Transition Structures for [2,3]-Wittig Rearrangement
--Solvent Effect on the Stereochemistry--

Toshiya OKAJIMA* and Yoshimasa FUKAZAWA

Department of Chemistry, Faculty of Culture and Education, Saga University;
1 Honjo-machi, Saga-shi 840-8502 Japan
Department of Chemistry, Faculty of Science, Hiroshima University;
Kagamiyama 1-3, Higashi-Hiroshima-shi 739-0046 Japan

[2,3]-Wittig rearrangement is one of the powerful tools for carbon-carbon bond forming reaction (Chart 1). In this paper, ab initio MO calculation was performed to locate the type I (Rautenstrach) and II (Trost) transition structures for [2,3]-Wittig rearrangement of cis- and trans olefin (1 and 2, respectively), in which Li atom is coordinated with CH3OCH3 molecules as the model for the coordination of etheral solvents commonly used in experiments, and obtained the new insight to the solvent effect attributable to the observed stereochemistry. Transition structures were located at the RHF/3-21G basis set and energies are evaluated by using density functional theory (Becke3LYP/3-21({)G level//RHF/3-21G).
The results of this study were summarized as follows.
(1) Solvation by three CH3OCH3 molecules are possible only for the Rautenstrach's type transition geometries (I), because of larger coordination space of solvent molecules around Li atom than that of Trost's type envelop (II).
(2) Coulomb interaction between growing negative charge on C4 and Li cation in Trost's type transition structures (II) could be maintained even by the coordination of two oxygen atoms of the solvent molecules to Li atom, indicating that the stabilization of growing anion on C4 by this contact is considerably large.
(3) The energy difference between each transition structure leading to the major and minor stereoisomeric products became smaller as the number of coordinating solvent molecules to Li atom increases.
This study shows that the stereoselectivity of [2,3]-Wittig rearrangement is controlled under the involvement of the solvent molecules in transition state and comprehensive, self-consistent interpretation of cis-erythro selectivity and trans-threo selectivity become possible by considering the solvation effect in addition to the substituent effect.

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