Hydroboration of metal–carbon triple bonds

Abstract

Hydroboration reactions of Fischer carbyne metal complexes [M(CR)X(CO)₂] (X = η-C₅R₅ or tpb′; tpb′ = tris(3,5-dimethylpyrazolyl)hydroborate) were studied. Depending on R and X two types of hydroboration products were formed, namely the α-boryl-η³-benzyl metal complexes [M(C₅R₅)(CO)₂{(BEt₂)C(H)(C₆H₄Me-4)}] (M = W, R₅ = Me₄Et; M = Mo, R = Me) and the novel borylmetal complexes [M(tpb′)(CO)₂{η²-B(R′)CH₂R}] (M = W, R′ = Et, R = C₆H₄Me-4 or Me; M = Mo, R′ = Et, R = C₆H₄Me-4; M = W, R′ = Ph, R = C₆H₄Me-4 or Me). β-Agostic CHM interactions are present between the metal and the boryl ligands. Labelling experiments involving “DBEt₂” showed selective transfer of deuterium to the former carbyne carbon, with only deuterium participating in the agostic interaction. From [W(CR)(C₅Me₅)(CO)₂] 2 (R = Me or SiPh₃) and “HBEt₂” the complex trans-[W(C₅Me₅)(CO)₂(H)(C₂H₄)] was obtained as the sole organometallic compound. Deuterium labelling experiments confirmed transfer of hydrogen from the borane to the carbyne complex. In polar solvents, [W(η-C₅Me₅)(CO)₂{(BEt₂)C(H)C₆H₄Me-4}] and the η-C₅Me₄Et analogues were at least partially (30–100%) converted into [W(η-C₅R′₅)(CO)₂(H)(C₂H₄)] (R′ = Me, R′₅ = Me₄Et) and unknown boron containing products. From an acetone solution of the α-borylbenzyl η-C₅Me₄Et complex crystals of a new product were isolated. Its unusual structure results from disruption of the boryl group, incorporation of two molecules of acetone to give a 1,3-dioxa-2-boracyclohexane ring system, conversion of the benzyl ligand into an η⁴-2-methyl-5-exo-methylenehexa-1,3-diene ligand and the formation of an acetyl group on the tungsten.