期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 124, 期 18, 页码 5068-5073出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja0177206
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cis-Stilbene (1) has been epoxidized by a set of diverse oxygen donors [00], catalyzed by the Mn-III(salen)X complexes 3 (X = Cl, PF6), to afford a mixture of cis- and trans-epoxides 2. The cis/trans ratios range from 29:71 (extensive isomerization) to 92:8, which depends both on the oxygen source [OxD] and on the counterion X of the catalyst. When (1alpha,2beta,3alpha) -(2-ethenyl-3-methoxycyclopropyl) -benzene (4) is used as substrate, a mechanistic probe which differentiates between radical and cationic intermediates, no cationic ring-opening products are found in this epoxidation reaction; thus, isomerized epoxide product arises from intermediary radicals. The dependence of the diastereoselectivity on the oxygen source is rationalized in terms of a bifurcation step in the catalytic cycle, in which concerted Lewis-acid-activated oxygen transfer competes with stepwise epoxidation by the established Mn-V(oxo) species. The experimental counterion effect is attributed to the computationally assessed ligand-dependent reaction profiles and stereoselectivities of the singlet, triplet, and quintet spin states available to the manganese species.
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