Asymmetric Pauson–Khand reaction with chiral, electron-deficient mono- and bis-phosphine ligands

Enantioselective synthesis of 5,7-bicyclic ring systems from axially chiral allenes using a Rh(I)-catalyzed cyclocarbonylation reaction

A transfer of chirality in an intramolecular Rh(I)-catalyzed allenic Pauson-Khand reaction (APKR) to access tetrahydroazulenones, tetrahydrocyclopenta[c]azepinones and dihydrocyclopenta[c]oxepinones enantioselectively (22-99% ee) is described. The substitution pattern of the allene affected the transfer of chiral information. Complete transfer of chirality was obtained for all trisubstituted allenes, but loss of chiral information was observed for disubstituted allenes. This work constitutes the first demonstration of a transfer of chiral information from an allene to the 5-position of a cyclopentenone using a cyclocarbonylation reaction. The absolute configuration of the corresponding cyclocarbonylation product was also established, something that is rarely done.

Phosphoramidites: privileged ligands in asymmetric catalysis

Asymmetric catalysis with transition-metal complexes is the basis for a vast array of stereoselective transformations and has changed the face of modern synthetic chemistry. Key to this success has been the design of chiral ligands to control the regio-, diastereo-, and enantioselectivity. Phosphoramidites have emerged as a highly versatile and readily accessible class of chiral ligands. Their modular structure enables the formation of ligand libraries and easy fine-tuning for a specific catalytic reaction. Phosphoramidites frequently show exceptional levels of stereocontrol, and their monodentate nature is essential in combinatorial catalysis, where a ligand-mixture approach is used. In this Review, recent developments in asymmetric catalysis with phosphoramidites used as ligands are discussed, with a focus on the formation of carbon-carbon and carbon-heteroatom bonds.

NMR measurements of scalar-coupling distributions in disordered solids

The measurement of scalar (J) couplings by solid-state NMR is a field of great interest, since this interaction is a rich source of local structural information, complementary to dipolar and chemical shift interactions. Here, we first demonstrate that J-coupling distributions exist and can be observed in disordered solids, as illustrated with the observation of a pair-specific distribution of (2)J((31)P-N-(31)P) couplings in a bis-phosphino amine, and we investigate the potential effects of such distributions on the measurement of average J-coupling constants. Second, we show that the measurement of two-dimensional (2D) distributions of J-couplings provides a much richer probe of local structural disorder than one-dimensional distributions, and we introduce new methods that provide different (selective or non-selective) ways of measuring 2D J distributions in a wide range of disordered systems. These methods are finally applied to a slightly disordered polymorphic sample of fully (13)C-enriched cellulose, and then to the bis-phosphino amine sample, from which 2D (2)J(PP)-coupling distributions are clearly identified and interpreted.

The intermolecular Pauson-Khand reaction

Five membered carbocycles are important building blocks for many biologically active molecules. Moreover, substituted cyclopentenones (e.g. cyclopentenone prostaglandins) exhibit characteristic biological activity. The efficiency and atom economy of the Pauson-Khand reaction render this process potentially one of the most attractive methods for the synthesis of such compounds. Although it was discovered in its intermolecular form, the scope of the intermolecular Pauson-Khand reaction has always been limited by the poor reactivity and selectivity of the alkene component. The past decade, especially the last three years, has seen concerted efforts to broaden the scope of this reaction. In this overview, we provide a comprehensive and critical coverage of the intermolecular Pauson-Khand reaction based on the reactivity characteristics of different classes of alkenes and a rationalization of successes and misfortunes in this area.

Rhodium-Catalyzed Asymmetric Aqueous Pauson-Khand-Type Reaction

An interesting rhodium-catalyzed asymmetric aqueous Pauson-Khand-type reaction was developed. A chiral atropisomeric dipyridyldiphosphane ligand was found to be highly effective in this system. This operationally simple protocol allows both catalyst and reactants to be handled under air without precautions. Various enynes were transformed to the corresponding bicyclic cyclopentenones in good yield and enantiomeric excess (up to 95 % ee). A study of the electronic effects of the enyne substrates revealed a correlation between the electronic properties of the substrates and the ee value obtained in the product of the Pauson-Khand-type reaction. A linear free-energy relationship was observed from a Hammett study.