Highly Selective Halide Anion-Promoted Palladium-Catalyzed Hydroformylation of Internal Alkenes to Linear Alcohols

Here we report on our study of the palladium-catalyzed hydroformylation of alkenes. A (bcope)Pd(OTf)2 complex (bcope = bis(cyclooctyl)phosphine ethane, 2) with substoichiometrically added halide anions is a highly efficient homogeneous catalyst (precursor) to selectively convert internal linear alkenes into predominantly linear (detergent) alcohols under mild conditions. Halide anion-dependent effects on the hydroformylation reaction rate as well as its chemo- and regioselectivity are observed. Thus, the rate of hydroformylation of thermally equilibrated internal higher alkenes increases by a factor of about 6−7 with chloride/bromide and about a factor 3−4 with iodide, while the selectivity toward alcohols increases to almost 100% upon addition of a substoichiometric quantity (with respect to palladium) of the halide anion source. Curiously, the regioselectivity toward linear alcohol increases in the reverse order, i.e., iodide > bromide > chloride. From a detailed analysis of the products obtained with model substrates, it is concluded that hydrogenolysis of (bcope)palladium-acyl intermediates is strongly accelerated by the presence of halide anions. From a comparison of the catalytic performance with some related L2Pd(OTf)2 complexes, in which L2 are bidentate phosphines closely related to bcope, it also appears that the ligand plays a critical role in the promoting effect of halide anions.