Brønsted Acid Catalyzed 1,2-Silyl Shift in Propargyl Silanes: Synthesis of Silyl Dienes and Silyl Indenes

Abstract

A general method for generation of allyl carbenium ions from propargyl silanes via a 1,2-silyl shift by Brønsted acids is reported. Two possible reaction pathways are described. Deprotonation results in silyl dienes with yields from 52% to 92%. Intramolecular Friedel−Crafts reactions of aryl-substituted systems give access to silyl indenes with yields of 18−90% depending on the substitution pattern. The obtained products have been shown to react as alkenyl silanes in Hiyama coupling and electrophilic substitution and as dienes in Diels− Alder cycloaddition.

Boron and Silicon-Substituted 1,3-Dienes and Dienophiles and Their Use in Diels-Alder Reactions

“…Turks and co-workers reported an acid-catalyzed 1,2-silyl shift in propargyl silanes ( 142 ) as a route to silyl-substituted dienes ( 143 ) ( Scheme 38 ) [ 35 ]. A variety of metal triflates were first investigated as catalysts for this reaction but the authors ultimately determined that small amounts of triflic acid sonicated in CH 2 Cl 2 caused rapid isomerizations in high yields for a variety of silanes.…”

“…Turks and co-workers reported an acid-catalyzed 1,2-silyl shift in propargyl silanes (142) as a route to silyl-substituted dienes (143) (Scheme 38) [35]. A variety of metal triflates were first Johnson, Hendrix and Jennings reported syntheses of a number of silyl ketene acetals (141) from deprotonation and TMSCl trapping of α-silyl-α,β-unsaturated esters (150) (Scheme 37) [34].…”

“…No Diels-Alder reactions of these dienes (141) were reported but isolated yields of diene products ranged from 85% to 99% and E:Z ratios across both double binds in all products were >20:1. Turks and co-workers reported an acid-catalyzed 1,2-silyl shift in propargyl silanes (142) as a route to silyl-substituted dienes (143) (Scheme 38) [35]. A variety of metal triflates were first investigated as catalysts for this reaction but the authors ultimately determined that small amounts of triflic acid sonicated in CH2Cl2 caused rapid isomerizations in high yields for a variety of silanes.…”

Recent Advances in the Synthesis of Indenes

“…A general method for the generation of allyl carbenium ions from propargylsilanes 44 via a 1,2‐silyl shift by Brønsted acids has been reported by Turks . One possible reaction pathway of these cations is the intramolecular Friedel–Crafts reaction of aryl‐substituted systems to give access to 2‐silylindenes 45 with yields ranging from 18 to 90 % depending on the substitution pattern (Scheme ).…”

“…A general method for the generation of allyl carbenium ions from propargylsilanes 44 via a 1,2-silyl shift by Brønsted acids has been reported by Turks. [29] One possible reaction pathway of these cations is the intramolecular Friedel-Crafts reaction of aryl-substituted systems to give access to 2-silylindenes 45 with yields ranging from 18 to 90 % depending on the substitution pattern (Scheme 16). The right choice of the solvent is crucial to avoid the competing pathway leading to 2-silyl-1,4-dienes which, on the other hand, are useful as alkenylsilanes in Hiyama coupling and electrophilic substitution, and as dienes in Diels-Alder cycloadditions.…”