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 TS(15-->16), TS(17-->18), and TS(19-->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 TS(15-->16) was also found to have a highly planar, tight, geometry, whereas TS(17-->18) and TS(19-->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, TS(21-->22), TS(23-->24), TS(25-->26), TS(27-->28), TS(29-->30), and TS(31-->32), demonstrate the relative uniqueness of the pseudopericyclic one, TS(15-->16).
Forte
Lila
L
Department of Chemistry, Lewis & Clark College, Portland, Oregon 97219-7899, USA.
Lafortune
Marie C
MC
Bierzynski
Irena R
IR
Duncan
James A
JA