Reactivity and Regioselectivity of Methylacetylene Cyclotrimerization over the Phillips Cr/Silica Catalyst: A DFT Study

Abstract

The mechanism of the methylacetylene cyclotrimerization catalyzed by the Phillips Cr/silica catalyst has been studied by DFT investigations based on a Cr(II)/SiO2 cluster model and a silica supported cluster model. Twenty-one kinds of Cr(II)/SiO2·(C3H4)n (n = 1–3) complexes were first optimized successfully. Starting from the most stable chromium(methylacetylene) complex, the following cyclotrimerization of methylacetylene on the quintet surface is prohibited by the spontaneous coupling of the two coordinated methylacetylenes. Instead of overcoming a much higher Gibbs free energy barrier by about 40 kcal/mol on the quintet surface, a spin flipping to the triplet surface at the chromium(methylacetylene) complex only requires 16.9 kcal/mol in Gibbs free energy. After the spin transition, the methyl-chromacyclopropene species was formed immediately on the triplet surface. The triplet dimethyl-chromacyclopentadiene species was generated by insertion of a coordinated methylacetylene into the 3-membered metallacycle ring. The following reaction may follow two pathways: (a) a concerted [4 + 2] cycloaddition or (b) a stepwise pathway (insertion and reductive elimination) via a trimethyl-chromacycloheptatriene species. All the eight [4 + 2] cycloaddition reaction pathways are favored competing with the stepwise pathways. The reactivity of each reaction pathway can be examined in terms of the calculated TOF using the energetic span model. We found that only four [4 + 2] cycloaddition reaction pathways (PES-T1Da, PES-T2Da, PES-T3Da, and PES-T4Da) are responsible for the cyclotrimerization of methylacetylene. The PES-T4Da leads to the production of 1,3,5-trimethylbenzene, while the other three pathways generate 1,2,4-trimethylbenzene. Furthermore, the effects of the silica support and the dispersion correction have been considered for the most plausible reaction pathways PES-T1Da, PES-T2Da, PES-T3Da, and PES-T4Da, respectively. Finally, with a consideration of the effects of the silica support and inclusion of the dispersion correction in the final calculated energies, the ratio of the 1,3,5- to 1,2,4-TMB is 0.32 at 363 K predicting that the 1,2,4-TMB is the dominant product in the cyclotrimerization of methylacetylene catalyzed by the Phillips Cr/silica catalyst.