Regioselective Radical Borylation of α,β-Unsaturated Esters and Related Compounds by Visible Light Irradiation with an Organic Photocatalyst

Abstract

Radical hydroboration reactions have only recently been reported and are still rare. Here we describe a photoredox radical hydroboration of α,β-unsaturated esters, amides, ketones, and nitriles with NHC-boranes that uses only an organocatalyst and visible light. The conditions are mild, the substrate scope is broad, and the α/β regioselectivity is high. The reaction requires only the organocatalyst; there is no costly metal, and there are no other additives (base, cocatalyst, initiator).

Copper-Catalyzed Cyclization of N-Propargyl Ynamides with Borane Adducts through B–H Bond Insertion

“…Organoboron compounds have great potential for application in the fields of organic synthesis and material and medicinal research, and a series of powerful methods for the construction of the C–B bond have been developed . Among these, the insertions of the B–H bond have attracted a great deal of attention due to their comparatively mild reaction conditions, high efficiency, environmental friendliness, and atomic economy .…”

NHC induced radical formationviahomolytic cleavage of B–B bonds and its role in organic reactions

“…17a The alkyl/ aryl radical formed presumably adds to B 2 cat 2 and, with coordination of DMF (dimethylformamide), the weakened B-B bond homolyzes giving the alkyl/aryl boronic ester and a DMFstabilized boryl radical. 17 Just recently, the photocatalytic, regioselective radical borylation of a,b-unsaturated esters and related compounds has been reported, 18 and we reported the transition metal-free borylation of primary and secondary alkyl sulfones to yield alkylboronic esters. 19 For the latter, the reaction of alkyl sulfones with bis(neopentyl glycolato) diboron(4) (B 2 neop 2 ) and a stoichiometric amount of base as promotor afforded alkylboronic esters with a wide functional group tolerance.…”

A general photo-induced wide-scope regioselective hydroboration of alkenes without using a photocatalyst or an external initiator

“…These results revealed that the part of the source of the hydrogen atom comes from the residual water of the solvent through hydrogen exchange between thiol 2a and water, which is consistent with previous reports. 35,36,63,64 In contrast, the borylated organoboron product 5a was mixed with 10 equiv. D 2 O, and no deuteride compound was detected, indicating the deuterium incorporation before the formation of the product.…”

“…34 Understanding the selectivity patterns of boryl radical addition processes and applying this knowledge to other related reactions will add value to the boryl radical chemistry and beyond. [35][36][37][38][39][40][41]…”