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 L-2 are bidentate phosphines closely related to bcope, it also appears that the ligand plays a critical role in the promoting effect of halide anions.