Intermolecular Alkyne Hydroacylation. Mechanistic Insight from the Isolation of the Vinyl Intermediate That Precedes Reductive Elimination

The isolation of the branched alkenyl intermediate that directly precedes reductive elimination of the final α,β-unsaturated ketone product is reported for the hydroacylation reaction between the alkyne HC≡CArF (ArF = 3,5-(CF3)2C6H3) and the β-S-substituted aldehyde 2-(methylthio)benzaldehyde: [Rh(fac-κ3-DPEphos)(C(═CH2)ArF)(C(O)C6H4SMe)2][CB11H12]. The structure of this intermediate shows that, in this system at least, hydride migration rather than acyl migration occurs. Kinetic studies on the subsequent reductive elimination to form the crystallographically characterized ketone-bound product [Rh(cis-κ2-DPEphos)(η2:η2,κ1-H2C═C(ArF)C(═O)(C6H4SMe)][CB11H12] yield the following activation parameters for reductive elimination, which follows first-order kinetics (kobs = (6.14 ± 0.04) × 10–5 s–1, 324 K): ΔH⧧ = 95 ± 2 kJ mol–1, ΔS⧧ = −32 ± 7 J K–1 mol–1, ΔG⧧(298 K) = 105 ± 4 kJ mol–1. Mechanistic studies, including selective deuteration experiments, show that hydride insertion is not reversible and also reveal ...