Why are BINOL-based monophosphites such efficient ligands in Rh-catalyzed asymmetric olefin hydrogenation?

Whereas recent synthetic studies concerning Rh-catalyzed olefin hydrogenation based on BINOL-derived monodentate phosphites have resulted in an efficient and economically attractive preparative method, very little is known concerning the source of the unexpectedly high levels of enantioselectivity (ee often 90-99%). The present mechanistic study, which includes the NMR characterization of the precatalysts, kinetic measurements with focus on nonlinear effects, and DFT calculations, constitutes a first step in understanding this hydrogenation system. The two most important features which have emerged from these efforts are the following: (1) two monodentate P-ligands are attached to rhodium, and (2) the lock-and-key mechanism holds, in which the thermodynamics of Rh/olefin complexation with formation of the major and minor diastereomeric intermediates dictates the stereochemical outcome.

Anthryl-substituted heterocycles as acid-sensitive fluorescence probes.

[reaction: see text] Four pH-sensitive fluorescence probes are presented which consist of an anthracene fluorophore and a pi-conjugated oxazoline, benzoxazole, or pyridine substituent. The protonation of the heterocycles increases their acceptor properties and results in significant red-shifts of the absorption and emission maxima of the anthracene chromophore. The comparison between 2-[2'-(6'-methoxyanthryl)]-4,4-dimethyl-2-oxazoline and 2-[2'-(anthryl)]-4,4-dimethyl-2-oxazoline reveals that the donor-acceptor substitution pattern of the fluorophore is not required to achieve a red shift upon protonation.

Enantioselective Rh-catalyzed hydrogenation of vinyl carboxylates with monodentate phosphite ligands.

[reaction: see text] Alkyl-substituted vinylcarboxylates, which normally show poor enantioselectivity in Rh-catalyzed hydrogenation with traditional chiral diphosphines, undergo highly enantioselective reactions with BINOL- and carbohydrate-based monophosphite ligands.