Recovering from Grignard reactions

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

The use of ketoaminals based on pyrrolidine as chiral auxiliaries has been demonstrated as anotiner entry to optically active a-hydroxycarbonyls. 7 fhe aminals (76 Scheme 6) are readily obtained from a chiral diamine (75) and glyoxal. Addition of Grignard reagents to (76), followed by hydrolysis, provides a chiral a-hychoxy aldehyde (78) with the (5)-contiguration. (Optical purities are measured in die 94—95% range. The chiral diamine can be recovered unchanged from the reaction mixture. 

Chemically, the diazirines are remarkably unreactive and in this they contrast strongly with diazo compounds. They are unaffected by alkalies and are stable in the presence of quite strong acids. (They may be recovered unchanged from solution in lOiV sulfuric acid, though there is a slow reaction in more concentrated solutions.) They do not form complexes with inorganic cations in the way that many azo compounds do, and they are unreactive towards alcohols and dienophiles. They do, however, react with Grignard reagents to yield iV-substituted diazir-idines. 

Cr(CO)s is not present in the final reaction mixture. Hydrolysis yields the Cr(CO)g, which is recovered by steam distillation and further purified by sublimation. Cr(CO)3Pli3 as an intermediate has been proposed, which upon Cr—Ph bond cleavage would yield the Cr(CO)g precursor prior to hydrolysis. Several possibilities about the Cr(CO>6 precursor exist, e.g., that the final carbonylation product could be the anionic species arising from either the insertion reaction (b) or the nucleophilic attack by the Grignard reagent on a carbonyl group of CrfCO) according to equation (c) ... 

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