Activator-free single-component Co(I)-catalysts for regio- and enantioselective heterodimerization and hydroacylation reactions of 1,3-dienes. New reduction procedures for synthesis of [L]Co(I)-complexes and comparison to in situ generated catalysts.

Although cobalt(I) bis-phosphine complexes have been implicated in many selective C-C bond-forming reactions, until recently relatively few of these compounds have been fully characterized or have been shown to be intermediates in catalytic reactions. In this paper we present a new practical method for the synthesis and isolation of several cobalt(I)-bis-phosphine complexes and their use in Co(I)-catalyzed reactions. We find that easily prepared (in situ generated or isolated) bis-phosphine and (2,6-N-aryliminoethyl)pyridine (PDI) cobalt(II) halide complexes are readily reduced by 1,4-bis-trimethylsilyl-1,4-dihydropyrazine or commercially available lithium nitride (Li3N), leaving behind only innocuous volatile byproducts. Depending on the structures of the bis-phosphines, the cobalt(I) complex crystallizes as a phosphine-bridged species [(P∼P)(X)CoI[μ-(P∼P)]CoI(X)(P∼P)] or a halide-bridged species [(P∼P)CoI[μ-(X)]2CoI(P∼P)]. Because the side-products are innocuous, these methods can be used for the in situ generation of catalytically competent Co(I) complexes for a variety of low-valent cobalt-catalyzed reactions of even sensitive substrates. These complexes are also useful for the synthesis of rare cationic [(P∼P)CoI-η4-diene]+ X- or [(P∼P)CoI-η6-arene]+ X- complexes, which are shown to be excellent single-component catalysts for the following regioselective reactions of dienes: heterodimerizations with ethylene or methyl acrylate, hydroacylation and hydroboration. The reactivity of the single-component catalysts with the in situ generated species are also documented.