摘要/Abstract
报道了In（I）Cl/手性磷酸1a双酸体系催化的简单烯烃和β，γ-不饱和α-酮酸酯的不对称[4+2]环加成反应，可高产率、高选择性（最高99：1dr，99%ee）生成相应的exo-[4+2]环加成产物.InCl和磷酸的协同作用是反应成功的关键，单独使用InCl和磷酸都不能催化反应进行.
关键词: 不对称催化, [4+2]环加成反应, 双酸催化, InCl, 手性磷酸
Compared with indium(Ⅲ), indium(I) has both vacant p-orbitals and an electron lone pair, showing distinctive catalytic behaviors. However, chiral indium(I) catalysis has been rarely reported. Previously, we have developed asymmetric binary acid catalysis with indium(Ⅲ) and chiral phosphoric acid for a number of enantioselective transformations. Asymmetric binary-acid catalysis in[4+2] cycloaddition of β,γ-unsaturated α-keto esters with different olefins have been reported by our groups over the past five years. In 2013, we developed exo-selective and enantioselective[4+2] cycloaddition of simple industrial feedstock olefins, such as propene and isobutene, styrene and so on, catalyzed by In(BArF)3/1a. However, the reaction with electron-rich olefins, such as 4-methoxylstyrene did not work very well by indium(Ⅲ) catalysis due to uncontrolled polymerization side pathway. Very recently, we developed a new binary acid system InCl/1a, which could catalyze enantioselective[4+2] annulation of β,γ-unsaturated α-keto esters with much more electron-rich alkoxyallenes. In this study, we reported that the binary acid InCl and 1a was an effective and exo-selective catalyst for the[4+2] cycloaddition of simple olefins. In the presence of InCl (10 mol%) and chiral phosphoric acid 1a (10 mol%), the reaction occurred smoothly to afford the desired cycloadducts in moderate to good yields (20%~93%), with excellent diastereoselectivity (>95:5, exo/endo) and enantioselectivity (up to 99% ee) under the room temperature in CHCl3. Different olefins, such as styrenes 2, ring-strained norbornene 5a, norbornadiene 5b, and cyclopentadiene dimer 5c all worked well with excellent stereoselectivity under the optimal reaction conditions. More importantly, when 4-methoxylstyrene is used, the reaction can proceeded smoothly to afford[4+2] adduct 4k in 70% yield and good stereoselectivity (>95:5 dr, and 88% ee). The typical procedure for asymmetric[4+2] cycloaddition is as follows:To a dry reaction tube was added chiral phosphoric acid 1a (0.005 mmol, 5 mol%), InCl (0.005 mmol, 5 mol%), 4 Å MS (10 mg), 3 (0.1 mmol), then CHCl3 (0.5 mL) and 2 or 5 (0.5 mmol) was added to the mixture. The mixture was stirred for 24 h at room temperature. The mixture was purified by column chromatography to give the desired cycloaddition products 4 or 6.
Key words: asymmetric catalysis, [4+2]cycloaddition, binary-acid catalysis, InCl, chiral phosphoric acid
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