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 alpha,beta-unsaturated ketone product is reported for the hydroacylation reaction between the alkyne HC CArF (Ar-F = 3,5-(CF3)(2)C6H3) and the beta-S-substituted aldehyde 2-(methylthio)benzaldehyde: [Rh(fac-kappa(3)-DPEphos)(C(=CH2)-Ar-F)(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-kappa(2)-DPEphos)(eta(2):eta(2),kappa(1)-H2C=C(Ar-F)C(=O)(C6H4SMe)-[CB11H12] yield the following activation parameters for reductive elimination, which follows first-order kinetics (k(obs) = (6.14 +/- 0.04) x 10(-5) s(-1), 324 K): Delta H-double dagger = 95 +/- 2 kJ mol(-1), Delta S-double dagger = -32 +/- 7J K-1 mol(-1), Delta G(double dagger)(298 K) = 105 +/- 4 kJ mol(-1). Mechanistic studies, including selective deuteration experiments, show that hydride insertion is not reversible and also reveal that an interesting isomerization process is occurring between the two branched alkenyl protons that is suggested to occur via a metallocyclopropene intermediate. During catalysis, the consumption of substrates and evolution of products follow pseudo zero-order kinetics. The observation of both linear and branched products under stoichiometric and catalytic regimes, in combination with kinetic modeling, allows for an overall mechanistic scheme to be presented. Partitioning of linear and branched pathways at the hydride insertion step occurs with an approximate 2:1 selectivity, while reductive elimination of the linear product is at least 3 orders of magnitude faster than that from the branched. An explanation for the large difference in rate of reductive elimination in this system, as recently outlined by Goldman, Krogh-Jespersen, and Brookhart, is that steric crowding in branched intermediates can slow C-C reductive elimination even though such species are higher in energy than their linear analogues, if the rotation of the vinyl group to the appropriate orientation is inhibited by steric crowding in the branched isomers.