Alpha-selective Ni-catalyzed hydroalumination of aryl- and alkyl-substituted terminal alkynes: practical syntheses of internal vinyl aluminums, halides, or boronates

Abstract

A method for Ni-catalyzed hydroalumination of terminal alkynes, leading to the formation of alpha-vinylaluminum isomers efficiently (>98% conv in 2-12 h) and with high selectivity (95% to >98% alpha), is described. Catalytic alpha-selective hydroalumination reactions proceed in the presence of a reagent (diisobutylaluminum hydride; dibal-H) and 3.0 mol % metal complex (Ni(dppp)Cl(2)) that are commercially available and inexpensive. Under the same conditions, but with Ni(PPh(3))(2)Cl(2), hydroalumination becomes highly beta-selective, and, unlike uncatalyzed transformations with dibal-H, generates little or no alkynylaluminum byproducts. All hydrometalation reactions are reliable, operationally simple, and practical and afford an assortment of vinylaluminums that are otherwise not easily accessible. The derived alpha-vinyl halides and boronates can be synthesized through direct treatment with the appropriate electrophiles [e.g., Br(2) and methoxy(pinacolato)boron, respectively]. Ni-catalyzed hydroaluminations can be performed with as little as 0.1 mol % catalyst and on gram scale with equally high efficiency and selectivity.

Scheme 1

Applications of Ni-Catalyzed Hydroaluminationsa ^a Synthesis of 5 and 6 at –15 °C. Ar = 2,4,6-(i-Pr)₃-C₆H₂.

Scheme 2

Conversion of Terminal Alkynes to α-Vinyl Halides ^a Reaction with N-iodosuccinimide under otherwise identical conditions. See the Supporting Information for experimental details.

Scheme 3

One Pot Synthesis of α-Vinyl Boronatesa ^a Yields refer to purified α-vinylboronates; see the Supporting Information for experimental details. B(pin) = pinacolatoboron.