Copper-Catalyzed Double C(sp³)–Si Coupling of Geminal Dibromides: Ionic-to-Radical Switch in the Reaction Mechanism

Abstract

A method for converting geminal dibromides into 1,1-disilylated alkanes is reported. The reaction is promoted by a copper(I) catalyst generated in situ from CuBr·SMe as a precatalyst and 4,4'-di- tert-butyl-2,2'-bipyridine (dtbpy) as a ligand. A Si-B reagent is used as the silicon pronucleophile. It is shown that the two C(sp)-Si bond-forming events differ in mechanism, with the first being ionic and the second being radical.

Activation of the Si–B interelement bond related to catalysis

“…When using geminal dibromides as electrophilic reaction partners, a combination of those two mechanistically different approaches (nucleophilic substitution and radical crosscoupling) was accomplished by Hazrati and Oestreich (Scheme 136). 155 This copper-catalysed process worked well with several functionalised terminal dibromides with no branching in the proximity (388a-c -389a-c). While branching closer to the reaction site as in 388d with a cyclohexyl group did not thwart the silylation, the yield dropped substantially with tert-butyl or phenyl as the R group (388e -389e; 388f -389f).…”

Regiodivergent and Stereospecific Aziridine Opening by Copper‐Catalyzed Addition of Silicon Grignard Reagents

“…Evidence for this scenario comes from the related, yet unprecedented, reaction of aziridine 1 a and the silicon pronucleophile Me 2 PhSiBpin ( 7 a , Scheme , bottom) . The isolated yield of 3 aa was 38 % under typical reaction conditions– but increased to 82 % with added LiCl. The regioselectivity is explained by steric arguments (gray box).…”

Recent advances in Cu-catalyzed C(sp3)–Si and C(sp3)–B bond formation

“…The reaction supposedly occurred partly via ionic silylation (of the first bromide), and partly via a radical pathway (silylation of the second bromide), effected by the presence of catalytic amounts of CuBr·SMe 2 along with dtbpy as ligand. Whatever the mechanism(s), a variety of substrates were suit-able for this transformation, giving the desired products 42-44 in good chemical yields (Scheme 10) [32].…”