Reactivity of cyclopentenyl anion analogous heterocycles: 1,5‐electrocyclization of 2‐oxa‐, 2‐thia‐, 2‐aza‐ and 2‐phosphabicyclo[3.2.0]hept‐3‐ene. A sigmatropic [1,3] carbon shift

Abstract

Carbonyl ylide-like intermediates are involved in the 1,5-electrocyclization of the bicyclo[3.2.0]heptenes 3a-c. The activation barriers analyzed by the time-and temperaturedependence of the exo * endo isomerization of specifically deuterated derivatives or of the racemization of optically active derivatives turned out to be higher by AAG* 3 11 kcal/ mol than those determined for the corresponding bicyclo [3.1.0]hexenes la-c. This result can be considered as an evidence for the electrocyclic nature of these ring openings due Carbonyl ylides, thiocarbonyl ylides, and azomethine ylides -formally the products of the oxirane, thiirane, or aziridine ring opening -have attracted attention as reaction partners in 1,3-dipolar cycloaddition~ [~-~]. The corresponding ylides 2a-d have recently been shown to be involved in the 1,5-electrocyclization of the heterocycles la-d[6371. At 120°C the ring opening of homothiophene l b and homopyrrole l c proceeds 63100 and 72 times faster and that of homophosphole 4,5-(CH3)2-ld slower by a factor of 0.0016 than that of homofuran l a . In the corresponding benzo-substituted the heteroatom effect on the rate of ring opening is less pronounced than in the parent systems la-d. At 120°C the ring opening in benzohomothiophene is faster only by a factor of 8.7 and in homoindole even slower by a factor of 0.033 than that in benzohomofuran. This observation has led to the conclusion that the different rates observed for ring opening in la-c is the result of the heteroatom effect on the ground state rather than on the carbonyl ylide-like intermediates.In this paper we report on the ring opening of the 2-hetera-substituted bicyclo[3.2.0]hept-3-enes 3a-d. The replacement of the three-membered by a four-membered ring should provide further insight into the nature of the ringopening reaction. In the case of a simple homolytic bond dissociation similar activation barriers are expected for the cleavage of corresponding cyclopropane and cyclobutane derivatives due to the comparable cyclopropane and cyclobutane strain energy [']. In the case of an orbital symmetrycontrolled electrocyclic ring opening the activation barriers to the diminished Walsh character of cyclobutane bonds compared to cyclopropane bonds. A stereochemical analysis of the fragmentation of 2-oxabicyclo[3.2.0]heptene 3a to furan and ethene leads to the conclusion that a sigmatropic [1.3] carbon shift proceeding with inversion of the migarting carbon followed by stereospecific retro-Diels-Alder reaction is the major pathway for this reaction similar to the rearrangement and fragmentation of the corresponding carbocycle 3e.of the cyclopropane derivatives should be significantly lower than those of the corresponding cyclobutane derivatives, because the Walsh-type molecular orbitals of the cyclopropane ring can interact with the heteroatom lone pair and the molecular orbitals of the bond in an earlier stage of the reaction than those of the cyclobutane ring which show only little Walsh character. D2-3a, endo-D2-3a) and the