Aryl Grignard reagents groups

Preparation of the chiral biphenyls and binaphthyls with high enantiose-lectivity can be achieved via substitution of an aromatic methoxyl group with an aryl Grignard reagent using oxazoline as the chiral auxiliary.38 Schemes 8-10 and 8 11 outline the asymmetric synthesis of such chiral biaryl compounds. 

The behavior of BnMgBr (5d) is similar to that observed for compounds with higher alkyl groups, i.e. only the coupling product was detected and the earlier report on the additional formation of benzyl alcohol was not confirmed. On the other hand, reactions of Ar" radicals formed in the anodic oxidation of aryl Grignard reagents are different from those established for Aik, as is evident from the percent distribution of parent radicals in major products given in Table 7. 

Later, Molander et al. extended this reaction to aryl-Grignard reagents by lowering the temperature and optimizing the solvent [9]. Futhermore, use of excess of alkenyl halide, as described in the initial mechanistic work of Kochi s group, proved to be unnecessary. 

In contrast to the carbometallation with aryl-Grignard reagents to unfunctionalized alkynes, this reaction does not require a co-catalyst such as copper. Only in one example, applying PhMgBr, was CuBr added in accord with the protocol reported by Hayashi s group. Although the nature of the catalytically active species remains unclear, an alkoxide-directed carbometallation which yields a (vinyl)iron intermediate is proposed. 

After exchange of the protecting group, the Weinreb amide 111 (an alternative to nitriles for the formation of ketones discussed in detail in Strategy and Control) reacted with the aryl Grignard reagent to give, after reduction, 112—the protected version of the alcohol 106 required for cyclisation. 

---