Theoretical study of the mechanisms for epoxidation and ketene addition reactions of strained cyclic olefins.

摘要

This work is directed at the use of ab initio computational methods to help understand the properties and reactions of strained olefins. Of particular interest are (1) the I-strain energy of olefins and its structural cause, (2) the effect of I-strain on the reactivities of these olefins, (3) the I-strain effect on the mechanism for peracid epoxidation of strained olefins and subsequent epoxide rearrangements, and (4) the mechanism for addition of ketenes to cyclopropenes.;The I-strain contents of several strained cyclic olefins were calculated and compared with the experimentally obtained values. Additionally, our calculations support the idea that the extra strength of the sp2 C-H bond compared to the sp3 C-H bond, is the factor mainly responsible for I-strain rather than smaller angular-distortion effects.;Rearrangement reactions have been found experimentally in the epoxidation of some highly-strained cyclobutenes, and we have computationally tested the hypothesis that such reactions might involve bifurcations at or near the transition states.;The calculated reaction surface for the reaction of bicyclo[2.2.0]hex-1(4)-ene with peroxyformic acid at the MP2/3-21G and MP2/6-31+G(d,p) levels of theory shows a planar transition state, and a hydroxylated carbocation structure, the latter being a point on the surface downhill from the former that corresponds to transfer of an OH+ moiety, leading to a rearranged spiroketone. Another transition state connecting the products could be close to the structure of a hypothetical point of bifurcation, on the surface.;We have studied the energy surfaces for acid-catalyzed rearrangements for oxirane, 2,2-dimethyloxirane, 2-oxabicyclo[1.1.0]butane, and 5-oxabicyclo[2.1.0]pentane. Computation of the energy surface for the acid-catalyzed ring opening of the exo-5-oxabicyclo[2.1.0]pentane indicates that the pathway to the cyclopropanecarboxaldehyde product concerted, while cyclobutanone product is formed through a stepwise mechanism involving an unusual cationic intermediate with a bridged hydrogen. The two transition-state structures for the above-mentioned transformations are very similar in their energies and geometries. These structures are in a flat region of the surface, near to a rarely observed bifurcation point, between the aldehyde and the intermediate.;Calculations on transition states for cyanoketene additions to cyclopropenes show the expected "orthogonal" polar transition-state structures, but with no evidence of the rearrangement reactions observed experimentally on the way to the product cyclobutanones.