Imines enantioselective allylation

Accordingly, chiral 7r-allylpalladium complex 204 was found to catalyze enantioselective allylation of imines with good to high selectivity (Equation (98)).4S7c 457e 457g 4S71... 

Enantiomerically pure homoallylic amines are very important chiral building blocks for the synthesis of natural products. However, enantioselective methods for homoallylamine are quite undeveloped. In 1995, Itsuno and co-workers reported the first example of enantioselective allylation of an imine (Scheme 7) [13]. The reaction of N-trimethylsilylbenzaldimine 19 with a chiral allylboron reagent 20 in ether at -78 °C afforded the corresponding homoallylamine 22 in 73% ee. 

In 1998, Yamamoto et al. reported the first catalytic enantioselective allylation of imines with allyltributylstannane in the presence of a chiral 7i-allylpalladium complex 23 (Scheme 9) [15]. The imines derived from aromatic aldehydes underwent the allylation with high ee values. Unfortunately, the allylation reaction of aliphatic imines resulted in modest enantioselectivities. They proposed that a bis-Jt-allylpalladium complex is a reactive intermediate for the allylation and reacts with imines as a nucleophile. The bis-Jt-allylpalladium complex seemed the most likely candidate for the Stille coupling [16]. Indeed, the Stille coupling reaction takes place in the presence of triphenylphosphine even if imines are present, whereas the allylation of imines occurs in the absence of the phosphine [17]. They suggested the phosphine ligand played a key role in controlling the... 

Highly activated imines such as u-N-losylimirio esters can be catalytically allylated wifh a Lewis acid. j0rgensen [420] and Lectka [227 b] have reported the Lewis acid-catalyzed enantioselective allylation of an a-imino ester wifh allylsilanes (Scheme 10.148). Tol-BINAP-Cu(I) complex is effective in this asymmetric process as well as the Mannich-type reaction wifh silyl enolates. Aromatic substituents on fhe allyl group dramatically improve fhe enantioselectivity. 

The first example of enantioselective allylation of an azomethine function was reported in 1995 by Itsuno and co-workers [42a]. These researchers studied the addition of preformed chirally modified allylboranes to N -(trimethylsilyl)ben-zaldehyde imine (5a) (<2 g of imine, ca. 0.27 M). Of the wide range of chirally modified allylboron reagents reported in the literature, the use of chiral allylbo-ronates 42a-c and 5-allyldialkylborane 43 were logical first choices given their utility in the enantioselective addition to carbonyl substrates (Scheme 20). 

The first example of truly catalytic, enantioselective allylation of imines has been reported by Yamamoto [46a]. These authors had first discovered that imines could undergo allylation reaction via a palladium-catalyzed allylstannane reaction to afford the corresponding homoallylamines in high yields [47a, 47b,... 

Genet has studied the enantioselective allylation of acyclic Shiff bases. Given the discussion and results above, one would not expect significant selectivity to result from use of simple chiral hgands. However, a respectable ee of 57% was determined for the reaction at -60 °C using chiral DIOP as hgand with the hthium enolate of the imine, Eq. (11) [42]. Further work indicated that use of two moles of DIOP vs. Pd, use of palladium acetate as catalyst precursor, and hthium hex-amethyldisilazide instead of LDA as base provided an increase in ee to 68% [43,... 

The enantioselective allylation of aldimines 8 with the tetraallylsilane-TBAF-MeOH system with use of the chiral bis-7i-allylpalladium catalyst 20a under catalytic, non-Lewis acidic, essentially neutral, and very mild reaction conditions has been achieved (Eq. 11) [9]. The reaction of imines 8 with 1.2 equivalents of tetraallylsilane 25d in the presence of 5 mol% of the chiral bis-Ti-allylpalladium catalyst 20a, 25 mol% of TBAF, and 1 equivalent of methanol in THF-hexane (1 2) cosolvent furnished the corresponding homoallylamines 9 in high yields and good to excellent enantioselectivities. 

SCHEME 11.40. Enantioselective allylic amination of MBH carbonates with henzophenone imine and Al-acetyl enamides. 

Yus M, Gonzalez-Gomez JC, Foubelo F (2011) Catalytic enantioselective allylation of carbonyl compounds and imines. Chem Rev 111 7774-7854... 

Recent reviews include catalytic enantioselective aUylation of carbonyl compounds and imines (224 referencesand enantioselective allylation of aldehydes, ketones, aldimines, and ketimines. (g)... 

Ir -aUyl complexes can also act as nucleophiles in additing to various electrophiles such as aldehydes, ketones, or imines. Krische et al. [143d] have developed a broad new family of enantioselective allylations via hydrogenations and transfer... 

More recently, the Pd-catalyzed allylation of enamines and imines has been applied to the enantioselective a-allylation of aldehydes and ketones in ee values up to Until several years ago, the results obtained in this investigation represented some of the most favorable cases of the Pd-catalyzed enantioselective allylation. Several representative results are summarized in Scheme 17 U8H40] p j. (jjg pd-catalyzed allylation of 2-phenylpropionaldehyde with (5)-proline allyl ester, a mechanism involving an organopaUadium complex shown in Scheme 18 has been proposed, and it does appear very plausible. 

Synthetic highlight Diastereoselective production of rac-menthol from its aromatic precursor is achieved by site-selective isopropylation and diastereoselective hydrogenation to the all-trans racemate. Enantioselective allylic amine-enamine-imine rearrangement of an acyclic diene-allylic amine, catalyzed by an Rh(I)-(—)-BINAP complex, affords (—)-menthol the process has been scaled-up to production of 1,000 tons/year. 

Enantioselective Allylic Amine-Enamine-Imine Rearrangement... 

Stereoselective Allylation Reaction. The enantioselective allylation of aldehydes with allyltrimethylsilane is now possible with good control. One example is the use of a chiral acyloxy borane (CAB) catalyst. Besides that, chiral Ti(OiPr)2X2-BINOL and TiFa-BINOL catalysts are effective for the enantioselective allylation of glyoxylates and aldehydes, respectively (eq 35). Chiral homoallylamines can be prepared from the reaction of imines and allyltrimethylsilane using chiral tt-allylpalladium complexes with TBAF (eq 36) or an (5)-Tol-BINAP-CuPFe system. Free radical allyl transfers from allyltrimethylsilane provides another method for this enantioselective C-C bond formation. Promoted by chiral Lewis acids, both... 

This book chapter is limited to Lewis acid-mediated reactions, and does not discuss the important field of Lewis base-mediated allylations, nor does it describe the reactions of allylsilanes with other electrophiles such as epoxides, imines, and allyl-X (X = -Cl, -OR, -OAc). The SaJcurai reaction has been covered under different forms in reviews focusing on The Stereochemistry of the Sakurai reaction , Intramolecular Addition Reactions of Allylic and Propargylic Silanes ," Selective Reactions Using Allylic Metals , Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , and Modem Carbonyl Chemistry . ... 

The startingpoint for the preparation of 1 was the commercial aldehyde 6. Enantioselective allylation followed by silylation delivered 7, which on cross metathesis with methacrolein gave the diene aldehyde 8. Imine formation then completed the construction of 1. 

Scheme 1.12 Enantioselective allylation of phenol-derived imines by Leighton group.

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