Carbon-Hydrogen Activation of Cycloalkanes by Cyclopentadienylcarbonylrhodium-A Lifetime Enigma

Abstract

Carbon-hydrogen bond activation reactions of four cycloalkanes (C5H10, C6H12, C7H14, and C8H16) by the Cp'Rh(CO) fragments (Cp' = eta(5)-C5H5 (Cp) or eta(5)-C5Me5 (Cp*)) were modeled theoretically by combining density functional and coupled cluster theories, and their reaction rates were measured by fast time-resolved infrared spectroscopy. The reaction has two steps, starting with the formation of a a-complex intermediate, followed by oxidative addition of the C-H bond by the rhodium. A range of a-complex stabilities among the electronically unique C-H bonds in a cycloalkane were calculated and are related to the individual strengths of the C-H bond's interactions with the Rh fragment and the steric repulsion that is incurred upon forming the specific a-complex. The unexpectedly large increase in the lifetimes of the a-complexes from cyclohexane to cycloheptane was predicted to be due to the large range of stabilities of the different sigma-complexes found for cycloheptane.. The reaction lifetimes were simulated with two mechanisms, with and without migrations among the different complexes, to determine if ring migrations prior to C-H activation were influencing the rate. Both mechanisms predicted similar lifetimes for cyclopentane, cyclohexane, and, to a lesser extent, cycloheptane, suggesting ring migrations do not have a large impact on the rate of C-H activation for these cycloalkanes. For cyclooctane, the inclusion of ring migrations in the reaction mechanism led to a more accurate prediction of the lifetime, indicating that ring migrations did have an effect on the rate of C-H activation for this alkane, and that migration among the a-complexes is faster than the C-H activation for this larger cycloalkane.