12 Alicyclic Chemistry By A. COX DepalZment of Chemistry and Molecular Sciences University of Warwick Coventry CV# 7AL 1 Introduction Reviews have been published on preparative aspects of vinyl cation chemistry with particular reference to their application to the synthesis of some cycloalkanones,' the preparative chemistry of cyclobutenedione and derivative~,~'~ the synthesis of carbocyclic spiro compound^,^ spiroc~njugation,~ conformational analysis of cyclo- hexa-l,4-diene~,~ metal-induced rearrangements of cyclopropyl olefins,' nucleo- philic eliminative ring fission' and aspects of prostaglandin chemistry.' 2 Synthesis Electrochemical reduction of aa'-dibromoalkanedioates containing alkoxycarbonyl groups at both termini has been shown" to lead to three- to seven-membered ring cycloalkane-l,2-dicarboxylicacid esters.Although the detailed mechanism has not yet been elucidated in the case of the dibromoglutaric ester the ratio of trans to cis isomers is affected considerably by solvent cathode material and electrolysis poten- tial. Three-membered Rings.-A highly enantioselective synthesis of cyclopro-pane derivatives has been reported' involving carbenoid type reactions between olefins and diazoalkanes and catalysed by bis[( -)-camphorquinone-a-dioxi-mato]cobalt(~~). Enantioselectivities of 88% optical yield and chemical yields of 90-95% have been recorded. The results suggest the intermediacy of a cobalt- carbene complex. A mechanistic study12 reveals an interesting sequence of reactions consisting of carbene complex formation enantioselective attack upon the olefin and epimerization during the cyclization.It has also been shown that a correlation ' M. Hanack Angew. Chem. Internat. Edn. 1978,17,333. 'A. H. Schmidt and W. Ried Synthesis 1978. 1. H.Knorr and W. Ried Synthesis 1978,649. A. P.Krapcho Synthesis 1978 77. H.Diirrand R. Gleiter Angew. Chem. Infernat.Edn. 1978,17,559. 'P. W. Rabideau AccountsChem. Res. 1978,11 141. S. Sarel Accounts Chem. Res.. 1978,11 204. * C.J. M. Stirling Chem. Rev. 1978 78 517. K. H. Gibson Chem. SOC.Rev. 1977,6,489. S. Satoh M.Itoh and M. Tokuda J.C.S. Chem. Comm. 1978 481. A. Nakamura A. Konishi Y. Tatsuno and S. Otsuka J. Amer. Chem. SOC.1978,100,3443. I' A. Nakamura A. Konishi R.Tsujitani M.-A. Kudo and S. Otsuka J. Amer. Chem. SOC.,1978,100. 3449. 279 280 A. Cox exists between the chirality of cyclopropane products and catalyst structure enabling selective production of the desired enantiomer. An extension has been made13 to the scope of the addition of carbenes to allenes to produce a variety of substituted methylenecyclopropanes. Both photolytically generated carboethoxycarbene as well as the copper carbenoid species have been studied and the intervention of perpendicular trimethylenemethane intermediates is discussed. Use has been made14 of the addition reactions of various carbanions of ketones and nitriles to chloromethyloxirane as a convenient synthesis of bifunctional vicinal cyclopropanes. Good yields are reported involving translcis ratios varying from 95 :5 to 10:90 depending on the substituents.The synthesis has been described” of a new class of compounds silylcyclopropyl ketones (Scheme 1). Methyl-Me,Si Me,SiCH=SMe, yo-R R Scheme 1 thiomethyltrimethylsilane is converted into its methiodide and following depro- tonation this gives an ylide treatment of which with a variety of ap-unsaturated carbonyl systems leads to the desired silylcyclopropyl ketones. In view of the ability of the Si atom to stabilize a P-carbonium ion these ketones may find use synthetic- ally. High yields are reported16 for the synthesis of activated cyclopropanes by the phase-transfer catalysed condensation of diethyl bromomalonate with suitable Michael acceptors.16 Although these syntheses appear to be of limited scope only products of 1,4-addition are formed.Reaction of methyl 4-bromocrotonate with lithium s-butyl- or t-butyl-mercaptide leads17 to the trans-cyclopropane (1). The R = s-butyl or t-butyl (1) course of the reaction is found to be critically dependent upon both solvent and mercaptide gegenion. The reaction occurs by Michael addition of mercaptide to methyl 4-bromocrotonate. It is suggested that the Li metal is co-ordinated to both S and ester carbonyl oxygen thereby holding the attacking S atom in close proximity to the @-carbon. The stability of this complex is solvent dependent and also dependent on the ability of the metal to co-ordinate effectively with the oxygen atom allowing more access to the &carbon.l3 X. Creary J. Org. Chem. 1978 43 1777. l4 G. Mouzin H. Cousse and B. Bonnaud Synthesis 1978 304. ’’ F. Cooke P. Magnus and G. L. Bundy J.C.S. Chem. Comm. 1978,714. J. M. McIntosh and H. Khalil Canad. J. Chem. 1978,56 2134. l7 R. D. Little and J. R. Dawson J. Amer. Chem. SOC.,1978 100 4607. Alicyclic Chemistry An interesting variation of a recent route to cyclopropanes has been announced" and involves electroreductive as opposed to metallic reductive elimination. For example electrochemical reduction of a citral derivative (Scheme 2) yields a mixture of the cyclopropane and acyclic hydrocarbon (85 :15in 8 1o/o yield). Any decrease in the amount of proton donors promotes enhancement of the formation of cyclo- propane at the expense of undesired acyclic compounds.Scheme 2 The photochemistry of gem-di-iodides has been investigated. l9 Irradiation of CH212in the presence of cyclohexene or other alkenes gives the corresponding cyclopropanes. This convenient method complements the familiar Simmons-Smith procedure but does not suffer from the disadvantageous sensitivity to steric effects; the transformation probably occurs via a H2C :I2 complex. A report has appeared2' describing the use of some rhodium carboxylates as catalysts for cyclopropanation of acetylenes (Scheme 3). Although the reaction fails for R' = CF3 and Ph yields of H CO,CH R'C=CR2 +N2CHC02CH3 Rhz(02CR3) +N, ' A R' R2 Scheme 3 86% are recorded for R' = C4H9. The results are consistent with a bimolecular carbenoid mechanism involving competitive cyclopropanation and insertion reac- tions.The intramolecular reductive coupling of dibenzoylalkanes with TiC13- LiA1H4 gives21 cyclopropenes in yields of between 40 and 60%. Four- Five- and Six-membered Rings.-A synthesis has been reported22 of a-methylenecyclobutanones involving the reaction with olefins of methyl(pheny1thio)- keten generated in situ by dehydrochlorination of a-(pheny1thio)propanoyl chloride with triethylamine. All of the cyclobutanones formed contained an endo-phenylthio group suggesting a concerted [2 +21 cycloaddition mechanism. Silyl ethers are found to react23 readily with dichloroketene a compound generated from trichloroacetyl chloride and activated zinc to afford good yields of cyclobutanones (Scheme 4).Mild acid hydrolysis of the siloxycyclobutanones leads to the corresponding 3-hydroxydichlorocyclobutanones. T. Shono Y.Matsumura S. Kashimura and H. Kyutoku Tetrahedron Letters 1978 1205. l9 N. J. Pienta and P. J. Kropp J. Amer. Chem. SOC.,1978,100,655. 2o N. Petiniot A. J. Anciaux A. F. Noels A. J. Hubert and Ph. Teyssie Tetrahedron Letters 1978,1239. 21 A. K. Baumstark C. J. McCloskey and K. E. Witt J. Org. Chem. 1978 43 3609. 22 T. Minami M. Ishida and T. Agawa J.C.S. Chem. Comm. 1978 12. 23 L.R. Krepski and A. Hassner J. Org. Chem. 1978,43,3173. 282 A. Cox Scheme 4 Continued interest is shown in the use of oxycyclopropanones in synthesis. In particular (*)-grandisol(2) has been from 4-methoxy-3,6,6-tri-methylcyclohexa-2,4-dienone by a procedure involving regiospecific cyclo- propanation of the dienone acid-catalysed hydrolytic rearrangement of the product and opening of the five-membered ring of the resultant bicyclo[3,2,0]heptanone system.A three step synthetic sequence of a cyclopentenone has been described25 (Scheme 5) also involving a p-oxycyclopropyl ketone and this has been used to -0 Scheme 5 prepare a number of natural products including a prostanoid intermediate (&)-a-cuparenone and (*) -p-vetivone. It is reported26 that epoxysulphones 0 /\ PhS02(CH2),CH-CH2 (n= 1or 2) react with CH3MgI in THF to give 3-phenyl- sulphonylcyclobutanols and 3-phenylsulphonylcyclopentanols.Yields are often in excess of 90% and oxidation of the product by the Jones reagent followed by treatment with triethylamine leads to 3-substituted cyclobutenones and cyclopent-2- enones.A new facile route to functionally substituted cyclopentenones has been described2' which starts from ethyl p-(1-pyrrolidiny1)acrylate (Scheme 6). By use of t-butyl-lithium this is almost quantitatively transformed into the P-alkoxycar- 24 E. Wenkert D. A. Berges and N. F. Golob J. Amer. Chem. Soc. 1978,100 1263. 25 E. Wenkert B. L. Buckwalter A. A. Craveiro E. L. Sanchez and S. S. Sathe J. Amer. Chem. Soc. 1978 100,1267. 26 B.Corbel J. M. Decesare and T. Durst Canad. J Chem. 1978,56,505. 27 R.R.Schmidt and J. Talbiersky Angew. Chem. Internat. Edn. 1978,17 204. Alicyclic Chemistry 0 Scheme 6 bonylvinyl-lithium reaction of which with @-unsaturated carbonyl compounds leads to functionalized cyclopent-2-en-1 -ones.Electrolysis of the anion of 3-methoxycarbonyl-7-oxabicyclo[2,2,l]heptane-2-carboxylic acids gives2* the 7-oxabicyclo[2,2,l]heptyl-2-cation,a species otherwise difficult to obtain (Scheme 7). Wagner-Meerwein rearrangement into the 2-oxabi- cyclo[2,2,l]heptyl-3-cation occurs by virtue of the participation of the oxygen atom 0 0 &C02CH3coy -2c0,-2e+ fiC0*CH R R -co I R Scheme 7 (R= H or Me) T. Akiyama T. Fujii H. Ishiwari T. Imagawa and M. Kawanishi TetrahedronLetters 1978 2165. 284 A. Cox and this sequence provides a novel stereoselective synthesis of 1,2,3-trisubstituted cyclopentanes.A significant feature of this route is that each substituent is present in a different oxidation state.Reaction of a diethyl alkylidenemalonate with an alkynyl bromide and zinc followed by heating and subsequently by hydrolysis leads2' to a good yield of a dialkyl cyclopent-2-ene-l,l-dicarboxylate(Scheme 8). It is possible that the product arises by thermal cyclization of the intermediate (3)for which supportive i.r. evidence is available. R' C0,Et R2)!?(C02Et (3) Scheme 8 A new method has been reported3* for the preparation of 2,2-dialkylcyclo- pentane-1,3-dione. The procedure uses intramolecular oxidative coupling of di- lithium enolates of 3,3-dialkylpentane-2,4-dioneby CU(OSO~CF~)~ and is charac- terized by the ready availability of the starting materials and experimental simplicity.Functionalized spiro [4 n]ring systems can also be elaborated by this method. Reaction of ketones with 1,l-dichloroallyl-lithium leads to the dichlorohomoallyl alcohol (4) which following acid dehydration and solvolysis gives the chloro- pentadienyl cation (5). Thermal conrotatory ring closure of this cation proceeds in excellent yield (Scheme 9). This method3' is applicable to both cyclic and acyclic ketones and is of particular use for cyclopentenone annulation; some regioselectivity seems to occur. It is reported3* that epoxy ketone (6) on treatment with hydrazine in methanol undergoes cyclization to the hydrindenol (7) in 85% yield. Substitution on the side-chain double bond does not appear to be crucial and the cyclization is also successful with acyclic systems (Scheme 10).This method can be modified for the preparation ofsix-membered rings as demonstrated by the cyclization of epoxide (8). Two serious mechanistic possibilities have been suggested concerted collapse of the vinyldiazene (9) and decomposition of the diazene to give a radical (lo) which then adds to the double bond. Eight-membered Rings.-Treatment of benzocyclo-octene with one mole of bromine results in the known 5,6-dibromide which on reaction with 10 molar equivalents of KOBu' leads to 67% of 7-t-butoxybenzocyclo-octene. This implies the intermediacy of 6,7-didehydrobenzocyclo-octene,a cumulene which has however proved to be too unstable for isolation or independent trapping.33 29 B.Bellassoued Y. Frangin and M. Gaudemar Synthesis 1978 150. 30 Y. Kobayashi T. Taguchi and T. Morikawa Tetrahedron Letters 1978 3555. 31 T. Hiyama M. Shinoda and H. Nozaki Tetrahedron Letters 1978 771. 32 G. Stork and P. G. Williard J. Amer. Chem. SOC., 1977,99 7067. 33 H. N. C. Wong T.-L. Chan and F. Sondheimer Tetrahedron Letters 1978,667. Alicyclic Chemistry 'yo LiCCI,CH=CH; R2 R2 Scheme 9 (8) OH OH v-HC < H (10) (9) Scheme 10 286 A. Cox A new annulating procedure has been announced34 involving the electrophilic addition of dichloromethyl methyl ether to dienes. Lewis acid catalysed conden- sation with 2,5-dimethylhexa-1,5-dienefollowed by treatment with base leads to a mixture of dimethylcycloheptatrienes.A related condensation with cyclo-octa- 13-diene yields a stereoisomeric mixture of dichloro-9-methoxybicyclo[3,3,1] nonanes. In an improved synthesis3’ of cyclo-octyne treatment of 4,5,6,7,8,9-hexahydro-cyclo-octa-1,2,3-thiadiazolor 4,5,6,7,8,9-hexahydrocyclo-octa-1,2,3-selenadiazol with butyl-lithium at low temperature results in up to 85% of the cyclo-octyne. A similar thermolysis of 1,2,3-~elenadiazoles has been to prepare cyclo-oct- 1-en-3-yne. Comparison of the 13C-n.m.r. chemical shifts with those of the unstrained cyclododecyne shows that ring strain causes an overall downfield shift of the signals. In this series the A6 values of the sp carbons appear to be capable of serving as a direct experimental measure of ring strain.Medium Ring Compounds.-It has been reported3’ that photolysis of the tri- cyclodecanone (11) leads to a mixture of dienes cis,cis-cyclonona-l,4-dieneand the previously unknown cis,trans-cyclonona-1,4-diene.Although the structural con- straints of (11)disfavour concerted processes they do permit a biradical mechanism to operate with selective formation of the trans double bond. This mechanism is based on stereoelectronic controlled fragmentation of an intermediate acyl cyclo- propyl radical to the isomeric trans-homoallylic species. A new synthesis of medium ring cycloalkene-1-carboxylic acids has been announced.38 The E isomers of cyclodecene-1-carboxylicacid and the E and 2 isomers of cycloundecene-1-carboxylic acid and cyclododecene- 1-carboxylic acid have been synthesized by aqueous alkaline hydrolysis of the corresponding 4-chloro- 3,4-poly(methylene)-2-pyrazolin-5-ones.Although the 2 isomer predominates determination of thermodynamic data show that for isomerization of the E/Z-cycloundecene-1-carboxylicacid the E isomer is the more stable.Annu1enes.-A synthesis has been reported3’ of the cis,cis,cis,trans-[ 9lannulene anion by reaction of 9-anfi-chloro-cis-bicyclo[6,l,O]nona-2,4,6-triene with alkali metals in THF. The conformational mobility of the compound has been investi- gated4’ and at three different temperatures by deuterium equilibration saturation transfer and dynamic n.m.r. spectroscopy. Almost identical activation enthalpies ” C. F. Garbers H. S. C. Spies H. E. Visagie J. C.A. Boeyens and A. A. Chalrners Tetrahedron Letters 1978 81. ’’ H. Biihl H. Gugel H. Kolshorn and M. Meier Synthesis 1978 536. 36 H. Petersen H. Kolshorn and H. Meier Angew. Chem. Internat. Edn. 1978,17 461. ’’ I. M. Takakis and W. C. Agosta Tetrahedron Letters 1978 2387. 38 A. Silveira Y. R. Mehra and W. A. Atwell J. Org. Chem. 1977 42 3892. 39 G. Boche H. Weber D. Martens and A. Bieberbach Chem. Ber. 1978,111 2480. 40 G. Boche H. Weber and A. Bieberbach Chem. Ber. 1978,111 2833. Alicyclic Chemistry 287 were found. It is concluded that the mechanism of topomerization probably doesnot involve reversible valence isomerizations of the corresponding all trans-anion. The isomerization of the cis-cis,cis,trans-[9]annulene anion into the thermodynamically more stable all-cis-[9lannulene anion has been studied41 kinetically under different conditions.It is catalysed by acids and also by alkali metals and a lower limit of 145.6 kJ mol-' found for the thermal isomerization. Dienyne ketones such as 2,2-dimethyl-7-t-butylnona-4,6-dien-8-yn-3-one give4* the corresponding tetrasubstituted 14-membered cyclic glycols on treatment with KOH in liquid NH3. Reaction of the cyclic glycols with tin(I1) chloride in organic solvents saturated with HCl affords the strongly diatropic tetrasubstituted bis- dehydro[ 14lannulenes. Using an already published method43 naphtho[2,3-i]-5,14-dimethyl-1,3-bisde-hydro[ 141annulene and nap h tho[ 1,2- i]- 5,14-dime thyl- 1,3-bisdehydro[ 14lannulene have been ~ynthesized.~~ These compounds are both analogues of anthracene and phenanthrene in which a terminal 6r-ring has in a formal sense been expanded to a 14r ring.Inspection of the 'H n.m.r. parameters of these two naphthodimethylbis- dehydro[ 14lannulenes reveals that the phenanthrene analogue is more diatropic than the anthracene analogue. This can be explained simply on the basis of KekulC structures. The synthesis of 1,6-methanofluorene and 9-methyl-1,6-methanofluorene and their anions has been acc~mplished.~~ By use of 'H n.m.r. the anions were shown to be delocalized aromatic systems existing in the 'cyclo- heptatriene' rather than norcaradiene form. Thus the anions exhibited electron delocalization over the entire r-system but a decided downfield shift of bridge protons relative to those in the methanoindenyl anion.The synthesis of 3-methoxy-1,5-methano[ 101annulene has been achieved46 and its 'Hn.m.r. spectrum measured. Comparison with the 'H n.m.r. spectrum of 1$-methano[ 101annulene reveals striking similarities and argues against any significant difference in the ability of the two r-systems to support an induced diamagnetic ring current. It is therefore,-concluded that both ring systems are of comparable aromatic character. A study of the molecular structure of 5,8,16,19-tetra-t-butyl-6,17,23-trisdehydro[ 10,10,2][ 14]annuleno[ 14lannulene has been ~ndertaken~~ to deter- mine if the r-electron delocalization in this compound is located within two separate 14-carbon ring systems or whether it embraces the entire 22-carbon ring.Measure- ment of bond distances show the former to be the correct model. Related work describes4* the preparation of trisdehydro[ 14,10,2][ 14]annuleno[ 18lannu- lene a compound consisting of two different (4n +2)-membered rings. The n.m.r. parameters of this compound reveal that it is strongly diatropic. Interestingly there is a gradual high-field shift of outer protons with increasing distance from the bridge and the same trend is observed with the inner protons of the 18-membered ring. 41 G. Boche and A. Bieberbach Chem. Ber. 1978,111,2850. 42 K. Fukui T. Nomoto S. Nakatsuji S. Akiyama and M. Nakagawa Bull. Chem. SOC.Japan 1977,SO 2758. 43 N. Darby T. M. Crisp and F. Sondheimer J. Org. Chem. 1977,42 1960. 44 T.C.Walsgrove and F.Sondheimer Tetrahedron Letters 1978,2719. 45 R. J. Hunadi and G. K. Helmkamp J. Org. Chem. 1978,43,1586. 46 L.T.Scott and W. R. Brunsvold J. Amer. Chem. SOC.,1978,100,4320. 47 Y. Kai N. Yasuoka N. Kasai S. Akiyama and M. Nakagawa Tetrahedron Letters 1978 1703. 48 S. Nakatsuji S. Akiyama and M. Nakagawa Tetrahedron Letters 1978 3723. 288 A. Cox These observations are difficult to understand if the ring current is confined to the periphery. Another trisdehydro[4tn +2]annuleno[4n‘ +21 annulene which has been s~nthesized~~ and examined spectroscopically is trisdehydro[ 18,10,2][ 14lannu- leno[22]annulene. As with the [18]annulene discussed above the signals of both the inner and outer protons in the 22-membered ring move to higher field as the distance from the common bond is increased.This can be explained in terms of an indepen- dent current being induced in each ring as is assumed in polycondensed benzenoid systems. Trisdehydro[ 14,14,2][ 18]annuleno[ 18lannulene has been reported” to be fairly stable and the electronic spectrum of this 30~-electron system found to show a close similarity with the analogous trisdehydror 10,10,2][ 14]annuleno[ 14lannulene. The n.m.r. spectrum reveals a strong diatropicity and once again displays the remarkable characteristic of a gradual high-field shift of all signals with increasing distance from the bridge. The synthesis and properties of l0-methylbenzo[d]-6,8-bisdehydro[151-annulenone (12) and 12-methylbenzo[ f]-8,1O-bisdehydro[ 1Slannulenone (13) have been ann~unced.~~ The electron absorption spectra are similar to one another OH OH indicating that the position of the benzene ring has little effect on the conjugation between the benzene nucleus and the bisdehydro[ 1Slannulenone skeleton.The n.m.r. shows that both compounds are atropic but similar measurements on the analogous protonated species show them to be diatropic and that they can be represented by delocalized formulae. Aromaticity in annulenoannulenes has been studied5*by several different theoretical approaches. The main conclusion is that the aromaticity is determined by the nature of the fused rings rather than by the size of the periphery. Po1ycyclics.-The preparation and properties of the highly strained olefin bicy- clo[2,2,0]hex-l(4)-ene have been rep~rted,’~ Controlled current electroreduction of the bromochloride (14) in DMF at -14°C gives volatile effluents which were Br 49 S.Nakatsuji S. Akiyama and M. Nakagawa Tetrahedron Letters 1978 1483. 50 M. Osuka Y. Yoshikawa S. Akiyama and M. Nakagawa Tetrahedron Letters 1977 3719. 51 J. Ojima and Y. Shiroishi Bull. Chem. SOC.Japan 1978 51 1204. ” B. A. Hess L. J. Schaad and I. Agranat J. Amer. Chem. SOC.,1978,100,5268. 53 J. Casanova J. Bragin and F. D. Cottrell J. Amer. Chem. SOC.,1978,100 2264. Alicyclic Chemistry 289 fractionally collected in cold traps. The substance obtained at -95 "C is the desired product and is found to be extremely reactive having t~< 10 s at -23 "C.Although the olefin reacts with cyclopentadiene at temperatures below those necessary for polymerization it fails to undergo cycloaddition with 2,3-dimethylbuta-1,3-dienein CD2C12 solution.The 13C n.m.r. displays an unusual downfield shift of the olefinic carbon suggesting a tendency toward allene-like sp character. Unlike 2,3,4-tri-t-butylcyclopentadienone,which on irradiation cyclizes to 'housenone' irradiation of tetra-t-butylcyclopentadienone(A = 254 nm) does not undergo C-2/C-5 bridge formation but rather leads to tricyclopentanone the criss-cross addition product (15). On prolonged irradiation this leadss4 to a hydro- carbon whose spectroscopic properties are in complete agreement with those expected for tetra-t-butyltetrahedrane (Scheme 11).The stability of this compound hu +++ 0 0 (15) Scheme 11 is attributed to the fact that any attempt at bond lengthening as a precursory step to bond cleavage is resisted because movement apart of any two t-butyl groups is opposed by the others.A tetrahedral structure permits the four substituent groups to adopt a regular spherical distribution and hence a maximum separation. Scheme 12 Model calculations have been described" discussing the possible collapse of bicyclobutan-2,4-diyl to tetrahedrane and allied experimental works6 shows that irradiation of some bicyclobutane derivatives offers little hope of access to tetra- hedrane. A report has been presented" summarizing both the theoretical and experimental approaches towards a synthesis of tetralithiotetrahedrane.Irradiation of dilithioacetylene in liquid NH3 at -45 "C leads to a compound having a 13C n.m.r. displaying a singlet (8 57.2) consistent with the expected sp3 hybridization of the tetrahedrane carbon and evaporation of the NH3 yields a product which by field desorption mass spectrometry is concluded to be C4Li4. Ab initio MO calculations indicate that face-centred tetralithiotetrahedrane (16) is 273 kJ mol-' more stable than the corresponding structure bearing the Li atoms at the vertices and this is " G. Maier S. Pfriem U. Schiifer and R. Matusch Angew. Chem. Internat. Edn. 1978,17,520. 55 M. C.Bohm and R. Gleiter Tetrahedron Letters 1978 1179. 56 G.Maier H. P. Reisenaure and H.-A. Freitag Tetrahedron Letters 1978 121. " G.Rauscher T. Clark D. Poppinger and P.von R. Schleyer Angew Chem. Internat. Edn. 1978,17 276. 290 A. Cox Li attributed to stabilization of the tetrahedrane 'bent bond' orbitals by Li. The nature of the photochemical process involving C2Li2 is at present unclear. Stimulated by the isolation of spiro[4,5]decanes possessing interesting biological activity a new general method (Scheme 13) for preparing this system has been anno~nced.~' Although yields are only moderate the reaction appears to be especially suited for the synthesis of 2-alkyl spiro[4,5]decane sesquiterpenes. For example when the sodium salt of a-formycyclohexanone (17) and (18)are allowed to react in (Me2N),P0 a spiro keto ester is produced as the major non-phosphorous- containing compound. The reaction has been applied to the total synthesis of (+)-P-vetivone (+)-p-vetispirene and (+)-a-vetispirene.,CO,Et BF,- - C0,Et+Q (17) (18) Scheme 13 A novel synthesis of spiro[4,5]deca-6,9-dien-2,8-dionehas been reported59 involving the intramolecular cyclization of phenolic a-diazoketones in the presence of copper(1) halides (Scheme 14). Evidence is presented to show that catalytic decomposition proceeds uit 3 norcaradiene. 0 ?H OH 0 li 8-RR R 0 "OQ L Scheme 14 (R=H or Me) W. G. Dauben and D. J. Hart J. Amer. Chem. SOC.,1977,99,7307. 59 C. Iwata M. Yamada Y. Shinoo K. Kobayashi and H. Okada J.C.S. Chem. Cornrn. 1977,888. Alicyclic Chemistry 291 As part of a programme to synthesize a molecule displaying neutral homoaroma- ticity the synthesis of elassovalene 2a,8b-dihydrocyclopent[cd]azulene (19) its Cr(C0)3 complex and its 5-and 6-methoxy derivatives was undertaken.60 Measurements of the ‘H- 13C-n.m.r.U.V. and p.e. spectra together with the diamagnetic susceptibility reveal that at least some small degree of homoaromaticity is present in the bridged cycloheptatriene part of their structure. A full paper has appeared6’ describing the use of multiple Diels-Alder cycloaddition reactions (of the domino or pincer type) in the construction of highly condensed alicyclic frameworks such as hexahydro-3,4,7-methenocyclopenta[a]pentalene (20) octahydro-3,6-dimethylenedicyclopenta-[cd,ghlpentalene (21) triquinacene(22) and a number of previously unknown (CH), isomers.The results suggest that there is considerable potential for the synthesis of highly condensed carbocyclic structures by the use of multistage cycloaddition reactions of 0-coupled cyclopolyolefins. The synthesis has been described6* of a tetracyclo[5,5,1 ,04.13,010,*3]tride~ane-tetraone (23) for which the generic name ‘staurene’ has been proposed. Generality has been an aim of the work with the immediate objective being the synthesis of the diketo-acid (24) by means of the reaction between 3-ketoglutarate and 1,2-dicar- bony1 compounds. H02CvC0,H H 6o L. A. Paquette C. C. Liao R. L. Burson R. E. Wingard,jun. C. N. Shih J. Fayos and J. Clardy,J. Amer. Chem. SOC.,1977,99,6935. L. A. Paquette M. J. Wyvratt H. C. Berk and R. E. Moerck J. Amer.Chem. SOC.,1978,100,5845. 62 R.Mitschka J. M. Cook,and U. Weiss J. Amer. Chem. SOC.,1978,100,3973. 0-0 292 A. Cox A new simple route to the [4,l,l]propellane system has been De-hydrohalogenation of. 1-chlorotricyclo[4,1 ,0,02*7]heptane in the presence of anthracene using lithium 2,2,6,6-tetramethylpiperidide leads to 9,10-dihydro-9,10- (1,7-tricyc10[4,1,0,02*7]heptano)anthracene. Between 160 and 180 "C this cleanly rearranges to 9,10-dihydro-9,10-[2,7-(3-methylenecyclohex-l-eno)]anthracene. The thermal rearrangement probably proceeds uia a retro-carbene ring opening of the bicyclo[ 1,l ,O]butane unit. Two hitherto unknown members of the tricyclic C10H16 cage compounds anti-and syn-tricycl0[4,2,1 ,12qdecane have been ~ynthesized.~~ These compounds are of interest because of their structural relationship to adamantane.A new approach to the synthesis of diamondoid hydrocarbons has been de~cribed.~' The method which may serve as either a single or double homologation has been used to synthesize anti-tetramantane but it may be applicable generally to higher members of the series. The sequence employs a keto-carbenoid insertion reaction to attach one or two cyclopentane rings to the diamantane periphery and this is followed by an unusual gas phase rearrangement-cyclization on a platinum-silica catalyst. 3 Stereochemistry An improved force field for molecular mechanics calculations of the structures and energies of hydrocarbons has been reported.66 With the aid of one-fold and two-fold rotational barriers a solution was achieved to the problem of obtaining a sufficiently large gauche butane interaction energy while keeping the hydrogens small enough for good structural predictions.For a varied list of hydrocarbons the results compared favourably with those obtained experimentally. Based upon MO cal-culations an analysis has been pre~ented~~ of l,3(non-bonded) carbon-carbon interactions in the cyclobutane series. The results suggest that this effect is dominant and that along with Baeyer strain these are the main factors determining the total strain of cyclobutanes and account for many of their exceptional properties. In a study6' of the conformational analysis of tertiary cycloalkyl (c6 c7 c,) carbocations it has been shown that tertiary cyclohexyl cations have a twist-boat ground state which is 2 kJ mol-' more stable than the chair.The tertiary cycloheptyl cation is fluxionally mobile but the methoxycyclo-octyl cation has an unsymmetrical chair-twist-boat conformation. Other cations are also discussed. A report has been made6' of the use of the lanthanide-induced shift reagent Eu(fod) in the direct determination of the solution structure of the conformationally mobile system cyclohexanecarbonitrile. The results show that the conformation with an equatorial cyano group exists to an extent of 54% in good agreement with earlier w~rk.~~,'~ 63 U. Szeimies-Seebach and G. Szeimies J. Amer. Chem. SOC. 1978,100 3966. 6A B. Ernst and C. Ganter Helu. Chim. Acta 1978,61 1107. 65 W.Burns M. A. McKervey T. R. B. Mitchell and J. J. Rooney J.Amer. Chem. SOC. 1978,100,906. 66 N. L. Allinger J. Amer. Chem. SOC.,1977,99 8127. 67 N. L. Bauld J. Cessac and R. L. Holloway J. Amer. Chem. SOC., 1977,99 8140. " R. P. Kirchen and T. S. Sorensen J. Amer. Chem. SOC., 1978 100 1487. 69 D. J. Raber M. D. Johnston and M. A. Schwalke J. Amer. Chem. SOC. 1977,99 7671. 'O J. A. Hirsch Topics in Stereochem. 1967,1 199. 71 For example B. Rickborn and F. R. Jensen J. Org. Chem. 1962,27,4606. Alicyclic Chemistry 293 In an investigation of the temperature dependence of the e.s.r. spectrum of the cyclohexadienyl radicals (25) generated by hydrogen abstraction from the cor- responding cyclohexa- 1,4-dienes it is shown7* that the cyclohexadienyl radical is planar but that it vibrates between bent structures.The vibrational spectrum of cycloheptanone has been examined73 and the results of the analysis show that cycloheptanone is a non-rigid pseudorotating molecule having a twist-chair struc- ture. Calculation of the pseudorotation energy surface indicates that the symmetric twist-chair conformer and the two adjacent twist chairs have almost the same energy. The other twist chair forms are separated by barriers of different heights. H (25) a; R=H b; R=SiMe3 Molecular mechanics calculations have been performed74 to fit the vibrational and rotational spectra of cyclo-octanone. In this way the lowest energy conformation was shown to be the boat-chair with the carbonyl in the 3-position (position 1 is the symmetric position at the chair end).The p.e. surface of this molecule emerged as being very complex and the various boat-chairs shown to interconvert by pseudoro- tation with barriers of about 33.5 kJmol-’. Use has been made” of dynamic n.m.r. spectroscopy and iterative strain energy calculations to determine the confor- mational properties of cis,cis-cyclo-octa- 1,4-diene. This work shows that the compound exists as a mixture of twist-boat (flexible) and boat-chair (relatively rigid) conformations having nearly the same energies. The calculated barrier to intercon- version is 37.6 kJ mol-’. It is of interest to note that since the interaction of the r-orbitals of the double bonds is mainly through space in the boat-chair and largely by hyperconjugative interaction via 3-methylene in the twist-boat this should have significance for the interpretation of the photoelectron spectrum.The same investigative techniques have been applied76 to probe the confor- mational features of cyclonona-1,2-diene cycl~nona-1,2,6-triene (26) and cyclo- deca-1,2,6,7-tetraene. The first compound exists in solution as a mixture of two conformations and undergoes two conformational processes a ring pseudorotation of the major conformation leading to time-average Czsymmetry and interconversion of the major conformation with the minor (symmetrical) conformation. The second compound exists as an unsymmetrical conformation that can undergo a strongly hindered ring pseudorotation (Scheme 15). The ground-state conformation of (*)-cyclodeca-1,2,6,7-tetraene is found to have a ‘non-intersecting’ two-fold axis of symmetry which is not maintained during conformational interconversions.72 M. Kira and H. Sakurai J. Amer. Chem. SOC.,1977,99,3892. 73 D. F. Bocian and H. L. Strauss J. Chem. Phys. 1977,67 1071. 74 T. C. Rounds and H. L. Strauss J. Chem. Phys. 1978,69,268. ’’ F. A. L. Anet and 1. Yavari J. Amer. Chem. SOC.,1977,99,6986. 76 F. A. L. Anet and I. Yavari J. Amer. Chem. SOC.,1977,99 7640. 294 A. Cox (26) Scheme 15 A study has been made77 of the conformational changes induced by europium shift reagents in medium ring 3-methoxycycloalkanones. The H2-H3 coupling constants which are approximately equal for trans and cis protons in the free cyclo-octanone show a marked change on addition of shift reagent consequent on bidentate co-ordination of the shift reagent.Reduction in ring size (c8+C7-+C,) induces a series of spectral changes consistent with a reduction of bidentate co-ordination. Molecular mechanics calculations have been to assess the relative stabilities of the conformers of humulene. The results show that the CT and CC conformations (27) and (28) are significantly more stable than the others. An investigation of the conformational interconversions of truns7truns,truns-cyc1ododeca-1,5,9-triene by 'H n.m.r. measured between -5 to -180 "C reveals79 a dynamic effect associated with a conformational process having a free energy of activation of 36.0 kJ mol-' and consistent with a single conformation with D3 symmetry for the triene.The conformational energy surface for ring inversion of the D3conformation to its mirror image has been investigated by iterative force-field calculations and the strain energy barrier for the best path is calculated to be 39.7 kJ mol-' in good agreement with the value from n.m.r. data. By use of line shape analysis of the temperature-dependent n.m.r. spectrum the isodynamical interconversion between the quasiplanar equilibrium conformers 77 E. Dunkelblum and H. Hart J. Org. Chem. 1977,42 3958. H. Shirahama E. bsawa and T. Matsumoto Tetrahedron Letters 1978 1987. 79 F.A. L. Anet and T. N. Rawdah J. Amer. Chem. SOC.,1978,100,5003. Alicyclic Chemistry (29b) and (29c) of 1,5-bisdehydro[l2]annulene has been found to involve" AG* = 18.8kO.84kJ mol-' (120 K).The magnitude of this barrier is consistent with the energy needed to disrupt the cyclic T-conjugation on transformation to the non- planar transition state (29a).Scheme 16 4 Structural Properties and Orbital Interactions Cyclobutadiene continues to excite considerable interest. A theoretical study has been undertaken" involving ab initio calculations both on the SCF level and with electronic correlation for both the square singlet and triplet and also the rectangular singlet states of the planar molecule. The square singlet is more stable than the corresponding triplet by about 29 kJ mol-' and the energy difference between the rectangular and square singlets is about 58.5 kJ mol-'. It is concluded that cyclo- butadiene should have a rectangular singlet ground state with very long C-C single bonds (1.57w).These resuIts are confirmed by other authorsg2 whose CI cal- culations additionally suggest that the square is the lowest energy intermediate for interconversion of rectangular singlets. This is supported in related workg3 from which it emerges that a square geometry represents a transition state for bond flipping in singlet cyclobutadiene. Theoretical determination of the i.r. spectrum of cyclobutadiene using ab initio SCF calculations of the vibration frequzncies and intensities of the i.r. active vibrations have showns4 that the observed i.r. spectrum is compatible with a rectangular structure and that the band at 1240cm-' is an in-plane CCH bending deformation of cyclobutadiene.In a study designed to elucidate the orbital interactions present between two and three linked cyclopropane rings as a function of dihedral angles the p.e. spectra of syn-and anti-bishomocyclohepta-3,f-trieneand syn,syn- anti,anti- and anti,syn-8o R. Gygax J. Wirz J. T. Sprague and N. L. Allinger Helv. Chim. Acra 1977,60 2522. 81 H. Kollmar and V. Staemmler J. Amer. Chem. SOC., 1977,99 3583. 82 J. A. Jafri and M. D. Newton J. Amer. Chem. SOC.,1978 100 5012. 83 W. T. Borden E. R. Davidson and P. Hart J. Amer. Chem. SOC.,1978,100 388. 84 H. Kollmar and V. Staemmler J. Amer. Chem. Soc. 1978,100,4304. 296 A. Cox trishomocycloheptatriene have been meas~red.'~ Satisfactory agreement between the p.e.measurements and model calculations could however only be achieved if 'radially' oriented components were mixed into the highest occupied 'tangential' Walsh orbitals. The p.e. spectra of 1,1,3,3-tetramethyl-2-vinyl-,3-ethynyl-1,l-dimethyl- and 2-ethynyl-1,1,3,3-tetramethyl-cyclobutanehave been measured.86 An interaction term of HRs= -52 kJ mol-' was used to describe the conjugation and from the spectra it appears that this interaction does not depend on conformation. The influence of the cyclobutane ring on the r-system is comparable to that of an alkyl substituent but may be considerably enhanced by alkyl substitution of the ring which lowers the energy difference between cyclobutane and r-orbitals. For a selected number of systems a comparison has been mades7 between the through-bond interaction of two mutually perpendicular r-systems and spirocon- jugation.Such systems can interact quite effectively via the c+-bonds of the four- membered ring as a consequence of the strong interaction of the Walsh orbitals of the cyclobutane ring and the r-orbitals. In some cases the HOMO or LUMO is influenced resulting in changes in both energy and symmetry. This should have consequences for reactivity. Dispiro[2,2,2,2]deca-4,9-dienehas been synthesized and its properties and reac- tions investigated." The compound exhibits a relatively large bathochromic shift of the absorption maximum (222nm) and a lowering of the ionization potential (698 kJ mol-') as compared to cis-1,2-dicyclopropylethylene.However no cyclic conjugation appears to be present as shown by n.m.r.A paper has appearedsg discussing the electronic properties and the spectra of cyclobutanone and its derivatives spiro[2,3]hexan-4-one spiro[2,3]hexan-5-one dispiro[2,1,2,l]octan-4-one and dispiro[2,0,2,2]octan-7-one. The theoretical approach used is an all-valence electron semiempirical method and the results are discussed in terms of charge-donation and charge-transfer from the 3-ring hypsochromic and bathochromic shifts of electronic transitions related to the carbonyl direct substituent effects on the carbonyl stretching frequency and the across-ring interaction in cyclobutanone. Single determinant ab initio MO theory has been usedgo to study equilibrium geometries enthalpies strain energies and spiro interactions for spiropentane spiropentene spiropentadiene spiro[2,4]hepta- 4,6-diene spiro[2,4]heptatriene and spiro[4,4]nonatetraene.The calculated values are in good agreement with experimental results. Spiro interaction is also apparent in other calculated molecular properties such as dipole moments and ionization potentials. In a paperg1 concerned with the dependence of transmission of con-jugation through a cyclopropane ring into a vinyl group the chemical consequences resulting from placement of substituents of high electron demand on the cyclo- propane ring in spiro-vinylcyclopropaneson the reactivity of the vinyl group toward electrophilic olefins are discussed. A %5 J. Spanget-Larsen,R. Gleiter M. R. Detty and L. A. Paquette J. Amer.Chem. Soc. 1978,100,3005. 86 P. Bruckmann and M. Klessinger Chem. Ber. 1978 111 944. *' P. Bischof R. Gleiter and R. Haider J. Amer. Chem. Soc. 1978 100 1036. T. Tsuji T. Shibata Y. Hienuki and S. Nishida J. Amer. Chem. Soc. 1978 100 1806. 'I. Y. Meyer and R. Pasternak Theoret. Chim. Acta 1978,47 27. 90 J. Kao and L. Radom J. Amer. Chem. Soc. 1978,100,760. 91 M. Langbeheim and S. Sarel Tetrahedron Letters 1978 1219. 13' Alicyclic Chemistry The a,.rr-electron system in (30) is shown to be highly sensitive to substitution effects and the cyclopropane ring to be capable of effectively transmitting con- jugation from the polar substituent through the ring into the vinyl group. The first (30) X = CN or C02CH3 theoretical calculations that reproduce the observed U.V.spectrum of barrelene are reported.92 Both through-space interaction which determines the ordering of the orbitals and is predominantly T,and through-bond interaction which mixes cr with .rr and inserts antibonding (T among antibonding T are present. The lowest energy transition (>200nm) is assigned as T-+(T* and the first excitation which is essentially .rr -+ .rr* is predicted at about 180 nm. The effect of substituents on the reactivity of the double bond in (31) towards [,4 + ,2,] and epoxidation reactions has been in~estigated.~~ The observed rate sequence is (31c) > (31a)> (31b) in both Me2S0 and CHCI3,and this is interpreted (31) a; X=Y=H b; X,Y=O c; X=OMe,Y=H in terms of orbital interactions through space operating between the substituents and the double bond.An examination has been made,94 using a series of CND0/2 semiempirical MO calculations of substituent effects on model exchange reactions of 4-substituted bicyclo[2,2,2]octylcarbinyl and cubylcarbinyl chlorides. The trends produced in both sets of compounds are similar in magnitude and probably in mechanism. Should through-bond coupling via framework orbitals be important it cannot be appreci- ably charge-transfer in character. The present results do not allow estimation of the relative importance of polarization and of the electrostatic effect. A theoretical study has been made95 of the cubane molecule using ab initio STO-3G SCF-X, MIND013 and INDO methods and the p.e. ionization energies calculated from the above by Koopman’s theorem.Utilizing the concept of interactions between symmetry-adapted combinations of localized CC and CH orbitals an analysis of the MO energy splitting pattern has been made. 92 A. Y. Meyer and R. Pasternak Tetrahedron 1977,33 3239. 93 M. N. Paddon-Row H. K. Patney and R. N. Warrener J.C.S. Chem. Comm. 1978,296. 94 R. B. Davidson and C. R. Williams J. Amer. Chem. SOC.,1978,100,2017. ’’ J. M. Schulman C. R. Fischer P. Solomon and T. J. Venanzi J. Amer. Chem. SOC.,1978,100,2949. 298 A. Cox 5 Reactions Metal-promoted Reactions.-It is that reaction of cyclopropene with ( qS-C5H5),NbCl leads to a new complex (q'-C5H5)NbC1(C3H4) which has been fully characterized spectroscopically. Treatment of this complex with HCl yields almost pure cyclopropane (>90°/0) and represents the first reported reduction of cyclopropene via an isolable metal complex.An investigation9' of the permethyl substituted A1Cl3 u complex (Scheme 17) has shown that a dynamic process is occurring consisting of consecutive 1-2 shifts of the Tl T1 ;;-#-AlC1,-AlCl; Scheme 17 AlC13 group. Complexes having an oligomethylene chain in some cases give struc- tures involving an AlC13 residue bonded exclusively to the bridgehead and in others a mixture having either this feature or one involving the AlC13 at some other position. The rates of cleavage of (32) and (33) by an equivalent of Rh(N0R)acac areg8 1.3x 1mol-' s-l and 1.3 x 1mol-' s-' respectively; introduction of a second endomethyl group (34) stops the reaction altogether (Scheme 18).The fgj-[Rh(NOR)CI]z '\ /' (32) R= R' -H \/ (33) R=H,R'=CH3 Rh (34) R = R'= CH~ NaBH,/CH,OH 1 Scheme 18 (NOR = norbornadiene) 96 S. Fredericks and J. L. Thomas J. Amer. Chem. Soc. 1978,100,350. 97 P.B.J. Driessen and H. Hogeveen J. Amer. Chem. Soc. 1978,100,1193. 98 P.E.Eaton and D. R. Patterson,I.Amer. Chem. Soc. 1978,100,2573. Alicyclic Chemistry remarkable effect resulting from introduction of a methyl group so far from the reaction site probably arises from steric interference to the internuclear movements which accompany opening of the cage. The first example of a six-electron Rh-catalysed isomerization of a dicyclopropyl compound has appeared.* Thus syn-l,3-bishomocycloheptatriene(35) leads to a 59 :41 mixture of bicyclo[6,l,0]nona-2,5-diene (36) and cis,cis,cis-cyclonona- 1,3,6-triene.An investigation is reported'00 of the effect of 1-trimethylsilyl and l-tri-methylgermyl substitution on the outcome of Ag+- and €3'-catalysed rearrange-ments in tricyclo[4,1,0,0,02*7] heptanes. The results can be interpreted in terms of the dependence of an electrophilic attack at a given edge bicyclobutane bond upon the position of the alkyl substituent and the hyperconjugative and homoconjugative stabilizationof cationic intermediates by C-Si and C-Ge. The binding of unsaturated propellanes to transition metals has been studied. lo' Among the group six metal carbonyls investigated the specific catalytic effectiveness correlates directly with metal-carbon bond strengths (Cr >W >Mo) and the order of wback donation (W-C >Cr-C >Mo-C).Isotopic labelling studieslo2 have established that in the Mo(CO),-promoted isomerizations the induced [1,5]-carbon shifts involve specific co-ordination of the unsaturated propellane to Mo with suprafacial migration operating under the control of the transition metal. Thermalv Promoted.-Thermal isomerization of phenylcyclopropane leads'03 to trans-and cis-p-methylstyrene and alkylbenzene. From kinetic and labelling studies it has been shown that there is synchronous stereomutation of the phenyl-bearing carbon and also of Cz and C3. The MO method PRDDO has been used'" to investigate the surface for the degenerate thermal rearrangement of methyl-enecyclopropane.The calculated activation energy is in agreement with that determined experimentally and the barrier to ring closure found to be c7.84 kJ mol-'. The rate of interconversion of endo-and exo-2-methyl-6-methylenebicyclo[3,1 ,O]hexane is c~rnparable'~~ with that for cis to trans isomeriz-ation of the 2,3-dimethylenecyclopropanes,suggesting the intermediacy of a planar trimethylenemethane singlet. Tracer experiments suggest exo methylene rotation succeeds rate-determining formation of a planar trimethylenemethane biradical. Mainly orthogonal and to a lesser extent planar or bisorthogonal trimethylene 99 L. A. Paquette and M. R. Detty Tetrahedron Letters 1978,713. loo R. T. Taylor and L. A. Paquette J. Ore.Chem. 1978,43,242. lo' L. A. Paquette J. M. Photis and R. P. Micheli J. Amer. Chem. SOC., 1977,99 7899. lo2 L. A. Paquette R. P. Micheli and J. M. Photis J. Amer. Chem. SOC.,1977,99 7911. lo3 J. T. Wood J. S. Amey D. Cortes and J. A. Berson J. Amer. Chem. Soc. 1978,100,3855. D. A. Dixon R. Foster T. A. Halgren and W. N. Lipscomb J. Amer. Chem. SOC.,1978,100 1359. lo' J. J. Gajewski and S. K. Chou J. Amer. Chem. Soc.. 1977,99,5696. 300 A. Cox biradicals are involved in the pyrolysis of both cis- and trans-2,3-dimethyl- me thylenecyclopropanes. An investigation'06 of the Cope rearrangement of allyl-substituted cyclopropenes in particular 1,2-diphenyl-3-allyl-3-methylcyclopropene, has shown that the reac- tion proceeds via an intermediate analogous to the ly4-cyclohexylene biradical; a pericyclic process does not therefore appear to be implicated.cis-Divinylcyclo- propane has been synthesized"' by Wittig reaction of cis-2-vinylcyclopropanecar-boxaldehyde with methylenetriphenylphosphorane,despite earlier reports of its reactivity. The compound is stable at low temperatures rearranging quantitatively to cyclohepta-1,4-diene with ti = 11min at 288.5 K. Complexes involving hexafluoroacetylacetonatorhodium are discussed. The thermal reactions of cis-1-aryl-2-vinylcyclopropaneshave been reported"' and have provided unequivocal evidence for the participation of an aromatic double bond in the Cope rearrangement. For 4-phenylbut-l-ene AH' is estimated as 125.52 kJ mol-' and for replacement of a double bond by a phenyl ring in the Cope rearrangement 4HA-41.8 kJ mol-'.By using the semiempirical MIND0/2 method the potential surface for the thermal reaction of methylenecyclobutane has been e~aluated.''~ Electronic factors are shown to be the least likely cause of the observed stereospecificity and strong support is provided for Gajewski's views"' that the ring opening pathway is similar to that of other cyclobutane derivatives and that a flat energy profile exists in the region of the transition state. An investigation has been begun'" of the largely unexplored area of the Chem- istry of the bis- and tris-homocycloheptatrienes. In particular a study of 7,7-diduterio-anti-1,5-bishomocycloheptatriene has revealed that the thermal reor- ganization of this system occurs via a two-fold 1,5-homodienyl rearrangement the second stage of which involves prior conformational ring inversion and geminal hydrogen interchange.Results have been reported112 of the first homogeneous gas phase thermolysis study of the 1,2,4-trideuterio derivative of bicyclo[2,2,0]hexa- 2,5-diene (Dewar benzene). These reveal that there is no single step pericyclic alternative to disrotatory cyclodissociation and that potential multistep mechanisms are limited to those that would require rate limiting formation of either trans cis cis-cyclohexa- 1,3,5-triene or trans-Dewar benzene. Gas phase pyrolysis of (+)-[3-2H]bicyclo[3,2,1]oct-2-en-7-one in a degenerate rearrangement to (-)-[ 1-2H]bicyclo[3,2,1]oct-2-en-7-one as observed by 'H-and 13C-n.m.r.spectroscopy and by polarimetry. It is however presently unclear whether the rearrangement is a concerted [1,3] sigmatropic acyl shift or a stepwise reaction passing over an allylic-acyl biradical intermediate derived by cleavage of the C1-C7 bond. An investigation of the automerization of (-)-[3-2H,9-'3C]-6-methylene-bicyclo[3,2,l]oct-2-ene has been shown114 to proceed via a non-random mixed A. PadwaandT. J. Blacklock J. Amer. Chem. SOC.,1978,100 1321. lo' J. M. Brown B. T. Golding and J. J. Stofko J.C.S. Perkin I. 1978,436. lo* E. N. Marvel1 and C. Lin J. Amer. Chem. Soc. 1978,100 877. lo9 W. W. Schoeller J. Amer. Chem. SOC.,1978,99 5919. 'lo J. J. Gajewski J. Amer. Chem. SOC.,1976 98 5254. 'I1 M. R. Detty and L. A. Paquette J.C.S.Chem. Comm. 1978 365. M. J. Goldstein and R. S. Leight J. Amer. Chem. SOC.,1977 99 8113. '13 J. M. Janusz L. J. Gardiner and J. J. Berson,J. Amer. Chem. SOC.,1977,99 8509. J. M. Janusz and J. A. Berson J. Amer. Chem. SOC.,1978 100 2237. Alicyclic Chemistry 301 mechanism consisting of (37)*(38) and (37)dequal amounts of (39) and (40) of which the major component is the Cope rearrangement. Detailed interpretation of the minor component depends on the relative values of the two [1,3]sigmatropic rate constants k and k7. . 2 .D (37) 11 11 .= (38) 13C (39) Activation parameters of spironorcaradiene-spirocycloheptatrieneequilibria (Scheme 19) have been determined1I5 by line shape analysis of 'H and I3C n.m.r. R' = R2= C1 or R' R2= benzo R3=HorCF3 Scheme 19 spectra.The results reveal that whereas the values obtained for the dibenzo- spironorcaradienes are comparable with the data of simple norcaradiene derivatives the tetrachloroderivatives show considerable differences. M. Kausch and H.Diirr Chem. Ber. 1978 111 1.