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(Sp)PdCl2 [Sp = (−)-sparteine] catalyzes a number of different aerobic oxidation reactions, and reaction of O2 with a PdII–hydride intermediate, (Sp)Pd(H)Cl (1), is a key step in the proposed catalytic mechanism. Previous computational studies suggest that O2 inserts into the PdII–H bond, initiated by abstraction of the hydrogen atom by O2. Experimental and computational results obtained in the present study challenge this conclusion. Oxygenation of in-situ-generated (Sp)Pd(H)Cl exhibits a zero-order dependence on [O2]. This result is inconsistent with a bimolecular H-atom-abstraction pathway, and DFT computational studies identify a novel “reductive elimination” mechanism, in which the chelating nitrogen ligand undergoes intramolecular deprotonation of the PdII–hydride. The relevance of this mechanism to other PdII oxidation catalysts with chelating nitrogen ligands is evaluated.

Benzoquinone (BQ) and O(2) are among the most common stoichiometric oxidants in Pd-catalyzed oxidation reactions. The present study provides rare insights into mechanistic differences between BQ and O(2) in their reactivity with a well-defined Pd-hydride complex, Pd(IMes)(2)(H)(O(2)CPh) (1). BQ promotes the reductive elimination of PhCO(2)H from 1 and catalyzes the formation of a Pd(II)-OOH complex when this reaction is carried out under aerobic conditions. These results have important implications for Pd-catalyzed oxidation reactions.

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Temperature. -Investigation of the asymmetric addition of trimethylsilyl cyanide to aldehydes in the presence of catalytic amounts of Ti(OiPr)4 and various BINOL-salen type ligands (I) reveals that good enantioselectivities are obtained with derivative (Id). -(LI, Z.-B.; RAJARAM, A. R.; DECHARIN, N.; QIN, Y.-C.; PU*, L.; Tetrahedron Lett. 46 (2005) 13, 2223-2226; Dep. Chem., Univ. Va., Charlottesville, VA 22904, USA; Eng.) -Mais 29-031