Structure-selectivity comparisons are made between chiral dirhodium(I1) tetrakis( methyl 2-oxopyrrolidine-5-carboxylates), R~z (~S -M E P Y )~ and Rhz(5R-MEPY)d (5), and dirhodium(I1) tetrakis(4-benzyl-2-oxazolidinones), Rh2(4R-BNOX)4 and Rh2(4S-BNOX)4 (6), to ascertain and understand their relative effectiveness as catalysts for enantiocontrol in metal-carbene transformations. The syntheses, spectral characteristics, and X-ray structures for these dirhodium(I1) compounds are reported. Each possesses two oxygen-and two nitrogen-donor atoms bound to each octahedral rhodium with a cis orientation of the nitrogen ligands. The Rhz(MEPY)4 catalysts are significantly more effective than those of Rh2(BNOX)4 in providing a high level of enantiocontrol in intermolecular and intramolecular cyclopropanation reactions, in intermolecular cyclopropenation reactions, and in intramolecular C-H insertion reactions of diazoacetates and diazoacetamides, often reaching >90% enantiomeric excesses. Molecular mechanics calculations that were able to reproduce the X-ray structures of R~z (~S -M E P Y )~ and Rh2(4R-BNOX)4 have been employed to obtain the preferred conformation of the intermediate metal-carbene, but the absolute configurations of cyclopropanation products are opposite to those predicted from the preferred metal-carbene conformation. However, conformational energy minima of the styrene-carbene complex predict the observed enantiomer preference. Dirhodium(I1) compounds possessing four chiral pyrrolidone or oxazolidinone ligands are effective catalysts for highly enantioselective metal-carbene transformations.14 Optical yields of greater than 90% have been achieved in olefin cyclopropanation with diazoacetate esters,* alkyne cyclopropenation,3 and even carbon-hydrogen insertion reactions.4 The basis for this high level of enantiocontrol has been attributed to the organization of the four chiral amide ligands around the dirhodium nucleus and the electronic orientation and the stabilization of the intermediate metal-carbene.5 A limited number of dirhodium(I1) tetrakis(carb0xamidate) compounds have been synthesized and characterized. Those formed from acetamide6 and trifl~oroacetamide~ are constructed so that in the major isomer two oxygen-and two nitrogen-donor atoms (2,2) are bonded to each octadehedral rhodium in a cis configuration (eq 1). When prepared in an acetamide melt by tH, 1 successive substitutions of acetate from dirhodium(I1) tetraacetate, only one tetrakis(acetamidate) 1 has been obtained: but Doyle, M. P.; Brandes, B. D.; Kazala, A. P.; Pieters, R. J.; Jarstfer, M. B.; Watkins, L. M.; Eagle, C. T. Tetrahedron Lett. 1990, 31, 6613. (2) Doyle, M. P.; Pieters, R. J.; Martin, S. F.; Austin, R. E.; Oalmann, C. J.; Miiller, P.

During the oxidation of toluene under semibatch conditions, the formation of benzyl alcohol is initially equal to the rate of formation of benzaldehyde. As the overall conversion increases the benzyl alcohol concentration at first decreases much faster than benzaldehyde, but this decrease slows down causing the benzyl alcohol concentration to reduce to zero only very slowly. To account for this phenomenon a new reaction pathway has been proposed where the formation of benzaldehyde out of benzylhydroperoxide is catalysed by benzoic acid. Incorporation of this new reaction in a model improves the description of benzyl alcohol concentration prophiles while maintaining good predictions for benzaldehyde and benzoic acid.

The diffusion-reaction equations for different model versions have been solved using a finite-differencing technique. In all models a reactant A is transferred from the gas to the liquid phase and reacts in the liquid with B to form P. The calculations comprised a simple stoichiometric model, a system with radical intermediates involved in the propagation steps and a version where also the termination reactions were included. The results show that the diffusion coefficients of radical intermediates can have significant influence on the profiles of concentrations and reaction rates near the G/L interface. Furthermore, it is shown that for very fast reactions differences in diffusion coefficients of the intermediates influence the by-product formation. For systems of two radical intermediates, the so-called mixed termination product is only formed in low quantities whereas the other two termination products dominate. The calculation of enhancement factors required in the design of a G/L reaction system can be performed with simplified models where the reactive intermediates do not occur in the expressions for the reaction rates. The optimum model for a specific design purpose can be found by tuning the functions that correlate the parameters of the complex model to the parameters of the simplified model. In principle it is possible to very easily evaluate a large number of alternatives. ᭧