10 Aromatic Compounds By R. G. COOMBES Department of Chemistry The City University St.John Street London EC1 V 4PB 1 Introduction It has been suggested' that the contribution to the magnetic susceptibility component perpendicular to the molecular plane serves as a more reliable aromaticity index than that based on magnetic anisotropy or n.m.r. data and that high magnetic anisotropy does not necessarily imply aromaticity. The approach seems similar to that in more recent papers by Flygare and co-workers* than those cited where the importance of relating to a hypothetical localized model is also emphasized. A small advanced text on aromaticity3 is worthy of attention particularly for its laudable separation of experimental and theoretical aspects. The full details of the preparation of cyclo-octa[def]biphenylene have a~peared.~" This molecule which might be expected to have simultaneously olefinic aromatic and antiaromatic properties exhibits a marked paramagnetic ring shift indicative of the planar 4n7r rings but is thermally stable and seems to exhibit a high reactivity towards electrophiles.A related compound (1) has three linearly fused 4n7r rings is thermally stable and is diatropi~.~~ It is concluded that the application of the simple n.m.r. criterion of aromaticity to polycyclic systems is hazardous. The importance of the local aniso- tropic contribution to proton shieldings has also been emphasized this being held responsible for the observed shifts in paratropic [4n]annulenes.' R. Benassi P.Lazzeretti. and F. Taddei J. Phys. Chern. 1975,79,848. 2-(a)T. G. Schmalz C. L. Norris and W. H. Flygare J. Amer. Chem. SOC.,1973 95 7961; (b) T. G. Schmalz T. D. Gierke P. Beak and W. H. Flygare Tetrahedron Lerters 1974 2885. D. Lewis and D. Peters 'Facts and Theories of Aromaticity,' Macmillan London 1975. (a)C. F. Wilcox jun. J. P. Uetrecht G. D. Grantham and K. G. Grohmann J. Arner. Chern. SOC.,1975 97 1914; (b) C. F. Wilcox jun. and G. D. Grantham Tetrahedron 1975,31,2889. M. Barfield D. M. Grant and D. Ikenberry J. Amer. Chem. SOC.,1975,97,6956. 215 216 R. G. Coombes Details of the improved version of the MIND0 semi-empirical SCF-MO method (MIND0/3) have been published6 and the approach has been criticized.' A general rule of aromatic stability of non-alternant polycyclic conjugated hydrocarbons has been proposed' and supported by aromatic stability indices.The application of PMO theory to homoaromaticity has been developed,' and the paper contains a useful review of homoaromatic systems. Application of MIND0/3 to the homo- tropenylium cation" indicates a long homoconjugate linkage and deformation of the seven-membered ring leading to an energy gain of 3 kcal mol-I. Full details of the study of cyclobutenyl cations have appeared.'' The 1,3-diphenyl-2,4-di-R-cyclobutenyl cations (R =Ph Me or H) have negligible contributions from 1,3-~ overlap but greater overlap occurs for exclusively methyl-substituted cations. The parent cyclobutenyl cation (2) exhibits truly aromatic delocalization is the simplest homoaromatic system and has AG* = 8.4kcal mol-' for the ring-flipping process.N.m.r. data on symmetrically trisubstituted 1,3,5-trineopentylbenzenescan be consistently interpreted in terms of the predominance of a rotamer with all three neopentyl groups on the same side of the ring thus providing evidence for attractive steric effects amongst the alkyl groups.l* The barrier to rotation of the dimethylamino-group in NN-dimethyl-2,4-dinitroaniline(7.1 kcal mol-') is similar to that calculated for NN-dimethyl-4-nitroaniline,indicating little mesomeric interaction of the out-of-plane ortho-nitro-group. l3 N.m.r. measurement~~~ indicate that trans-stilbenes become more nearly coplanar if a vinylic hydrogen is replaced by deuterium whereas the corresponding cis-stilbenes differ little from one another.This suggests that a C-*H bond has a smaller steric requirement than a C-'H bond presumably due to the smaller vibrational amplitude. ESCA measurement^'^^ show that the carbon adjacent to a trifluoromethyl group substituted in a benzene ring is negative in agreement with earlier CND0/2 predictions and more surprisingly that the group donates electrons to the ring overall. The fact that this charge is localized at the ipso-position is suggested as an explanation of the apparent contradiction to chemical evidence. The use of the aryl fluorine atom as a probe for investigating electronic effects of substituents must involve some qualifications. 15' 19Fchemical shifts involve factors different from and of a different order of complexity to those encountered in the study of substituent effectson conventional chemical properties.One problem is their apparent sensitiv- ity to substituent-induced structural distortions which are not necessarily a function of substituent size. The earlier demonstration that linear free-energy relationships and the Hammond Postulate are incompatible [Ann. Reports (B),1973,70,74,392] has been expanded R. C. Bingham,M. J. S. Dewar andD. H. L0,J.Amer. Chem. SOC.,1975,97,1285,1294,1302,1307. J. A. Pople J. Amer. Chem. SOC.,1975,97,5306;W. J. Hehre ibid. p. 5308; cf. M. J. S. Dewar ibid.,p. 6591. 8 J. Kruszewski and T. M. Krygowski Canad. J. Chem. 1975,53,945. R. C. Haddon J. Amer. Chem. SOC.,1975,97,3608. lo R.C. Haddon Tetrahedron Lefters 1975,863. G. A. Olah J. S. Staral R. J. Spear and G. Liang J. Amer. Chem. SOC.,1975 97 5489. l2 R. E. Carter B. Nilsson and K. Olsson J. Amer. Chem. Soc. 1975,97,6155. l3 D. D. MacNicol Tetrahedron Leners 1975 2599. l4 P. J. Mitchell and L. Phillips J.C.S. Chem. Comm. 1975 908. l5 (a) S. A. Holmes and T. D. Thomas J. Amer. Chem. SOC.,1975 97 2337; (b)W. Adcock M. J. S. Dewar R. Golden and M. A. Zeb J. Amer. Chem. SOC.,1975,97,2198. Aromatic Compounds 217 in an important analysis by C. D. Johnson.16 It is noted that free-energy relation- ships are linear over very wide ranges and that selectivity is clearly independent of reactivity which may lead to the deduction that transition-state structure is indepen- dent of reaction rate; a conclusion which may also be drawn from isotope studies.The question remains of whether this deduction is correct or whether transition-state structures do vary but selectivity parameters and isotope effects fail to respond in the expected manner. The long-standing problem of the extent of dissociation of the triphenylmethyl dimer has been res01ved.l~ 2 Benzene Isomers Oxides and Homobenzenes Gas-phase electron-diffraction studies of the structures of benzvalene,lga and hexa -methyl prismane,186 hexaAuoroDewarbenzenel8‘and a microwave study” of pris- mane have been performed. A complete assignment of the fundamental vibrations of Dewarbenzene and benzvalene has been reported.20 Both molecules exhibit abnor- mal C=C stretching frequencies indicative of o-.rr interaction between the strained bridge bonds and the ?r-bonds.The first reports have appeared of bicyclopropenyl Cope rearrangements. Pyrolysis of (3a) gave (3b) which exists as two diastereomers2* Compound (3b) was mainly responsible for the rn-and p-xylenes observed amongst the products of the thermal aromatization of (3a). Compounds (4a) and (4b) also rearranged thermally to give (5a) and (Sb) respectively and the reverse of these reactions (5)+(4) R Ph Ph Ph Ph R MePh Ph Me Ph Ph (3) a; R’ = H R2 = Me (4) a; R = Me (5) a; R = Me b; R’ = Me R2 = H b;R=H b;R=H occurs photochemically.22 In the latter case however it seems that only a chair transition state is involved and that the product is the d,Z-isomer. Supporting evidence has been presented to confirm that thermal bicyclopropenyl to benzene rearrangements proceed via Dewarbenzene~.~~’~~ Chemiluminescence and kinetic techniques were used to identify 1,4-dimethyIDewarbenzeneas an intermediate in the aromatization of 3,3’-dimethylbi~yclopropenyl.*~ A study of the photochemical rearrangement of (5b) into 2,3,4,6- and 2,3,4,5-tetraphenyltol~enes~~ has clearly l6 C.D. Johnson Chem. Rev. 1975,75 755. l7 K. S. Colle P. S. Glaspie. and E. S. Lewis J C.S. Chem. Comm. 1975,266. l8 (a)R. R. Karl jun. and S. H. Bauer J. Mol. Structure 1975,25 1; (b)R. R. Karl jun. Y. C. Wang and S. H. Bauer ibid. p. 17; (c) K. L. Gallaher Y. C. Wang and S. H. Bauer ibid.,p. 35. l9 R. D. Suenram J. Amer. Chem. SOC.,1975,97,4869.*O D. W. T. Griffith J. E. Kent and M. F. O’Dwyer Austral. J. Chem. 1975,28 1397. 21 W. H. de Wolf 1. J. Landheer and F. Bickelhaupt Tetrahedron Leners 1975 179. 22 R. Weiss and H. Koelbl J. Amer. Chem. SOC.,1975,97 3224. 23 N. J. Turro G. B. Schuster R. G. Bergman K. J. Shea and J. H. Davies J. Amer. Chem. Soc. 1975,97 4758. z4 R. Weiss and H. Koelbl J. Amer. Chem. Soc. 1975,97 3222. 218 R. G.Coombes demonstrated that prismanes are not intermediates in the photochemical rearrange- ment either and the suggestion has been made22 that species analogous to (6) are important in both photochemical Cope and aromatization processes. (6) (7) (8) 1,4-TrimethyleneDewarbenzene,which remains intact at 300 "C has been prepared'' by the silver-ion-catalysed valence isomerization of 1,l'-trimethylenebicyclopropenyl(7) and in methanol as solvent trapping of the inter- mediate cation (8)was observed in support of the previously suggested mechanism for these reactions [Ann.Reports (B),1973,70,390].Further confirmation26 comes from capture by the solvent of the other proposed intermediate in the reaction of 1,l'-dimethylenebicyclopropenyl. 1,4-PentamethyleneDewarbenzene has been prepared by a similar method to that mentioned above and on thermal aromatiza- tion it gave the unexpected rearrangement product benzo~ycloheptene.~' Me (9) The syntheses and resolutions of the Dewarbenzene (9),28"the first optically active derivative of a valence-bond isomer of benzene and related compounds286,c have been reported.Compound (9) undergoes a remarkably high-yield photochemical conversion into the corresponding achiral prismane. Further examples of the unusually stable Dewarbenzene isomers of perfluorohexa-alkylbenzeneshave been prepared,29" and a detailed of the hexakis(pentafluoroethy1)benzenesystem shows that the equilibrium favours the Dewar isomer above 551 K. The application of MIND0/3 theory to the conversion of benzvalene into benzene suggests an extremely exothermic 'allowed' The photoreactions of benzvalene involve several unusual processes in which the extraordinary unreac- tivity of the benzvalene skeleton is notew~rthy.~' Hexakis(trifluoromethy1)-25 1. J. Landheer W. H. de Wolf and F. Bickelhaupt Tetrahedron Letters 1975 349; see also L. A.Paquette Synthesis 1975,347. 26 F. C. Peelan G. G. A. Rietveld I. J. Landheer W. H. de Wolf and F. Bickelhaupt Tetrahedron Letters 1975,4187. 27 J. W. van Straten I. J. Landheer W. H. de Wolf and F. Bickelhaupt Tetrahedron Letters 1975,4499. 28 (a) J. H. Dopper B. Greijdanus and H. Wynberg J. Amer. Chem. Soc.,1975,97,216; (b)J. H. Dopper B. Greijdanus D. Oudman and H. Wynberg Terruahedron Letters 1975,4297; (c) J. H. Dopper B. Greijdanus D. Oudman and H. Wynberg J.C.S. Chem. Comm 1975,972. 29 (a) M. G. Barlow R. N. Haszeldine and M. J. Kershaw Tetrahedron 1975 31 1649; (b) A. M. Dabbagh W. T. Flowers R. N. Haszeldine and P. J. Robinson J.C.S. Chem. Comm. 1975,323. 3O M. J. S. Dewar and S. Kirschner J. Amer. Chem. Soc.,1975,97,2932. 31 C.A. Renner T. J. Katz J. Pouliquen and N. J. Turro J. Amer. Chem. SOC.,1975,97,2568. Aromatic Compounds 219 benzvalene ozonide has been prepared32" and in contrast to the unsubstituted analogue is stable at room temperature. Photolysis of this compound to yield the syn-tetrakis(trifluoromethy1)cyclobutadiene dimer (10) involved the free cyc- lobutadiene which could be trapped by diethyl azodicarboxylate. Flash thermolysis of (1l) prepared by a route involving cycloaddition to the benzvalene also yielded (10) via the same intermediate cy~lobutadiene.~~' (10) R = CF (11) R = CF Prinzbach and co-workers have continued their studies of benzene oxides and related compounds. These have led to simple syntheses of ~treptamine,~~" 2-de~xystreptamine,~~' ~is-trithia-,~~" (f )-hy~samine,~~~ trans-a,a -dioxa-P -thia-,33d and trans -p -oxa-a,a-di thia- tris- a-homo benzenes33 (12a) (13 a) and (13 b) and two to trans-trioxatris-a-homobenzene(13c).The reaction of cis-trioxatris-u-homobenzene (12b) with hydrobromic acid has been as has the thermal isomerization of cis-triazatris-a-homobenzene to the 1,4,7-triazacyclonona-2,5,8-triene The cis -diazabis-a -homobenzenes undergo rapid 2a-27r valence isomerization to give 1,4-diaza[8]annulenes (14) and it appears that (14c) sustains a diamagnetic ring current in contrast to (14a) and (14 b) .34c R R \ / xvXNuN X- (12) a;X = Sx Y b;X=O (13) a; X = 0,Y = S (14) a; R = Ts b;X=S,Y=O b; R = C0,Me c;X=Y=O c;R=Me trans-Bishomobenzene is readily available35 by the reaction of dibromocarbene with 3-norcarene followed by methyl-lithium-promoted carbenoid cyclization and acid-catalysed ring opening of the resultant bicyclobutane (13 which also provides a preferred route to 2,3-homotropilidene by silver-ion-catalysed rearrangement.In 32 (0) Y. Kobayashi I. Kumadaki A. Ohsawa Y. Hanzawa and M. Honda Tetrahedron Letters,1975 3001; (6) ibid. p. 3819. 33 (a) R. Schwesinger and H. Prinzbach Angew. Chern. Internut. Edn. 1975,14,630; (6) H. Prinzbach R. Keller and R. Schwesinger ibid. p. 632; (c) S. Kagabu and H. Prinzbach ibid. p. 252; (d) H. Prinzbach C. Kaiser and H. Fritz ibid. p. 253; (e)H. Prinzbach R. Keller and R. Schwesinger ibid. p. 633; cf)H. Prinzbach and H.-W. Schneider Tetrahedron Letters 1975,3073.34 (a)L. Knothe and H. Prinzbach Tetruhedron Letters 1975,1319; (b) H. Prinzbach R. Schwesinger M. Breuninger B. Gallenkamp and D. Hunkler Angew. Chern. Internat. Edn. 1975 14 347; (c) H. Prinzbach M. Breuninger B. Gallenkamp R. Schwesinger and D. Hunkler ibid. p. 348. 35 R. T. Taylor and L. A. Paquette Angew. Chem. Internut. Edn. 1975,14 496. 220 R. G.Coombes contrast to previously known trans-a-homobenzenes the hexamethyl derivative undergoes reversible valence isomerization and skeletal rearrangement to (16) above 120°C presumably because of steric interaction between the cis-isopropylidene moieties.36 3 Benzene and its Derivatives General.-A useful review of the photosubstitution reactions of aromatic com- pounds has appea~ed.~' It has been pointed out that there is sparse evidence concerning the nature of the intermediates in phototransposition reactions of six-membered aromatic rings and that attempts to trace the bonding relationships of all six atoms should provide informative restrictions on the possible intermediates The immediate precursor of the 2 1photoadduct of maleic acid and benzene is the originally proposed bicyclo-octadiene (17) which is formed from the zwitterion (18) in unsensitized reactions.39 Heavy-atom solvents induce an alternative triplet pathway to (17).The conflicting reports on the photoaddition of furan to benzene have been largely rec~nciled,~~ and the proportions of adducts are dependent on the light source and the reactant concentrations.Compound (19) and its Cope- rearranged isomer are the major products with high concentrations of benzene if a 0 +&o 0-& (17) (18) (19) low-intensity near-monochromatic source (254 nm) is used. The fixed cis -diene 1,2-dimethylenecyclohexaneadds photolytically to benzene to produce the novel 1,4-1',3'-benzene adduct (20) and the 1,4-1',4'-add~ct,~l" whereas the latter type of adduct was the only one isolated from the reaction of benzene and cis-1,2- dihydrophthalic anhydride.416 The truns-diene 3-methylenecyclohexene gave only very low yields of unidentified product^.^^‘' Benzene has been forced to act as a dienophile towards hexachlorocyclopentadieneat 220-240 "Cunder extreme pres- sure (10000 atm) to give the 1,2-adducts (21).42 36 P.Binger and J. McMeeking Angew. Chem. Internat. Edn. 1975,14 371. 37 J. Cornelisse and E. Havinga Chem. Rev. 1975,75 353. 38 J. A. Barltrop and A. C. Day J.C.S. Chem. Comm. 1975 177. 39 D. Bryce-Smith R. R. Deshpande and A. Gilbert Tetrahedron Letters 1975 1627. 40 J. C. Berridge D. Bryce-Smith A. Gilbert andT. S. Cantrell J.C.S. Chem. Comm. 1975,611. 41 (a)J. C. Berridge D. Bryce-Smith and A. Gilbert Tetrahedron Leners 1975,2325;(b)N. C. Yang C. V.Neywick and K. Srinivasachar ibid. p. 4313. 42 W. Jarre D. Bieniek and F. Korte Angew. Chem. Internat Edn. 1975,14 181. Aromatic Compounds 221 (21) R' and R2 = H or C1 The first spirocyclopropabenzenes (22) have been ~ynthesized~~ by 1,3-dipolar cycloaddition of diazocyclopentadienes to cyclo-octyne and subsequent photofrag- mentation.Compounds (22a) and (22c) are thermally stable but (22b) rearranges to the 1-arylcyclo-octene rapidly at room temperature. The benzene ring has been subjected to further strain and the synthesis of cyclopropa[4,5]benzocyclobutene (23) has been acc~mplished.~~ (22) a; R = Ph b;R=H (23) c; R = p-BrC,H4 The new soluble catalyst q3-C3H,Co[P(OMe),] leads to specific cis-hydrogenation of arenes and pure all-cis-[2HJcyclohexane has been prepared for the first time by hydrogenation of [*H,]ben~ene.~~ Transition-metal catalysis is also involved in a new general of indans and tetralins by acetylene cyclization (Scheme 1). Reagents i (n-Cp)Co(CO,)-n-C,HI Scheme 1 The rate coefficient for reaction of phenyl radicals from phenylazotriphenyl- methane and benzene has been measured4' by a technique involving competition with N-(t-buty1)-a-phenylnitrone.The value of ca.lo5dm3 mol-* s-' suggests that the rate of decarboxylation of the benzoyloxy radical is considerably faster than previously assumed. Studies of the dependence of product ratios on temperature and on the addition of Cu" salts in the phenylation of 0-and p-dichlorobenzenes support the contention that the formation of the cyclohexadienyl radical inter- mediate may be reversible particularly when the incoming radical is ortho to a 43 H. Durr and H. Schmitz. Angew. Chem. Internat. Edn. 1975,14 647. 44 D. Davalian and P. J. Garratt J. Amer. Chem. SOC.,1975,97,6883;See also C. J. Saward and K.P. C. Vollhardt Tetrahedron Letters 1975 4539. 45 E. L. Muetterties M. C. Rakowski F. J. Hirsekorn W. D. Larson V. J. Basus and F. A. L. Anet J. Amer. Chem. Soc. 1975,97 1266. 4h R. L. Hillard tert. and K. P. C. Vollhardt Angew. Chem. Infernat. Edn. 1975 14,712. 47 E. G. Janzen and C. A. Evans J. Amer. Chem. SOC.,1975,97,205. 222 R. G.Coombes sub~tituent.~~ A variety of persistent cyclohexadienyl radicals have been generated by addition of some carbon- silicon- oxygen- and phosphorus-centred radicals to various sterically hindered aromatic Species(24) was stable for several days at room temperature. Bu Bu' (24) A preferable alternative to the Rosenmund reduction for aromatic acid chlorides has been described?' Their reactions with phospholens give arlyphospholenium salts which yield the corresponding aldehydes and phospholen oxides on hydrolysis.The reaction of acyl halides with lithium aryltrialkylborates readily available from the reaction of aryl-lithium with trialkylboranes produces the corresponding aryl ketones in high yields.51 The demonstration that N-tosyloxyphthalimide readily undergoes photolysis in aromatic solvents to give the N-arylphthalimides by a radical path has opened up a Gabriel-type route to primary aromatic amine~.~~ The Schiemann reaction does not involve prior dissociation of the fluoroborate anion but involves direct reaction of the anion probably with the singlet aryl cation (see p. 227).53An example of the unknown possibly aromatic benzochlorophenium ions (25) is implicated as an intermediate in the thermal dediazoniation of o-(@,p-dichloroetheny1)phenyldiazonium hexafluorophosphate to give chloro(o-chloropheny1)e thyne .54 (26) An interesting report is of the selective aliphatic hydrogen exchange in 2,4,6-tri- isopropyltoluene on treatment with deuteriated trifluoroacetic acid in carbon tetrachl~ride.~~ In addition to aromatic exchange the @-hydrogen atoms in the 4-isopropyl substituent only were exchanged and a mechanism involving exchange in the most stable tertiary carbonium ion (26) formed by oxidation was suggested.48 R. Henriquez and D. C. Nonhebel Tetrahedron Letters 1975,3855,3857. 49 D. Griller K. Dimroth T. M. Fyles and K. U. Ingold J. Amer. Chem. SOC.,1975,97,5526. 50 D.G.Smith and D. J. H. Smith J.C.S. Chem. Comm. 1975,459. 51 E.-I. Negishi A. Abrarnovitch and R. E. Merrill J.C.S. Chem. Comm. 1975 138. 52 J. I. G. Cadogan and A. G. Rowley J.C.S. Perkin I 1975 1069. 53 C.G.Swain and R. J. Rogers J. Amer. Chem. SOC.,1975,97 799. 54 G.A.Olah and Y. Yamada J. Org. Chem. 1975,40 1107. 55 A.Nilsson and K. Olsson Acra Gem. Scand. (B),1975,29,752. Aromatic Compounds 223 Electrophilic Substitution.-The kinetically controlled gas-phase reaction of the t-butyl cation with rn-xylene gives predominantly 1,3-dimethyl-4-t-butylbenzene, in contrast to the orientation observed in conventional Friedel-Crafts alkylati~n.~~ Photochemically induced nitrosation reactions of phenols and naphthols in neutral solution in the presence of sodium nitrite have been rep~rted.~' It is questionable whether this is due to the enhanced acidity of 2-naphthol on excitation to the excited singlet state producing a local increase in acidity which allows protonation of nitrite ion and consequent nitrosation of 2-naphthol at the 1-position.The presence of a cation radical under conditions appropriate to the side-chain chlorination of polyalkylbenzenes in acetic acid has been reported (twice!)'" and the involvement of such species in the side-chain process has been suggested. The high positional selectivity and the product distribution from for example 1,2,3,5- tetramethylbenzene is evidence against this hyp~thesis,~'" and detailed studies of the products have been interpreted in terms of ion pair involvement after loss of chloride ion from the cyclohexadiene intermediate.59b The full report6' of the nitration of $-cumene in aqueous sulphuric acid convinc- ingly demonstrates that selectivity may remain between positions in a molecule of similar steric requiremects but of different intrinsic reactivity when reaction is occurring at the encounter rate.The selectivity however was not found to be sufficient to require the presence of any intermediate involving attractive interaction prior to the Wheland intermediate. The rates of nitration of the acids Ph(CH,),SO,H indicate that the sulphonate anions are the reactive species in <98% sulphuric acid and that even when n =3 the sulphonate ion exerts a deactivating influence.61 Tertiary aromatic phosphine oxides [ArP(O)R,] react as their conjugate acids leading to metal ortho-orientation with considerable discrimination against para -substitution.62A rein~estigation~~" of the temperature dependence of the HR acidity scale has revealed that contrary to an earlier report the graph of -HR versus %H,SO does indeed become less steep with increasing temperature and hence the apparent qualitative difference from the behsviour of rate profiles for nitration disappears.A compilation of the extensive literature data on nitration reactions in aqueous sulphuric acid has been presented with rates extrapolated to similar reaction conditions.636 Nitration of p-t-butyltoluene in acetic anhydride yields inter alia the first characterized 1,2-adduct (27) presumably because of steric inhibition of nucleo- philic capture at the 4-po~ition.~~ The adducts (28) from 3,4-dimethylaceto- or benzo-phenone form the 2- and 5-nitro-derivatives in strongly acidic solution by 1,2- or 1,3-nitro shifts from the intermediate cyclohexadienyl cation which can be 56 P.Giacomello and F. Cacace J.C.S. Chem. Comm. 1975,379. 57 F. D. Saeva and G. R.Oh 3. Amer. Chem. Soc.,1975,97,5631. 58 J. K. Kochi Tetrahedron Letters 1974,4305; 1975,41. 59 (a)E. Baciocchi and G. Illuminati Tetrahedron Letters 1975,2265; (b)E. Baciocchi L. Mandolini and A. Patara ibid. p. 2268. 6o J. W. Barnett R. B. Moodie K. Schofield and J. B. Weston J.C.S. Perkin XI 1975 648. 61 R. B. Moodie K. Schofield and T. Yoshida J.C.S. Perkin XI 1975,788.62 E. Malinski A. Piekos and T. A. Modro Canad. J. Chem. 1975,53 1468. 63 (a)M. J. Cook N. L. Dassanayake C. D. Johnson A. R. Katritzky and T. W. Toone J. Amer. Chem. Soc. 1975,97,760; (b)A. R. Katritzky B. Terem E. V. Scriven S. Clementi and H. 0.Tarhan J.C.S. Perkin ZX 1975 1600; A. R. Katritzky S. Clementi and H. 0.Tarhan ibid. p. 1624. 64 A. Fischer and R. Roderer J.C.S. Chem. Comm.,1975 798. 224 R. G.Coombes trapped by reactive arenes to give biphenyl derivative^.^' @so-Nitration and subse- quent nucieophilic capture is indicated in the observation66 of 2-cyclopropyl-4- hydroxytoluene amongst the products of nitration of 2-cyclopropyltoluene. Nuclear and side-chain nitrates have been isolated from the reactions of tetrachloro-o- and -p-xylenes respectively with fuming nitric Compound (29) is claimed as the first nuclear nitrate ester from nitration reactions.Bu' H OAc 0 (27) (28) (29) A thorough study of the nitration of pentamethylbenzene by impure nitronium hexafluorophosphate in nitromethane indicates two modes of nitration.68 The first involves the nitronium ion produced in the rate-limiting stage and the second involves attack by the incipient nitronium ion formed by oxidation by nitrogen dioxide of the molecular complex formed between the arene and the nitrosonium ion. Both processes form products uia ipso-substituted cT-complexes. Selective nitration can be achieved by electrophilic displacement of thallium(rr1) from aryl thallium(rII) trifluoroacetates with dinitrogen tetr~xide.~~ High proportions of p-nitroalkylbenzenes can be prepared in this way but arenes with substituents orienting thallium(1Ir) to the ortho-position failed to react.A study7' of the effect of nitrous acid on the nitration of 2,4-di-iodomesitylene and 2,4-di-iodo-3,5-dimethylanisole concludes that in the former case nitrodeiodination occurs whereas in the latter nitrosodeiodination and oxidation is the preferred pathway perhaps due to the greater aromatic reactivity of the substrate. The formation of 4-cyclopropyl-3-nitrobiphenyl as a significant product of nitration of 4-cyclopropylbiphenyl with no report of the 4,2-isomer may suggest a very large discrimination between ortho- and rneta-activation by the cyclopropyl gr~up.~' The prevalence of side-chain substitution in the sulphonation of 9-methyl- and 9,lO-dimethyl-anthraceneshas been inter~reted~~ in terms of loss of a proton from the 9-methyl group in the Wheland intermediate formed by electrophilic attack at the 10-position.Side-chain substitution however is not significant for nitration of toluene in acetic anhydride as has been believed.73 Considerable deviations from additivity on sulphonation of trimethylbenzenes have been inter~reted~~ in terms of 65 A.Fischer C.C. Greig and R. Roderer Canad. J. Chem. 1975 53 1570. 66 Yu. S.Shabarov,S. S. Mochalov N. B. Matveeva and I. P. Stepanova J. Org. Chem. (U.S.S.R.),1975 11 565. 67 H. Suzuki,K. Ishizaki S. Maruyama and T. Hanafusa J.C.S. Chem. Comm. 1975,5 1 ;Bull. Chem. SOC. Japan 1975,48,2112.6s E. Hunziker P.C. Myhre J. R. Penton and H. Zollinger Helv. Chim. Acta 1975,58 230. 69 B.Davies and C. B. Thomas J.C.S. Perkin I 1975 65. 70 K. Olsson Acta Chem. Scand. (B),1975,29,405. 71 S. A. Ermishkina S. S. Mochalov and Yu. S. Shabarov,J. Org. Chem. (U.S.S.R.),1975,11 369. 72 H. Cerfontain A. Koeberg-Telder C. Ris and C. Schenk J.C.S. Perkin 11 1975,966. 73 S. C. Narang and M. J. Thompson Austral. J. Chem. 1975,28,385. 74 H. Cerfontain A. Koeberg-Telder k. Ris and Z. R. H. Schaasberg-Neinhuis J.C.S. Perkin 11 1975 970. Aromatic Compounds 225 steric effects and the postulated early position of the transition state along the reaction co-ordinate compared with that for toluene. It has been ~uggested~~ that in non-catalytic bromination reactions the power- series rate equation is the mathematical equivalent of that expected from a conven- tional A-S,2 process in which the rate of the reverse of the first step becomes significant during the course of the reaction and need have no other special mechanistic significance.Evidence from uncatalysed para-bromination of N-methylacetanilide was interpreted in this way. from the diazo-coupling reactions of 4-substituted phenolate anions suggest that the methoxy-group activates the position meta to it to electrophile attack in contrast to the behaviour expected from its u+value. This was associated with the presence of a strongly activating substituent and the weak electrophilicity of the reagent. A new value of 'u+'for the metu-methyl substituent (-0.098) determined from the pyrolysis of 1-arylethyl acetates gives better correlations of electrophilic substitution reaction~.~~ Friedel-Crafts reactions involving alkanes and cycloalkanes have been ~eviewed.~' The catalysed alkylation of chlorobenzene by cyclopropane gives n-propyl- and isopropyl-chlorobenzenes.79The high proportion of rneta-substitution under some circumstances was presumably due to thermodynamic control.A kinetic study of Friedel-Crafts alkylation in n-hexane indicates that previous values had been affected by the heterogeneity of reaction solutions.8o The new results for toluene accord with the Brown Selectivity Relationship. The role of solvation energy in related reactions has been emphasized." The alkylation of benzene with t-butyl chloride in the presence of aluminium trichloride and a saturated hydrocarbon containing a tertiary carbon atom leads not only to t-butylbenzene but also to alkylation by a moiety derived from the saturated hydrocarbon.82 Nucleophilic Substitution.-Gas-phase reactions of fluorine-substituted aromatics with hydroxide and alkoxide ions have been interpreted in terms of two reaction pathway^.'^ The Meisenheimer complex can either lose fluoride ion to give the phenol or ether or can lose an alkyl fluoride or hydrogen fluoride to give the phenoxide ion.Short-lived radical species formed in reactions of para -substituted nitrobenzenes with nucleophilic reagents have been trapped by t-nitrosobutane to give nitroxide radical^.'^ The normally difficult formation of the complex (30)from aniline and trinitroben- zene is catalysed by the presence of a tertiary amine (e.g.DABCO)." Equilibrium and kinetic measurements on the formation of 1,l-dimethoxy-complexes from activated anisoles and sodium methoxide in the presence and absence of 18-crown-6 ether suggest that the sodium ion is normally associated with the anion by interaction 75 W. M. Schubert and J. L. Dial J. Amer. Chem. SOC.,1975 97 3877. 76 J. KuliE M. Titz and M. VekrB Coil. Czech. Chem. Comm. 1975,40,405. 77 E. Glyde and R. Taylor J.C.S. Perkin 11 1975 1463. R. Miethchen and C.-F. Kroger 2. Chem. 1975,15 135. 79 M. Khosrovi I. Partchamazed and M. Fakhrai Tetrahedron Letters 1975 2619. 8o B. J. Carter W. D. Covey and F. P.DeHaan J. Amer. Chem. Soc.,1975,97,4783. C. Decoret J. Royer and 0.Chalvet Tetrahedron,1975,31 973. 82 L. Schmerling J. Amer. Chem. SOC.,1975,97,6134. S. M. J. Briscese and J. M. Riveros J. Amer. Chem. Sac. 1975 97 230. 84 I. I. Bilkis and S. M. Shein Tetrahedron,1975,31,969. 85 E. Buncel and H. W. hung J.C.S. Chem. Comm. 1975 19. R. G.Coombes with the oxygen atoms of the C-1 methoxy-groups and the ortho-substituents.s6 In the presence of a crown ether in benzene solution trinitrobenzene gives Meisenheimer complexes with a number of anions some of which (e.g. F-) do not react in aqueous The anion from cyclohexanone reacts with 33-dinitrovenzoic acid and derivatives to form 2-and 4-substituted complexes.88 The more stable 2-complex is formed more quickly.The spiro-complex anion (31) has been spectrophotometrically observeds9 as an intermediate in the Smiles rearrange- ment of N-acetyl-@ -aminoethyl 2,4-dinitrophenyl ether and (32) is formed at -70 "C by the reaction of 4-(4-~hlorobutyl)biphenylwith lithium." n 0 NCOMe . I QN02. fj. I.-' NO* Ph (31) (32) A kinetic investigation of the displacement of halide from 0-and p-bromo- and p-fluoro-nitrobenzenes by thiophenoxide ion has shown that a crown ether consider- ably accelerates reaction in t-butyl alcohol but not in mefhan01.~~ This is interpreted in terms of nucleophile ion pairing in the former solvent. Phenoxide and thiophenoxide ions interact strongly with micelles of cetyltrimethylammonium bromide which also strongly catalyses reactions of these ions with 2,4-dinitroflu~robenzene.~' Thiophenoxide ion can rather surprisingly cause aryl- oxygen bond fission in competition with attack at the carbonyl group on reaction with some nitrophenyl The products of reaction of the ambident nitrite nucleophile with halogeno- nitrobenzenes are nitrophenols whether initial attack is by the oxygen or nitrogen centre.94 Nitrogen attack has been demonstrated and is preferred for displacement of C1 Br and I.Oxygen attack which is favoured for fluorine displacement is also 86 M. R. Crampton J.C.S. Perkin ZZ 1975 825. 87 A. R. Butler J.C.S. Perkin Z 1975 1557. A. Ashiaq V. MachiEek and V. Sterba Coll. Czech. Chem. Comm. 1975,40,1910. 89 S. Sekiguchi and K. Okada J. Org. Chem. 1975,40 2782.90 E. Grovenstein jun. and S. Akabori J. Amer. Chem. SOC.,1975,97,4620. 91 G. Guanti C. Dell'Erba S. Thea and G. Leandri J.C.S. Perkin ZZ 1975 389. 92 H. Chaimovich A. Blanco L. Chayet L. M. Costa P. M. Monteiro C. A. Bunton and C. Paik Tetrahedron 1975,31 1139. 93 G. Gaunti C. Dell'Erba F. Pero and G. Leandri J.C.S. Chem. Cumm. 1975,823. 94 T. J. Broxton D. M. Muir and A. J. Parker J. Org. Chem. 1975,40,2037 3230. Aromatic Compounds 227 enhanced more than nitrogen attack by transfer to a dipolar aprotic solvent. The ratios of rate coefficients for reactions of 6-substituted 2,4-dinitrochlorobenenes with aniline and N-methylaniline can be attributed to the steric effect of the substituent the ratio increasing with the size of the 6-s~bstituent.~~ The correspond- ing rates for the 4-substituted analogues show that the faster aniline reaction is subject to considerably larger substituent effects than the N-methylaniline reaction.An earlier transition state caused by a primary steric effect for the slower N-methylaniline reaction is suggested in apparent contradiction to the expectations of the Hammond Postulate. The specific catalysing effect of copper(1) salts exemplified in the displacement of iodide ion from 2-iodoazobenzene by cuprous cyanide or alkoxide has been attri- buted to ~helation.~~ The nickel-catalysed displacement reactions of aryl halides have also been in~estigated.~' Direct substitution by an alkyl group from an alkyl-lithium into a benzenoid ring can be achieved by carrying out the reaction with m-benzenetricarbonylchromium(0) and subsequently adding iodine to give the alkylben~ene.~~ The full report implicating the phenyl cation as the reactive intermediate in dispiacement reactions of phenyldiazonium salts in the absence of bases reducing agents or light has appeared,"" and this interpretation is supported by nitrogen isotope-eff ect measurements on the dediazoniation of phenyl diazonium boroflu~ride~~~ and by the largest secondary aromatic hydrogen isotope effect yet Ab initiu calculations suggest that the p-amino-substituent will stabilize a singlet phenyl cation but probably not to the extent that the ion would be expected to be formed in solvolytic reactions.loo The heterolytic dediazoniation of some substituted phenyldiazonium ions with halide ions in pyridinium polyhydrogen fluoride solution has been studied.The o-nitrophenyldiazonium ion gives isomeric chloronitrobenzenes (48% u- 44% rn- and 8% ps and these results were discussed1o1u partly in terms of the ambident nature of the diazonium ions."" Biaryk-Tris(triphenylphosphine)nickel(O) the presumed reactive species from the reagent tetrakis(triphenylphosphine)nickel(O) may now be conveniently gener- ated in situ by reduction of bis(triphenylphosphine)nickel(rI) dichloride with zinc in the presence of triphenylphosphine."* Treatment with aryl halides produces biaryls in good yields. The earlier method has been applied to a synthesis of cyclic ortho-bridged biphenyls with rings of up to 14 member^."^ Aromatic iodides couple readily when treated with a catalytic quantity of palladium(@ acetate in triethylamine or tri-n-butylamine at 100OC,'04 and rapid Ullmann couplings of 95 W.Eggiman P. Schmid and H. Zollinger Helv. Chim. Acfa 1975,58 257. 96 P. V. Roling J. Org. Chem. 1975 40 2421. 97 R. Cramer and D. R. Coulson J. Org. Chem. 1975,40,2267. 98 M. F. Semmelhack H. T. Hall M. Yoshifuji and G. Clark J. Amer. Chem. Soc. 1975,97 1247. 99 (a)C. G.Swain,J. E. Sheats and K. G. Harbison J. Amer. Chem. Soc. 1975,97,783; (b)C. G. Swain J. E. Sheats and K. G. Harbison ibid. p. 796; (c)C. G. Swain J. E. Sheats D. G.Gorenstein and K. G. Harbison ibid. p. 791. loo J. D. Dill and P. von R. Schleyer Tetrahedron Letters 1975,2857. Io1 (a)G.A. Olah and J. Welch J. Amer. Chem. SOC. 1975,97,208; (b)G. A. Olah and J. L. Grant ibid. p. 1546. *02 A. S. Kende L. S. Liebeskind and D. M. Braitsch Tetrahedron Letters 1975 3375. lo3 M. F. Semmelhack and L. S. Ryono J. Amer. am.SOC.,1975,97 3873. lo4 F. R. S. Clark R. 0.C. Norman and C. B. Thomas J.CS. Perkin I 1975 121. 228 R. G.Cbombes activated aryl halides at room temperature in homogeneous solution can be accom- plishedlo5 by reaction with copper(1) trifluoromethylsulphonate in aqueous ammoniacal acetone. Diarylthallium(m) derivatives are not as inert as previously believed and the trifluoroacetates are useful synthetic intermediates for the synth- esis of unsymmetrical biphenyls. '06 Conclusive evidence has been presented to rule out ionization as the process responsible for the dynamic n.m.r.behaviour of some 9-arylfluorenes. lo' Mechan-isms involving change of configuration at C-9 or intramolecular hydrogen shifts were discounted and all the evidence is consistent with simple hindered rotation about the aryl-C-9 bond. The two rotamers of 9,9':9',9"-terfluorenyl(33)formed on Michael addition of fluorene to 9,9'-bifluorenylidene have been assigned the s-cis,s-cis-and s-cis,s-trans-conformations.'08The bis(propel1ane) (34) has been ~ynthesized.'~~ Quinones and Related Compounds.-An authoritative collection of reviews of quinonoid chemistry has appeared. lo The microwave spectrum of o-benzoquinone is consistent with an essentially planar classical o-quinonoid structure in the gas phase."' Cerium(1v) oxide-hydrogen peroxide is a useful reagent for the conver- sion of phenols into hydroperoxycyclohexa-2,5-dienones.112a Diphenylselenic anhydride [(PhSeO),O] reacts with phenols under neutral conditions to give 0-and p-hydroxycyclohexadienones whereas with the phenolate anions only the ortho-orientation is observed. ''2b The N-chlorosuccinimide-triethylaminecomplex is an effective oxidizing agent for the conversion of catechols into o-quinones and hydroquinones into p-q~inones."~ Oxidative demethylation of 2,5-dimethoxybenzaldehyde by the silver(I1) oxide method was unsuccessful but the corresponding diacetate gave 1,4-benzoquinone aldehyde diacetate quantita- tively.' l4 Io5 T. Cohen and J. G. Tirpack Tetrahedron Letters 1975 143.lo6 E. C. Taylor H.W. Altland and A. McKillop J. Org. Chem. 1975,40 2351. lo7 W. T. Ford T. B. Thompson K. A. J. Snoble and J. M. Timko J. Amer. Chem. Soc. 1975,97,95. lo8 M. Minabe and K. Suzuki J. Org. Chem. 1975,40 1298; Bull. Chem. SOC.Japan 1975,48 1480. Io9 G. Wittig and W. Schoch Annalen 1975,600. ]lo 'Chemistry of Quinonoid Compounds' Part I ed. S. Patai Wiley Chichester England 1974. G. L. Blackman R. D. Brown and A. P. Porter J.C.S. Chem. Comm. 1975,499. lI2 (a)D. H. R. Barton P. D. Magnus and J. C. Quinney J.C.S. PerkinI 1975,1610; (b)D. H. R. Barton P. D. Magnus and M. N. Rosenfeld J.C.S. Chem. Comm. 1975,301. 113 H. D. Durst M. P. Mack and F. Wudl J. Org. Chem. 1975,40 268. D. V. Rao H. Ulrich and A. A. R. Sayigh J. Org. Chem. 1975 40 2548.Aromatic Compounds (35) (36) The singlet m-quinone (35) is s~ggested''~ as an intermediate in the oxidative decarbonylation of 2,4,6- tri- t -butylresorcinol to give 2,3,5-tri- t- butylcyclopentadienone. p-Quinones co-ordinated to palladium(0) show distinctive behaviour and in the case of p-benzoquinone its reaction with butadiene forms a complex (36)where two molecules of butadiene are cyclized across one double bond.' l6 Lewis-acid catalysis reverses the regiospecificity of the Diels-Alder addition of alkylated 1,4-quinones to certain substituted 1,3-dienes. 'I7 p-Benzoquinones undergo photochemical addi- tion to tetramethyl- or 1,l -dimethyl-allene to form 5-hydroxyindan-2-one deriva- tives by a mechanism involving subsequent rearrangement.ll8 The Thiele-Winter acetoxylation of a series of methoxyphenyl- hydroxyphenyl- and aryl-substituted p-benzoquinones has been studied.The acetoxy-group always enters ortho or para to the phenyl or aryl group and never ortho to the hydroxy- or methoxy-substituent. A related study involves analogues bearing both bromine and phenyl groups. 'lY6 Chloro-p-benzoquinone reacts with veratrole both in aqueous sulphuric acid'20a and in the presence of aluminium chloride'206 to give various triphenylene derivatives. The thermal behaviour of diazido- 1,4-quinones has been studied.'" The 2,5- and 2,6-compounds undergo a unique thermal cleavage via a cyclopentene- 1,3-dione intermediate to give the very reactive cyanoketens. The He(r1) photoelectron spectrum of p-quinodimethane formed by the flash vacuum pyrolysis of [2,2]paracyclophane has been observed and it indicates that the ground-state structure is that of a polyene.'22 Photodecarbonylation of 1,3-diphenylinden-2-one-substitutedmaleimide adducts gave the o-quinodimethanes (37) which undergo a thermal 1,5-sigmatropic shift of the imido-group rather than alkyl migration.'23a Compounds analogous to (37a) but with less than two of the sterically stabilizing phenyl substituents are unstable but undergo photochemical conversion into cyclobuta[3,4]cyclobuta[1,2]benzenes which revert to o-quinodimethanes on heating.1236 The intermediacy of the twisted and strained o-quinodimethane (38) is indicated in a study of the photochemical decarbonylation W.H.Starnes jun. D. A. Plank and J. C. Floyd J. Org. Chem. 1975,40 1124. 116 H.Minematsu S.Takahashi and N. Hagihara J.C.S. Chem. Comm. 1975,466. 117 Z. Stojanac R.A. Dickinson N. Stojanac R. J. Woznow and Z. Valenta Canad. J. Chem. 1975,53 617. N. Ishibe K. Hashimoto and Y. Yamaguchi J.C.S.Perkin I 1975 318. Ily (a)J. M. Blatchly. R. J. S. Green J. F. W. McOmie and S. A. Saleh J.C.S.Perkin I 1975,309;(b)J. F. W. McOmie J. B. Searle and S. A. Saleh ibid. p. 314. 120 (a)R.Buchan and 0.C. Musgrave J.C.S. Perkin I 1975 811; (6)ibid. p. 2185. lz1 D. S. Pearce M. J. Locke and H. W. Moore J. Amer. Chem. SOC.,1975,97,6181; W. Weyler jun. W. G. Duncan and H. W. Moore ibid. p. 6187. lZ2 T. Koenig R.Wielesek W. Snell and T. Balle J. Amer. Chem. Soc. 1975,97 3225.123 (a)D. W.Jones and G. Kneen J.C.S. Perkin I 1975 171; (b) ibid. p. 175. 230 R. G. Cbornbes of the methyl benzobicyclo[3,2,l]octen-8-one-3-endo-and -exo-~arboxylates.~~~ The isoindenes [e.g. (39)]previously proposed as intermediates in the photo- rearrangement of 1,l-diarylindenes have been directly observed by flash photo- lysis.12' (37) a; R' = R2 = Me b; R'-RZ = (CH,) A synthetically useful route to p-benzoquinone ethylene acetal the approaches to which were previously limited is provided'26 by the photolysis of 2-phenoxyethanol in the presence of mercuric oxide and iodine via 2-(p-iodophenoxy)ethanol. Diquinocyclopropanones are formed by oxidation of bis(hydroxyaryl(cyc1opro-penones from the reaction of trichlorocyclopropenium tetrachloroaluminate with the appropriate hindered phen01s.l~~" They undergo spontaneous decarbonylation to the corresponding cumulene derivatives (40).Compound (40c) dimerizes thermally to the tetraquinocyclobutane (4 1). R R .(I)=C=COO R R (40) a; R = Me b; R = Pr' C; R = Bu' (41) Cyclophanes.-The reaction involving thermal extrusion of sulphur dioxide to form ethano-bridges has been extended to give a new method for synthesis of medium and large hydrocarbon rings,128 and this has been applied to the synthesis of [4](4,4")-orthoterphenylophane. Some [2,2]cyclophanes are now much more readily prepared.12'" The experimental procedure for production of dithia[3,3]cyclophanes has been markedly improved and subsequent Wittig rearrangement to substituted [2,2]cyclophanes is a route to be preferred to that involving Stevens rearrangement.For paracyclophanes and metacyclophanes where 1,6-elimination is a possibility the conversion is best accomplished by the 'benzene-Stevens' route (Scheme 2).129b Iz4 D. S. Weiss J. Amer. Chem. SOC.,1975,97 2550. Iz5 J. J. McCullough and A. J. Yarwood J.C.S. am. Comm. 1975 485. A. Goosen and C. W. McCleland J.C.S. Chem. Comm. 1975 655. (a)R. West D. C.Zeder S. K. Koster and D. Eggerding J. Org. Chem.,1975,40,2295; (b)S. K. Koster and R. West ibid. p. 2300. F. Vogtle and J. Grutze Angew. Chem. Znternat. Edn. 1975,14 559. (a)R.H. Mitchell T. Osubo and V. Boekelheide TetrahedronLetters 1975,219; (b)T. Otsubo and v. Boekelheide ibid. p. 3881. Aromatic Compounds 231 Ph-S PhSO 1 to1 I -CHZ-S-CHz-Benzyne+ -CH-CH,-+ -CH-CH2-% -CH=CH-Scheme 2 A simple synthesis of [2,2,2,2]paracyclophane-1,9,17,25-tetraene from terephthalaldehyde and p-bisbromomethylberzene by a Wittig procedure has been rep~rted,'~' and the series of [2"]paracyclophanes has been extended131 to include n =5,6 and 8.Energies for the axial-equatorial conformational change of the ethylene bridge protons have been established for the series. Studies of a series of new disubstituted [2,2]paracyclophanes in which each ring bears one substituent have been carried out. 132 Spectral characteristics of such compounds were attributed to transannular electronic effects but no transannular directive effects were observed for the nitration of 4-cyano- or 4-acetyl-[2,2]paracyclophane.Tetra- and octa-fluoro[4,2]paracyclophanes have been prepared,'33 and comparison with the corresponding octafluoro[2,2]paracyclophane may indicate ring distortions in the latter with the increased proximity of the aromatic rings. A one-step synthesis of [nJmetacyclophanes(n= 8-12) and 2,6-pyridinophanes by nickel catalysis of the cyclocoupling of diGrignard reagents with aromatic dihalides has been rep~rted.'~~ A new preparative route to metacyclophanes (Scheme 3) can be applied even to [6]-and [7]-metacyclophane~.~~~ Various (42) Reagents i CHBr,-Bu'OK. Scheme 3 derivatives are then available via the (6 +n)-lithio[n]metacyclophane. Photolysis of (42) produces transannular products. Studies of transannular reactions of [2,2]metacyclophane have revealed'36 a new type of disproportionation reaction with aluminium chloride giving rise to hydropyrenes and pyrene.Metacyclo-polynuclear carbophanes can be synthesized by the transannular reaction of mul- tilayered metacy~lophanes'~~ [e.g. (43)-+(44) and (45)]. The quadruply bridged [2,2,2,2]( 1,2,4,5)cyclophane (46) has been re~0rted.l~~ Two bridges were formed I3O B. Thulin 0.Wennerstrom and H. E. Hogberg Acta Chem. Scad. (B),1975,29,138. 131 I. Tabushi H. Yamada and Y. Kuroda J. Org. Chem. 1975,40 1946. 132 H. Allgeier M. G. Siegel,R. Helgeson E. Schmidt and D. J. Cram J. Amer. Chem. Soc. 1975,97,3782. 133 R. Filler and E. W. Choe Canad. J. Chem. 1975,53 1491. 134 K. Tamao S.-I. Kodama T.Nakatsuka Y. Kiso and M. Kumada J.Amer. Gem. Soc. 1975,97,4405. 135 S. Hirano H. Hara T. Hiyama S. Fujita and H. Nozaki Tetrahedron,1975,31 2219. 136 K. Nishiyama K. Hata and T. Sato Tetrahedron,1975,31,239. 137 T. Umemoto T. Kawashima Y. Sakata and S. Misumi Tetrahedron Letters 1975,463. 138 R. Gray and V. Boekelheide Angew. Chem. Zntemat. Edn. 1975,14 107. R. G.Coombes (45) by the photochemical sulphur-extrusion procedure and two by transannular car- benoid insertion reactions. [2,2](2,7)Pyrenophane (47) and [2,2]( 1,3)pyrenophane (48) have been prepared by routes initially involving [2,2]metacyclophane deriva- tives and the formation of the pyrene structure by transannular ~eacti0n.l~~ Attempts14' to synthesize the triple-layered compound (49) produced the com-pound with structure (50).This may be formed via (49) if the strained inner benzene nucleus reacts intramolecularly as a dienophile with the anthracene nucleus. The triply clamped helical compound (51)has been prepared via the trithia-compound and Stevens rearrangement.I4l The two triphenylethane systems are fixed in propel- ler form but mutally displaced giving a new type of helically chiral molecular skeleton. 139 T. Umemoto S. Satani Y. Sakata and S. Misumi Tetrahedron Letters 1975 3159; T. Umemoto T. Kawashima Y. Sakata and S. Misumi Chem. Letters 1975 837. 140 T. Toyoda A. Iwama Y. Sakata and S. Misumi Tetrahedron Letters 1975,3203. l4I F.Vogtle and G. Hohner Angew. Chem. Internut. Edn. 1975 14,497. Aromatic Compounds Me I .C C I Me (49) (51) (50) 4 Molecular Rearrangements The Claisen rearrangement has been reviewed,14* as has the multiplicity of rear- rangement paths undergone by cyclohe~adienones.~~~ The naphthalenone (52a) rearranges in acidic acetic anhydride to give (53) in good yield the product of a 'forbidden' suprafacial [1,4] shift whereas it gives symmetry-allowed products arising from [3,4] and [1,5] shifts in aqueous sulphuric acid as does the allyl analogue (52b) under both sets of ~0nditions.l~~ The thermal rearrangement of (I?)-1- propenyl-2-naphthyl allyl ether to give (54) proceeds by a double [3s,3s] path no d y d;(eCH=CH2 \/ \/ I (52) a;X =H,Y =Me Me (54) b; X =Y =H or D (53) evidence for a sigmatropic [1,5s] allyl shift being Ally1 groups in some ortho-semibenzenes [e.g.(55)] rearrange so rapidly [to (56)]that it is not possible to isolate the former.146 This process involves a [3,3] sigmatropic shift rather than the radical-chain mechanism favoured with para-semibenzenes. 1,3-Benzyl migration in analogous compounds that are stable however presumably proceeds by the free-radical process. The Stevens rearrangement of benzyldimethylphenacylammonium ylide does not proceed by consecutive [1,4] and [1,3] shifts.'47 Stere~chemical'~~~ and quantitative CIDNP on the rearrangement of closely related ylides are compatible with two possible mechanisms (a) the generally accepted radical-pair mechanism but with an average geminate recombination which is extremely fast or (b) dual 14* S.J. Rhoads and N. R. Raulins org. Reactions 1975 22 1. 143 B. Miller Accounts Chem. Res. 1975 8 245. 144 B. Miller and M. R. Saidi Tetrahedron Letters 1975 1365. 145 M. Muelly J. Zsindely and H. Schmid Heiv. Chim. Acta 1975 58 610. 146 B. Miller and M. R. Saidi Tetrahedron Letters 1975 1691. S. H. Pine and J. Cheney J. Org. Chem. 1975 40 870. 148 (a)W. D. Ollis M. Rey I. 0.Sutherland and G. L.Closs J.C.S. Chem. Comm. 1975,543; (6)U. H. Dolling G. L. Closs A. H. Cohen and W. D. Ollis ibid. p. 545. 234 R. G.Coombes (55) R = H or Me (56)R = H or Me pathways involving concurrent radical-pair and concerted processes where the latter predominates. The extent (7%) of 1,2-phenyl shift in the triphenylvinyl cation during silver-ion- catalysed acetolysis of the corresponding bromide is similar to that observed during acetolysis of the t~iflate.'~~' The second example of a 1,2-hydride shift in a vinyl cation has been noted 1496 during the acetolysis catalysed by silver ion of cis-or trans-p-bromostyrene.The data again precluded the intermediacy of a hydrogen- bridged ion. The ratios of benzaldehyde to substituted benzaldehyde from the reactions of substituted benzhydryl azides with nitrosonium tetrafluoroborate and with substituted benzhydryl tetrafluoroborates are similar and provide confirmation that aldehyde products are formed from benzhydryl azide by a Curtius rearrange- ment with the benzhydryl ~ati0n.l~' There is steric interference to the aryl migration by the benzhydryl group.Products of the acid-catalysed rearrangement of N-acetylhydrazobenzene have been interpretedl'l in terms of the involvement of phenylnitrene in either the rearrangement or disproportionation reactions. These data are quoted in support of one variant of the controversial C-N diprotonation hypothesis [cf. Ann. Reports (B) 1972,69,263]. A kinetic study of the two-proton-catalysed rearrangement of hydrazobenzene to benzidine in moderately concentrated acid suggests that the second protonation may well be part of the rate-limiting No conclusion could be drawn from this work on the site of protonation. E.s.r. studies that cation radicals are involved in the mechanism of reductive scission of NN'-dimethylhydrazobenzenes which occurs concurrently with rearrangement.The rates of reactions proceeding by both one- and two-proton-catalysed mechanisms were enhanced by anionic micelles of Na' -O,SO(CH,) ,Me.'54 A kinetic study of the unusal Wallach rearrangement undergone by 2,2',4,4',6,6'- hexame thylazobenzene to 4- h ydroxyme t hyl- 2,2',4',6,6'-pen tamet hylazobenzene 149 (a)F H. A. Rummens R. D. Green A. J. Cessna M. Oka and C. C. Lee Canad. J. Chem 1975,53 314;(b) C.C. Lee and E. C. F. KO J. Org. Chem. 1975,40,2132. 150 M. P. Doyle D. M. Hedstrand S. C. Busman and D. Alexander,J. Amer. Chem. SOC.,1975,97,5554. 151 Z.J. Allan Monatsh. 1975,106 429. 152 C. A. Bunton and R. J. Rubin Tetrahedron Letters 1975 59. 153 J.-D. Cheng and H. J. Shine J. Org. Chem 1975,40,703.C. A. Bunton and R. J. Rubin Tetrahedron Letters 1975,55. Aromatic Compounds 235 has demonstrated the operation of two mechanistic pathways depending on solvent a~idity.’~’” One involves a dicatonic intermediate and the other a quinoid species. This dichotomy of mechanism has been further illustrated in conventional rearrange- ments in the azoxynaphthalene series. ”” Interestingly in moderately concentrated sulphuric acid both (57) and (58)yield (S9) the former via a quinoid intermediate and the latter via the dicationic mechanism.155c The rearrangement has been reviewed.lSsd The evidence concerning the mechanism of the Fischer-Hepp rearrangement of N-nitroso-amines has been discu~~ed.”~” The intramolecular nature of the rear- rangement process occurring concurrently with a reversible denitrosation reaction has been further demonstrated by studies of halide-ion catalysi~’’~~ and of the rearrangement :denitrosation ~atio.”~~ The latter study has yielded quantitative information on the efficacy of various nitrite traps.The denitrosation process in hydrochloric acid solution involves nucleophilic attack on the protonated nitro- samine and the rate is strongly dependent upon the nucleophile Denitrosation in sulphuric acid solution is not due to the hydrogen sulphate ion but both water and H30+ are suggested to be reactive entities. Protonated N-methyl-N- nitrosoaniline transfers a nitroso-group directly to thiourea without the intermediacy of a free nitrosating 5 Condensed Systems Resonance and localization energies of benzenoid hydrocarbons can,be obtained by an algorithm based on counting resonance structures and the results are in good agreement with those from highly parametrized SCF-LCAO-MO calculation^.^^^ It has been by the graph-theoretical approach that the early success of resonance theory rested on the fortunate fact that all KekulC structures for ben- zenoid hydrocarbons and acyclic polyenes have the same parity and a similar has yielded an explanation for the success of the theory for predicting relative rates of aromatic substitution.An index based on counting Kekul6 structures has been propo~ed”~ which describes the local benzenoid character of individual rings in condensed benzenoid hydrocarbons and which when summed over all rings indicates the degree of ‘benzenoidicity’ of the molecule.5-Methylene-l,2-benzocyclohexa- 1,3-diene (60) has been synthesized by the reaction of 3,4-benzotricyc10[4,1 ,O,O*”]heptene with v-allylpalladium(II) chloride dimer in chloroform.’60 It is sufficiently stable to permit isolation but in the pure liquid it undergoes valence isomerization to 2-methylnaphthalene at a moderate rate at room temperature. Substituted 1,2-dihydronaphthalenes can be synthesized in good yield by con- trolled gas-phase pyrolysis of the appropriate l-phenylbuta- l,3-diene.l6’ A route to lS5 (a)R. A. Cox and E. Buncel J. Amer. Chem. SOC.,1975,97,1871; (b)R. A. Cox A. J. Dolenko and E. Buncel J.C.S. Perkin fI 1975,471 ;(c) E. Buncel R. A. Cox and A.Dolenko Tetrahedron Letters 1975 215; (d)E. Buncel Accounts Chem. Res. 1975,8 132. lS6 (a)D. L. H. Williams Tetrahedron,1975,31 1343; (b)D. L. H. Williams Internut. J. Chem. Kinetics 1975,7,215; (c)D. L. H. Williams J.C.S. Perkin If 1975,655;(d)I. D. Biggs and D. L. H. William< ibid. p. 107; (e)D. L. H. Williams J.C.S. Chem.Comm. 1975 375. 157 R. Swinborne-Sheldrake W. C. Herndon and I. Gutman TetrahedronLetters 1975,755. 158 (a)J. Gutman N. Trinajstic and C. F. Wilcox jun. Tetruhedron 1975,31 143; (b)ibid. p. 147. lS9 M. Randic Tetrahedron,1975,31 1477. lM) I. Murata J. Nakazawa M. Kato T. Tatsuoka and Y. Sugihara Tetrahedron &tters 1975 1647. P. B. Valkovich J. L. Conger F. A. Castiello T. D. Brodie,and W. P. Weber J. Amer. Chern.Soc. 1975 97 901.236 R. G. Coornbes 3-substituted 1-methylnaphthalenes is provided by the reaction of substituted 1-methylindenes with dibromocarbene,'62 and the reaction of pentacar-bonyl[methoxy(phenyl)carbene]chromium(0) with diphenylacetylene yields the complex (61) in which the naphthol skeleton has been ~ynthesized.'~~ A new approach to functionalized naphthalene^'^^ involves the reaction of substituted benzaldehydes with the appropriate Grignard derivative followed by the cyclization (62) -+(63). Phenyl radicals react with dimethyl but-2-ynedioate to give tetramethyl n 00 H' OH naphthalene-1,2,3,4-tetracarboxylatevia intermediate styryl radi~a1s.l~~ A direct method for the preparation of substituted naphthalenes and naphthols from benzenes and conjugated ketones and esters respectively uiu dienolate anions has been described.'66 The same benzyne is formed from both 2-and 3-bromoanisole and this reacts with the anion from mesityl oxide to give 8-methoxy-1,3- dimethylnaphthalene both ct-and y-attack by the anion on the rnetu-position being important.Symmetrical 9,lO-unsubstituted anthracenes can be synthesized in one step in good yield from substituted bromobenzenes by using N-lithio-2,2,6,6- tetramethylpiperidine in THF.167 The sequence of reactions involves benzyne and the enolate ion of acetaldehyde arising from the base-catalysed cleavage of the solvent. 162 S.J. Gillespie jun. S. P. Acharya and D. A. Shamblee J. Org. Chem. 1975,40 1838. K. H. DOG,Angew. Chem.Internat.Edn. 1975,14,644. 164 H.J. J. Loozen J. Org. Chrn. 1975,40 520. 165 B. D. Baigrie J. Brenan J. I. G. Cadogan,J. Cook and J. T. Sharp J.C.S. Perkin I 1975 1060. 166 P. G. Sammes and T. W. Wallace J.C.S. Perkin I 1975 1377. 167 I. Fleming and T. Mah J.C.S. Perkin I 1975 964. Aromatic Compounds 237 The first unequivocal 1,5-naphthoquinone (64) has been synthesized16* and 1,6-methano[ 101annulene has been converted into 9,lO-homonaphthodiquinone (65) by a pathway entailing electrochemical reaction. 16' The reaction of some naphthalenes containing more than two methyl substituents with dichlorocarbene yielded relatively stable polymethyl-substituted 1,2-benzoheptafulvenes as well as bis(dichloromethano)tetrahydronaphthalene~.'~~~ Naphthalene underwent ring opening to 6-chlorobenzocycloheptatrienes anthracene to 6-chloro- 11H-dibenzo[a,e]cycloheptatriene and phenanthrene gave the 9,10-nor~aradiene.'~~~ The efficient photocyanation of phenanthrene and naphthalene with sodium cyanide in the necessary presence of 1,4-dicyanobenzene has been rep~rted,'~' and irradiation of phenanthrene in the presence of various amines causes the photofixation of carbon dioxide to yield 9,lO-dihydrophenenthrene-9-carboxylicacid.172 Measurements of enthalpies of activa- tion are held to provide proof that the Diels-Alder reaction of tetracyanoethylene with 9,lO-dimethylanthracene passes through the formation of a complex between the reactants.173 Kinetic of the reaction of sodium naphthalene with n-hexyl halides accord with the usual electron-transfer mechanism but the hexyldihydronaphthyl anion is a discrete intermediate and reacts relatively slowly with the halide in contrast to the corresponding reaction with water.With n-hexyl fluoride the solvent effects are opposite to those with the other halides and it is that in the former case the negative charge in the transition state is slightly localized relative to that of the naphthalene radical anion. Low-field CIDNP studied7& of the reactions of sodium naphthalene and anthracene with water provide evidence not observed at high field and suggest that a significant fraction of the reaction proceeds through direct radical-anion protonation. Pyrene-like molecules [e.g. (66)] have been prepared from corresponding mul- tilayered metacyclophanes by routes involving transannular reaction and subsequent dehydr~genation'~' (see also refs.137 and 139). A new synthesis of mronene from 168 H. L. K. Schmand and P. Boldt J. Amer. Chem. SOC.,1975,97,447. 169 W. Bornatsch and E. Vogel Angew. Chem. Internat. Edn. 1975,14,420. 170 (a)A. Oku T. Hino and K. Matsumoto J. Org. Chern.,1975,40,695;(b)G. Blume T.Neumann and P. Weyerstahl Annalen 1975,201. 171 K. Mizuno C. Pac and H. Sakurai J.C.S. Chem. Cbmm. 1975,553. 172 S. Tazuke and H. Ozawa J.C.S. Chem. Cbmm. 1975,237. 173 V. D. Kiselev and J. G. Miller J. Amer. Chem. SOC.,1975,97,4036. 174 (a)S. Bank and D. A. Juckett J. Amer. Chem.Soc. 1975,97,567; (b)J. F. Garst R. D. Roberts and B. N. Abels ibid. p. 4925; (c)J.F. Garst and J. A. Pacifici ibid. p. 1802. 175 T. Umemoto T. Kawashima Y.Sakata and S. Misumi TetrahedronLetters 1975 1005. 238 R. G.Coombes readily available starting materials has been reported.176 3,6-Bis(bromome thy1)phenan threne reacts with p -xylene-a,a '-di thiol and after photochemical sulphur extrusion to form a cyclophane intermediate treatment with aluminium trichloride in carbon disulphide and dehydrogenation yields coronene. The cyclohepta[def]phenanthrenylium ion the second 14~-perimeter cationic species has been synthesized. 177u The indications are that (67) contributes largely to its ground state structure and that a peripheral diamagnetic ring current is of no consequence. By contrast studies of the cyclopenta[cd]phenalenyl anion indicate that a reasonable representation of the ground state is the delocalized (68) a perturbed [13lannulenide ion.1776 The properties of various derivatives support the utilization of a peripheral electronic model which describes the dibenzo[ cd,gh]pen- talene system as a perturbed [121ann~lene.~'~ [8H]-Cyclopent[a]acenaphthylene (69) is a new example of the relatively small group of planar acidic hydrocarbon^.'^^ @ / \ (67) (68) (69) N.m.r. studies of its anion which takes up a deuteron at C-8 suggest that the charge resides not only on the cyclopentadienyl moiety but also to an extent on the naphthalene skeleton. T-SCF force-field calculations of [5]- [6]- and [7]-helicenes have yielded geometries and energies of racemization the latter showing agreement with experi- mental data.'" A novel simple synthesis of [6]helicene in 55% yield from a 2-naphthaleneacetonitrile by a route involving initial condensation with 2-dimethylamino-1,l-bis(dimethyliminomethy1)ethene diperchlorate has been described,'" and [6]helicene has also been prepared by photochemical ring closure of polymer-supported 1,2-diarylethenes lS2 The all-benzene helicene series up to [14]helicene is now c~mplete."~ Examples of [11]- [121- and [14]-helicenes have been synthesized in one operational step by the previously used photo-induced double cyclodehydrogenations of bis(arylviny1)arenes.183u A systematic study of these photochemical ring closures using circularly polarized light has been reported,183b although no asymmetric syntheses were observed in the cases of the higher benzoiogues of [lO]helicenes.1-Hydroxymethyl[6]helicene rearranges to the spiroindene (70)on treatment with and the corresponding aldehyde yields 176 J. T. Craig B. Halton and S.-F. Lo,Austral. J. Chem. 1975,28 913. 177 (a)I. Murata K. Yamamoto Y. Kayane and H. Ori Tetrahedron Letters 1975 131; (b)I. Murata K. Yamamoto M. Morioka M. Tamura and T. Hirotsu ibid. p. 2287. 178 B. M. Trost and P. L. Kinson J. Amer. Chem. SOC.,1975 97 2438. 179 K. Yamamoto M. Morioka and I. Murata Tetrahedron Letters 1975 3009. 180 H. J. Lindner Tetrahedron 1975,31 281. l*l C. Jutz and H.-G. Liibering Angew. Chem. Znternat. Edn. 1975,14,418. IS2-J. M. Vanest M. Gorsane V. Libert J. Pecher and R. H.Martin Chimiu (Switz.) 1975 29 343. IS3 (a)R. H. Martin and M. Baes Tetrahedron 1975,31,2135;(b)A. Moradpour H. Kagan M. Baes G. Morren and R. H. Martin ibid. p. 2139. Aromatic Compounds indene (70) on treatment with acid,'84a and the corresponding aldehyde yields mainly (71) on trwtment with the ylide of (EtO)2POCH2C0,Et.'84b 1,16-Didehydrohexahelicene (hexa[7]circulene) (72) has been synthesized by a cyclo- phanediene route.185 This molecule is probably saddle-shaped and consequently dissymmetric. (71) R = C0,Et 6 Non-benzeneSystems The first volume of a proposed series of compilations of topics in non-benzenoid aromatic chemistry has appeared.186 Three and Four-membered &@.-The simplest known antiaromatic system is now claimedlg7 to be the di-t-butyl-(3,5-di-t-butylphenyl)cyclopropenylradical (73).The (73) antiaromaticity of the 3~-electron system is held to be demonstrated both by the fact that delocalization of the unpaired electron in the cyclopropenyl ring is less than in the allylic portion of an open-chain analogue and also by the fact that the unpaired electron resides to a greater extent in the phenyl ring of (73) than does that of the benzyl radical. An improved synthesis of tris-dialkylaminocyclopropeniumchlorides involving an exchange procedure has been reported and oxidation of these ions by anhydrous antimony pentachloride leads to the precipitation of brick-red stable salts of the radical dications (74).'88 Total ferrocenyl substitution has been shown to be an alternative to using dialkylamino-groups for example as stabilizing substituents in the cyclopropenium ion and triferrocenylcyclopropenium perchlorate has been prepared.'89 The tri-t-butylcyclopropenium ion substitutes electrophilically in the cyclopentadienyl of some Group VI metal carbonyl derivatives {e.g.[(q'-(a)R.H.Martin J. Jespers and N. Defay TetrahedronLetters 1974,1093;(6)Helv. Chim.Acra 1975 58 776. 185 P. J. Jessup and J. A. Reiss Tetrahedron Letters 1975 1453. 'Topics in Nonbquenoid Aromatic Chemistry' ed. T. Nozoe R. Breslow K. Hafner S. Ito and I. Murata Wiley New York 1975. lg7 K. Schreiner W. Ahrens and A. Berndt Angew. Gem. Internat. Edn. 1975,14 550. IE8 R.Weiss and K. Schloter Tetruhedron Letters 1975 3491. 189 1. Agranat and E.Aharon-Shalom J. Amer. Chern. SOC.,1975,97 3829. 240 R. G.Coombes (74) (75) R = Pr' C,H,)M(CO)J} and in other cases forms the oxocyclobutenyl ligand.lgO Reactions of the 1,2-bis(di-isopropylamino)-3-chlorocyclopropeniumion with the cyclopen- tadienide and indenide ions lead to what is c1aimedl9la as a new type of tripolar compound [e.g. (75)]. However similar compounds containing a cyclopentadienide ring that is 1,3-linked to two N-alkylpyridinium rings have been prepared,"lb and evidence has been presented that these at least are best represented as hybrids of monocationic forms. MINDO/3 calculations agree with some earlier results in predicting a singlet ground state for cyclobutadiene and the suggestion has been made that the square or effectively square species obtained by the matrix-isolation technique [Ann.Reports (B) 1973 70 4151 is the excited triplet.19* Another theoretical treatment,193 however suggests that when cognizance is taken of the effects of electron repulsion in open-shell systems the ground state of the species can be a square or effectively square singlet. An X-ray analysis of the cyclobutadiene (76) chosen to be as stable but as unperturbed as possible provides clear evidence that the most stable conformation is a rectangle distorted slightly by the substituent~.~~~ A comparative photoelectron spectroscopic study of tri-t-butylcyclobutadiene and (77) for which Me0,C DBU' Bu' Bu' (76) u = 1.506A b = 1.376.A c = 1.547.A (77) d = 1.4068 an X-ray analysis is available suggests a rectangular structure of the butadiene ring here al~0.l~' INDO calculations on the stable donor-acceptor cyclobutadienes suggest that their most stable geometry is that of the D2,, paralle10gram.l~~ A new method has been described'97 for the detection of reaction intermediates involving the generation of an intermediate from an insoluble polymer-bound precursor and its trapping by a second solid phase suspended in the same medium.190 M. Green and R. P. Hughes J.C.S. Chem. Comm. 1975,862. 191 (a)Z. Yoshida S. Araki and H. Ogoshi Tetrahedron Letters 1975 19; (b)W. D. Erhardt and H. L. Amrnon ibid. p 3997. 192 M. J. S. Dewar and H. W. Kollmar J. Amer. Chem. SOC.,1975,97 2933. 193 W. T. Borden J. Amer. Chem. SOC.,1975,97 5968.194 L. T. J. Delbaere M. N. G. James N. Nakamura and S. Masamune J. Amer. Chem. SOC.,1975,97 1973. 195 G. Lauer C. Muller K. W. Schulte A. Schweig G. Maier and A. Alzerreca Angew. Chem. Internat. Edn. 1975 14 173. 196 C.U. Pittman jun. K. L. Douglas Q. Y. Ng W. Hunter D. Pace and L. D. Kispert J. Org. Gem. 1975 40 2121. 197 J. Rebek and F. Gavina J. Amer. Chem. SOC.,1975,97 3453. Aromatic Compounds 241 Oxidation of a polymer-bound derivative of cyclobutadieneiron carbonyl in the presence of a polymer-bound maleimide derivative has been shown by this technique to proceed with 96% transfer of free cyclobutadiene. Stereochemical studies on the oxidation of the tricarbonyl(cyclobutadieny1)iron complex (78) in the presence of dimethyl maleate to give the bicyclo[2,2,0]hexene have also implicated the inter- mediacy of a strongly selective free cy~lobutadiene.'~~ Tetrafluorocyclobutadiene has been revealed'- as an intermediate in the photolysis of (79) from the nature of transformation and trapping products.1,3-Bis(diethylamino)-2,4-diphenylcyclobutadiene which does not fit the usual donor-acceptor substituent structure for stability has been synthesizedzm and is stable for more than 1h in the solid at room temperature. E! (78) (79) Tri-t- butylcyclobutadiene undergoes ionic as well as synchronous and radical additions and it exhibits unusual activation parameters for its dimerization reaction in accord with an extremely sterically hindered transition state.20' Thermolysis of syn-and anti-dimers of cyclobutadiene to yield cyclo-octatetraene and the activa- tion parameters obtained have been discussed2o2a in terms of a biradical mechanism rather than the unusual major direct triplet pathway proposed elsewhere.202b Butalene (80),which has two fused 4nm rings is a transient intermediate in some reactions of 3-chloro[2,2,0]bicyclohexane induced by certain bases at moderately low The vigorous conditions necessary support the conclusion that such fused ring systems are not strongly stabilized by their overall content of (4n+2) m-electrons.The second benzocyclobutadiene (81) has been as blue needles on heating the cyclobutene (82). It is fairly stable in the absence of oxygen and e.s.r. and magnetic measurements indicate a singlet ground state.204b Bu' Bu' 19* E.K. G. Schmidt Chem. Ber. 1975,1OS,1609. 199 M.J. Gerace D. M. Lemal and H. Ed,J. Amer. Chem Sm. 1975,97,5584. R. Gompper S. Mensch and G. Seybold Angew. Gem. Intemat. an. 1975,14,704. 201 G.Maier and W. Sauer Angew. Chem. Intemat. Edn. 1975,14,648. 202 (a)H. M. Frey H. D. Martin and M. Hekman J.C.S. Chem Cornm. 1975,204;(6)R.S.Case M. J. S. Dewar S.Kirschner R.Pettit and W. Slegeir J. Amer. Chem Soc. 1975,196 7581. *03 R.Breslow J. Napierski and T. C. Clarke 3. Amer. chehz. Soc. 1975,97,6275. 204 (a)F.Toda and M. Ohi J.C.S. Chem Comm. 1975,506;(b)F.Toda and K. Mukai Chem.Letters 1975, 777. 242 R. G.Coombes Five-and Seven-membered Rings.-Ionization potential measurements on cyclo- pentyl and cyclopentadienyl radicals confirm2o5 that the cyclopentadienyl cation is antiaromatic and that it is destabilized by 7.6 kcal mol-' with respect to the cyclopen- ty1 cation.A convenient synthesis of alkylated fulvenes (83) from a,P-unsaturated acid chlorides (R'CH=CR2COCI) and alkynes (R3CH,CECH) via the 4-alkylidene-2-cyclopentenoneand a lithium alkyl (R4Li) has been reported.206 6,6-Dimethylfulvene dimerizes on the metal template on direct reaction with iron vapour using a low-temperature co-condensation procedure to give 1,l'- tetramethylethyleneferrocene (84) accompanied by 1,l-di-isopropylferro~ene.~~~ Me Fe Me Me (84) The first example of isomerization of 1,4-endoperoxides into 1,2-dioxetans with characterization and isolation of intermediates has been observed2'* in the sensitized photo-oxygenation of polyarylfulvenes.X-ray analysis of a series of pentafulvalenes [e.g. (SS)] has been used to develop relationships between the charge density on the five-membered ring and carbon-carbon bond lengths.209 The 2- and 3-halogenotropone radical ions are generally unstable and give tropone uia a radical path that is of potential synthetic utility.210 The previously unknown 'p-tropoquinone' cyclohepta-3,6-diene- 1,2,5-trione (86) has been pre- pared2'la by several routes the most convenient of which involves chemical oxida- tion of 5-hydroxytropolone. Attempts to prepare '0-tropoquinone' by a similar method led to the 2-hydrate. Spectral evidence however was obtained for the unhydr ated species in solution.lb The bicyclo[ 3,2 ,O]hepta- 3,6-dien-2-one photo- chemical rearrangement products of some 2,S-disubstituted troponoids with electron-withdrawing groups at the 5-position revert to the original troponoids in the dark even below room temperature presumably by an ionic process.212 205 F. P. Lossing and J. C. Traeger J. Amer. Chem. Soc. 1975,97 1579. 206 C. Rabiller and G. J. Martin Tetrahedron Letters 1975,3713. 20' T. S. Tan J. L. Fletcher and M. J. McGlinchey J.C.S. Chem. Comm. 1975 771. 208 J. P. Le Roux and C. Goasdoue Tetrahedron 1975,31,2761. 209 H. L. Ammon and G. L. Wheeler J. Amer. Chem.Soc. 1975,97,2326. 210 M. Martinelli L. Nucci L. Pardi F. Pietra and S. Santucci Tetruhedron Letters 1975 2089. 211 (a)S. Ito Y.Shoji H. Takeshita M.Hirama and K. Takahashi TetrahedronLetten,1975,1075; (b) M. Hirama and S.Ito ibid. p. 1071. 2** T. Kobayashi T. Hirai J. Tsunetsugu H. Hayashi and T. Nozoe Tetrahedron 1975,31 1483. Aromatic Compounds The intermediacy of A’-pyrazoline species in the aerobic reaction of tropones with diazoalkanes to yield cyclo-octatrienones has been dernon~trated,’’~~ and this reaction with the tropone-tricarbonylion(0) complex to yield a stable A’-pyrazoline has been as part of a new synthesis of 2,3-homotropones (Scheme 4). Hexame thyl- tris-cr-homotropone is formed from 3,3-Reagents i R,CN,; ii Me,NO. Scheme 4 dimethylcyclopropene and carbon monoxide in the presence of a p hosphine- modified palladium(0) catalyst.214 The first syntheses of the oxygen analogues of homotropones uiz.tropone 2,3-oxide and 4,5-oxide have been described.215 An efficient synthesis of 4,5-benzotropone from o-xylylene dibromide involving bisalkylation with lithio-t-butyl acetate and Dieckmann cyclization to 43-benzocycloheptenone has been reported.216 The second benzo[3,4)-cyclobuta[l,2Jtropone (87) has been ~ynthesized,~” and n.m.r. studies indicate that the benzenoid protons resonate in the normal aromatic region. This suggests that there is no paramagnetic contribution from the four-membered ring. The formation and trapping of the benzyne analogue 4,5-didehydrotropone has been repOrted218 from the lead tetra-acetate oxidation of 1-amino-1H-cycloheptatriazol-6-one. It forms a 9,lO-adduct with anthracene. The properties of 2-hydroxy-2,4,6- and -2,4,7-cyclo-octatrienoneprovide some evidence for homoaromatic interaction and they have accordingly been termed ‘homotropolones’.219 Heptafulvene (88) has been synthesized2” and normally decomposes in a few seconds at room temperature whereas the 1,8-condensed heptafulvene (89)is stable 213 (a)M.Franck-Neumann and D. Martina Te@ahedmnktters 1975,1755; (b) ibid. p. 1759. 214 P. Binger and U. Schuchardt Angew. Gem. Infernat. Edn. 1975,14 706. ,15 R. Miyamoto T. Tezuka and T. Mukai Tetrahedron Letters 1975,891. 216 G. D. Ewing and L. A. Paquette J. Org. Chem. 1975,40 2966. 217 L. Lombard0 and D. Wege Tetrahedron Letters,1975,115. T. Nakazawa and I. Murata Angew. Chem. Znremaf. Edn. 1975,14,711. 219 Y. Kitahara M.Oda and S. Miyakoshi Tetrahedron Letters 1975 4141; Y. Kitahara M. Oda S. Miyakoshi and S. Nakanishi ibid. p. 4145. 220 W. K. Schenk R. Kyburz and M. Neuenschwander Helu. Chim. Acm,1975,58,1099. 244 R. G.Coombes 0 (88) (89) for several days.221 X-ray analysis of cu-(6-fulvenyl)diben[u,e]heptafulvene indi-cated222a boat-shaped seven-membered ring and a pronounced alternation of bond lengths. Photochemical dimerization of 1,4-dialkynylbenzenes gives azulene~~~~ [e.g. (90) from 1,4-bis(phenyIethynyl)benzene]. The remarkable anchimeric assistance shown by the 1-azulyl substituent in arylethyl arenesulphonate solvolysis is not shown224 by the 2-azulyl group and the relative abilities of the ring positions 1> 2> 6=r 4 to participate is also the order of HMO cation localization energies for these positions.The thermal rearrangements of azulenes to naphthalenes have been the subject of and it is suggested that most products can be accommodated by a mechan- ism involving attack of a radical (R.) on the seven-membered ring migration of the attacked carbon and substituents (i.e. CHRor CRMe) into the other ring and loss of R H or Me. Azulene is nitrosated at the 1-position at a rate which is that of encounter of the reactants but the reaction of 1-nitroazulene which shows a primary .kinetic isotope effect involves rate-limiting proton loss from the Wheland inter- mediate.226 AMulenes.-An account of the renaissance in cyclo-octatetraene chemistry has appeared227" and two examples of stable cyclo-octatetraene bond-shift isomers have been reported.Compounds (91) and (92)were prepared2276 by Mo(CO),-catalysed rearrangement of the propellatriene (93),which also forms (94),the product of a unique two-fold circumambulatory rearrangement. The former rearrangement probably proceeds by [1,5]-sigmatropic shift of a trigonal cyclobutene carbon within a molybdenum complex.227c 1,2,3,4-Tetramethylcyclo-octatetraeneand the bond- shift isomer 1,2,3,8-tetramethylcyclo-octatetraenehave also been prepared,227d 221 S. Kuroda M. Funamizu Y. Kitahara anbT. Asao Tetrahedron Letters 1975,3197. 222 H. J. Lindner K. Hafner M. Romer and B. Von Gross Annalen 1975,731. 223 G. Clauss and W. Ried Chem. Ber. 1975,108,528. 224 R. N. McDonald and J. M. Richmond J.Org. Chem. 1975,40 1689. 225 R. W. Alder and G. Whittaker J.C.S. Perkin II 1975,714; R. W. Alder and C. Wilshire,ibid. p. 1464. 2z6 B. C. Challis and R. J. Higgins J.C.S. Perkin II 1975 1498. 227 (a)L. A. Paquette Tetrahedron 1975,31,2855; (b) L. A. Paquette J. M. Photis K. B. Gifkins and J. Clardy J. Amer. Chem. Soc.,1975,97,3536; (c) L. A. Paquette and J. M. Photis TetrahedronLetters 1975,1145; (d)L. A. Paquette J. M. Photis and G. D. Ewing J. Amer. Chem.Soc. 1975,97,3538. Aromatic Compounds 245 Me + (93) (92) (94) and the activation energies for ring inversion and bond shifting are sufficiently large to be attainable only with difficulty. The reaction of cyclo-octatetraene with liquid sulphur dioxide promoted by antimony pentafluoride leads228a to the formation of 9-thiabarbaralane 9,g-dioxide (95) by an unprecedented 1,5-cycloaddition.Studies of the hydrolysis of cyclo-octatetraenylcarbinyl chloride have produced2286 no evidence for the involvement of the homotropylium ion but allylic stabilization as in (96) is more likely. Evidence has been presented228c that #I-cyclo-octatetraenylethylbrosylates undergo solvolysis to give initially spiro[7,2]nonatrienyl cations [e.g. (97)] and finally tetrahydro- azulenyl acetates. Whether the ring in (97) retains its tub conformation or partially (95) (96) (97) flattens to permit homoaromatic stabilization remains an open question. 1,4-Dinitrocyclo-octatetraene,the first example of this type of compound has been Nonafulvene has been prepared230 by the reaction of lithium cyclononatetraenide with acetoxybromomethane at low temperatures followed by elimination of acetic acid.It seems that the non-planar compound which shows no ring current exists as a racemic mixture of two enantiomeric conformers. 1,6-Methano[lO]annulene is a product of the hydrolysis of (98).231 An ub initio theoretical study of this molecule it to be of weak or intermediate aromaticity contrasting somewhat with its known properties. Indeed the first quantitative of the aromatic reactivity of annulenes has involved the H Br (98) (99) z28 (a)L. A. Paquette U. Jacobsson and M. Oku J.C.S. Chem. Comm 1975,115; (b)W. Kitching K. A. Henzel and L. A. Paquette,J. Amer. Chem. SOC.,1975,M 4643; (c) L. A. Paquette and K.A. Henzel ibid. p. 4649. 2z9 N. N. Podgornova E. S. Lipina and V. V. Perekalin J. Org. Chem.(U.S.S.R.),1975,11,209. 230 M. Neuenschwander and A. Frey Chimia (Switz.) 1975,29,212. 231 D. B. Ledlie and L. Bowers J. 0%.Chem.,1975,40,792. z32 G. L. Grunewald I. M. Uwaydah R. E. Christoffersen and D. Spangler TetrahedronLetters 1975,933. 233 R.Taylor J.C.S. Perkin IZ 1975 1287. 246 R. G. Coombes detritiation and desilylation of 1,6-methano[ lolannulene and its 11,l l-difluoro- derivative and has demonstrated that the reactivities and response to demand of electrons at the 2-positions of these molecules are closely similar to corresponding quantities for similar positions in the very reactive aromatics thiophen and diben- zofuran respectively.The 107r analogue (99) of a-tropolone has been synth- e~ized~~~ and it shows close similarity in behaviour to its analogue. However this analogy does not extend to ring contraction in the presence of strong bases. The steric compression of the two inner bridge hydrogen atoms has been overcome and the elusive syn-1,6:8,13-bismethano[14lannulene (100) has been prepared.235a It is clearly an arene having very similar properties to the 1,6:8,13-propanediylidene[ 14lannulene (101; n = 1).The contrast with the cyclopolyolefinic anti-isomer provides a most impressive demonstration that molecular geometry is of crucial importance for the occurrence of aromaticity in cyclically conjugated ring systems. In a series of related compounds (101) no indications of a significant (101) reduction of 7r-electron delocalization with bridge size were found and in the case of (101; n =5)a conformational mobility of the bridge was An X-ray analysis of the related 15,16-dimethyl-l,6:8,13-ethanediylidene[14lannulene indi- cates a nearly planar annulene ring with a C-C bond length (1.39 A) close to the ‘aromatic’ The effects of annelation on the properties of annulenes have been the subject of considerable study.Annelation of a 61r ring onto monodehydro[ 12lannulene and bisdehydro[ 14lannulenes reduces the paratropicity and diatropicity respectively,236 and the latter effect is also observed with tran~-lS,16-dihydropyrene.~” Benzanne-lation of the conformationally fixed protonated bisdehydror 1Slannulenone ring (102) (103) 234 E.Vogel J. Ippen andV. Buck Angew. Gem. Internat. Edn. 1975,14,566. 235 (a)E. Vogel J. Sombroek and W. Wagemann Angew. Chem. Inkmar. Edn. 1975,14 564; (6)A. Alscher W. Bremser D. Cremer H. Guenther H. Schmickler W. Sturm and E. Vogel Chem. Ber. 1975 lOS 640; (c) R.Bianchi G. Casalone and M. Simonetta,Acta ctysr. 1975 B31 1207. 236 R. H. Wightman and F. Sondheimer Tetrahedron Letters 1975,4179;R.R.Jones J. M. Brown and F. Sondheimer ibid. p. 4183. 237 R.H.Mitchell and R. J. Carruthers Tetrahedron Letters 1975,4331. Aromatic Compounds 247 reduces the diamagnetic ring current and (102) is in fact atropi~.’~~ Double benzannelation reduces the paramagnetic ring current in the bisdehydro[ 17lannulenone (103).239 The n.m.r. spectrum of the highly unstable dinaphtho-di-t-butyldidehydro[18lannulene (104) indicates a diatropicity compara- ble to that of a non-annelated analogue.24oa This behaviour is similar to that of the [14]annulene series [Ann.Reports (B) 1974,71 3081 where the mono-annelated derivative was less diatropic and is contrary to the usual effect of annelation. The dibenzobisdehydro[ 14lannulene (105) was too unstable for n.m.r. study but the (104) (105) benzo-naphtho-analogue was a little more diatropic than the mononaphtho- derivative,240b The unusually high diatropicity of these conformationally stable diannelated systems was attributed to the fact that it is possible to draw two equivalent Kekul6 structures. In accord with this idea the diatropicity of a dinaphthotetradehydror 18]annulene where this is not possible was strongly sup- A trisdehydro[16]annulene (106; n = l) containing formal acetylenic and cumulenic linkages has been and the strong paratropicity of the compound was attributed to the planar structure and the decrease in bond alterna- tion.The trisdehydro[2O]annulene (106; n = 2) had a lower paramagnetic ring Some 13Cn.m.r. spectroscopic analyses of these compounds suggested the presence of two types of acetylenic carbon atoms,indicating that (106a) may be the better structural representation. A study of three new bridged [18]ann~lenes~~~ has provided important evidence that apart from planarity the overall shape of the annulene loop is a factor in the magnitude of the detected ring current which was greatest in (107).The strong ring current detected in (107) appears to contradict the predictions of MIND0/3 calculations on [18lannulenea itself [Ann. Reports (B) 1974,71 3101. Finally macrocyclic analogues of naphthalene have been prepared for the first time. A study of the tetrakisdehydro[ 14]annuleno[ 14lannulene (108) demon- that strate~~~~the annelation reduces the diamagnetic ring current of the 238 R. T. Weavers R. R. Jones and F. Sondheimer Tetrahedron Letters 1975,1043. 239 J. Ojima A. Kimura and T. Tokoyama Chem. Leners 1975,207. 240 (a) M. Morigaki M. Iyoda and M. Nakagawa Tetrahedron Letters 1975 2315; (6) A. Yasuhara M. Iyoda T. Satake and M. Nakagawa ibid. p. 3931; (c)M. Iyoda and M. Nakagawa Chem.Letters 1975 815. 241 (a)S.Nakatsuji M. Morigaki S. Akiyama and M. Nakagawa TetrahedronLeners 1975,1233; (b)S. Nakatsuji and M. Nakagawa ibid. p. 3927. 242 R. B. DuVernet T. Otsubu J. A. Lawson and V. Boekelheide J. Amer. Chem. Soc. 1975,97,1629. 243 T. M. Cresp and F. Sondheimer J. Amer. Chem. Soc. 1975,97,4412. 248 R. G.Cbombes @-H (106a) Bu' w B u t \/ (106b) bisdehydro[l4]annulene ring but to a lesser extent than the reduction caused by annelation of benzene. The second example (109) consists of two tetradehydro[l8]annulene systems which n.m.r. measurements suggest are best regarded as two fused aromatic nuclei.244 Me -Me II Ft I II F II C \-But 244 T. Kashitani S. Akiyama M. Iyoda and M. Nakagawa J. Amer. Chem. Soc.,1975,97 4424.