Publications

A new approach to determination of the stereochemical structure of bis-selenium-substituted alkenes using experimental ⁷⁷Se NMR studies and B3LYP/6-311G(d) quantum-chemical calculations is developed. Joint analysis of experimental and calculated data allows assignment of signals in the ⁷⁷Se NMR spectrum. The method was evaluated taking the model compounds (PhSe)HC=C(SePh)R (R = COOMe, CH₂NMe₂, CH₂OH, Ph) as examples.

A mechanistic study of the hydroselenation of alkynes catalyzed by Pd(PPh ₃)₄ and Pt(PPh₃)₄ has shown that the palladium complex gives products of both Se-H and Se-Se bond addition to the triple bond of alkynes, while the platinum complex selectively catalyzes Se-H bond addition. The key intermediate of PhSeH addition to the metal center, namely Pt(H)(SePh)(PPh₃)₂, was detected by ¹H-NMR spectroscopy. The analogous palladium complex rapidly decomposes with evolution of molecular hydrogen. A convenient method was developed for the preparation of Markovnikov hydroselenation products H₂C-C(SePh)R, and the scope of this reaction was investigated. The first X-ray structure of the Markovnikov product H₂C-C(SePh)CH₂N⁺HMe₂HOOC-COO⁻ is reported.

Palladium catalyzed hydroselenation of alkynes gives the products of both Se-H and Se-Se bonds addition to the triple bond, while platinum complex selectively catalyzes Se-H bond addition.

Comparative study of the intramolecular alkyne triple bond addition reaction to the conjugated ­C≡C—CH X moiety (X = CH₂, O, S, NH) revealed that two different pathways are possible in the system, namely [4 + 2] and [3 + 2] cycloaddition reactions. The energetically preferred pathway for enynes (X = CH₂) involves [4 + 2] cycloaddition leading to benzene derivatives, whereas heteroatom-substituted substrates undergo [3 + 2] cycloaddition resulting in a five-membered aromatic ring in the final product. This paper reports a detailed mechanistic study based on full potential energy surface calculations at the MP2 and B3LYP theory levels, with MP4(SDTQ) energy evaluation. The effect of solvent was included within the PCM approach.

The present study explains the different catalytic activities of platinum and palladium in Se−Se addition reactions with alkynes. Under the catalytic conditions cis-[Pt(SePh)₂(PPh₃)₂] undergoes fast isomerization to the trans isomer, which does not react with alkynes. Palladium complexes maintain their catalytic activity, due to the formation of the dinuclear structure [Pd₂(SePh)₄(PPh₃)₂]. It was shown that the palladium intermediate involved in the catalytic cycle can be prepared directly in the reaction mixture starting from the simple [PdCl₂(PPh₃)₂] precursor, thus allowing replacement for the traditional Pd(PPh₃)₄ catalyst. X-ray analysis shows that the products of Se−Se addition reactions with alkynes possess the necessary geometry parameters for coordination as bidentate ligands.

The mechanistic study of palladium catalyzed S–S and Se–Se bonds addition to alkynes revealed the involvement of dinuclear transition metal complexes in the catalytic cycle. Coordination of alkyne to dinuclear transition metal complex was found to be the rate determining step of the reaction. An unusual phosphine ligand effect increasing the yield of addition reaction was found in the studied system. A new synthetic procedure was developed to perform the catalytic reaction using easily available Pd(II) complex. The scope of the reaction and the reactivity of S–S and Se–Se bonds toward alkynes were investigated. The X-ray structure of the product of S–S bond addition reaction showed favorable geometry for the possible application as a chelate ligand.

An unusual phosphine ligand effect increasing the yield of the Ar ₂E₂ addition reaction to alkynes was found. The catalytic reaction involves intermediate formation of dinuclear palladium complexes, which may be a subject of further polymerization.