Highly efficient monophosphine platinum catalysts for alkyne diboration

Abstract

Automated parallel screening using a series of in situ generated platinum(0) phosphine complexes has allowed the identification of improved catalysts for the diboration of alkynes using bis(pinacolato)diboron (B₂(pin)₂, pin = OCMe₂CMe₂O). A selection of phosphines were added to [Pt(NBE)₃] (NBE = norbornene), which contains only labile mono-olefin ligands, and the activity of the resulting solutions as catalysts for the diboration of 4-CF₃C₆H₄CCC₆H₄CF₃-4′ 1 by B₂(pin)₂ was investigated by in situ GC-MS and/or NMR spectroscopy. This allowed the optimum phosphine ∶ platinum stoichiometry to be identified as 1 ∶ 1, and the large differences in catalyst activity depending on the nature of the phosphine to be quantified. The best phosphines employed in the study, PCy₃ and PPh₂(o-Tol) (o-Tol = C₆H₄Me-o), give activities orders of magnitude greater than the worst, such as P(C₆F₅)₃ and PBu^t₃. The monophosphine catalysts function much more efficiently than previous catalysts for a range of alkynes allowing diborations to be performed at ambient temperatures. The diboration of strained cyclic alkenes and some vinyl- and allyl-arenes proceeded well, although the catalysts were inactive for other olefinic systems examined. As a result of these studies, the isolable and stable compound [Pt(PCy₃)(η²-C₂H₄)₂] was identified as an excellent catalyst for alkyne diboration even at room temperature.

p