Rhenium acyl complexes deprotonation

However, the more hindered, less basic lithium hexamethyldisilazamide reacts slowly with 1 at 0 °C to provide chemoselectively the desired enolate species 5. The a-protons of these rhenium-acyl complexes are believed to have a lower pKa than the cyclopentadienyl protons, but unless treated with hulky, selective hases the cyclopentadienyl protons exhibit greater kinetic acidity due to statistical factors and an earlier, reactant-like transition state since minimal rchybridiza-tion is required at the anionic center after cyclopentadienyl deprotonation. Equilibration of the cyclopentadienyl anion to the thermodynamically more stable enolate species cannot compete with the rapid acyl migration84. 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

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