期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 135, 期 38, 页码 14390-14400出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja407348s
关键词
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资金
- National Science Foundation [CHE-075006]
Chiral organometallic reagents are useful in asymmetric synthesis, and configurational stability of these species is critical to success. In this study we followed the epimerization of a chiral Grignard reagent, prepared by Mg/Br exchange of bromonitrile trans-2b. This compound underwent highly retentive Mg/Br exchange in Et2O; less retention was observed in 2-MeTHF and THF. Epimerization rate constants k(tc), were determined at 195 K by measuring the diastereomer ratio of deuteration product d(1)-3b as a function of the delay time before quench. Studies were also performed at varying concentrations of Et2O in toluene. Remarkable dynamic range in k(tc) was seen: relative to reaction at 0.12 M Et2O in toluene, epimerization was 26-, 800-, and 1300-fold faster in Et2O, 2-MeTHF, and THF, respectively. Thus, the identity and concentration of an ethereal solvent can dramatically affect configurational stability. Reaction stoichiometry experiments suggested that, in Et2O, the Grignard reagent derived from trans-2b exists as an i-PrMgCl heterodimer; the invariance of k(tc) over a 20-fold range in [Mg](total) ruled out mandatory deaggregation (or aggregation) on the epimerization path. Analysis of the dependency of k(tc) on [Et2O] and temperature in Et2O/toluene solution at 195, 212, and 231 K indicated fast incremental solvation before rate-limiting ion-pair separation and provided an estimate of the entropic cost of capturing a solvent ligand (-13 +/- 3 eu). Calculations at the MP2/6-31G*(PCM)//B3LYP/6-31G* level provide support for these conclusions and map out a possible ionogenic conducted tour pathway for epimerization.
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