10 Aromatic Compounds By R. G. COOMBES Department of Chemistry The City University St. John Street London EC1 V 4PB 1 Introduction The graph-theoretical approach'" to conjugation and resonance energies of hydrocarbons involving the concept of conjugated circuits [see Ann. Reports (B) 1976 73 p. 2231 has been developed as a new approach to the aromaticity" of polycyclic systems those having only conjugated circuits with (4n +2) r-electrons when the Kekulk structures are examined being defined as aromatic. The approach allows the prediction of the relative aromaticity of some systems contain- ing both (4n +2) and (4n) conjugated circuits. Resonance energies obtained2" using Clark aromatic sextet theory correlate well with the Dewar-type resonance energies from graph theory [see Ann.Reports (B) 1976 73 p. 201],26 and the latter approach has been applied to a wide range of systems.*" An essentially identical graph-theoretical approach derived however as the result of a different line of argument has also been rep~rted.~ The degree of electron delocalization in cyclic systems has been propo~ed,~ with theoretical justification to be a function both of orbital phase continuity requirements and of the disposition of donor and acceptor component systems. The condition under which the normal criterion for delocalization based on the number of m-electrons becomes invalid in 'effectively discontinuous systems' has been presented. An interesting polycyclic system with different possible delocalized structures is the dicyclo-octatetraenot 1,2 :4,5]benzene dianion which has been shown' not to sustain a localized 14welectron diamagnetic ring current.It is probably best described by the planar 20m-electron structure (l) where the electrostatic repul- sion is adequate to overcome the strain energy and the antiaromatic character of the ion. The diamagnetic susceptibility exhalt at ions of several 7-substituted cyclo- heptatrienes vary as a function of the size of the substituent are very large and are indicative of a substantial ring current.6 It has been suggested that in view of the low resonance energies and the bond alternation these systems should simply be M. RandiC (a) Tetrahedron 1977,33 1905; (6)J. Amer. Chem. SOC.,1977,99.444. J.-I. Aihara (a) Bull.Chem. SOC.Japan 1977 50 2010; (b) ibid. p. 3057; (c) J. Amer. Chem. Soc. 1977,99,2048. I. Gutrnan M. Milun and N. TrinajstiC J. Amer. Chem. Soc. 1977 99 1692. S. Inagaki and Y. Hirabayashi,J. Amer. Chem. SOC.,1977,99,7418. L. A. Paquette G. D. Ewing S. Traynor. and J. M. Gardlik I. Amer. Chem. Soc. 1977,99 6115. R. F. Childs and I. Pikulik Canad. J. Chem. 1977,55259. 215 R. G. Coombes (3) R =P-NO~C~H~ (2) described as diatropic rather than homoaromatic and the authors question whether diatropicity is a useful or even valid criterion of aromaticity. Two homoaromatic ions have been reported' which involve the interruption of the cyclic conjugated system by a three-carbon bridge. The benzannelated homocyclopentadienyl anion (2) is formed by treatment of the parent hydrocarbon with butyl-lithium.Examination of several systems' has provided no evidence for trishomoaromatic stabilization. Kinetic and product studies of the hydrolysis of (3) for example indicate86 no participation of the double bonds. A mathematical model has been devised which enables the magnitude of the interaction between homoaromatic centres to be evaluated.8c The sites of protonation of simple substituted benzenes in the gaseous phase have been receiving attention. Ring protonation of phenol has been calculated9 by ab initio MO theory to be 63 kJ mol-' more favourable than 0-protonation and a deuterium-exchange experiment gave an approximate value in agreement with this. Measurements of N1,core ionization energy suggest that aniline is protonated on nitrogen in the gaseous phase but anomalies exhibited by NN-dimethylaniline may be due to ring protonation.lo" Pulsed ion cyclotron resonance spectroscopy confirms1ob the former result although ab initio MO theory suggests that ring protonation is only 4-13 kJ mol-' less favourable. After comparison of the rela- tive base strengths of some rneta- and para-substituted anilines in the gaseous phase with the values for these compounds in aqueous solutions it has been suggested that for rn-toluidine and m-anisidine protonation occurs on the ring."' The opposing views concerning the nature of aryl "F n.m.r. polar field effects have now been largely reconciled. Calculations (CND0/2) on the 1-substituted 4-p-fluorophenylbicyclo[2,2,2]octaneshave indicated"" the dominance of 7r-polarization effects direct field effects accounting for only about 25% of the polar effects on "F chemical shifts.In this connection the distinction between T-polarization and the 7r-inductive effect has been emphasized."b 13CN.m.r. data A. Dagan D. Bruck and M. Rabinovitz Tetrahedron Letters 1977 2995. (a) L. A. Paquette M. J. Kukla S. V. Ley and S. G. Traynor J. Amer. Chem. SOC.,1977,99,4756;L. A. Paquette H. C. Berk C. R. Degenhardt and G. D. Ewing ibid. p. 4764; (6)L. A. Paquette P. B. Lavrik and R. H. Summerville J. Org. Chem. 1977 42 2659; (c) L. A. Paquette T. G. Wallis T. Kempe G. G. Christoph J. P. Springer and J. Clardy J. Amer. Chem. SOC.,1977 99 6946. D. J. DeFrees R.T. McIver and W. J. Hehre 1.Amer. Chem. Soc. 1977,99 3853. (a) R. G. Cave11 and D. A. Allison J. Amer. Chem. SOC.,1977,99,4203; (b)S. K. Pollack J. L. Deviin tert K. D. Surnrnerhays R. W. Taft and W. J. Hehre ibid. p. 4583; (c)K. D. Summerhays S. K. Pollack R. W. Taft and W. J. Hehre ibid. p. 4585. (a)W. F. Reynolds Tetrahedron Letters 1977 675; (6) W. Kitching M. Bullpit D. Gartshore W. Adcock T.-C. Khor D. Doddrell and I. D. Rae J. Org. Chem. 1977 42 2411; (c) D. F. Ewing S. Sotheeswaran and K. J. Toyne Tetruhedron Letters 1977,2041; (d)W. Adcock andT.-C. Khor ibid. p. 3769. Aromatic Compounds 217 provide unequivocal supporting evidence for 7r-polarization in the phenyl group of 1-substituted phenylbicyclo[2,2,2]octanes although there is not quantitative agreement with the above calculations."' The relative importance of the w-polarization and direct field effects has also been estimated by comparison of 19F n.m.r.data on the para-fluoro- and meta-fluoro-systems and the result is in good agreement with the estimate mentioned above."d Reversed I9Fn.m.r. polar field effects are realized in the orrho-fluoro-analogues. The modified version of the general theory of structure-property relationships previously proposed,'2n has been applied'26 to the problem of the relationships between substituent chemical shifts and substituent reactivity parameters and some success has been achieved in predicting whether a single parameter or a pair of parameters should be utilized in a particular situation. The non-additive behaviour of I3C chemical shifts in para-disubstituted benzenes [see Ann.Reports (B) 1976,73,p. 2031 has been reinter~reted.'~ The data for the 1-and 2-positions are in fact predicted additively and those for the 4-positions may be accurately described by linear proportionality relationships with the appropriate substituent chemical shifts in monosubstituted benzenes. The proportionality relationships were interpreted in terms of localized changes in the excitation term of the paramagnetic shielding expression for 13Cshifts determined largely by the ion- ization potential of the 4-substituent. Substituent effects on the 13C shifts of the side-chain C atoms in meta- and para-disubstituted benzenes have also been studied and inter~reted.'~ Further reviews illuminate the reactivity-selectivity debate.150~6 Considerations concerning change of solvent and temperature have been emphasized as has the fact that if substituents influence bond cleavage as well as bond formation a more reactive species can be more selective than a less reactive one in spite of an 'earlier' transition state.An illustrative example is the competition of aniline and 3-chloroaniline for substituted benzenesulphonyl chlorides where selectivity increases with reactivity. This argument is similar to that concerning desolvation of ions in reactions of charged species [see Ann. Reports (B) 1976 73 p. 2031.This latter effect has however been regarded as unimportant. l6 2 Benzene Isomers Oxides and Homobenzenes The photoelectron spectrum of benzvalene has been reported17 and this has led to the suggestion that the unusual U.V.absorption (217nm) may be due to the superimposition of the first member of a low-lying Rydberg series on a valence- shell transition. Full details of the preparation separation and aromatization of meso- and dl-l,l'-dimethyl-3,3'-bicyclopropenyls have now appeared." The aromatization l2 M. Godfrey J.C.S. Perkin IZ,(a)1975 1016; (b) 1977 769. 13 B. M. Lynch Canad.J. Chem. 1977,55,541. l4 N. Inamoto S. Masuda K. Tori and Y. Yoshimura Tetrahedron Letters 1977,737;J. Bromilow R. T. C. Brownlee and D. J. Craik Austral. J. Chem. 1977 30 355. Is (a)B. Giese Angew Chem. Internat. Edn. 1977,16 125; (6)A. Pross Ado. Phys. Org. Chem. 1977 14,69.l6 C. D. Ritchie and M. Sawada J.Amer. Chem. SOC.,1977,99 3754. l7 P. J. Harman J. E. Kent T. H. Gan J. B. Peel and G. D. Willett J. Amer. Chem. Soc. 1977,99,943. Is J. H. Davis K. J. Shea and R. G. Bergman J. Amer. Chem. SOC.,1977,99 1499. R. G. Coombes passes through an intermediate in which the stereochemical distinction between the diastereoisomers has disappeared. The data are most consistent with Scheme 1for gas-phase aromatization and probably for reaction in solution also. 1,4-Hexamethylene-Dewar-benzene is formed slowly and apparently quantitatively on irradiation of a solution of [6 Jparacyclophane in cyclohexane; warming the solution to 50-90 “Cresults in a clean and rapid reversion to the thermodynamic- ally more stable precursor.” A polemical reminder of the limitatiop of the use of correlation diagrams illustrated by that for the ‘allowed’ but not spontaneous thermal aromatization of benzvalene has been given;20 the words ‘allowed’ and ‘forbidden’ are only of use when there is a choice between two stereochemically different modes of reaction.Benzvalene has been shown2* to undergo a degenerate rearrangement catalysed by silver(1) ion in competition with the known catalysed aromatization. Evidence was presented that the reaction of the silver(1) ion with the bicyclobutane moiety is a reversible process involving the intermediacy of (4). Scrambling does not occur with other bicyclobutane derivatives. (4) (5) Ts=p-MeC6H4S02 (6) The synthesis of 3-t-butoxycarbonylbenzeneoxide has been accomplished” and in contrast to the 4-isomer it exists mainly as the benzene oxide valence tautomer.Attempts to effect nucleophilic addition on this molecule failed although the 4-isomer reacts uia its oxide tautomer. Benzene oxide itself is converted into (Z,Z)-hexa-2,4-dienedial on oxidation with pero~y-acids,~~ the oxidative ring opening involving the oxepin tautomer. The reaction of [Rh(C0)2C1]2 with ben- zene oxide and arene oxides at room or lower temperatures in chloroform or methanol leads to both deoxygenation and the formation of phenol by mechanisms involving initial Lewis acid catalysis.24 The equilibrium between the azepine (5) and its bicyclic valence tautomer (6) has been directly observed2’ in the range -70 to +40 “C [AG*,(S)-D (6) =46 kJ mol-’I.l9 S. L. Kammula L. D. Iroff M. Jones jun. J. W. van Straten W. H. de Wolf and F. Bickelhaupt J. Amer. Chem. SOC.,1977.99,5815. 2o J. J. C. Mulder J. Amer. Chem. SOC.,1977 99 5177. 21 U. Burger and F. Mazenod Tetrahedron Letters 1977,1757. 22 B. A. Chiasson and G. A. Berchtold J. Org. Chem. 1977,42 2008. 23 S. G. Davies and G. H. Whitham. J.C.S. Perkin I 1977 1346. ” R. W. Ashworth and G. A. Berchtold Tetrahedron Letters 1977,343. ” H. Prinzbach H. Babsch H. Fritz and P. Hug Tetrahedron Letters 1977 1355. Aromatic Compounds A new synthetic pathway to substituted trans-bishomobenzenes is now avail- able.26 Thermolysis of di bromote tracyclo[ 5,1,0 02*4,03*5]octanes readily available from tricyclo[4,1 ,0,02v7]hept-3-enes gave the species (7) which was readily Reagents i Li-Bu‘OH in THF Scheme 2 dehalogenated (Scheme 2).Studies of the flash pyrolysis (485“C)of 2H1-labelled hexahomobenzene (8) have shown2’ that there is not sufficient interaction energy amongst the three constituent rings to allow for bond shifting a result in agreement with last year’s report concerning trioxahexahomobenzenes [see Ann. Reports (B) 1976,73 p. 2051. 3 Benzene and its Derivatives General.-The second part28 of the review of the organic photochemistry of ben- zene is concerned with photoreactions with other molecules. The photoaddition of methyl phenylpropiolate to benzene has been shown29a to form the tetra-cyclo[3,3,0,02*4,03~6]oct-7-ene (9) as major product in addition to the known 1-methoxycarbonyl-8-phenylcyclo-octatetraene, and in proportions which vary with Ph the wavelength of irradiation.The products are also photochemically intercon- vertible. The presumed precursor to both products at least in the photoaddition reaction is the corresponding bicyclo[4,2,0]octatriene,and species with this struc-ture have now been from the photoaddition of alkynes to hexafluoro- benzene. 26 R. T. Taylor and L. A. Paquette J. Amer. Chern. SOC.,1977,99,5824. ” M.R. Detty and L. A. Paquette Tetrahedron Letters 1977 347. D.Bryce-Smith and A. Gilbert Tetrahedron 1977,33,2459. 29 (a)A. H. A. Tinnemans and D. C. Neckers J. Amer. Chem. SOC.,1977,99,6459;(b)B. Sket and M. Zupan ibid. p. 3504. 220 R. G. Coombes The major product of photocycloaddition of 2,3-dihydropyran to benzene is the endo-1,2-adduct formed in a process of surprising selectivity having a quantum efficiency greater than those previously observed for any mode of addition of an ethene to ben~ene.~'" The 1,3-photocycloadditions of ethyl vinyl ether to toluene and in particular those to anisole are promoted by polar solvents indicating that the 1,3-addition is not the general homopolar process as was previously believed.306 The synthetic utility of the 1,3-addition of alkenes to anisole has been explored as a route to the bicyclo[3,2,1 jocten-8-one sy~tern.~" 1,3-Cycloadducts are the major products of photochemical reactions30d between benzene and both (10) ethene [to give (lo)] and propene in contrast to the behaviour expected from the ionization potentials of these alkenes.28 The authors suggest that electron affinities are more relevant for these alkenes which are electron acceptors relative to benzene.2,2-Dimethyl- 1,3-dioxole undergoes 1,2-photoaddition to benzene as expected from its ionization potential but gives in contrast to normal alkenes the exo- addu~t.~'' Photoreactions of hexafluorobenzene have been the subject of some attenti~n.*~~*~'~-~ The [2 +2 Jcycloaddition of cyclopentene in cyclohexane solution proceeds3'" to give predominantly the exo-adduct (85YO) and reaction with norbornadiene gives predominantly (80%) the exu-~yn-isomer.~~~ It may be that these results reflect the stereochemistry of precursor complexes formed in the absence of irradiation.28 Studies of the photoaddition of aliphatic amines to ben- zene have been reported in An important covers the field of cobalt-catalysed cyclo-oligomeriza- tion of alkynes and full details326 of the formation of benzocycloalkenes using this approach and of their synthetic utility have appeared.The method has also been applied in an elegant synthesis of the steroid nucleus.32c The key step involves the reaction of the precursor (1 1) (Scheme 3) which is available by convergent series of reactions from hexa- 1,5 -diyne and 2-methylcyclopent-2-enone.The overall yield is 28% based on the former. Some attention has been paid to the mechanism(s) of the trimerization. Evidence has been presented33a that there exist at least two distinct mechanisms for the production of aromatic compounds from the reaction of alkynes with metallocyclopentadienes if these are intermediates.One path 30 (a)A. Gilbert and G. Taylor Tetrahedron Letters 1977,469; (b) J.C.S. Chem. Comm. 1977 242; (c) J. A. Ors and R. Srinivasan J. Org. Chem. 1977,42 1321; (d)M. F. Mirbach M. J. Mirbach and A. Saus Tetrahedron Letters 1977 959; (e)H.-D. Scharf and J. Mattay ibid. p. 401. 31 (a)B. Sket and M. Zupan J.C.S. Chem. Comm. 1977,365; (b) Tetrahedron Letters 1977,281 1; (c)M. Zupan B. Sket and B. Pahor ibid. p. 4541; (d)M. Bellas D. Bryce-Smith M. T. Clarke A. Gilbert G. Klunkin S. Krestonosich C. Manning and S. Wilso,i J.C.S. Perkin Z 1977 2571. 32 (a) K. P. C. Vollhardt Accounts Chem. Res. 1977,JO 1; (b) R. L. Hillard tert.and K. P. C. Vollhardt J. Amer. Chem. Soc. 1977 99 4058; (c)R. L. Funk and K. P. C. Vollhardt ibid. p. 5483. 33 (a)D. R. McAlister J. E. Bercaw and R. G. Bergman J. Amer. Chem. SOC.,1977 99 1666; (b)P. Caddy M. Green E. O'Brien L. E. Smart and P. Woodward Angew. Chem. Znternat. Edn. 1977,16 648. Aromatic Compounds Reagents i bis(trimethylsilyl)acetylene-CpCo(C0)2 Scheme 3 involves initial co-ordination of the alkyne and the other direct pathway may involve Diels-Alder addition. Other of 5-indenylrhodium(~) complexes have however suggested the intermediacy of metallocyclopent-2-enes. Ynamines (R'C-CNR;) undergo cyclotrimerization in moderate yields on warming in acetonitrile solution over Ni" or Ni' but not Cu' catalysts and they form the unsymmetrical triaminobenzene (12).34a A useful synthesis of hindered Rtot Rt /NR; NR; (1 2) aromatic amine~~~' also involves ynamines which react with a-pyrone carbon dioxide being lost spontaneously; methyl 2-(NN-diethylamino)-3-methylbenzoate for example is formed in 79% yield from 1-diethylaminoprop-1-yne.Two procedures have been reported for the preparation of metu-bisannelated benzenes (13). The first procedure35a (Scheme 4)utilizes Corey's modified pinacol x= (CH2)rn (13) y= (CH2)n Reagents i Mg(HgkTiCl4; ii POC13-pyridine; iii acrylic acid; iv 1O0/o Pd/C A Scheme 4 reduction and the involves the initial reaction of 2-cyclo-alkenylcycloalkanones with malonodinitrile acid-catalysed cyclization and efficient removal of the nitrile function from the ortho-amino-nitrile.A simpler albeit low-yield preparation of benzo[ 1,2 :3,4]dicyclobutane (13; n = m = 1) has been de~cribed,~~" and the procedure of the earlier Diels-Alder method has been (a)J. Ficini J. d'Angelo and S. Falou Tetrahedron Letters 1977 1645; (6) T. A. Bryson and D. M. Donelson J. Org. Chem. 1977,42 2930. 35 (a)M. E. Isabelle D. H. Lake and R. H. Wightman Canad.J. Chem. 1977,55,3268; (b)J. Sepiol B. Kawalek and J. Mirek Synthesis 1977 701. " (a)E. Giovannini and H. Vuilleumier Helu. Chim. Acta 1977 60 1452; (b)R. P. Thummell and W. Nutakul J. Org. Chem. 1977 42 300. 222 R. G. Coombes applied'6b to a series of meta- and para-bisannelated benzenes involving fused 4-and 5-membered rings. Syntheses of benzocyclobutenes abound.The flash vacuum pyrolysis of tolyl propargyl ethers provides a low-yield source of aromatic-ring-methylated benzo-cyclopropene~.~' Full details of the thermal and electron-impact syntheses of benzocyclobutene itself and examples having oxygenated substituents on the aryl ring have appeared.38" The latter type of compound however can indeed be prepared38b by the ready and efficient cyclization of the corresponding ortho- lithio(2-bromoethy1)benzenes. 3,4-Bis(trimethylsilyl)benzocylobutene has been synthesized for the first time.39 Interestingly the trimethylsilyl group at the 4-position is five-hundred times more susceptible to hydrolysis than that at the 3-position. The reaction of aaaa-tetrabromo-o-xylene with pentacarbonyliron gave4' a facile synthesis of mainly trans- 1,2-dibromobenzocyclobutene.Pyrolysis (600"C; 0.3 s) of 2-methylbenzoyl chlorides gave the corresponding benzocyclobutenones in good yield^.^' Benzocycloalkenones have been synthesized4*" in good yield by treatment of o-bromophenylalkanoic acids with butyl-lithium at low temperatures and the procedure allows for the presence of some functionality in the ring being formed. An extension of the oxidative rear- rangement of styrene derivatives by thallium(II1) nitrate a general method for the ring expansion of benzocycloalkanones of this type. They are first subjected to a Wittig procedure whereby a methylene carbon which may be substituted is inserted selectively between the aromatic ring and the carbonyl group.The reactions of polyfluoro-aromatic compounds with aryl and alkyl radicals have been reviewed.43 Some scavenging reactions of phenyl radicals from the thermal decomposition of p heny lazo trip hen ylme t hane are nearly diffusion-controlled in media of low viscosity and are so controlled in media of high viscosity leading to relative rates approaching unity for pairs of scavenger^.^^ Competition with slower reactions at higher viscosities has given absolute rate constants. The reactions of the phenyl radicals do not show a clear selectivity- reactivity relationship. Correlations for the rate of hydrogen abstraction from substituted toluenes by isopropyl and t-butyl radicals by a Hammett equation have been re~orded.~' The authors suggest that substituent effects on both the structure of the transition state (consideration of polar resonance structures) and of the initial state (bond dissociation energy) are important and that contrary to common practice both should be considered.The ability of aromatic nitro-compounds to increase the yields of biaryls in arylation reactions is now believed46 to involve 37 J. M. Riemann and W. S. Trahanovksy Tetrahedron Letters 1977 1863. (a) R. J. Spangler B. G. Beckmann and J. H. Kim J. Urg. Chem. 1977,42,2989;(6)P. D. Brewster J. Tagat C. A. Hergrueter and P. Helquist Tetrahedron Letters 1977,4573. 39 R. L. Hillard tert. and K. P. C. Vollhardt Angew. Chem. Internat. Edn. 1977 16 399. 40 R. Victor Transition Metal Chem. (Weinheim) 1977 2 2.41 P. Schiess and M. Heitzmann Angew. Chem. Internat. Edn. 1977 16 469. 42 (a) R. J. Boatman B. J. Whitlock and H. W. Whitlock jun. J. Amer. Chem. SOC.,1977,99,4822; (b) E. C. Taylor C.-S. Chiang and A. McKillop Tetrahedron Letters 1977 1827. " L. S. Kobrina Russ. Chem. Rev. 1977 46 348. 44 R. G. Kryger J. P. Lorand N. R. Stevens and N. R. Herron J. Amer. Chem. Soc. 1977,99 7589. 45 W. H. Davis jun. and W. A. Pryor J. Amer. Chem. SOC.,1977,99 6365. 46 R. Henriquez and D. C. Nonhebel J. Chem. Res. (S) 1977,253. Aromatic Compounds 223 oxidation of the intermediate arylcyclohexadienyl radicals by an electron-transfer mechanism examples having electron-withdrawing substituents being more effective. The authors of the original experimental work on the photobromination of dihalogeno-benzenes have reacted4’ to the suggestion that ipso-substitution is involved [see Ann.Reports (B),1976,73 p. 2081 and show that such substitution is neither required nor precluded by their results. A deamination reaction that uses alkyl nitrite and copper(r1) halide has been used in several processes that are of synthetic imp~rtance.~~ The reagent rapidly converts arylamines into aryl chlorides and bromides in high yield,48b and a Meer- wein-type arylation of alkenic substrates giving a-halogeno-P-aryl compounds occurs if the reaction is carried out in acetonitrile or acetone solutions containing the sub~trate.~~‘ Good to excellent yields of phenols are formed from aryl bromides under basic conditions on reaction of the derived Grignard reagent with molybdenum peroxide-pyridine-hexamethylphosphoramide.4g A procedure for the formation of phenols from diazonium ions which avoids the usual highly acidic conditions and is considered to be the method of choice for new cases consists in adding copper(!) oxide to a dilute solution of the arenediazonium salt in the presence of a large excess of copper(I1) nitrate.” Electrophilic Substitution.-Studies of the gas-phase pyrolysis of some 1-arylethyl acetates have leds1 to the conclusion that the Baker-Nathan order of electron release by alkyl groups is a solvation phenomenon and also that hyperconjugation from C-C bonds is greater than that from C-H bonds.These studies also suggest that the solvolysis of aa-dimethylbenzyl chlorides is exceptionally sensitive to steric hindrance to solvation and that this limits the suitability of the derived (++ constants for the description of electrophilic aromatic substitution.The pro- tonations of toluene and ethylbenzene in super-acid media have been rein- vestigateds2 and the presence of the ortho-protonated species has been established; the results are now in agreement with gas-phase studies. Details of the gas-phase reaction of the t-butyl cation with phenol and anisole to give initially predominantly t-butylated oxonium ions have appeared.53a In contrast to the t-butyl cation which exhibits both substrate and positional selec- tivity the isopropyl cation does not show substrate selectivity in its reactions with arene~.~~~ It does however exhibit a measurable degree of positional selectivity and its reactions are complicated by secondary dealkylation and isomerization processes.Chlorobenzene is isopropylated at a rate only half that of toluene and the predominant ortho-substitution is again attributeds3‘ to initial attack at the substituent. The attack of CH5+ and CzH5+ ions on halogeno-benzenes causess4 47 P. Gouverneur and J. P. Soumillion Bull. SOC. chim. belges 1977 86 647. 48 M. P. Doyle J. F. Dellaria jun. B. Siegfried and S. W. Bishop J. Org. Chem. 1977 42 3494; M. P. Doyle B. Siegfried and J. F. Dellaria jun. ibid. p. 2426; M. P. Doyle B. Siegfried R. C. Elliott and J. F. Dellaria jun. ibid. p. 2431. 49 N. J. Lewis S. Y. Gabhe and M. R. DeLaMater J. Org. Chem.1977,42 1479. 50 T. Cohen A. G. Dietz jun. and J. R. Miser J. Org. Chem. 1977,42 2053. 51 E. Glyde and R. Taylor J.C.S. Perkin ZZ 1977 678. ’* D. FgrcaSiu M. T. Melchior and L. Craine Angew. Chem. Znrernat. Edn. 1977,16 315. 53 (a)M. Attins F. Cacace G. Ciranni and P. Giacomello J. Amer. Chem. Soc. 1977,99 4101 5022; (6) ibid. p. 2611; (c)M. Attina and P. Giacomello Tetrahedron Letters 1977 2373. 54 M. Speranza and F. Cacace J. Amer. Chem. Soc. 1977,99 3051. 224 R. G.Coombes extensive dehalogenation by two competitive routes which lead to pro-todehalogenation and methyldehalogenation and are a reflection of the ambident nucleophilic character of the substrates. Mono- and di-substituted benzenes have been brominated by 80Br+ in the gas phase.” The relative rates led to good u+,p+ plots (p’ = -0.9) and the data on toluene agreed with the Brown selectivity rela- tionship.Perrin has asserteds6 that aromatic nitration is not always a conventional elec- trophilic substitution but that for some reactive substrates the mechanism involves the transfer of an electron from the aromatic to the nitronium ion followed by collapse of the radical pair. Experiments quoted in support of this proposal involve the determination of anodic half-wave potentials for nitrogen dioxide and some aromatic compounds and one involving the generation of the naphthalene radical cation in the presence of nitrogen dioxide which gives the normal isomer ratio of nitro-naphthalenes. The author appears to confuse and combine the different approaches of others to encounter pairs and wcomplexes and ignores the possi- bility of catalysis by nitrous acid which must surely affect the results obtained for some substrates which are used to support the proposed mechanism.Catalysis by nitrous acid is evident in the formation of the ipso-Substituted u-complex (14)that is formed on nitration of NN-dimethyl-p-toluidine in 70-77% sulphuric acid.57 It slowly rearranges to the NN-dimethyl-4-methyl-2-nitroaniliniumion. 6MeZ OzN Me Q It seems that the problem of the identity of the electrophile for nitration by nitric acid in acetic anhydride has at last been The observed rates have been shown to be sensitive to the presence of trace impurities at [HN03]< 0.9 mol dmP3 and a comparison of the true rate constant for the reaction of the electrophile and toluene with that for the reaction in acetic acid indicates that reaction indeed involves the nitronium ion.A study of some model compounds has led to a reconsiderati~n~~~ of the nitration of acetanilide and it now seems clear that this molecule is in fact nitrated as the unprotonated species by nitric acid in aqueous sulphuric acid. For anisole in the same mediums9* the ortho :para ratio varies considerably and an explanation has been offered that involves the combined effects of hydrogen bonding and a diffusion-controlled reaction. With 0-and p-methylanisole ipso-substitution at C-Me is of importance and in the latter case this leads to 4-methyl-2-nitrophenol ” E. J. Knust J.Amer. Chem. SOC.,1977,99 3037. s6 C. L. Perrin J. Amer. Chem. Soc. 1977 99,5516. *’ K. Fujiwara J. C. Giffney and J. H. Ridd J.C.S. Chem. Comm. 1977 301. 58 N. C. Marziano R. Passerini J. H. Rees and J. H. Ridd J.C.S. Perkin 11 1977 1361. 59 (a)R. B. Moodie P. N. Thomas and K. Schofield J.C.S.Perkin 11,1977 1693; (b)J. W. Barnett R. B. Moodie K. Schofield J. B. Weston R. G. Coombes J. G. Golding and G.D. Tobin J.C.S. Perkin ZZ 1977.248. Aromatic Compounds as a major product. 4-Methyl-4-nitrocyclohexa-2,5-dienone,a postulated inter- mediate in this reaction has been observed60a as a product from the nitration of p-methylanisole and some other para-substituted toluenes in acetic anhydride. Nitration ips0 to an ethyl group has been observed60b in p-diethylbenzene p-ethyltoluene and almost exclusively in ethylmesitylene.Attempts to activate a biphenyl to nitration ips0 to a phenyl group have however failed. The sensitivity of ipso-nitration to substituent effects has been examined6'" by the competitive study of a series of 5-X-hemimellitene derivatives (X =H F Br OAc OMe and NHAc) and a p+ value (-8>p+ > -12) has been estimated showing a satisfactory agreement with that for nitrodeprotonation. Some synthetic aspects of side-reactions attending aromatic nitration have been reviewed.61 An interesting example of intramolecular trapping of a Wheland intermediate has been observed6*" in the isolation of (15) from the nitration of methyl P-(2,3,4-trimethylphenyl)isovalerate. Another reaction involving initial ipso-substitution is the conversion626 of some p-xylonitrile derivatives into 1-imino- 1,3 -di hydroiso benzofurans [e.g.(1 6) from 2,3,4,6- te trame thy1 benzonitrile]. Solvolytic of the ipso-adducts formed on nitration of hemimellitene have reinforced the view [see Ann. Reports (B) 1976 73,p. 2101 that in >50% sulphuric acid adducts of this type yield the ipso-Wheland intermediate which is then partitioned between trapping by water and rearrangement by in this case sequer.tia1 1,2-shifts of the nitro-group. Full details of the reactions of nitronium salts with pentamethylbenzene have been given,64 confirming that the initially detected species is the l-nitro-l,2,3,4,6-pentamethylbenzenoniumion. The 1,3- dinitro-l,2,4,5,6-pentamethylbenzenonium ion formed from pentamethyl-nitrobenzene undergoes a degenerate rearrangement of a nitro-group from posi- tion 1 to position 5.(15) The mercuric-acetate-catalysed nitration of toluene in acetic acid at 80 "C,which produces 36% 0-,12% m-,and 52% p-nitrotoluenes has been to involve initial mercuration followed by nitrosodemercuration and subsequent oxidation of the nitroso-toluenes. The sulphonations of some polyethyl- and polyisopropyl-benzenes in concen- trated sulphuric acid have been studied.66 The sulphonic acids from 1,2,3,5- and 6o A. H. Clernens M. P. Hartshorn K. E. Richards and G. J. Wright Austral. J. Chem. 1977,30 (a)p. 113; (b)p. 103. 61 H. Suzuki Synthesis 1977 217. 62 (a)M. Shinoda and H. Suzuki J.C.S. Chem.Comm. 1977 479; (6) H. Suzuki M. Koge and T. Hanafusa ibid. p. 341. 63 T. Banwell C. S. Morse P. C. Myhre and A. Vollmar J. Amer. Chem. SOC.,1977,99 3042. 64 A. N. Detsina V. I. Marnatyuk and V. A. Koptyug J. Urg. Chem. (U.S.S.R.),1977,13 122. 65 L. M. Stock and T. L. Wright J. Org. Chem. 1977,42 2875. 66 A. Koeberg-Telder and H. Cerfontain J.C.S. Perkin IZ 1977 717; H. Cerfontain A. Koeberg-Telder and C. Ris ibid. p. 720. 226 R. G. Coombes 1,2,4,5-tetraethylbenzeneare unstable and rearrange into 2,3,4,5-tetraethylben- zenesulphonic acid. With 1,3,5-tri-isopropylbenzene,both rearrangement and dealkylation reactions become of importance. Kinetic and product have shown that the sulphonation of aniline in a large excess of 100-102% sulphuric acid at 25°C proceeds by direct attack of the electrophile (H&O,+) on the aromatic ring of the anilinium ion.Under more aqueous conditions (-97% sulphuric acid) at higher temperatures some reaction occurs by the same mechanism as shown by the formation of rn-aminobenzenesulphonicacid but reaction involving the intermediacy of phenylsulphamic acid may also occur. The latter compound itself at 25 "C,yields67b only u-and p-aminobenzenesulphonic acids by an intermolecular path involving C-sulphonation of the minority 0-protonated species PhNHS03H and subsequent N-desulphonation although at lower acidities solvolysis to the anilinium ion is a competing process.67c The sulphonation of m-aminobenzenesulphonic acid in fuming sulphuric acid has also been studied.68" Studies of methanesulphonanilide (PhNHS0,Me) indicate6*' that sulphonation occurs on the unprotonated species which is in the minority above 84% sulphuric acid.For non-catalytic bromination reactions the process may follow the power-series rate equation and involve a reaction pathway in which more than one molecule of bromine is concerned in the transition state. The suggestion [see Ann. Reports (B) 1975 72 p. 2251 that this will show the same kinetic behaviour as an A-SE2 process in which the rate of the reverse of the first step becomes significant and that the rate expressions for these processes are equivalent is in~orrect.~~" The circumstances under which a distinction may be made have been established and the authors also briefly review the mechanisms available for brominations by molecular bromine.The effect of addition of bromide and perchlorate to the bromination of polymethylbenzenes in 90% aqueous acetic acid shows clearly6" that in this case reaction involves a transition state which includes more than one molecule of bromine. Flow 'H n.m.r. studies have clearly confirmed7' the general- ity of the occurrence of cyclohexadienone intermediates in the bromination of substituted phenols in aqueous acetic acid the lifetime of the intermediates decreasing with the size of alkyl substituents at 2-and 6-positions. 'Free' CT3+cations have been generated from the decay of a radioactive atom in a tritiated methane solution in toluene and ben~ene.~' The attack of the cation on the arene is likely to occur in a much shorter time than is necessary for the formation of an organised solvent sphere.The intermolecular selectivity was indicated by an approximate to1uene:benzene rate ratio of 2.3:1 and a very low but slightly electrophilic intramolecular selectivity was observed The vinylation of some aromatic substrates by a variety of vinyl triflates [e.g. PhC(Me)=C(Ph)OS02CF3] lacking a p C-H bond is possible and it results in " P. K. Maarsen and H. Cerfontain J.C.S. Perkin 11 1977 (a)p. 1008; (b)p. 921; (c) p. 929. (a) P. K. Maarsen R. Bregman and H. Cerfontain J.C.S. Perkin II 1977 1863; (b)P. K.Maarsen and H. Cerfontain ibid. p. 1003. " (a)N. H. Briggs P. B. D. de la Mare and D. HaII J.C.S. Perkin ZZ 1977 106; (6) R.M. Keefer and L. J. Andrews J. Amer. Chem. SOC., 1977,99 5693. 70 C. A. Fyfe and L. Van Veen jun. J. Amer. Chem. SOC., 1977,99 3366. 71 F. Cacace and P. GiacomeIIo J. Amer. Chem. SOC.,2977,99 5477. Aromatic Compounds 227 good yields of styrenes under moderate conditions even in the absence of Friedel-Crafts Nucleophilic Substitution.-Recent developments in the application of linear free- energy relationships between reactivity and physical properties of leaving groups and substrates have been A comparison of some substitution reactions of both neutral and negative nucleophiles in the 2,4-dinitrohalogeno-benzeneswith reactions with organic cations has provided16 further support for the general applicability of the N+ nucleophilicity scale.Selectivity again appears not to be a function of reactivity in these reactions. The photostimulated SRNlreaction between iodobenzene and potassium diethylphosphite in DMSO has been st~died,'~" with particular attention being paid to the initiation and termination stages. The main reaction for the latter is first-order in propagating radical. The initiation however may involve initial absorption by iodobenzene or by a molecular complex of iodobenzene with diethylphosphite. An SRNl reaction in aqueous t-butyl alcohol has been identified746 in the reaction of halogeno-benzenes with phenoxide ion in the presence of sodium amalgam to form benzene and diphenyl ether. A thermally induced SRNl reaction has been in the reaction of bromo- and iodo- benzenes with ketone enolate anions in DMSO at 25°C in the dark and in one example this was well separated from the photostimulated process.Large rate accelerations by added oxygen nitrobenzene and ferric nitrate were however not explained. The reactions of picryl chloride and bromide with the diethylmalonate anion have been studied7' by the stopped-flow technique which enables the stages involving fast formation of a-complexes followed by slower decomposition to the picryl esters to be identified. In the reaction of 2,4,6-trinitroanisol& with n-butylamine to give in the absence of a large excess of the amine N-&-butyl)picramide the 'H n.m.r spectrum of the intermediate u-complex has been recorded using a flow technique.76 The stable intramolecular a-complex (17) formed under basic conditions from N-benzyl-N-(2-hydroxyethyl)picramide cyclizes under the conditions of the Smiles' rearrangement without prior rear- rangement to form 4-benzyl-5,7-dinitro-2,3-dihydrobenzoxazine.77 Kinetic and equilibrium data have been reported7* for the addition of sodium ethoxide to a series of &substituted 2,4-dinitrophenetoles to give a-complexes.The results support the suggestion that interaction with the cation for 1,l -diethoxy-complexes involves the oxygen atoms of the alkoxy-groups attached to C-1 and the ortho-substituents. 72 P. J. Stang and A. G. Anderson Tetrahedron Letters 1977 1485. 73 G. Bartoli and P. E. Todesco Accounts Chem. Res. 1977,10 125; see also G. Bartoli P. E. Todesco and M.Fiorentino J. Amer. Chem. SOC.,1977,99,6874. 74 (a) S. Hoz and J. F. Bunnett J. Amer. Chem. Soc. 1977 99 4690; (b) S. Rajan and P. Sridaran Tetrahedron Letters 1977 2177; (c) R. G. Scamehorn and J. F. Bunnett J. Org. Chem. 1977 42 1449,1457. 75 K. T. Leffek and A. E. Matinopoulous-Scordou Canad. J. Chem. 1977,SS 2656,2664. 76 C. A. Fyfe A. Koll S. W. H. Damji C. D. Malkiewich and P. A. Forte J.C.S. Chem. Comm. 1977 335. 77 V. N. Drozd V. N. Knyazev and V. M. Minov J. Org. Chem. (U.S.S.R.) 1977,13,357. 78 M. R. Crampton J.C.S. Perkin ZI 1977 1442. R. G. Coombes (17) (1 8) (1 9) The formation of the a-complex (18; R = NHPh) from 1,3,5-trinitrobenzene and aniline catalysed by a tertiary amine in DMSO has been shown79a to involve rate-limiting deprotonation by the catalyst of the zwitterionic intermediate (19) and it shows only a small primary kinetic isotope effect.The magnitude of the effect should have relevance to the identification of rate-limiting stages in SNAr reactions (see below). The complex (18; R=NHPh) may also be formed by the reaction of aniline with the complex (18; R = OMe) and the full evidence that in this example a dissociative mechanism involving the intermediacy of tri-nitrobenzene is involved has been given.79b Methoxide ion acts in a similar fashion to the tertiary amine catalyst in the previous reaction but in this case the formation of the intermediate (19) is rate-limiting. It seems that the unreactivity of aromatic as opposed to aliphatic amines towards nitro-aromatic compounds is due to ther- modynamic rather than kinetic factors.Arguments have been presentedsoo that currently available evidence indicates that the specific base-general acid-catalysed mechanism for substitution in activated aromatic compounds by amines involving fast deprotonation and rate- limiting catalysed expulsion of the leaving group is not a significant pathway in protic solvents. Instead the mechanism involves a rate-limiting deprotonation of (20) when the expulsion of the leaving group is relatively fast (e.g.X = OPh) or a rapid equilibrium deprotonation of (20) followed by a rate-limiting non-catalysed H Y (20) expulsion of a very poor leaving group (e.g.on occasions when X = OMe). Catalysis by base in the reaction of piperidine with 2,4-dinitrophenyl4-nitrophenylether in benzene solution has been discussedsob in terms of concerted proton transfer and expulsion of the leaving group the protic base acting as a bifunctional catalyst.Evidence from the uncatalysed reaction of morpholine with 2,4-dinitrophenyl phenyl ether in dipolar aprotic solvents suggests that here also fast deprotonation and slow expulsion of the leaving group does not occur,soc and the authors also 79 (a) E. Buncel W. Eggimann and H. W. Leung J.C.S. Chem. Comm. 1977 55; E. Buncel and W. Eggimann J. Arner. Chem. Soc. 1977 99 5958; (b)E. Buncel J. G. K. Webb and J. F. Wiltshire ibid. p. 4429. (a)C. F. Bernasconi R.H. De Rossi and P.Schmid J. Amer. Chem. SOC.,1977 99 4090; (b) D. Spinelli G.Consiglio and R. Noto J.C.S. Perkin 11 1977 1316; (c) D. Ayediran T. 0.Bamkole J. Hirst and I. Onyido ibid. p. 597. Aromatic Compounds 229 discuss shortcomings of the concerted mechanism for media of low dielectric constant and suggest a modification involving the formation of aggregates within which bond-making and -breaking is not synchronous. Biary1s.-An important modification of the nickel-phosphine-complex-catalysed homo-coupling of aryl halides has been reported,'l and the reaction proceeds catalytically in nickel in the presence of zinc which reduces the Ni" species formed to the active Nio species. The molar ratio PhBr :Zn :PPh3:Ni(PPh3)2CI2 of 1:1:0.3:0.5 in DMF solution at 50°C gave an 89% yield of biphenyl. The presence of iodine greatly accelerated the reaction.Efficient formation of the symmetrical biaryl has been observeds2 when some arylcopper cluster compounds [e.g. (4-MeC6H4)4C~4] react with equimolar quantities of copper triflate. These reactions involve intermediate complexes which may be isolated in some cases Treatment of a variety of aromatic substrates with thallium(rI1) trifluoroacetate in trifluoroacetic acid results in oxidative coupling to giveg3 biaryls in good yield in the absence of powerfully electron-withdrawing substituents probably by a radical cation mechanism; 2,5-dimethylanisole for example is converted into 2,2',5,5'- te trame t hyl-4,4'-dime t hoxybiphen yl. An improved me thods4 has been reported for the conversion of arylmercury (11) chlorides into biaryls using [ClRh(CO),] as a catalyst in the presence of lithium chloride.Thermal decomposition of tetra-aryl-tellurium compounds at 140 "C forms biaryls by a non-radical path~ay.~' The preparation of a wide variety of mixed biaryls in improved yields is possible by the application of phase-transfer catalysis to a Gomberg reaction.86 Potassium acetate in the presence of 18-crown-6 apparently reacts with an arenediazonium ion in non-polar solution to form a diazoanhydride which then decomposes in the presence of various aromatic compounds to form biaryls. The reaction of some arylzinc derivatives with aryl bromides or iodides in the presence of a small amount of a nickel or palladium catalyst [e.g. Ni(PPh,),] provides a general and highly selective route to unsymmetrical biaryl~.'~ Palladium(I1) chloride ( < 0.001 mol equivalent) appears" to be a preferred catalyst in Kharasch-type reactions of aryl Grignard reagents with iodobenzenes in THF.Aromatic nitrogen compounds (nitroso- azo- azoxy- and nitro-benzenes and NN-dialkylaniline N-oxides in addition to aryl-hydroxylamines) are susceptible to nucleophilic attack by benzene to give biphenyls in varying yields under strongly acidic condition^;'^ S-(NN-dimethylamino)-2-methylbiphenyl,for example is formed in 46% yield from NN-dimethyl-p-toluidine N-oxide. A comparison of the rates of solvolysis of some a-(7-X-fluoren-2-yl)benzyl chlorides and cr-(4'-X-biphenyl-4-yl)benzyl halides is consistent with regarding '' M. Zembayashi K. Tamao J.-I. Yoshida and M.Kumada Tetrahedron Letters 1977 4089. '' G. van Koten J. T. B. H. Jastrzebski and J. G. Noltes J.C.S. Chem. Comm. 1977,203; J. Org. Chem. 1977,42,2047. " A. McKillop A. G. Turrell and E. C. Taylor J. Org. Chem. 1977,42 764. 84 R. C. Larock and J. C. Bernhardt J. Org. Chem. 1977,42 1680. 85 D. H. R. Barton S. A. Glover and S. V. Ley J.C.S. Chem. Comm. 1977,266. 86 S. H. Korzeniowski L. Blum and G. W. Gokel Tetrahedron Letters 1977 1871. " E.-I. Negishi. A. 0.King and N. Okukado J. Org. Chem. 1977,42 1821. '' A. Sekiya and N. Ishikawa J. Organometallic Chem. 1977 125 281. 89 T. Ohta K. Shudo and T. Okamoto Tetrahedron tetters 1977 101; K. Shudo T. Ohta Y. Endo and T. Okamoto ibid. p. 105. 230 R. G. Coombes fluorene as a planar biphenyl structure and yields an interplanar angle for biphenyl in accord with other estimates." Both systems respond to substituent effects in a similar way.The overall reactivity to protiodetritiation of 9,lo-dihydrophenanth-rene relative to that of fluorene is consistent with the differences in coplanarity between the and other results confirm that the low reactivity of the a-position of the latter arises from an increase in strain on going to the correspond- ing transition state. The reversible rearrangement of 1-fluoro-9H-fluoren-9-one into the 3-fluoro-isomer in polyphosphoric acid provides the first direct evidence for the complete reversibility of a Friedel-Crafts a~ylation.~~' Quinones and Related Compounds.-A review of photoinitiated reactions of quinones has appeared.92 A convenient synthesis93 of 2-alkyl p-benzoquinones in 65-70°/0 yields involves the oxidation of the p-alkylphenol with thallium(II1) perchlorate in perchloric acid.The reaction of allyl-silanes with various p-benzo- quinones in the presence of titanium tetrachloride gave94 allyl-substituted hydro- quinones whilst 2,fi-disubstituted quinones gave p-allyl-quinols which were presumed also to be intermediates in the former cases. An interesting base- catalysed rearrangement of p-peroxyquinol ethers to give p-quinoxyacetic acid derivatives has been and is shown formally in (21). The suggested mechanism involves initiation by homolysis of the 0-0 bond even at -60 "C. The quinone methide (22) which has a vinyl group in conjugation with the quinone system forms in the solid or in concentrated solution in an aprotic solvent a dimer involving Diels-Alder addition of the terminal double bond of one mole- cule to the side-chain diene system of another molecule.96a The products of nucleophilic addition to (22) have been and with water and methanol both 1,6- and 1,B-adducts are observed.Kinetic studies of the related methylene compound have also appeared.96" Some reactive 1,3-dipoles (R-b=N-X) add to 2,6-di-t-butyl-4-methylenecyclohexa-2,5-dienone to give the 1:1 cycloadducts (23) in high yield providing ready access to the hetero-~pir0[4,5]decatrienone system.96d 90 R. Bolton and R. E. M. Burley J.C.S. Perkin ZZ,1977,426. 91 (a)H. V. Ansell and R. Taylor I.C.S. Perkin ZZ,1977,866;(6)I.Agranat Y. Bentor and Y.4. Shih I. Amer. Chem. SOC.,1977,99,7068. 92 A. V. El'tsov 0.P. Studzinskii and V. M.Grebenkina Rum. Chem. Rev. 1977 46 93. 93 Y. Yamada and K. Hosaka Synthesis 1977 53. 94 A. Hosomi and H. Sakurai Tetrahedron Letters 1977,4041. 95 A. Nishinaga K. Nakamura K. Yoshida and T. Matsuura Chem. Letters 1977 303. 96 (a)J. A. Hemingson Tetrahedron Letters 1977,2967;(b)J.C.S. Perkin ZZ,1977,616; (c)G.Leary I. J. Miller W. Thomas and A. D. Woolhouse ibid. p. 1737; (d) A. D. Woolhouse Austral. J. Chem. 1977,30 1145. Aromatic Compounds 231 An electron-diffraction study of p-xylylene which is widely accepted to have a singlet structure has been made,97a and the conclusion reached that the molecule does not simply contain conjugated double and single bonds.The average C=C bond length is longer than typical values for conjugated double bonds and in good agreement with simple HMO theory. The i.r. and Raman spectra of o-xylylene have been observed for the first time.97b The molecule was generated in the gaseous phase by the reaction of aa'-dibromo-o-xylene with sodium or potassium vapour which had been excited by a microwave discharge and it was then collected in an argon matrix (at 8-30 K). 2,2-Dimethyl-2H-indene (24) shows more than a (24) transient existence.98a It was generated and isolated in a matrix at 77 K but was also generated in a solution at room temperature where it was suficiently stable for its 'H n.m.r. spectra to be recorded. Transient isoindenes have been observed and studied in the flash photolysis of a series of l,l-diaryl-indene~.~~~ Isoindene itself has been im~licated~~' as an intermediate in a photobisdecarbonylation reaction which may be suitable for future matrix-isolation studies.o-Xylylenes have also been observed as transients in the flash photolysis of methylated indan-2-0nes.~~ The intermediacy of the 2H-indenes (26) in the thermal rearrangement of the 1-acyl- 1,3-diphenyl- and -dimethyl-indenes (25) to the corresponding 2-acyl- indenes (27) has been established,'00" and migratory aptitudes have been described. Other studies of the related photochemical rearrangement of 3-substituted 1,l- diphenylindenes have been reported. loob (25) (26) (27) R = Ph or Me; X = e.g. CHO COMe COPh or C02Me Oxidation of 4-aryl-2,6-di-t-butylphenolswith oxygen in the presence of a cobalt(r1) complex followed by decomposition of the intermediate hydroperoxides with toluene-p-sulphonic acid gave the 5-aryl-3-t-butyl-o-benzoquinones in good yield."' Oxidation of a series of o-benzoquinones with lead(1v) acetate in methanol to yield the corresponding dimethyl hexa-2,4-dienedioates in 60-90% 97 (a)P.G. Mahaffy J. D. Wieser and L. K. Montgomery J. Arner. Chem. Soc. 1977,99,4514; (b)K. L. Tseng and J. Michl ibid.,pp. 4840 6154. 98 (a) W. R. Dolbier jun. K. Matsui J. Michl and D. V. Horak J. Amer. Chem. SOC.,1977,99,3876; (b) K. K. de Fonseka C. Manning J. J. McCullough and A. J. Yarwood ibid. p. 8257; (c) R. N. Warrener R. A. Russell and T. S.Lee Tetrahedron Letters 1977,49. 99 K. K. de Fonseka J. J. McCullough and A. J. Yarwood J.C.S. Chem. Comm. 1977,721. loo (a) D. W. Jones and G. Kneen J.C.S. Perkin I 1977 1313; (6)W. A. Pettit and J. W. Wilson J. Amer. Chem. SOC.,1977,99,6372. A. Nishinaga K. Nishizawa H. Tomita and T. Matsuura Synthesis 1977 270. R. G. Coombes yield has been reported."* A pronounced and unexpected solvent effect has been in the reaction of some o-benzoquinones with butadiene; 4-methoxy- 5-methyl-o-benzoquinone for example gave mainly the abnormal spiro-compound (28) in benzene and (29) in methanol. (28) (29) An unusually stable orange-red crystalline o-quinone methide probably (30) has been from the oxidation of 2-cinnamyl-4,5-methylenedioxyphenol with silver oxide and the corresponding methide from 2-(4-methoxybenzyl)-4,5-methylenedioxyphenol rapidly dimerizes in polar solvents.Another stable o-quinone methide (m.pt. 284-5 "C)is (31) prepared'046 by the reaction of 2,4-dimethylphenol with two equivalents of 1,3-benzodithiolylium tetrafluoroborate in (30) (31) acetonitrile and subsequent addition of triethylamine. 6-Acetoxycyclohexa-2,4-dien-1-ones react with Grignard and lithium reagents to give in addition to the expected 1,2-and 1,4-addition an ether which is the product of the formal addition of a carbanion to the carbonyl oxygen.'o5a Ether formation largely supersedes conjugate addition with tertiary and benzylic organometallics. Substituent effects support an electron-transfer mechanism,'056 and further evidencelosc suggests that dialkylmagnesium species are concerned in the case of Grignard reagents.The metallation reaction of electrochemically derived quinone bisacetals [see Ann. Reports (B) 1976 73,p. 2151 has been modified'06 to produce organo- copper-lithium derivatives which will react efficiently with allylic halides to provide a route to some isoprenoid natural products (e.g. cymopol). 4 Cyclophanes A review of the information to be gained concerning steric effects of substituents from conformational studies of cyclophanes has appeared.lo7 [5]Metacyclophane Io2 M. Wiessler Tetrahedron Letters 1977 233. Io3 S. Danishefsky P. F. Schuda and W. Caruthers J. Org. Chem. 1977 42 2179. (a) L. Jurd Tetruhedron 1977 33 163; (b) J. Nakayama K.Yamashita M. Hoshino and T. Takemasa Chem. Letters 1977 789. lo' (a)B. Miller J. Org. Chem. 1977 42 1402; (b) ibid. p. 1408; (c) B. Miller E. R. Matjeka and J. G. Haggerty Tetrahedron Letters 1977 323. '06 P. W. Raynolds M. J. Manning and J. S. Swenton J.C.S. Chem. Comm. 1977 499. lo' H. Forster and F. Vogtle Angew. Chem. Internut. Edn. 1977,16,429. Aromatic Compounds the lowest member of this series to date is a by-product of a synthesis of 3,3'-hexamethylenebicyclopropenyl.'08 Its spectral properties are indicative of unusu-ally high strain and also of high conformational rigidity of the pentamethylene chain. A novel synthesislog of [71-and [8]-paracyclophanes involves flow pyrolysis of the spiroquinodimethanes (32; n =6 and 7) and a simple synthesis"o of some [lO]paracyclophanes [e.g.(33)] involves the reaction of methyl propiolate and aluminium chloride with cyclododecene and thermal dehydrogenation of the resul- ting 2 :1 adducts. [3.3]Paracyclophane is now readily available by a route"'a'b involving the surprisingly efficient (94%) pyrolysis of the disulphone of 2,13-di thia[4.4]paracyclop hane. The di thia[4.4]cyclophanes may also yield' ' the [3.3]cyclophanes on photodesulphurization in triethyl phosphite. Another approach to [3.3]paracyclophanes also suitable for higher cyclic oligomers has involved the construction of aliphatic bridges by a reaction between a bromide and a carbanionic species. Charge-transfer interactions in some [3.3]cyclophanes have been studied.'1'b~1'2a Such interactions may be transmitted through intervening rings in multilayered cyclophanes."2b The general usefulness of the sulphone pyrolysis procedure for the synthesis of many-membered hydrocarbon rings has also been illustrated.' l3 Representatives of the highly strained cyclophanes which have two benzene rings held with three or more ethano-bridges have been synthesized.A new approach to [2.2.2](1,2,4)cyclophane involves the formation of the third bridge in a paracy- clophane deri~ative."~ The chiral [2.2.2](1,2,4)(1,3,5)cyclophane has been synthesized,"' and it has a marked tendency to resinify at room temperature. Its skew geometry causes an upfield shift of one aromatic proton to 74.96. [2.2.2.2](1,2,3,5)Cyclophane, the first to have a consecutive arrangement of three lo* J.W. van Straten W. H. de Wolf and F. Bickelhaupt Tetrahedron Letters 1977,4667. 109 J. W. van Straten W. H. de Wolf and F. Bickelhaupt Rec. Trav. chim. 1977,96 88. 'lo B. B. Snider and N. J. Hrib Tetrahedron Letters 1977 1725. '11 (a) D. T. Longone S. H. Kiisefogiu and J. A. Gladysz J. Org. Chem 1977 42 2787; (6) M. W. Haenel A. Flatow V. Taglieber and H. A. Staab Tetrahedron Letters 1977 1733; (c)T. Otsubo M. Kitasawa and S. Misumi Chem. Letters 1977 977; (d)T. Synmyozu K. Kumagae T. Inazu and T. Yoshino ibid. p. 43. (a) T. Shinmyozu T. Inazu and T. Yoshino Chem. Letters 1977 1347; (6)H. A. Staab U. Zapf and A. Gurke Angew. Chem. Internat. Edn. 1977 16 801; see also H. A. Staab and C. P. Herz ibid. p. 392; C.P. Herz and H. A. Staab ibid. p. 394. J. Grutze and F. Vogtle Chem. Ber. 1977,110 1978; L. Rossa and F. Vogtle J. Chem. Res. (S) 1977 264. 114 S. Trampe K. Menke and H. Hopf Chem. Ber. 1977,110,371. 115 M. Nakazaki K. Yamamoto and Y. Miura J.C.S. Chem. Comm. 1977,206. R. G. Coombes ethano-bridges has been synthesized"6a by a procedure analogous to that applied to the (1,2,4,5)-isomer [see Ann. Reports (B) 1975,72 p. 2311; an X-ray structure of the latter has now appeared."6b [2.1.2.1]Paracyclophane has been synthesized in 14% yield by a modified Wurtz reaction under high dilution on 4,4'-dichlorome th yldiphenylme thane. l7 The application of the Wittig procedure to the synthesis of [2.2.2.2]cyclophanetetraenes [see Ann.Reports (B) 1975,72 p. 2311 has been extended,118a but an attempt to form [2.2.2]paracyclophanediene from 4,4'-bibenzyldicarbaldehyde and the bisphosphonium salt of 1,4-di(bromoethyl)benzene failed but gave a small yield of all-ci~-[2~]paracyclophanetetraene, providing an improved route to [26]paracyclophane. The Wittig approach has also led to the synthesis"sb of [2.0.2.0]metacyclophanediene,which on photolysis in the presence of iodine gave bi-4,5-phenanthrylene (34) a non-planar analogue of [8]circulene. The reaction of benzene-l,3,5,-tricarbaldehyde and the bisphosphonium salt of 1,4-di(bromo-methy1)benzene has given1l8' a route to the bicyclophane (35) which should exist as a rapidly equilibrating mixture of enantiomeric conformations. The 'benzene-Stevens' route [see Ann.Reports (B) 1975,72 p. 231 Scheme 21 has been applied"' to the syntheses of both syn-and anti-[2.2](1,4)naphthaleno-phane-1,13-dienes. [2](1,5)Naphthaleno[2]paracyclophane and its 2,6-isomer where the bridging is from one benzene ring of the naphthalene moiety to the other have been synthesized for the first time.120a The naphthaleno-paracyclo- phane (36) has also been prepared'20b for the first time by two routes and in spite 116 (a)W. Gilb K. Menke and H. Hopf Angew. Chem. Internat. Edn. 1977.16 191; @)A.W. Hanson Acta Cryst. 1977 833 2003. 117 S. Sergheraert P. Marcinal and E. Cuingnet Tetrahedron Letters 1977 2879. 118 (a)B. Thulin 0.Wennerstrom I. Somfai and B. Chmielarz Acta Chem. Scand. (B) 1977 31 135; (6) B. Thulin and 0.Wennerstrom Tetrahedron Letters 1977,929; (c)H.E. Hogberg B. Thulin and 0.Wennerstrom ibid. p. 93 1. 119 T. Otsubo and V. Boekelheide J. Org. Chem. 1977,42 1085. 120 (a) M. W. Haenel Tetrahedron Letters 1977 4191; (b) W. Bieber and F. Vogtle Angew. Chem. Internat. Edn. 1977 16 175. Aromatic Compounds of the similarity of its U.V. spectrum to that of a 1,8-diarylnaphthalene the 'Hn.m.r. spectrum shows an enhancement of the anisotropic effect due to the closer approach of the p-phenylene rings. (37) A further synthesis and an X-ray structure of [2.2](2,7)pyrenophane have appeared.lZ1 [2.21 (1,4)tropylio( 1,4)cycloheptatrienop hane (37) te trafluoro borate has been formedlZ2" from [2.2]paracyclophane by a route involving ring expansions using dibromocarbene.In contrast to its normal behaviour the cycloheptatriene ring acts as a good intramolecular charge-transfer donor. New cyclophanes having tropylium rings have been reported.lZZb A new series of compounds which have the combined attributes of helicenes and cyclophanes are the 4,4'"-o-guaterphenylophanes (38; n = 2 and 6) which have been ~ynthesized'~~ by a sulphone-extrusion procedure. Species (38; n = 2) exhi- bits an unexpectedly high conformational flexibility. 5 Molecular Rearrangements The thermal and acid-catalysed Claisen rearrangements of N-allyl-anilines have been invesfigated.lZ4 The rearrangement of N-(l',l'-dimethylallyl)-2,6-dimethyl-aniline into 4-( l',l'-dimethylallyl)-2,6-dimethylaniline cannot be achieved ther- mally but occurs readily in 4moldm-3 sulphuric acid.A new regioselective synthesis of anilides having carbonyl-functionalized alkyl groups in the ortho-position has been ac~omplished'~~" in 40-70% yields by decarboxylative 1-aza-1'-oxa[3.3]~igmatropic'~~~ Claisen rearrangements of enolizable or enolized N-aryl- NO-diacyl-hydroxylamines i.e. (39)-+(40). N-Acyl-N-aryl-azasulphoniumsalts 0 R' R'KNH 121 H. Irngartinger R. G. H. Kirrstetter C. Krieger H. Rodewald and H. A. Staab Terruhedron Letters 1977 1425. 122 (a)J. G. O'Connor and P.M.Keehn Tetrahedron Letters 1977,3711;(b)H. Horita T. Otsubo and S. Misumi Chem. Letters 1977 1309. 123 F.Vogtle M.Atzmiiller W. Wehner and J. Grutze Angew. Chem. Znternat. Edn. 1977,16 325. 124 S.Jolidon and H.-J. Hansen Chimia (Switz.) 1977.31,46;Helv. Chim. Acfu 1977,60 978. 125 (a)R. M.Coates and I. M. Said J Amer. Chem. SOC. 1977 99 2355; (b) F. Vogtle and E. Goldschmidt Chem. Ber. 1976,109,1; (c) P.G.Gassman and R. J. Balchunis Tetrahedron Letters 1977.2235. R. G. Coombes have been prepared for the first time and when treated with base they yield ylides (41) which undergo spontaneous [2,3]sigmatropic rearrangement to produce o-methylthiomethyl-anilides and thus provide another route to ortho-substituted anilide~.'~'' The assumption that the severe conditions necessary for [3,3]sig- matropic rearrangement of 0-aryl or 0-vinyl oximes refer to the preliminary tautomerism stage have been confirmed,126 and some N-aryl-O-vinyl-hydro-xylamines [e.g.(4211 and 0-aryl-N-vinyl-hydroxylaminesundergo spontaneous rearrangement demonstrating the ease of cleavage of N-0 bonds when a Cope rearrangement path is available. It has been by deuterium labelling that 2-vinyl-phenols at 143 "C undergo reversible 1,5-hydrogen shifts to give o-quinone methides and the same process leads to rapid (E,Z)isomerization of for example 2-(prop- 1-eny1)-phenols. X I COMe MeO,( (41) (42) It has been noted that an autoxidation process to form dimethylbenzylamine and benzoic acid occurs during Stevens rearrangement of benzy Idime th yIp henacylam- monium ylide at low temperatures and that this predominates under an atmos- phere of oxygen.'** The solvent hexamethylphosphoramide enables the extension of the application of the Smiles' rearrangement of 2-aryloxy-2-methyl- pro- panamide~'~~~ into N-aryl-2-hydroxy-2-methyl-propanamidesto examples which possess deactivated aromatic This reaction is the basis of a new method for the conversion of phenols into anilines.1296 The kinetic nitrogen isotope effect for the benzidine rearrangement of hydra-zobenzene is kI4/kl5= 1.0203 (*0.0007) demonstrating at last unequivocally that breaking of the N-N bond in this two-proton process is part of the rate- limiting step. This result'30 rules out the recent proposal involving a second pro- tonation at the C-1 atom but cannot distinguish between the traditional mechanism having pre-equilibria at the two nitrogen atoms followed by rate- limiting rearrangement or some concerted process involving N-N bond scission and almost complete second protonation in the rate-limiting stage.The synthetic usefulness of the oxygen analogue of the benzidine rearrangement has been examined.131 The reactions of salts of N-aryl-hydroxamic acids with activated halogeno-benzenes yield 4-amino-4'-hydroxybiphenyls uia intermediate NO-diaryl-hydroxylamines. Biaryls are also formed by a new ~earrangement'~~ of arylhydrazones of aromatic aldehydes and diary1 ketones which is stated to be of 12' T. Sheradsky E. Nov,S. Segal and A. Frank J.C.S. Perkin I 1977 1827. H.-J. Hansen Helv. Chim. Acta 1977,60 2007. lZ8 S. H. Pine and E. Fujita J. Org. Chem. 1977 42 1460. 129 R. Bayles M. C. Johnson R. F. Maisey and R. W. Turner Synthesis 1977 (a)p.31; (6)p. 33. I3O H. J. Shine G. N. Henderson A. Cu and P. Schmid J. Amer. Chem. Soc. 1977,99 3719. 13' T. Sheradsky and E. Nov,J.C.S. Perkin I 1977 1296. '32 F. Fusco and F. Sannicolo Tetrahedron Letters 1977 3163. Aromatic Compounds 237 general applicability and which is exemplified by the formation of 4-amino-3'-formyl-6'-methoxy-3,5-dimethylbiphenyl (35%) on heating the 2,6-dimethyl-phenylhydrazone of anisaldehyde in polyphosphoric acid. The product data from the nitramine rearrangement of N-methyl-N-nitro-m-chloroaniline have been with those from the Claisen rearrangement of ally1 m-chlorophenyl ether and it is claimed that the 'cartwheel' mechanism for the former is not able to accommodate the findings that isomer formation except for a small steric effect and the requirement for ortho,puru-substitution is random.Photochemical rearrangement of the sulphonyl group has been in the conversion of phenyl benzenesulphonate into 0-and p-hydroxyphenyl phenyl sul- phones by U.V. irradiation and a mechanism involving a caged radical pair has been proposed. Further of the Fischer-Hepp rearrangement and concurrent denitrosation have involved methyl-substituted N-methyl-N-nitroso-anilines, with a view to the evaluation of steric effects; in particular those on the enigmatic intramolecular rearrangement step. A steric effect was apparent for the 3,s- dimethyl compound. N-Nitrosodiphenylamine is denitrosated by various nucleo- philes and can also act as a direct ring-nitrosating agent towards aniline~.'~~' 6 Condensed Systems A new route'36 to polymethylated naphthalenes anthracenes and phenanthrenes involves the reaction of 3,4-dichloro-1,2,3,4-tetramethylcyclobutenewith aromatic hydrocarbons in the presence of aluminium tribromide and for example reaction with naphthalene gave 1,2,3,4-tetramethylphenanthrene(60%).The usefulness of photodehydrocyclization reactions of stilbenes and analogues to give phenanth- renes has been increased as reaction occurs much more cleanly137 in the presence of certain 7r-acceptor molecules (e.g. tetracyanoethylene) under anaerobic condi- tions and if necessary at low temperature. The generation of a'-bromo-1,2-naphthoquinodimethane from 1-bromomethyl-2-dibromomethylnaphthaleneis in an improved one-pot synthesis of phenanthrenes.The species then undergoes Diels-Alder reaction with N-phenylmaleimide elimination of hydrogen bromide and fortuitous oxidation to give (43). A new phenanthrene synthesis is NPh 0 (43) (45) a=99" (44) p=138" 133 W. N. White and J. R. Klink J. Org. Chem. 1977 42 166. 134 Y. Ogata K. Takagi and S. Yamada. J.C.S. Perkin II 1977 1629. 13' (a)I. D. Biggs and D. L. H. Williams J.C.S. Perkin II 1977 44; (b)J. T. Thompson and D. L. H. Williams ibid. p. 1932. 136 A. P. Krysin N. V. Bodoev and V. A. Koptyug J. Org. Chem. (U.S.S.R.), 1977,13 1183. 137 J. Bendig M. Beyerrnann and D. Kreysig Tetrahedron Letters 1977 3659. 13' G. W. Gribble E. J. Holubowitch and M. C. Venuti Tetrahedron Letters 1977 2857. 238 R.G. Coombes in the direct conversion of a 2,2'-di(chloromethy1)-substituted biphenyl into the phenanthrene with sodamide in ammonia using an iron catalyst. Vanadium trifluoride oxide has been to convert a variety of 1,2-diaryl-ethene derivatives into phenanthrenes in high yield. Full details of the preparation of 1,l-dichlorocyclopropa[b]naphthalenefrom 1,l,la,7a-tetrachloro-la,2,7,7a-tetrahydro-2,7diphenylcyclopropa[~]naphthalene have appeared.I4* The gem-dibromo-derivative cannot be formed in an analogous way but the reaction of the dichloro-compound with a small excess of ethyl- or phenyl-magnesium bromide leads to halogen exchange the isolable gem-diethyl compound only being formed with a large excess of the Grignard reagent. The unisolable gem-diphenyl compound undergoes ready cleavage of the cyclopropane ring.Acenaphthylene is formed (41% yield) by the elimination of carbon monoxide from 1H-phenalen-1-one on flow pyrolysis at 900°C.141 This method is of more general utility and for example provides the preferred route to indeno[2,1- alindene (dibenzopentalene). Acenaphth[ 1,2-a]acenaphthylene (44) has been synthesi~ed'~~ by a new route and the increasingly strained compounds obtained by introducing ethano-bridges across the peri-positions of (44) have been made for the first time. Spectral effects due to strain were noted and the authors remark on the lack of calculations on these systems. The structure of 1-bromo-lH-cyclo-buta[de ]naphthalene (45) has been determined143a and surprisingly the three rings are essentially coplanar.The strain due to peri-bridging is accommodated by adjustments particularly in bond angles [see (45)] throughout the molecule. The first determination of the crystal structure of a Dewar molecule has been per- formed'43b on 1,3,6,8-tetra-t-butylhemi-Dewar-naphthalene. The molecule has a syn-structure as the substituents cause considerable distortion and the benzo- cyclobutene system is not planar. Decamethylanthracene has been prepared for the first time,'44 utilizing (in part) the removal of a nitrogen bridge in a 1,4-imine by oxidation with peroxide [see Ann. Reports (B) 1976,73 p. 2241. The strain in the structure was illustrated by the ease of photochemical conversion into the 9,lO-Dewar-isomer. A convenient method has been for the introduction of a tertiary carbon carrying an elec- tronegative group into the 9-position of anthracene.The appropriate alkyl radical generated thermally from an azo-compound reacts with anthrone under basic conditions to give the 10-alkyl-anthrone and the alcohol formed on subsequent reduction could be aromatized to the desired 9-substituted anthracene by phos- phorus pentoxide. Dehydrogenation of a series of mono-and di-t-butyldihy- droanthracenes has been in~estigated'~~ as a potential synthetic route to t-butyl- anthracenes. 1-and 2-t-Butylanthracene were prepared in this way although rearrangements complicated other cases. 2,6-Di-t-butylanthracene could be pre- pared in high yield by direct t-butylation of anthracene.13' (a) M. S. Newman and H. M. Dali J. Org. Chem. 1977,42,734; (b)A. J. Liepa and R. E. Summons J.C.S. Chem. Comm. 1977 826. 140 A. R. Browne and B. Halton J.C.S. Perkin I 1977 1177. 14' G. Shaden Angew. Chem. Internat. Edn. 1977,16 50. R. H. Mitchell T. Fyles and L. M. Ralph Canad. J. Chem. 1977 55 1480. 143 (a) M. Gessner P. Card H. Shechter and G. G. Christoph J. Amer. Chem. SOC.,1977,99,2371; (b) R. W. Franck R. Gruska and J. G. White Tetrahedron Letters 1977 509. 14* H. Hart and B. Ruge Tetrahedron Letters 1977 3143. 14' T. Mitsuhashi S. Otsuka and M. bki Tetrahedron Letters 1977 2441. "'P. P. Fu and R. G.Harvey J. Org. Chem. 1977,42 2407. Aromatic Compounds Some unusual structures in the anthracene series have been stabilized.2,4,9-Trichloro- 1,lO-anthraquinone is a stable crystalline solid prepared14' by refluxing 9-chloro-1O-hydroxy- 1,4-anthraquinone with thionyl chloride and triethylamine and some stable 9,lO-quinodimethanes [e.g. (46) m.pt. 105 "C] have been pre- pared148a by the initial reaction of the anthraquinone with methylmagnesium iodide followed by treatment with phosphoryl chloride in pyridine. Related compounds (47) have been formed'48b from the appropriate anthrone. Compounds which had at least two peri-methyl-methyl interactions were formed with the alkylidenedi- hydroanthracene structures shown whereas simpler compounds were formed as the anthracene tautomer. Other examples have been rep~rted'~~.'~*~ of the ease of loss of aromaticity of the central rings of anthracenes caused by double peri interactions.(47)R' R2 R3= Me H Me; Me Me H; H Me Me; or Me Me Me A'3C-labelling experiment has revealed'490 the scrambling of a-and p-(but not angular) carbon atoms in naphthalene at 1035 "C ('automerization') and a reason- able (but unproven) hypothesis involves the intermediacy of azulene. Mass spec- trometric studies indicate'496 that complete scrambling of the carbon atoms of naphthalene occurs in an energy range between the ionization potential and 18.8eV and prior to loss of C2H2. The oxidation of naphthalene with m-chloroperbenzoic acid under carefully controlled conditions has led'5oa to a synthesis of unti-naph-thalene 1,2 :3,4-dioxide which complements that of the syn-isomer reported last year [see Ann.Reports (B) 1976,73 p. 2251. A naphthalene pentaoxide has been from 1,6-epoxy[ 101annulene by two successive sequences of addi- tion of singlet oxygen and thermolysis and it has been shown to have the syn,syn,syn,syn-1,2;3,4;5,6;7,8-unti-9,10-structure. The mechanism proposed for the photocatalysed nitrosation of 2-naphthol appeared ~nlikely'~'" [see Ann. Reports (I?),1975,72 p. 2231 but new evidence for acid-catalytic action of singlet excited 2-naphthol on a ground-state reaction has been presented for the production of acetals from the irradiation of alkanals in Although 1-naphthol in contrast to phenol does not undergo pho- todimerization it has now been reportedlS2 that irradiation of the 1-naphthoxide ion 14' M. V. Gorelik S. P. Titova and V.A. Trdatyan J. Org. Chem. (U.S.S.R.),1977,13 424. (a)B. F. Bowden and D. W. Cameron Tetrahedron Letters 1977,383; (6)J.C.S. Chem. Comm. 1977 78; (c)H. Hart and H. Wachi ibid.,p. 409. 149 (a)L. T. Scott and G. K. Agopian J. Amer. Chem. SOC., 1977,99,4506;(6) H. Budzikiewicz and R. Stoize Monatsh. 1977 108,869. lS0 (a)K. Ishikawa and G. W. Griffin Angew. Chem. Inrernat. Edn. 1977,16 171; see also K. Ishikawa H. C. Charles and G. W. Griffin Tetrahedron Letters 1977 427; (b) E. Vogel A. Brewer C.-D. Sornmerfeld R. E. Davis and L.-K. Liu Angew. Chem. Internat. Edn. 1977 16 169. (a)E. A. Chandross J. Amer. Chem. SOC.,1976 98 1053; (b)M. Hisaoka and K. Tokurnaru Chem. Letters 1977 533. T. Kitamura T. Irnagawa and M. Kawanisi J.C.S. Chem. Comm. 1977 81. 14' R.G. Coombes remarkably gives a dimer (48),probably via a photoinduced nucleophilic addition reaction. In a seemingly unprecedented nucleophilic substitution reaction the formation of 1-piperidhometh yl-4-meth yl-3-nitronap h thalene has been recor-ded’53 from the reaction of 1,4-dimethyl-2,3-dinitronaphthalene with piperidine and the authors suggest redefining the term ‘tele-substitution’ to cover reactions where the entering group occupies a position one or two atoms away from that vacated by the leaving group. (48) Heating potassium 2-naphthoxide in a primary alcohol (benzyl or containing more than three carbon atoms) has been to give a novel route to the 1-alkyl-2-naphthol in good yield. The syntheses of 1-t-butyl- and l-t-pentyl-2- naphthol have been and although these molecules are considerably strained any ketone tautomer present was undetectable.The bulk of the alkyl group does lead however to an extremely rapid autoxidation to the hydro- peroxynaphthalenone (49). It has been that the unreactivity to oxygen of some 1-aryl- and 1-aralkyl-2-naphthols is due to intramolecular hydrogen bonding of the hydroxy-group to the aryl group of the substituent. The first quantitative determination of the reactivity towards electrophiles of anthracene results from studies of pr~tiodetritiation.’~~ The 1-and 2- positions are more reactive than the corresponding positions in naphthalene but are closer in reactivity than are those in naphthalene. Irradiation of 2-~tyrylbenzo[c]phenanthrenein chiral solvents gives”’ the optic- ally active hexahelicene albeit with low optical yields (0.2-2%).3,15-Ethano- and 3,15-(2-oxapropano)-[7]helicenehave been synthe~ized,’~~ and their conformations lS3 G. Guanti S. Thea M. Novi and C. Dell’Erba Tetrahedron Letters 1977 1429. lS4 T. Kit0 and K. Ota J. Org. Chem. 1977,42 2020. 155 (a)J. Carnduff and P. A. Brady J. Chem. Res. (S) 1977 235; (6)P. A. Brady J. Carnduff and F. Monaghan Tetrahedron Letters 1977 3295. 156 H. V. Ansell M. M. Hirschler and R. Taylor J.C.S. Perkin IZ 1977 353. lS7 W. H. Laarhoven and Th. J. H. M. Cuppen J.C.S. Chem. Comm. 1977,47. lS8 M. Joly N. Defay R. H. Martin J. P. Declerq and G. Cermain Helv. Chim. Acta 1977,60 537. Aromatic Compounds 241 and 'H n.m.r.spectra compared with those of [7]helicene and 3,15-dimethyl[7] helicene. Thermolysis of l-formyl[6]helicene tosylhydrazone in benzene in the presence of sodium hydride gavels9 the carbene-insertion product (50),which was resolved using h.p.1.c. [see Ann. Reports (B) 1976,73 p. 2261. 7 Non-benzene Systems Three- and Four-membered Rings.-The 11-methyl-1l-tricyclo[4,4 1,01*6]undecyl cation has been formed,160 below -60 "C and its 13Cn.m.r. spectrum suggests that it has significant 2~-homoaromatic character leading to an unsymmetrical structure (51). The planar l-(2,3-diphenylcycloprop-2-enylidenemethyl)-2,3-diphenylcyclopropenium ion has been synthesized'61 by treatment of bis(2,3-diphenylcycloprop-2-eny1)methane with trityl perchlorate. Spectral measurements indicate a significant contribution of (52) to its structure.Tetra(NN-dialkyl-amino)triafulvalene dications [e.g. (53)] have been synthesized'62 for the first time. PriN-NPr; *______ Me;.i3 Ph*Ph Ph Ph PriN N Pr; (52) (53) (51) The mono- bis- and tris-(p-NN-di-isopropylamino)triphenylcyclopropeniumions have been formed163a by the reaction of NN-di-isopropylaniline with the appro- priate chlorocyclopropenium ion; the fully substituted ion is one of the most stable cyclopropenium ions yet reported. Studies of these ions allow an interesting comparison to be made with analogous triphenylmethyl cations showing for example the reduced importance of the immonium canonical form in the former cases. The reactions of 1,2-bisdialkylarnino-3-halogenocyclopropeniumions and aryl Grignard reagents have been and reactions of an intermediate cyclopropenium Grignard reagent identified.Two electrochemical have yielded values for 'antiaromatic' destabiliza- tion upon the formation of a cyclobutadiene ring. The value for cyclobutenedione upon reduction to the radical anion was at least 59kJm0l-'.'~~~ Dewar has reinforced'65a his suggestion that the cyclobutadiene formed by the matrix-isolation technique is a metastable triplet [see Ann. Reports (B),1975,72 p. 2401 and he has used MIND0/3 theory to calculate (with apparent success) the i.r. 159 J. Jespers N. Defay and R. H. Martin Tetrahedron 1977 33 2141. G. A. Olah G. Liang D. B. Ledlie and M. G. Costopoulos J. Amer. Chem. SOC., 1977,99,4196.161 K. Komatsu K. Masumoto and K. Okamoto J.C.S. Chem. Comm. 1977 232. 162 Z.-I. Yoshida H. Konishi S.-I. Sawada and H. Ogoshi J.C.S.Chem. Comm. 1977 850. (a) K. Komatsu R. West and D. Stanislawski J. Amer. Chem. Sac. 1977,99,6286; (b)Z.-I. Yoshida H. Konishi Y. Miura and H. Ogoshi Tetrahedron Letters 1977 4319. 164 (a) M. Horner and S. Hiinig Angew. Chem. Internat. Edn. 1977 16 410; (6) R. D. Rieke C. K. White L. D. Rhyne M. S. Gordon J. F. W. McOmie and N. P. Hacker J. Amer. Chem. SOC., 1977 99 5387. (a)M. J. S. Dewar and A. Komornicki J. Amer. Chem. Soc. 1977 99 6174; (6) H. Kollmar and V. Staemmler ibid. p. 3583. R. G. Coombes spectrum of such a species. The paper seems however to have been overtaken in publication by the doubts raised concerning the experimental values [see Ann.Reports (B) 1976 73,p. 2281. In contrast to Dewar's calculations and with relevance to his arguments concerning metastability another treatment that even for a square geometry the singlet is lower in energy than the triplet which is in violation of Hund's rule due to spin polarization effects. Both observed and calculated electron densities for a rectangular cyclobutadiene for which the struc- ture is known have shown166n that the density maxima for the four-membered ring lie significantly off the lines joining the carbon atoms indicating that the bonds are 'bent'. The symmetrical and apparently uncomplexed tetrakis(trifluoromethy1)cyclobutadiene has been fully characterized'66h spectrally in solution and matrix at low temperatures.It has a singlet ground state and appears to be rectangular. Tri-t-butylcyclobutadiene for which a rectangular structure is to be assumed has now been shown'66c to undergo reactions typical of a triplet diradical species; for example it undergoes addition with carbon tetrachloride. It was suggested that the two structures may be in rapid equilibrium or that the reaction is initiated by transfer of an electron from the cyclobutadiene within a donor-acceptor complex. Labelling experiment^^^' have confirmed the intermediacy of butalene (54) in the reaction of l-chloro[2,2,0]bicyclohexa-2,5-dienewith lithium dialkylamides to give dialkyl-anilines but this pathway only accounts for about 50% of the product. -SiMe (54) (55) 1,2-Bis(trimethylsilyl)benzocyclobutadienehas been prepared'68 by the vacuum pyrolysis of the &,cis-dienediyne (55) as a thermally stable air-sensitive orange-red oil.The 'H n.m.r. spectrum provides convincing evidence of strong paramagnetic ring-current contributions to the induced ring-current. The pathways of oxidation of benzocyclobutadienes have been ~larified'~~" by the observation that in solution the tetraphenylbenzo[ 1,2 :4,5]dicyclobutadiene (56) is oxidized initially to the oxide (57). In the solid however the stable o-quinone methide (58) is formed. Bu' BU' 166 (a)H. Irngartinger H.-L. Hase K.-W. Schulte and A. Schweig Angew. Chem. Internat. Edn. 1977 16 187; (b)S. Masamune T. Machiguchi and M. Aratani J. Amer. Chem.SOC.,1977,99,3524;(c)G. Maier and W. Sauer Azgew. Chem. Internat. Edn. 1977 16 51. 16' R. Breslow and P. L. Khanna Tetrahedron Letters 1977 3429. K. P. C. Vollhardt and L. S. Yee J. Amer. Chem. Soc. 1977 99 2010. 169 (a)F. Toda N. Dan K. Tanaka and Y. Takehira J. Amer. Chem. SOC.,1977 99,4529; (b)F. Toda and T. Yoshioka Chem. Letters 1977 561. 243 Aromatic Cornpounds Cycloadditions of tetracyanoethylene to substituted analogues of (56) occur in a [2 + 21manner across the bond indicated (1,2) to give highly strained propel lane^.'^^^ Five-and Seven-membered Rings.-A fulvene synthesis has ~esulted’~’ from a study of the reactions of thiobenzophenones with the dicarbonylcyclopentadienyl-iron anion. Yields are improved by the inclusion of para-electron-releasing substituents in the aryl group.The synthesis of fulvenes from the reaction of 1-chloroalkyl acetates and sodium pentadienide followed by base-catalysed eli- mination has been extended171a to some 6-substituted fulvenes and 1,2-benzo- fulvene 1,2,3,4-dibenzofulvene and the corresponding 6-methyl- and 6-phenyl- derivatives have been prepared by an analogous procedurei71* from sodium indenide or sodium fluorenide. 1,2-Benzofulvene has also been formed”* by the flash vacuum pyrolysis of the 11-diphenylmethyleue derivative (59) which was prepared from the benzyne adduct of 6,6-diphenylfulvene by selective hydro- genation. t (59) (40) (61) X = C02Me CHO,or CN A series of 6,6-disubstituted fulvenes has been protonated under super-acid conditions at low temperatures.173 Protonation takes place exclusively at the C-2 position of the fulvene ring in accord with calculated electron densities. The crystalline isolable 1,2,3,4-tetraphenyi- and 1,2 :3,4-dibenzofulvalenes and 2,3- diphenylfulvalene (60) which is only stable in solution have been synthesized. 174 Species (60) readily dimerizes by [4 +2laddition. The scope of the facile pentalene synthesis from 1,3-di-t-butyl-6-dimethylaminofulvene[see Ann. Reports (B) 1976 73,p. 2301 has been by using other alkynes to give the pentalenes (61). These pentalenes are formed as dimers with which the monomers are in rapid equilibrium and which form the monomers in organic solvents. The dimerization reactions and other dimerization products that are formed after longer times have been st~died,”~“ as have the reactivities of the pentalene monomers to acids bases dienophiles and dienes.175b 3-Amino-tropones have been f~rmed”~ by rapid pyrolysis of a corresponding enamine; 3-amino-4-methyltropone,for example being formed from (62). 3-Ethoxy- and 3-phenoxy-tropones have been prepared’77 by the reaction of 3- tosyloxytropone with sodium ethoxide and lithium phenoxide respectively. A 170 H. Alper and H.-N. Paik J.C.S. Chem. Comm. 1977 126. (a)M. Neuenschwander and R. Iseli Helv. Chim. Acra 1977,60 1061; (b)M. Neuenschwander R. VBgeli H.-P. Fahrni H. Lehmann and J.-P. Ruder ibid. p. 1073. 172 R. N. Warrener K. I. Gell and M. N. Paddon-Row Tetrahedron Letters 1977 53. G. A. Olah G. K. S.Prakash and G. Liang J. Org. Chem. 1977,42,661. 174 H. Prinzbach H. Sauter H.-G. Horster H.-H. Limbach and L. Knothe Annalen 1977 869. 17’ hl. Suda and K. Hafner Tetrahedron Letters 1977 (a)p. 2449; (b)p. 2453. 17‘ J. Ficini and A. Durkault Tetrahedron Letters 1977 809. 177 M. Cavazza R. Cabrino and F. Pietra Synthesis 1977 298. R. G. Coombes reported preparation'78a of 3-hydroxytropone derivatives appears however to be incorrect,'786 the products being 2-hydroxyacetophenones. Electrophilic substitution of 5-methyltropolone by arenediazonium ions occurs first at the 3-and then at the 7-position. 179a 5-Arylazotropolones undergo arylazo exchange (61-92%) on reaction with a different arenediazonium ion. 1796 The reaction of 5-nitrosotropolone with cyclopentadiene gives (63) and provides an example of a stable tropolone derivative reacting as a 27~ compound in a [4+ 2]~ycloaddition.*~~' 5-Phenylazotropolone however which is known to react as its diketo-form adds to cyclopentadiene utilizing its N=N-C=C linkage to give (64).179d Further reactions with cyclopropene have established the endo-stereo- chemistry required for the latter reaction.179e An analogous side-chain reaction has also been established'79f as a minor pathway in one case in a homodiene system on reaction of 2-methoxy-6-styryltropone with maleic anhydride. The first benzo[3,4]cyclobuta[ 1,2-c]tropolone7 the 9-chloro-derivative7 has been synthesizedlaO and appears to exist predominantly or exclusively as the tautomer (65).The geometrically isomeric 2,3 :6,7-dioxa-bis-cr-homotropones [e.g. (66)] have been formed18' by the direct oxidation of tropone with alkaline hydrogen peroxide. The unreactivity of the remaining double bond however limits their synthetic utility. A series of methoxy-substituted heptalene-l,8- and -3,8-diones have been formed"*" by oxidation of 8H-cyclohepta[~tropolone.182b In solution in fluorosulphonic acid these compounds are diprotonated and provide the first examples of the lop-heptalenium dication structure [e.g. (67)]. The MIND0/3 method has been applied to the prediction of the rearrangement of benzyl and substituted benzyl cations to tropylium ions and related processes and "I3 (a)J. H. Clark and J. M. Miller Tetrahedron Letters 1977 139; (6)H.Takeshita ibid.,p. 1657. 179 (a)T. Ide K. Imafuku and H. Matsumura Chem. Letters 1977 717; (6) ibid. p. 511; (c)I. Saito K. Sakan. and S. It8 ibid. p. 253; (d)S. It8 and I. Saito Tetrahedron Letters 1977 1203; (e)Y. Fujise M. Sakaino and S. It8 ibid. p. 2663; cf> I. Saito Y. Watanabe and S. It8 ibid. p. 3049. 'I3' M. Sato S. Ebine and J. Tsunetsugu Tetrahedron Letters 1977 855. H. Prinzbach W. Seppelt and H. Fritz Angew. Chem. Znternat. Edn. 1977,16 198. S. Kuroda and T. Asao Tetrahedron Letters 1977 (a)p. 289; (6)p. 285. Aromatic Compounds of the properties of these ions.'83 A pure heptamethyltropylium salt has been prepared'84 for the first time by the use of phosphorus pentachloride as a reagent for hydride abstraction from 1,2,3,4,5,6,7 -heptame t hyltropilidene.The 9,lO-dihydr0-9-10-(1,2-tropylio)anthraceneion (68) has been synthesi~ed,'~~ and the U.V.spectrum is indicative of strong intermolecular charge-transfer interaction despite the small overlap of the orbital systems involved. 8-Phenylheptafulvene has been preparedlg6 by the reaction of the benzyl- tropylium ion with an equivalent of triethylamine in dichloromethane. It is stable for several days in solution at room temperature under nitrogen. U.V. spectroscopy suggests even less r-electron delocalization than in heptafulvene itself and only a small contribution of the dipolar canonical form for this system. 8-Amino- and 8-methoxy-8-(triethylsi1oxy)heptafulvene have also been synthesized; they are hygroscopic and thermolabile.18' A facile synthesis of 8-azaheptafulvenes involves the reaction of tropylium borofluoride with anilines to give 8-aryl-8-azahep- tafulvenium salts which are deprotonated with aqueous sodium carbonate.'88a The cycloaddition reactions of these compounds with inter alia isocyanates and ketens to give (usually) [8+ 2]cycloadducts have also been studied.'88b Two independent report^'^' have been given of an improved route to azulenes by [6+ 4lcycloaddition of 6-NN-dimethylaminofulvenewith thiophen 1,l-dioxides; 3,4-dichlorothiophen for example yielding the hitherto unknown 5,6-dichloroazulene in 60% or 46% 1896 yield. 2,6-Dihydroxyazulene has been preparedlgoa from diethyl 2,6-dihydroxyazulene- 1,3-di~arboxylate'~~~ and undergoes a heavily solvent-dependent keto-enol tautomerism.In chloroform the species exists mainly as 1,3-dihydroazulene-2,6-dione.1,3-Dicarbethoxyazulene-2,6-dione has also been ~ynthesized,'~~' but was isolated only in the form of a dimer. 1-Alkyl-azulenes are converted into 1-acyl-azulenes in 59-98 yields by O/O a novel oxidation using DDQ.I9*' C. Cone M. J. S. Dewar and D. Landman J. Amer. Chem. Soc. 1977,99,372; M. J. S. Dewar and D. Landman ibid. pp. 2446 4633 7439. K.-I. Takeuchi Y. Yokomichi and K. Okamoto Chem. Letters 1977 11 77. T. Nakazawa and I. Murata J. Amer. Chem. SOC.,1977,99 1996. K. Komatsu M. Fujimori and K. Okamoto Tetrahedron 1977 2791. '13' K. M. Rapp and J. Daub Tetrahedron Letters 1977 227. "'(a) K.-I. Sanechika S. Kajigaeshi and S. Kanemasa Synthesis 1977 202; (6) K.Yamamoto S. Kajigaeshi and S. Kanemasa Chem. Letters 1977 85 91. (a)S. E. Reiter L. C. Dunn and K. N. Houk J. Amer. Chem. Soc. 1977,99,4199;(b)D. Copland D. Leaver and W. B. Menzies Tetrahedrm Letters 1977,639. 19* (a)T. Morita H. Kanzawa. and K. Takase Chem. Letters 1977 753; (b)T. Morita and K. Takase ibid. p. 513; (c)T. Amemiya M. Yasunami and K. Takase ibid. p. 587. 246 R. G. Coombes Annulenes.-A series of substituted cyclo-octatetraene dications has now been prepared'" by oxidation of the parent hydrocarbon. Most are stable only at low temperatures but the sym-dibenzocyqlo-octatetraene dication is stable even at 0 "C,and spectral evidence suggests that delocalization of all fourteen .rr-electrons about the sixteen-carbon-atom periphery is occurring.Oxidation of cyclo-octa[deflfluorene by a different method gave'92 the protonated cyclo-octa[deflfluorenone dication (69) a 14~electron aromatic system which is stable at room temperature. The 9,10-diphenylbicyclo[6,2,0]decapentaenes(70) have been ~ynthesized'~~ by a route involving initial photoaddition of diphenylacetylene to bicyclo[4,2,0]octa-3,7-diene,and other stages one of these involving the thermal isomerization of bicyclo[4,2,0]octatriene to cyclo-octatetraene. The preliminary evidence suggests that welectron delocalization in (70; R = H) but not in (70; R = Me) overcomes the strain involved in attaining (near) planarity. R (70) R =H,D or Me PMO theory has demon~trated'~~ that the small variations in the perimeter bond lengths of 1,6-methano[ 101annulene are due mainly to transannular 1,6-homoaromatic interaction.This type of interaction is smaller but still important for the 1,6-methano[ 1l]annulenium cation [cf. Ann. Reports (B) 1976 73,p. 2341. 1,&Methano[ 1Olannulene and some hetero-bridged analogues undergo Diels-Alder addition with 4-substituted-1,2,4-triazoline-3,5-diones, giving both mono- and bis-adducts formed by anti-attack. 195a The methylene bridge also hinders sterically the approach of NNN'N'-tetramethyl-p-phenylenediamineto 2,7-dinitro-1,6-methano[lO]annulene,leading to the formation of only a 1:1 donor-acceptor complex.'95b Unsubstituted 1,5-methano[ lO]annulene has been synthesized and appears to maintain a diamagnetic ring current.'96 In contrast to its 1,&analogue however its electronic spectrum extends into the visible region and it is an orange crystalline solid.Two sets of correlation^'^' have been reported between experimentally deter- mined properties of annulenes and variously calculated values of 'resonance ener- gies per v-electron' (REPE). In the first case197n the correlation involved the rates of formation of some annulenes determined by Sondheimer et al. in connection 19' G. A. Olah J. S. Staral G. Liang L. A. Paquette W. P. Melega and M. J. Carmody J. Amer. Chern. SOC.,1977,99,3349. 19* I. Willner A. L. Gutman and M. Rabinovitz J. Amer. Chem. SOC.,1977,99,4167. M. Oda H. Oikawa N. Fukazawa and Y. Kitahara Tetrahedron Letters 1977 4409. 194 R.C. Haddon J. Org. Chem. 1977 42 2017. lQ5 (a) P. Ashkenazi D. Ginsburg and E. Vogel Tetrahedron 1977 33 1169; (b) J. A. Chudek R. Foster and E. Vogel J.C.S. Perkin ZI 1977 994. S. Masamune and D. W. Brooks Tetrahedron Letters 1977 3239. 19' (a) B. A. Hess jun. and L. J. Schaad J.C.S. Chem. Comm. 1977 243; (b) B. A. Hess jun. L. J. Schaad and M. Nakagawa J. Org. Chem. 1977,42,1669. 19' Aromatic Compounds with their 'reactivity criterion' of aromaticity [see Ann Reports (B) 1976 73,p. 2371. In the second the differences in 'Hn.m.r. chemical shifts of the inner (i) and outer (0)protons of some dehydroannulenes (71 ;rn =p =0 1 2 or 3) gave a good linear correlation with REPE suggesting that chemical-shift data at least if used in this way can give quantitative information concerning aromaticity.(71) n=l or3 Studies of some newly synthesized model compounds have led to the conclusion that the [12lannulene rings in some previously studied di- and tri-benzannelated bisdehydro[ 12lannulenes are paratropic. '98a A necessarily planar benzo-naphtho- analogue was also synthesi~ed,'~'' and the 'H n.m.r. shifts were interpreted in terms of paratropicity of the [12]annulene ring. The 2,3 :6,7 8,9 12,13-tetra- benzo[l3]annulenyl anion (72) the first such ion to contain only formal double and single bonds has been prepared'99a from the corresponding hydrocarbon. *99b The 'H n.m.r. spectrum is indicative of the reduced diatropicity caused by benzan- nelation and in addition of the considerable change in ring conformation from that of the parent hydrocarbon.Full details*"" of the synthesis of 3,7,10,14-tetrasubstituted 1,8-bisdehy-dro[ 14lannulenes by the method involving cyclic dimerization of 2,3-dien-4-yne ketones and the details2"' of the greatly improved synthetic route to bisde- hydrobenzannulenes have appeared. In the latter publication the syntheses of bisdehydrobenz-[141-,-[161- -[181- and -[20]-annulenes as well as of some methyl derivatives are described. It seems that the structural features which allow one to draw equivalent Kekul6 structures for some dehydroannulenes [e.g. (73)] have little effect on their properties.201 Diatropicity differences between (73) and (73) 198 (a)H. A. Staab and P. Gunthert Chem. Ber. 1977,110 619; (b)H.A. Staab and H. J. Shin ibid. p. 631. 199 I. Willner A. Gamliel and M. Rabinovitz (a) Chem. Letters 1977 1273; (6) Synthesis 1977 410. 200 (a)K. Fukui T. Nomoto S. Nakatsuji S. Akiyama and M. Nakagawa Bull. Chem. SOC.Japan 1977 50 2758; (b)N. Darby T. M. Cresp and F. Sondheimer J. Org. Chem. 1977,42 1960. 201 (a)M. Osuka S. Akiyama and M. Nakagawa Tetrahedron Letters 1977 1649; (b)Y. Yoshikawa S. Nakatsuji F. Iwatani S. Akiyama and M. Nakagawa ibid. p. 1737. R. G. Coombes (71);rn = 0,n = 3 p = l) for example appear to be insignificant,201” and similar conclusions were also drawn from studies of a paratropic bisdehydro[ 16lannulene system.z01b It seems that energy differences between non-equivalent canonical forms [e.g. in (71;rn = 1 n = 3 p = l)] are not enough to depress significantly the T-electron delocalization.Benzo[l8]annulene has been synthesized,*02 in low yield by a route involving prototropic isomerization of an intermediate 13,14,15,16-bisdehydro-derivative. Again annelation of the benzene ring reduces the diatropicity of the macrocyclic ring. Two further annulenes have been synthesized203 where extensive benzan- nelation has removed the diatropicity of (4n+2)~-electron macrocyclic rings and for example tribenzo[a,g,rn]-l5,17-bisdehydro[ 18lannulene is atropic. The dimethyldihydrobenzo[e]pyrene (74) sustainszo4 about 55% of the ring-current of the parent non-benzannelated dimethyldihydropyrene. This amount is much greater than the value for other benzannelated [14]annulenes and reflects the greater aromaticity of the rigid dihydropyrene nucleus.All-~is-[2~](2,5)thio-phenophanetetraene (75) has been prepared205 in a one-step reaction from thio- phen-2,5-dicarbaldehyde and the Wittig reagent from 2,5-bis(chloromethyl)thio-phen. The compound shows a small paramagnetic ring current over the 24~-electron periphery; this is enhanced on cooling when planar conformations become of more importance. The presence of the thiophen nuclei does not seem to interfere with the paratropicity of the molecule. The syntheses of the dibenzannulenones (76;n = 1 2 and 3) have been completedzo6“ and the results for (76;n = 1) and (76;n =2) have now been in full The paratropicity exhibited by (76;n = 1) and (76;n = 2) which are however less paratropic than monobenzannelated analogues disap- pears in (76; n = 3) and comparison with an acyclic analogue indicates that the latter is atropic.The first monocyclic large-ring annulenones (77;R =H)and (77; R = Me) have been synthesized.z06‘ The introduction of the additional methyl group (77;R=Me) causes a change in conformation at the other trans double bond. Both species exhibit paratropicity and this is enhanced on dissolution in *02 U. E. Meissner A. Gensler and H. A. Staab Tetrahedron Letters 1977 3. 203 J. Ojima M. Enkaku and C. Uwai Bull. Chem. SOC.Japan 1977,50 933. 204 S. Icli V. J. Nowlan P. M. Rahimi C. Thankachan and T. T. Tidwell Canad. J. Chem. 1977 55 3347. 205 A. Strand B. Thulin and 0.Wennerstrom Actu Chem. Scund. (B), 1977,31 521.206 (a)J. Ojima M. Enkaku and M. Ishiyama J.C.S. Perkin I 1977 1548; (b)J. Ojima Y. Yokoyama and M. Enkaku Bull. Chem. SOC.Jupan 1977,50 1522; J. Ojima M. Ishiyama and A. Kimura ibid. p. 1584; (c)T. M. Cresp J. Ojima and F. Sondheimer J. Org. Chem. 1977 42 2130. Aromatic Compounds 0 0 deuteriotrifluoroacetic acid. syn-4,13;6,11 -Dimethano[ 15lannulenone (78) has been synthesized as an air-stable red crystalline solid and an X-ray crystallographic analysis has been performed. The molecule is protonated in trifluoroacetic acid to give a delocalized annulenium The syntheses of the first bridged [22]annulenes i.e. syn- and anti-(79) have been accomplished,208 in one step by the dimerization of 6,ll -methano[ 1llannu-lenylidene followed by ring closure and loss of hydrogen.'H N.m.r. measurements show no diatropicity and suggest that there is localization of bonds as shown in (79). The first derivative to be formed that is derived from the hitherto unknown nonalene skeleton is the planar delocalized diatropic dibenzo[gh oplnonalenide dianion (80) formed209 from the reaction of 6,14-dihydrodibenzo[gh oplnonalene with butyl-lithium. Further bicyclic compounds containing two fused macrocyclic conjugated 7r-systems have been Octalene (81) has been prepared2'Oa from 1,4,5,8- tetrahydronaphthalene as a lemon-yellow air-sensitive compound. Structures involving a central double bond or aromatic structures were ruled out. Peripheral C=C bond shifting occurs above 80"C.ortho-Fused [14]annuleno-[16]- and -[18]-annulenes analogous to the original [14]annulene [see Ann.Reports (B) 1975,72 p. 2481 have been synthesized.*"' The diatropicity of the 14-membered ring is reduced as the size of the fused annulene is decreased irrespective of whether this latter annulene is (4n +2)-or 4n-membered. Other annuleno-annulenes are (82; rn = 1)210c and (82; m = 2),210dwhich are both strongly diatropic and quite stable in 207 W. Wagemann K. Miillen E. Vogel T. Pilati and M. Simonetta Angew. Chem. Internat. Edn. 1977 16 170. 208 U. H. Brinker R. W. King and W. M. Jones J. Amer. Chem. SOC.,1977,99 3175. *09 I. Willner and M. Rabinovitz J. Amer. Chem. Soc. 1977,99 4507. *lo (a) E. Vogel H.-V. Runzheimer F. Hogrefe B. Baasner and J. Lex Angew. Chem. Internat.Edn. 1977,16,871; J. F. M. Oth K. Miillen H.-V. Runzheimer P. Mues and E. Vogel ibid. p. 872; (b)T. M. Cresp and F. Sondheimer J. Amer. Chem. SOC.,1977,99 194; (c) S. Nakatsuji S. Akiyama and M. Nakagawa Tetrahedron Letters 1977 3723; (d) M. Osuka Y. Yoshikawa S. Akiyama and M. Nakagawa ibid.,p. 37 19. 250 R. G. Coombes (82) (83) contrast to the appropriate [26]-and [30]-annulenes. In the 'H n.m.r. spectra the inner protons of the 14-membered ring appear at a much higher field than those of the 18-membered ring and the latter and all outer protons shift to higher field as the distance from the central bridge increases. Finally the first examples of annulenophanes e.g. (83) have been reported.211 The benzenoid protons of (83) show the expected upfield shift due to the ring current in the [lolannulene ring.The indications are however that in this example transannular interaction of welectrons is small. *" M. Matsumoto T. Otsubo Y. Sakata and S. Misumi Tetrahedron Letters 1977 4425.