(Chiral) lithium-(magnesium-)zinc and lithium-cobalt combinations as dual reagents for aromatic deproto-metalation and aryl transfer to aldehydes.
Résumé
The deprotonating ability of mixed lithium-zinc or lithium-magnesium-zinc combinations contg. amido and alkyl ligands in THF were compared using anisole as substrate and iodine to quant. trap the formed arylmetal species. The results showed that the deprotonating ability is hampered if a Grignard reagent is employed to introduce the alkyl ligand, and is reduced when 2,2,6,6-tetramethylpiperidino ligands are replaced by less hindered/basic chiral amido or alkyls. Concerning the interception of the generated lithium-zinc aryl species by aldehydes, the presence of amido ligands leads to side reactions/lower yields, and no clear improvement was obsd. if lithium-magnesium-zinc aryl species are used. Racemic mixts. to very low enantioselectivities were noted when chiral amido ligands were incorporated in the compn. of the bases. Still with enantioselective aryl transfer to aldehyde as purpose, the deprotonating ability of mixed lithium-cobalt combinations contg. amido and alkyl ligands were compared using anisole as substrate and anisaldehyde to trap the formed arylmetal species. As before, the deprotonating ability is reduced when 2,2,6,6-tetramethylpiperidino ligands are replaced by less hindered/basic alkyls or chiral amido. The trapping step using aldehydes being in this case more efficient, even in the presence of amido ligands, the alcs. were obtained in higher yields. With recourse to a lower interception temp., and using only bis[(R)-1-phenylethyl]amino as ligands, 32 and 22% yield, and 69 and 65% ee were obtained using, resp., anisaldehyde and 3,4,5-trimethoxybenzaldehyde to intercept the metalated anisole. [on SciFinder(R)]