Jaume Castro

Toward the understanding of the mechanism and enantioselectivity of the Pauson-Khand reaction. Theoretical and experimental studies*

Semiempirical and density functional theory (DFT) calculations have been performed on the key steps of the commonly accepted mechanism of the PausonKhand reaction (PKR). In this context, the high reactivity of ynamine complexes in the cycloaddition process has been rationalized on the basis of an anomerically assisted dissociation of CO. Moreover, an explanation has been provided for the correlation between olefin strain and reactivity in the PKR. Inspired by these results, new selective syntheses of cyclopentanones and phenols based on PKR with cyclopropene have been developed. On the other hand, the theoretical analysis of phosphine-substituted dicobalt carbonyl complexes of alkynes has helped in the development of efficient chelating (P,N) and bridging (P,S) ligands for the stereochemical control of the reaction and in the understanding of their action modes.

ASYMMETRIC INDUCTION STUDIES IN THE INTRAMOLECULAR PAUSON-KHAND CYCLIZATION OF 7-ALKOXY-1-HEPTEN-6-YNES

Abstract Asymmetric induction has been achieved in the intramolecular Pauson-Khand reaction of 7-alkoxy-1-hepten-6-ynes derived from chiral alcohols. The stereochemical outcome of the reaction can be explained by the intermediacy of a cis-cobaltabicyclooctane.

Asymmetric synthesis of bicyclo[4.3.0]nonan-8-ones by intramolecular Pauson-Khand reaction

Abstract The intramolecular Pauson-Khand reaction of (E)-1-alkoxy-1-octen-7-ynes derived from (1S,2R)-2-phenylcyclohexanol is described. 7-Alkoxybicyclo[4.3.0]non-1(9)-en-8-ones are obtained in yields up to 65% and with diastereoselectivities higher than 10:1. The absolute configuration of the major diastereomer obtained has been ascertained and can be rationalized on the basis of the preferential conformation of the precursor.

Divergent stereoselective synthesis of (E) and (Z) O-Alkyl enol ethers

Abstract While the reduction of alkynyl ethers with lithium aluminum hydride (or with sodium bis(2-methoxyethoxy) aluminum hydride) gives almost exclusively O-alkyl enol ethers with (E) configuration, the reaction with sodium bis(2-methoxyethoxy) aluminum hydride pretreated with one equivalent of a free alcohol leads stereoselectively to enol ethers with (Z) configuration.

A qualitative molecular mechanics approach to the stereoselectivity of intramolecular Pauson-Khand reactions

Abstract Molecular mechanics calculations on the proposed cobaltacycle intermediates of the Pauson-Khand bicyclization of substituted 6-heptenynes are reported, providing the first theoretical evidence supporting the currently accepted mechanism for the Pauson-Khand reaction. In all instances, the experimentally observed configuration of the predominant bicyclo[3.3.0]oct-1-en-3-one product can be correlated with that of the most stable putative product-determining cobaltacycle, which contains a trans-fused five-membered ring Co-metallocycle.

THE FIRST ALKYNE-DICOBALTCARBONYL COMPLEX WITH A BIDENTATE CHIRAL LIGAND WITH CO-P AND CO-N COORDINATION

Abstract Treatment of the phenylacetylene dicobalthexacarbonyl complex with (R)-2-(2-diphenylphosphinophenyl)-4-phenyl-4,5-dihydrooxazole produces a 85:15 mixture of the diastereomer complexes 2 and 3 that can be separated by recrystallization; the X-ray structure of 2 shows that one cobalt atom is coordinated to both the phosphorus and nitrogen atoms of the ligand.