Beyond butadiene: activation parameters for thermal stereomutations and cycloreversion of cyclobutanes from a photochemical cyclodimerization of the rigid, all-trans triene 4a-methyl-2,3,4,4a,5,6-hexahydro-2-methylenenaphthalene

Abstract

Enthalpies of stabilization of polyenyl radicals, recently determined by thermal cis-trans isomerization about double bonds, are incorporated into a model for thermal cyclodimerization (and its reverse, cycloreversion) of polyene based on the ethylene-cyclobutane paradigm. This model accommodates the butadiene-divinylcyclobutane equilibrium satisfactorily. When the model is applied to polyenes of higher order, enthalpies of activation in both directions are predicted to be strikingly lowered as the order of the polyene increases. Photochemical dimerization at -75 O C of optically pure triene (R)-3 of the title, produces a single dimer (of three allowed), to which a cyclobutane structure in the anti and, tentatively, endo,endo configuration, (R,R)-4a,n,n, is assigned. The remarkable stereospecificity is ascribed to an intermediate excimer, in which maximum overlap and minimal steric repulsion are hypothetically achieved in either of the two incipiently anti arrangements. Irradiation of racemic (R,S)-3 at -75 O C produces two of six possible isomers, now racemic 4 , , , and racemic 4,,,. Each of the photochemically favored cyclobutanes begins to equilibrate above -20 O C (very rapidly at 20 "C and above) with the other two stereoisomers in its set. From rates measured between -22.7 and 0.0 O C , Eyring parameters, AH* = 21.2 kcal/mol and AS* = 2.7 eu, are obtained. In the temperature range of 0.0 to 22.0 O C , the mixture of dimers, continuously maintained at equilibrium, undergoes thermal, [2 + 21 cycloreversion to the triene (R)-3 with Eyring parameters, AH* = 24.2 kcal/mol and AS* = 5.8 eu.Although syn-anti rearrangement competes with cycloreversion in divinylcyclobutanes, the gap in enthalpy of activation between the two paths is significantly larger in these dibutadienylcyclobutanes. (7) Energies in Schemes I and I1 are in units of kcalfmol. AfHo 2(+12.56) = +25.1 exp AfH* +68.6 AH* +61.8 H k AfHo +6.8 I i 2(-3.74) t -% AfHo 2(+26.11) = +52.2 exp AH* +23.0

Evidence for Pericyclic and Stepwise Processes in the Cyclodimerization of Chloroprene and 1,3-Butadiene from Pressure Dependence and Stereochemistry. Experimental and Theoretical Volumes of Activation and Reaction

“…The enthalpies of formation derived for the transition states of the [4 + 2] as well as the [2 + 2] cycloaddition from the known kinetic and thermochemical data (Table 3) are close to or even higher than those calculated for the potential diradical intermediates ( , -), ( , -), and ( , )-19 by using the force-field MM2ERW parametrized by Roth et al18 for the allyl radical. Doering, Belfield, and He19 obtained similar results by estimating the activation enthalpy for the dimerization of 1,3-butadiene to the diradical 19 starting from the activation enthalpy of the dimerization of ethene to cyclobutane (Table 3b). In their ab initio study, Li and Houk20 also predict the stepwise mechanism to be slightly favored, by 1.3 kcal/mol, over the pericyclic pathway (Figure 3).…”

“…Pressure Dependence and Stereochemistry of the Dimerization of 1,3-Butadiene In the dimerization of 1,3-butadiene, competition among [4 + 2], [2 + 2], and a formal [4 + 4] cycloaddition leads to 4-vinylcyclohexene (16), frans-divinylcyclobutane (17), and 1,5cyclooctadiene (18), respectively.16 Many studies have been undertaken to answer the question of whether the Diels-Alder product, 4-vinylcyclohexene (16), is formed in a pericyclic or Mff°(TS) = A//f°(starting material) + AH*(298 K) and the enthalpies of formation AHflCaicde calculated for the diradicals 19 (a) by MM2ERW18b or (b) by a thermochemical estimate starting from the enthalpy of activation of the ethene dimerization19 (all enthalpy values in kcal/mol). b Enthalpy of activation determined for the dimerization of ethene to cyclobutane.c Energies of conjugation between a pair of conjugated double bonds.…”

Analysis of highly disfavored processes through pathway-specific correlated fluorescence

“…In the context of the Woodward and Hoffmann rules, the thermal retro-[2ϩ2]-cycloaddition is symmetry forbidden and therefore proceeds through an intermediate whose character lies between that of a diradical and zwitterion (4)(5)(6). As shown in Scheme I, 3-methylenecyclohexene is thermally stable; however, its photochemically generated spirocyclobutanes readily undergo thermal reversal (7). For the most part, the regiochemical outcome of methyl-and vinyl-substituted cyclobutane fragmentation can be described by the stability of its intrinsic diradical intermediate(s) (8,9).…”

The Cyclohexadienyl Radical in the Thermal Syn−Anti Isomerization of Two Crossed Pentaenes of the Type of Bis-Homofulvalene

“…6 Another sighting on the W and U configurations of the pentadienyl radicals is obtained by examination of the cycloreversion of cyclobutane 9 and comparison with 10. 35 As shown in Scheme 4, the cyclobutane 9 can be produced by sensitized irradiation at -75 °C of 4,4-dimethylmethylenecyclohexa-2,5diene. Although no distinction can be made between head-tohead or head-to-tail structure on the basis of the NMR spectrum, the rapid rate of disappearance of 9 and reappearance of the monomeric triene at -31.6 °C is better reconciled by the head- to-head structure.…”

“…Another sighting on the W and U configurations of the pentadienyl radicals is obtained by examination of the cycloreversion of cyclobutane 9 and comparison with 10 . As shown in Scheme , the cyclobutane 9 can be produced by sensitized irradiation at −75 °C of 4,4-dimethylmethylenecyclohexa-2,5-diene.…”