Group 6 pentacarbonyl acetates

GROUP 6 PENTACARBONYL ACETATES AS THEIR p-NITRIDO-BIS(TRIPHENYLPHOSPHORUS)(l +) SALTS... 

Group 6 pentacarbonyl acetato complexes have been also found to undergo addition of alkyl halides (Mel) to produce alkyl acetates and the pentacarbonyl halide metal complex. When allyl halides are used instead of alkyl halides, an oxidative addition product is formed, which contains the allyl, halide, and acetate ligands bonded to the metal. 

Aminolysis of methoxy carbene metal pentacarbonyl complexes of the chromium group yields amino carbene compounds. If the aminolysis is performed with amino acid esters, amino carbene derivatives of amino acids are obtained [162,163]. The metal carbene is relatively stable under a variety of conditions but can be removed with TFA. It may therefore be regarded as an amino protecting group in peptide synthesis. The metal amino carbene amino acid may be activated and coupled to other amino acid esters. Following this idea, Weiss and Fischer prepared various dipeptides, the tripeptide (OC)5Cr(Ph)-Ala-Ala-Ala-OMe 69 [162] and the tetrapeptide (OC)5Cr(Ph)-Gly-Gly-Pro-Gly-OMe 70 (Scheme 5.35) [163]. Treatment with cone. TFA at 20 °C for 10 min. removes the metal carbene group and furnishes the free peptide esters along with Cr(CO)g. Removal of the metal carbene is also possible with 80% acetic acid (80 °C, 30 min.), which leaves Boc... 

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