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 6 D 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.