Zinc is the 24th most abundant element in the Earth’s crust. Since the discovery of pure zinc metal in the 18th century, the main application of zinc metal is as materials. Because zinc salts are abundant, cheap, non-toxic, and exhibit environmentally benign properties, organic chemists have been interested in using zinc salts as catalysts in organic synthesis during the last three decades. Within this review, we have summarized the progress on applications of zinc salts in organic reactions. Our the target is to emphasize the special properties of zinc catalysts, and further promote the interest of organic chemists.

Zinc-Catalyzed Organic Synthesis: CC, CN, CO Bond Formation Reactions

The current syntheses of imines from benzylamines are often performed in organic solvents or under harsh reaction conditions. Clean oxidation of primary benzylamines to imines has been successfully achieved using H2O2 in water at room temperature catalyzed by V2O5. Among the 10 imine products, 5 of them precipitated from the reaction and led pure products after simple filtration. No organic solvents are needed in the whole process. The yields are good to quantitative. This represents an efficient and green procedure of the synthesis of imines. A similar green oxidation of benzylamines to aromatic aldehydes is also reported. A benzylic anion-involved mechanism is proposed based on the experiments.

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Convenient and clean synthesis of imines from primary benzylamines.

Four purely chiral supramolecular frameworks were constructed from a new chiral terpyridyl ligand, (S)-2{4-[2,6-bis(pyridin-2-yl)pyridin-4-yl]benzylamino}propanoic acid (H2ptpy). The resulting compounds 1–4, namely [Cd2(Hptpy)2(Ac)2]·3CH3OH·H2O (1) [Ni2(Hptpy)2(CH3OH)(H2O)](ClO4)2·CH3OH·H2O (2), [Ni2(Hptpy)2(H2O)2](SCN)2 (3), and [Cu2(Hptpy)2](dca)2·4H2O (4) (Ac = acetate and dca = dicyanamide) all consist of those chiral metallocyclic units. As expected, those chiral nanometallocycles not only induced a chiral array or helicates but also amplified chirality through hydrogen-bonding interactions, and finally aggregated into complicated higher-dimensional chiral supramolecular architectures with chiral voids for further use. The formation of the metallocyclic units is easily monitored by UV/Vis spectra. Spectrophotometric titrations were performed by titrating Na(Hptpy) with a soltution of Cd(Ac)2 in methanol, and the global stability constants of [Cd2(Hptpy)2]2+ (log β = 17.60) were calculated. In combination with the structural analyses and UV/Vis and ES mass spectra, it is tentatively deduced that metallocycle units in 1–4 are the thermodynamically favored species in solution. Both the chiral H2ptpy ligand and 1 in the solid state and in solution display purple emission. The bathochromic shift of 1 is mainly due to the bichelation and intrametallocyclic π–π interactions.

Chiral Supramolecular Frameworks Based on Chiral Metallocycles: Crystal Structures and Photophysical Properties

Oxidant controlled ionic liquid mediated cross-dehydrogenative coupling (CDC) of benzylamines with N-heterocycles having sp2 or sp3 carbon resulted in the formation of C-benzoylated or alkenylated products. Benzoylation of N-heterocycles occurs via (NH4)2S2O8 catalyzed benzoyl radical formation. An oxidative alkenylation of N-heterocycles having C-sp3 carbon (2-methylaza-arenes) occurs via deamination of benzylamine followed by C-sp3-H bond activation in high stereoselectivity. Both benzoylation and alkenylation protocols are metal-free, green, simple, efficient, and tolerate a wide variety of functional groups.

Oxidant-Controlled C-sp2/sp3–H Cross-Dehydrogenative Coupling of N-Heterocycles with Benzylamines

An insight into a base-free Michael addition reaction as catalyzed by a bifunctional nickel N-heterocyclic carbene complex using density functional theory studies

Michael addition reactions of bis(amino acidato)metal(II) complexes (metal = copper, nickel, zinc; amino acid = glycine, dl-alanine, l-alanine) with acrylonitrile have been carried out under variou...

V. Rajalakshmi

Papers

Novel Michael Addition Products of Bis(amino acidato)metal(II) Complexes: Synthesis, Characterization, Dye Degradation, and Oxidation Properties