Scope and Mechanism of Formal SN2' Substitution Reactions of a Monomeric Imidozirconium Complex with Allylic Electrophiles
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- 作者：Gojko Lalic ; Jamin L. Krinsky ; Robert G. Bergman
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：April 2, 2008
- 年：2008
- 卷：130
- 期：13
- 页码：4459 - 4465
- 全文大小：281K
- 年卷期：v.130,no.13(April 2, 2008)
- ISSN：1520-5126
文摘
The zirconium imido complex Cp2(THF)Zr=NSi(t-Bu)Me2 (1) reacts with allylic ethers, chlorides,and bromides to give exclusively the products of the SN2' reaction; i.e., attack at the allylic position remotefrom the leaving group with migration of the double bond. The primary amine products can be isolated inexcellent yields, after in situ Cbz protection, in the presence of variety of functional groups. Gooddiastereoselectivity and complete stereoselectivity allowed the formation of enantioenriched allylic aminesfrom enantioenriched allylic ethers. Regiospecific substitution with 1 has also been achieved with allylicfluorides, which are notoriously poor substrates in other substitution reactions. On the basis of rate andkinetic isotope effect studies, we propose a general mechanism for the allylic substitution reactions with 1which involves dissociation of THF and binding of the substrate, followed by the substitution step. In a DFTstudy of the substitution reaction, we identified a six-membered closed transition state for the substitutionstep and other relevant stationary points along the reaction coordinate. This study shows that the substitutionreaction can be described as a concerted asynchronous [3,3]-sigmatropic rearrangement. This detailedknowledge of the reaction mechanism provides a rationale for the origins of the observed regio-, diastereo-,and stereoselectivity and of the unusual reactivity profile observed in the reaction.