3-Silyl-3-Borylhex-4-Enoate: A Chiral Reagent for Asymmetric Crotylboration of Aldehydes

Abstract

A nonracemic 3-silyl-3-borylhex-4-enoate reagent has been developed. Its asymmetric crotylboration of aldehydes provides Z-anti-homoallylic alcohols possessing a trisubstituted vinylsilane in high yields with excellent stereo- and enantioselectivity. Diverse decoration of vinylsilane and ester groups, as well as formation of functionalized THF rings, showcase the potential of the approach in the synthesis of polyketide natural products.

Synthesis of (1-silyl)allylboronates by KOtBu-catalyzed ring-opening gem-silylborylation of cyclopropenes

“…3 The high potential of (1-silyl)allylboranes has been empirically evidenced by the broad scope of transformations through regio-, chemo-, and stereoselective C-C bond forming reactions. 3,4 Despite the utility of these compounds, their efficient synthetic methods remain to be further developed. Most of the existing synthetic strategies rely on the use of preformed organosilicon and/or organoboron compounds as exemplified by silylation of allylboranes (Scheme 1a), 5a hydroboration of allenylsilanes (Scheme 1b) 5b or silylcyclopropenes, 5c homologation of alkenylboronic acid derivatives with silylmethylene donors (Scheme 1c), 5d,e and cross-coupling of bromoalkenes with diborylsilylmethanes (Scheme 1d).…”

“…7b A high yield of 3aa was also achieved by using NaOtBu instead of KOtBu (entry 3), but LiOtBu was found to be ineffective presumably due to the lower basicity (entry 4). It is worth noting that the use of a transition-metal alkoxide catalyst such as Cu(OtBu)(IPr) 8 led to the formation of a dimer of 1a in the form of 3,3,6,6-tetraphenyltricyclo[3.1.0.0 2,4 ]hexane ( 4) with no formation of 3aa (entry 5). 9 Under simple and mild conditions using KOtBu as the catalyst, various 3,3-diarylcyclopropenes 1 could be transformed into the corresponding (1-silyl)allylboronates 3 by the reaction with silylboronate 2a (Table 2).…”

“…a Determined by 1 H NMR against internal standard (dimethyl terephthalate). b 3,3,6,6-Tetraphenyltricyclo[3.1.0.0 2,4 ]hexane ( 4) was obtained as the major product in 59% yield.…”