Tropylium-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 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.

Unusual Alternating Crystallization-Induced Emission Enhancement Behavior in Nonconjugated ω-Phenylalkyl Tropylium Salts

“…From our previous work in the chemistry of tropylium ions, we have reported the use of nonbenzenoid cationic tropylium moiety as a convenient electron-deficient building block for photoactive materials . Tropylium ion has also been long known to form inter- or intramolecular π–π complexes with phenyl rings in arenes .…”

Tropylium Salt-Promoted Vinylogous Aza-Michael Addition of Carbamates to para-Quinone Methides: Elaboration to Diastereomerically Pure α,α′-Diarylmethyl Carbamates

“…For example, Lambert and co-workers demonstrated the utility of the tropylium cation as a mediator in the α-cyanation of amines with KCN . Later, Nguyen’s group demonstrated the catalytic utility of the tropylium cation (mostly as a Lewis acid) in several synthetic methodologies, such as acetalization and trans -acetalization reactions, carbonyl–olefin metathesis reactions, retro-Claisen-type reactions, O–H insertion reactions of carboxylic acids with diazo compounds, etc. − In this manuscript, we intend to demonstrate the catalytic utility of tropylium salts in the vinylogous aza-Michael reactions of para -quinone methides ( p -QMs) − to access unsymmetrical α,α′-diarylmethyl amine derivatives. These compounds have been well-recognized in the area of medicinal chemistry, and many α,α′-diarylmethyl amine-based drugs have been already commercialized (Figure ).…”

Markovnikov alcohols via epoxide hydroboration by molecular alkali metal catalysts

“…Such methods usually suffer from poor regioselectivities by providing a mixture of both primary and secondary alcohols, poor functional group tolerance, and the generation of enormous amounts of waste. Heterogeneous ( Duval et al., 2021 ; Thiery et al., 2007 ; Nandi et al., 2017 ) and particularly homogeneous transition-metal catalyzed hydrogenation ( Yao et al., 2019 ; Liu et al., 2019 ), hydroboration ( Patnaik and Sadow, 2019 ; Song et al., 2017 ; Desnoyer et al., 2017 ), and hydrosilylation ( Zhang et al., 2017 , 2018 ; Wenz et al., 2017 ; Henriques et al., 2016 ; Gansäuer et al., 2012 ; Park and Brookhart, 2011 ; Nagashima et al., 2000 ) of epoxides as well as transition-metal-free approaches ( Patnaik and Sadow, 2019 ; Zhang et al., 2018 ; Huang et al., 2022 ; Ton et al., 2021 ) were developed by several laboratories leading preferentially to anti -Markovnikov alcohols. Many fewer catalysts have been reported for the selective reduction of epoxides into Markovnikov alcohols.…”