cis-Stilbene and (1,2,3
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- 作者：Waldemar Adam ; Konrad J. Roschmann ; Chantu R. Saha-Mö ; ller ; and Dieter Seebach
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：May 8, 2002
- 年：2002
- 卷：124
- 期：18
- 页码：5068 - 5073
- 全文大小：81K
- 年卷期：v.124,no.18(May 8, 2002)
- ISSN：1520-5126
文摘
cis-Stilbene (1) has been epoxidized by a set of diverse oxygen donors [OxD], catalyzed by theMnIII(salen)X complexes 3 (X = Cl, PF6), to afford a mixture of cis- and trans-epoxides 2. The cis/transratios 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 (1
,2
beta2.gif" BORDER=0 ALIGN="middle">,3)-(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 productarises from intermediary radicals. The dependence of the diastereoselectivity on the oxygen source isrationalized in terms of a bifurcation step in the catalytic cycle, in which concerted Lewis-acid-activatedoxygen transfer competes with stepwise epoxidation by the established MnV(oxo) species. The experimentalcounterion effect is attributed to the computationally assessed ligand-dependent reaction profiles andstereoselectivities of the singlet, triplet, and quintet spin states available to the manganese species.
,2beta2.gif" BORDER=0 ALIGN="middle">,3)-(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 productarises from intermediary radicals. The dependence of the diastereoselectivity on the oxygen source isrationalized in terms of a bifurcation step in the catalytic cycle, in which concerted Lewis-acid-activatedoxygen transfer competes with stepwise epoxidation by the established MnV(oxo) species. The experimentalcounterion effect is attributed to the computationally assessed ligand-dependent reaction profiles andstereoselectivities of the singlet, triplet, and quintet spin states available to the manganese species.