DIFFERENT TRANSITION STRUCTURES FOR [2,3]-WITTIG REARRANGEMENTS OF STABILIZED AND UNSTABILIZED ALLYLOXY METHYL ANIONS - RATIONALE FOR THE DICHOTOMOUS SENSE OF STEREOSELECTION

Transition structures of [2,3]-Wittig rearrangements of allyloxymethyl anion, allyloxypropargyl anion, allyloxyacetaldehyde anion, and their 1-methyl (crotyl) and 3-methyl analogs have been located. The transition structure of the rearrangement of allyloxymethyl anion is extremely early, with the C---C bond nearly unformed, and largely reflects inversion of the carbanion center and breaking of C-3---O-4 bond. The transition structure becomes much different when the anion is stabilized by an ethynyl substituent; now the C---C forming bond is 2.3 Angstrom A, and the C---O bond is only slightly broken. The ethynyl group is calculated to strongly prefer exo orientation. In agreement with experimental observations, formation of Z-alkene is calculated to be slightly favored for the rearrangment of 3-methyl-allyloxymethyl anion, while E-alkene is formed exclusively when the anion is stabilized by an ethyl group. The ethynyl group also shows the exo preference in E- and Z-crotyl ether cases, while the formyl group prefers the endo position in the E-crotyl system. Thus, not only the general trend of E to anti and Z to syn diastereoselection but also the anomalous sense of E to syn selectivity for carbonyl substituents can be rationalized.