钯配合物和手性磷酸连续催化的烯丙醇和醛的不对称羰基烯丙基化反应

Asymmetric Carbonyl Allylation of Aldehydes with Allylic Alcohols under the Sequential Catalysis of Palladium Complex and Chiral Phosphoric Acid

利用钯配合物/手性磷酸催化剂体系,通过不对称连续催化,实现了烯丙醇和醛的不对称羰基烯丙基化反应.该反应使用廉价易得的烯丙醇底物,在催化量的手性磷酸作用下,以优秀的产率和立体选择性得到了高烯丙醇产物.

The asymmetric carbonyl allylation of aldehydes with allylmetal reagents presents one of the most efficient and straightforward methods for the synthesis of optically active homoallylic alcohols, which have found widespread applications in organic synthesis. As such, a wide range of chiral catalysts, including Lewis acids, Lewis bases and Bronsted acids have been reported to enable highly stereoselective carbonyl allylation of aldehydes with allylmetal reagents. Among them, chiral phosphoric acid-catalyzed carbonyl allylation of aldehydes with pinacol aUylboronates represents a promising method, whereas an additional operations required for the preparation of allylboronates from allyl halides or highly active allylmetal- lics impose some constraints on the carbonyl allylation process. In this context, the asymmetric addition of allylboronates, in situ generated from palladium-catalyzed allylborylation, to aldehydes has been reported, while stoichiometric amounts of chiral diboronate reagents are basically required. Allylic alcohols are readily available feedstock. The direct use of allylic alcohols as starting materials in asymme.tric allylborylation of carbonyls is highly valuable. Herein, we will report an asym- metric carbonyl allylation of aldehydes with allylic alcohols in the presence of octamethyl-2,2＇-bi（1,3,2-dioxaborolane） under the sequential catalysis of a palladium complex and chiral phosphoric acid. The presence of 2.5 mol% （η3-C3FIs）2Pd2C12, 5 mol% P（OPh）3 and 10 tool% chiral phosphoric acid B＊H-1 enabled 4-nitrobenzaldehyde 2a to smoothly undergo the asym- metric carbonyl allylation reaction with 2-buten-l-ol la and octamethyl-2,2＇-bi（1,3,2-dioxaborolane）, giving rise to the de- sired homoallylic alcohol product 3aa in a 99% yield and with 〉20 ： 1 dr and 92% ee. Under the optimal conditions, the generality for allylic alcohol substrates was investigated to reveal that the installation of either of saturated alkyl substituents, carbon-carbon double bond or heteroatom group in the allylic alcohols allowed the target products （3ca-3fa, 3ha-3ja） to be obtained in high yields and with excellent stereoselectivities. A （Z）-allylic alcohol and branched allylic alcohols were also able to generate the target products （3ba, 3ga）, successfully. Although cinnamic alcohols participated in a clean reaction, relatively lower yields and stereoselectivity were delivered （3ka and 31a）. The examination of aldehydes suggested that the introduction of either electronically deficient or rich substituents to the benzene ring of benzaldehydes was tolerant and led to corresponding homoallylic alcohols in excellent yields and stereoselectivities （3ab-3ak and 3m）, with the exception of o-anisaldehyde （3al）. In addition, 2-naphthaldehyde, aliphatic aldehydes and enals are all good substrates and provide high yields and enantiomeric excesses as exemplified by 3-phenylpropanal and 4-methoxycinnamaldehyde （3an-3ap）.