Uncovering the Potential of Boronic Acid and Derivatives as Radical Source in Photo(electro)chemical Reactions

Abstract

The chemistry of boron compounds has been profoundly investigated in the last decades, leading to ubiquity in many synthetic applications as well as biologically active molecules. Since the advent of photoredox catalysis, an upsurge in the discovery of suitable radical precursors has been witnessed, and boron is becoming a protagonist in this field. Despite many studies focused on trifluoroborates, the use of boronic acids and esters have received much less attention in this regard. Because of their high oxidation potential, the development of methods to enable their involvement in photocatalyzed reactions is only recent. Nonetheless, this review summarizes novel strategies developed to unlock their role as radical precursors in photochemical reactions and discloses the potential that boronic acids and esters own, when their intrinsic chemical properties are exploited.

Merging dual photoredox/cobalt catalysis and boronic acid (derivatives) activation for the Minisci reaction

“…37 When subjecting benzyl boronic acid (19) to the optimal conditions, the product could only be isolated in 20% yield, and the concomitant formation of unidentified side products was observed. Tertiary boronic esters reacted smoothly, but in moderate yield, under the reported conditions (17)(18); in addition to the adamantyl moiety mentioned before, a tert-butyl group was also coupled. Heterocyclic boronic acid (derivatives) also proved to be successful radical sources, and a tetrahydropyran moiety, together with N-Boc-protected piperidine (one of the most common scaffolds found in drugs 40 ) were coupled in good yields (20)(21).…”

“…Overall, the unattainable oxidation potential of these species has hindered their utilization as suitable radical precursors. Only recently, expedient strategies have been devised for this purpose [17][18][19][20] , including our previous work on free boronic acid activation under photo-flow conditions, which relied on the mediation of amide based solvents to form alkyl radicals (Figure 1B). 18 The application of these strategies is nevertheless mostly limited to redox-neutral additions on activated alkenes and a few related reactions.…”

Cu-Catalysed Coupling of Aliphatic Amines with Alkylboronic Esters

“…[32] While formation of alkyl radicals from alkylboronic esters has been previously reported, this typically requires prior formation of a boron 'ate' complex through coordination of an alkoxide, pyridine or aryl lithium. [33][34][35][36][37] However, such an 'ate' complex was not observed by 1 H or 11 B NMR spectroscopy when analysing mixtures of boronic ester 1 with morpholine in the presence of isopropanol. [26] To investigate this further, we conducted a series of cyclic voltammetry experiments.…”

Merging dual photoredox/cobalt catalysis and boronic acid (derivatives) activation for the Minisci reaction

“…Overall, the unattainable oxidation potential of these species has hindered their utilization as suitable radical precursors. Only recently, expedient strategies have been devised for this purpose, [17][18][19][20][21] including our previous work on free boronic acid activation under photo-flow conditions, which relied on the mediation of amide based solvents to form alkyl radicals (Fig. 1B).…”