TY - JOUR
T1 - Carbon-hydrogen activation of cycloalkanes by cyclopentadienylcarbonylrhodium-A lifetime enigma
AU - Pitts, Amanda L.
AU - Wriglesworth, Alisdair
AU - Sun, Xue Zhong
AU - Calladine, James A.
AU - Zarić, Snežana D.
AU - George, Michael W.
AU - Hall, Michael B.
PY - 2014/6/18
Y1 - 2014/6/18
N2 - Carbon-hydrogen bond activation reactions of four cycloalkanes (C 5H10, C6H12, C7H 14, 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.
AB - Carbon-hydrogen bond activation reactions of four cycloalkanes (C 5H10, C6H12, C7H 14, 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.
UR - http://www.scopus.com/inward/record.url?scp=84902687617&partnerID=8YFLogxK
U2 - 10.1021/ja5014773
DO - 10.1021/ja5014773
M3 - Article
AN - SCOPUS:84902687617
SN - 0002-7863
VL - 136
SP - 8614
EP - 8625
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 24
ER -