36495-37-1Relevant articles and documents
Kinetics and thermodynamics of H? transfer from (η 5-C5R5)Cr(CO)3H (R = Ph, Me, H) to methyl methacrylate and styrene
Tang, Lihao,Papish, Elizabeth T.,Abramo, Graham P.,Norton, Jack R.,Baik, Mu-Hyun,Friesner, Richard A.,Rappe, Anthony
, p. 10093 - 10102 (2003)
The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d5 and styrene-d8, and (b) the Cr hydrides (η5-C5Ph5)Cr(CO) 3H (2a), (η5-C5Me5)Cr(CO) 3H (2b), and (θ5-C5H 5)Cr(CO)3H (2c). With a large excess of the deuterated olefin the first exchange goes to completion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenation is insignificant except with styrene and CpCr(CO)3H; in most cases, the radicals arising from the first H? transfer are too hindered to abstract another HH?.) Statistical corrections give the rate constants kreint for H? transfer to the olefin from the hydride. With MMA, kreint decreases substantially as the steric bulk of the hydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reaction of MMA or styrene with 2a gives the corresponding metalloradical 1a as termination depletes the concentration of the methyl isobutyryl radical 3 or the α-methylbenzyl radical 4; computer simulation of [1a] as f(t) gives an estimate of ktr, the rate constant for H? transfer from 3 or 4 back to Cr. These rate constants imply a ΔG (50 °C) of +11 kcal/mol for H? transfer from 2a to MMA, and a ΔG (50 °C) of +10 kcal/mol for H? transfer from 2a to styrene. The CH3CN pKa of 2a, 11.7, implies a BDE for its Cr-H bond of 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr-H bond in 2c. In combination the kinetic ΔG values, the experimental BDE for 2a, and the calculated ΔS values for H? transfer imply a C-H BDE of 45.6 kcal/mol for the methyl isobutyryl radical 3 (close to the DFT-calculated 49.5 kcal/mol), and a C-H BDE of 47.9 kcal/mol for the α-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 MMA polymerization.
Hydrogen atom transfer reactions of transition-metal hydrides. Kinetics and mechanism of the hydrogenation of α-cyclopropylstyrene by metal carbonyl hydrides
Morris Bullock,Samsel, Edward G.
, p. 6886 - 6898 (2007/10/02)
The hydrogenation of α-cyclopropylstyrene (CPS) by a series of metal carbonyl hydrides (MH) gives a mixture of the unrearranged hydrogenation product Ph(CH3)(c-C3H5)CH (UN) and the rearranged hydrogenation product (E)-Ph(CH 3)C=CHCH2CH3 (RE). With the exception of HCr(CO)3Cp, second-order kinetics are found, conforming to the rate law -d[CPS]/dt = k[CPS][MH]. The proposed mechanism involves hydrogenation by sequential hydrogen atom transfers from the metal hydride to the organic substrate. The rate-determining step is the first hydrogen atom transfer in which a carbon-centered radical and a metal-centered radical are formed. In the case of HCr(CO)3Cp at 22 °C, the equilibrium constant for this step is K ～ 10-12. The effect of the significant amount of 17-electron *Cr(CO)3Cp radical formed in the hydrogenation of CPS by HCr(CO)3Cp is accommodated by the kinetic analysis. Since the initially formed carbon-centered radical undergoes first-order ring-opening rearrangement in competition with second-order trapping by MH, analysis of the product ratio as a function of [MH] concentration provides relative rates of hydrogen atom transfer from metal hydrides to a carbon-centered radical. Relative rates of hydrogen atom transfer at 60 °C from MH to 1 are as follows: krel = 1 for HMn(CO)4PPh3, krel = 4 for HMo(CO)3(C5Me5), krel = 93 for HMo(CO)3Cp, krel = 94 for HFe(CO)2(C5Me5). Comparison of the hydrogenation of CPS by HW(CO)3Cp and DW(CO)3Cp indicates that the kinetic isotope effect is inverse (kHW/kDW = 0.55) for the first hydrogen atom transfer but normal (kHW/kDW = 1.8-2.2) for the second hydrogen atom transfer. The first hydrogen atom transfer is endothermic, and its rate is largely influenced by the strength of the M-H bond. Steric effects appear to exert a dominant influence on the rate of the second hydrogen atom transfer, which is exothermic. Kinetic and mechanistic experiments indicate that hydrogenation of 2-cyclopropylpropene by HCr(CO)3Cp also occurs by a radical pathway.
