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 CC bond nearly unformed, and largely reflects inversion of the carbanion center and breaking of C₃O₄ bond. The transition structure becomes much different when the anion is stabilized by an ethynyl substituent; now the CC forming bond is 2.3 Å, and the CO 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 ethynyl 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.