6 Aromatic Compounds By I. G. C. COUlTS Department of Chemistry and Physics Nottingham Polytechnic Nottingham NGl 1 8NS 1 General and Theoretical Studies The ready formation of a trimethylenemethane dianion from isobutene has hitherto been attributed to a conjugative stabilization ('Y-aromaticity') involving all four carbon atoms but recent calculations suggest that the anion is non planar and that the stabilization is caused by maximally distant localization of the T electrons rather than by delocalization.' However the comparative stability of dications formed in SbF5/SO2C1F by rigid tetramethylenes such as pagodane has been explained in terms of nonclassical 'aromatic' structures.2 From a re-examination of the Mills-Nixon effect it was concluded3 that when strain is imposed on a benzene the system localizes its bonds in the Mills-Nixon manner but on annulation of benzene with small rings the effect diminishes due to the formation of 'banana bonds'.The aromaticity of benzenes in which a methine has been replaced by a transition metal and associated ligands (the metallobenzenes) has been reviewed4 and the thermochemical equivalence of substituted benzenes and ethylenes has been used in a study of the aromaticity of heterocycles.' Supersonic jet mass resolved excitation spectroscopy has been employed to determine minimum energy conformations of ethoxybenzenes6 Steric hindrance in crowded benzenehexacarboxylates induces an unusual dual fluore~cence,~ and the ring distortion in triaminotrinitrobenzenes' and in potassium salts of trinitrophloro- glucinol' have been determined by X-ray crystal analysis.Surprisingly the crystal structure of the benzene-picric acid complex has only now been reported." Triaryl- boranes (1) containing ethano bridges can be resolved at room temperature." In a comprehensive investigationt2 the MNDO-calculated hydride affinities of substituted aromatic cations and the proton affinities of substituted benzenes have ' A. Gobbi P. J. MacDougall and G. Frenking Angew. Chem. Znt. Ed. Engl. 1991 30,1001. R. Herges P.v. R. Schleyer M. Schindler and W.-D. Fessner J. Am. Chem. Soc. 1991 113 3649. A. Stanger J. Am. Chem. Soc. 1991 113 8277. J. R. Bleeke Acc. Chem. Rex 1991 24 271. R. S. Hosmane and J. F. Liebman Tetrahedron Lett. 1991 32 3949.'E. R. Bernstein H.4. Im M. A. Young H. V. Secor R. L. Bassfield and J. I. Seeman J. Org. Chem. 1991 56 6059. ' N. Yamasaki Y. Inoue T. Yokoyama A. Tai A. Ishida and S. Takamuku J. Am. Chem. Soc. 1991 113 1933. J. J. Wolff S. F. Nelsen P. A. Petillo and D. R. Powell Chem. Ber. 1991 124 1719. J. J. Wolff S. F. Nelsen D. R. Powell and J. M. Desper Chem. Ber. 1991 124 1727. 10 H. Takayanagi Y. Toubai M. Goto S.-I. Yamaguchi and H. Ogura Chem. Pharm. Bull. 1991,39,2491. K. Okada H. Inokawa and M. Oda Tetrahedron Lett. 1991 32 6363. 12 R. Karaman J.-T. L. Huang and J. L. Fry J. Org. Chern 1991 56 188. 131 I. G. C. Courts Y Y / w \ (1) a X = Me Y = H b X Y = -HC=CH-CH=CH-SiMe3 -2-SiMe3 [Li(THF)I2+ Me3Si LMe3siQ SiMe3 SiMe3 -l3 K.Krogh-Jespersen J. Am. Chem. SOC.,1991 113 417. A. Sekiguchi K. Ebata C. Kabuto and H. Sakurai J. Am. Chem. SOC.,1991 113 1464; 7081. l5 T. Lund and H. Lund Acta Chem. Scad. 1991,45 655. Aromatic Compounds A theoretical study16 indicates that cyclopentadienylidenecarbene may be a stable isomer of o-benzyne; the heat of formation of the latter is calculated” to be 105 kcal/mole. Arynes may be generated from phenol triflates using lithium diisopropy1amidel8 or by halogen-lithium exchange of o-haloaryl triflates.” Methoxydehydrobenzenes react regioselectively with methoxyfurans.20-22 2 Preparation of Benzenes from Non-aromatic Precursors The chemistry and biology of the enediyne anticancer antibiotics have been the subject of a comprehensive review.23 Terminal acetylenes and enynes undergo palladium-catalysed tandem bond formation leading to aromatic rings24 (Scheme 1).Lithiation2’ of 2,3-dibromobicyclo-[2,2,2]octanegave a trimer which by a SET Scheme 1 process yielded the benzene (4). Trisannelated benzenes are also obtained by the cyclodehydration of cycloalkanones catalysed by zirconium26 halides or by tetra- chl~rosilane.~~ Phenols are produced by the Nio-promoted reaction of cyclo-butenones with alkynes,28 and by PdC12( PPh,),-catalysed cyclocarbonylation of penta-2,4-dienyl acetates2’ Thermal conversion of bicycle (5) to fluorobenzene proceeds via 1,l-difluorocyclohexa-2,5-dieneformed by a rate determining homolytic H-~hift.~’ 9dF (4) (5) 16 N.A. Burton G. E. Quelch M. M. Gallo and H. F. Schaefer 111 J. Am. Chem. Soc. 1991 113 764. 17 Y. Guo and J. J. Grabowski J. Am. Chem. Soc. 1991 113 5923. 18 P. P. Wickham K. H. Hazen H. Guo G. Jones K. H. Reuter and W. J. Scott J. Org. Chem. 1991 56 2045. 19 T. Matsumoto T. Hosoya M. Katsuki and K. Suzuki Tetrahedron Lett. 1991 32 6735. 20 R. G. F. Giles M. V. Sargent and H. Sianipar J. Chem. Soc. Perkin Trans. 1 1991 1571. R. G. F. Giles A. B. Hughes and M. V. Sargent J. Chem. Soc. Perkin Trans. 1 1991 1581. 22 R. W. Baker T. M. Baker A. A. Birkbeck R. G. F. Giles M. V. Sargent B. W. Skelton and A. H. White J. Chem. Soc. Perkin Trans. 1 1991 1589. 23 K. C. Nicolaou and W.-M. Dai Angew. Chem. Znt. Ed. Engl. 1991 30,1387. 24 S. Torii H.Okumoto and A. Nishimura Tetrahedron Lett. 1991 32 4167. 25 K. Komatsu S. Aonuma Y. Jinbu R. Tsuji C. Hirosawa and K. Takeuchi J. Org. Chem. 1991,56,195. 26 H.Shirai N. Amano Y. Hashimoto E. Fukui Y. Ishii and M. Ogawa J. Org. Chem. 1991 56 2253. 27 S. S. Elmorsy A. Pelter and K. Smith Tetrahedron Lett. 1991 32 4175. 28 M. A. Huffman and L. S. Liebeskind J. Am. Chem. Soc. 1991 113 2771. 29 Y. Ishii C. Gao M. Iwasaki and M. Hidai J. Chem. Soc. Chem. Commun. 1991 695. 30 W. R. Dolbier Jr J. J. Keaffaber C. R. Burkholder S. F. Sellers H. Koroniak and J. Pradhan Tetrahedron Lett. 1991 32 3933. 134 I. G. C. Coutts In the dehydrogenation of cyclohexane to benzene on platinum a stable surface intermediate C6H9 has been identified.31 1,3-Cyclohexanediones are transformed to dimethyl resorcinols with 12/CH30H,32 while aryl alkyl ethers are obtained from the treatment of a,P -unsaturated cyclohexenones with 12/ROH/ceric ammon- ium nitrate33 or VO( OEt)Cl,/AgOTf/ EtOH.34 Various steroids react with [(C,M~,)RUOM~)~]CF,SO,H to give selective aromatization of the A ring by demethylation,, and Bu,SnH promotes C(9)-C( 10) bond cleavage in representatives of the 3-0x0- 1,4-diene series of steroids to afford36 A-ring aromatic 9,lO-secosteroids.Acid-catalysed dienone-phenol rearrangement of 4-methyl-4-cyanocyc-lohexadienone occurs very ~lowly,~' the dienone being stable for months in CF3C02H. A synthesis of C-aryl glycosides is based on the reductive aromatization of quinone ketal~.~* 3 Substitution in the Benzene Ring Electrophilic Substitution.-Catalysis by cyclodextrins of the coupling of phenol to diazonium cations may be due to the electron-rich character of the cyclodextrin interior stabilizing the a-c~mplex.~~ Interesting new insights continue to be gained into the venerable Friedel-Crafts alkylation of benzenes.Clay-supported metal chlorides are effective catalysts for the alkylation of alkylbenzenes.'"' Laszlo and co-workers have reported41 unusual reactivities in alkylations catalysed by ZnC12- montmorillonite. Toluene is more reactive towards benzyl chloride than mesitylene when the substrates are reacted separately but in one-pot reactions mesitylene is favoured and in both cases benzyl alcohol is a more effective alkylating agent than benzyl chloride.Combinations of alcohols and protonic acids useful for the alkylation of nitroben- zene will also effect substitution of aromatic aldehydes ketones and esters.42 Further examples have appeared of the use of perfluorinated resinsulfonic acid as Friedel- Craft Mixtures of CC13F and AlCl replace H by CF3 in polychloroben- ~enes.~~ Reaction of 1,3,5-triisopropylbenzenewith TaC15/CH2C12 gives the stable complex (6) resulting from an intermolecular isopropyl group migration.46 Ally1 31 C. L. Pettiette-Hall D. P. Land R. T. McIver Jr and J. C. Hemminger J. Am. Chem. SOC.,1991 113 2755. 32 A. S. Kotnis Tetrahedron Lett. 1991 32,3441. 33 C. A. Horiuchi H. Fukunishi M. Kajita A. Yamaguchi H. Kiyomiya and S. Kiji Chem.Lett. 1991 1921. 34 T. Hirao M. Mori and Y. Ohshiro Chem. Lett. 1991 783. 35 F. Urbanos J. Fernandez-Baeza and B. Chaudret J. Chem. SOC.,Chem. Cornmun. 1991 1739. 36 H. Kiinzer G. Sauer and R. Wiechert Tetrahedron Lett. 1991 32,7247. 37 J. N. Marx J. Zuerker and Y.-S. P. Hahn Tetrahedron Lett. 1991 32 1921. 38 K. A. Parker and C. A. Coburn J. Am. Chem. SOC., 1991 113 8516. 39 H. Ye D. Rong and V. T. D'Souza Tetrahedron Lett. 1991 32,5231. 40 S. J. Barlow J. H. Clark M. R. Darby A. P. Kybett P. Landon and K. Martin J. Chem. Res. (S) 1991 74. 41 A. Cornelis C. Dony P. Laszlo and K. M. Nsunda Tetrahedron Lett. 1991 32,2901; 2903. 42 Y.-S. Shen H.-X. Liu M. Wu W.-Q. Du Y.-Q. Chen and N.-P. Li J. Org Chem. 1991 56 7160. 43 T.Yamato C. Hideshima G. K. S. Prakash and G. A. Olah J. Org. Chem. 1991 56 2089; 3955. 44 T. Yamato N. Sakaue T. Furusawa M. Tashiro G. K. S. Prakash and G. A. Olah J. Chem. Res. (S) 1991 242. 45 J. Castaner J. Riera J. Carilla A. Robert E. Molins and C. Miravitlles J. Org. Chem. 1991 56 103. 46 E. Solari C. Floriani A. Chiesi-Villa and C. Rizzoli J. Chem. SOC.,Chem. Cornmun. 1991 841. Aromatic Compounds arenes are obtained from the SnC1,-promoted reaction of allylstannanes with arene~,”~ and vinyl cations stabilized by the sulfur of thiophenyl substituents can take part in the alkylation of benzenes.48 A similar stabilizing effect with selenium is in the reaction of aromatic hydrocarbons with PhSeCH( Br)COzEt/TiC14 yielding PhSeCH(Ar)CO,Et.Stable carbocations such as that formed on the protonation of acetophenone do not alkylate unreactive benzenes but cinnamaldehyde its oxime and imine form dications in CF3S03H with sufficient reactivity to do so,” and in CF3S03H/SbF5 nitriles diprotonate to give RC+=N+Hz capable of Houben-Hoesch and Gatter- mann reactions with ben~ene.~’ It is proposed5’ that the t-butylation of benzo- phenones and benzaldehyde with t-BuLi/ SOClz proceeds by a single electron transfer mechanism (Scheme 2) as does the CoC1,-catalysed acylation of anisoles with acid chlorides.53 0 Scheme 2 Contrary to earlier reports diethyl ether is an effective solvent for the two-phase nitration of ani~ole.’~ By suitable choice of catalyst ozone-mediated reactions of nitrogen oxides can lead to ortho-nitration of acetanilides” and to or 41 J.4.Yamaguchi Y. Takagi A. Nakayama T. Fujiwara and T. Takeda Chem. Lett. 1991 133. 48 T. Takeda F. Kanamori H. Matsusita and T. Fujiwara Tetrahedron Lett. 1991 32 6563. 49 C. C. Silveira E. J. Lenardao J. V. Comasseto and M. J. Dabdoub Tetrahedron Lett. 1991 32 5741. 50 T. Ohwada N. Yamagata and K. Shudo J. Am. Chem. Soc. 1991 113 1364. s1 M. Yato T. Ohwada and K. Shudo 1.Am. Chem. Soc. 1991 113 691. 52 G. A. Olah A.-h. Wu and 0.Farooq Synthesis 1991 1179. 53 J. Iqbal M. A. Khan and N. K. Nayyar Tetrahedron Lett. 1991 32 5179. s4 M. J. Thompson and P. J. Zeegers Tetrahedron 1991 47 8787. 55 H. Suzuki T. Ishibashi T. Murashima and K. Tsukamoto Tetrahedron Lett.1991 32 6591. 56 H. Suzuki T. Murashima K. Shimizu and K. Tsukamoto Chem. Lett. 1991 817. I. G. C. Coutts p~ly-nitration~~ of benzenes. A novel family of stable solid electrophilic fluorinating agents the bisammonium salts (7) has been prepared,58 and the influence of Lewis acids on the directed fluorination of aromatic substrates has been ~tudied.~’ Silver-assisted decomposition of aryldiazosulfides ArN=NSPh in the presence of sub-stoichiometric quantities of fluoride may be a useful route to ‘8F-labelled aromatic compounds.60 Results of the competitive iodination of durene and mesitylene under a variety of conditions support the intermediacy of an I+ species?l The regioselective monoiodination of resorcinol and phloroglucinol has been described?2 R In trifluoromethanesulfonic acid arenes undergo electrophilic hydroxylation with NaBO ,63 phenol triflates are formed by the thermal or photolytic decomposition of diazonium fl~oroborates~~ and hydrazoic acid reacts with aromatic compounds to give primary arylamines by a concerted process involving nucleophilic attack of the arene on the conjugate acid of the azide and elimination of nitrogen.65 The chlorosulfonation of aromatic systems has been reviewed.66 Substitution uia Organometallic Intermediates.-1,2-Bis(bromomagnesio)benzene crystallizes from THF as a tetramer the first to be reported for a diorganylmag- ne~ium.~~ Phenyl Grignard reagents react with 2-methoxybenzoic esters derived from 2,6-dialkylphenols to give l,l’-biphenyl-2-~arboxylates,6~ and undergo nickel-cata- lysed cross-coupling with neopentyl iodides?’ Benzaldehydes can be induced to undergo ortho-substitution by Grignard reagents7’ as shown in Scheme 3 and phenols may be ortho-alkylated by condensation with 1-hydroxymethylbenzotriazole and subsequent nucleophilic displacement of the benzotriazole by Grignard reagents .71 57 H.Suzuki T. Murashima K. Shimizu and K. Tsukamoto J. Chem. Sac. Chem. Cammun. 1991 1049. 58 R. E. Banks S. N. Mohialdin-Khaffat G. S. Lal I. Sharif and R. G. Syvret J. Chem. Sac. Chem. Carnmun. 1992 595. 59 S. T. Purrington and D. L. Woodward J. Org. Chem. 1991 56 142. 60 S. A. Haroutounian J. P. DiZio and J. A. Katzenellenbogen J. Org. Chem. 1991 56 4993. 61 C.Galli J. Org. Chem. 1991 56 3238. 62 I. Thomsen and K. B. G. Torssell Acta Chem. Scand. 1991 45 539. 63 G. K. S. Prakash N. Krass Q. Wang and G. A. Olah SYNLEn 1991 39. 64 N. Yoneda T. Fukuhara T. Mizokami and A. Suzuki Chem. Lett. 1991 459. 65 H. Takeuchi T. Adachi and H. Nishiguchi J. Chem. Sac. Chem. Cammun. 1991 1524. 66 J. P. Bassin R. J. Cremlyn and E. J. Swinbourne Phosphorus Sulfur Silicon 1991 56 245. 67 M. A. G. M. Tinga 0. S. Akkerman F. Bickelhaupt E. Horn and A. L. Spek J. Am. Chem. Sac. 1991 113 3604. 68 T. Hattori T. Suzuki and S. Miyano J. Chem. Sac. Chem. Cornmun. 1991 1375. 69 K. Yuan and W. J. Scott Tetrahedron Lett. 1991 32 189. 70 H. Jendralla Liebigs Ann. Chem. 1991 295. 71 A. R. Katritzky X. Lan and J.N. Lam Chem. Ber. 1991 124 1809. Aromatic Compounds CHO CH=NPh CHO i P~(OAC)~ AcOH ii Ph3P R3MgBr iii HCl H20 Scheme 3 A remarkable synthesis of poly-( p-phenylene) in aqueous media employs72 the Pd-mediated reaction of diboronate (8) and bromodiphenic acid (9) Bisflavonoids have been obtained73 from flavoneboronic acids and iodoflavones and arylboronic acids couple with a variety of 7r-deficient heteroaryl chlorides in the presence of palladium bearing a bidentate phosphorus ligand.74 COzH [ O > W B [ ) Br*Br HOzC 0 (8) (9) In a one-pot preparation of ibuprofen from p-xylene three consecutive selective metalations with BuLi-t-BuOK alternate with three electrophilic substitution^.^^ Selective metal-halogen exchange in 2,4,6-tribromoani~ole,~~ the generation of C,O,O-trilithiated derivatives of dihydric phenols,77 and the p~lylithiation~~ of trimethoxybenzenes have been reported.The tetrazole moiety is a useful directing group for the lithiation of 5-aryl sub~tituents,~~ and Meyers has described" naphthyl- oxazolines which are potent chiral auxiliaries in ortho-lithiation. N,N'-dimethyl- ethylenediamine adducts of salicylaldehydes metalate ortho to the modified alde- hyde group.81 Substituted saccharins may be readily obtained via ortho-lithiative sulfonamidation of N,N-diethylbenzamides.g2Metalation of N-pivaloyl 2-(3-methoxyphenyl) ethylamine with butyllithium occurs between the substituents giving a route to inaccessible 8-methoxyisoquinolinesg3 while dilithiation of N-t- BOC-2-methylbenzylamines affords 3-hydroxytetrahydroisoquinolines.84 A variation of the Friedlander quinoline synthesisg5 starts from ortho-lithiated N-t-BOC-72 T.I. Wallow and B. M. Novak J. Am. Chem. SOC.,1991 113,7411. 73 D.Muller and J.-P. Fleury Tetrahedron Lett. 1991 32,2229. 74 M. B. Mitchell and P. J. Wallbank Tetrahedron Lett. 1991 32,2273. 75 F. Faigl and M. Schlosser Tetrahedron Lett. 1991 32,3369. 76 K. Green J. Org. Chem. 1991 56 4325. 77 J. M. Saa J. Morey G. Suner A. Frontera and A. Costa Tetrahedron Lett. 1991 32,7313. 78 S. Cabbidu L. Contini C. Fattuoni C. Floris and G. Gelli Tetrahedron 1991 47 9279. 79 L. A. Flippin Tetrahedron Lett. 1991 32,6857. 80 D. J. Rawson and A. I. Meyers J. Org. Chem.1991 56 2292. 81 M. Gray and P. J. Parsons SYNLET 1991 729. 82 D. J. Hlasta J. J. Court and R.C. Desai Tetrahedron Lett. 1991 32,7179. 83 M. Schlosser and G. Simig Tetrahedron Lett. 1991 32,1965. 84 R.D.Clark and Jahangir Heterocycles 1991 32,1699. 85 I.-S. Cho L. Gong and J. M. Muchowski J. Org. Chem. 1991 56 7288. I. G. C. Coutts or N-pivaloylanilines. The combined use of metallation and Suzuki cross-coupling has been applied to syntheses of the phenanthrene gymnopusin,86 the azafluoran- thene imel~teine,~~ and dibenzo [b,d] pyran-6-ones related to ellagic acid.88 Aryltin reagents undergo Pdo-catalysed cross coupling with aryl fl~orosulfonates,8~ and with vinyl triflates in polar aprotic solvents.90 A Pd0(R3Sn) catalyst system effects intramolecular coupling of aryl or benzylic halides and has been applied9* to the synthesis of (10) (Scheme 4).(i) Pd(OAc),/PPh,/ (Me3Sn) Scheme 4 (Trifluoromethyl) copper species produced from CF3SiR3/F-/Cu(I) in DMF substitute CF3 for iodine in a variety of aryl iodides,92 and arylcopper intermediates react with trifluoroacetic anhydride to give aryl trifluoromethyl ketones.93 In the oxidative coupling of R'R2CuLi and R'R2Cu(CN)Li2 high yields of unsymmetric biaryls R' R2 can be obtained by temperature control.94 Template-directed intra- molecular Ullmann coupling has been used in the synthesis of unsymmetrical diphenic acids,95 and the preparation of unsymmetric and symmetric biaryls from cis-diarylgold(111) complexes has been reported.96 Upon q2-coordination with osmium phenols and anilines show enhanced reactivity towards electrophiles e.g.in the formation of (12) from (11) and maleic anhydride;97 the reactions of 0 86 X. Wang and V. Snieckus Tetrahedron Lett. 1991 32 4879. 87 B. Zhao and V. Snieckus Tetrahedron Lett. 1991 32 5277. 88 B. I. No A. Kandil P. A. Patil M. J. Sharp M. A. Siddiqui V. Snieckus and P. D. Josephy J. Org. Chem. 1991 56 3763. 89 G. P. Roth and C. E. Fuller J. Org. Chem. 1991 56 3493. 90 V. Farina and G. P. Roth Tetrahedron Left. 1991 32 4243. 91 R. Grigg A. Teasdale and V. Sridharan Tetrahedron Lett. 1991 32 3859. 92 H. Urata and T. Fuchikami Tetrahedron Lett. 1991 32 91. 93 F. A. J. Kerdesky and A. Basha Tetrahedron Lett.1991 32 2003. 94 B. H. Lipshutz K. Siegmann and E. Garcia J. Am. Chem. SOC.,1991 113 8161. 95 M. Takahashi T. Kuroda T. Ogiku H. Ohmizu K. Kondo and T. Iwasaki Tetrahedron Lett. 1991 32 6919. 96 J. Vicente M. D. Bermudez and J. Escribano Organometallics 1991 10 3380. 97 M. E. Kopach J. Gonzalez and W. D. Harman J. Am. Chem. SOC.,1991 113 8972. Aromatic Compounds nucleophiles with chloroarene -Mn( CO) and fluoroarene -Cr( CO) complexes have been exploited in syntheses of diary1 ethers98 and thi~ethers~~ respectively. Dimethyldioxirane liberates arenes quantitatively from Cr(C0)3 complexes."' The chemistry of aryllead(IV) tricarboxylates has been reviewed."' These reagents effect mono-N-arylation of aromatic amines under mild conditions.lo2 Radical Reactions.-Oxidation potentials of phenoxide ions combined with pKHA values provide good estimates of the homolytic bond dissociation energies for 0-H bonds in phenol^."^ Fluorinated biaryl derivatives 4-RC6H4hOH (R = F CF, OCF,) are obtained by coupling 4-RC6H4Br with phenol anions via a photostimu- lated SRNl reaction.lW Liquid ammonia is a suitable solvent for a study of the reaction of aryl radicals with pyrrole anions,'05 and for the electrosynthesis by SRNl processes of 4'-hydroxybiphenyl- 1 -~ulfones,~~~ but in the addition of aryl radicals to alkenes the presence of a proton donor is necessary for reasonable yields.'" Consideration of the product ratio of bitolyls obtained by the sonochemical coupling of bromotoluenes over lithium suggests a radical mechanism for the reaction.lo8 Cyclizations of aryl halides generated by the reaction of bromobenzenes with Bu3SnH have been used in syntheses of quinolines and ben~azepinones,"~ of the oxindole portion of gelsernine,"O and of aporphines."' In an approach112 to the alkaloid ungeremine the benzoyl dihydroindole ( 13) on treatment with DMSO/K2C03 gave a radical which cyclized to (14).0 0 (33) (14) The nucleophilicity of acyl radicals has been used to explore the mechanism of acylation of ferrocene and production of 1,l'-diacylferrocenes points to initial chemistry occurring at the iron centre rather than at the ring carbon^."^ Dediazoniz-ation of an aryldiazonium salt by PI1compounds probably proceeds by a radical 98 A.J. Pearson and P. R. Bruhn J. Org. Chem. 1991 56 7092. 99 M. J. Dickems J. P. Gilday T. J. Mowlern and D. A. Widdowson Tetrahedron 1991 47 8621. 100 A.-M. Lluch F. Sanchez-Bacza F. Camps and A. Messeguer Tetrahedron Lett. 1991 32,5629. 101 J. T.Pinhey Aust. J. Chem. 1991 44 1353. 102 D. H. R. Barton D. M. X. Donnelly J.-P. Finet and P. J. Guiry J. Chem. Soc. Perkin Trans. I 1991 2095. 103 F. G. Bordwell and J.-P. Cheng 1Am. Chem. Soc. 1991 113,1736. 104 R. Beugelmans and J. Chastanet Tetrahedron Lett. 1991 32,3487. 105 M. Chahma C. Combellas H. Marzouk and A. Thibbault Tetrahedron Lett. 1991 32 6121. 106 P. Boy C. Combellas S. Fielding and A. Thiibault Tetrahedron Lett. 1991 32,6705. Z. Chami M. Gareil J. Pinson J.-M.SavCant and A. ThiCbault J. Org. Chem. 1991 56 586. G. J. Price and A. A. Clifton Tetrahedron Lett. 1991 32,7133. A. J. Clark K. Jones C. McCarthy and J. M. D. Storey Tetrahedron Lett. 1991 32,2829. 107 108 109 110 D.J. Hart and S. C. Wu Tetrahedron Lett. 1991 32,4099. 111 J. C. Estevez M. Carmen Villaverde R.-J. Estevez and L. Castedo Tetrahedron Lett. 1991 32,529. 112 U. Lauk D. Diirst and W. Fischer Tetrahedron Lett. 1991 32,65. C. Lampard J. A. Murphy and N. Lewis Tetrahedron Lett. 1991 32,4993. 113 I. G. C. Coutts chain me~hanism."~ by reaction Heck arylation of camphene is smoothly effe~ted"~ with ArN2BF4/ P~(OAC)~. Acetophenones protonated by solution in oleum undergo specific para-substitution by alkyl radicals albeit in poor yield,"6 while electron rich aromatics are trifluoromethylated by radicals obtained' l7 from the oxidation of CF3S02Na.Nucleophilic Substitution.-From a review of the literature and new experimental data an alternative mechanism involving a bimolecular displacement process has been proposed for the reaction of nucleophiles with aromatic radical anions contain- ing leaving groups.' l8 Several workers have reported on the nucleophilic displace- ment of nitro groups from nitroarenes. Fluorodenitration occurs in sulfolane at 180 "C side reactions being suppressed by trapping the liberated nitrile anion with phthaloyl dichloride' l9 or difluoride.12' Conversion of nitrobenzenes or naph- thalenes to the corresponding phenols can be effected by reaction with benzyl alcohol in tetramethylurea followed by debenzylation.'21 A recent monograph'22 discusses the influence of the nitro group in nucleophilic aromatic substitutions.In N-methyl-2-pyrrolidone chlorobenzenes react with sodium alkanethiolates to give thi~phenols,'~~ and potassium thiophenolate displaces chloride from chloroanilines and acetanilides to afford the corresponding thiophenyl corn pound^.'^^ Under phase- transfer conditions hydroxide is sufficient nucleophilic to displace one fluoride from polyfluorobenzenes with Bu4NHS04 being the preferred cata1y~t.l~~ Formation of dinitrophenoxides from hydroxide attack on chlorodinitrobenzenes is claimed to be another reaction proceeding by collapse of a radical anion to a Meisenheimer complex.'26 Anilines are converted directly to fluorobenzenes by treatment'27 with NaNO,/pyridine.HF.Deprotonation of aziridine (15) with butyllithum gives (16) the product of intramolecular nucleophilic aromatic addition.'28 114 S. Yasui M. Fujii C. Kawano Y. Nishimura and A. Ohno Tetrahedron Lett. 1991 32 5601. 115 W. Yong P. Yi Z. Zhuangyu and H. Hongwen Synthesis 1991 967. 116 L. B. Din 0.Meth-Cohn and N. D. A. Walshe J. Chem. SOC Perkin Trans. 1 1991 781. 117 B. R. Langlois E. Laurent and N. Roidot Tetrahedron Lett. 1991 32 7525. 118 D. B. Denney and D. Z. Denney Tetrahedron 1991,47 6577. 119 F. Effenberger and W. Streicher Chem. Ber. 1991 124 157. 120 M. Maggini M. Passudetti G. Gonzalez-Trueba M.Prato U. Quintily and G. Scorrano J. Org. Chem. 1991 32 6406. 121 F. Effenberger M. Koch and W. Streicher Chem. Ber. 1991 124 163. 122 F. Terrier 'Nucleophilic Aromatic Displacement the Influence of the Nitro Group' VCH Publishers New York 1991. 123 J. E. Shaw J. Org. Chem 1991 56 3728. 124 A. J. Caruso A. M. Colley and G. L. Bryant J. Org. Chem. 1991 56 862. D. Feldman D. Segal-Lew and M. Rabinovitz J. Org. Chem. 1991,56 7350. 126 R. Bacaloglu A. Blasko C. Bunton E. Dorwin F. Ortega and C. Zucco J. Am. Chem. Soc. 1992 113 238. 127 T. Fukuhara N. Yoneda K. Tamamura and A. Suzuki J. Fluorine Chem. 1991,51,299. 128 H.-J. Breternitz E. Schaumann and G. Adiwidjaja Tetrahedron Lett. 1991 32 1299. Aromutic Compounds Oxidized Benzenes.-Mixed anodic trimerization of 1,2-dialkoxybenzenes and ben- zocrown ethers leads to the formation of triphenylenes possessing one or two complexing sites.'29 A key step in the synthesis of neoisostegane (17) is a biaryl oxidative coupling induced13' by RuO,/TFA and synthetic appro ache^'^' to the cytotoxic alkaloid discorhabdin C use oxidation by hypervalent iodine of (18) to (19).OH Anodic oxidation gives reasonable yields of verangiaquinol~~~~ from 2,6-dihalo-4- hydroxy phenylacetamides and directly of mixed quinone monoketals from ally1 or propargyl ethers of 4-methoxyphen01.l~~ 1-4-t-Butylperoxy-2,5-cyclohexadiene- ones are obtained by treatment of 2,4,6-trialkylphenols with t-BuOOH in the presence of 12-molybdophosphoric The indium-mediated Reformatsky reaction of quinones gives good yields of p-quinols including jacaranone.135 Benzoquinone bis(dimethy1 ketals) couple to the ortho position of titanium or bromomagnesium phenolates to produce unsymmetrical hydroxylated biphenyl^,'^^ and treatment of arenes with 1,4-dimethyl- 1,4-dimethoxycyclohexadienein the presence of ZnC12 yields arylated p-xylene~.'~' and Reports have appeared of transformations of fl~oro-'~~ halogen^'^^ cis-cyclohexa-3,5-diene-l 2-diols available from the Pseudomonus putidu oxidation of 129 J.-M. Chapuzet N. Simonet-Gueguen I. Taillepied and J. Simonet Tetrahedron Lett. 1991 32 7405. 130 Y. Landais J.-P. Robin and A. Leburn Tetrahedron 1991 47 3787. 131 Y. Kita H. Tohma M. Inagaki K.Hatanaka K. Kikuchi and T. Yakura Tetrahedron Lett. 1991 32 2035. 132 N. Bicchierini M. Cavazza L. Nucci F. Pergola and F. Pietra Tetrahedron Lett. 1991 32 4039. 133 S. Dhanalekshmi K. K. Balasubramanian and C. S. Venkatachalam Tetrahedron Lett. 1991,32 7591. 134 M. Shimizu H. Orita T. Hayakawa Y. Watanabe and K. Takehira Bull. Chem. SOC.Jpn. 1991,64,2583. 135 S. Araki N. Katsumura K. Kawasaki and Y. Butsugan J. Chem. Soc. Perkin Trans. I 1991 499. 136 G. Sartori R. Maggi F. Bigi and G. Casnati J. Chem. SOC.,Perkin Trans. I 1991 3059. 137 F. Alonso and M. Yus Tetrahedron 1991 47 313. 138 H. A. J. Carless and 0.Z. Oak J. Chem. Soc. Chem. Commun. 1991 61. 139 D. R. Boyd M. V. Hand N. D. Sharma J. Chima H. Dalton and G. N. Sheldrake J.Chem. SOC. Chem. Commun. 1991 1630. I. G.C.Coutts benzenes and of cycloaddition reactions of isopropylidene derivatives of related cyclohexadienediols.14' The dimeric product (20) has been isolated from the bacterial degradation of 2,6-~ylenol.'~~ Electron-rich methoxyarenes are effectively oxidized to p-benzoquinones by magnesium monoperoxyphthalate with a water-soluble iron porphyrin as and the Ag' cation induces coupling of 4,4'-dihydroxystil- benes followed by further oxidation to yield phenanthrene-2,7-q~inones.'~~ Phenols are oxidized by dimethyldioxirane to o-quinones'u and catechol is cleaved to 2,Z-muconic acid.'45 The mechanism of the catalytic hydroxylation of aromatic hydrocarbons by hydrogen peroxide has been reviewed.'46 Phenols are converted to catechols by reaction with dioxygen in the presence of a tetrahydroborato Cu' complex,'47 and 4-methoxyphenols are oxidized'48 to the corresponding o-acetoxyphenols by Cu( OAc),/ HOAc.A binuclear three-coordinate Cu' complex containing a polybenz- imidazole ligand exhibits tyrosinase-like activity towards Fluorine reacts with wet acetonitrile to produce an oxidizing agent which converts aromatic amines to the corresponding nitroarenes."' 4 Condensed Polycyclic Aromatic Compounds Naphthalenes.-Useful new approaches to substituted naphthalenes continue to be developed. Nitrile thioethers R-CBr=C( SEt)CN react like a-alkynenitriles with ambiphilic derivatives to yield naphthylamine~.'~' Irradiation of a-diazoketones (21) gives 2-naphthols (22) via arylketen intermediate^,'^^ and phenylbutadienes (23) in which the 2-cis moiety is incorporated into a cyclic structure undergo oxidative photocyclization to na~htha1enes.l~~ Two synthetic approaches to the 140 M.F. Mahon K. Molloy C. A. Pittol R. J. Pryce S. M. Roberts G. Ryback V. Sik J. 0. Williams and J. A. Winders J. Chem. SOC.,Perkin Trans. 1 1991 1255. 141 H. Kneifel C. Poszich-Buscher S. Rittich and E. Breitmaier Angew. Chem. Int. Ed. EngL 1991,30,202. 142 I. Artaud K. B. Aziza C. Chopard and D. Mansuy J. Chem. Soc. Chem. Commun. 1991 31. 143 F. R. Hewgill R. Slamet and J. M. Stewart J. Chem. SOC.,Perkin Trans. 1 1991 3033. 144 J. K. Crandall M. Zucco R. S. Kirsch and D. M. Coppert Tetrahedron Lett. 1991 32 5441.145 A. Altamura C. Fusco L. D'Accolti R. Mello T. Prencipe and R. Curci Tetrahedron Lett. 1991 32 5445. 146 E. A. Kharakhanov S Y. Narin and A. G. Dedov App. Organomet. Chem. 1991 5 445. 147 F. Chioccara P. Di Gennaro G. La Monica R. Sebastiano and B. Rindone Tetrahedron 1991,47,4429. 148 Y. Takizawa A. Tateishi J. Sugiyama H. Yoshida and N. Yoshihara J. Chem. SOC.,Chem. Commun. 1991 104. 149 L. Casella M. Gullotti R. Radaelli and P. Di Gennaro J. Chem. SOC.,Chem. Commun. 1991 1611. M. Kol and S. Rozen J. Chem. SOC.,Chem. Commun. 1991 567. 151 D. Reux and F. Pochat J. Chem. SOC.,Chem. Commun. 1991 1419. 152 A. Padwa D. J. Austin U. Chiacchio J. M. Kassir A. Rescifina and S. L. Xu,Tetrahedron Lett. 1991 32 5923. 153 R.S. Olsen J. C. Minniear W. M. Overton and J. M. Sherrick J. Org. Chem. 1991 56 989. Aromatic Compounds protein kinase C inhibitors the calphostins are based on de novo formation of highly functionalized naphthalenes. In one route 1,1,3-trioxygenated butadienes form Diels-Alder adducts with o-quinol acetate~’’~; the other relies”’ on the anion from (24) reacting with (S)-6-methyl-5,6-dihydropyran-2-one to yield (25) which MOMO MOMO OH 0 C02Et Me0 Me0 can be aromatized with DDQ. Lactones resulting from reaction of a succinyl chloride with benzenes undergo an intramolecular Friedel-Crafts acylation on addition of BBr3 to afford 4-phenylnaphthalen-l-ols, intermediates in the preparation of gossy- ~01.”~ Homophthalic anhydride on treatment with MeOH/ MeI/ K2C03 is conver- directly to 12-methoxybenzo[d]-naphtho[2,3-b]pyran-5-one.’” With an excess of sodium cyclohexanethiolate perfluorodecalin yields octakis(cyc1o-hexylthio) 11aphtha1ene.l’~ 2,2’-Dimethoxy-l,l’-binaphthaleneundergoes spon-taneous resolution on crystallization from ani~ole.’’~ Regioselective alkylation of naphthalene with propan-2-01 or propene over a mordenite catalyst affords 2,6- diisopropylnaphthalene.160 From a kinetic study’61 on the acetylation of naphthalene with CH3COC1/CH2Cl2/AlCl3 it is proposed that the mechanism for P-naphthyl acetylation involves a two-stage process with the second loss of a proton being rate-limiting while a-substitution proceeds through two a-complexes and the decomposition of the second is partly rate-determining.Acylnaphthalenes are also obtained from the products of the reaction of nitriles with zirconocene complexes of substituted naphthalynes.’62 Radical cations are intermediates in the HN02- ~atalysed’~~ and charge-transfer’@ nitration of naphthalenes. 154 R. S. Coleman and E. B. Grant J. Org. Chem. 1991 56 1357. C. A. Broka Tetrahedron Lett. 1991 32 859. 156 J.-T. Huang T.-L. Su and K. A. Watanabe J. Org. Chem. 1991 56 4811. 157 W. V. Murray and S. K. Hadden J. Chem. Res. (S) 1991 279. 158 D. D. MacNicol W. M. McGregor P. R. Mallinson and C. D. Robertson J. Chem. SOC.,Perkin Trans. 1 1991 3380. 159 G. Gottarelli and G. P. Spada J. Org. Chem. 1991 56 2096. 160 A. Katayama M. Toba G. Takeuchi F.Mizukami S. Niwa and S. Mitamura J. Chem. Soc. Chem. Commun. 1991 39. 161 D. D. Dowdy P. H. Gore and D. N. Waters J. Chem. SOC.,Perkin Trans. 2 1991 1149. 162 S. L. Buchwald and S. M. King J. Am. Chem SOC.,1991 113 258. 163 P. J. Gross and J. H. Ridd J. Chem. SOC.,Perkin Trans. 2 1991 1773. 164 S. Sankararaman and J. K. Kochi J. Chem. SOC.,Perkin Trans 2 1991 165. 144 I. G. C. Coutts 1,5-Diodonaphthalene undergoes halogen-metal exchange with t-BuLi to give mono- or di1ithioderi~atives.l~~ Diethyl 1-naphthyl phosphate on treatment with LDA rearranges to diethyl 1-hydroxy-2-naphthyl phosphonate.'66 Reaction of the carbene (CH30C0)& with naphthalene gives a mixture of products originating from addition of the carbene to the 2,3-b0nd,'~~ and cheletropic addition of dichlorocarbene to 1-H-cyclopropa[ blnaphthalene yields 1 l-dichloronaphtho- [b]cy~lobutene.'~~ Tri-and Polycyclic Compounds.-The reaction of arynes with anions of ethyl cyanoacetates constitutes a brief synthesis of anthraquin~nes.'~~ Phenanthrenes are obtained by heating the dilithium salts of the bistosylhydrazones of biphenyl-2,2'- dicarboxaldehydes'70 and from treatment of 2,2'-dialkoxystilbenes with low valent titanium.171 Dicyclopropanthracenes or phenanthrenes may be ~ynthesized'~~ from tetramethylenecyclohexanes as illustrated in Scheme (5).Alkylation of enamines by Cl(Br) Br c1 2Lw c1 Br(C1) CI(Br) DDQ Br:; -benzene c1 Br(C1) \\ Scheme 5 benzylic or (p-haloethy1)aryl halides followed by cyclodehydration and dehydroge- nation provides an efficient route to a wide range of polycyclic corn pound^.'^^ Corannulene is formed directly by flash vacuum pyrolysis of 7,lO-diethynylfluoran- ther~e,'~~ The and an efficient synthesis of circumanthracene has been re~0rted.l~~ previously inaccessible compound (26) containing a dienophilic central double 165 W.Wang S. V. D'Andrea J. P. Freeman and J. Szmuszkovicz J. Org. Chem. 1991 56 2914. 166 B. Dhawan and D. Redmore J. Org. Chem. 1991 56 833. 167 M. Pomerantz and M. Levanon Tetrahedron Lett. 1991 32 995. 168 I. Durucasu N. Saracoglu and M. Balci Tetrahedron Lett. 1991 32 7097. 169 B. M. Bhawal S. P. Khanapure H. Zhang and E. R. Biehl J Org. Chem. 1991,56 2846. I7O M.E.Jung and A.Hagiwara Tetrahedron Lett. 1991 32 3025. 171 A. Banerji and S. K. Nayak J. Chem. SOC.,Chem. Commun. 1991 1432. 17' W.E.Billups M. M. Haley R. C. Claussen and W. A. Rodin J. Am. Chem. SOC.,1991 113 4331. 173 R. G. Harvey J. Pataki C. Cortez P. Di Raddo and C. X.Yang J. Org. Chem. 1991 56 1210. 174 L. T. Scott M. M. Hashemi D. T. Meyer and H. B. Warren J. Am. Chem SOC.,1991 113 7082. 175 R. D. Broen and F. Diedrich Tetrahedron Lett. 1991 32 5227. Aromatic Compounds bond is easily obtained by Heck coupling of 1,8-diiodonaphthalene with acenaph- tha1er1e.l~~ Triptycenes can be prepared’77 in good yield by sequential reaction of quinone-anthracene adducts with LiAlH4 and tosyl ch10ride.l~~Fusing 9,lO- anthradiyl moieties to the benzenoid bonds of triptycene gives rise to a family of iptycenes; the ultimate structure with six fused residues ‘supertriptycene’ (27) has been obtained from a 10-step synthetic sequen~e.’’~ (27) From ab initio calculations it is concluded that kekulene gains extra stability (‘superaromaticity’) from the presence of the circular formation of benzenoid rings.’79A study’” of the dinaphtho[ a,j]anthracene (28) in which rotational motion of the internal methoxy group is restricted suggests that the barrier to linear oxygen inversion is >70 kJ mol-’.OH Thermal reactions of polycyclic aromatic hydrocarbons usually involving radical intermediates has been reviewed.181 The radical cation generated on dissolving bis(pentamethylpheny1)methanein CFJCOzHis that of 1,2,3,4,5,6,7,8-octamethylan-thracene,lS2 and diatropic cations such as (29) are produced on protonation of 176 G.Dyker Tetrahedron Lett. 1991 32 7241. 177 H. K. Patney Synthesis 1991 694. 178 K. Shahlai and H. Hart J. Org. Chem. 1991 56 6905. 179 J. Cioslowski P. B. O’Connor and E. D. Fleischmann J. Am. Chem. SOC.,1991 113 1086. R. B. Gupta M. K. Kaloustian R. W. Franck and J. F. Blount J. Am Chem. SOC.,1991 113 359. 181 S. E. Stein Acc. Chem Res. 1991 24 350. 182 L. Eberson and F. Radner J. Chem. SOC.,Chem. Commun. 1991 1233. 146 I. G. C. Coutts cyclopenta[ ~]phenalenediones.'~~ Formation of 9,lO-anthraquinone on N204nitra-tion of anthracene probably proceeds by a 1,4-addition mechanism rather than via a radical cation.'84 The radical anion obtained on irradiation of 9,lO-dicyanoan- thracene is stable for weeks in the presence of unreactive counter-i~ns.'~~ Reduction of the helicene 5-isopropyl-l,4,8-trimethylphenanthreneyields a 4nr conjugated dianion which preserves its helicity despite the decreased barriers to racemization.'86 Ful1erenes.-This year has seen an explosive increase in papers dealing with the chemical and physical properties of the fullerenes the family of closed carbon cages containing 12 pentagons and two or more hexagons in an sp2 network.Kroto and co-worker~'~~ have given an authoritative account of the pioneering studies on fullerene-60 and more recent work has been reviewed.'88 There has been debate about the aromaticity of the fullerenes.The vanishingly small welectron ring current magnetic susceptibility of c60 has been attributed to cancellation of diamagnetic and paramagnetic contributions leading to 'ambiguous' aromatic character. Using a valence-bond approach it has been s~ggested"~ that c60 is particularly stable as the only fullerene in which it is possible to isolate all the bonds in six-membered rings and none in five-membered rings. C,o is electron-deficient being easily reduced and reacts with nucle~philes.'~' It undergoes stepwise flu~rination'~~ to yield eventually CmF60 and C60(H' C6H5),,with benzene/AlCl,. The availability of fullerenes has been increased by development of a simple bench-top reactor,'94 and of improved separations based on solient extraction^."^ 5 Non-benzenoid Aromatics Some newer methods for the synthesis of tropenoid compounds have been reviewed.Ig6 Tropones may be obtained by reaction of Rh"-stabilized vinylcarbenoids with l-methoxy-l-[trimethylsilyloxy]buta-1,3-diene,'97 and a-homo-o-benzoquin- ones rearrange on treatment with BF,.Et,O to give boron difluoride derivatives of 183 S.Kuroda Y. Kanbata Y. Fukuyama S. Hirooka H. Takeda T. Tsuchida Y. Furuki T. Sumi 0. Hanida M. Yamada and I. Shimao Bull. Chem. SOC., Jpn. 1991 64 971. F. Radner Acta Chem. Scand. 1991 45 49. 185 M. A. Kellett D. G. Whitten 1. R. Gould and W. R. Bergmark J. Am. Chem. SOC.,1991 113 358. R. Frim G. Zilber and M. Rabinovitz J. Chem. SOC.,Chem. Cornmun. 1991 1202. 187 H. W. Kroto A. W. Allaf and S.P. Balm Chem. Rev. 1991,91 1213. F. Diederich and R. L. Whetten Angew. Chem. Int. Ed. EngL 1991 30,678. 189 R. C. Haddon L. F. Schneemeyer J. V. Waszczak S. H. Glahm R. Tycho G. Dabbagh A. R. Kortan A. J. Muller A. M. Mujsce M. J. Rosseinsky S. M. Zahurak A. V. Makhija F. A. Thiel K. Raghavachari E. Cockayne and V. Eiser Nature 1991,350,46. I9O R.Taylor Tetrahedron Lett. 1991 30,3731. 191 J. W. Bausch G. K. S. Prakash G. A. Olah D. S. Tse D. C. Lorents Y. K. Bae and R. Malhotra J. Am. Chem SOC.,1991 113 3205. 19* J. H.Holloway E. G. Hope R. Taylor G. J. Langley A. G. Avent T. J. Dennis J. P. Hare H. W. Kroto and D. R. M. Walton J. Chem. SOC.,Chem. Cornmun. 1991 966 '93 G. A. Olah I. Bucsi C. Lambert R. Aniszfeld N. J. Trivedi D. K. Sensharma and G.K. S. Prakash J. Am. Chem. Soc. 1991 113 9387. 194 A. S. Koch K. C. Khemani and F. Wudl J. Org Chem. 1991 56 4543. 195 D. H. Parker P. Wurz K. Chatterjee K. R. Lykke J. E. Hunt M. J. Pellin J. C. Hemminger D. M. Gruen and L. M. Stock J. Am. Chem. SOC.,1991 113 7499. 196 M. G. Banwell Aust. J. Chem. 1991 44 1. 197 H. M. L. Davies T. J. Clark and G. F. Kimmer J. Org. Chem. 1991 56 6440. Aromatic Compounds 147 a-tropolone~.’~~ Alkoxyazulenes are formed by the reaction of 2H-cyclohepta[ b] -furan-2-ones with or tho ester^.'^^ A synthesis of a new [14lannulene-dione acepleiadylene- 1,2-dione has been reported.200 Oxidation of isopyrene with rn-chloroperbenzoic acid yields quinone (30) to which canonical structures such as (31) make a significant contribution.’” A variety of bridged [14]annulenes are available by LUMO-controlled nucleophilic addition of organometallic reagents to dicyclopenta[ ef,kZ]heptalene and subsequent quenching of the resulting anions.’02 The aromaticity of cyclopentadienide has been probed by examination of the annelation effects of fusion of the anion to a [14]annulene dimethyldihydropyrene; it is concluded that C5H5- has about 50% of the effective aromaticity of ben~ene.”~ Successive replacement of phenyl groups in the tritryl cation by 1-azulenyl residues leads to increased stability with the Pk,+ value of 11.3 for the tri( 1-azuleny1)methyl cation being the highest so far reportedzo4 for methyl cations substituted with hydrocarbon groups.Cremer and co-workers have published results of ab initio calculations of I3C NMR shifts as probes of the structure and homoaromaticity of the tropylium and bis-’06 homocations.The hydrocarbons (32) and (33) valence isomers of homoannulenes undergo interconversion on flow-vacuum pyroly- S~S.’~’ Complexation of the diatropic bridged annulene (34) with Fez(C0)9 occurs in the 14~ annulene system rather than the benzene ring giving a formally 19* M. G. Banwell and M. P. Collis J. Chem. SOC.,Chem. Commun. 1991 1343. 199 T. Nozoe H. Wakabayashi K. Shindo S. Ishikawa C.-P. Wu and P.-W. Yang Heterocycles 1991,32 213. 200 S. Suzuki,S. Tanaka J. Tsunetsugu M. Higashi and H. Yamaguchi J. Chem. Res. (S) 1991 168. 201 E. Vogel L. Schmalstieg P. Henk 0. Wilmes J. Lex R.Gleiter and M. Langer Angew. Chem. Int. Ed. Engl. 1991 30,681. ’02 J. Alexander M. Baumgarten K.-U. Klabunde and K. Mullen Tetrahedron Leu. 1991 32 735. 203 R. H. Mitchell N. A. Khalifa and T. W. Dingle J. Am. Chem. Soc. 1991 113 6696. ’04 S. Ito N. Morita and T. Asao Tetrahedron Lett. 1991 32 773. ’05 D. Cremer F. Reichel and E. Kraka J. Am. Chem. Soc. 1991 113 9459. ’06 P. Svensson F. Reichel P. Ahlberg and D. Cremer J. Chem. Soc. Perkin Trans. 2 1991 1463. 207 M. D. Banciu M. D. Stanescu C. Florea A. Petride C. Draghici and E. Cioranescu Bull. SOC.Chem. Fr. 1991 128 919. 148 I. G. C.Coutts anti-aromatic product.208 An acetoxyl group is introduced into dialkylaminotropones or azulenes by treatment with Pd(OAc) ,209 and palladium mediates the cross-coup- ling of bromotropolones with organostannanes,210 and the Heck vinylation of haloazulenes.211 6 Cyclophanes Cyclophanes have been the subject of a recent comprehensive monograph.212 Reac- tion of readily available difurano-annelated [2,2] paracyclophanes with arynes yields a variety of compounds such as (35) containing alternatingly orthogonal arene units.213 The syn conformer of (36) is more stable than the anti suggesting that Hex -Hex Me0 go ’\ -0Me (37) (38) through-space interactions between 0-and p-benzoquinones are attractive.,14 1,lo-0-Benzo[2,2]orthocyclophane-o-quinones215 and the metacyclophane216 (37) have also been used in studies of intramolecular change-transfer interactions.The interesting diphenoquinonophane (38) has been ~repared,~” and found to be unexpectedly stab1e.Reaction of tetracyanoethylene with [6]( 1,4)anthracenophane occurs at the strained bridge ring to give the first reported [2 + 2]cycloadduct of anthracene.218 Studies of binding between naproxen and chiral mono- and bis( 1,l’-binaphthyl) cyclophanes highlight the difficulty of predicting the conformational preferences of large flexible macro cycle^.^^^ 208 R. H. Mitchell and P. Zhou Angew. Chem. Int. Ed. Engl. 1991,30,1013. ‘09 K. Saito M. Kozaki K. Uenishi N. Abe and K. Takahashi Chem. Pharm. Bull 1991 39 1843. 210 M. G.Banwell J. M. Cameron M. P. Collis G. T. Crisp R. W. Gable E. Hamel J. N. Lambert M. F. Mackay M. E. Reum and J. A. Scoble Aust. J. Chem. 1991,44,705.211 H. Horino T. Asao and N. Inoue Bull. Chem. SOC.Jpn. 1991,64,183. 212 F. Diederich ‘Cyclophanes’ Royal Society of Chemistry Monographs in Supramolecular Chemistry 1991,Cambridge. 213 B. Konig J. Heinze K. Meerholz and A. de Meijere Angew. Chem. Int. Ed. Engl. 1991,30 1361. 214 Y.Fukazawa T. Nakamura and S. Usui Tetrahedron Lett. 1991 32 3183. 21s Y.Fukazawa T.Nakamura T. Haino and S. Usui Chem. Lett. 1991 957. 216 H. A. Staab A. Dohling and C. Krieger Tetrahedron Lett. 1991 32 2215. 217 K. Tani J. Takano T. Tohda M. Ariga H. Higuchi and S. Misumi Chern. Lett. 1991 1461. 218 Y.Tobe T. Takahashi K. Kobiro and K. Kakiuchi Tetrahedron Lett. 1991 32 359. 219 P. P.Castro and F. Diederich Tetrahedron Lett. 1991 32 6277.