Visible-light-induced hydroalkoxymethylation of electron-deficient alkenes by photoredox catalysis

Abstract

Hydroalkoxymethylation of electron-deficient alkenes using alkoxymethyltrifluoroborates by photoredox catalysis has been developed. Highly reactive alkoxymethyl radicals can be easily generated from oxidation of alkoxymethyltrifluoroborates via a visible-light-induced SET process.

Single-electron transmetalation in organoboron cross-coupling by photoredox/nickel dual catalysis

“…1. However, the radical nature of the activation mode is strongly supported by the previous studies of Akita (20, 21) and our own experiments using chiral ligand scaffolds and racemic secondary alkyl nucleophiles (see below).…”

“…Subsequent capture of the alkyl radical at Ni(II) would then yield high-valent Ni(III) intermediate 9 , which was expected to undergo reductive elimination to generate the desired cross-coupled product 10 and Ni(I) complex 11 . From here, reduction of 11 [ E red > −1.10 V (24, 25)] by the reduced form of the photocatalyst 8 [ E ox = +1.37 V (21)] would regenerate both the Ni(0) catalyst 1 and the Ir catalyst 4 , closing the dual catalytic cycle.…”

“…Potassium organotrifluoroborates were identified as promising partners in this new class of cross-couplings, as previous reports have documented their ability to function as carbon radical sources upon single-electron oxidation (18, 19). Furthermore, Akita and co-workers have used Ir[dFCF 3 ppy] 2 (bpy)PF 6 (dFCF 3 ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = bipyridine) as a catalyst for the oxidation of activated potassium organotrifluoroborates, demonstrating the feasibility of their implementation in the proposed single-electron transmetalation manifold (20, 21). …”

“…3). Comparable single-electron oxidation potentials (20, 21) serve as the singular commonality between these structurally dissimilar reagents. Thus, C-C bond formation via single-electron transmetalation proceeded smoothly under these extremely mild and unoptimized conditions.…”

Improving Transformations Through Organotrifluoroborates

“…The scope of this protocol has been extended to include alkoxymethyltrifluoroborates (Scheme 49) [84]. The olefins participating in this C-C bond-forming process bear ketone, ester, amide, and nitrile moieties and also include cyclic enones, demonstrating the mild nature of the photoredox method which, therefore, boasts a high degree of functional group tolerance.…”