A [(sec-allyloxy)dimesitylsilyl]stannane bearing a phenyl group on the olefin part reacts with n-BuLi in THF to give a cyclopropylsilane as a single diastereomer, in contrast to the [2,3]-sila-Wittig rearrangement affording an allylsilane, previously observed for a substrate bearing an alkyl group on the olefin part. The striking substituent effect is revealed by ab initio calculations in terms of the regioselectivity in the reaction of silyllithiums with an olefin.