Chromium complexes lithium derivatives

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

With A-pivaloyl 2,6-dimethylaniline chromium complex 69, the chiral lithium amide 70 derived from l-phenyl-2(4/-methylpiperazinyl)ethylamine turned out to be the base of choice for the asymmetric lithiation at the benzylic methyls (Table 3). With almost all electrophiles, chromium complexes bearing different substituents on the nitrogen atom could be obtained in up to 99% ee92,93. In contrast, the chiral bases (R,R)-3, (R)-27 and (S)-4 resulted in modest asymmetric induction ranging from 44% to 78% ee. 

The lithiated derivative (LiCgH5)Cr(CO)3 has been prepared in high yield by the reaction of [(CO)3Cr](CaHgHgC6H5)[Cr(CO)3] with re-butyl lithium. Complexes, such as 2-phenylpyridine chromium tricarbonyl and (Ph2PCgHg)Cr(CO)3, which are not otherwise obtainable were prepared by the reaction of the lithiated derivative with pyridine and PhaPCl, respectively 348). Benzene chromium tricarbonyl has been metalated by treatment with re-butyl lithium in THF and after carbona-tion yielded w-benzoic acid chromium tricarbonyl 304). 

Removai of the methoxy group and its substitution by cyanomethyl in the protected estrone derivative shown in the form of the arene chromium tricarbonyl complex in tetrahydrofuran solution, was achieved in 46% yield by treatment at -78 C with lithioacetonitrile (from acetonitrile in tetrahydrofuran containing hexamethylphosphorictriamide and lithium di-isopropylamide at -78°C) followed by stirring for 4 hours at ambient temperature (ref. 104). 

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