Shedding new light on ZnCl2-mediated addition reactions of Grignard reagents to ketones: structural authentication of key intermediates and diffusion-ordered NMR studies

Building on recent advances in synthesis showing that the addition of inorganic salts to Grignard reagents can greatly enhance their performance in alkylation reactions to ketones, this study explores the reactions of EtMgCl with benzophenone in the presence of stoichiometric or catalytic amounts of ZnCl(2) with the aim of furthering the understanding of the role and constitution of the organometallic species involved in these transformations. Investigations into the metathesis reactions of three molar equivalents of EtMgCl with ZnCl(2) led to the isolation and characterisation (X-ray crystallography and (1)H and (13)C NMR spectroscopy) of novel magnesium "zinc-rich" zincate [{(THF)(6)Mg(2)Cl(3)}(+){Zn(2)Et(5)}(-)] (1), whose complicated constitution in THF solutions was assessed by variable-temperature (1)H DOSY NMR studies. Compound 1 reacted with one equivalent of benzophenone to yield magnesium magnesiate [{(THF)(6)Mg(2)Cl(3)}(+){Mg(2)(OC(Et)Ph(2))(2)Cl(3)(THF)}(-)] (3), whose structure was determined by X-ray crystallography. (1)H NMR monitoring of this reaction showed two equivalents of ZnEt(2) formed as a co-product, which together with the "magnesium only constitution" of 3 provides experimental insights into how zinc can be efficiently recycled in these reactions, and therefore used catalytically. The chemoselectivity of this reaction can be rationalised in terms of the synergic effect of magnesium and zinc and contrasts with the results obtained when benzophenone was allowed to react with EtMgCl in the absence of ZnCl(2), where the reduction of the ketone takes place preferentially. The reduction product [{(THF)(5)Mg(3)Cl(4){OC(H)Ph(CF(3))}(2)] (4) obtained from the reaction of EtMgCl with 2,2,2-trifluoroacetophenone was established by X-ray crystallography and multinuclear ((1)H, (13)C and (19)F) NMR spectroscopy. Compounds 3 and 4 exhibit new structural motifs in magnesium chemistry having MgCl(2) integrated within their constitution, which highlights the new role of this inorganic salt in providing structural support for the newly generated alkoxide ligand.

Structurally defined potassium-mediated zincation of pyridine and 4-R-substituted pyridines (R = Et, iPr, tBu, Ph, and Me2N) by using dialkyl-TMP-zincate bases

Two potassium-dialkyl-TMP-zincate bases [(pmdeta)K(mu-Et)(mu-tmp)Zn(Et)] (1) (PMDETA = N,N,N',N'',N''-pentamethyldiethylenetriamine, TMP = 2,2,6,6-tetramethylpiperidide), and [(pmdeta)K(mu-nBu)(mu-tmp)Zn(nBu)] (2), have been synthesized by a simple co-complexation procedure. Treatment of 1 with a series of substituted 4-R-pyridines (R = Me(2)N, H, Et, iPr, tBu, and Ph) gave 2-zincated products of the general formula [{2-Zn(Et)(2)-mu-4-R-C(5)H(3)N}(2)2{K(pmdeta)}] (3-8, respectively) in isolated crystalline yields of 53, 16, 7, 23, 67, and 51%, respectively; the treatment of 2 with 4-tBu-pyridine gave [{2-Zn(nBu)(2)-mu-4-tBu-C(5)H(3)N}(2)2{K(pmdeta)}] (9) in an isolated crystalline yield of 58%. Single-crystal X-ray crystallographic and NMR spectroscopic characterization of 3-9 revealed a novel structural motif consisting of a dianionic dihydroanthracene-like tricyclic ring system with a central diazadicarbadizinca (ZnCN)(2) ring, face-capped on either side by PMDETA-wrapped K(+) cations. All the new metalated pyridine complexes share this dimeric arrangement. As determined by NMR spectroscopic investigations of the reaction filtrates, those solutions producing 3, 7, 8, and 9 appear to be essentially clean reactions, in contrast to those producing 4, 5, and 6, which also contain laterally zincated coproducts. In all of these metalation reactions, the potassium-zincate base acts as an amido transfer agent with a subsequent ligand-exchange mechanism (amido replacing alkyl) inhibited by the coordinative saturation, and thus, low Lewis acidity of the 4-coordinate Zn centers in these dimeric molecules. Studies on analogous trialkyl-zincate reagents in the absence and presence of stoichiometric or substoichiometric amounts of TMP(H) established the importance of Zn-N bonds for efficient zincation.

Direct bonds between metal atoms: Zn, Cd, and Hg compounds with metal-metal bonds

The synthesis and characterization of [Zn(2)(eta(5)-C(5)Me(5))(2)], a stable molecular compound with a Zn-Zn bond and the first example of a dimetallocene structure, has opened a new chapter in the organometallic chemistry of zinc and in metallocene chemistry. The existence of two directly bonded zinc atoms demonstrates that the [Zn-Zn](2+) unit, the lightest Group 12 homologue of the well-known [Hg-Hg](2+) ion, can be stabilized by appropriate ligands. Activity in this area has increased enormously in the few years since the determination of the structure of this molecule. Numerous theoretical studies have been devoted to the investigation of the electronic, structural, and spectroscopic properties of this and related compounds, and new metal-metal coordination and organometallic compounds of zinc, cadmium, and mercury have been synthesized and structurally characterized. This Minireview gives an overview of activity in this field during the past three to four years.