Carbon-carbon and carbon-hydrogen bond activation at ditungsten centers supported by alkoxide ligands

Abstract

Alkoxide ligands, which are π-donating ligands, complement the behavior of traditional organometallic ligands, which are π-acceptors. Steric control in the determination of coordination number and substrate binding can be introduced by the specific choice of alkoxide ligand. The compounds W 2 (0R) g contain two important functionalities, the alkoxide group and the (W=W) 6+ unit, and form versatile templates for organometallic chemistry. The reactions between W 2 (0R) 6 compounds and alkynes give rise to a variety of products derived from either metathesis of the W=W and C=C bonds, carbon-carbon or carbon-tungsten coupling reactions depending upon the alkoxide ligand and the substituents on the alkyne. The addition of ethylene to W 2 (0R) 6 compounds proceeds via a sequence of reversible steps involving olefin adducts, metallacyclopentanes and ultimately alkylidyne ligands by C-H activation and elimination of ethane. The compounds 1,2-R 2 W 2 (OR) 4 contain three functionalities of interest: (i) the ancillary alkoxide ligands, (ii) the central (W=W) 6+ moiety and (iii) metal-alkyl bonds. The reactions between 1,2-R 2 W 2 (OR) 4 compounds and alkynes give rise to a variety of products depending upon the steric properties of R and R’, the lack or presence of β-H atoms in the R’ group and the nature of the alkyne. Examples include (i) (Pr i 0) 2 (Me 3 CCH 2)W=CEt by W=W and C=C bond metathesis, (ii) W 2 (μ-CCMe 3) (H) (OPr 1) 4 (μ-C 4 Me 4) by C-H activation and C-C bond formation, (iii) alkyne adduct formation, e.g. W 2 (OPr i) 4 (CH 2 Ph) 2 -(r/ 2 -C 2 R 2) 2, where R - Me and Et, and (iv) W 2 (μ-C 2 Me 2) 2 (OPr i) 4 by alkyl group disproportionation or M-C bond homolysis. Mechanistic studies of the above are discussed on the basis of results obtained from both isotopic labelling experiments and kinetic studies. © 1989 IUPAC