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 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 pathway, which is triggered by the hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of in situ counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.
1,1-Diboryl
alkenes are versatile building blocks in organic synthesis
and medicinal chemistry. However, there have been only a small number
of established methods to prepare this class of compounds, and most
of them used transition-metal catalysts, which are undesirable in
the preparation of biologically relevant compounds. Herein, we report
an unprecedented application of P1–
t
Bu phosphazene as a superbasic organocatalyst to
promote 1,1-diboration reactions of unactivated aromatic and electron-deficient
terminal alkynes. The dual Brønsted and Lewis basicity of this
phosphazene enables the activation of reaction substrates and allows
for high regio- and stereoselectivity to be obtained. A combination
of thorough experimental and computational studies suggests interesting
mechanistic insights for these phosphazene-catalyzed diboration reactions,
which are also discussed in detail.
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