The Development of Aldehyde Catalytic System

Aldehyde catalysts have proven to be highly effective in facilitating and accelerating a wide range of challenging transformations in organic chemistry. This article is structured into three main sections, focusing on the utilization of aldehydes as organocatalysts, the aldehydes/transition metals catalytic systems, and photochemical initiators. Finally, we provide a concise summary of the advancements in this fascinating research field, offering our perspectives and insights.

Remote Radical 1,3-, 1,4-, 1,5-, 1,6- and 1,7-Difunctionalization Reactions

Radical transformations are powerful in organic synthesis for the construction of molecular scaffolds and introduction of functional groups. In radical difunctionalization reactions, the radicals in the first functionalized intermediates can be relocated through resonance, hydrogen atom or group transfer, and ring opening. The resulting radical intermediates can undertake the following paths for the second functionalization: (1) couple with other radical groups, (2) oxidize to cations and then react with nucleophiles, (3) reduce to anions and then react with electrophiles, (4) couple with metal-complexes. The rearrangements of radicals provide the opportunity for the synthesis of 1,3-, 1,4-, 1,5-, 1,6-, and 1,7-difunctionalization products. Multiple ways to initiate the radical reaction coupling with intermediate radical rearrangements make the radical reactions good for difunctionalization at the remote positions. These reactions offer the advantages of synthetic efficiency, operation simplicity, and product diversity.

Palladium-catalyzed oxidative Heck reaction of non-activated alkenes directed by fluorinated alcohol

A new reactivity pattern for the regio- and stereoselective oxidative arylation of non-activated alkenes by introducing a trifluoromethyl group in the substrate enol has been established.

Radical addition to the CC bond meets (1,n)-HAT: recent advances in the remote C(sp3)–H or C(sp2)–H functionalization of alkenes

This review summarizes the recent development of remote C(sp3)–H bond or aldehydic C(sp2)–H functionalizations enabled by intermolecular radical addition to CC bond/(1,n)-HAT tandem sequences.

Excited-state cobaloxime catalysis enabled scalable oxidant-free dehydrogenative C–H phosphinoylation of undirected heterocycles

Visible-light-induced excited-state cobalt catalysis enables C(sp2)–H/C(sp3)–H phosphinoylation accompanied by H2 evolution. The reaction achieves the late-stage modification of more than 10 distinct classes of heterocycles and arenes.

Electrochemically induced Markovnikov-type selective hydro/deuterophosphonylation of electron-rich alkenes

An electrochemically induced Markovnikov-type selective hydro/deuterophosphonylation of electron-rich alkenes with P(O)H compounds to generate various organophosphorus compounds has been achieved.

Mild olefin formation via bio-inspired vitamin B12 photocatalysis

Dehydrohalogenation, or elimination of hydrogen-halide equivalents, remains one of the simplest methods for the installation of the biologically-important olefin functionality. However, this transformation often requires harsh, strongly-basic conditions, rare noble metals, or both, limiting its applicability in the synthesis of complex molecules. Nature has pursued a complementary approach in the novel vitamin B₁₂-dependent photoreceptor CarH, where photolysis of a cobalt–carbon bond leads to selective olefin formation under mild, physiologically-relevant conditions. Herein we report a light-driven B₁₂-based catalytic system that leverages this reactivity to convert alkyl electrophiles to olefins under incredibly mild conditions using only earth abundant elements. Further, this process exhibits a high level of regioselectivity, producing terminal olefins in moderate to excellent yield and exceptional selectivity. Finally, we are able to access a hitherto-unknown transformation, remote elimination, using two cobalt catalysts in tandem to produce subterminal olefins with excellent regioselectivity. Together, we show vitamin B₁₂ to be a powerful platform for developing mild olefin-forming reactions.

Terminal or subterminal olefins can be selectively formed from alkyl electrophiles via bio-inspired vitamin B₁₂ photocatalysis.

C-H functionalization reactions enabled by hydrogen atom transfer to carbon-centered radicals

Selective functionalization of ubiquitous unactivated C-H bonds is a continuous quest for synthetic organic chemists. In addition to transition metal catalysis, which typically operates under a two-electron manifold, a recent renaissance in the radical approach relying on the hydrogen atom transfer (HAT) process has led to tremendous growth in the area. Despite several challenges, protocols proceeding via HAT are highly sought after as they allow for relatively easy activation of inert C-H bonds under mild conditions leading to a broader scope and higher functional group tolerance and sometimes complementary reactivity over methods relying on traditional transition metal catalysis. A number of methods operating via heteroatom-based HAT have been extensively reported over the past few years, while methods employing more challenging carbon analogues have been less explored. Recent developments of mild methodologies for generation of various carbon-centered radical species enabled their utilization in the HAT process, which, in turn, led to the development of remote C(sp3)-H functionalization reactions of alcohols, amines, amides and related compounds. This review covers mostly recent advances in C-H functionalization reactions involving the HAT step to carbon-centered radicals.

Photoredox-Catalyzed Remote Difunctionalizations of Alkenes To Synthesize Fluoroalkyl Ketones with Dimethyl Sulfoxide as the Oxidant

Visible light-mediated cascade remote oxyfluoroalkylation of alkenes under mild conditions is developed for the first time. The key point of this transformation is the incorporation of alkene fluoroalkylation-initiated remote benzyl C-H bond activation via a 1,5-H shift in a highly controlled site-selective manner and Kornblum reaction with dimethyl sulfoxide as the oxidant. With this method, a broad array of fluoroalkyl groups were introduced into a double bond to produce 1,6-fluoroalkylated ketones at room temperature.

K2S2O8-Mediated Hydroxyalkylation of Benzothiazoles with Alcohols in Aqueous Solution

The K2S2O8-mediated hydroxyalkylation of 2H-benzothiazoles with aliphatic alcohols in aqueous solution was described. The mild and convenient protocol generated a series of hydroxyalkylated benzothiazoles in moderate to good yields. Besides, benzimidazole and ethers were also compatible in this reaction, leading to corresponding C2 ether-substituted heteroarenes.

Modular Synthesis of Alkylarylazo Compounds via Iron(III)-Catalyzed Olefin Hydroamination

A novel Fe-catalyzed olefin hydroamination with aryldiazo sulfones for accessing alkylarylazo compounds has been successfully developed. Aryldiazo sulfones are used as radical acceptors, and N-N double bonds will be regenerated when an arene sulfonyl group leaves. The reaction features mild reaction conditions and a broad substrate scope, allowing access to many azo compounds that would be difficult or perhaps impossible to access using other methods.