INVESTIGATION OF THE 6-METHYLENEBICYCLO(3.1.0)HEX-3-EN-2-ONE META-QUINOMETHANE VALENCY TAUTOMERIC PAIR.

摘要

The mechanism of the reaction of 6-methylenebicyclo 3.1.0 hex-3-en-2-one (1) with electron rich trapping reagents (alcohols, alkyl-and alkoxy-substituted olefins) has been studied. These reactions and first-order in 1 and zero-order in the added trapping reagent, as determined by kinetic methods which give the Arrhenius equation: k(sec('-1)) = 10('14.0)exp(-30,600 + 600/RT for the disappearance of 1. The kinetic behavior requires the production of an intermediate in the rate-determining step with the bimolecular product-forming process occurring in a subsequent step. Three candidates for the kinetically-formed intermediate, triplet m-quinomethane (2-T), which has been determined to be the ground state of this system by a Curie Law study of the triplet EPR signal, 2-methylenebicyclo 3.1.0 hex-3-en-2-one (76), a valency tautomer of m-quinomethane, and singlet m-quinomethane (2-S), are critically examined. Only 2-S is consistent with the kinetic data.;Further support for the singlet nature of the product-determining intermediate is obtained by comparison of the products observed from the reaction of 1 or m-xylylene with cis, cis- or trans, trans-2,4-hexadiene. The retention of stereochemistry observed in the reaction of 1 compared to the loss of stereochemistry observed in the reaction of m-xylylene is consistent with the reactions of 1 proceeding along the singlet manifold.;Reaction of 1 (115(DEGREES)C) with cis-1,2-dimethoxyethene gives rise to a 17:1 mixture of cis- and trans-1,2-dimethoxy-7-hydroxyindan and a 14:1 mixture of cis- and trans1,2-dimethoxy-5-hydroxyindan while reaction with the trans olefin gives rise to the two regioisomeric indans possessing only trans stereochemistry. The effect of temperature, solvent, and molecular oxygen on the product stereospecificity suggests that the product-determining intermediate is the singlet m-quinomethane. The reactivity toward electron-rich trapping reagents suggests substantial zwitterionic character in this species. These studies further indicate that the products are formed in a two-step process where the second step very rapidly follows the first. The relative rate of the second step has been probed by examining the products obtained from the reaction of 1 with (beta)-pinene. None of the products expected from a fragmentation competitive with ring-closure are observed. This is consistent with a very short-lived intermediate.