The mechanistic pathways for the [1,4] and [1,2] Wittig rearrangements of 2-silyl-6-aryl-5,6-dihydro-(2H)-pyrans
have been studied at the M06-2X/6-31+G(d,p), 6-311++G(d,p) and cc-pVTZ level of theory.
The crucial C-O bond cleavage step in the mechanism has been analysed initially, using
two model reactions covering aliphatic as well as cyclic allylic ethers. The barriers
for the one-step as well as two-step pathways have been calculated and the mechanisms
for both the [1,4] and [1,2] Wittig rearrangement reactions are predicted to occur
through a two-step mode. An energetic analysis of the reaction pathways reveals that
the [1,4]-rearrangement has a lower barrier than the [1,2]-Wittig rearrangement. The
C-O cleavage transition state was found to have the highest barrier and is thus the
rate determining transition state for all of the studied molecules. This is in agreement
with the previously published experimental studies. The role of the allylic trimethylsilane
group in the stabilization of the intermediate anions of the Wittig reactants has
also been investigated while comparing it with the phenyl and allylic t-butyl groups
through Natural Bond Orbital (NBO) calculations.