Transition-metal-free synthesis of arylboronates via thermal generation of aryl radicals from triarylbismuthines in air

Abstract

A simple and versatile synthesis of arylboronates has been achieved by using triarylbismuthines as aryl radical sources under transition-metal-free and open-air conditions. Conventional methods required photoirradiation or electrolysis to generate aryl radicals from triarylbismuthines. In this study, it was found that simply heating the solution of triarylbismuthines in benzotrifluoride (BTF) in air successfully led to the generation of aryl radicals, and the subsequent reaction with bis(pinacolato)diboron afforded a variety of arylboronates in moderate to good yields.

Keywords: arylboronates; bis(pinacolato)diboron; radical reactions; transition-metal-free synthesis; triarylbismuthines.

Scheme 1

(a) Conventional methods for the generation of Ar^• from Ar₃Bi, (b) our previous studies, and (c) this work.

Scheme 2

Scope for transition-metal-free synthesis of arylboronates 3 using triaylbismuthines 1 and diboron 2. Yields were determined by ¹H NMR spectroscopy based on 1 as three transferable aryl groups (internal standard: 1,3,5-trioxane). Isolated yield was shown in parentheses. ^aCHCl₃ (0.4 mL) was used as the solvent.

Scheme 3

Control experiment of the metal-free borylation under an argon atmosphere.

Figure 1

Comparison of the crude mixture of the reactions under (a) argon atmosphere or (b) open-air.

Scheme 4

Radical-trapping experiments using TEMPO as a radical scavenger.

Scheme 5

A proposed reaction pathway for the synthesis of arylboronates.