Ricochet inter-ring haptotropic rearrangement of sigma-methyl-(eta(5)-indenyl)chromium tricarbonyls. Experimental kinetic and theoretical DFT study

摘要

sigma-Methyl-(eta(5)-indenyl) chromium tricarbonyl (III) rearranges quantitatively into (eta(6)-1-endo-methylindene) chromium tricarbonyl (TV) in C6D6 solution at 30-60 degrees 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 (Delta G# = 23.6 kcal mol(-1); Delta H# = 18.9 +/- 0.2 kcal mol(-1);Delta S# = -18.6 +/- 0.2 cal K-1 mol(-1)). (IV) undergoes further irreversible isomerization at 60-120 degrees into (eta(6)-3-methylindene)chromium tricarbonyl (V) with a higher activation barrier (Delta G# = 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) --> (TV) 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. (C) 1998 Elsevier Science S.A. All rights reserved. [References: 30]