The development of positive charge β to silicon in a saturated five‐membered ring has been studied in the cis‐ and trans‐2‐(trimethylsilyl)cyclopentyl trifluoroacetates. The cis substrate solvolyzes in 97% trifluoroethanol at 25° C about 5 times faster than the analogous six‐membered ring, after correction for differences in ring strain. The trans substrate solvolyzes about 360 times more slowly than the analogous six‐membered ring. These changes are in agreement with a hyperconjugative mechanism for interaction between the silyl group and the developing positive charge. The expected cosine‐squared dependence of hyperconjugation on the SiCCX dihedral angle suggests that the cis dihedral angle is reduced somewhat from the 60° in the six‐membered ring, and the trans dihedral angle is reduced substantially from the 180° in the six‐membered ring.

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