12 Alicyclic Chernistry ByA. COX Department of Molecular Sciences Universityof Warwick Coventry8 CV4 7AL 1Introduction This year has seen the publication of a further volume of ‘Organic Syntheses’’ and also the Cumulative Indices2 to Collective Volumes 1-5. A survey of the 1974 literature on mechanisms has appeared3 as has another volume4 of ‘Organic Reac- tions’ together with several supplements to R~dd.~ Reviews have been published on the chemistry of rings of various sizes,6 bridgehead carbonium ions,7 bridgehead olefins’ and the chemistry of the prostaglandins.’ Finally a set of rules have been announced” which predict the relative ease of ring formation. 2 Synthesis Three- and Four-membered Rings.-A number of syntheses of cyclopropanes by routes involving carbenes have appeared.In particular it has been found” that reaction of olefins with organic gem-dihalides and copper proceeds smoothly at moderate temperature to give cyclopropanes in good yields. The cis-isomer is obtained predominantly from terminal olefins such as hex-1-ene. gem-Difluorocyclopropanes are formed’* in high yields from chlorodifluoromethane alkanolate ions and reactive alkenes provided that the concentration of alkanolate is small. Alkanolates of the necessary small concentration are found in a solution of halide ions in epoxides such as oxiran and (chloromethy1)oxiran. The first successful cyclopropanations using zinc chloride-catalysed decompositions of diphenyl-diazomethane have been rep~rted.’~ Conjugated dienes and electron-rich olefins 1 ‘Organic Syntheses’ ed.S. Masamune Wiley New York 1976 Vol. 55. 2 ‘Organic Syntheses Collective Volumes 1-5 Cumulative Indices’ Wiley New York 1976. 3 ‘Organic Reaction Mechanisms’ ed. A. R. Butler and M. J. Perkins Interscience London 1974. 4 ‘Organic Reactions’ Wiley New York 1976 Vol. 23. 5 Rodd’s Chemistry of Carbon Compounds 2 Supplements A B C D and E Elsevier Amsterdam 1974. 6 (a)S. A. Math. in ‘Alicyclic Chemistry’ ed. W. Parker (Specialist Periodical Reports) The Chemical Society London 1976 Vol. 4 p. 1; (6)D. G. Morris ibid. p. 196; (c) E. J. Thomas ibid. p. 278. 7 J. Carnduff in ‘Alicyclic Chemistry’ ed. W. Parker (Specialist Periodical Reports) The Chemical Society London 1976 Vol. 4 p. 345. 8 R. Keese Angew.Chem.1975,87 568. 9 G.Pattenden in ‘Aliphatic Chemistry’ ed. A. McKillop (Specialist Periodical Reports) The Chemical Society London 1976 Vol. 4 p. 243. 10 J. E. Baldwin J.C.S. Chem. Comm. 1976 734 736 738. 11 N. Kawabata M. Naka and S. Yamashita J. Amer. Chem. SOC.,1976,98 2676. 12 M. Kamel W. Kimpenhaus and J. Buddrus Chem. Ber. 1976,109,2351. 13 D. S. Crumrine T. J. Haberkamp and D. J. Suther J. Org. Chem. 1975,40,2274. 279 280 A. Cox were found to be good substrates and the relatively high yields ease of isolation and ease of scaling-up compared with the carbene route make this the preferred method for synthesizing 6,6-diarylbicyclo[ 3,l ,O]hex-2-enes. Mechanistic studies suggest that the results are best explained by a diphenylcarbenoid intermediate which reacts competitively with the olefin to produce the cyclopropane product.A series of papers on various aspects of metal salt-catalysed synthesis of carbenoids has appeared.I4 The synthesis and some chemistry of perfluorohexamethylbicyclo-propenyl the final member of the first complete set of benzene valence isomers has been described." The synthetic strategy (Scheme 1)entailed coupling of a suitably F3cbCF3 I ,Iv Reagents i OC1- C1- DMSO H20; ii F~CCEZCCF,,A; iii NaI MeCN; iv hv Hg. Scheme 1 functionalized perfluoro- 1,2,3-trimethylcyclopropene, prepared in turn via addition of a carbene to perfluorobut-2-yne. The thermal stability of perfluorohexamethyl- bicyclopropenyl is remarkable in that it aromatizes cleanly with 2h at 360 "C.Hence the thermodynamically least-stable member of the valence isomer set is by a wide margin the most stable kinetically. The Favorskii-type reaction of 2-dialkylamino-3-halogenocycloalkenes,which provides an elegant entry into bicy- clopropane chemistry has now been extended16 to the synthesis of simpler systems. For example when enamine (l),AgBF4 and dimethylamine react in a 1 1 2 molar ratio in ether the bisamine (2) is formed almost quantitatively. Me AgBFq c MezNH CH2Cl2 ' Me2NGz2 Me 20% Me (1) (2) The first light-induced cyclization of allylmagnesium halides to cyclopropylmag-nesium halides has been rep~rted.'~ This transformation which proceeds in good 14 (a)D. S. Wulfman Tetrahedron,1976,32 1231; (6)D. S. Wulfman R.S. McDaniel and B. W. Peace ibid.,p. 1241; (c) D. S. Wulfman B. W. Peace and R. S. McDaniel ibid. p. 1251; (d)D.S. Wulfman B. G. McGibboney E. K. Steffen N. V. Thinh R. S. McDaniel and B. W. Peace ibid.,p. 1257. 15 M. W. Grayston and D. M. Lemal J. Amer. Chem. SOC.,1976,98,1278. 16 E. Jongejan H. Steinberg and Th. J. de Boer Tetrahedron Leffers 1976 397. 17 S. Cohen and A. Yogev J. Amer. Chem. SOC.,1976,98,2013. Alicyclic Chemistry 28 1 yield is of interest because since allylmagnesium halides are known to exist in solution as aggregates of tight anion-cation pairs the reaction formally amounts to a light-induced interconversion of ally1 carbanions into cyclopropanyl carbanions. Although the degree of selectivity could not be established theoretical considera- tions suggest a disrotatory course for the transformation.A number of new substituted cyclopropanes have been prepared'* by trapping various metallated alkyl-substituted cyclopropanes. Although a recent p~blication'~ indicates that iodocyclopropenes are expected to be very unstable this year has brought2' the syntheses of l-chloro-2-iodo-3,3-difluorocyclopropeneand of 1,2-di-iodo-3,3-difluorocyclopropene. The interest in these compounds stems from their potential use in copper coupling reactions. These studies are still in progress. A method of cyclization of dienes based on the silver-ion-induced oxidation of organoboranes has been reported (Scheme 2).21 For example 2,3-dimethylbuta- R3B [R-Ag] -B R' % R-R Scheme2 1,3-diene is cyclized to trans-dimethylcyclobutane in 79% yield on reaction of the diene with diborane followed by alkaline AgNO,.The transformation has been shown to be applicable up to eight-membered rings. A typical procedure is given. Reactions of anhydrous hydrogen chloride with mixtures of propyne and but- 1-yne and of propyne and pent-1-yne produce22 the corresponding 1,3-dialkyl- 1,3- dichlorocyclobutane cross-cyclization products together with the addition and cy- clodimerization products of the individual acetylenes. The amounts of the total (i.e. homo-and cross-)cyclization products range from 9 to 35%. 1,l-Dipyrrolidinocyclopropanes are known to react with ketones to form addition products. It has now been that these aminals undergo ready alkylation with nitromethane and the reduction products of this process yield primary amines which may serve as substrates for the Tiff eneau-Demjanov ring enlargement forming fused-ring cyclobutanones (Scheme 3).A new and general method for the synthesis a,n=l b,n=O a n =l;X= N3 b,n=O;X=N Scheme3 18 D. Bauer and G. Kobrich Chem. Ber. 1976,109,2185. l9 D. C. F. Law S. W. Tobey and R. West J. Org. Chem. 1973,38,768. 20 J. Sepiol and R. L. Soulen J. Org. Chem. 1975,40 3791. 21 R. Murphy and R. H. Prager Tetrahedron Letters 1976,463. 22 K. Griesbaum and W. Seiter J. Org. Chem. 1976 41 937. 23 H. H. Wasserman M. J. Hearn B. Haveaux and M. Thyes J. Org. Chem. 1976,41 153. 282 A. Cox of 1-substituted cyclobutenes has been described24 and its utility illustrated by synthesis of a natural product.The method (Scheme 4) involves metallation of RLi +El fE + TMEDA Scheme 4 methylenecyclobutane followed by reaction with electrophiles of the ambident anion sogenerated. Suitable variation of the reaction parameters make it possible to adjust the regiomer ratio from 0 100 to 61 :39 in favour of the desired product. A high yield synthetic procedure (Scheme 5) has now been announcedz5 leading to ben- a; R1=OMe; R2=H b; R' =R2= OMe c; R' R2 = OCHzO Scheme 5 zocyclobutene itself and also to the same system bearing oxygen substituents on the aryl ring. The experimental procedure involves sublimation of isochromanones in a stream of nitrogen at 3 Torr over a nichrome wire at 500°C and collection of the pyrolysate in a cold trap at -78°C.The intramolecular reductive coupling of 1,4-dicarbonyl compounds using TiC1,-LiAlH forms the basis of a one-step prep- aration of 1,2-diphenylcyclob~tene.~~ This transformation which proceeds in -50% yield can also be adapted to the synthesis of 1,2-diphenylcyclohexene from a 1,6-dicarbonyl compound. A synthesis of the short-lived tetrafluorocyclobutadiene has been Irradiation of tetrafluorocyclobutene-3,4-dicarboxylic anhy-dride at 253.7 nm in the presence of -100 m mol of furan yielded in addition to octafluorocyclo-octatetraene a compound having the composition of a 1:1adduct of furan with tetrafluorocyclobutadiene. These observations are summarized in Scheme 6. Five- Six- Seven- and Eight-membered Rings.-A method of cyclopentanone formation using the recently developed reagent 1-1ithiocyclopropyl phenyl sulphide has been reported.'* The overall transformation which proceeds with high stereoselectivity is illustrated in Scheme 7.Closely related is the cyclopentane annelation represented in Scheme 8 and brought about under the agency of lithium phenylthio(~yclopropy1)cuprate.~~ 24 S. R. Wilson and L. R. Phillips Tetrahedron Letters 1975 3047. 25 R. J. Spangler and B. G. Beckmann Tetrahedron Letters 1976 2517. z6 A. L. Baumstark E. J. H. Bechara and M. J. Semigran Tetrahedron Letters 1976 3265. M. J. Gerace D. M. Lemal and H. Erti J. Amer. Chem. SOC.,1975,97,5584. 28 B. M. Trost and D. E. Keeley J. Amer. Chem. Soc. 1976,98 248. 29 E. Piers C.K. Lau,and I. Nagakura Tetrahedron Letters 1976 3233. 2' Alicyclic Chemistry ‘0 1 “qa* co’‘irionrJ 1 F8 FF FF Scbeme 6 Reagent i PhS Scbeme7 Reagent i LiCu+PhS q)2 Scbemell The stoicheiometric reaction of dialkyl- or diaryl-acetylenes with tetracarbonyl- nickel in the presence of HCI aff ords3’ tetrasubstituted cyclopentenones in yields ranging from 25-70%. A dependence of cyclopentenone formation on acidity has been found and a mechanism suggested to account for this. The thiazolium ion- catalysed dimerization of aldehydes to acyloins has been used3’ to cyclize pentane- dials and hexanediols in the presence of triethylamine using acetonitrile as solvent. Oxidation gives 2-hydroxy-2-enones several of which are important flavouring materials.30 W. Best B.Fell and G. Schmitt Chem. Ber. 1976 109 2914. 3l R. C. Cookson and R. M. Lane J.C.S. Chem. Comm. 1976,804. 284 A. Cox A preliminary report3* shows that the reaction of P-iodo-cY,&unsaturated ketones with lithium phenylthio(2-vinylcyclopropyl)cuprate followed by thermal rearrange- ment of the resulting ~-(2-vinylcyclopropyl)enones,provides a short efficient synthesis of cycloheptatriene systems. This conversion (Scheme 9) constitutes a \ A I Scheme 9 seven-membered ring annelation. The synthesis and properties of 2-hydroxycyclo-octa-2,4,6-trienone(3) and 2-hydroxycyclo-octa-2,4,7-trienone(4) have been It is concluded that (3)seems to have some 7r-interaction and hence may be called a 1,7-7r-homotropolone and similarly that (4)has some homoconjugation and likewise may be called a 5,7-7r-homotropolone.8x5 \/ (3) (4) Medium and Large Rings.-A new method has been for the preparation of medium-ring cycloalkynes. These compounds are strained and therefore excep- tionally reactive. Consequently mild conditions are demanded €or their synthesis. The reaction sequence is as outlined in Scheme 10. Reaction of the bicyclic system c"3 Br gg* OMe Me0 Scheme 10 (5) for 2 h with a slight excess of silver tosylate in refluxing acetonitrile has been found36 to be a highly stereoselective method of generating tosylated medium-sized rings in excellent yields. The interesting hydrocarbon cyclododeca- 1,5,9-triyne (6) has been ~ynthesized.~~ Although aspects of the electronic spectrum of (6)point to 32 E.Piers and 1. Nagakura Tetrahedron Leners 1976,3237. 33 Y. Kitahara M. Oda and S. Miyakoshi Tetrahedron Letters 1976,4141. 34 Y. Kitahara M. Oda S. Miyakoshi and S. Nakanishi Tetrahedron Letters 1976,4145. 35 C. B. Reese and A. Shaw J.C.S. Perkin I 1976,890. 36 H. J. J. Loozen W. M. M. Robben T. L. Richter and H. M. Buck J. Org. Chem. 1976.41.384. 37 A. J. Barkovich and K. P. C. Vollhardt J. Amer. Chem. SOC.,1976,98,2667. Alicyclic Chemistry 285 (5) n = 1,2,or3 effective interaction of the three acetylene groups proof of tlle triacetylene structure is provided by the i.r. spectrum and l3C-H coupling constant which appears normal for propargylic moieties. This rules out the possibility of structures such as (7a) and (7b).The 24-w-electron system 3,11,16,24-tetra-t-butyl-1,12-24-trisdehydro-0 Bra _.__I, 1.3Brz.CC14 2.3NaOEt. EtOH \ NaOMe-MeOH H20 I Br ve(/j$z (yj A 7 (7b) (74 (6) [24]annulene has been ~ynthesized.~~ Its n.m.r. spectrum clearly indicates it to be strongly paratropic and its essentially temperature-independent nature demon- strates it to have high conformational stability. A group of papers on the synthesis and properties of dehydroannulenes has also appea~ed.~’ Amongst the conclusions is that the tetramethyltetrakisdehydro[18lannulene (8) sustains a strong diamagnetic ring current and has high conformational stability. Me-Melil II Polycyclic Systems.-A communication has appeared4’ in which the first successful synthesis of a compound 1,2-diphenyltricyclo[2,2,0,02~5]hexan-2-ol (1l),containing S.Nakatsuji S. Akiyama and M. Nakagawa Tetrahedron Letters 1976 2623. 39 (a)T. Nomoto K. Fukui and M. Nakagawa Bull. Chem.Soc.Japan 1976,49,305; (b)T.Katakami K. Fukui T. Okamoto and M. Nakagawa ibid.,p. 297; (c)S. Tomita and M. Nakagawa,ibid.,p. 302; (d)J. Ojima T. Katakami G. Nakaminami and M. Nakagawa ibid. p. 292. 40 E. C. Alexander and J. Uliana J. Amer. Chem. SOC.,1976,98,4324. 286 A.Cox the hitherto unknown tricycl0[2,2,O,O~*~]hexane ring system is announced. The synthesis which proceeds in five stages from 2-phenylbicyclo[ 1,l,l]pentan-2-01 contains the interesting mandelate ester photodecarboxylation (9) -+ (10).The (9) Ph (1 1) syntheses of a number of monosubstituted and 1,4-disubstituted cubanes have been de~cribed,~' together with the measurement and analysis of their 100 MHz 'H n.m.r. spectra. Typical coupling constants observed are quoted and a simple additivity rule is described whereby chemical shifts in CDC13 can be predicted. The derived correlations allow quick and effective identification of cubane derivatives from their 'Hn.m.r. spectra and also aid in the interpretations of the more complex spectra of less symmetrical cage molecules. A second paper4* by the same authors reports that aromatic solvent-induced shifts have been measured for a number of substituted cubanes in benzene and pyridine and an additivity rule again derived which allows accurate prediction of these shifts.These arise from independent transient 1:1 associations of solvent molecules with the electron-deficient sites of all local dipoles in the solute. On exposure to n-butyl-lithium 1,2-di-iodoadamantane yields43 a highly reactive intermediate which can be intercepted in a Diels-Alder reaction with butadiene at -78 "Cand which in the absence of butadiene dimerizes spontaneously in a [2 + 21 fashion. This intermediate is believed to be adamantene the tricyclic analogue of trans-cyclohexene. As such it represents a much more extreme case of a highly inflexible system in which the opportunity for distributing the large angle strain over several bonds is at a minimum. Of the theoretically possible 2486 tetracycloun- decanes empirical force field calculations that 2,4-ethanonorademantane (13) and 2,8-ethanonoradamantane (14) should be the most stable and should have comparable stability.It has been found that AIBr rearrangement of tetracyclo[6,3,0,02*6,05~g]undecane (12) gives (13)and (14) in the ratio 97 :3,thus identifying (13)as the thermodynamically more stable isomer. A description of the synthesis of 3-oxobicyclo[3,3,l]nonan-6-ylmethyl methanesulphonate and its cycli- 41 J. T. Edward P. G. Farrell and G. E. Langford J. Amer. Chem. SOC.,1976,98,3075. 42 J. T.Edward P. G. Farrell. and G. E. Langford J. Amer. Chem. SOC.,1976,98,3085. 43 W.Burns D. Grant M. A. McKervey and G. Step J.C.S. Perkin Z,1976 234. 44 S.A.Godleski P. von R. Schleyer and E.Osawa J.C.S. Chem. Comm. 1976 38. Alicyclic Chemistry zation to twistan-4-one has been rep~rted.~’ This synthesis illustrates the remaining single -single bond process for constructing the twistane skeleton and along with the detailed analysis of the structure of twistane exemplifies the analytical approach to synthesis as expounded by Corey. Two papers have a~peared,~~.~’ which make contributions to the dodecahedrane skeleton as a synthetic goal. The first reports the synthesis of chiral triquinacene-2-carboxylic acid and (+)-2,3-dihydrotriquinacen-2-one (15) together with the first configurational assignments to members of the triquinacene family for which configurational assignments are available. The second paper reports conversion of hexahydrotriquinacen-2-one into the pentaseco- dodecahedrane (16)for which system the name ‘meso-bivalvane’ has been suggested.3 Stereochemistry A theoretical analysis has been of the effects of geometry optimization upon computed barriers to rotation and pyramidal inversion from which it appears that if AE is the true energy difference between the energy minimum and the transition state for a particular conformational process then (barrier using the optimized geometry of the lower state) >AE> (barrier using the optimized geometry of the higher state). The effect of carbonyl substitution on ring planarity and ring-puckering vibration has been analy~ed~~ for cyclobutane in terms of a one-dimensional potential model incorporating a ‘torsional strain’ parameter in addition to the contributions of ring angle deformations and unstrained torsions about the ring bonds.The potential constants determined indicate that the increased planarity is due mainly to reduction in torsional repulsion. An investigation has been undertaken” of the effect of the t-butyl group on the cyclopentane ring and values of Av(Av = v(OH)free-Y(OH)bnded) measured for a series of 1,2-disubstituted-4-t-butylcyclopentanes of structural type (17). Comparison of the Av values reveals a pronounced effect of the t-butyl group on the dihedral angle of the substituents in 45 D. P. G. Hamon and R. N. Young Austral. J. Chem. 1976 29 145. 46 L. A.Paquette W. B. Farnham and S. V. Ley J. Amer. Chem. SOC.,1976,97,7273. 47 L.A.Paquette I.Itoh and W. B. Farnham J. Amer. Chem. SOC.,1976,97 7280. 48 J. P. Colpa H. B. Schlegel and S. Wolfe Canad. J. Chem. 1976,54 526. 49 A.L.Meinzer and W. C. Pringle J. Phys. Chem. 1976,80 1178. 50 G.Bernath and L. Gera Tetrahedron Letters 1976 1615. 288 A. Cox I Bu‘ (17) positions 1and 2. The results of an investigation of the geometry of four conforma- tions of cyclohexane chair (D3d),boat (&) half-chair (Q and planar (06) by means of the FSGO model are reported.’l There are four coplanar carbon atoms in the half-chair conformation and this gives rise to considerable eclipsing of the hydrogens of neighbouring methylene groups. The FSGO model calculations indicate that some of these eclipsing interactions are relieved by a slight ‘rocking’ of these methylene groups.However although the relative energies of the various conformations are given their expected ordering the energy differences between the chair and the other conformations are larger than those previously reported. A study has been presenteds2 determining the steric role of 7r-electrons on conformational equilibria. Whereas an em-methylene group alters the polar character of the molecule only slightly it is found to have an enormous influence on the conforma- tional preference of substituents at the 3-position. Moreover the results show the ‘steric effect’ of the 7r-electrons to be either attractive or repulsive in comparison to a cyclohexane axial-axial interaction depending on the nature of the interacting substituent and of the solvent.By measuring53 the low-temperature pulse Fourier transform 13C n.m.r. spectrum of rnethylcyclohexane enriched with 13C in the methyl substituent the ratio of the major to the minor conformation has been shown to be 164 :1. This corresponds to a conformational free energy difference at 172 K of 7.30k0.25 kJ mol-’. Work has also been reporteds4 in support of the view that the standard kinetic method of conformational analysis using 4-t-butyl derivatives is unreliable when atom-1 is directly involved in the reaction. These results derive from measurements of the combined rate constants for ethoxy exchange in 1,l-disubstituted cyclohexane ethyl carboxylates. Proton-decoupled 31P n.m.r. spectra have been for cyclohexyl cis-and truns-4-methylcyclohexyl and cis-and trans-4-t-butylcyclohexyl derivatives of a number of different phosphorus functions and the 31P shifts for the conformationally rigid t-butyl derivatives found to reveal a strong dependence on the steric disposition of the group.By the principle of additivity of group conformational free energies the first A values for phosphorus functions were obtained and these were found to be larger than for comparable nitrogen and sulphur functions. ‘H n.m.r. studies of dimethylcycloheptanones at low temperatures together with conformational calculations using the PCILO approach indicates6 that the most stable conformations are twist chairs with the carbonyl group 51 T. D. Davis and A. A. Frost J. Amer. Chem. SOC., 1975,97,7410. 52 J.B. Lambert and R. R. Clikeman J. Amer. Chem. SOC.,1976,98,4203. 53 H. Booth and J. R. Everett J.C.S. Chem. Comm. 1976 278. 54 D. R. Brown P. G. Leviston J. McKenna J. M. McKenna R. A. Melia J. C. Pratt and B. G. Hutley J.C.S. Perkin ZZ 1976 838. 55 M. D. Gordon and L. D. Quin J. Amer. Chem. SOC.,1976,98 15. 56 M. St.-Jacques C. Vaziri D. A. Frenette A. Goursot and S. Fliszar J. Amer. Chem. SOC.,1976,98 5759. Alicyclic Chemistry located at position 2 i.e. (18) (19) and (20) in accord with a greater competitive conformational preference for the carbonyl group relative to the gem-dimethyl group. Cyclo-octa-1,3-diene and the 1,4- and 1,5-isomers have been studied" by (18) (19) (20) the force field method. The former is calculated to be a mixture of two conforma- tions of which one (21) has a C axis and one (22) is irregular (Cl).For the 1,4-isomer there are two conformations a boat-boat (23) and a boat-chair (24) both (21) (22) (23) (24) of which have C symmetry. The 1,4-isomer is substantially all in the boat-chair conformation whereas the 1,5-isomer is known to be preferentially a twist-boat (C,) conformation. The calculations indicate the 1,3-isomer to be much more stable followed in turn by the 1,4- and the 1,5-isomer. Another force-field study of &,cis- cyclo-octa-1,5-diene has also been the results of which although corn- patible with work of Anet,59 is less so with that reported by Alli~~ger.~' Three potential energy minima and four transition states have been found to be relevant for a description of the conformational properties of cyclo-octa- 1,5-diene and the seven calculated conformations are essentially characterized by different distributions of angle and torsion strain.It is concluded that the most favourable calculated conformation of cis,cis-cyclo-octa- 1,5-diene is a twist-boat structure of symmetry C2(C-CH2-CH,-C torsion angles of 52.5") in agreement with Anet's experi- mental evidence. "C-n.m.r. has been used6' to show that cyclonona- 1,2,6-triene exists in an unsymmetrical twist-boat-chair conformation (25) that can undergo a (25) hindered pseudorotation (barrier height 53 kJ mol-') to achieve a C time-averaged symmetry. A conformational analysis of 3,4-homotropilidene has been carried out6* by means of 'H n.m.r.spectroscopy and if compared with cyclohepta-1,3,5-triene having the three-membered ring in the ex0 position (trans-conformation) the s7 N. L. Allinger J. F. Viskocil U. Burkert and Y. Yuh Tetrahedron,1976,32 33. s8 0.Ermer J. Amer. Chem. SOC., 1976,98 3964. 59 F. A. L. Anet and L. Kozerski J. Amer. Chem. SOC.,1973,95 3407. 60 F. A. L. Anet and I. Yavari J.C.S. Chem. Comm. 1975 927. 61 H. Giinther and J. Ulmen Chem. Ber. 1975,108 3132. 290 A. Cox molecule has been shown to exist in a flattened boat conformation. Polycyclic annulenes are among the molecules which show .rr-systems highly deformed from planarity. A simplified force-field has been discussed62 for such compounds and the ratios of the twist and out-of-plane bending force constants involved are derived from structural data.4 Structural Properties and Orbital Interactions Theoretical studies have been of the planar (26) and twisted (27) forms of vinylmethylene together with the corresponding calculation on cyclopropene. The calculated parameters are of sufficient accuracy to be compared directly with the experimental results. The calculations show that it is the 1,3-biradical singlet state ('A ") rather than the methylene-like singlet ('A ') which correlates well with cyclo- propene and the suggestion is made that (lA") (26) is the state most important in the H H H (26a) (26b) (27) isomerization. Ab inifioMO theory has been to study the relative energies of various isomers of C3H3+. The cyclopropenium ion is found to be the most stable C3H3+ isomer with a resonance energy >247 kJ mol-'.The propargyl cation is the next most stable form with an energy 140 kJ mol-' above the cyclopropenium cation. Results of similar calculations have also been reported for two acyclic C3H' systems. Single determinant ab initio MO theory has also been applied6' to the study of the structures and relative energies of all possible singlet and triplet isomers of formula C3H2. Within the manifold of closed shell singlets cyclopropenylidene is found to be the most stable being some 70 kJ mol-' below vinylidenecarbene. Comparison is drawn between the relative energies of the most stable of each of the singlet and triplet manifolds and it has been estimated that the lowest energy singlet isomer cyclopropylidene is ca.54kJ mol-' more stable than the best triplet form propar- gylene. A series of cyclobutenyl cations has been prepared66 and studied by 13C and 'H n.m.r. spectroscopy. The data for the 1,3-diphenyl-2,4-diR-cyclobutenyl cations (28) are characteristic of alkenyl (allyl) cations with undetectably small contribu- tions from 1,3~overlap. Exclusive methyl substitution as in the 1,3-dimethylcyclobutenyl cation (29) and the 1,2,3,4-tetramethylcyclobutenylcation (30) lead to decreased importance of allylic delocalization and accordingly greater 1,3~-overlapand homoaromatic charge delocalization. The series culminates in the parent cyclobutenyl cation which exhibits truly aromatic delocalization. However results6' of an ab inifiocalculation do not appear to be in entire agreement with these experimental conclusions for it is claimed that the calculated geometry of the H.B. Burgi and E. Shefter Tetrahedron 1975 31,2976. 63 J. H. Davis W. A. Goddard and R. G. Bergman J. Amer. Chem. SOC.,1976,98,4015. 64 L. Radom P. C. Hariharan J. A. Pople and P. von R. Schleyer J. Amer. Chem. SOC.,1976,98 10. 65 W. J. Hehre J. A. Pople W. A. Lathan L. Radom E. Wasserman and Z. R. Wasserman J. Amer. Chem. SOC.,1976,98,4378. 66 G.A. Olah J. S. Staral R. J. Spear and G. Liang J. Amer. Chem. SOC.,1975 97 5489. 67 W. J. Hehre and A. J. P. Devaquet J. Amer. Chem. SOC.,1976,98,4370. Alicyclic Chemistry 291 R)-fR Ph Me Me Me Ph H Me (28) R = Ph Me or But (29) (30) R =H D or CI homocyclopropenyl cation is closer to that expected of an open ‘cyclobutenyl’ ;.ucture than a tightly bridged bicyclic form.The lowest energy form on the C4H5+ potential surface is a methyl derivative of the 27r electron aromatic cyclopropenyl Y cation. A series of remarkably stable 1-methoxy-2-R-3,4,4-trifluorocyclobutenyl cations (R = F Cl or OMe) has been prepared and studied68 by ‘H and 19F n.m.r. in sdution each cation is found to exist as an equilibrating pair of isomers which differ only by the 1-methoxy-group conformation. The n.m.r. data establish the impor- tance of 1,3~-overlap and also from a complete lineshape analysis a series of AG’ values was obtained. The generalized cyclobutene anion radical/butadiene anion radical electrocyclic reaction has been explored69 experimentally and theoretically and its preferred stereochemical course found to be conrotation.Among the theoretical approaches evaluated the HOMO method alone was found to be un-satisfactory. A number of papers have appeared once again on the structure of cyclobutadiene. Ab initio MO theory finds7’ singlet cyclobutadiene to be a rectangular molecule in contrast with the square geometry assumed by the triplet species. Neither state of cyclobutadiene is as stable as another cyclic form on the C4H4 potential surface namely methylenecyclopropene but both are lower in energy than yet another possibility tetrahedrane. Another paper71 on the geometry of cyclobutadiene concludes that when the important effects of electron repulsion in open-shell systems are considered a square or effectively square singlet can be the ground state of cyclobutadiene.Repulsion between the two electrons in the non- bonding MOs of cyclobutadiene is considered to be minimized in a square geometry resulting in a flat curve for rectangular distortion in the lowest singlet state. Experimental evidence however seems to favour the conclusions reached in ref. 71 for it is now that cyclobutadiene has a strong propensity to form a charge-transfer complex whenever a species possessing acceptor properties is availa- ble. Consequently it is probable that CO generated in the photolysis of (31) or (32) interacts weakly with cyclobutadiene and causes a weak splitting of the band associated with the degenerate bending vibration.0 0 B. E. Smart and G. S. Reddy J. Amer. Chem. SOC.,1976,98,5593. 69 N. L. Bauld J. Cessac C.4. Chang F. R. Farr and R. Holloway J. Amer. Chem. Soc. 1976,98,4561. 7O W. J. Hehre and J. A. Pople J. Amer. Chem. SOC.,1975,97,6941. 71 W. T. Borden J. Amer. Chem. SOC.,1975,97 5968. 72 S. Masamune Y. Sugihara K. Morio and J. E. Bertie Canad. J. Chem. 1976 54 2679. 73 G. Maier H.-G. Hartan and T. Sayrac Angew. Chem. Internat. Edn. 1976,15 226. 292 A.Cox The results of a kinetic study of the reaction of substituted alkenylidenecyclo- propanes with 4-phenyl- 1,2,4-triazoline-3,5-dione (PTAD) have been and discussed in terms of the ionization potentials of the HOMOSand the results of CNDO calculations and frontier MO theory. The detailed analysis of the cycloaddi- tion leads to the conclusion that it is concerted and occurs uia a very early transition state which is orbitally controlled (Scheme 11).A second paper under the same R3 Scheme 11 authorship reports7' details of an investigation using He(1) photoelectron spectro- scopy and CNDO calculations of the electronic structure and bonding in alkenylidenecyclopropanes.The nature and magnitude of the interactions between the Walsh orbitals of the three-membered ring and the two 7r-bonds of the allene chromophore are analysed and the forms of the five highest energy MOs are outlined. In addition to the interaction of the cr-type in-plane Walsh orbital with the terminal allene double bond the analysis indicates that one of the lower-lying degenerate pz 7r-type orbitals interacts significantly with the exocyclic double bond.Tworeviews have in the field of non-classical carbonium ions both of which are mainly concerned with the 2-norbornyl cation. In a more general arti~le,~' Hogeveen discusses pyramidal mono- and di-cations. Structural data for the geometries of the l-triquinacenyl cation (33) and the 10-triquinacenyl cation (34) and for the corresponding radicals have been obtained7' by a MIND0/3 calculation. This reveals that the cation (33) is ca. 95 kJ mol-' more stable than (34) and the upper limit of the activation energy for the transformation of + (33) (34) (34) into (33) is calculated to be 37 kJ mol-'. The three chlorides (35)-(37) have been prepared" and allowed to react with antimony pentafluoride in S0,FCl at -78 "C and the 'H and 13Cn.m.r.spectra of the reaction mixture measured. Only the monocations were found to be present in each case their kinetic stability 74 D. J. Pasto J. K. Borchardt T. P. Fehlner H. F. Baney and M. E. Schwartz,J. Amer. Chem. SOC.,1976 98 526. 75 D. J. Pasto T. P. Fehlner M. E. Schwartz and H. F. Baney J. Amer. Chem. Soc. 1976,98 530. 76 G. A. Olah Accounts Chem. Res. 1976,9 41. 77 H. C. Brown Tetrahedron 1976 32 179. H. Hogeveen and P. W. Kwant Accounts Chem. Res. 1975,8,413. 79 P. Bischof Angew. Chem. Internat. Edn. 1976,15,556. Bo D. Bosse and A. de Meijere Angew. Chem. Internat. Edn. 1976 15 557. 293 Alicyclic Chemistry c1 c1 @z$ (35) (36) (37) increasing with chlorine substitution.The ‘H and I3C n.m.r. spectroscopic data for the free carbenium ions show that almost planar divinylcarbenium ion units are present in these species. The He(1) photoelectron spectra of a series of nine cycloalkenes have been reported.8’ Along with the calculated coefficients for the frontier orbitals of these species measurement or estimation of experimental vertical IPSor EAs has made possible determination of reactivity indices. The behaviour of cycloalkenes toward ‘one-bond’ and ‘two-bond’ nucleophiles is quite different so that a theoretical criterion for concertedness in cycloaddition is established. In another papers2 on cycloadditions it is shown that orbital interaction between antisymmetric MOs can furnish cyclic additions with concerted character.The density analysis locates a clear-cut distinction of the consequence of the interaction between two antisymmetric MOs from that of the interaction between two symmet- ric MOs in cyclic additions. According to the analysis concerted formation of two bonds can be expected only if the dominant orbital interaction is the one between two antisymmetricMOs. 5 Reactions Metal-promoted Reactions.-A review containing 132 references on the transition- metal-catalysed rearrangement of small-ring organic compounds has appearedeS3 The rhodium-catalysed addition of CO to reactive dienes and enones with the formation of five-membered rings has been For example reaction of diene (38) with [Rh(C0)2C1J2 leads to 4,5,6,7,-tetramethylindan-2-one, besides the products resulting from the rearrangement of the bicyclobutane moiety.It is probable that the mechanistic route proceeds via intermediate (39) which may [Rh(CO),C112 co + I I CI-Rh-CO + (38) (39) ‘Rh(C0)CI’ possibly exist in polymeric form. An interesting paper has appeared8’ describing the nickel(0)-catalysed reaction of a 1,8-bishomocubane. The ease with which the isomerization occurs is profoundly influenced by the catalyst system employed and in the case of certain ligands at least it appears that formation of soluble 81 K. N. Houk and L. L. Munchausen J. Amer. Chem. SOC.,1976,98,937. 82 H. Fujimoto S. Inagaki and K. Fukui J. Amer. Chem. Soc. 1976,98 2670. 83 K. C. Bishop Chem. Rev. 1976,76,461. 84 R. F.Heldeweg and H. Hogeveen J. Amer. Chem. Soc. 1976,98,6040. 85 R. Noyori M. Yamakawa and H. Takaya J. Amer. Chem. SOC.,1976,98,1471 294 A. Cbx co-ordinatively unsaturated nickel(0) complexes is crucial. The catalytic two-bond cleavage of (40) is considered to proceed by a mechanism involving the C-2-C-5 edge-on co-ordination complex and the possibility of a concerted process through C0,Me C0,Me (40) face-on metal co-ordination is ruled out. The synthesis of the bipyridylnickelocyc- lopentane derivatives (41)-(43) and their displacement reactions with ring-strained (41) (42) (43) olefins have been reported.86 These metallo-cyclopentane complexes are thought to be relevant for the cyclodimerizations shown in Scheme 12. Scheme12 A number of examples of silver-ion-promoted rearrangements of strained polycyclic systems have appeared.Dehydronoriceane (44),for example has been shown” to rearrange to 2,4-ethenonoradamantane.However on the basis of calculations presented it appears that rationalization of the observed relative reactivity of (44)and bicyclopentane (45)towards Ag’ ion has to be founded on considerations other than strain. It is suggested that an intermediate resulting from 86 M.J. Doyle J. McMeeking and P. Binger J.C.S. Chem. Comm. 1976 376. 87 T. Katsushima R. Yamaguchi and M. Kawanisi J.C.S. Chem. Comm. 1976 39. Alicyclic Chemistry (44) (45) breaking of the central bond a is stabilized by the hyperconjugative effect of the three C-C bonds connecting the bicyclopentane portion to the cyclohexane ring [see (44) bold lines].Homopentaprismane (46a) has been synthesized;88 it is not rearranged to the less strained undecanes by silver salts. Comparison of the hypothetical course of this unobserved conversion of homopentaprismane with examples of known conversions in the cubane series leads the authors to conclude that the rate-determining step in these silver-ion-catalysed rearrangements is the bond migration b-+c (Scheme 13). -2 cyclobutanes (-2 cyclobutanes) [+ 1cyclopropane] (+1cyclopropane) bb bd a X = zero bridge :cubane d cuneane a X = -CH2-:homocubane d homocuneane (norsnoutane) a X = -CH2CH2-:1,8-bishomocubane d snoutane a X = no bridge :secocubane d secocuneane (46a) (-2 cyclobutanes) (no ring loss) (46c) (+1 cyclopropane) (46b) -’(46d) Scheme 13 A review has been published8’ of olefin metathesis.Treatment of 1-methylcyclobutene with (dipheny1carbene)pentacarbonyltungsten leads” to polymer the structure of which enables the preference for alternating sequences of monomeric units to be measured and in which the stereochemistry of trisubstituted olefin metathesis can be determined. The selectivity was found to be small compared with that for other additions or for metatheses of terminal olefins and the stereochemistry was interpreted as that for 1,2-disubstituted olefins The first report has appeared’’ of the conversion of a structure possessing two non-conjugated 88 P. E. Eaton L. Cassar R. A. Hudson and D. R. Hwang J. Org. Chem.1976 4X 1445. 89 N. Calderon E. A. Ofstead and W. A. Judy Angew. Chem. Internat. Edn. 1976,15,401. 90 T. J. Katz J. McGinnis and C. Altus J. Amer. Chem. SOC.,1976 98 606. 91 P. G. Gassman and T. H. Johnson,J. Amer. Chem. SOC.,1976,98 861. 296 A. Cox olefinic groups (47)into (48) by a commonly accepted metathesis catalyst. Experi- mental evidence has been announced92 showing that some olefin metatheses involve formation of metal-carbene complexes from non-carbenoid precursors. This prem- ise is based on the selective trapping of such carbenes by Michael acceptors e.g. ethyl (CH2)8 - $(cH2)$; H' - CF3 CF3 CF3 CF3 (47) (48) acrylate. Indeed in certain cases they appear to be so efficient that a complete quenching of the metathesis can occur.The well-established olefin metathesis catalyst derived from phenyltungsten trichloride and aluminium trichloride has been found93 to be successful in promoting retrocarbene additions from certain simple cyclopropanes under mild conditions. In the light of this knowledge a cyclopropane-olefin cross metathesis reaction has been acc~mplished~~ (Scheme 14) in the presence of catalysts formed from the reaction of phenyltungsten trichloride with either aluminium trichloride or ethylaluminium dichloride. 4"' kR2 +CH2=CHR2 -+CH2=CHR'+ H H Scheme14 Thermally Induced Reactions.-A paper has appeared9' which is aimed at studying the overall reaction isomerization of cyclopropane by means of a complete potential energy surface including simultaneously all the important geometrical degrees of freedom of the molecule.Among the conclusions drawn are that the initial methylene vibrational energy for a trajectory to be reactive is much greater than generally expected and that also for most reactive trajectories a single rotation of 180"of one or both terminal CH2groups occurs within the radical species before ring closure. Berson has inve~tigated~~ the thermal stereomutation of phenyl cyclo- propanes. Analysis of the kinetics leads to results which are expected if the stereomutation pathway passes over a 0,O-trimethylene ('wcyclopropane'). The accelerating effect of a 2-dimethylamino-substituent on the thermal 1,3-sigmatropic rearrangement of vinylcyclopropane to cyclopentene has been measured and found9' to exceed even the large effect of a 2-methoxyl substituent.This rate enhancement is attributed to stabilization of a developing radical centre at C2-by the 2-dimethylamino-group. The optically active 2-E-benzylidene-4-t-butyl-4-cyano-3-phenylcyclobutanone (49) has been to rearrange thermally to the 2-2-benzylidene isomer (50) by a [1,3]-sigmatropic shift with inversion of the 92 P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC., 1976,98,6055. 93 P. G. Gassman and T. H. Johnson J. Amer. Chem. Soc. 1976 98,6057. 94 P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC., 1976,98 6058. 95 X. Chapuisat and Y. Jean J. Amer. Chem. SOC.,1975 97,6325. 96 J. A. Berson L. D. Pedersen and B. K. Carpenter J. Amer. Chem. SOC., 1976 98 122. 97 H.G. Richey and D. W. Shull Tetrahedron Letters 1976 575. 98 H. A. Bampfield P. R. Brook and K. Hunt J.C.S. Chem. Comm. 1976 146. 297 Alicyclic Chemistry R Rkc2 XdR X Y Y Y (49) X=Y=Ph X=CN;Y=Bu' migrating centre. A planar and hence achiral intermediate is excluded by the results which allow a twisted conformation as transition state or intermediate in the rearrangement. The thermal dimerization of 1,l-dimethylallene gives99 three different substituted cyclobutanes and thermal decomposition of 4,5-dimethylene-3,3,6,6-tetramethyl-3,4,5,6-tetrahydropyridazinegives the same products in the same ratios. The results are interpreted in terms of the 2,2'-bis-( 1,l-dimethylallyl) biradical being a common intermediate (Scheme 15).5-Alkylpentakis(methoxycarbonyl)cyclopentadienes \ Scheme 15 equilibrate loo at 100"Cwith 1-alkyl-pentakis(methoxycarbony1)cyclopentadienes uia [ 1,5]-sigmatropic ester shifts. At higher temperatures a further ester shift is observed leading to 2-alkyl-pentakis(methoxycarbonyl)cyclopentadienes. The intramolecular character of the rearrangement is demonstrated by crossover experi- ments. Trisection has been undertaken'" of the thermal automerization of 4-vinylcyclohexene. In an attempt to provide information bearing on the question of the temporal succession of bond breaking and bond formation as represented by the three extreme cases of transition states (5 1)-(53) several 2,6-disubstituted homo- (.\U Initial bond bond formation step breaking (51) (53) tropilidenes have been synthesized and their rearrangements studied'" by 'H n.m.r.spectroscopy. Evidence is cited attesting to the conclusion that in (bridged) homo- tropilidenes the transition state is like (54)or (55). A related study has been 99 T. J. Levek and E. F. Kiefer J. Amer. Chem. Soc. 1976,98 1875. 100 R. W. Hoffmann P. Schmidt and J. Backes Chem. Ber. 1976,109,1918. 101 W. von. E. Doering and D. M. Brenner Terruhedron Letters 1976 899. 102 H. Kessler and W. Ott J. Amer. Chem. Soc. 1976,98 5014. 298 A. Cox (54) (55) ~ndertaken"~ as a model system. It is using the tricyclo[3,3 1,02*8]nona-3,6-dienes concluded that heteroatoms such as chlorine and oxygen seem to cause little if any perturbation of the barbaralane framework provided that they are attached to C-3 and C-7.This negligible effect contrasts with the rate-enhancing influence of oxygen in the 3-hydroxy Cope rearrangement of hexa-1,5-dienes and especially in the 3-oxido Cope rearrangement. The activation parameters of the Cope rearrange- ment of 1,2,3,4,5,6,7,7'-octadeuterio-3,4-homotropilidene have been deter-minedlM between -20 and +102"C by a complete lineshape analysis of the temperature-dependent 'H n.m.r. spectrum. In addition it was found that the isomer with the CH2 group at the three-membered ring is more stable than the isomer with the diallylic CH2 group by ca. 1kJ mol-l. A Cope rearrangement has also been imp1icatedlo5 in the transformation of (56) into (57). The reaction which is important synthetically in regiospecific quinone isoprenylation is an exceptionally ready allyl-p-quinol Cope rearrangement.It should be viewed as good precedent for the possible reinterpretation of the mechanism of other ally1 quinol rearrangements together with related transformations. Vapour-phase pyrolysis of pentacyclo[5,5,0,02"2,06'8,O3~9]dodeca-4,10-diene has been shownlo6 not to give detectable amounts of any other (CH2)12isomer. Instead the starting material fragments to benzene at temperatures above 160 "C. Experimental support is cited to account for this behaviour in terms of Scheme 16. For pericyclic reactions it has Scheme16 103 A. Busch and H. M. R. Hoffmann Tetrahedron Letters 1976 2379. 104 H.Giinther J.-B. Pawliczek J. Ulmen and W.Grimrne Chem. Ber. 1975,108,3141. 105 D.A. Evans and J. M. Hoffman,J. Amer. Chem. SOC.,1976,98,1983. 106 E.Vedejs and R. A. Shepherd,J. Org. Chem. 1976,41,742. Alicyclic Chemistry 299 been suggested that substitution of one component with electron-releasing groups (raising the MOs of that component) and of the other component with electron- withdrawing substituents (lowering the MOs) effectively reduces the energy barrier for disallowed processes. In this context the thermal [a2,+ ds]ring-opening of substituted 1,4,5-endo-6-endo-tetramethylbicyclo[2,2,O]hex-2-enes(58) to give (59),has been investigated.'" In each case the product expected for [dS+dr] Me Me Me I Me ... 1 (58) R' = R2= Me (59) R' = C02Me R2= Me R' = R2= C02Me R1= C02Me R2 = Ph cycloreversion was selectively formed.Approximate values of activation parameters are given. In solution at 180"Cor on passage through a hot tube (>370"C 18s residence time) as-homobasketene has been observed'08 to fragment cleanly to benzene and cyclopentadiene. Evidence is presented supporting the view that fragmentation of as-homobasketene starts with a (2+ 4)-cycloreversion to syn-tricyclo[5,4,0,02~6]undeca-3,8,10-triene. Classification of the second step as a (2 + 2) or a (4 + 4)process cannot be made at this stage. A report has appeared"' on the butadienylcyclopropane rearrangement in tricyclo[5,3,0,02*'0]deca-3,5-diene (60) the cisoid geometry of which is fixed by the ethano bridge. Detection of the rearrangement was accomplished with the 2H,-labelled compound.The free energy of activation was found to be CQ. 21 kJ mol-' higher than in (61).Bearing in mind that (61)must first be transformed into its higher energy transoid conformer (62),the energy difference for the pericyclic reaction itself in the cisoid as against the transoid transition state should be taken as AAG' a25 kJ mol-'. (60) (61) (62) Photochemically Induced Reactions.-A study has been made''' of the stereochem- ical fate upon rearrangement of the optically active 2-phenyl-3-methylenecyclopropane when reaction has been induced either thermally or photo- chemically. The results clearly demonstrate that chirality is maintained during the course of the transformation whether thermally or photochemically induced and also exclude the development of the planar geometry calculated to be favoured for triplet 10' F.van Rantwijk and H. van Bekkum Tetrahedron Letters 1976.3341. W. Mauer and W. Grimme Tetrahedron Letters 1976,1835. 109 K.Heger and W. Grimme Angew. Chem. Infernat.Edn. 1976,15,53. 110 W.A.Gros T. Luo,and J. C. Giibert J. Amr. Chem. SOC.,1976,98,2019. 300 A. Cox trimethylenemethane. Irradiation of a series of tetrasubstituted olefins leads"' to positional migration of the double bond e.g. (63)+(64),and labelling studies have (63) (64) indicated that the migration process is intramolecular and is therefore a [1,3]-sigmatropic shift of hydrogen. That the migration process is apparent in cyclopen- tenes may be due at least in part to the emergence of the .np* state as the lowest-lying singlet excited state.An investigation of the photoreactions of methyl cis-and trans-4,4,6-trimethylhepta-2,5-dienoatehas provided"* the first example of a di-.n-methane rearrangement where the ring-opening goes in one direction on direct irradiation and in the other direction when a sensitizer is used (Scheme 17). 0 0 3oo nm Acetone Scheme 17 The results show that vinylcyclopropane formation competes with triplet energy dissipation by the free-rotor effect and which products are formed depends only on relative rates. The stereoselective photochemical photodimerization of liquid but-2- ene has been in~estigated."~ The experimentally observed distribution of the four isomers as a function of the cisltrans ratio in liquid but-2-ene has been quantitatively reproduced by calculation.The calculations were based on theoretical rate expres- 111 P.J. Kropp H. G. Fravel and T. R. Fields J. Amer. Chem. SOC.,1976,98 840. 112 P. Baeckstrom J.C.S. Chem. Comm. 1976,476. lf3 H. Yamazaki R. J. Cvetanovic and R. S. Irwin J. Amer. Chem. SOC.,1976,98 2198. Alicyclic Chemistry sions derived from a reaction mechanism which considers the statistical probability of encounters between the electronically excited and the ground-state cis-and trans-but-2-ene molecules. The products and mechanisms of the reaction of O(3P)atoms with inter alia 1:methylcyclohexene have been investigated. '14 A general mechan- ism is discussed for this oxidation which is of interest not only from the fundamental grounds of structure and reactivity but also because of its significance for atmos- pheric pollution.Experimental evidence has been pr~duced"~ for a twisted form of 1-phenylcyclohexene presenting a double bond past orthogonality and commonly called trans-1-phenylcyclohexene. Laser photolysis of 1-phenylcyclohexene in methanol at room temperature forms a transient absorbing in the range 300- 430 nm with a maximum near 380 nm and having a lifetime of 9 ps. 114 J. S. Gaffney R. Atkinson and J. N. Pitts J. Arner. Chem. SOC.,1976,98 1828. 1x5 R. Bonneau J. Joussot-Dubien L. Salem and A. J. Yarwood J. Amer. Chern. SOC.,1976,98,4329.