Title: 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′ = η5-C5H5 (Cp) or η5-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 σ-complex intermediate, followed by oxidative addition of the C–H bond by the rhodium. A range of σ-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 σ-complex. The unexpectedly large increase in the lifetimes of the σ-complexes from cyclohexane to cycloheptane was predicted to be due to the large range of stabilities of the different σ-complexes found for cycloheptane. The reaction lifetimes were simulated with two mechanisms, with and without migrations among the different σ-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 σ-complexes is faster than the C–H activation for this larger cycloalkane.
Publication Year: 2014
Publication Date: 2014-06-09
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 35
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