Helicenes: synthesis and applications.

Allenes are the simplest class of cumulenes, with two contiguous CC bonds, and show unique physical and chemical properties. These features make allenes particularly attractive in modern organic chemistry. In this Review, attention is paid to the advances made in catalytic asymmetric synthesis and natural product syntheses based on well-established reactions of allenes, such as propargylation, addition, cycloaddition, cycloisomerization, cyclization, etc., with or without catalysts. Their versatile reactivity, substituent-loading ability, axial to center chirality transfer, and controllable selectivity allow access to target molecules by unique and efficient approaches. The main topics in this Review are presented with selected examples from 2003 to 2011.

Allenes in Catalytic Asymmetric Synthesis and Natural Product Syntheses

This critical review covers advances in anion complexation in the year 2010. The review covers both organic and inorganic systems and also highlights the applications to which anion receptors can be applied such as sensing, anion transport, control of molecular motion and gelation (179 references).

Anion receptor chemistry: highlights from 2010

Much attention has been paid to the cross-coupling reaction of organosilicon compounds due to their stability, non-toxicity, and natural abundance of silicon. In addition, the silicon-based cross-coupling has many advantages over other cross-coupling protocols. Successful examples of the silicon-based cross-coupling reaction are reviewed, focusing especially on the advances made in the last decade. Having had a number of highly effective palladium catalysts developed mainly for other cross-coupling reactions, the development of the silicon-based protocol owes heavily to the design of organosilicon reagents which effectively undergo transmetalation, a key elemental step of the silicon-based cross-coupling reaction. This tutorial review thus classifies various organosilicon reagents depending on substituents on silicon and surveys their cross-coupling reactions with various electrophiles.

Silicon-based cross-coupling reaction: an environmentally benign version

Multisubstituted isoquinoline and pyridine N-oxides have been prepared by Rh(III)-catalyzed cyclization of oximes and diazo compounds via aryl and vinylic C–H activation. This intermolecular annulation involving tandem C–H activation, cyclization, and condensation steps proceeds under mild conditions, obviates the need for oxidants, releases N2 and H2O as the byproducts, and displays a broad substituent scope.

Rh(III)-catalyzed synthesis of multisubstituted isoquinoline and pyridine N-oxides from oximes and diazo compounds.

Helical chiral 2-aminopyridinium ions were designed as a significantly more acidic (active) dual hydrogen-bonding catalyst than commonly used (thio)urea-based systems. The helicene framework was specifically utilized to position an inherently chiral barrier on the hydrogen-bonding side of the catalyst. The catalyst reactivity and enantioselectivity were successfully demonstrated in additions of 4,7-dihydroindoles to nitroalkenes (0.5−2 mol % catalyst loadings, up to 98:2 er).

Helical Chiral 2-Aminopyridinium Ions: A New Class of Hydrogen Bond Donor Catalysts

Helical chiral 2,2'-bipyridine N-monoxides as catalysts in the enantioselective propargylation of aldehydes with allenyltrichlorosilane

Stable and reusable tetraorganosilicon reagents, alkenyl-, aryl-, and silyl[2-(hydroxymethyl)phenyl]dimethylsilanes, undergo 1,4-addition reactions to α,β-unsaturated carbonyl acceptors under mild rhodium-catalysis. The reaction tolerates a diverse range of functional groups and is applicable to gram-scale synthesis. Use of a chiral diene ligand allows the achievement of the corresponding enantioselective transformations using the tetraorganosilicon reagents, providing the silicon-based approach to optically active ketones and substituted piperidones that serve as synthetic intermediates of pharmaceuticals. A rhodium alkoxide species is suggested to be responsible for a transmetalation step on the basis of the observed kinetic resolution of a racemic chiral phenylsilane in the enantioselective 1,4-addition reaction under the rhodium−chiral diene catalysis.

Organo[2-(hydroxymethyl)phenyl]dimethylsilanes as mild and reproducible agents for rhodium-catalyzed 1,4-addition reactions.

Highly efficient synthesis of silyl-substituted oligoarenes with defined structures is achieved by an iterative cross-coupling reaction and deprotection sequence using organo[(2-hydroxymethyl)phenyl]dimethylsilanes. The excellent stability of the tetraorganosilicon compounds as well as mild and divergent conditions for cross-coupling and deprotection steps allows preparation of highly conjugated oligoarenylsilanes such as unsymmetrically disilylated quinquethiophenes.

A silicon-based approach to oligoarenes by iterative cross-coupling reactions of halogenated organo[(2-hydroxymethyl)phenyl]dimethylsilanes.

Optically active chiral alkyl chlorides are valuable compounds because of their bioactivity and versatile synthetic utility. Accordingly, the ring opening of epoxides with a chloride nucleophile stands as an important goal in asymmetric catalysis. We describe herein recent advances in the design and development of chiral pyridine N-oxide catalysts for the enantioselective synthesis of chlorohydrins.

Jinshui Chen

Papers

Helical Chiral 2-Aminopyridinium Ions: A New Class of Hydrogen Bond Donor Catalysts

Helical chiral 2,2'-bipyridine N-monoxides as catalysts in the enantioselective propargylation of aldehydes with allenyltrichlorosilane

Organo[2-(hydroxymethyl)phenyl]dimethylsilanes as mild and reproducible agents for rhodium-catalyzed 1,4-addition reactions.

A silicon-based approach to oligoarenes by iterative cross-coupling reactions of halogenated organo[(2-hydroxymethyl)phenyl]dimethylsilanes.

Helical chiral pyridine N-oxides: a new family of asymmetric catalysts.