Exploring small bite-angle ligands for the rhodium-catalyzed intermolecular hydroacylation of βs-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)][BAr^F₄], 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(^tBu₂PCH₂P^tBu₂)(η⁶-C₆H₅F)][BAr^F₄] 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)][BAr^F₄], 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)][BAr^F₄].