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Chemical Catalysis towards a Sustainable Development:
Transformation of Bio-Resources and Atom-Efficient Reactions
Catalyzed by Bio-Metals

Final Report Summary - CHEMCATSUSDE (ChemCatSusDe: Chemical Catalysis towards a Sustainable Development: Transformation of Bio-Resources and Atom-Efficient Reactions Catalyzed by Bio-Metals)

The ChemCatSusDe project (PIIF-GA-2010-271877) aimed at addressing the fundamentals of some of the industrial and economical issues of our society: fossil fuel depletion, utilization of renewable resources derived from the biomass, and environmentally friendly chemistry for a sustainable future. At its core lied the versatile chemistry of alkaline-earth elements: magnesium, calcium (two bio-metals) and strontium and barium (two less-known benign metals) for the preparation of a new family of green catalysts capable of performing both the polymerization of cyclic esters and highly atom-efficient hydroelementation reactions. During the 18 months of this project, a series of tasks revolving around synthetic organometallic chemistry and catalytic applications were achieved successfully, as summarized below:

1) Series of heteroleptic alkaline-earth metal amides and alkyls have been isolated on large scales, by exploiting the stabilizing effect of secondary intramolecular interactions. Meanwhile, the first stable and readily accessed barium complexes, including an imino-anilide barium alkyl complex, were synthesized, offering real potential as catalysts for a variety of organic transformations.

2) In the presence of alcohol, the above complexes were demonstrated to mediate the immortal ring-opening polymerization (iROP) of L-lactide (a fully bioresourced monomer) highly efficiently, converting thousands of monomer equiv in a highly controlled fashion, with very high catalytic activity (up to 21,200 molL-LA•(molAe•h)–1 (Scheme 1).

3) These complexes display high aptitude in the catalysis of three key transformations: the cyclohydroamination of amino-alkenes, and the intermolecular hydroamination and hydrophosphination of activated alkenes. Perfect anti-Markovnikov regioselectivity was observed in the case of intermolecular hydroamination and hydrophosphination of styrene derivatives, whereas ring-closure is also strictly regioselective (5-exo-trig) during the cyclohydroamination of terminal amino-alkenes. In reactions of styrene with pyrrolidine, the catalytic activity of an imino-anilide barium complex, measured under mild conditions, exceeded by 1 to 2 orders of magnitude those reported to date for intermolecular hydroamination reactions catalyzed by alkaline-earth, rare-earth or even late-transition metal complexes. Details of mechanisms of these processes have been obtained through kinetic studies, which allowed to rationalize most of the experimental results.

These results have been published in top-ranked, peer-reviewed international journals such as Angew. Chem. Int. Ed., J. Am. Chem. Soc., Chem. Eur. J., etc.
They have opened avenues for the development of alkaline earth organometallic chemistry –a very rich, still nascent research field-, for further optimization of catalytic performance in known processes, with possible breakthrough as compared to known catalysts, both in terms of selectivity and activity.

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