Toward an Understanding of the High Enantioselectivity in the Osmium-Catalyzed Asymmetric Dihydroxylation. 4. Electronic Effects in Amine-Accelerated Osmylations

Abstract

Electronic effects in osmylation reactions accelerated by pyridine and quinuclidine derivatives were investigated by varying the substituents on the amine ligand as well as on the alkene substrate. Ligand substituent effects were gauged by determination of the equilibrium constants for coordination of the amines to OsO 4 , evaluation of structural properties and reduction potentials of the amine-OsO 4 complexes, and analysis of the kinetics of osmylations in the presence of the amines. Substrate substituent effects were gauged by kinetic Hammett studies using several different amine/alkene combinations. Nonlinear Hammett relationships resulting from alkene substituent effects were observed, and the deviation from a linear free energy relationship was found to depend on the structure, binding capacity, and concentration of the amine. The results were evaluated in terms of the contending "[3 + 2]" and "[2 + 2]" mechanisms currently under consideration. A change in mechanism that depends on the structural and electronic properties of both alkene and amine is proposed.

Electronic Effects in Olefin Oxidation by Imidoosmium(VIII) Compounds

“…Such a behaviour was also observed by Sharpless for related Os-catalysed reactions in the presence of a chiral tertiary amine ligand. [26] If the data for the p-NO 2 /p-H pair is omitted, a linear regression gives a p-value of Ϫ0.55 (Figure 3) which is in good agreement with the expected electrophilic character of the 16e-oxidant 1. A comparable result had been determined by Henbest for dihydroxylation of styrenes.…”

Improving enantioselectivity via rationally tuning electronic effect of catalysts in the organocatalytic asymmetric aldol reaction

“…1 H NMR and 13 C NMR spectra were recorded on a Varian Mercury 200 (200 MHz), or a Varian Mercury Plus 300 (300 MHz) spectrometer in CDCl 3 with TMS as an internal standard or in [d 6 ]DMSO. Mass spectra were obtained on a Brucker ESQUIRE~LC TM ESI ion trap mass spectrometer.…”

Diradical versus Concerted Mechanisms for the Dihydroxylation of Protoanemonin by OsO4 and OsO4−NH3 − The Effect of the Base in the Reaction

“…These results are in agreement with experiment, although experimental data show greater decreases in the activation barriers. [16,17] Of all the olefins studied, protoanemonin shows the greatest reduction in activation energy by the coordination of a base to the catalyst. The activation barriers are reduced by 3.2 or 3.1 kcal/mol depending on the regioisomer.…”

“…[14,15] Experiments show that adding a base to the osmium tetroxide increases the reaction rate. [16,17] The fact that the presence of an amine accelerates the reaction rate has been [a] Unitat de Química Física, Departament de Química, Edifici C.n., Universitat Autò noma de Barcelona, 08193 Bellaterra, Catalonia, Spain Fax: (internat.) ϩ 34-93/581-2920 E-mail: gregori@klingon.uab.es…”