthalene (68). O2 It may be mentioned here that the photolysis of 2 Ph Ph Ph (48) trans-1,2-diphenylbenzocyclobutenehas given the dihydroazulene (69) by a rearrangement thought to be initiated by homolytic cleavage of the 1,2-bond.lo3 Thermolysis of the cis-dideuteriated sulphone (70)at 430 "Cgives almost entirely 99 W. Oppolzer and K. Keller Angew. Chem. Internar. Edn. 1972 11 729. lo" (a)R.Hug H. J. Hansen and H. Schmid Helv. Chim. Acta 1972,55 10;(6)W. Oppol-zer Angew. Chem. Internat. Edn. 1972 11 1031. N. L. Bauld C. S. Chang and F. R. Farr J. Amer. Chem. SOC.,1972,94 7164. lo2 A. R. Browne and B. Halton J.C.S. Chem. Comm. 1972 1341. '03 M. Sauerbier Tetrahedron Letters 1972 547. Aromatic Compounds 579 trans-1,2-dideuteriobenzocyclobutene,this being consistent with disrotatory formation of o-quinone dimethide by extrusion of sulphur dioxide followed by conrotatory closure to benzocyclobutene. 'O4 Thermal ring-opening of benzo- cyclobuten-1-01s is via a conrotatory process where the (E)-dienols (71)are formed esc; H P @ \ \ \H H 'D H (70) (71) R = H or Ph preferentially as shown by the adducts formed with maleic anhydride.'05 The o-quinone dimethide (72) has been characterized in solution ;on irradiation it yields 9,lO-phenanthrocyclobutene.'O6 An elegant synthesis of the [2,2]para- cyclophane (73) has been achieved by dimerization of the p-quinone dimethide (74) generated in situ from biallenyl and dimethyl acetylenedicarboxylate. O7 (72) Me0,C' C0,Me Quinones.-The y-radiolysis of 1,4-quinones in benzene results in nuclear phenylation ;'O8 n-allylnickel bromide complexes have been used to introduce ally1 substituents. 'O9 The Thiele-Winter acetoxylation of quinones has been reviewed;' lo' further studies of the reaction show that t-butyl groups are often replaced by acetoxy and that acetoxylation rarely occurs ortho to a t-butyl substituent.lob Oxidation of 2-benzylphenol with 2,3-dichloro-5,6-dicyanoben-zoquinone in methanol gives 2-benzyl-l,4-benzoquinone, not 4-hydroxybenzo- phenone as previously reported ; the coupled product (75) is an isolable intermediate.' ' ' The oxidative rearrangement of polyporic acid (76) to the pulvinic acid dilactone (77) takes place smoothly in dimethyl sulphoxide-acetic anhydride ; the mechanism of the reaction has been discussed in relation to the lo' J. R. Du Manoir J. F. King and R. R. Fraser J.C.S.Chem. Comm. 1972 541. B. J. Arnold and P. G. Sammes J.C.S. Chem. Comm.1972 1034. lo' J. P. Anhalt E. W. Friend and E. H. White J. Org. Chem. 1972 37 1015. lo' H. Hopf Angew. Chem. Internat. Edn. 1972 11 419. J. G. Wilson and J. W. Sweeting Austral. J. Chem. 1972 25 1877 2383. log L. S. Hegedus E. L. Waterman and J. Catlin J. Amer. Chem. SOC.,1972,94 7155. 110 (a) J. F. W. McOmie and J. M. Blatchly Org. Reactions 1972 19 199; (h) J. M Blatchly R. J. S. Green and J. F. W. McOmie J.C.S. Perkin I 1972 2286. 'I' J. M. Singh and A. B. Turner J.C.S. Perkin I 1972 2294. 580 J. W.Barton OH CH2Ph 0 0 HOp h ~0 oPh H 0 ph$0 Ph NC OH (76) (77) known biosynthetic conversion of (76) into (77). ' ' Photochemical and thermal reactions of 2-substituted naphthoquinones with ynamines have given inter alia the cyclobutenes (78).Eliminations with removal of the angular (R') sub-stituent were only possible with (78 ;R' = SEt R2 = Ph) and (78 ;R' = OC0,-Me R2 = Ph) when dimers of the corresponding naphthocyclobutadienes 0 (78) R' = OMe OAc OCO,Me or SEt R2= CO,Me CN;or Ph were obtained. '' The thermal reaction of phenanthrene-9,lO-quinonewith sodium chlorodifluoroacetate gives the carbonate (79 ; Z = >C=O) probably via (79;Z = >CF,) formed by the 1,4-addition of difluorocarbene. Surprisingly when the lithium salt is employed the main product is the hydroxy-ketone (80); it is suggested that this results from attack of chloride ion on epoxide (81) the corresponding 1,2-adduct of difluorocarbene. ' 'I2 R. J. Wikholm and H. W. Moore J. Amer. Chem.SOC.,1972,94 6152. 'I3 M. E. Kuehne and H. Linde J. Org. Chem. 1972,37 4031. 'I4 M. Derenberg and P. Hodge J.C.S.Perkin I 1972 1056. Aromatic Compounds 58 1 Cyc1ophanes.-The stereochemistry of [2,2]metacyclophanes has been re-viewed' ' and an alternative nomenclature for cyclophanes has been put for- ward.' l6 Syntheses of [7]meta-' 170 and [7]para-cyclophanes,' 17* of the syn- and anti-forms of [2,2] (1,4)anthracenophane lI8 and of optically active [2,2]meta- cyclophane' and doubly bridged cyclophanes l2 have been described. Further studies of multilayered [2,2]paracyclophanes are reported including an X-ray crystallographic study of a four-layered compound ' syntheses of optically active derivatives with three and four layers,12' and of a six-layered compound.123 Cycloalkylation of benzene using 2,Z'-bis(hydroxymethy1)diphenylmethane gives a remarkably good yield (75%) of the nine-membered ring compound (82; Z = CH,); the analogues (82; Z = 0 or S) are obtainable in the same way.'24 (82) Nuclear magnetic resonance methods have been used to study the rates of in- ternal rotation of the benzene rings in bridge-substituted paracyclophanes. ' Additions of bromine and deuterium bromide to 1,2-dehydro[2,2]paracyclophane follow a cis stereochemical course. The addition products and their diastereomers undergo silver-ion-assisted acetolysis with retention of configuration. 26 As with the solvolysis of l-tosyloxy[2,2]paracyclophane discussed in last year's Report,'27 these results are best interpreted as involving the formation of ben- zylic ions which are delocalized over both benzene rings.Photo-oxygenation of benzo[2,2]paracyclophane in methanol gives rise to the [2,2]metaparacyclo- phane (84),presumably by solvolytic rearrangement of the initially formed endoxide (83). Thermal rearrangement of l-~iny1[2,2]paracyclophanein the 115 F. Vogtle and P. Neumann Angew. Chem. Internat. Edn. 1972 11 73. K. Hirayama Tetrahedron Letters 1972 2 109. 'I7 (u)S. Fujita S. Hirano and H. Nozaki Tetrahedron Leffers 1972 403; (b) N. L. Allinger and T. J. Walter J. Amer. Chem. SOC.,1972 94 9268. " T. Toyoda 1. Otsubo Y. Sakata and S. Misumi Terruhedron Letters 1972 I73 I. H. W. Gschwend J. Amer. Chem. SOC. 1972 94 8430. M. Nakazaki K. Yamamoto and M. Ito J.C.S.Chem. Comm. 1972. 433. 12' H. Mizuno K. Nishiguchi T. Otsubo S. Misumi and N. Morimoto Terrahedrnn Letters 1972 498 I. 122 M. Nakazaki K. Yamamoto and S. Tanak J.C.S. Chem. Comm. 1972,433. T. Otsubo Z. Tozuka and S. Mizogami Tetrahedron Letters 1972 2927. I 24 T. Sato K. Uno and M. Kainosho J.C.S. Chem. Comm. 1972 579. 125 M. Nakazaki K. Yamamoto and S. Okamoto Bull. Chem. SOC.Japan 1972 45, 1562; S. E. Potter and I. 0.Sutherland J.C.S. Chem. Comm. 1972 754. R. E. Singler and D. J. Cram J. Amer. Chem. SOC.,1972 94 3512. H. Heaney Ann. Reports (B) 1972. 68 539. 12' H. H. Wasserman and P. M. Keehn J. Amer. Chem. SOC.,1972 94. 298. 582 J. W.Barton range 100-160 "C gives cis-l,2-dehydro[4,2]paracyclophane(85) via biradical intermediates.29 Transannular reactions of [2,2]metacyclophane to give 4,5,9,10-tetrahydropyrenederivatives are well known. Recently quantitative rearrangement to 1,2,3,3a,4,5-hexahydropyrene (86) has been observed when @/ (85) [2,2]metacyclophane is heated at 60 "C with iodine in benzene. 130 Deuterium-labelling experiments indicate the operation of an intermolecular hydrogen- transfer mechanism. 3 Non-benzene Systems Three- and Four-membered Rings.-Cyclopropenone has now been obtained in a pure stateI3' and many of its reactions have been studied.'32 When it is treated with bromine at -30 "C attack occurs at carbonyl rather than at the ring double-bond forming thecyclopropeniumsalt (87 ;R = X = Br) which rearranges to trans-p-bromoacryloyl bromide on warming to 0 "C ; with triethyloxonium fluoroborate at 0 "C the ethoxypropenium fluoroborate (87 ;R = Et X = BF,) is obtained.Diethoxycyclopropenone (88 ;R = OEt) 'three-cornered acid' diethyl ester has been obtained in 107; yield by irradiation of diethoxycyclobutene-1,2-dione (89 ;R = OEt) the diethyl ester of squaric acid. '33 Attempted hydrolysis of (88; R = OEt) resulted in decomposition and gave some squaric acid (89; R = OH). The corresponding photo-induced ring-contraction of 4,4-dichloro-2 (87) (88) (89) M. H. Delton and D. J. Cram J. Amer. Chem. SOC.,1972 94 1669. 130 T. Sat0 and K. Nishiyama J.C.S. Chem. Comm. 1972 163; J. Org. Chem. 1972. 37 3254. j3' R. Breslow and M. Oda J. Amer. Chem. SOC.,1972 94. 4787. 13' R. Breslow M.Oda and J. Pecoraro Tetrahedron Letters 1972 4415 4419. '33 E. V. Dehmlow Tetrahedron Letters 1972 1271. Aromatic Compounds 3-diphenylcyclobutenone gives 3,3-dichloro-1,2-diphenylcyclopropene,easily hydrolysed by water to diphenylcyclopropenone (88; R =Ph).'33 Whereas the condensation of diphenylcyclopropenone with phenylmalonodinitrile in acetic anhydride leads to the quinone dimethide (90) that with phenylcyanoacetone gives the triafulvene (9 1). 34 Calculations and n.m.r. spectroscopic studies have suggested that the preferred KekulC structure for benzocyclopropene is (92); Ph Ph however an X-ray crystallographic study of the ester (93) gives no indication of this type of bond fixation and if anything the 3,4 and 5,6 bonds are longer than the mean value and the fused rings are not quite coplanar.'35 The chemistry of phenylcyclobutenediones has been reviewed.l3 Low-temperature matrix preparations of cyclobutadiene by the photolysis of a-pyr~ne'~'~ and of the bridged cyclobutene (94),' 37b have been reported.Treat- ment of the aromatic-type palladium complex (95) with ethylenebis(dipheny1- phosphane) gives the highly hindered cyclobutadiene (96) which although \/ \/ \/ \/ isolable reacts rapidly with oxygen to give inter alia the corresponding furan. 13* An X-ray study is in progress to determine the bond orders in the four-membered ring of (96). There is evidence that the dianion of cyclobutadiene (97) a Huckel aromatic system is generated when cis-3,4-dichlorocyclobutenereacts with sodium naphthalide at -40 "C as quenching with deuteriomethanol gives H.U. Wagner R. Seidl and H. Fauss Tetrahedron Lerters 1972 3883. '35 E. Carstensen-Oeser B. Muller and H. Durr Angew. Chem. Internat. Edn. 1972 11 422. '36 W. Ried and A. H. Schmidt Angew. Chem. Internat. Edn. 1972 11 997. 13' (a) C. Y. Lin and A. Krantz J.C.S.Chem. Comm. 1972 1111; (b) S. Masamune M. Suda H. Ona and L. M. Leichter J.C.S. Chem. Comm. 1972 1268. 13' H. Kimling and A. Krebs Angew. Chem. Internat. Edn. 1972 11 932. 584 J. W.Barton 3,4-deuteriocyclobutene,together with the syn and anti dimers of cyclobuta-diene.13' A direct synthesis of the dianion of squaric acid (98) has been achieved by the electrolytic reduction of carbon monoxide dissolved under pressure in an aprotic solvent containing tetrabutylammonium bromide.40 The thieno- cyclobutadiene derivative (99) results from a Wittig reaction of 3,4-diphenylcyclo- butenedione. There appears to be extensive conjugation in the molecule and the n.m.r. spectrum indicates the presence of a paramagnetic ring current arising from the four-membered ring.141 Predictably the reaction of (99) with tetra- cyanoethylene is a [2 + 21 cycloaddition to the four-membered ring rather than a [4 + 21 addition to the hetero-ring. Vacuum pyrolyses of the cis-and truns-isomers of the epoxide (100) have given the parent furocyclobutadiene (101). 142 Compared with (99) it is highly reactive and although moderately stable in solution it dimerizes slowly to the bisfurocyclo-octatetraene(102).Five- Seven- and Nine-membered Rings.-Thermogravimetric analysis of reactions of tetraphenyldiazocyclopentadienewith various substrates containing heteroatoms indicates that carbene formation is the initial common step. 143 Several salts of the pentachlorocyclopentadienideanion with large cations have been isolated and characterized spectroscopically. 144 Dechlorination of the perchlorocyclopentadiene (103) with triethyl phosphite gives hexachlorofulvene (104) in 94 %yield.145" Under Diels-Alder conditions (104) can act as an electron- deficient diene and as a dienophile; it reacts with nucleophiles at C-6 and is protonated at C-l.'45b Fulvenes have been obtained from the ring-expansion 139 J. S. McKennis L. Breuer J. R. Schweiger and R.Pettit J.C.S. Chem. Comm. 1972 365. 140 G. Silvestri S. Gambino G. Filardo M. Guainazzi and R. Ercoli Gazzetta. 1972 102 818. 141 P. J. Garratt and K. P. C. Vollhardt J. Amer. Chem. SOC.,1972,94 1023. 142 K. P. C. Vollhardt and R. G. Bergman J. Amer. Chem. Soc. 1972 94 8950. I43 B. H. Freemann G. S. Harris B. W. Kennedy and D. Lloyd J.C.S. Chem. Cumm. 1972 912. 144 G. Wulfsberg and R. West J. Amer. Chem. SOC.,1972 94 6069. 145 (a)E. T. McBee E. P. Wesseler D. L. Crain R. Hurnaus and T. Hodgkins J. Org. Chem. 1972 37,683; (6)E. T. McBee E. P. Wesseler R. Hurnaus and T. Hodgkins ihid. p. 1100. Aromatic Compounds c1 clFlc1 \I cl&cl \I CI c1 CI c1 (103) ( 104) of methylenecyclopropenes with ynamines.These products themselves behave as enamines and undergo further ring-expansion on treatment with dimethyl acetylenedicarboxylate to give heptafulvenes (Scheme 6). 46 The vinylogous NC CN Ph-CSC-NEt PhNgh Ph Ph Ph NEt MeO,CC~CCO Me I NC CN Scheme 6 fulvalene (105) has been isolated in pure form. In solution at room temperature it isomerizes rapidly and quantitatively to (106) which rearranges to the ben- zenoid isomer (107) at 80 "C.14' Full details of the synthesis of 1-methylpentalene have been published; the spectra of the compound and its reactivity confirm (105) 14' T. Eicher and T. Pfister Tetrahedron Letters 1972 3969. 14' H. Sauter and H. Prinzbach Angew. Chem. Internat. Edn. 1972 11 296. 586 J. W. Barton that it is non-aromatic.148 The isolation of sandwich complexes of pentalene with iron cobalt and nickel has been re~0rted.I~~ Treatment of octachloro-bicyclo[3,3,0]octa-l,3,7-trieneor its 1,4,6-isomer with antimony pentachloride has given the bis(hexach1oroantimonate) of a dication C8Cl,2+,which from the simplicity of its infrared spectrum is thought to be the hexachloropentalenyl dication (108). '50 Cl c1 Cycloheptatrienyldiazomethane decomposes thermally or photochemically by three pathways to yield heptafulvene (109) cyclo-octatetraene or benzene together with acetylene. 15' Perchloroheptafulvene has been obtained by the thermal isomerization of (1 10). 52 Treatment of cycloheptatrienyl-7-carboxylic acid chloride with triethylamine at -20°C generates the highly reactive hepta- fulvene keten (111).153" A number of cycloadditions of (111) have been des- cribed with 2-methoxytropones it reacts to give inter aka the heptafulvalenes (1 12).153bOn the basis of its n.m.r.spectrum and dipole moment it has been HH (109) (110) (1 11) (112) R = HorBr suggested that methylenenorbornadiene (1 13) should be regarded as a bicyclo- heptafulvene with a significant contribution from the charge-separated form (114).154 Tropolone reacts with the 1,2-diphenylcyclopropeniumion to give ion (1 15) isolable as the perchlorate salt which on treatment with triethylamine gives the stable fulvalenedione (1 16) formally a quinone of this system.' Dissolution of (116) in acids regenerates cation (115) and there is evidence of further protonation to dication (1 17) at high acidities.14* R. Bloch R. A. Marty and P. de Mayo Buff. Soc. chim. France 1972 2031. 14' T. J. Katz N. Acton and J. McGinnis J. Amer. Chem. Soc. 1972 94 6206; T. J. Katz and N. Acton ibid. p. 3281. 150 K. Kusuda and N. Osaka J.C.S. Chem. Comm. 1972 508. lJ1H. E. Zimmerman and L. R. Sousa J. Amer. Chem. SOC.,1972 94 834. 152 A. Roedig M. Forsch B. Haveux and D. Scheutzow Tetrahedron Letters 1972 2613. 153 (a)T. Asao N. Morita and Y. Kitahara J. Amer. Chem. SOC.,1972 94 3655; (b)T. Asao N. Morita C. Kabuto and Y. Kitahara Tetrahedron Letters 1972 4379. lJ4R. W. Hoffmann R. Schiittler W. Schafer and A. Schweig Angew. Chem. Internat. Edn. 1972 11 512. 15' K. Takahashi and K. Takase Tetrahedron Letters 1972 2227.Aromatic Compounds Ph . HO Ph The cyclization of a doubly-vinylogous fulvalene is the basis of the Hafner-type synthesis of azulenes some particularly facile examples of which have been des- cribed recently,' 56 as have examples of the alternative approach the cyclization of a vinylogous heptafulvene.'57 It is found that the relative acidities of the azuloic acids (1 < 2 < 5 < 6) are consistent with decreasing n-electron density at these positions of the azulene ring system. 15* Stable dibenzononafulvenes ( 1 18) have been obtained by reactions of aromatic aldehydes with the anion of the parent dibenzononatetraene or the ylide (119)' 59 Although the latter reacts readily with oxygen it shows relatively low reactivity towards carbonyl groups and is considered to be intermediate in structure between the corresponding cyclopentadienyl ylide which shows aromatic stabilization and a normal unstabilized phosphorane.' 56 W. Bauer and U. Miiller-Westerhoff Tetrahedron Letters 1972 1021. Is' H. Prinzbach and H. J. Herr Angew. Chem. Interoat. Edn. 1972 11 135. R. N. McDonald and R. R. Reitz J. Org. Chem. 1972 37 2703. M. Rabinowitz and A. Gazit Tetrahedron Letters 1972 721 3361. 588 J. W.Barton A~ulenes.-New reviews of the annulene~,'~' one limited to [lO]annulenes and other (CH) hydrocarbons,"' and the heteronins'62 have been published. The relationship between ring current and proton chemical shifts in annulenes has been in~estigated,'~~ as has the use of the benzenoid proton coupling constants to study the x-election structure of the annulene ring in benzo[n]annulenes.164 New syntheses of bisdehydro-[ 141- -[181- -[22]- and -[26]-annulenes have been reported. 165 Dehydrogenation of the [6 + 41-cycloadduct (120) of butadiene with tropone has given 1,6-rnethano[lO]annulen-ll-one(121).'66 The results of a 0 (120) (121) three-dimensional X-ray crystallographic study on [14lannulene have been reported ;16' the deduced structure is compatible with the n.m.r. spectrum measured at low temperature and shows a considerable departure from planarity. Electron spin resonance has been used to study the relative planarity in a series of bridged [14]ann~lenes.'~* The synthesis of a bisdehydro[ 1S]annulenium cation has been reported;'69 as expected it shows the aromatic character of a 14~- electron system.Favorski rearrangement of the bridged [14lannulene (122) has given after removal of the bridge carboxy-group the hydrocarbon (123).' It is stable and surprisingly resistant towards dehydrogenation. The expected dehydrogenation product (124) was eventually obtained l7Ob by hydride abstrac- tion from (123) with trityl borofluoride and treatment of the resulting cation with "O F. Sondheimer Accounts Chem. Res. 1972 5 81. ''I S. Masamune and N. Darby Accounts Chem. Res. 1972 5 272. '62 A. G. Anastassiou Accounts Chem. Res. 1972 5 281. '63 R. C. Haddon Tetrahedron 1972 28 3613 3635. D. Creme and H. Giinther Annulen 1972 763 87. ''' K. Fukui T.Nomoto S. Nakatsuji and M. Nakagawa. Tetrahedron Letters 1972 3 157; M. Iyodo and M. Nakagawa ibid. pp. 3 161,4253 ; M. Iyodo and M. Nakagawa J.C.S. Chem. Comm. 1972 1003. 16' S. Ito H. Ohtani S. Narita and H. Honma Tefrahedron Letters 1972 2223. lb7 C. C. Chiang and I. C. Paul J. Amer. Chem. Soc. 1972 94 4741. '" F. Gerson K. Miillen and E. Vogel J. Amer. Chem. SOC.,1972 94 2924. P. D. Howes and F. Sondheimer J. Amer. Chem. SOC.,1972 94 8263. ''O (a)E. Vogel and H. Reel J. Amer. Chem. SOC.,1972,94,4388; (b)H. Reel and E. Vogel Angew. Chem. Internat. Edn. 1972 11 1013. 16' Aromatic Compounds water. Its spectra are in keeping with those of (123) and other bridged annulenes of this type and indicate an important contribution by the Kekule structures with a peripheral 147c system.Electrochemical reduction of [16lannulene has given the radical anion and the dianion,I7' both of which appear to be planar. Unlike that of [18]annulene the n.m.r. spectrum ofthe dianion is not temperature- dependent and the resonance of the inner protons appears at very high field; it is concluded that the delocalization is better in the dianion than in [181annulene by ca. 6 kcal mol- '. The thermolysis of [18]annulene at 90 "Cgives an isomeric material probably containing (125) and angular isomers of it which on further heating gives benzene and 1,2-benzo-1,3,7-cyclo-octatriene. Low-temperature photolysis of the isomer mixture gives benzene and [12]annulene the bicyclododecapentaene (126) being a likely intermediate in both processes (Scheme 7).'72 Syntheses designed to A 90 "C A d -or hv -100°C L J 1 A Scheme 7 lead to the tetradehydro[l8]annulenes (127; R' = Ph R2 = Bu') and (127; R' = Bu' R2 = Ph) give the same product thus providing chemical evidence of the identity of the acetylenic and cumulenic linkages in this system.'73 The upper limit for aromaticity in [4n + 2lannulenes was discussed in last year's Report.'74 Another [26]annulene a bisdehydro-derivative has now been shown to sustain a diamagnetic ring current. 75 "I J. F. M. Oth H. Baumann J. M. Gilles and G. Schroder J. Amer. Chem. Soc. 1972 94 3498. K. Stockel P. J. Garratt F. Sondheimer Y. de Julien de Zelicourt and J. M. Oth J. Amer. Chem. SOC.,1972 94 8644. T.Nomoto K. Fukui and M. Nakagawa Tetrahedron Letters 1972 3253. H. Heaney Ann. Reports (B) 1971 68 543. M. Ioda and M. Nakagawa Tetrahedron Letters 1972 4253. 590 J. W.Barton Ill II I II Ill II R2 R' (127) The aromaticity of annulenones has been investigated using an HMO method. 176 A simple route to certain bisdehydroannulenones has been des- cribed.' 77 Base-catalysed condensation of the acetylenic aldehyde (128) with acetone gives the diacetylene (129) which undergoes oxidative coupling to the [13]annulenone (130) in 45-5004 yield. The method can be extended for the 0 0 synthesis of [151- and [17]-annulenones ; similar syntheses have given bis- dehydro[ 17lannulenones containing ring heteroatoms. 78 A l0n-electron analogue of tropolone (131) has been synthesized.' 79 Like tropolone it forms salts with both acids and bases giving rise to the delocalized ions (132) and (133).Syntheses of the cyclopenta- and cyclohepta-phenalenones (134) and (135) are reported.'*' Their n.m.r. spectra indicate that the peripheral conjugated systems can sustain a diamagnetic ring current and they may be regarded as bridged annulenones. B. A. Hess and L. J. Schaad Tetrahedron 1972 28 5299. 17' P. D. Howes E. LeGoff and F. Sondheimer Tetrahedron Letters 1972 3691 3695. P. J. Beeby and F. Sondheimer J. Amer. Chem. Soc. 1972 94 2128; R. H. McGirk and F. Sondheimer Angew. Chem. Internat. Edn. 1972 11 834. 179 J. Reisdorf and E. Vogel Angew. Chem. Internat. Edn. 1972 11 2 19. I. Murata K.Yamamoto T. Hirotsu and M. Morioka Tetrahedron Letters 1972. 33 I 3389. Aromatic Compounds 4 Polycyclic Compounds The modern concept of aromaticity and its application to polycyclic benzenoid systems forms the subject of a recent monograph embodying a wealth of ex- perience in this field.’*l An annelation reaction of general utility has been reported. For example the trimethine (136) reacts with the carbanion from 1-naphthoa.cetonitri1e to give the base (137) which undergoes thermal cyclization to 4-cyanophenanthrene in 91 % yield.182 The relative ease of photocyclization of diarylethylenes has been related to the sum of the free valence indices in the first excited state (CF*) at the two positions which become bonded. There is further evidence of a lower limit for CF* below which cyclization does not occur.183 Photolysis of 2,2’-distyrylbiphenyl gives the cyclobutane (138) as the product of kinetic control but a tetrahydropyrene probably (139) as that of thermodynamic control. By irradiation under oxidative conditions the benzo[c]chrysene derivative (140) is formed in addition to the dehydrogenation Ph Ph ’” E. Clar ‘The Aromatic Sextet,’ Wiley London 1972. C. Jutz and J. M. Wagner Angew. Chem. Internat. Edn. 1972 11 315. D. D. Morgan S. W. Horgan and M. Orchin Tetrahedron Letters 1972 1789; T. Sat0 and T. Morita Bull. Chem. SOC.Japan I972,45 1548. 592 J. W.Barton product of (139). 184 The corresponding thermal reaction gave the all-cis isomer of (1 38).Intramolecular thermal rearrangements of 1,8-distyryInaphthalenesand related compounds have been reported. Octamethylnaphthalene has been synthesized ;'86a reactions of this and other polymethylnaphthalenes have been described.' 86b Thedianions ofnaphthalene '87 and acenaphthylene'" have been generated by the action of n-butyl-lithium on 1,4-dihydronaphthalene and acenaphthene respectively. The former has been characterized as bis[(tetramethylethylenediamine)lithium(~)]naphthalenide and the crystal structure of this salt has been determined. There is continued interest in the photochemical reactions of naphthalenes. 1,4-endo-Peroxides are formed in good yield by the dye-sensitized photo-oxygenation of di- and poly-methyl- na~htha1enes.I~~ The structure of the photodimer of methyl 2-naphthoate has been determined ;190 irradiation of the methyl naphthoates in the presence of sodium borohydride results in reduction of the unsubstituted ring to give the 5,8-dihydro-derivatives; naphthalene itself gives a mixture of 1,6dihydronaph- thalene and tetralin."' The photoaddition of 2,3-dimethylbut-2-ene to 2-naph- thonitrile in methanol gives adducts (141) and (142) incorporating a molecule of the solvent ; a mechanism involving ion-pair formation is suggested.lg2 -4-H' 'H OMe (141) ( 142) Full reports have been published of the synthesis and reactions of the macro- cyclic diacetylene (143) and related compounds having acetylenic functions in close proximity.193 Electrophilic additions to (143) and hydrogenation over a Lindlar catalyst proceed with transannular coupling of the acetylene groups as does the reaction with iron pentacarbonyl which gives the complex (144) of 184 W.H. Laarhoven and J. H. M. Cuppen J.C.S. Perkin I 1972. 2074. 185 J. Meinwald and J. A. Kapecki J. Amer. Chem. Soc. 1972,94 6235; S. F. Nelson and J. P. Gillespie J. Amer. Chem. SOC.,1972 94 6237 6238. I86 (a)H. Hart and A. Oku J. Org. Chem. 1972 37 4269; (6)A. Oku Y. Ohnishi and F. Mashio ibid. p. 4265; H. Hart and A. Oku ibid. p. 4275. I87 J. J. Brooks W. Rhine and G. D. Stucky J. Amer. Chem. SOC.,1972 94 7346. 188 L. D. Kershner J. M. Gaidis and H. H. Freedman J. Amer. Chem. Sor. 1972 94 985,4400. 189 H. H. Wasserman and D. L. Larsen J.C.S. ChPm. Comm. 1972 253 H. Hart and A.Oku ibid. p. 254. 1 YO P. J. Collin D. B. Roberts G. Sugowdz D. Wells and W. H. F. Sasse Tetrahedron Letters 1972 32 1. 191 J. A. Barltrop and R. J. Owers J.C.S. Chem. Comm. 1972 592. 192 J. J. McCullough and W. S. Wu J.C.S. Chem. Comm.. 1972 1136. I93 H. A. Staab E. Wehinger. and W. Thorwart Chem. Ber. 1972. 105 2290; H. A. Staab H. Mack and A. Nissen. ibid. p. 23 10;H. A. Staab and B. Draeger. ibid.. p. 2320. 593 Aromatic Compounds tetrabenzo[a,c,g,i]biphenylene.Birch reduction of biphenylene gives the tetra- hydro-compound (143 together with (147) which is presumably formed by valence isomerization of the tetrahydrobiphenylene (146) 194 compound (147) was previously isolated from the addition of benzyne to cyclohexa-l,3-diene.Full details of the synthesis of 2-thianorbiphenylene (148) have appeared ;I9' 2,6-diazobiphenylene(149)or the 2.7-isomer has been made by the flash photolysis of pyridine-3-diazonium-4-carboxylate'9h and the corresponding perfluoro- compounds by pyrolysis of the silver salt of 2,5,6-trifluoropyridine-3,4-dicarboxyl-ic acid.'97 A Wittig reaction of glyoxal with ylide (150) has given cyclo-octa- [d,ef]biphenylene (151) which contains a fused [8 + 4]n-electron system and shows definite antiaromatic properties.'98 I" C. A. Matusak and L. Dickson J. Org. Chem. 1972,37 3345. P. J. Garratt and K. P. C. Vollhardt J. Amer. Chem. Soc. 1972 94 7087. Ig6 J. Kramer and R. S. Berry J. Amer. Chem. Soc. 1972 94 8336. 19' E. G. Bartoch A. Golloch and P.Sartori Chem. Ber. 1972 105 3464. C. F. Wilcox J. P. Uetrecht and K. K. Grohman J. Amer. Chem. Soc. 1972.94.2532. 594 J. W.Barton The generation and reactions of naphtho[b]cyclobutadiene (152) have been investigated,199 and an interesting synthesis of cyclobutadienopleiadene (1 54) is reported.200 In the final step treatment of the diol(153) with hydrochloric acid in tetrahydrofuran resulted in decomplexation and concomitant elimination to F;e(CO) \ I & give (154). The cycloheptatriene derivative (155 ; R = H) has been synthesized. It rearranges to fluorene at 170°C and is converted into the salt (156) of the homobiphenylene cation by reaction with trityl hexafluorophosphate. The salt reacts with sodium methoxide to give 9-methoxyfluorene together with the ether (155; R = OMe).20' PF,-The acidities of a number of 9-alkylfluorenes have been measured.O2 A synthesis of 1H-cyclopent[c,d]indene (157) is reported,203 but attempts to iso- merize it to the 2aH-compound which is expected to be relatively acidic failed. The carbene phenalenylidene (158) has been generated. It is found to give 4 ., 1 2 ( 157) (1 58) (159) ''' M. P. Cava and A. C. Hsu J:Amer. Chem. SOC.,1972,94 6441. 'O0 B. W. Roberts and A. Wissner J. Amer. Chem. SOC.,1972 94 7168. 'O' L. Lombard0 and D. Wege Tetrahedron Letters 1972 4859. '02 A. Streitweiser C. J. Chang and D. M. E. Reuben J. Amer. Chem. Soc. 1972 94 5730. 203 B. L. McDowell and H. Rapoport. J. Org. Chem.. 1972 37 3261. Aromatic Compounds 1,2- and 1,6-cycloadducts to the peri-positions with a~rylonitrile~~~" and cyclo- he~tatriene,''~' the results obtained being best interpreted as involving the triplet species (159).The addition of dichlorocarbene generated from chloroform and aqueous sodium hydroxide in an emulsifying system to phenanthrene has given the stable dibenzonorcaradiene (160) which rearranges to 6-chlorodibenzo[a c]-tropylium chloride at 140 OC.'05 The mechanism of photodimerization of 9-anthroic acid has been investigated.206 Conjugated acyclic dienes and cyclohexa- 1,3-diene undergo [4 + 41 photoaddition to the 9,lO-positions of anthracene. With acyclic dienes the metastable trans-adducts (161) are primary ( 160) (161) products being converted into the stable cis-adducts on sensitized irradiation ; 9-cyanoanthracene and 9-anthraldehyde undergo stereospecific [4 + 21 addition In the photoreaction of anthracene with cyclopentadiene both [4 + 41 and [4 + 21 additions are observed the former being reversible.208 Analogous [4+ 41 photoadditions to naphthalene have been reported very recently.209 Syntheses of dibenzobarrelene2 lo and polycyclic triptycene2 derivatives from linear acenes have been described.1-Bromomethyltriptycene is found to be extremely unreactive under solvolysis conditions being some 10' 2-fold less reactive than the unbridged l-chlor0-2,2,2-triphenylethane.~ Pentacene reacts with sulphur to give a stable hexasulphide for which the structure (162) is pro-posed ;hexacene behaves similarly.' ' The kinetics of protonation of perylene radical anions have been inve~tigated.''~ The reaction of perylene radical cation with cyanide ion gives equal amounts of 1- and 3-cyanoperylene together with the parent hydrocarbon.'l' It is now shown that electrophilic nitration of '04 (a) I.Murata T. Nakazawa and S. Yamamoto Tetrahedron Letters 1972 2749; (6)I. Murata T. Nakazawa and T. Amanishi ibid. p. 5089. '05 G. C. Joshi N. Singh and L. M. Pande Synthesis 1972 317. 206 D. 0.Cowan and W. W. Schmiegel J. Amer. Chem. Soc. 1972 94 6779. '07 N. C. Yangand J. Libman J. Amer. Chem. SOC.,1972,94,1405; N. C. Yang J. Libman L. Barrett M. H. Hui and R. L. Loeschen J. Amer. Chem. Soc. 1972 94. 1408. '08 G. Kaup Angew. Chem. Internat. Edn. 1972 11 718.209 N. C. Yang J. Libman and M. Savitzky J. Amer. Chem. SOC.,1972 94 9226. O H. P. Figeys and A. Dralants Tetrahedron 1972 28 303 1. *I1 M. Sugihashi R. Kawagita T. Otsubo Y. Sakata and S. Misumi Bull. Chem. SOC. Japan 1972,45 2836. 'I2 J. W. Wilt and T. P. Mallory J. Org. Chem. 1972 37 2781. '13 E. P. Goodings D. A. Mitchard and G. Owen J.C.S. Perkin I 1972 1310. G. Levin C. Sutphen and M. Szwarc J. Amer. Chem. Sac. 1972. 94. 2652. 215 H. J. Shine and C. V. Ristagno J. Org. Chem. 1972 37 3424. 596 J. W.Barton s-s-s perylene gives the I-nitro-derivative’ l6 in addition to the 3-nitroperylene which was obtained previously. The major product of photoaddition of thymine to 3,4-benzopyrene has the singly-bonded structure (163) and not a cyclobutane bridge as previously thought.217 The theory has been advanced that 3,4- benzopyrene may be able to couple at other positions and thus cross-link DNA strands.H (163) The name ‘circulene’ is suggested for polycyclic compounds which possess a closed ring of aromatic or heteroaromatic nuclei. In this class are coronene (planar) corannulene (bowl-shaped) and the thiophen derivative (la),the synthesis of which is reported.218 Because of the relationship between the inner and outer diameters of (164) it is expected to have a ‘corrugated’ outer edge. ’I6 J. J. Looker J. Org. Chem. 1972 37 3379. ’I7 G. M. Blackburn R. G. Fenwick and M. H. Thompson Tetrrrhedron Letters 1972 589. ‘IM J. H. Dopper and H. Wynberg Tetrahedron Letters 1972 763.