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Carbon-hydrogen activation of cycloalkanes by cyclopentadienylcarbonylrhodium--a lifetime enigma.

Journal of the American Chemical Society (2014-05-16)
Amanda L Pitts, Alisdair Wriglesworth, Xue-Zhong Sun, James A Calladine, Snežana D Zarić, Michael W George, Michael B Hall
ABSTRACT

Carbon-hydrogen bond activation reactions of four cycloalkanes (C5H10, C6H12, C7H14, and C8H16) by the Cp'Rh(CO) fragments (Cp' = η(5)-C5H5 (Cp) or η(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 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.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Cyclohexane, anhydrous, 99.5%
Sigma-Aldrich
Cyclohexane, ACS reagent, ≥99%
Sigma-Aldrich
Cyclohexane, suitable for HPLC, ≥99.9%
Sigma-Aldrich
Cyclohexane, suitable for HPLC, ≥99.7%
Sigma-Aldrich
Cyclohexane, puriss. p.a., ACS reagent, ≥99.5% (GC)
Sigma-Aldrich
Cyclopentane, reagent grade, 98%
Sigma-Aldrich
Cyclohexane, Laboratory Reagent, ≥99.8%
Sigma-Aldrich
Cyclooctane, ≥99%
Supelco
Cyclopentane, analytical standard
Sigma-Aldrich
Cyclohexane, ACS reagent, ≥99%
Sigma-Aldrich
Cycloheptane, 98%
Supelco
Cyclopentane, suitable for HPLC, ≥75% cyclopentane basis