= Solution
The opposite insertion orientation gives a benzylic <metal>–<carbon> bond, $\mathrm{Co{-}CH(Ph)CH_3}$. C–Si <reductive elimination> then gives the branched product
$$
\boxed{\mathrm{PhCH(SiR_3)CH_3}.}
$$
A plausible explanation is stabilization of the developing and final benzylic Co–C interaction by the adjacent aromatic pi system, which can outweigh the steric preference for a primary alkyl. That makes an electronically different choice from an ordinary alkyl substituent. This is a mechanistic rationale for the proposed cycle, not proof of its rate-determining step. In the absence of a chiral <catalyst> or another source of asymmetric induction, the new stereogenic <carbon> would give a <racemic mixture>.
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