基于手性负离子置换策略的异喹啉不对称转移氢化研究

Application of Chiral Anion Metathesis Strategy in Asymmetric Transfer Hydrogenation of Isoquinolines

报道了一种手性负离子置换策略,并将其成功应用于异喹啉化合物的不对称转移氢化反应.以手性磷酸作为催化剂,氯代甲酸酯作为底物活化试剂,Hantzsch酯为有机负氢供体的条件下,该反应能以优良收率得到手性1-取代-1,2-二氢异喹啉产物,ee值最高为79%.这是首次实现异喹啉化合物不对称转移氢化.

Asymmetric hydrogenation of N-hetero aromatics offers a very straightforward and efficient method to obtain the corresponding chiral N-hetero cyclic saturated or partially saturated compounds. As one of the most challenging substrates, asymmetric hydrogenation of isoquinolines has met with limited success probably because of lower reactivity and the catalyst deactivation resulted from strong coordination. Considering the prevalence of the chiral 1,2,3,4-tetrahydroisoquinoline motif in natural alkaloids and drug molecules, the development of new catalyst system for asymmetric hydrogenation of isoquinolines is highly desirable and significant. Herein, a novel chiral anion metathesis strategy successfully applied for asymmetric transfer hydrogenation of isoquinolines is reported. N-Protected 1-substituted 1,2-dihydroisoquinolines were obtained with high yield and up to 79% ee in the presence of Hantzsch ester and chloroformate using chiral phosphoric acid as catalyst. The phosphate salt and the activated N-acyl isoquinolinium chloride undergo anion metathesis to form chiral contact ion pair, which leads to a highly enantioselective transfer hydrogenation of isoquinolines. After systematically investigating the effects of activating reagent, solvent, base, hydride donor and catalyst on this transfer hydrogenation reaction, the best result was achieved under the optimized condition as follows: 5 mol% H8-BINOL-derived chiral phosphoric acid as catalyst, 1.2 equivalent 2,2,2-trichloroethyl chloroformate as activator, 1.5 equivalent dimethyl 2,6-diethyl-1,4-dihydropyridine-3,5-dicarboxylate as hydride donor, 1.5 equivalent sodium carbonate as base and cyclohexane as solvent. The reaction is tolerant toward a broad range of aryl or alkyl 1-substituted isoquinoline substrates. This methodology represents one of the rare examples of asymmetric hydrogenation of this challenging substrate. The utilizing of chiral anion metathesis strategy could enable chiral phosphoric acid to catalyze more asymmetric transformation process and further researching is ongoing in our laboratory.