摘要/Abstract

以N-烷氧基邻苯二甲酰亚胺为底物, 利用光与铜共催化, 在蓝光照射下, 实现了远程C—H键不对称氰基化反应. 反应中涉及氧自由基的1,5-氢迁移过程, 结合铜催化自由基接力的策略, 以良好到优秀的对映选择性、单一的区域选择性得到了芳基取代的烷基醇类衍生物δ-位C—H键不对称氰基化产物. 该方法条件温和, 具有较好的官能团兼容性以及底物普适性, 为光学活性的δ-氰基醇类化合物的合成提供了高效方法.
关键词: 不对称氰基化, C—H键, 铜催化, 光催化, 1,5-氢迁移
Optically pure alkylnitriles are important structural motifs found in agrochemicals, pharmaceuticals, and natural products, which can be further transferred to acids, amines and amides. Direct asymmetric cyanation of sp ³ C—H bonds represents the most efficient synthetic pathway to these optically pure alkylnitriles. However, selective functionalization of sp ³ C—H bonds remains a crucial issue due to the inertness of sp ³ C—H bonds as well as the difficulties in the control of stereo- and regioselectivity. Inspired by enzymatic oxygenases and halogenases, such as cytochrome P450 and nonheme iron enzymes, the radical-based C—H functionalization has received much attention, which was initiated with a hydrogen atom transfer (HAT) process. Recently, numerous reports have been disclosed for the highly efficient functionalization of C—H bonds with an intramolecular HAT process as a key step to govern the reactivity and site selectivity. Our group has developed a copper-catalyzed radical relay process for the enantioselective cyanation and arylation of benzylic C—H bonds using TMSCN and ArB(OH)₂ as nucleophiles respectively. Mechanistic studies indicated that a benzylic radical generated via a radical replay process can be trapped by a reactive chiral copper(II) cyanide enantioselectively, delivering optically pure benzyl nitriles efficiently. Herein, we communicate the catalytic asymmetric cyanation of remote C—H bonds by merging photoredox catalysis with copper catalysis. This reaction proceeds under mild reaction conditions and exhibits good functional group compatibility as well as wide substrates scope. Additionally, the nitrile group was further reduced to amide under hydrogen atmosphere. This reaction provides an efficient pathway to synthesize chiral δ-cyano alcohols and 1,6-amino alcohols. The general procedure is as following: in a dried sealed tube, substrate 1 (0.2 mmol, 1.0 equiv.), Cu(CH₃CN)₄PF₆ (0.01 mmol, 5 mol%), L (0.015 mmol, 7.5 mol%) and Ir(ppy)₃ (0.002 mmol, 1 mol%) were dissolved in dichloromethane (4.0 mL) under N₂ atmosphere, and stirred for 30 min. Then, TMSCN (50 μL, 0.4 mmol, 2 equiv.) was added slowly under N₂ atmosphere. The tube was sealed with a Teflon-lined cap, and the mixture was stirred under the irradiation of blue LED for 1~7 d. The reaction mixture was diluted with dichloromethane, filtered through a short pad of celite. A solution of TBAF (3 equiv.) and HOAc (3 equiv.) was added to the filtration. The mixture was stirred for 5 min and then washed with water (3×10 mL) and dried over anhydrous Na₂SO₄. After filtration and concentration, the residue was purified by silica gel chromatography (eluent: petroleum ether/ethyl acetate) to afford target product.
Key words: enantioselective cyanation, C—H bonds, copper catalysis, photoredox catalysis, 1,5-hydrogen atom transfer


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