CpCo(I)- and Cp*Ru(II)Cl-catalyzed [2+2+2] cycloadditions of siladiynes and alkynes: A combined experimental and theoretical study

Benzo-2-silaindanes (3a-g) and benzo-2,3-tetralines (4a-g) were synthesized by CpCo(CO)(2) and Cp*Ru(cod)Cl catalyzed cyclotrimerization of siladiynes (1-2a,b) with alkynes (RC)-C-1 CR2 (R-1, R-2 = H, CO2Me, Ph, Ts). X-ray analyses of 3f and 4g confirmed the formation of silicon-carbon heterocycles (silacycles). The different reactivity of alkynes depend on Co- and Ru-catalysts suggesting the different mechanism. Stoichiometric NMR studies as well as DFT calculations revealed that heterolytic coupling occurs between the siladiyne and alkyne followed by an internal [4+2] cycloaddition for CpCo(I)-catalyzed reactions, whereas Cp*Ru(II)Cl-catalyzed reactions proceeded via homolytic coupling of siladiynes to afford a ruthenacyclopentatriene followed by insertion of the alkynes. Minimum energy crossing point (MECP) calculations for the former heterocoupling reaction disclosed different two-state reaction compared to known two-state one. The latter intermediate was characterized in NMR experiments as the first reported complex of a ruthenacyclopentatriene formed from terminal diyne. (C) 2019 Elsevier B.V. All rights reserved.