Exploring Small Bite-Angle Ligands for the Rhodium-Catalyzed Intermolecular Hydroacylation of β‑S-Substituted Aldehydes with 1‑Octene and 1‑Octyne

A comparative study of seven crystallographically
characterized
rhodium precatalysts, which contain a variety of chelating diphosphine
ligands, for the hydroacylation of 1-octyne or 1-octene with 2-(methylthio)­benzaldehyde
has been undertaken. These studies show that the best performing catalyst
for 1-octyne, [Rh­(L)­(η6-C6H5F)]­[BArF4], L = iPr2PNMePiPr2, delivers alkyne selective hydroacylation with
high efficiencies at low loadings (1 mol %, 2.0 M aldehyde, 25 °C,
ToN = 100, 97% conversion in 5 min), and also shows high selectivity
for the linear product. Experiments suggest that the alkyne selectivity
arises from the alkyne being more competitive for metal binding compared
to the alkene. Labeling experiments using the [Rh­(tBu2PCH2PtBu2)­(η6-C6H5F)]­[BArF4] system,
that gives the final product in a linear:branched ratio of 6:1, indicate
that the pathway that produces the branched product operates via an
irreversible hydride insertion. Intermediate acyl hydride complexes,
[Rh­(L)­(H)­(COC6H4SMe)­(acetone)]­[BArF4], have been characterized by low temperature NMR spectroscopy,
as have their subsequent reductive decarbonylation products, one of
which has also been crystallographically characterized: [Rh­(iPr2PNMePiPr2)­(SMePh)­(CO)]­[BArF4].