摘要/Abstract
利用钯配合物、有机胺和手性布朗斯特酸三元催化剂体系实现了α-取代丙醛参与的1，4-二烯烃的烯丙基碳氢不对称烷基化反应，以良好的产率、优良的区域选择性和立体选择性得到了结构多样的手性α-羰基化合物.此外发现非手性膦配体对钯配合物的催化活性和反应的选择性有十分显著的影响，为后续相关研究提供了新的思路.
关键词: 不对称催化, 钯催化, 手性磷酸, 1,4-二烯烃, 烯丙基碳氢键官能化
In the recent decade, the palladium-catalyzed allylic C-H alkylation reaction of simple alkenes has been well-established as an efficient and atom-economical synthetic alternative for the fine chemical synthesis without the requirement of any prefunctionalization, in comparison with the conventional procedures. We recently established a highly enantioselective α-allylation reaction of enolizable aldehydes with terminal alkenes by using a ternary catalyst system, including palladium, amine, and chiral Brønsted acid, wherein the chiral anion controls the enantioselectivity. Although this protocol provides allylic alkylation products with high levels of enantioselectivity of up to 90% ee, the extension of the optimal conditions to a 1,4-diene led to a very low regioslectivity. In the presence of a palladium complex, an oxidant and a chiral phosphoric acid, the 1,4-diene could be smoothly oxidized and principally generated two regiomeric π-vinylallyl-palladium phosphate intermediates, either of which led to different products. Therefore, the simultaneous control of regio-and stereoselectivities in the allylic C-H alkylation reaction of aldehydes with 1,4-dienes would pose additional challenge in comparison with a similar reaction of allylarenes. Herein, we will report an asymmetric α-allylation of enolizable aldehydes with a wide range of 1,4-dienes enabled by cooperative catalysis of a palladium complex, amine, and a chiral Brønsted acid. The presence of 6 mol% Pd(OAc)2, 24 mol% P(4-MeOC6H4)3, 6 mol% chiral phosphoric acid (R)-TRIP, 80 mol% cumylamine and 1.50 equivalents of 2,6-dimethylbenzoquinone enabled (E)-penta-1,4-dien-1-ylbenzene (1a) to smoothly undergo the asymmetric allylic C-H alkylation reaction with 2-phenylpropanal (2a), giving rise to the desired α-allylated aldehyde 3a in a 77% isolated yield, 11:1 regioselectivity, 20:1 E/Z and 93% ee. Under the optimal conditions, the generality for enolizable aldehydes was investigated and revealed that 2-aryl propinonaldehydes bearing either electron-donating or electron-withdrawing substituent at the para-(3b~3f) or meta-(3g~3i) position of the phenyl moiety were nicely tolerated, giving rise to the desired allylation products in moderate to good yields with excellent regio-, E/Z-and enantioselectivities. Moreover, 2-naphthyl propinonaldehyde was also able to participate in the asymmetric allylic C-H alkylation reaction, providing the allylation product (3j) with 73% yield, 10:1 regioselectivity, 12:1 E/Z and 91% ee. The examination of 1,4-dienes found that this protocol tolerated a wide scope of aryl and alkyl substituted 1,4-dienes, which showed broad adaptability for the facile construction of a broad spectrum of chiral α-quaternary carbonyl compounds. In addition to the terminal aryl-substituted 1,4-dienes (3k~3q), different substitution patterns were allowed to offer excellent enantioselectivities ranging from 92% ee to 95% ee. Interestingly, this protocol was also amenable to a long chain aliphatic substituent. Although a terminal phenyl (3r) group showed detrimental effect on the regio-and E/Z-selectivities, while the ester (3s), chloride (3t), ether (3u) and cyclohexyl (3v) groups were nicely compatible with the protocol, affording the products with satisfactory results in terms of yields, regio-and stereoselectivities.
Key words: asymmetric catalysis, palladium catalysis, chiral phosphoric acid, 1,4-diene, allylic C-H functionalization
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