Highly Regio‐, Stereo‐, and Enantioselective Copper‐Catalyzed B−H Bond Insertion of α‐Silylcarbenes: Efficient Access to Chiral Allylic gem‐Silylboranes

Abstract

Herein, we report the development of a method for highly regio-, stereo-, and enantioselective BÀ H bond insertion reactions of α-silylcarbenes generated from 1-silylcyclopropenes in the presence of a chiral copper(I)/bisoxazoline catalyst for the construction of chiral γ,γ-disubstituted allylic gem-silylboranes, which cannot be prepared by any other known methods. This reaction is the first highly enantioselective carbene insertion reaction of α-silylcarbenes ever to be reported. The method shows general applicability for various 3,3disubstituted silylcyclopropenes and exclusively affords E-products. The novel chiral γ,γ-disubstituted allylic gem-silylborane products are versatile allylic bimetallic reagents with high stability and have great synthetic potential, especially for the construction of complex molecules with continuous chiral centers.Carbenes are important intermediates with diverse reactivities. α-Silylcarbenes are a class of functionalized carbene species with relatively good stability, [1] and their asymmetric transfer reactions constitute a promising tool for facilely introducing the silyl-methylene moiety and preparing structurally diverse chiral organosilanes. [2] α-Carbon carbenes were broadly utilized in the enantioselective carbene transfer reactions; [3] however, the analogue carbene transfer reactions of α-silylcarbenes are much underdeveloped probably owing to the restricted source of α-silylcarbene precursors [4] and the lack of efficient catalytic protocols (Scheme 1a). Dirhodium or copper-catalyzed asymmetric cyclopropanation reactions of α-silylcarbenes derived from corresponding α-silyl diazocompounds provided a reliable access to chiral silylcyclopropanes, [5] yet highly enantioselec-tive insertion reactions of α-silylcarbenes are basically unknown. [6] Recently, our group developed a method of coppercatalyzed BÀ H insertion reaction of cyclopropene-derived αvinylcarbenes. [7] We hypothesized that 1-silylcyclopropenes might serve as a suitable precursor for α-silyl-αvinylcarbene, [8] which could undergo enantioselective BÀ H insertion to afford chiral gem-silylboranes (Scheme 1b). To achieve this transformation, at least the following three challenges need to be addressed. ( 1) We needed to regioselectively cleave the C1-C3 bond to form an α-silyl-α-vinyl metal carbene intermediate in the ring-opening process. [9] (2) We need to control the stereoselectivity of the C=C bond if R 1 and R 2 were different. (3) We needed to achieve efficient chiral induction to control the enantioselectivity of the BÀ H bond insertion. [10] By using a Cu I /chiral bisoxazoline complex, we established a method for highly regio-, stereo-, and enantioselective BÀ H bond insertion reactions of α-silyl-α-vinylcarbenes derived from 1-silylcyclopropenes, representing the first highly enantioselective XÀ H bond insertion reaction of αsilyl-substituted carbenes. This protocol affords chiral allylic[*] M.

Photocatalyzed regioselective hydrosilylation for the divergent synthesis of geminal and vicinal borosilanes

“…The synthesis of borosilanes, mainly through transition-metal catalysis, has attracted much attention in the last decade (Fig. 1A ) 3 – 18 . For instance, Morken and co-workers reported the synthesis of geminal borosilanes through Pt-catalyzed hydrosilylation of β-alkyl-substituted alkenyl boronates 8 .…”

Stereoselective synthesis of 1,3- and 1,4-dicarbonyl-alkenes from cyclopropenes in a catalytic zinc system

“…5 Several elegant studies since then have been disclosed that focus on non-precious metals catalysing vinyl carbene reactions. 6 Inspired by these pioneering studies, we envisioned that a zinc catalyst would have the potential to work in a module “cutting/insertion” cascade of cyclopropenes and enaminones to afford different regioselective products. 7…”

Cu(I)-Catalyzed Highly Diastereo- and Enantioselective Constructions of Boron/Carbon Vicinal Stereogenic Centers via Insertion Reaction

“…All of the deuterium atoms in product 3aa-d 2 were dispersed either on the benzyl carbon atom or on the B atom. The kinetic isotope effect (KIE) experiment (k H /k D = 1.8,) suggested that the hydrogen-transfer step was fast 18 (Figure 5B). Based on the labeling experiments, a plausible mechanism is proposed to explain this insertion reaction (Figure 5).…”