Pyrazolylborate−Zinc Alkoxide Complexes. 1. Basic Properties, Methylations, and Heterocumulene Insertions

Abstract

While a solution of TpPh,MeZn-OH in methanol contains only traces of TpPh,MeZn-OMe, according to the equilibrium constant K = 5.8 x 10(-4), the reactions of TpPh,MeZn-OH with the electronegative alcohols trifluoroethanol and hexafluoro-2-propanol easily yield TpPh,MeZn-OCH2CF3 and TpPh,MeZn-OCH(CF3)2. The extremely hydrolytically sensitive TpPh,MeZn-OR complexes, with R = Me, Et, i-Pr, and CH2CH2F, as well as TpCum,MeZn-OR, with R = Me and i-Pr, are accessible from the Tp*Zn-hydride complexes and the corresponding alcohol. Alkylations with methyl iodide have revealed the high nucleophilicity of TpPh,MeZn-OMe by conversion to dimethyl ether and TpPh,MeZn-I. This conversion occurs rapidly not only with pure TpPh,MeZn-OMe but also with TpPh,MeZn-OH (as such or in the presence of methanol) and with TpPh,MeZn-OCOOMe. A relation of the Tp*Zn-alkoxides to the function of the zinc enzyme alcoholdehydrogenase exists in the reaction of TpPh,MeZn-OCH(CH3)2 with aromatic aldehydes, which yields acetone and the corresponding benzyl oxides TpPh,MeZn-OH2Ar. The heterocumulenes carbon dioxide, carbon disulfide, isothiocyanates, and one isocyanate are inserted into the Zn-OR bonds, yielding one alkyl carbonate complex (TpPh,MeZn-OC(O)OMe), two xanthogenate complexes (TpPh,MeZn-SC(S)OR), three iminothiocarbonate complexes (TpPh,MeZn-SC(NR')OR), and one alkyl carbamate complex (TpPh,MeZn-NR-COOMe). All insertion reactions can be described by a common mechanism involving a four-center intermediate in which the most basic heteroatom of the heterocumulene is attached to zinc.

Zinc Thiolate Complexes [ZnLn(SR)]+ with Azamacrocyclic Ligands, Part III: The Influence of the Ligand Ln on the Reactivity of Zinc‐Bound Thiolate

“…[12][13][14][15]26,27] In this case, the methylation (as well as any other reaction) is restricted to occur at the thiolate side of the complex cation. As the coordination of the reacting molecules to Zn II implicates a steric demand, the degree of obstruction of the zinc(II) ion by the macrocycle should have a pivotal influence on the geometry of the transition state.…”

Zinc Oxide‐Catalyzed Dehydrogenation of Primary Alcohols into Carboxylic Acids

“…As ar esult, the homogeneous speciesi sm ost likely az inc alkoxide which would also explain why the transformationc an be catalyzed by avariety of other zinc(II) speciestog ive similar resultsa sw ith zinc oxide.T here is to the best of our knowledge no evidence in the literature for ac lassical b-hydride elimination pathway with zinc alkoxides to form zinc hydrides. [25] The enzyme alcohol dehydrogenase reacts by hydride elimination from az inc alkoxide, but the hydride is transferred to NAD + . [26] This, however,d oes not rule out that az inc hydride can be formed at elevated temperature and especially if the carbonyl compound is removed from the mixture.…”

Designing Heterogeneous Catalysts by Incorporating Enzyme-Like Functionalities into MOFs

“…The active site is proposed to contain a Zn atom in a tetrahedral coordination sphere connected to histidine ligands. Model studies [44,45] have so far activated CO 2 , but the process stopped at the bicarbonate complex (''product inhibition''). Figure 17 is an illustration of the Zn-complex from Ref.…”