4 Reaction Mechanisms Part (i) Orbital Symmetry Correlations and Pericyclic Reactions By D. W. JONES Department of Organic Chemistry The University Leeds LS2 9JT 1 General and Theoretical Aspects A valuable new text’ illustrates the utility of the frontier molecular orbital (FMO) approach in treating reactivity in ionic free-radical and pericyclic reactions. Several useful reviews have also appeared.* Application of ab initio MO theory to the Diels-Alder addition of butadiene to ethylene3” provides predictions which contrast with those of earlier MIND0/3 calc~lations.~~ The addition is now calculated to proceed in a concerted manner via a symmetric transition state (TS); the hex-2-ene- 1,6-diyl diradical although of lower energy than this TS is only connected to the reaction by a TS 8.36 kJmol-’ higher in energy than the concerted TS.3“ A distinction has been discerned3‘ between cycloadditions in which the dominant FMO interaction is between antisymmetric orbitals and those cycloadditions in which the principal FMO interaction is between symmetric orbitals.For the first type e.g. normal Diels-Alder reactions concerted formation of two bonds is expected. However for the second type e.g. Diels- Alder addition with inverse electron demand the addition will be less concerted or involve complex formation. For additions of a series of cyano-olefins to e.g. cyclopentadiene a test has been made3d of the relationship between log(addition rate constant) and the interaction energy (AE) of the diene HOMO and cyanoethylene LUMO.The interaction energy was calculated using equation (l),where p is the resonance integral and CAand CDare 1 I. Fleming ‘Frontier Orbitals and Organic Chemical Reactions’ Wiley London 1976. 2 (a ) J. J. Dannenberg ‘Predictive Molecular Orbital Calculations in Organic Chemistry’ Angew. Chem. Intentat. Edn. 1976,15,519; (b)N. Dennis A. R. Katritzky and Y. Takeuchi ‘Synt‘hetic Applications of Heteroaromatic Betaines with Six-Membered Rings’ ibid. p. 1; (c) A. Padwa ‘Intramolecular 1,3-Dipolar Cycioaddition Reactions’ ibid. p. 123; (d)E. A. Halevi ‘Orbital Correspondence Analysis in Maximum Symmetry’ ibid. p. 593; (e) T. Wagner-Jaurregg ‘Reactions of Azines and Imines with Dienophiles’ Synthesis 1976 349; cf) F. McCapra ‘Chemical Mechanisms in Bioluminescence’ Accounts Chem.Res. 1976,9,201;(g)W. J. Hehre ‘Ab Initio Molecular Orbital Theory’ ibid.,p. 399; P. D. Bartlett ‘Four-membered Rings and Reaction Mechanisms’ Chem. SOC.Rev. 1976 5 149; D. Bryce-Smith and A. Gilbert ‘The Organic Photochemistry of Benzene’ Tepahedron 1976,32,1309;c. W. Spangler ‘Thermal [l,j] Sigmatropic Rearrangement’ Chem. Reu. 1976 76 187. (a) R. E. Townshend G. Ramunni G. Segal W. J. Hehre and L. Salem J. Amer. Chem. SOC.,1976,98 2190; (b)M. J. S. Dewar A. C. Griffin and S. Kirschener ibid. 1974 96 6225; (c) H. Fujimoto S. Inagaki and K. Fukui ibid. 1976,98,2670;(d)K. N. Houk and L. L. Munchausen ibid. 1976,98,937. 41 D. W.Jones the coefficients of the electron acceptor and donor respectively at the sites of interaction (CND0/2 calculations).In the denominator of the equation the experi- mentally determined donor ionization potential (IP,) and acceptor electron affinity (EA,) replace the energies of the diene HOMO and cyano-olefin LUMO respec- tively. The quantity C is the amount by which the charge-transfer configuration drops in energy as the molecules approach; it is estimated as 4eV. Plots of log(rate constant) against hE are not linear there being a levelling off of reactivity for more reactive olefins. Earlier TSs for the more reactive olefins would result in different values of /3 and C and so account for this result; the neglect of other MO interactions could also be more serious for one cyano-olefin than another. In nucleophilic attack on cyano-olefins (or two-step Diels-Alder reactions) AE will depend only on the larger cyanoalkene LUMO coefficient.As a result the order of reactivity of a series of cyanoalkenes to nucleophilic attack will differ from the order of reactivity in the Diels-Alder reaction. An orbital mixing rule has been derived4 for the interaction of three MOs; with the aid of the rule orbital bias can be estimated and the results applied to explain preferred ex0 -attack on norbornene syn -selectivity in the Diels-Alder addition of certain cyclopentadienes and regioselectivity in the attack of diazomethane on thiocarbonyl compounds and vinyl and ethynyl thioethers. Structural data from several X-ray analyses of 1,6-methanoannulenes (1) and their cyclized forms (2) have been used in an interesting attempt to map the reaction path for the ring-cl~sure.~" The 1,6-interaction is attractive over the range of distances (1.5 A covalently bonded to 2.48 A non-bonded) considered.The results represent a structural expression of an attractive interaction that follows immediately from the rules of orbital symmetry conser~ation.~" Related X-ray as well as calculations5' suggest that the preferred attack on a carbonyl group by a nucleophile is as in (3) whereas attack on an acetylene should proceed as in (4).These considerations have led Bald~in'~ to suggest rules for ring-closure; favoured ring-closures being those in which the linking chain enables the reacting centres to achieve the trajectories indicated in (3)and (4). Based on a 'banana-bond' model of unsaturation the processes (3) and (4) imply inversions at the reacting carbons.Since geometric processes other than inversions are often observed in concerted electrocyclic reactions the rules may not apply to them. However the preferred approach to an acetylenic bond (4)may serve to explain why the linear 4 S. Inagaki H. Fujimoto and K. Fukui J. Amer Chem. Soc. 1976,98,4054. 5 (u)H. B. Burgi E. Shefter and J. D. Dunitz Tetrahedron 1975,31,3089;(b) H. B. Burgi J. D. Dunitz J. M. Lehn andG. Wipff ibid.,1974,30,1563;G. Wegner PolymerLetters 1971,9,133;R. H. Baughman J. Appl. Phys. 1972,43,4362; (c)H. B. Burgi J. M. Lehn and G. Wipff J. Amer. Chem. Soc. 1974,% 1956; (d)J. E. Baldwin J. C.S. Chem. Comm. 1976,734,738;(e)D. B. Bigley and R.H. Weatherhead J.C.S.Perkin ZZ 1976,592; (f) G.Stork and G. Kraus J. Amer. Gem. SOC.,1976,98 6747. Reaction Mechanisms -Part (i) Orbital Symmetry Correlations acetylene may so readily replace the bent olefin in pericyclic processes e.g. the Cope rearrangement (5) and the decarboxylation (6).'' It seems possible that other reactivity problems in pericyclic reactions could benefit from consideration of preferred angles of approach of reacting centres. An intramolecular ene-reaction employing an acetylenic enophile (7; arrows) is useful in the construction of five-mem bered rings. sf "-pI & 8 CSHII H 0R' (5) (6) (7) In structure (8)orbital interaction between the aromatic n-system and the olefinic bond operating uia the C-1 -C-2 and C-5 -C-6 a-bonds results in a lowering of the LUMO energy so that the olefinic bond in (8) is readily reduced under Birch conditions.6" Orbital interaction through space in the anion (9)imparts 6~-electron aromatic character and accounts for exclusive methylation syn to the methoxy- group.6b This effect is related to the unusual stability of cis-dihalogeno- and dialkoxy-ethylenes which has been treated theoretically.6' Similar through space interaction between entering nucleophile and leaving group will secure 6~-electron aromaticity for the preferred syn-TS in SN2' reactions.6d The endo-preference of alk-2-enyl anions6= may have a similar origin.Quantitative application of the principle of least motion7" to the 1,3-shift implied in (10) favours the inversion pathway shown rather than the intuitively preferred retention pathway; thus the inversion observed to accompany such rearrangemet~t~~ is consistent with either orbital symmetry or least-motion control.2 Cycloaddition and Cheletropic Reactions From the reaction of the very reactive diene (11)with sulphur dioxide the (4+2) w-adduct (12) is formed under conditions of kinetic control and the (n +4)w-adduct 6 (a)M. N. Paddon-Row R. Hartcher and R. N. Warrener J.C.S. Chem Comm.,1976 305; (b) R. R. Fraser and K. L. Dhawan ibid.,p. 674; (c)R. Hoffmann and R. A. OlofsonJ. Amer. Chem.Suc. 1966.88 943; N. D. Epiotis S. Sarkanen D. Bjorkuist L. Bjorkuist and R. Yates ibid. 1974,% 4075; (d)cf. R. L. Yates N. D. Epiotis and F. Bernardi ibid.1975,97 6615; (e)M. Schlosser and J. Hartmann ibid.. 1976,98,4674. 7 (a)J. A. Altmann 0.S. Tee and K. Yates J.Amer. Chem. Soc. 1976,98,7132;(b)J. A. Berson and G. L. Nelson ibid.,1967,89 5503. D. W.Jones / Me Me Me Me (11) (12) (13) (13) is formed under conditions of thermodynamic control.*" Non-linear cheletropic extrusion of sulphur monoxide from (14) occurs in boiling dichloromethane and in the presence of diazo-compounds (R,CN,) sulphines (15; R = Aryl) are obtained.8* Addition of dimethoxycarbene to dimethyl maleate gave an adduct in which the methoxycarbonyl groups are trans; addition of the carbene to cis-and trans+-deuteriostyrene is now shown to be stereospecific in accord with a non-linear cheletropic process.*c Further examples' of carbene-like addition of 1,3-dipoles to double bonds have appeared; (16) affords (17) possibly via a dipole produced as in (16; arrows).(14) (15) (16) (17) In the reaction of singlet oxygen with olefins and dienes the most important FMO interaction is between the HOMO of the olefin and the LUMO of singlet oxygen. Ionization potentials of olefins and dienes accordingly provide a useful guide in predicting the preferred site of singlet oxygen attack;lon in (18) the exocyclic double bond (IP = 8.27 eV) is reactive and the endocyclic double bond (IP = 8.63 eV) is passive. Diels-Alder type addition of singlet oxygen to (19) occurs from the more open anti direction but ene-type attack on (20)occurs from the syn direction. In (20) interaction between the oxygen LUMO and N-1 -N-2 HOMO is preferred (energy anti 8 (a) R.F. Heldeweg and H. Hogeveen J. Amer. Chem. Soc. 1976 98 2341; (b) B. F. Bonini G. Maccagnani and G. Mazzanti J.C.S. Chem. Comm. 1976,431; (c)R. A. Moss and J. K. Huselton ibid. p. 950. 9 A. Padwa A. Ku A. Mazzu and S. I. Wetmore J. Amer. Chem. SOC.,1976 98 1048; L. Garanti A. Vigevani and G. Zecchi Tetrahedron Letters 1976 1527. lo (a)L. A. Paquette D. C. Liotta and A. D. Baker Tetrahedron Letters 1976,2681; (b)L. A. Paquette D. C. Liotta C. C. Liao T. G. Wallis N. Eickman J. Clardy and R. Gleiter J. Amer. Gem. SOC., 1976,98 6413; L. A. Paquette C. C. Liao D. C. Liotta and W. E. Fristad ibid. p. 6412. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations gap 8.2 eV) to interaction between the double-bond HOMO and oxygen LUMO (energy gap 8.7 eV).The singlet oxygen is thereby deactivated and attack on the anti-face of (20) suppressed. In (19) interaction between the oxygen LUMO and diene HOMO is preferred (energy gap 7.4 eV) to deactivation by the hydrazine unit so that anti attack is observed.lob In a theoretical study"" of the addition shown in (21) C-1-C-6 and C-4-C-5 bonding was found to be improved when atoms C-2 and C-3 were brought into proximity with C-7 and 0-8respectively; this supports the novel view that secondary interactions function by improving interaction at the primary bonding sites. The FMO method has provided an explanation for the well-established 'ortho -para ' regioselectivity shown in the majority of Diels-Alder reactions.The hitherto unknown addition of an electron-rich diene to an electron-rich dienophile provides an excellent test as matching the larger coefficients in both HOMO-LUMO pairs as in (22) and (23) predicts predominant formation of the 'meta' isomer. If a diradical X.. .. 3(*...i. :c 2 \..:* xGg '1 08 HOMO LUMO LUMO HOMO (21) (22) (23) were an intermediate in such an addition the para-isomer would be expected to predominate. Trapping the very reactive o -quinodimethanes (24) and (25) with prop-1-yne and ethoxyethylene gave in both cases predominant formation of the unusual 'meta '-adduct e.g. (26).' lb Preparations of several potentially useful Diels- Alder dienes have*appeared.'2 Thus (27) available by electrocyclic ring-opening of the related cyclobutene adds to methyl vinyl ketone to give (28) in which the R2 (24) R' = H R2= Me (25) R' = OMe R2= H phenylthio-group rather than the methoxy-group has controlled the direction of addition.'*" The adduct (29) from 1-trimethylsilylbutadiene undergoes proto- desilylation (29; arrows) offering a potentially useful method of changing the position of the double bond in an adduct.12' The intramolecular Diels-Alder reaction continues to attract attenti~n.'~ An intriguing example'3a is provided by the 11 (a)T. Sugimoto Y. Kobuke J. Furukawa and T. Fueno Tetrahedron Lefrers 1976 1587; (b)I. Fleming F. L. Gianni andT. Mah ibid. p. 881. l* (a)B. M. Trost and A. J. Bridges J. Amer. Chem.SOC.,1976,98 5017 (6) M. J. Carter and I. Fleming J.C.S. Chem. Comm. 1976 679; A. Percival and I. Fleming ibid. p. 681; (c) M. E. Jung and C. A. McCombs Tetrahedron Letters 1976,2935;L. E. Overman G. F. Taylor and P. J. Jessup ibid.,p. 3089; J. F. W. Keana and P. E. Eckler J. Org. Chem. 1976,41,257 1; L. E. Overman and L. A. Clizbe J. Amer. Chem. SOC.,1976 98 2352. l3 (a)R. L. Funk and K. P. C. Vollhart,J. Amer. Chem. SOC.,1976,98,6755; (6) T. Kametani H. Nemoto H Ishikawa K. Shiroyama and K. Fukumoto ibid. p. 3378; P. Yates and H. Auksi,J.C.S.Chem. Comm. 1976 1016; W. Oppolzer R. Achini E. Pfenninger and H. P. Weber Helu. Chim. Acfa 1976,59,1186; L. H. Klemm T. M. McGuire and K. W. Gopinath J. Org. Chem. 1976 41 2571. 46 D. W. Jones addition of bis-trimethylsilylacetylene to compounds of type (30); this proceeds under catalysis by cyclopentadienyldicarbonylcobalt,and probably affords inter- mediate o -quinodimethanes which then undergo intramolecular addition to the C-2 double bond affording products (31).Reaction of the optically active nitroso- (29) (30) (31) compound (32) with E,E-hexa-2,4-diene in methanol gives 3R,6S-(33) of 39% optical p~rity;'~ the configurations of the product accord with preferred exo -addition to the diene which approaches (32) from the front. The reverse Diels-Alder reaction CN Me CI Me (32) (33) (34; arrows) has been used to prepare ~entatetraene,'~~ and fragmentation of (35) involving initial reverse Diels-Alder reaction (35 ; arrows) has provided evidence for the intermediacy of methyleneketen."' The FMO method suggests that electron- rich dienes will add to simple fulvenes in (6 + 4)7r fashion; this prediction has been verified16 by reaction of 1-diethylaminobutadiene with 6-phenylfulvene and dehyd- rogenation of the product to 4-phenylazulene (36).(34) (35) (36) Huisgen has considered in detail17" the case for concerted 1,3-dipolar addition and concludes that the large bulk of the data is in agreement with a ,4 + ,2 process. The observation that diazomethane adds to ethoxyethylene to give (37)17' agrees with a concerted process in which the dipole LUMO-dipolarophile HOMO interac-tion is dominant.' Synthetic applications of six-membered ring heteroaromatic 14 H. Nitsch and G. Kresze Angew Chem.Internat Edn. 1976 15 760. 15 (a) J. L. Ripoll J.C.S. Chem. Comm. 1976 235; (b) R. F. C. Brown F. W. Eastwood and G. L. McMullen J. Amer. Chem. SOC.,1976 98 7421. 16 L. C. Dunn Y. M. Chang and K. N. Houk J. Amer. Chem. SOC.,1976,98,7095. 17 (a)R. Huisgen J. Org. Chem. 1976 41 403; (b)R. A. Firestone ibid. p. 2212; (c)N. Dennis A. R. Katritzky and M. Ramaiah J.C.S. Perkin I 1976,2281 and following papers; (d)P.G. Sammes and R. A. Watt J.C.S. Chem. Comm. 1976.367; (e)W. Oppolzer and M. Petrzilka J. Amer. Chem. SOC.,1976,98 6722; (f) J. Leitich Angew. Chem. Internat. Edn. 1976,15 372; (g) G. Bianchi and D. Maggi J.C.S. Perkin (11),1976 1030. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations (37) betaines have been reviewed,2b and a series of papers describes the substantial contribution of Dennis Katritzky and their collaborators.17' Intramolecular 1,3- dipolar cycloadditions have also been reviewed,*' and a neat example involving addition of a simple olefin to a pyridinium betaine (38) (39) has been pr~vided."~ 0 (39) An elegant synthesis of (f)-l~ciduline'~' has as its key step regioselective intramolecular nitrone addition (40; arrows). The first 1,3-dipolar cycloaddition to a nitro-group in its electronic ground state has been achieved17f in the reaction of 2,E-cyclo-octa- 1,5-diene with p-cyanonitrobenzene giving (41). The high reactivity of E-cyclo-octene towards nitrile oxides may be due to pyramidalization of the olefinic carbon atoms [see (42)].'7g 3 Sigmatropic Reactions The only example18n of a predominantly antara 1,3-shift has been challenged.18' Kinetic data for the thermolysis of (43) were originally taken to indicate 65% rearrangement by 1,3-shift of C-3 to Car with antarafacial use of the ally1 system.The data can be reinterpreted in terms of as much as 100% suprafacial migration; rearrangement could involve 36% suprafacial migration with inversion at C-3 and 64% randomization of C-3 stereochemistry via the biradical (44)produced by ring-opening of (43) in a conrotatory-bevel sense [see arrows in (43)]. It is proposed that cleavage of cyclobutane derivatives generally involves simultaneous 18 (a)J. E. Baldwin and R. H. Fleming J.Amer. Chem. SOC.,1973,95,5261; (b)J. J. Gajewski ibid. 1976 98 5254; (c) H.A. Bampfield P. R. Brook and K. Hunt J.C.S. Chem. Comm. 1976 146; (d)J. A. Berson P. B. Dervan R.Malherbe and J. A. Jenkins J. Amer. Chem. Soc. 1976,98,5937; (e)G. D. Andrews and J. E. Baldwin ibid. p. 6705,6706; (f)J. A. Ross R. P. Seiders and D. M. Lemal ibid. p. 4325; (g) W. Thies and E. P. Seitz,J.C.S. Gem. Comm. 1976,846; (h) B. Franzus M. L. Scheinbaum D. L. Waters and H. B. Bowlin,J. Amer. Chem. Soc.,1976,98,1241; (i) J. F. Garst and C. D. Smith ibid.,p. 1526; (j) M. B. Rubin M. Weiner and H. D. Scharf ibid. p. 5699. D. W.Jones (431 (44) rotation (bevel) about the bond opposite the cleaving bond [C-l-C-4 in (43)]; this serves to minimize steric repulsions and maximize overlap between originally bonded atoms.18b The 1,3-shift with inversion at the migrating centre converting (45) into (46) proceeds with 99% retention of optical purity.18,' The rearrangement is thought to involve the chiral intermediate or TS (47) produced by a conrotatory- bevel ring-opening [see (45)] which gives maximum relief of steric strain.Suprafacial LUMO Bu' CN& ally]cation H Ph Ph H Ph Bu' HOMO Bu' 0 0 enolate (45) (46) (47) pathways are also dominant in the 1,3-rearrangement of 1,2-divinylcyclobutanes to vinylcyclohexenes e.g. (48;arrows),lgd and the vinylcyclopropane rearrangement of (49).Ige In both these cases the allowed suprafacial-inversion pathway is only slightly preferred to the forbidden suprafacial-retention route. It is pointed out18e that the experimental facts regarding vinylcyclopropane rearrangements are directly con- trary to the prediction of MIND0/3 calculations.It has been suggestedlSf that the very rapid 1,3-migration of sulphur in certain episulphoxides is a 'pseudopericyclic' process e.g. (50; arrows) in which there is an interchange of roles between bonding and non-bonding orbitals. Hydroboration (51; arrows) in which the vacant boron orbital switches roles with the C-H bond orbital is a related process. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations Following the discovery that the oxy-Cope rearrangement is enormously acceler- ated in related potassium alkoxides it has now been observedlgg that rearrangement of the potassium salt (52; R = K) is much more rapid than that of the silyl ether (52; R =SiMe,).The formal 1,3-shift (52; arrows) may be less likely than ring-cleavage to an ally1 anion and recombination by Michael addition; this would agree with the failure of 1-vinylcyclononanol to undergo ring-expansion. However a mechanism involving radical-ketyl pairs should be considered for this and a related rearrange- ment.18" Only 16% of the products of Wittig rearrangement of the anion (53)contain cyclopentylmethyl groups formed by cyclization of intermediate hex-5-enyl radicals. The remaining 84% of the reaction is intramolecular giving products containing the hex-5-enyl group. The intramolecular process was thought to differ from secondary recombination of alkyl radical and ketyl and the term 'radical-concerted' was suggested to describe it.18' Photodecarbonylation of (54) is preceded by 1,3-acyl shift (54; arrows) to a cyclobutane- 1,2-dione which on photolysis loses carbon monoxide.18' Ph2C-O-(CH2).+CH=CHZ 0 (52) (53) (54) The ~uggestion'~~ that Cope rearrangement of 2,5-diphenylhexa- 1,5-diene (55) involves an intermediate (56) receives support from MIND0/2 calculations.19' Although 2,5-diphenyl substitution in hexa- 1,5-diene markedly accelerates Cope rearrangement analogous substitution in homotropilidenes (57) and barbaralanes slightly slows rearrangement,'" suggesting that the rearrangement TS (or inter- mediate?) is not of type (58). On the other hand attempts to detect dipolar intermediates of type (59) in the Cope rearrangement of hexa-1,5-dienes with a (55) (56) (57) (58) (59) donor group at C-2 and an acceptor group at C-5 were fruitf~l.''~ Thus (60; R' = H R2=D) rearranges rapidly to (60; R' = D R2= H) and the putative dipolar inter- mediate (61) could be trapped with benzaldehyde or acrylonitrile.Evidence for a dipolar intermediate in the 3-~ulpha[3,3]-sigmatropic shift'" of (62) was also obtained.lgd In an intriguing transformation i.r. laser irradiation of (63) at 10.8 pm results in complete conversion into (64) which is transparent at 10.8pm.'9f 19 (a)M. J. S. Dewar and L. E. Wade J. Amer. Chem. Soc. 1973,95 290; (b)A. Kormornicki and J. W. McIver ibid. 1976,98 4553; (c)H. Kessler and W. Ott ibid. 1976,98 5014; A. Busch and H. M. R. Hoffmann Tetrahedron Letters 1976,2379; (d)R. Gompper and W. R.Ulrich Angew. Chem. Internat. Edn. 1976,15,299,301;(e)Fornomenclature see F. Vogtle and E. Goldschmidt Chem. Ber. 1976,109 1; (f)I. Glatt and A. Yogev J. Amer. Chem. SOC.,1976,98 7087. D. W.Jones 0 N -CO,Me k&' &D2 ~I ',@,Me COPh D D (60) (61) (62) (63) (64) There is increasing evidence that unsaturated groups undergo particularly easy 1,54gmatropy. Thus 3a-H-benzimidazoles e.g. (65) generated as reactive intermediates,"" are believed to undergo a sequence involving [131-butadienyl shift [1,5]-imidoyl shift and [1,5]-hydrogen shift (Scheme). Only under Scheme vigorous thermal conditions are the products of alternative [131- and [1,9]- sigmatropy of the ring-junction methyl of (65)observed.20a In both (65)and (66)"' migration of the unsaturated group to carbon rather than nitrogen is observed.However in the pyrazolenines (67) an acyl group (MeCO or C02Me) migrates to both nitrogen and carbon with the latter process being more important.20C The more rapid acetyl than methoxycarbonyl migration observed in this study accords with the relative migratory aptitudes of these groups in [1,5]-shifts in indenes and cyclohex- adienes;20d this supports the view that more rapid 1,5-migration of acetyl compared to vinyl is in part due to a secondary interaction between the diene HOMO and the T* orbital of the unsaturated group.20d The more rapid 1,Srearrangement of the formyl group compared to hydrogen suggests that formyl may replace hydrogen in other pericyclic processes; the benzoyl group which also migrates more rapidly than hydrogen in the 1,5-shift has already replaced hydrogen in a process (68; arrows) analogous to the ene-reaction.'Oe 20 (a)T.L. Gilchrist C. J. Moody and C. W. Rees J.C.S. Chem. Comm. 1976,414; (b)C. D. Anderson J. T. Sharp E. Stefaniuk and R. S. Strathdee TetruhedronLetters 1976,305; (c)M. Franck-Neumann and C. D. Buchecker ibid.,p. 2069; (d) D. J. Field D. W. Jones and G. Kneen J.C.S. Chem. Comm. 1976 873; (e) A. E. Baydar G.V. Boyd R. L. Monteil P. F. Lindley and M. M. Mahmoud ibid. p 650; (f) M. Kato M. Funakura M. Tsuji and T. Miwa ibid. p. 63; (g) R. F. Childs and C. V. Rogerson J. Amer. Chem. Soc. 1976,98,6391. 51 Reaction Mechanisms-Part (i) Orbital Symmetry Correlations In accord with orbital symmetry considerations there is inversion of the migrating carbon in the photochemical Berson-Willcott rearrangement of (69)which first gives the intermediate (70).20f Photo-rearrangement of the cation (71; R' =OH R2=H) to its isomer (71;R'=H R2=OH) can be viewed as 1,6-sigmatropy in the canonical form (72).20g Although rearrangements of both (69)and (72) proceed with inversion in accord with orbital symmetry control this is also the least-motion pathway.Meoh (LJ 0 d::;H-e Me H Me H ., \/ \\ -_-R2 R' OH (69) (70) (71) (72) 4 ElectrocyclicReactions Marked substituent effects on the rate of opening of cyclobutenones (73; arrows) to vinylketens parallel substituent effects on the opening of cyclobutenes to butadienes;21" introduction of an alkyl group at C-2 slows ring-opening whereas a strong acceleration attends replacement of alkyl groups at C-4 by phenyl groups.It appears that the TSs for ring-opening benefit more from phenyl conjugation than the products. Related vinyl participation may be important in the ring-closure of (74; R =vinyl) which is more rapid than closure of (74; R =Et) (AAH' =13 kJ mo1-').21b It was suggested by Epiotis that the barrier to forbidden pericyclic reactions is lowered by configuration interaction which becomes more important when one component of the pericyclic reaction carries electron-releasing groups and the other component carries electron-withdrawing groups. Support for this idea is provided by the observation2" that (75; R =C02Me) undergoes more rapid disrotatory cyc- lobutene ring-opening than (75; R =Me) (AAH* =35 kJ mol-'); similar substituent Me (a)R.Huisgen and H. Mayr J.C.S. Chem. Comm. 1976,55,57; (6)C. W. Spangler Terrahedron,1976 32,2681; (c)F. van Rantwijk and H. van Bekkum Tetrahedron Letters 1976,3341. D. W.Jones effects were observed for the rearrangement of Dewar-benzene derivatives. The thermal isomerization of 1-and 2-methylbicyclo[2,l,O]pent-2-enehas been the subject of a detailed mechanistic study22 which supports initial ring-opening to a chemically activated cyclopentadiene [Ann.Reports (B) 1975 72 561. Photochemical ring-closure involving an aromatic ring (76; arrows) may be involved in the biogenesis of apolignans. The parent compound (77) is formed by irradiation of (76).230In accord with the principle of least motion cyclohexadienes e.g.(78) in which the pseudo-axial disposition of the methyl group is preferred undergo photochemical opening to E-hexatrienes e.g. (79).236A new index for predicting the preferred mode of photochemical closure of substituted cycloheptat- rienes is derived from the stability gain of the FMOs. In several cases e.g. (80)+(81) the observed pathway is correctly ~redicted.~~' Disrotatory opening of the radical (82) to a cyclohexadienyl radical is slower than sigmatopic rearrangement to (83).24aNeither cyclization of (84)to (85)24bnor of (86)to (87)24cis consistent with orbital symmetry control. Reduction of cup-unsaturated ketones (Ac20 Zn HCI-Et20) to cyclopropyl acetates is believed to involve conrotatory cyclization of intermediate ally1 anions.24d In several cases the product stereochemistry is consis- tent with this mechanism e.g.(88)+(89). 22 W. E. Farneth M. B. D'Amore and J. L. Brauman J. Amer. Chem. SOC.,1976,9% 5546. 23 (a) H G.Heller and P. J. Strydom J.C.S. Chem. Comm. 1976,50;(b)P. Courtot and J. Y. Salaun ibid. p. 124; (c) T. Tezuka and 0.Kikuchi Tetrahedron Letters 1976 1125. Z4 (a)R. Sustmann and F. Lubbe J. Amer. Chem. Soc. 1976,98,6037; (6)G. A. Olah J. S. Staral and L. A. Paquette ibid. p. 1267; (c) R. E. Lehr J. M. Wilson J. W. Harder and P. T. Cohenour ibid. p. 4867; (d) C. W. Jefford and A. F. Boschung Helv. Chim Acfa. 1976,59 962. Reaction Mechanisms -Part (i) Orbital Symmetry Correlations