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Final Report Summary - HYDROXCAT (Novel catalytic reactions involving O-H bonds)

The main goals of our original proposal can be summarized as the development of mechanistic understanding of the catalytic alcohol dehydrogenation to esters and catalytic amidation of alcohols with amines by PNP and PNN Ru pincer complexes (Scheme 1). The aforementioned is a very significant “green” catalytic dehydrogenation reaction whose only byproduct is hydrogen gas, that was developed earlier in our group.1 we planned to design and synthesize superior pyridine-based pincer ligands and complexes for these reactions towards improving their outcomes, and perhaps lead to the discovery of new catalytic pathways. Our studies indicated that the amidation and esterification catalysis occurred via an unprecedented cooperation between the metal and the ligand involving aromatization-dearomatization of the pincer ligand (Scheme 2).

It proved difficult to obtain kinetic information due to the nature of the reaction (i.e. it must be carried out at high temperatures under a flow of Argon gas, thus ruling out the use of NMR techniques). We approached the problem by attempting to design new pincer ligands that would create a more stable dearomatized complex, believed to be the active form of the catalyst that had stubstituents such as methyl or pyridyl groups on the phosphorous and nitrogen atoms. Towards this goal, we developed a new technique for the synthesis of modified PNN and PNP ligand scaffolds with a variety of substituents, such as those shown in Scheme 2.

The ligands of Scheme 2 were synthesiszed, and ruthenium complexes thereof were prepared and studied. For example, the Me-PNN ligand was complexed to Ru and the activation of substrates such as alcohols and ammonia was studied. According to our initial predictions, activation of the substrate by a dearomatized Me-PNN-Ru complex would lead to one single species with the methyl group on the phosphorous arm facing upwards toward the hydride ligand. This stereospecific reaction would hinder catalysis, as one of the most crucial steps in the catalytic cycle is hydrogen transfer from the arm to the metal hydride ligand, eventually generating dihydrogen. With a methyl group on the same side as the hydride ligand, the coupling would be blocked and further reactivity would not occur.

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WEIZMANN INSTITUTE OF SCIENCE
Israel
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