摘要
研究了双酸催化剂对吲哚和β,γ-不饱和α-酮酸酯的不对称Friedel-Crafts烷基化反应.金属铟(III)盐和手性磷酸组成的双酸催化体系可以高效、高选择性地催化吲哚与β,γ-不饱和α-酮酸酯的Friedel-Crafts烷基化反应.简单改变铟(III)盐的抗衡阴离子从F-到Br-,就能使产物的构型发生反转,并同时获得相应的高产率(最高达98%)和高对映选择性(最高大于99%)的1,4-加成产物.
Effective access to both enantiomers of any targeted products from a single chiral source of catalyst is highly desirable in asymmetric catalysis. Such dual stereocontrol has normally been encountered with structural modifications on the chiral ligands or catalysts skeletons. Recently, simple variations of reactions conditions such as solvents, temperature, additive and catalytic metal center, can lead to remarkable reversal of enantioselectivity with minimum structural modifications, thus providing a modular and synthetic appealing approach in asymmetric catalysis and synthesis. Previously, we have developed asymmetric binary acid catalysis wherein chiral Br?nsted acids, mostly phosphoric acids, are utilized as dual acids and ligands in concert with metal catalysts, and this type of catalysis demonstrated tunable, even switchable stereoselectivity due to the combinatorial and synergistic features. In particular, simple swap of counteranion(from F- to Br-) of Indium(III) salts led to complete switch of regioselectivity(1,2- vs. 1,4- addition) in the reaction of N-methyl indole 2a' and ketoester 3a with high enantioselectivity achieved for both regioisomers. In our further studies, we have found that when indole 2a was employed instead of N-methyl indole 2a', the counter anion effect on regioselectivity was not observed and both InF3 and InBr3 promoted exclusively 1,4-addition reactions. Interestingly, reversal of enantioselectivity of the 1,4-conjugate adduct was observed by simple swap of counter anions of indium(III). The obtained optimal binary-acids combination, InF3(1c)2 and InBr3(1d)2 were found to be R- and S-selective catalyst for the 1,4-addition reactions, respectively. In the presence of asymmetric binary-acid catalysts(InX3/1, 2.5 mol%), indoles and β,γ-unsaturated α-ketoesters were stirred at-70 ℃ for 24 h to afford the various indole esters 4 in good to excellent yield(up to 98% yield) and enantioselectivities(up to>99% ee).
出处
《化学学报》
SCIE
CAS
CSCD
北大核心
2014年第7期809-814,共6页
Acta Chimica Sinica
基金
国家自然科学基金(Nos.21025208
21202170
21390400)
973项目(No.2012CB821600)资助~~
