Vinyl phosphonates are well-known constituents of the family of phosphorus containing organic compounds. They are of extensive importance in synthetic organic chemistry both as intermediates and as final products, and have different applications as monomers and co-monomers in polymeric materials. At present, main methods for the synthesis of vinyl phosphonates are based on Pd, Cu or Ni catalysts, but most of them require the use of phosphine ligands and/or severe reaction conditions. In the last years, we have actively been working in the development of new and mild methodologies based on the use of bare or supported copper nanoparticles (CuNPs) for their application in the construction of C-C and C-heteroatom bonds. In a recent publication, we informed the direct synthesis of vinyl phosphonates catalyzed by CuNPs supported on ZnO starting from aliphatic alkynes and commercial diethyl phosphite. The reactions were carried out in acetonitrile as solvent, in the absence of any additive or ligand, and under mild reaction conditions. Notably, the use of ZnO as support and MeCN as solvent was mandatory for the reaction to take place. Besides, we conducted a series of experiments in order to get some information about the mechanism involved. The addition of TEMPO, a radical scavenger, did not affect the formation of the vinyl phosphonate product; therefore, it should take place through a non-radical process under the reaction conditions. Additionally, the reaction carried out by using 1-deuterio-oct-1-yne as starting alkyne, led to the corresponding deuterated-vinyl phosphonate product, thus suggesting that copper acetylide species would not participate as intermediates, and thus the reaction is more likely to occur via a copper-catalyzed anti-Markovnikov hydrophosphorylation process, leading to the corresponding vinyl phosphonates through the addition of the (EtO)2(HO)P: nucleophile to the carbon-carbon triple bond. We assume that ZnO (Lewis basic sites) could be playing a non-innocent role in the catalytic system, probably through the P-H bond activation. With the aim to explain and understand these experimental results, we performed a computational analysis using DFT methods (GAUSSIAN09). Based on the experimental data, the previous reports by other authors and the results showed by our DFT studies, we have proposed a possible reaction mechanism that implies an active role of ZnO and MeCN in different steps of the reaction. As preliminary results, DFT studies showed a strong coordination between both metals, Cu and Zn, and P=O oxygen atom, consequently, the participation of cyclic structures as intermediates is favoured in these CuNPs/ZnO-catalyzed hydrophosphorylation of aliphatic alkynes.
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Synthesis of vinyl phosphonates from aliphatic alkynes catalyzed by CuNPs/ZnO. A DFT study of the reaction mechanism, effect of the catalyst and solvent.
Published: 30 October 2015 by MDPI in The 19th International Electronic Conference on Synthetic Organic Chemistry session Computational Chemistry
Keywords: DFT methods, phosphorus nucleophiles, vinyl phosphonates, CuNPs/ZnO