Tropylium Salt Promoted Hydroboration Reactions: Mechanistic Insights via Experimental and Computational Studies

Abstract

Hydroboration reaction of alkynes is one of the most synthetically powerful tools to access organoboron compounds, versatile precursors for cross coupling chemistry. This type of reaction has traditionally been mediated by transition metal or main group catalysts. Herein, we report a novel method using tropylium salts, typically known as organic oxidants and Lewis acids, to efficiently promote the hydroboration reaction of alkynes. A broad range of vinylboranes can be easily accessed via this metal-free protocol. Similar hydroboration reactions of alkenes and epoxides can also be efficiently catalyzed by the same tropylium catalysts. Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic paradigm, which is triggered by a hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of <i>in situ</i> counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

Organosuperbase Catalyzed 1,1-Diboration of Alkynes

“…[1][2][3][4] The former feature is incompatible with the desired Brønsted basicity, so it has to be the latter. Being informed by our earlier boration work 27 and chemistry with N-heterocyclic carbenes 28,29 and olefins, [30][31][32][33] which are very strong Brønsted bases and or s-Lewis donors themselves but unsuitable for this type of chemical transformation, we focus our initial effort on the investigation of 1,1-diboration reaction of terminal alkynes using phosphazene superbase P1-t Bu-tris(tetramethylene), denoted in this work as P1-t Bu, as reaction promoter (Scheme 1, bottom).…”