Tokunaga Norihito, Yoshida Kazuhiro, Hayashi Tamio
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan.
Proc Natl Acad Sci U S A. 2004 Apr 13;101(15):5445-9. doi: 10.1073/pnas.0307260101. Epub 2004 Feb 23.
Addition of lithium aryl(tetraisopropoxy)titanates [ArTi(OPr-i)(4)(-)Li(+)] to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (up to 99% ee) in the presence of an excess amount of chlorotrimethylsilane and a rhodium catalyst (3 mol % Rh), generated from RhCl(C(2)H(4))(2) and (S)-binap, in tetrahydrofuran at 20 degrees C to give high yields of the corresponding silyl enolates as 1,4-addition products. The presence of chlorotrimethylsilane is essential for the 1,4-addition to take place. (31)P NMR spectroscopic studies revealed that the catalytic cycle consists of three transformations, that is, (i) insertion of an enone into arylrhodium species forming (oxa-pi-allyl)rhodium intermediate, (ii) silylation of the (oxa-pi-allyl)rhodium with chlorotrimethylsilane giving silyl enolate and a chloro-rhodium complex, and (iii) transmetalation of aryl group from aryltitanate to the chloro-rhodium regenerating the aryl-rhodium.
在过量的三甲基氯硅烷和由[RhCl(C₂H₄)₂]₂与(S)-联萘二膦配体(3 mol% Rh)生成的铑催化剂存在下,于20℃的四氢呋喃中,将芳基(四异丙氧基)钛酸锂盐[ArTi(OPr-i)₄⁻Li⁺]添加到α,β-不饱和酮中,反应具有高对映选择性(高达99% ee),得到高产率的相应硅基烯醇盐作为1,4-加成产物。三甲基氯硅烷的存在对于1,4-加成反应的发生至关重要。³¹P NMR光谱研究表明,催化循环由三个转化步骤组成,即:(i) 烯酮插入芳基铑物种形成(氧杂-π-烯丙基)铑中间体;(ii) (氧杂-π-烯丙基)铑与三甲基氯硅烷进行硅烷化反应,生成硅基烯醇盐和氯铑配合物;(iii) 芳基从钛酸酯转移至氯铑上,使芳基铑再生。