Crystallographic characterisation of organolithium and organomagnesium intermediates in reactions of aldehydes and ketones

We report reactions of LiNacNac and NacNacMg(TMP) with common organic substrates (TMP=2,2,6,6-tetramethylpiperidide). Using bulky β-diketiminate compound NacNac (Me, Dipp), we have trapped metalated intermediates amenable to X-ray crystallographic study. LiNacNac and acetone produced the NacNac-free hexameric diacetone alkoxide [{LiO(MeC(=O)CH ₂C(Me) ₂} ₆]; whereas pinacolone gave the simple monomeric donor–acceptor complex [Li{(MeCN-2,6- ⁱPr ₂C ₆H ₃) ₂CH}{O=C(Me) ^tBu}]. Benzaldehyde produced dimeric [Li{(MeCN-2,6- ⁱPr ₂C ₆H ₃) ₂CC(=O)Ph}{O=C(Ph)OCH ₂Ph}] ₂, where one benzaldehyde molecule has inserted in to the γ-carbon position of the NacNac ligand, with a molecule of benzyl benzoate derived from a Tishchenko reaction of the aldehyde acting as a terminal donor to lithium. Tetranuclear, mixed dimer [{LiO(MeCN-2,6- ⁱPr ₂C ₆H ₃) ₂CH-CH(Ph)} ₂{LiO(PhC(=O)} ₂] containing the same modified NacNac ligand was the fortuitous product from repeating the reaction of LiNacNac and benzaldehyde in a solution presumably contaminated with benzoic acid. Combining NacNacMg(TMP) with pinacolone affords monomeric heteroleptic [Mg{(MeCN-2,6- ⁱPr ₂C ₆H ₃) ₂CH}{OC( ^tBu)(Me)CH ₂C(=O) ^tBu}] with an aldolate constituting the second anion. Completing the set is dimeric [Mg{(MeCN-2,6- ⁱPr ₂C ₆H ₃) ₂CH}{O(C=CH ₂)Ph} ₂], where the enolate derived from acetophenone bridges the magnesium centres.