Catalyst- and Silane-Controlled Enantioselective Hydrofunctionalization of Alkenes by Cobalt-Catalyzed Hydrogen Atom Transfer and Radical-Polar Crossover

Abstract

The catalytic enantioselective synthesis of tetrahydrofurans, which are found in the structures of many biologically active natural products, via a transition-metal-catalyzed hydrogen atom transfer (TM-HAT) and radical-polar crossover (RPC) mechanism is described herein. Hydroalkoxylation of nonconjugated alkenes proceeded efficiently with excellent enantioselectivity (up to 94% ee) using a suitable chiral cobalt catalyst, N-fluoro-2,4,6-collidinium tetrafluoroborate, and diethylsilane. Surprisingly, the absolute configuration of the product was highly dependent on the steric hindrance of the silane. Slow addition of the silane, the dioxygen effect on the solvent, thermal dependence, and DFT calculation results supported the unprecedented scenario of two competing selective mechanisms. For the less-hindered diethylsilane, a high concentration of diffused carbon-centered radicals invoked diastereoenrichment of an alkylcobalt(III) intermediate by a radical chain reaction, which eventually determined the absolute configuration of the product. On the other hand, a more hindered silane resulted in less opportunity for a radical chain reaction, instead facilitating enantioselective kinetic resolution during the late-stage nucleophilic displacement of the alkylcobalt(IV) intermediate.

Catalytic hydrogen atom transfer to alkenes: a roadmap for metal hydrides and radicals

“…Expanding upon a preliminary disclosure of asymmetric tetrahydrofuran formation via RPC, 92 the Shigehisa group explored the formation of enantioenriched tetrahydrofurans using alkenyl alcohols with substituted binaphthyl cobalt salen catalysts (Scheme 19, right). 140 An interesting kinetic effect was observed with the silane; sterically-hindered silanes or slow addition of silane led to a switch in enantioselectivity, indicative of two enantiodetermining steps in the mechanism. The rst step proposed was a radical chain mechanism leading to enantioenrichment of the (R)-organocobalt(III) species, which underwent oxidation to the organocobalt(IV) species and subsequent displacement by the pendant alcohol to form the (S)-enantiomer (63-94% ee) (Scheme 18c).…”

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

“…Mechanistically, the authors proposed the presence of cationic alkylcolbalt(IV) complexes in the enantio-determining step, where cation-π interactions between the radical cation of the Schiff base fragment and the biaryl substituents of the dibenzofuran motif, account for the superior asymmetric induction observed with 4d-Co [87]. Last year, Shigehisa and co-workers also explored the structural effect of chiral cobalt salen catalysts on their previously developed cyclohydroalkoxylation methodology of unactivated alkenes (Figure 11, bottom) [89]. They identified the chiral and bulky binaphtyl ligands as optimal for the reaction of gem-disubstituted alkenols bearing terminal olefin, leading to the formation of various 5-membered rings in good yields and with moderateto-high enantioinduction.…”

“…In 2014, taking advantage of the cobalt-based catalytic system developed for alkene hydroalkoxylation (vide supra), the Shigehisa group reported the intramolecular Last year, Shigehisa and co-workers also explored the structural effect of chiral cobalt salen catalysts on their previously developed cyclohydroalkoxylation methodology of unactivated alkenes (Figure 11, bottom) [89]. They identified the chiral and bulky binaphtyl ligands as optimal for the reaction of gem-disubstituted alkenols bearing terminal olefin, leading to the formation of various 5-membered rings in good yields and with moderateto-high enantioinduction.…”

Recent advances in CoSalen-catalyzed radical reactions

“…氧化, 实现自由基-极性交叉. 此基础上, 他们陆续将反 应范围扩展到分子内的含氮 [32] 、氧 [33] 、碳 [34] 、硫 [35] 等官能团参与的环化, CoSalen 1f是通用性最好的催化 2018年, Pronin课题组 [36] 几乎与Pronin的报道同时, Shigehisa等 [44] 也报道 最近, 叶克印、朱军等 [49] 利用CoSalen/N-氟代酰 6 基于Co(I)的还原偶联反应…”