Deuterium Kinetic Isotope Effects and Mechanism of the Thermal Isomerization of Bicyclo[4.2.0]oct-7-ene to 1,3-Cyclooctadiene

Abstract

The thermal conversion of cis-bicyclo[4.2.0]oct-7-ene to cis,cis-1,3-cyclooctadiene might involve a direct disrotatory ring opening, or it might possibly take place by way of cis,trans-1,3-cyclooctadiene. This cis,trans-diene might possibly form the more stable cis,cis isomer through a [1,5] hydrogen shift or a trans-to-cis isomerization about the trans double bond. Deuterium kinetic isotope effect determinations for the isomerizations of 2,2,5,5-d(4)-bicyclo[4.2.0]oct-7-ene and 7,8-d(2)-bicyclo[4.2.0]oct-7-ene rule out these two alternatives because the observed effects are much smaller than would be anticipated for these mechanisms: k(H)/k(D)(d(4)) at 250 degrees C is 1.17 (1.04 per D), and k(H)/k(D)(d(2)) at 238 degrees C is 1.20 (1.10 per D). The direct disrotatory ring opening route remains the preferred mechanism.

Electrocyclic Ring Opening of cis‐Bicyclo[m.n.0]alkenes: The Anti‐Woodward–Hoffmann Quest

“…This is in good agreement with recent experimental results from Baldwin [6] and previous experiments by Bramham and co-worker. [10] The concerted, disrotatory, ring opening seems to be noncompetitive as well.…”

“…Kinetic isotope effect experiments performed by Baldwin and co-workers. [5,6] www.chemeurj.org k D = 1.04 per deuterium, but we consider that this approximation could only apply if the reaction proceeds through the disrotatory ring opening, for which the four labels are roughly equivalent. But it is clearly inappropriate to analyze the [1,5]-H shift since only one deuterium label is involved in the primary KIE whereas the other three labels would contribute to the kinetic isotope effect in a lower extent (they would promote b and g secondary isotope effects, probably in opposite directions due to the gain of coordination at one terminus of the pericyclic reaction and the loss of coordination at the other).…”

“…In previous work [6] it was assumed that a considerable KIE would be expected, due to the deuterium label on the trans-double bond, if the ratelimiting step involves a cis,trans!cis,cis isomerization through internal rotation. In fact, several experimental KIE values for cis!trans isomerizations of substrates with deuterium labels on the isomerizing alkene groups have been reported with k H /k D values of around 2.0 at room temperature.…”

“…Very recently Baldwin and co-workers published a number of papers aimed to definitively address this issue. [5,6] After collecting data extracted from several kinetic studies, Baldwin and co-workers drew a tentative energy profile which combine the Arrhenius parameters for the overall reaction obtained by Branton and co-workers, [7] and are shown in Equation (1).…”

“…[6] Prompted by the relevance of the challenge, we theoretically addressed the mechanistic issues of the 1 a to 5 a isomerization. [11] We showed by high-level computations that the ring opening of 1 a obeys the Woodward-Hoffmann rules, and proceeds stepwise through a conrotatory motion followed by a double-bond isomerization (Scheme 1, right).…”