Trapping, Stabilization, and Characterization of an Enolate Anion of a 1,6-Adduct of Benzophenone Chelated by a Sodium Alkylamidozincate Cation

There has been a recent upsurge of activity in the study of alkali metal zincate reagents due to their often special reactivity/selectivity in, for example, deprotonative metalation and nucleophilic addition reactions. Heteroleptic dialkylamidozincates, [M+Zn(R)2(NR'2)-], usually transfer selectivity of the amide ligand to electrophiles. Here, in contrast, it is reported that the sodium zincate [TMEDA.Na(mu-tBu)(mu-TMP)Zn(tBu)] reacts as an alkylating agent toward the diaryl ketone benzophenone (Ph2C=O), selectively adding one of its tBu ligands to the para-C atom of one of the Ph rings. The reaction can be carried out at room temperature, which is a decided advantage over lithium reagents as these are generally utilized at subambient temperatures. The stabilizing effect of the bimetallic (Na, Zn) cationic residue of the starting zincate reagent in coordinating to the dearomatized enolate anion of the 1,6-addition adduct allows the adduct to be isolated in a pure crystalline form. An X-ray crystallographic study of the adduct reveals a molecular structure based on a near-planar, four-element (NaOZnN) ring with a TMP-N and an enolato-O bridge. The Na and Zn atoms also carry terminal TMEDA (N,N'-attached) and tBu (C-attached) ligands, respectively. Also included are 1H/13C NMR spectroscopic data for the adduct when dissolved in cyclohexane-d12 solution.