Ricochet inter-ring haptotropic rearrangement of σ-methyl-( η5-indenyl) chromium tricarbonyls. Experimental kinetic and theoretical DFT study

Abstract

σ-Methyl-( η 5-indenyl) chromium tricarbonyl ( III) rearranges quantitatively into η 6-1-endo-methylindene) chromium tricarbonyl ( IV) in C 6D 6 solution at 30–60°C. Methyl group attachment to the positions 2 or 3 of indenyl ligand in ( III) has no influence on the activation parameters of this ricochet inter-ring haptotropic rearrangement ( ΔG #=23.6 kcal mol −1; ΔH #=18.9±0.2 kcal mol −1; ΔS #=−18.6±0.2 cal K −1 mol −1). ( IV) undergoes further irreversible isomerization at 60–120° into ( ν 6-3-methylindene) chromium tricarbonyl ( V) with a higher activation barrier ( ΔG #=28.5±0.1 kcal mol −1) via two consecutive [1,5]-sigmatropic hydrogen shifts. The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with extended basis sets. Calculations show that the rearrangement ( III) → ( IV) proceeds in two steps. Methyl group migration from chromium into position 1 of the indenyl ligand is the rate-determining step leading to the formation of the 16-electron intermediate ( VII). The calculated activation barrier ( E a=19.6 kcal mol −1) is in good agreement with the experimental one. Further rearrangement ( VII) → ( V) proceeds via a trimethylenemethane-type transition state ( XVIII) with an activation barrier 11.8 kcal mol −1. The coordination of the chromium tricarbonyl group at the six-membered ring has only minor influence on the kinetic parameters of the hydrogen [1,5]-sigmatropic shift in indene.