Pyridine‐Based Organocatalysts for Regioselective syn‐1,2‐Silaboration of Terminal Alkynes and Allenes

Abstract

An organocatalytic silaboration of terminal alkynes and allenes was established using pyridine-based catalysts. In the presence of 4-cyanopyridine (1-2 mol%), alkyl propiolates underwent regio-and stereoselective addition of silylboronic esters in toluene at 135°C to afford (Z)-3-boryl-2-silylacrylates in good yields. 2,6-Dichloro-4,4'bipyridine and 4-(3,5-dichlorophenyl)pyridine also exhibited high catalyst efficiency for the 1,2-silaboration of ethyl propiolate, whereas 1,1-silaboration was induced by P(n-Bu) 3 , t-BuOK, and ICy to afford ethyl 3-boryl-3-silylacrylates as Z/E mixtures. The silaboration of ethynylbenzenes and terminal allenes was also catalyzed by 4-cyanopyridine to afford (Z)-β-boryl-α-silylstyrenes and β-borylallylsilanes in a regioselective manner.

Activation of the Si–B interelement bond related to catalysis

“…Suginome and co-workers applied pyridinebased organocatalysts to the silaboration of phenylacetylenes (34a, k-o -Z-41a, k-o, Scheme 11). 26 With 4-cyanopyridine as catalyst, the reaction required 135 1C to add MePh 2 Si-Bhg (40) across the C-C triple bond with consistently high regio-and stereoselectivity. The proposed mechanism commences with coordination of 4-cyanopyridine to the silylborane to form adduct XI, which undergoes homolytic cleavage to afford radical pair XII.…”

“…It ought to be mentioned that 1,1silaboration of ethyl propiolate mediated by a catalytic amount of an organocatalyst such as nBu 3 P, KOtBu or ICy was reported by Suginome and co-workers one year before (see Section 4.5). 26 Elevated reaction temperature seems to be typical for palladium-catalysed alkyne silaborations. By applying the palladium complex Pd(ITMe) 2 (PhCCPh) as precatalyst, Navarro and co-workers accomplished the title reaction at room temperature (34a, f, t -38a, f, t, Scheme 13, top).…”

“…Scheme 11). 26 In that work, terminal allenes were subjected to the same reaction conditions (131f-g -135f-g, Scheme 41). The silyl and boryl groups are delivered to the internal C-C double bond with the boryl moiety attached on the former C(sp) atom.…”

“…Schemes 11 and 41), 4-cyanopyridine can be used as a catalyst for the regioselective silaboration of terminal alkynes and allenes. 26 This protocol is also applicable to the addition of silylboronic esters to propiolates (249a-c -250a-c, Scheme 79, top). 26 Propiolate esters of primary alcohols reacted in good yields whereas the tert-butyl ester did not convert at all.…”

Recent Advances in Pyridine‐Based Organocatalysis and its Application towards Valuable Chemical Transformations

“…The protocol envisioned here is the first transition‐metal‐free strategy for silaboration of terminal alkynes and allenes. The catalyst 4‐cyanopyridine ( 4 c ) in low loading (1‐5 mol %) was sufficient enough to convert ethynylbenzenes and terminal allenes into ( Z )‐ β ‐boryl‐ α ‐silylstyrenes 12 a – c and β ‐borylallylsilanes 14 a – b , 15 a – b (Scheme ) …”