Rate Expressions for Abstraction of Hydrogen Atom from a Molybdenum Hydride by Alkyl Radicals

摘要

Rate expressions were determined for the abstraction of hydrogen atom from pentamethylcyclopentadienyltricarbonyl-molybdenum hydride (Me sub 5 CpMo(CO) sub 3 H). The radical clocks hept-6-en-2-yl, produced from bromine atom abstraction from 7-bromoheptene, and hex-5-enyl, produced from bromine atom abstraction from 6-bromohexene, were utilized to compete intramolecular cyclization (kcyc) with bimolecular abstraction (kabs) from Me sub 5 CpMo(CO) sub 3 H in n-octane. Using known rate expressions for the cyclization of the primary clock, and separate rate expressions for cyclization of the secondary clock to give cis- and trans-cyclized products, rate expressions for abstraction were developed from ratios of unrearranged and rearranged hydrocarbon products. For the primary clock, log(kabs/M (sup 1) s (sup -1)) = 9.0 +/- 0.3 - (0.93 +/- 0.2)/theta, theta = 2.3RT kcal/mole, kabs = 2.1 x 10 sup 8 M (sup -1) s (sup -1). For the secondary clock, log(kabs/M (sup -1) s (sup -1)) = 8.6 +/- 0.3 -(0.81 +/- 0.1)/theta, and kabs = 1.0 x 10 (sup 8) M sup -1 s (sup -1). The bimolecular rates are slower by a factor of ten than expected for diffusion control. The activation barriers are lower than activation barriers expected for diffusion-controlled hydrogen abstraction by ca. 1.5 kcal/mole. The results are discussed in terms of the thermochemistry of the reactions and in terms of rates of biomolecular reactions approaching diffusion control. The synthetic utility of the reduction reaction is discussed. (ERA citation 13:037090)