Experimental and theoretical studies on the (co)cyclotrimerization of alkynes (and ethylene) in a TpRh compound

Abstract

The reactions between the Rh(I) ethylene complex TpRh(C₂H₄)₂ (1; Tp = hydrotris(pyrazolyl)borate), and the alkynes di-tert-butylacetylene dicarboxylate (DTBAD), acetylene, phenylacetylene, and methyl propiolate (MP) have been studied, and the results compared with those obtained previously for dimethylacetylene dicarboxylate (DMAD). Electron withdrawing groups (DTBAD) at both termini of the triple bond stabilize η4-diene-Rh(I) species, while terminal alkynes (acetylene, phenylacetylene) easily lead to η3-allyl-Rh(III) species. The terminal alkyne methyl propiolate, with only one electron withdrawing substituent, exhibits an intermediate behavior, forming both η4-diene-Rh(I) and η3-allyl-Rh(III) species. Two types of intermediate octahedral TpRh(III) rhodacycles have been detected and characterized by NMR. Mechanistic investigations by DFT are also in agreement with the intermediate role of these types of species, which form via oxidative coupling of compounds of type [TpRh(C₂H₄)(C₂R₂)] and [TpRh(C₂R₂)₂]. Ligand exchange reactions to form these species from 1 are instrumental in the outcome of the reactions.