Carbon–Carbon Bond Forming Reactions with Tantalum Diamidophosphine Complexes That Incorporate Alkyne Ligands

Abstract

The incorporation of o-phenylene-linked diamidophosphine ligands onto the readily available alkyne complexes Ta(alkyne)Cl3(DME) (where alkyne = hex-3-yne or 1,2-bis(trimethylsilylacetylene); DME = 1,2-dimethoxyethane) results in the formation of a versatile set of starting materials of the general formula [PhNPN*]Ta(alkyne)Cl (where [PhNPN*] = PhP(2-(N-mesityl)-5-Me-C6H3)2). Upon reaction with KBEt3H, the synthesis of the corresponding hydride complexes [PhNPN*]Ta(alkyne)H can be achieved; these complexes feature extremely downfield (δ ∼21 ppm) doublet resonances (2JHP = ∼35 Hz) in the respective 1H NMR spectra that are assigned to the newly formed Ta–H moieties. Subsequent reaction of these Ta hydrides with 2,6-dimethylphenylisocyanide and phenylacetylene results in the insertion of these species into the Ta–H bond and the formation of the corresponding iminoformyl and phenylvinyl complexes, respectively. While the former intermediate cannot be detected, the latter was characterized by NMR spectroscopy. Both of these processes result in the further transformation to generate C–C coupled products by a reductive elimination sequence with the coordinated alkyne; in the case of the iminoformyl, an azadiene results, whereas with the phenylvinyl derivative a butadienyl fragment is generated. Single-crystal X-ray diffraction and a suite of NMR spectroscopic techniques were used to characterize these species. A discussion of the bonding of the products in the context of the process by which they form is presented. The rate of formation of the butadienyl moiety from the phenylvinyl intermediate results in the activation parameters of ΔH⧧ = 22.2 ± 0.3 kcal/mol and ΔS⧧ = −8.7 ± 0.2 cal/(mol)(K).