CARBENOID CHARACTER IN TRANSITION STRUCTURES FOR REACTIONS OF KETENES WITH ALKENES

The reactions of ketene and methylketene with ethylene and of ketene with propene were studied with ab initio molecular orbital calculations, with the STO-3G, 3-21G, and 6-31G* basis sets and correlation energy corrections at the MP2 level. The cycloadditions proceed via geometries that indicate that the reaction is of the 2 + 2 + 2 type, rather than a [π2s + π2a] cycloaddition. It is a quasi-pericyclic reaction, like carbene cycloadditions and hydroborations, with appreciable interaction of carbon ketene central carbon with both termini of the alkene. Bond formation is very asynchronous, with bond lengths of 1.78 and 2.43 Å. There is appreciable charge separation in the transition structure. The stereochemical preferences of the methylketene and propene reactions are consistent with experimental results on related substituted cases. © 1990, American Chemical Society. All rights reserved.