RICOCHET INTERRING HAPTOTROPIC REARRANGEMENT OF SIGMA-METHYL-(ETA(5)-INDENYL)CHROMIUM TRICARBONYLS - EXPERIMENTAL KINETIC AND THEORETICAL DFT STUDY

sigma-Methyl-(eta(5)-indenyl) chromium tricarbonyl (III) rearranges qu antitatively into (eta(6)-1-endo-methylindene) chromium tricarbonyl (T V) 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 a ctivation parameters of this ricochet inter-ring haptotropic rearrange ment (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 i rreversible isomerization at 60-120 degrees into (eta(6)-3-methylinden e)chromium tricarbonyl (V) with a higher activation barrier (Delta G# = 28.5 +/- 0.1 kcal mol(-1)) via two consecutive [ 1,5]-sigmatropic hy drogen shifts, The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with ext ended basis sets. Calculations show that the rearrangement (III) --> ( TV) proceeds in two steps. Methyl group migration from chromium into p osition 1 of the indenyl Ligand is the rate-determining step leading t o 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 activatio n barrier 11.8 kcal mol(-1). The coordination of the chromium tricarbo nyl group at the six-membered ring has only minor influence on the kin etic parameters of the hydrogen [ 1,5] -sigmatropic shift in indene. ( C) 1998 Elsevier Science S.A. All rights reserved.