Photo and copper dual catalysis for allene syntheses from propargylic derivatives via one-electron process

Abstract

Different from the traditional two-electron oxidative addition-transmetalation-reductive elimination coupling strategy, visible light has been successfully integrated into transition metal-catalyzed coupling reaction of propargylic alcohol derivatives highly selectively forming allenenitriles: specifically speaking, visible light-mediated Cu-catalyzed cyanation of propargylic oxalates has been realized for the general, efficient, and exclusive syntheses of di-, tri, and tetra-substituted allenenitriles bearing various synthetically versatile functional groups. A set of mechanistic studies, including fluorescence quenching experiments, cyclic voltammetric measurements, radical trapping experiments, control experiments with different photocatalyst, and DFT calculation studies have proven that the current reaction proceeds via visible light-induced redox-neutral reductive quenching radical mechanism, which is a completely different approach as compared to the traditional transition metal-catalyzed two-electron oxidative addition processes.

Interplay of diruthenium catalyst in controlling enantioselective propargylic substitution reactions with visible light-generated alkyl radicals

“…In this reaction system, both photoredox and palladium catalysts work as photosensitizers under visible light irradiation, where a single electron transfer (SET) between the propargylic esters and excited palladium catalysts occurs to afford the propargylic radicals, which react with the alkyl radicals, generated from 4-alkyl-1,4-dihydropyridines via a SET process with photoredox catalysts, to afford the corresponding propargylic alkylated products. Furthermore, photoinduced enantioselective propargylic substitution reactions were only reported for propargylic cyanation catalyzed by dual photoredox-and copper-catalyzed system, where a SET between the propargylic esters and photoinduced photoredox catalysts occurs to afford the propargylic radicals, which react with the nucleophilic copper cyanate complexes, formed via reactions of chiral copper catalysts and CNin situ, to afford the corresponding propargylic cyanated products [45][46][47] or cyanated allenes 48 . Thus, the results described in this present manuscript are the successful example of enantioselective propargylic alkylation of propargylic alcohols with free radicals.…”

“…Formation of other TEMPO-trapped compounds was not observed from the reaction mixture. At present, we cannot exclude the possiblity of the formation of propargylic radicals as reactive intermediates 37,[45][46][47][48][57][58][59][60] , because we used tertiary propargylic alcohols bearing the CF 3 moiety as the substrate, where the corresponding propargylic radicals are difficult to be trapped by TEMPO. Then, Stern-Volmer analysis for emission quenching of fac-[Ir(ppy) 3 ] by 4a, 3a, or the ruthenium-allenylidene complex [Cp*RuCl(μ-SMe) 2 Ru(C=C=CHPh)Cp*]BF 4 (16), prepared by the reaction of [{Cp*RuCl(μ-SMe)} 2 ] with 1 in the presence of NH 4 BF 4 and anhydrous MgSO 4 55 , was performed in ClCH 2 CH 2 Cl.…”

Cross-coupling of organic fluorides with allenes: a silyl-radical-relay pathway for the construction of α-alkynyl-substituted all-carbon quaternary centres

“…The silyl radical in B then directly abstracts a hydrogen atom from the allene ( 3 ) to form allenyl radical-containing frustrated radical pair C , which could easily isomerize to the sterically highly demanding propargylic radical-containing frustrated radical pair D . 25 Subsequently, D would attract organic fluoride 1 or 2 by preferential interaction between the F atom and the B center to afford TS-I . Finally, the desired ethynyl-containing product with a quaternary carbon center ( 4 or 5 ) would be obtained via C–C bond coupling, accompanied by the release of E ([Bpin(O t Bu)F]K), which would promptly react with another equivalent of KO t Bu to provide a stable [Bpin(O t Bu) 2 ] species and KF.…”

“… 16,22 Initially, R 3 SiBpin and potassium tert -butoxide (KO t Bu) in an ether-based solvent smoothly generate intermediate A. Owing to the radical-initiation properties of KO t Bu 23 and the steric demand of intermediate A, A splits into the bulky frustrated radical pair B, 24 which consists of a trialkylsilyl radical (˙SiR 3 ) and a boron-radical species (B˙), via homolytic cleavage of the Si–B bond. The silyl radical in B then directly abstracts a hydrogen atom from the allene (3) to form allenyl radical-containing frustrated radical pair C, which could easily isomerize to the sterically highly demanding propargylic radical-containing frustrated radical pair D. 25 Subsequently, D would attract organic fluoride 1 or 2 by preferential interaction between the F atom and the B center to afford TS-I. Finally, the desired ethynyl-containing product with a quaternary carbon center (4 or 5) would be obtained via C–C bond coupling, accompanied by the release of E ([Bpin(O t Bu)F]K), which would promptly react with another equivalent of KO t Bu to provide a stable [Bpin(O t Bu) 2 ] species and KF.…”

Modular and Selective Access to Functionalized Alkynes and Allenes via the Intermediacy of Propargylic Acetates

“…Functionalized alkynes and allenes are valuable building blocks in organic synthesis and are important core structures in many bioactive molecules, natural products, and organic functional materials. , α-Difunctionalized alkynes and trisubstituted allenes are representative of them, and their synthesis has attracted much attention over the past decade. , The established strategies for the preparation of α-difunctionalized alkynes include various transition-metal-catalyzed cross-couplings using palladium, ruthenium, nickel, copper, etc. (Scheme a,b). − The reliable synthesis of trisubstituted allenes has been realized by the cross-coupling of propargylic electrophiles with various nucleophiles under transition-metal catalysis (Scheme c). − Notably, in the presence of different catalysts, the substitution of propargylic substrates by organometallic reagents can provide access to either α-difunctionalized alkynes or trisubstituted allenes (the bifurcation between Scheme b and c). However, the formation of propargylic or allenic products in a highly selective manner remains difficult, mainly due to the inherent properties of the involved active transition metals, which has inevitably hampered their applications in practical synthesis .…”