The mechanism of an aromatic C-H coupling reaction between heteroarenes and arylboronic acids using a Pd catalyst was theoretically and experimentally investigated. We identified the C-B transmetalation as the rate-determining step. The (S)-catalyst-reactant complex was found to be stabilized by hyperconjugation between π-orbitals on the tolyl group and the S-O σ* antibonding orbital in the catalyst ligand. Our findings suggest routes for the design of new, improved Pd catalysts with higher stereoselectivity.