4 Reaction Mechanisms Part (i) Pericyclic Reactions By N. G. RAMSDEN Glaxo Group Research Green ford Middlesex UB6 OHE 1 Cycloaddition Reactions During the past year further experimental and theoretical data has been presented for the concertedness of cycloaddition reactions. The activation energy of the retro-Diels-Alder reaction of 1,2,3,6-tetrahydropyridineover a range of temperatures shows a slight increase in marked contrast with the drop usually seen in simple bond fission. This may reflect a change from a concerted to a biradical mechanism at high temperatures.’ The observed site selectivity of the reaction of butadiene with protoanemonin has been explained on the basis of a highly asynchronous or biradical mechanism with the crucial factor being the formation of a cyclic aromatic radical when attack is at the exocyclic double bond.* Cyclopropabenzene reacts with diphenylisobenzofuran to give the [47r + 27r] adducts.’ In addition the [47r + 2a] adduct is formed presumably via a biradical rearrangement (Scheme 1).High pressure FTIR spectroscopy of the reaction between isoprene and maleic anhydride suggests a two step mechani~m.~ N-Vinylcarbazole and (NC),C=C( CO,Me) react via a zwitterionic intermediate with kinetic evidence being found for both uni- molecular decomposition of the electron donor-acceptor complex and bimolecular reaction with N-vinyl ~arbazole.~ The use of More O’Ferrall diagrams has been modified by the incorporation of the topological theory of chemical reactivity to allow the classification of pericyclic reactions in terms of their degree of concertednes6 Hartree-Fock calculations have been performed to explain why tetrafluoroethylene undergoes [2 + 21 rather than [4 + 21 cycloadditions with b~tadiene.~ They show that whilst fluorine substituents have little effect on the energy of the Diels-Alder transition state they have a profound stabilizing effect on the energy of the 1,4 allylic biradical in the [2 + 21 cycloaddition.The relative energies of biradical versus synchronous pathways for S. S. Sidhu J. H. Kiefer A. Lifshitz C. Tamburu J. A. Walker and W. Tsang Int. J. Chem. Kinet. 1991 23,215. ’V.Branchadell J. Orti R. M. Ortuno A. Oliva J. Font J. Bertran and J. J. Dannenberg J. Org. Chem. 1991 56 2190. U. H.Brinker and H. Wuster Tetrahedron Let?. 1991 32,593. Y.Ikushima N. Saito and M. Arai Bull. Chem. SOC.Jpn. 1991 64 282. ’ T. Gotoh A. B. Padias and H. K. Hall jun. J. Am. Chem. SOC. 1991 113 1308. R. Ponec and M. Stmad J. Math. Chem. 1991,8 103. S. J. Getty and W. T. Borden J. Am. Chem. SOC.,1991 113,4334. 51 N. G. Ramsden Ph Ph Ph Ph Ph Scheme 1 the cycloaddition between the NO; ion and ethyne have been compared by semi- empirical and ab initio calculatiom8 The minimum estimate for the bias to the stepwise route is 11 kcal/mol. MNDO calculations indicate that the reaction of nitroethene and aromatic nitrile oxides is under charge transfer control.' Two regiostereomeric pathways exist. Both are concerted with early transition states.'o911 The dimerization of ketene to cyclobutanedione and diketene has been studied by ab initio SCF calculations.Both products are formed via an unsymmetrical transition state indicating that neither is formed via a 2s + 2a cycloaddition.'' The reaction of l-oxa-2,3-cyclohexadiene in both [4 + 21 and [2+ 21 cycloaddi-tions occurs with remarkable regio- and stereo-specificity. The authors suggest that this indicates a concerted non radical reaction me~hanism.'~ The importance of molecules being in a reactive conformation before undergoing the Diels- Alder reaction has been demonstrated. The reactivity of silyl pyrimidines (1)has been shown experimentally to decrease in the order X = CO >> 0 > CH2 >> NH. Conformational studies on the corresponding desilyl compounds show that this order reflects the probability of the molecule being in a reactive c~nformation.'~ The dicyano pyrimidines (2) are much more reactive in the Diels-Alder reaction than their methylene ana10gues.I~ A comparison of their crystal structures reveals H.S. Rzepa and W. A. Wylie J. Chem. SOC. Perkin Trans. 2 1991 939. A. Baranski and G. Banki Collect. Czech. Chem. Commun. 1991 56 425. 10 A. Baranski and J. Cioslowski Collect. Czech. Chem. Commun. 1991 56 1167. A. Baranski and E. Cholewka Pol. J. Chem. 1991 65 319. 12 E. T. Seidl and H. F. Schaefer 111 J. Am Chem. SOC. 1991 113 5195. 13 R. Ruzziconi Y. Naruse and M. Schlosser Tetrahedron 1991 47 4603. 14 W. A. W. Stolle A. T. M. Marcelis and H. C. Van der Plas ibid.1991 47 1753. 15 W. A. W. Stolle A. E. Frissen A. T. M. Marcelis H. C. Van der Plas Y. Wang L. Haming and C. H. Stam J. Org. Chem. 1991 56 2411. Reaction Mechanisms -Part (i) Pericyclic Reactions that the former have a conformation with the reacting centres much closer and MNDO calculations show that the dicyano compounds possess lower transition state energies. An extensive investigation of the rate of cyclization of 2-furfuryl methyl fumarates (3) has shown that the gem dialkyl effect is due primarily to the reactive rotamer effect and not angle compression.’6 The cyclobutane compound RR’ = (CH2)3 should have a similar reactive rotamer effect to the dimethyl com- pound and thus cyclize relatively rapidly if this effect was dominant.However its larger internal angle should lead to a slower cyclization than the dihydro compound. In fact it cyclizes some 200 times faster than the dihydro compound. (1) R = Me,Si (2) R = H (3) R=H R = CN The effect of solvents and co-solutes on cycloaddition reactions has been studied. A Diels-Alder reaction in the liquid phase has been modelled as a pair of coordinates one associated with chemical reaction motion and the other describing the contrac- tion of the medium cavity surrounding the reaction.” The corresponding kinetics show both equilibrium and non equilibrium behaviour. Cycloaddition of methyl acrylate to 2,6-bis(t-butyldimethylsilyloxy)-3,4-dihydropyridinedisplays 92 :8 exo selectivity in benzene. PM3 MO calculations suggest that this is due to solvent effects in the medium.” The use of the parameters Spand to account for the influence of the medium on the rate and selectivity of the Diels-Alder reaction is complicated by the fact that the effect of dipolar solute-solvent interactions may be included in both terns.” Monte Carlo statistical methods show that the free energy of solvation of the transition state of the reaction between methyl vinyl ketone and cyclo- pentadiene is of the order of 4kcal/mol lower in water than in propane.20 The rate enhancement arises from a hydrophobic effect and enhanced hydrogen bonding to the carbonyl in the more polar transition state.A variety of other cycloadditions are also accelerated in water due to a hydrophobic effect.21*22 Alkali metal salts increase the rate whilst agents capable of forming lipophillic micelles decrease it.The kinetics of the reaction of imine quinone (4)with dienes when plotted against the parameter Dpof solvent nucleophilicity show a very poor quality linear correla- tion with a negative slope.23 The intramolecular reaction of furfurylamides (5) in 16 M. E. Jung and J. Gervay J. Am. Chem. Soc. 1991 113 224. 17 M. V. Basilevski V. M. Ryaboi and N. N. Weinberg J. Phys. Chem. 1991 95 5533. 18 R. Sustman W. Sicking H. Lamy-Schelkens and L. Ghosez Tetrahedron Lett. 1991 32 1401. 19 c. Cativiela J. I. Garcia J. A. Mayoral A. Avenoza J. M. Peregrina and M. A. Roy J. fhys. Org. Chem. 1991 4 48. 20 J. F. Blake and W. L. Jorgensen J. Am. Chem. SOC.,1991 113 7430.21 I. Hunt and C. D. Johnson J. Chem. Soc. Perkin Trans. 2 1991 1051. 22 W. Blokziji M. J. Blandamer and J. B. F. N. Engberts J. Am. Chem. Soc. 1991 113 4241. 23 G. Desimoni G. Faita and P. P. Righetti Tetrahedron 1991 47 5857. N. G.Ramsden q i”CONH DNpco2H CO2H (5) the presence of various supramolecular catalysts has been studied.24 MM calculations suggest that the para compound (6) stabilizes starting material whilst the ortho compound (7) stabilizes the transition state. The effect of lithium ions on cycloaddition reactions continues to be a subject of investigation. No reaction was observed on treatment of 1,8,10-undecatrien-3-one with lithium perchlorate whereas lithium tetrafluoroborate gave the product in quantitative yield.This has been ascribed to the slow release of catalytic boron trifluoride rather than catalysis by the lithium cati01-1.’~ Lithium chloride accelerates the Diels- Alder reaction of N-ethylmaleimide to anthracene-9-carbinol but lithium perchlorate slows the rare. This can be accounted for by a modification of the hydrophobic effect and rules out the lithium cation as the catalyst.26 The Diels-Alder reaction of various substrates in lithium perchlorate/diethyl ether shows significant rate enhan~ement.~~ The catalytic activity of this system has been reviewed during the year.’* Further theoretical and experimental support has been given to the theory that transition state hyperconjugation is responsible for facial selectivity in cycloaddition reactions.A variety of cycloadditions involving 5-substituted adamantane derivatives (8) have shown clear but modest preference for attack at the Zu face.29 Hexa- chlorocyclopentadiene and its dimethoxy analogue react with 1,5-~yclooctadiene to produce a 1:4 syn/anti mixture of diadducts (Scheme 2).30,31 The initial mono- adducts have been shown to be fluxional by 13CNMR and MM calculations show no preferences for the face of attack. The facial selectivity must result from conforma- tionally dependant transition of T-u-T interactions. Hexacyclopentadecadiene 24 S. C. Hirst and A. D. Hamilton J. Am. Chem. SOC.,1991 113 382. 25 D. A. Smith and K. N. Houk Tetrahedron Lett. 1991,32 1549. 26 R. Breslow and C. J. Rizzo J. Am. Chem. SOC.,1991 113 4340.27 M. A. Forman and W. P. Dailey ibid. 1991 113 2761. 28 H. Waldmann Angew. Chem. 1991 103 1335. 29 H. Li J. E. Silver W. H. Watson R. P. Kashyap and W. J. Le Noble J. 0%.Chem 1991 56 5932. 30 J. G. Garcia F. R. Fronzek and M. L. McLaughlin Tetrahedron Lett. 1991 32 3289. 31 J. G. Garcia and M. L. McLaughlin ibid. 1991 32 3293. Reaction Mechanisms -Part (i) Pericyclic Reactions xx xx / C Ycl Anti SYn Scheme 2 derivatives (9) show a strong preference for addition to the carbonyl face.32 Orbital tilting and transition state steric and torsional effects cannot alone account for this and unfavourable orbital interactions between the closed shell of the carbonyl and the methylenes syn to the incoming orthogonal 7r orbital of the alkene may be important.Cycloadditions of N-sulfonyl-5-aza-l,3-butadienes give a single product derived from a >20 1 endo transition state.33 The endo selectivity may be partially due to secondary orbital stabilization but the exceptional selectivity suggests that the transition state with the lone pair on nitrogen and the carbon oxygen bond of the dienophile lying transperiplanar benefits from a n- 7r* stabilization. The addition of alkenes to [4,3,2]-propella-2,4,8,10-tetraen-7-one(10) syn to the five membered (8) X = F Ph (9) X = O,CH (10) X = 0,H ring has been rationalized in terms of the difference in dihedral angle between the cyclohexadiene ring and the two flanking rings.34 The 7r facial selectivity of reaction of spiro-(bicyclo[2,2,1]heptane-2,1’-[ 2,4]cyclopentadiene) has been surmized to be due to steric interactions in the transition state.35 The intramolecular Diels- Alder 32 J.M. Coxon R. G. A. R. Maclagan D. Q. Mcdonald and P. J. Steel J. Org. Chern 1991 56 2542. 33 D. L. Boger W. L. Corbett T. T. Curran and A. M. Kasper J. Am. Chem. SOC,1991 113 1713. 34 T. Tsuji M. Ohkita and S. Nishida J. Org. Chem 1991 56 997. 35 D. J. Burnell and Z. Valenta Can. J. Chem. 1991 69 179. N. G. Ramsden reaction of a series of 7-alkoxy-undecatrienes gives trans fused cycloadducts together with significant amounts of the cis fused ad duct^.^^ These must arise via a boatlike transition state and it would appear that the 7-alkoxy substituent electronically destabilizes the chairlike transition state.The enantioselectivity of p-lactam produc- tion by [2+ 21 cycloaddition of imines and chiral ketenes derived from oxazolidines and oxazolidinones is determined by the chiral a~xiliary.~' The relative stereochemistry is primarily determined by the structure of the imine and the free or bound nature of the ketene. In the non concerted [2 + 21 cycloaddition of chiral ketenes with the 2-imine component of diazepines the stereochemistry is determined by minimum steric interactions in the transition state leading to zwitterionic inter- mediates followed by a conrotatory ring closure to products.38 However reaction of diazepines with N-a-diphenylnitrone gives endo and exo products in the same ratio presumably due to an absence of secondary orbital interactions in the transition state.39 A series of enantiometrically pure vinyl ketene acetals which function as diastereoselective dienes in Diels- Alder reactions have been ~ynthesized.~' The relative order of the transition state energies was determined to be Re exo < Re endo < Si endo < Si exo rationalized by molecular mechanics and T charge distribution calculations.The cyclization reactions of allenes have been studied in some detail. The structures and product distribution of the cycloadducts from the reaction between 1,3-dimethylallene and alkenes indicate a two step reaction via a diradical ir~termediate.~~ The reaction can occur via an anti anti- or anti syn-diradical and product distribution appears to depend upon the degree of develop-ment of the diradical intermediate.Use of chirally enriched allenes gave all four possible cycloadducts with varying degrees of racemization. Molecular modelling calculations on the conformational energy surface for approach to give the activated complex and diradicals suggest that three minimum energy channels exist for the reaction and they differ in the extent of the facial selectivity of attack.42 p-Tolyl vinyl sulfoxide has been activated as a Diels-Alder dienophile and gives very good diastereomeric excess at -20 "C. Endo attack occurs from the Si face with the S-cis conformer of the sulfoxide being preferred.43 A bisoxazolidine/Fe"' couple (1 1) catalyses the chiral Diels- Alder reaction of 3-acryloxyl-l,3-oxazolidine-2-one with Me Me 36 W.R. Roush M. Kageyama R. Riva B. B. Brown J. S. Warmus and K. J. Moriarty J. Org. Chem. 1991 56 1192. 37 L.S. Hegedus J. Montgomery Y. Narukawa and D. C. Snustad J. Am. Chem. Soc. 1991 113 5784. 38 M. Muller D. Bur T. Tschamber and J. Streith Helu. Chim. Acta. 1991 74 767. 39 K. Saito A. Yoshino and K. Takahashi Heterocycles 1991 32 1. 40 M. A. Boehler and J. P. Konopelski Tetrahedron 1991 47 4519. 41 D. J. Pasto K. D. Sugi and J. L. Malandra J. Org. Chem. 1991 56 3781. 42 D. J. Pasto and K. D. Sugi ibid. 1991 56 6216. 43 B. Ronan and H. B. Kagan Tetrahedron; Asymmetry 1991 2 75. Reaction Mechanisms -Part (i) Pericyclic Reactions H cyclohexadiene.4 It is proposed that the major reaction pathway involves a stereoisomeric octahedral complex with the dienophile complexing via an axial and equatorial bond relative to the plane of the ligand.Oxazaborolidine catalysts have been devised which generate chiral adducts with >95 :5 enantioselectivity in the reaction of dienophiles with cyclopentadiene via the proposed adduct (12).45 N-acylnitroso dienophiles derived from C2 symmetric pyrrolidines react with cyclo- hexadiene to give adducts in excellent diastereomeric excess (Scheme 3).46 Use of pyrrolidines of opposite helicity gives products of the opposite configurations. n Scheme 3 Analysis of the microwave spectra of ozone/ethene mixtures shows that they are consistent only with a Van de Waals complex of C3 symmetry with ethene and ozone having near parallel planes:’ It is argued that this complex lies in a shallow minimum prior to the transition state.2 Sigmatropic Reactions The preference for chair-like transition states in the ester enolate Claisen rearrange- ment has been investigated using a variety of straight chain carbocyclic and heterocyclic propanoates. A novel stereoelectronic effect in pyranoid and furanoid 44 E. J. Corey N. Imai and H. Y. Zhang J. Am. Chem. SOC.,1991 113 728. 45 E. J. Corey and T. P. Leh ibid. 1991 113 8966. 46 A. Defoin A. Brouillard-Poichet and J. Streith Helv. Chim. Acta 1991 74 103. 47 C. W. Gillies J. Z. Gillies R. D. Suenram F. J. Lovas E. Kraka and D. Cremer J. Am. Chem. SOC. 1991 113 2412. N. G. Ramsden glycals appears to stabilize the boat-like transition state whilst the preferred transi- tion state in carbocycles is dependent upon steric factors?8 The products from the reaction of methyl hydroxydithioalkanoates with LDA and allylic bromides undergo a thio-Claisen rea1~angement.4~ Both E and 2 isomers give predominantly syn products and a rationalization in terms of transition state structure has been given (Scheme 4).Macrolides synthesized from p-amino acids undergo the Claisen rearrangement to give chiral pyrrolidines via a boatlike transition state (Scheme 5).50 The tri-s-butyl aluminium catalysed Claisen rearrangement of bicyclic allyl Scheme 4 I I CO2Et CO2Et Scheme 5 vinyl ethers gives rise to ring expanded products with exocyclic vinyl ethers reacting uia a chairlike transition state and endocyclic ethers reacting via a boatlike transition ~tate.~' The asymmetric Claisen rearrangement of allyl vinyl ethers has been effected with a chiral organoaluminium species (13).52Conformational analysis of the two chairlike transition states reveals that the aluminium species must differentiate between them on the basis of the orientation of the methylene group of the substrates.A method has been described for the enantioselective Ireland-Claisen rearrangement of achiral allylic esters with high asymmetric induction using a chiral br~moborane.~~ Thus reaction of E-crotyl propionoate and the bromoborane (14) gave either the threo or erythro product depending upon base and solvent. The absolute and relative 48 R.E. Ireland P. Wipf and J. N. Xiang J. Org. Chem. 1991 56 3572. 49 P. Beslin and S. Perrio Tetrahedron 1991 47 6275. 50 J. Cooper D. W. Knight and P. T. Gallagher J. Chem SOC.,Perkin 1 1991 705. 51 L.A. Paquetta D. Fredrich and R. D. Rogers J. Org. Chem. 1991 56 3841. 52 K.Maruoka H. Banno and H. Yamamoto Tetrahedron; Asymmetry 1991 2 647. 53 E. J. Corey and D. H. Lee J. Am. Chem. Soc. 1991 113,4026. Reaction Mechanisms -Part (i) Pericyclic Reactions stereochemistry of the products can be rationalized in terms of the steric bias of the chiral auxiliary. The para-Claisen rearrangement of hydroxypropenyloxy benzaldehydes (15) has been studied.54 Three products are obtained the expected dihydroxypropenyl benzaldehydes (16) dihydroxypropenyl benzene (17) and dihydroxypropenyl ben- zaldehyde (18).The formation of the benzene is rationalized as occurring via a [ 1,2] shift of a formyl group in the ortho-cyclohexadiene intermediate. The formation of the latter benzaldehyde is proposed to occur via a [2,3] sigmatropic shift of the allyl group in the para-cyclohexadiene intermediate. The free phenol group is essential for the formation of these abnormal products. OH OH 3 OH The rearrangement of allyl vinyl ethers in lithium perchlorate in ether occurs via a [ 1,3] sigmatropic rearrangement rather than the normal [3,3] sigmatropic rearrangement .55 The oxy-Cope rearrangement of 1,5-heptadiene-3-ols has been studied.56 The reaction proceeds via chairlike rather than boat-like transition states and the difference in energy levels between them appears to be determined by an equatorially orientated oxido group.Secondary deuterium kinetic isotope effects indicate a highly dissociative transition state with substantial bond breaking of the C-3-C-4 bond 54 S. N. Kilenyi J. M. Mahaux and E. Van Durme .I.Org. Chem. 1991 56 2591. 55 P. A. Grieco J. D. Clark and C. T. Jagoe J. Am. Chem. SOC.,1991 113 5488. 56 L. A. Paquette and G. D. Maynard Angew. Chem. 1991 103 1392. 60 N. G. Ramsden and little bond making between the allylic termini.” The E/Z stereoselection in the reaction of the analogous 4-01s has been determined allowing a reasonable transition state to be p~stulated.’~ The [3,3] sigmatropic shift of the oxy-Cope rearrangement of norbornane derivatives proceeds with 100% stereoselectivity uia an endo chairlike transition state.59 The E and syn stereochemistry of the product enolates is established during chirality transfer at this stage and the oxygen up conformation stems from structural features present in the starting material.The E syn up enolates are thermodynamically unstable with respect to the E syn down atropisomers and the products may arise from reaction of these with electrophiles. Modelling of the Cope rearrangement using a CASSCF wavefunction and a 6-31G* basis set shows both a symmetrical aromatic transition state and a dissymmetric transition state followed by a symmetrical biradical intermediate.60 The ene reactions between chiral aP unsaturated oxazolidines and alkenes do not proceed with high diastereoselectivity as the reaction can occur uia endo or exo transition states and both syn and anti hydrogens can be abstracted.61 The reverse ene reaction of the cyclobutane (19) occurs via a concerted mechanism and the stereochemical outcome is consistent with a transition state with the alkenyl moiety endo with respect to the ring and the breaking C-H bond aligned with the breaking C-C bond.62 A kinetic study of the ene reaction between methyl acrylate and P-pinene under aluminium trichloride catalysis in a variety of solvents suggests that the transition state has pronounced zwitterionic ~haracter.~~ The circumambulatory migration of acyloxy groups or bromine around a cyclopentadiene ring has been elucidated.64y65 Optically active reactant can be recovered unchanged after the thermal rearrangement of the dicyclopropane (20).66However the product (21) is HH 4 Mew H OMe Et 50% racemized.This rules out reaction passing exclusively through a symmetrical biradical intermediate and formally the reaction data can be explained as two competing allowed a2s + a2a reactions passing over diastereomeric transition states to enantiomeric products. 1-t-Butyl-3-methylallene undergoes a thermal [1,3] sig-matropic rearrangement to 1-t-b~tyl-l,3-butadiene.~~ The mechanism is proposed 57 J. J. Gajewski and K. R. Gee J. Am. Chem SOC.,1991 113,967. 58 K. Tomooka S. Y. Wei and T. Nakai Chem. Lett. 1991 43. 59 L. A. Paquette K. D. Combrink S.W. Elmore and R. D. Rogers J. Am. Chern. SOC.,1991 113 1335. 60 M. Dupuis C. Murray and E. R. Davidson ibid. 1991 113 9756. 61 B. B. Snider and Q. Zhang J. Org. Chem. 1991 56 4908. 62 S. J. Getty and J. A. Berson J. Am. Chem. SOC.,1991 113 4607. 63 P. Laszlo and M. Teston-Henry J. Phys. Org. Chem. 1991,4 605. 64 I. E. Mikhailov G. A. Dushenko I. A. Kamenetskaya 0.E. Kompan Y. T. Struchkov and V. I. Minkin Mendeleev Commun. 1991 83. 6s V. I. Minkin I. E. Mikhailov G. A. Dushenko I. A. Yudilevich R. M. Minyaev A. Zschunke and K. Muegge J. Phys. Org. Chem. 1991 4 31. 66 M. D. Wendt and J. A. Berson J. Am. Chem. SOC.,1991 113 4675. 67 D. J. Pasto and J. E. Brophy J. Org. Chem. 1991 56 4554. Reaction Mechanisms -Part (i) Pericyclic Reactions to be concerted with migration of hydrogen to the 2p antibonding orbital on the central carbon atom of the allene and concomitant 90" rotations at the termini of the 1,3-butadiene system.Early transition state structures of the [2,3] Wittig rear- rangements of allylsulfonium methylides have been located with the E3-21(G)* and 6-31 + G" basis sets.68 They have envelope conformations with the slightly formed C-C bond nearly eclipsed with the partially broken C-S bond. The rearrangement of vinyl cyclopropanes to cyclopentenes has been studied. The use of chiral deuterium labelled trans-1-ethenyl-2-methylcyclopropanesin thermal rearrangements has shown that four stereochemically distinct pathways exist.69 There is no significant energetic preference for concert in evidence and orbital symmetry control of the stereochemical outcome does not seem to be a plausible explanation.To avoid steric influences of alkyl substituents on the cyclopro- pane ring deuterium labelled substrates (22) were used.70 The reaction was >85% stereospecific consistent with a disrotatory ring opening followed by a rapid ring closure of the diradical. High stereospecificity at each of the three stereogenic sites is obtained during the rearrangement of cis-2- (2-propy1)- 1 (E) propenyl cyclopro- pane (23).'* This result is predicted if the reaction is controlled by optimal overlap of C-H and 7r bonding orbitals with the C3 syn component of the degenerate 3E' HOMO of the cyclopropane ring. D Dehydrofluorinative aromatization of (24) occurs via a two step rnechani~rn.~~ The first step is a rate determining homolytic hydrogen shift.The [1,3] hydrogen shifts in propene have been studied by MO calculations for both the neutral molecule and the radical cation. The migratory process occurs antarafacially via a C3transition state. The barrier height is reduced from 358 kJ/mol to 139 kJ/mol in the radical cation due to a weakened C-H bond and a more favourable orbital interaction in the transition state. The distribution of localized orbital centroids suggests that proton transfer occurs in propene whilst hydride transfer occurs in the radical 68 Y. D. Wu and K. N. Houk ibid. 1991 56 5657. 69 J. E. Baldwin and N. D. Ghatlia J. Am. Chem SOC.,1991 113 6273.70 J. J. Gajewski and L. P. Olson ibid. 1991 113 7432. 7' P. A. Parziale and J. A. Berson ibid. 1991 113 4595. 72 W. R. Dolbier J. J. Kaeffaber C. R. Burkholder S. F. Sellers H. Koroniak and J. Pradhan Tetrahedron Lett 1991 32 3933. 73 M.T. Nguyen L. Landuyt and L. G. Vanquickenborne Chem. Phys. Lett. 1991 182 225.