Titanocene dichloride complexes bonded to carbosilane dendrimers via a spacer of variable length – Molecular dynamics calculations and catalysis of allylic coupling reactions

Abstract

Carbosilane metallodendrimers of the first generation Si{CH2CH2CH2SiMe2CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5}4 and Si{CH2CH2CH2SiMe[CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5]2}4 (n=1, 5) and of the second generation Si{CH2CH2CH2SiMe[CH2CH2CH2SiMe2CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5]2}4 (n=1, 5) and Si{CH2CH2CH2SiMe[CH2CH2CH2SiMe(CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5)2]2}4 (n=5) were prepared, purified using GPC-NMR and GPC, and characterized by multinuclear NMR, IR and ESI-TOF mass spectrometry.Computer atomistic models of the second generation metallodendrimers with the length of carbon spacer n=1, 3, and 5 carbon atoms were created, parametrized, and consequently simulated in THF solvent using molecular dynamics. The equilibrated dendrimer model structures were analyzed to provide inter alia density distribution of selected molecular components. It was shown that (i) average distance between the closest pairs of Ti atoms (around 11Å), (ii) permanent accessibility of Ti atoms from many sites even if they were in close contact with another Ti atom, and (iii) high dynamics of titanocene groups were favorable conditions to fully use all Ti atoms for the catalytic reaction in case of the longest five carbon spacer.Dendrimers of the second generation with 8 and 16 end groups and the five carbon spacer were used in the catalytic system with manganese metal to catalyze allylic coupling reactions of cinnamyl and geranyl bromides with low catalyst loadings.