| 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. |
