CASSCF Molecular Orbital Calculations Reveal a Purely Pseudopericyclic Mechanism for a [3,3] Sigmatropic Rearrangement

A comparative CASSCF/6-31G*-level computational study of the concerted [3,3] sigmatropic rearrangements of cis-1-iminyl-2-ketenylcyclopropane (15), cis-1-iminyl-2-propadienylcyclopropane (17), and cis-1-iminyl-2-keteniminylcyclopropane (19) to give products 16,18, and 20, respectively, was conducted Analysis of the active space MOs of TS15 -> 16, TS17 -> 18, and TS19 -> 20 suggests that the 17 -> 18 and 19 -> 20 rearrangements are classically pericyclic, whereas the 15 -> 16 rearrangement is pseudopericyclic with two orbital disconnections-one involving the nitrogen lone-pair orbital and the other the carbonyl carbon of the ketene moiety The novel TS15 -> 16 was also found to have a highly planar, tight, geometry, whereas TS17 -> 18 and TS19 -> 20 were both shown to have the boat-shaped geometry expected for classically pericyclic [3,3] sigmatropic rearrangements Results of calculations on the [3,3] sigmatropic rearrangements involving additional transition structures, TS21 -> 22, TS23 -> 24, TS25 -> 26, TS27 -> 28, TS29 -> 30, and TS31 -> 32, demonstrate the relative uniqueness of the pseudopericyclic one, TS15 -> 16