Dimerization of aryl alkynes by Grubbs-Hoveyda complex: investigation of factors affecting the efficiency of the process based on experimental and theoretical approaches

Abstract

The 2nd generation Grubbs-Hoveyda complex in the presence of slight excess of triphenylphosphine was found as an efficient catalyst for the synthesis of 1,4-disubstituted enynes with preference for head-to-head coupling and Z-selectivity form in the dimerization of various terminal aryl alkynes. Selective product formation was affected by the steric and the electronic properties of the phosphine used as well as the substituents of the alkynes. In order to shed some light on the properties of the alkynes that may affect the yield of desired products a set of 14 alkyne derivatives was characterized on the basis of Density Functional Theory (DFT) using ωB97XD functional and 6-311++G(d,p) basis set. The Atoms in Molecules (AIM) theory was employed for electronic structure and topological analyses of the investigated compounds. The quantitative description of the substituent effect was carried out with assistance of the Harmonic Oscillator Model of Aromaticity (HOMA), charge of the Substituent Active Region (cSAR(X)) and Substituent Effect Stabilization Energy (SESE). It was found that substituents in the phenyl ring are able to affect the geometric and electronic structure of the alkynyl moiety. The experimental and theoretical results revealed that high activity of the investigated ruthenium complex towards dimerization of alkynes is related with σ-donor ability and bulkiness of added phosphine combined with small size aryl alkyne bearing electron-withdrawing group. However, the influence of the structure of aryl alkyne on the catalyst activity could not be easily explained only on the basis of steric and electronic factors.