Arene Activation by Calcium Hydride/Zinc Amide Equilibration

The β-diketiminato calcium hydride, [(BDI)CaH]2 (BDI = HC{(Me)CNDipp}2, where Dipp = 2,6-i-Pr2C6H3), reacts with [Zn{N(SiMe3)2}2] and [Zn(TMP)2] (TMP = 2,2,6,6-tetramethylpiperidide) to provide labile species and complex equilibria with a degree of commonality in ultimately providing the products of calcium to zinc β-diketiminate transmetalation. The silazide system provides the known [(BDI)Ca{N(SiMe3)2}] and the heterobimetallic, [(BDI)Ca{N(SiMe3)2}(μ-H)Zn]2, as a viable instrument for ligand transfer to yield [(BDI)ZnH] as the final reaction product. In contrast, the TMP-derived system evidences an enhancement in Brønsted basicity, yielding [(BDI)Zn(C6H5)] by deprotonation of the benzene solvent. This process occurs via the heterobimetallic [(BDI)Ca(μ-N{C(CH3)2CH2}2CH)(μ-H)Zn(μ-H)]2 and the previously unreported calcium amide [(BDI)Ca(TMP)]. The [(BDI)Zn(C6H5)] produced by this reaction may be exploited in biphenyl synthesis by a telescoped palladium-catalyzed cross-coupling with bromobenzene, which is unaffected by the presence of the other residual reaction products. This protocol has also been extended to the deprotonation of several further nonactivated arenes to yield the corresponding β-diketiminato arylzinc reagents with only mesitylene providing 3,5-dimethylbenzyl formation by C(sp3)-H deprotonation. Although [(BDI)Ca(TMP)] reacts with both benzene and toluene, with the latter reaction providing the calcium benzyl, [(BDI)Ca(CH2C6H5)], and it is plausible that the latter species arises from initial C(sp2)-H deprotonation and subsequent isomerization, density functional theory (DFT) calculations identify arene deprotonation to be kinetically and thermodynamically disfavored. The bimetallic derivative, [(BDI)Ca(μ-N{C(CH3)2CH2}2CH)(μ-H)Zn(μ-H)]2, cannot be definitively identified as the agent of arene deprotonation. We conclude, however, that the zinc arylation process requires the synergic cooperation of both metals.