Highly Stereoselective Grignard Addition to <i>Cis</i>-Substituted <i>C</i>-Cyclopropylaldonitrones. The Bisected <i>s</i>-<i>Trans</i> Transition State Can Be Stabilized Effectively by the Lewis Acid-Coordination
作者:Yuji Kazuta、Hiroshi Abe、Akira Matsuda、Satoshi Shuto
DOI:10.1021/jo048637e
日期:2004.12.1
We previously found that Grignard addition to a C-cyclopropylaldonitrone, C-[cis-2-(N,N-diethyl-carbamoyl)-trans-2-phenylcyclopropyl]-N-benzylaldonitrone (1), stereoselectively gave the antiproduct 3, in which the stereoselectivity was particularly high when MgBr2 was the additive. In this study, the reaction pathway was investigated in detail. The stereoselective addition was initially thought to occur via either a 1,5-chelation-controlled or a bisected s-trans conformation-controlled pathway. However, Grignard addition to a nonchelating silyl ether-type substrate, C-[cis-2-(tert-butyldiphenylsilyloxymethyl)-trans-2-phenylcyclopropyl]-N-benzylaldonitrone (7), also gave the antiproduct 9 with high stereoselectivity suggesting that chelation is not important in the reaction. Theoretical calculations of C-cyclopropylaldonitrones showed that the coordination of Mg2+ at the nitrone oxygen significantly stabilizes the bisected s-trans conformer due to the effective hyper-conjugation between the,pi* of the nitrone C=N bond and the electron-donating cyclopropane orbitals. This kind of orbital interaction is able to stabilize the transition state of the nucleophilic addition and is maximized in the bisected conformation, in which the orbitals of the forming bond and the cyclopropane C-C bond are in an almost planar arrangement. Thus, the high stereoselectivity can be explained by nucleophilic attack on the less hindered side of the C=N bond of the substrates in the Mg2+-coordinated bisected s-trans conformation.