Hirohisa Ohmiya

In nature, a number of enzymes use thiamine diphosphate as a coenzyme to catalyze the pyruvate decarboxylation. The resultant enamine, a so-called "Breslow intermediate," is known to perform single electron transfer to various electron acceptors. Inspired by this enzymatic catalysis, N-heterocyclic carbene (NHC)-catalyzed radical reactions have been developed. This minireview highlights the recent progress and developments in NHC-based radical catalysis. This minireview is categorized according

N-Heterocyclic carbene (NHC) organocatalysis to promote so-called umpolung reactions has emerged as a powerful tool for modern organic synthesis. NHC-bound nucleophiles known as Breslow intermediates can be obtained by reacting an NHC catalyst with an aldehyde and will add to carbon–heteroatom or carbon–carbon double bonds. On the other hand, the substitution-type cross-couplings of carbon electrophiles (that is, R–X species) with NHC-bound nucleophiles to generate carbon–carbon bonds have also

Visible-light-mediated chemical processes have been vigorously studied and have led to state-of-the-art synthetic chemistry since they enable the control of radical generation and excited-state-based transformations. The essential process is the generation of a radical species via single electron transfer (SET) between the substrate and catalyst. While photoredox chemistry is an important methodology, these systems essentially require photocatalysts and involve redox processes of the catalyst in

A cobalt-diamine complex catalyzes the cross-coupling reactions of primary and secondary alkyl halides with aryl Grignard reagents. It is confirmed that oxidative addition of alkyl halide to cobalt proceeds via a radical process. Optically pure Ueno-Stork halo acetals undergo diastereoselective cross-coupling reactions, the products of which are transformed into optically active THF derivatives. A sequential radical cyclization/arylation reaction under cobalt catalysis provides extremely short a

There have been significant advancements in radical reactions using organocatalysts in modern organic synthesis. Recently, NHC-catalyzed radical reactions initiated by single electron transfer processes have been actively studied. However, the reported examples have been limited to catalysis mediated by alkyl radicals. In this article, the NHC organocatalysis mediated by aryl radicals has been achieved. The enolate form of the Breslow intermediate derived from an aldehyde and thiazolium-type NHC

Allyl-aryl coupling between allylic acetates and arylboronic acids took place in the presence of catalytic amounts of Pd(OAc)(2), 1,10-phenanthroline, and AgSbF(6) with high gamma-selectivity and E/Z-selectivity. The reaction of an optically active allylic acetates with an alpha-stereogenic center proceeded with excellent alpha-to-gamma chirality transfer with syn-selectivity and gave the corresponding optically active allyl-aryl coupling products with a stereogenic center at the benzylic positi

Reactions between acyclic (E)-allylic acetates and arylboronic acids in the presence of a palladium catalyst prepared from Pd(OAc)2, phenanthroline (or bipyridine), and AgSbF6 (1:1.2:1) proceeded with excellent γ-selectivity to afford allyl−aryl coupling products with E-configuration. The reactions of α-chiral allylic acetates took place with excellent α-to-γ chirality transfer with syn stereochemistry to give allylated arenes with a stereogenic center at the benzylic position. The reaction tole

Details of cobalt-catalyzed cross-coupling reactions of alkyl halides with allylic Grignard reagents are disclosed. A combination of cobalt(II) chloride and 1,2-bis(diphenylphosphino)ethane (DPPE) or 1,3-bis(diphenylphosphino)propane (DPPP) is suitable as a precatalyst and allows secondary and tertiary alkyl halides--as well as primary ones--to be employed as coupling partners for allyl Grignard reagents. The reaction offers a facile synthesis of quaternary carbon centers, which has practically

[reaction: see text] This paper describes cobalt-mediated cross-coupling reactions of alkyl halides with 1-(trimethylsilyl)ethenylmagnesium bromide and 2-(trimethylsilyl)ethynylmagnesium bromide, respectively. The cobalt system allows for employing secondary as well as primary alkyl halides as the substrates. The reactions offer facile formations of alkyl-alkenyl and alkyl-alkynyl bonds. The reaction mechanism would include single-electron transfer from a cobalt complex to alkyl halide to genera

Carboxylation of alkylboron compounds (alkyl-9-BBN) with CO(2) proceeded in the presence of catalytic amounts of CuOAc/1,10-phenanthroline and a stoichiometric amount of KO(t)Bu. The alkylboranes are easily and widely available through the alkene hydroboration, and thus the overall process represents a reductive carboxylation of alkenes with CO(2). The broad functional group compatibility and the inexpensiveness of the Cu/1,10-phenathoroline catalyst system are attractive features of this protoc

Copper-catalyzed allyl-alkyl coupling between allylic phosphates and alkylboranes, prepared by hydroboration of alkenes with 9-BBN-H, takes place with complete gamma- and E-selectivities and with preferential 1,3-anti stereochemistry. The reaction tolerates various functional groups in both the allylic phosphate and alkylborane. Catalytic mechanisms involving transmetalation between a trialkyl(alkoxo)borate and a copper(I) complex to form an alkylcopper(I) species are proposed.

Radical–radical coupling, the selective reaction between two different radical species, has contributed to the methodology for connecting bulky units. Light-driven N-heterocyclic carbene (NHC) organocatalysis is recognized as a state-of-the-art methodology enabling radical–radical coupling. The catalytic process involves forming an acyl azolium intermediate from the NHC catalyst and an acyl donor, followed by single electron reduction of this key intermediate, which is largely dependent on the p

The Cu-Xantphos system [Cu(O-t-Bu)-Xantphos, 10-15 mol %] catalyzes the intramolecular hydroamination of unactivated terminal alkenes bearing an unprotected aminoalkyl substituent in alcoholic solvents, giving pyrrolidine and piperidine derivatives in excellent yields. This system is applicable to both primary and secondary amines and tolerates a variety of functional groups.

The generation of tertiary, secondary, and primary alkyl radicals has been achieved by the direct visible-light excitation of a boracene-based alkylborate. This system is based on the photophysical properties of the organoboron molecule. The protocol is applicable to decyanoalkylation, Giese addition, and nickel-catalyzed carbon-carbon bond formations such as alkyl-aryl cross-coupling or vicinal alkylarylation of alkenes, enabling the introduction of various C(sp³) fragments to organi

Copper-catalyzed γ-selective coupling between propargylic phosphates and alkylboron compounds (alkyl-9-BBN, prepared by hydroboration of alkenes with 9-BBN-H) affords multisubstituted allenes with various functional groups. The reaction of enantioenriched propargylic phosphates to give axially chiral allenes proceeds with excellent point-to-axial chirality transfer with 1,3-anti stereochemistry.