Kinetics and Thermodynamics of H Transfer from (5-C

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• 作者：Lihao Tang ; Elizabeth T. Papish ; Graham P. Abramo ; Jack R. Norton ; Mu-Hyun Baik ; Richard A. Friesner ; and Anthony Rappé
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：August 20, 2003
• 年：2003
• 卷：125
• 期：33
• 页码：10093 - 10102
• 全文大小：178K
• 年卷期：v.125,no.33(August 20, 2003)
• ISSN：1520-5126

文摘

The rates of H/D exchange have been measured between (a) the activated olefins methylmethacrylate-d₅ and styrene-d₈, and (b) the Cr hydrides (⁵-C₅Ph₅)Cr(CO)₃H (2a), (⁵-C₅Me₅)Cr(CO)₃H(2b), and (⁵-C₅H₅)Cr(CO)₃H (2c). With a large excess of the deuterated olefin the first exchange goes tocompletion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenationis insignificant except with styrene and CpCr(CO)₃H; in most cases, the radicals arising from the first Htransfer are too hindered to abstract another H.) Statistical corrections give the rate constants k_reinit for Htransfer to the olefin from the hydride. With MMA, k_reinit decreases substantially as the steric bulk of thehydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reactionof MMA or styrene with 2a gives the corresponding metalloradical 1a as termination depletes theconcentration of the methyl isobutyryl radical 3 or the -methylbenzyl radical 4; computer simulation of[1a] as f(t) gives an estimate of k_tr, the rate constant for H transfer from 3 or 4 back to Cr. These rateconstants imply a G (50 C) of +11 kcal/mol for H transfer from 2a to MMA, and a G (50 C) of +10kcal/mol for H transfer from 2a to styrene. The CH₃CN pK_a of 2a, 11.7, implies a BDE for its Cr-H bondof 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr-H bond in 2c. In combination thekinetic G values, the experimental BDE for 2a, and the calculated S values for H transfer imply a C-HBDE of 45.6 kcal/mol for the methyl isobutyryl radical 3 (close to the DFT-calculated 49.5 kcal/mol), anda C-H BDE of 47.9 kcal/mol for the -methylbenzyl radical 4 (close to the DFT-calculated 49.9 kcal/mol).A solvent cage model suggests 46.1 kcal/mol as the C-H BDE for the chain-carrying radical in MMApolymerization.