O-Benzyl Xanthate Esters under Ni/Photoredox Dual Catalysis: Selective Radical Generation and Csp³–Csp² Cross-Coupling

Abstract

Alkyl xanthate esters are perhaps best known for their use in deoxygenation chemistry. However, their use in cross-coupling chemistry has not been productive, which is due, in part, to inadequate xanthate activation strategies. Herein, we report the use of O-benzyl xanthate esters, readily derived from alcohols, as radical pronucleophiles in Csp3–Csp2 cross-couplings under Ni/photoredox dual catalysis. Xanthate (C–O) cleavage is found to be reliant on photogenerated (sec-butyl) radical activators to form new carbon-centered radicals primed for nickel-catalyzed cross-couplings. Mechanistic experiments support the fact that the key radical components are formed independently, and relative rates are carefully orchestrated, such that no cross reactivity is observed.

Alkyl Carbon–Carbon Bond Formation by Nickel/Photoredox Cross‐Coupling

“…[10] Subsequently,m ultiple strategies for C(sp 3 )r adical generation have been developed. Some approaches rely on redox-active groups,s uch as alkyltrifluoroborates,c arboxylates,b is(catecholato)silicates, and 1,4-dihydropyridines,t oa chieve programmed reactivity ( Figure 3), and others utilize the innate reactivity of substrates (via hydrogen or halogen atom transfer) to generate alkyl radicals.R adicals originating from the oxidation of sulfinate salts, [21] xanthates, [22] and a-silylamines [23] have also been employed in this cross-coupling strategy,a lthough they are not discussed in this Minireview.…”

Synergistic Photoredox/Transition‐Metal Catalysis for Carbon–Carbon Bond Formation Reactions

“…The radical could be generated In 2017, Molander and co-workers developed a dual photoredox/Ni catalysis methodology relying on O-benzyl xanthate esters 569, which are well-known radical precursors, [121] readily obtained from benzyl alcohols (Scheme 86). [122] The generation of the reactive radical relies on the use of sec-butyltrifluoroborates. The latter gets oxidized first, via a SET oxidation initiated by the Ir catalyst in its excited state, leading to a sec-butyl radical which reacts with the xanthate derivative in order to yield the desired radical 571, ready for Ni-catalysed cross-couplings (from 575 to 570).…”

Alkyl‐C‐C‐Bindungsbildung durch Nickel/Photoredox‐Kreuzkupplung

“…Andere nutzen die inhärente Reaktivitätv on Substraten (über Wasserstoff-oder Halogenatomtransfer) zur Erzeugung von Alkylradikalen. Es wurden auch Radikale aus der Oxidation von Sulfinatsalzen, [21] Xanthaten [22] und a-Silylaminen [23] in dieser Kreuzkupplungsstrategie verwendet, obgleich sie in diesem Kurzaufsatz nicht diskutiert werden.…”