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product, and excellent enantioselectivity was obtained.
Cordova reported an α‐allylation of an aldehyde with an allyl acetate catalyzed by a combination of diphenylprolinol silyl ether and Pd catalyst (Eq. 1.41). A π‐allyl complex would be generated from allyl acetate and Pd(PPh3)4, which reacts with a chiral enamine generated from an aldehyde and organocatalyst [73]. Excellent enantioselectivity was obtained.
Cozzi reported an asymmetric allylation reaction of an aldehyde and an allyl alcohol catalyzed by a combination of MacMillan’s catalyst and a Lewis acid such as InBr3 (Eq. 1.42) [74]. This is an SN1‐type reaction, and the chiral enamine, which is generated from an aldehyde and MacMillan’s catalyst, reacts with an allylic carbenium ion, which is generated from an allyl alcohol and InBr3 to afford the product after hydrolysis.
Dixon reported an asymmetric carbocyclization of aldehyde‐linked allene by a combined use of diarylprolinol silyl ether and Pd(OAc)2 to afford a substituted cyclopentanecarbaldehyde with good enantioselectivity (Eq. 1.43) [75]. The organocatalyst reacts with an aldehyde to generate a chiral enamine, which reacts with allene activated by Pd(II).
Gong reported that diphenylprolinol/Pd(II) cooperative catalysis has enabled a highly enantioselective addition of cyclic ketones to unactivated alkenes (Eq. 1.44) [76]. The reaction includes amide‐directed, regioselective activation of alkenes by Pd(II) and enhancing the nucleophilicity of α‐carbon of the ketones by enamine catalysis, which provides the γ‐addition products with good to high yields and efficient stereochemical control.
Jia reported an enantioselective α‐arylative desymmetrization of cyclohexanones using Pd(OAc)2 and proline as a chiral amine catalyst (Eq. 1.45) [77]. Morphan derivatives bearing α‐carbonyl tertiary stereocenters were produced in good yields with excellent enantioselectivities. The generated enamine reacts with Ar‐Pd‐X, and β‐hydride elimination affords another enamine, which is hydrolyzed to provide the product (Scheme 1.5).
Scheme 1.5. The reaction mechanism.
Nishibayashi reported an enantioselective propargylic alkylation of propargylic alcohols with aldehydes in the presence of a thiolate‐bridged diruthenium complex and diarylprolinol silyl ether as a co‐catalyst to afford the corresponding propargylic alkylated products in excellent yields with high enantioselectivity (Eq. 1.46) [78]. This is a new type of enantioselective propargylic substitution reaction, wherein the chiral enamines react with the ruthenium–allenylidene complexes, where both the transition metal catalyst (ruthenium complex) and organocatalyst (secondary amine) activate propargylic alcohols and aldehydes, respectively, and cooperatively.
1.7.2.2. Iminium Ion and Transition Metal Catalyst
Cordova reported an enantioselective silyl addition to enals using Me2PhSi‐B(pin) as a silyl source, and the reaction is catalyzed by a combination of diphenylprolinol silyl ether and a copper(I) salt (Eq. 1.47) [79]. The copper(I)–silyl intermediate is a nucleophile that reacts with an iminium ion to afford the Michael product with excellent enantioselectivity. The product is a synthetically useful β‐silylated aldehyde.
The same group extended the reaction using B2 (pin)2 and dialkyl zinc, respectively, by a combined use of diphenylprolinol silyl ether and Cu(OTf)2 to afford β‐borylated aldehydes (Eq. 1.48) [80] and β‐alkylated aldehydes [81], respectively, both with excellent enantioselectivity. The β‐arylation reaction also proceeded using ArB(OH)2, which is catalyzed by diphenylprolinol silyl ether and Pd(OAc)2 [82].
It is known that Hantzsch esters are useful hydride donors for the reduction of α,β‐unsaturated aldehydes to saturated aldehydes, and, in 2005, MacMillan reported the asymmetric reduction of β,β‐disubstituted α,β‐unsaturated aldehydes catalyzed by MacMillan’s catalyst (Eq. 1.49) [83].
In 2012, Hori reported an asymmetric hydrogenation of β,β‐disubstituted α,β‐unsaturated aldehyde, which is catalyzed by a combination of 2‐diarylmethylpyrrolidines and heterogeneous Pd/BaSO4 under an H2 atmosphere (Eq. 1.50) [84]. This reaction is successfully applied to citral. A mixture of E‐ and Z‐citral in any ratio afforded citronellal with high enantioselectivity. Citronellal is a key synthetic intermediate of l‐menthol.