No abstract

No abstract

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.

No abstract

Tris(triisopropylsilyl)silane (iPr 3 Si) 3 SiH has been synthesized and studied by X-ray and neutron diffraction. It possesses an unusual structure in which the four silicon atoms are nearly coplanar, ∠Si-Si-Si ) 118.41(5)°. The Si-H distance is found to have a normal value of 1.506(2) Å. Thermal and room-temperature photochemical decomposition of (iPr 3 -Si) 3 SiH leads to the elimination of iPr 3 SiH and the generation of bis(triisopropylsilyl)silylene, [(iPr 3 Si) 2 Si:]. Reactions of (iPr 3 Si) 2 Si: include precedented insertions into H-Si bonds and addition to the π-bonds of olefins, alkynes, and dienes. Despite theoretical predictions of a triplet ground state for [(iPr 3 Si) 2 Si:], stereospecific addition to cis-and trans-2-butene was observed.