Kinetics and thermodynamics of H center dot transfer from (eta(5)-C5R5)Cr(CO)(3)H (R = Ph, Me, H) to methyl methacrylate and styrene

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The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d(5) and styrene-d(8), and (b) the Cr hydrides (eta(5)-C5Ph5)Cr(CO)(3)H (2a), (eta(5)-C5Me5)Cr(CO)(3)H (2b), and (eta(5)-C5H5)Cr(CO)(3)H (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)(3)H; in most cases, the radicals arising from the first H. transfer are too hindered to abstract another He.) Statistical corrections give the rate constants k(reinit) for H. transfer to the olefin from the hydride. With MMA, k(reinit) 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 la as termination depletes the concentration of the methyl isobutyryl radical 3 or the alpha-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 rate constants imply a DeltaG (50 degreesC) of +11 kcal/mol for H. transfer from 2a to MMA, and a DeltaG (50 degreesC) of +10 kcal/mol for H. transfer from 2a to styrene. The CH3CN pK(a) 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 DeltaG values, the experimental BDE for 2a, and the calculated DeltaS 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 alpha-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.
Publisher
AMER CHEMICAL SOC
Issue Date
2003-08
Language
English
Article Type
Article
Keywords

CATALYTIC CHAIN TRANSFER; BOND-DISSOCIATION ENERGIES; FREE-RADICAL POLYMERIZATION; TRANSITION-METAL HYDRIDES; DENSITY-FUNCTIONAL THEORY; SOLVATION FREE-ENERGIES; ATOM TRANSFER-REACTIONS; HYDROGEN; COMPLEXES; MECHANISM

Citation

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, v.125, no.33, pp.10093 - 10102

ISSN
0002-7863
DOI
10.1021/ja0349271
URI
http://hdl.handle.net/10203/203364
Appears in Collection
CH-Journal Papers(저널논문)
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