摘要/Abstract

利用金属钯与非手性膦配体原位生成的钯配合物和手性方酰胺为协同催化体系实现了乙烯基碳酸乙烯酯（VECs）与甲醛的不对称脱羧环加成反应，以良好的产率和对映选择性得到了手性叔醇类化合物.发现了有机小分子催化剂-手性方酰胺可实现该反应的不对称诱导，为通过两性离子烯丙基钯中间体的环加成反应的研究提供了新的思路.
关键词: 协同催化, 钯催化, 手性方酰胺, 不对称环加成反应, 手性叔醇
Chiral tertiary alcohols are ubiquitous in medicinally relevant agents and biologically active natural products. Although some catalytic asymmetric approaches for the synthesis of chiral tertiary alcohols have been reported, the development of efficient methods for enantioselective construction of tertiary alcohols is still highly appealing. Most recently, we have developed Pd-catalyzed asymmetric decarboxylative cycloaddition of vinylethylene carbonates (VECs) with formaldehyde to construct tertiary alcohol derivatives. The reaction was catalyzed by the chiral palladium complex with a chiral phosphoramidite to afford methylene acetal protected tertiary vinylglycols in high efficiency. Since the pioneer works by Gong and Takemoto respectively for the allylic substitution under cooperative catalysis of palladium complex and chiral phase-transfer catalyst, the asymmetric allylic substitution synergistically catalyzed by transition metal and organocatalyst has recently attracted a great deal of attention. However, there have been no reports on the combination of transition-metal and squaramide for the allylic alkylation. In this communication, we will report the asymmetric decarboxylative cycloaddition of VECs with formaldehyde under cooperative catalytic system of achiral palladium complex and chiral squaramide. With combination of palladium complex in situ generated from Pd₂(dba)₃·CHCl₃ (2.5 mol%) and achiral phosphine ligand L4 (10 mol%) and chiral squaramide OC2 (25 mol%) as cooperative catalysts, the reaction of VECs with paraformaldehyde (10 equiv.) proceeded smoothly to give desired tertiary alcohol derivatives in good yields (51%~65%) with moderate enantioselectivities (62%~79% ee). The reaction conditions are also suitable for the reaction of VEC with electronic deficient arylaldehydes to afford desired products in high yields with good enantioselectivities, although the catalytic system is less effective for the control of the diastereoselectivities. Although the enantioselectivity of the reaction is not significantly high, we firstly demonstrated that the chiral induction for the cycloaddition reaction could be achieved under the cooperative catalytic system of achiral palladium complex and chiral squaramide. The detail reaction mechanism and stereochemical outcome are currently underway, and will be reported in due course.
Key words: cooperative catalysis, palladium-catalyzed reaction, chiral squaramide, asymmetric cycloaddition, chiral tertiary alcohol


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