Me 2 NC 2 H 4 C 5 Me 4 Li reacts with Cr(THF) 3 Cl 3 to give (η 1 :η 5 -Me 2 NC 2 H 4 C 5 Me 4 )CrCl 2 , in which the complexation of the N-donor atom to the metal atom has been confirmed by X-ray crystallography. A series of related compounds, e.g. (η 1 :η 5 -cyclo-C 4 H 8 NSiMe 2 OSiMe 2 C 5 H 4 )-CrCl 2 , has been prepared by varying the substituents on the organic ligand. Further reaction with organomagnesium reagents leads to formation of the corresponding dialkyl-Cr complexes. Related species have been prepared containing imine-, alkoxy-, and alkylthiosubstituted cyclopentadienyl groups as well as the C-donor ligand tetramethylimidazol-2ylidene. Treatment of these compounds with methylalumoxane (MAO) leads to the formation of highly active catalysts for the oligomerization, polymerization, and copolymerization of ethylene.

The chemical synthesis of organic molecules involves, at its very essence, the creation of carbon-carbon bonds. In this context, the aldol reaction is among the most important synthetic methods, and a wide variety of catalytic and stereoselective versions have been reported. However, aldolizations yielding tertiary aldols, which result from the reaction of an enolate with a ketone, are challenging and only a few catalytic asymmetric Mukaiyama aldol reactions with ketones as electrophiles have been described. These methods typically require relatively high catalyst loadings, deliver substandard enantioselectivity or need special reagents or additives. We now report extremely potent catalysts that readily enable the reaction of silyl ketene acetals with a diverse set of ketones to furnish the corresponding tertiary aldol products in excellent yields and enantioselectivities. Parts per million (ppm) levels of catalyst loadings can be routinely used and provide fast and quantitative product formation in high enantiopurity. In situ spectroscopic studies and acidity measurements suggest a silylium ion based, asymmetric counteranion-directed Lewis acid catalysis mechanism.

A series of phosphinoalkyl-substituted cyclopentadienylchromium complexes, e.g., (η 1 :η 5 -R 2 PC 2 H 4 C 5 H 4 )CrCl 2 , have been synthesized. The complexation of the P atom to the chromium has been confirmed by crystal structure determinations. Activated by methylalumoxane (MAO), these compounds catalyze the oligomerization and polymerization of ethylene. The catalytic activity and the degree of oligomerization are controlled by the steric properties of the substituents on the donor atom: the smaller the substituent, the greater the activity and the lower the molecular weight of the oligomer. Density functional calculations indicate that propagation and termination through β-hydrogen transfer are competitive for small substituents and that the latter process, which involves spin inversion, becomes selectively disfavored with increasing size of the substituents. For large substituents β-hydrogen elimination to the chromium atom is found to be the preferred route for termination, although this reaction is less facile than insertion.

We report a highly efficient, general and practical method for the aldol condensation of acetone with aromatic and aliphatic aldehydes, using morpholinium trifluoroacetate as a catalyst.