Borane-Catalyzed Selective Hydrosilylation of Internal Ynamides Leading to β-Silyl (Z)-Enamides

Abstract

We have developed a borane-catalyzed regio- and stereoselective hydrosilylation of internal ynamides for the first time. The scope of ynamide substrates and silane reactants was broad under mild reaction conditions, affording synthetically versatile β-silyl (Z)-enamide products. The observed stereoselectivity was reasoned to be due to the β-silicon effect on a postulated ketene iminium intermediate.

Tris(pentafluorophenyl)borane-Catalyzed Reactions Using Silanes

“…The substrate scope was further advanced when Chang and coworkers discovered that internal ynamides, the close relative to the enamine, were also susceptible to reduction using this catalytic system [67]. In their study, they examined a variety of sulfonamide protected ynamides and noted a unique but reproducible regiochemistry for the reduction (Figure 23).…”

Borane-catalyzed selective dihydrosilylation of terminal alkynes: reaction development and mechanistic insight

“…At room temperature, the hydrosilylation of phenylacetylene 1a using B(C 6 F 5 ) 3 /EtMe 2 SiH combination only affords Z -vinylsilane 3aa in 31% yield. 18 This may probably be attributed to the occurrence of 1,1-carboboration reaction between phenylacetylene and B(C 6 F 5 ) 3 . 16,17 From a mechanistic point of view, the hydrosilylation pathway, involving a concerted three-molecular Si-centered displacement of a boron-coordinated hydride by the alkyne transition state (S N 2 @Si), 13,19 requires more entropy-cost than the two-molecular transition state toward the carboboration pathway.…”

Recent advances in transition metal-free catalytic hydroelementation (E = B, Si, Ge, and Sn) of alkynes

“…95 The hydrosilylations of some terminal alkynes proceeded in the presence of 10 (0.04-0.1 mol%) at 25-50 C to bring about a complex mixture of the corresponding vinylsilanes (cis/trans) and alkynylsilanes in poor yields, while in the case of ethynyltrimethylsilane, it underwent both hydrosilylation and redistribution reactions to give various vinylsilanes as a mixture (Scheme 25). As similar as the borane-silane adduct 2 0 a, [68][69][70] the h 1 -silane adduct (10a) was proposed to act as an active catalyst for the outer-sphere hydrosilylation of alkynes. In fact, the h 1 -Si-H coordination to the cationic Si center at 10a was evidenced by the coalescence of the Si-H signal in the time-dependent 1 H NMR spectra.…”

“…Motivated by the precedent Lewis acidic Al-catalyzed alkyne hydrosilylation, Chang et al developed for the rst time the B(C 6 F 5 ) 3 (2 0 )-catalyzed hydrosilylation of internal ynamides, furnishing a broad range of b-silyl (Z)-enamides (Scheme 14). 68 The hydrosilylation of ynamides having various substituents in the b-position occurred at 60 C in the presence of 2 0 (3 mol%) with Et 2 SiH 2 (2 equiv.). Variation of the N-substituents in ynamides from N-methyl to -phenyl or -cycloalkyl, and from N-tosyl to -mesyl groups did not hamper the reactivity and selectivity (52-81%; Z/E, >20 : 1).…”

“…The observed (E)selectivity in the reaction of propiolates implied a mechanistic path different from the prototypical B(C 6 F 5 ) 3 -mediated outersphere trans-hydroelementation. 68,96 A reaction pathway for the cis-hydrogermylation of propiolates was proposed (Scheme 62). Initially, B(C 6 F 5 ) 3 is assumed to bind the carbonyl moiety of the propiolate to form zwitterionic complex 2 0 e, where the carbocation is reduced by a hydrogermane 184 to generate allenolate 2 0 f possessing a germylium ion.…”