Imines allylation, catalytic systems

Anisidine imines of aldehydes, p-McOCfJ I4-N=CI IR (R = Ar, alkyl) can be ally-lated with anti-selectivity, using triethylborane and a Pd(II)-phosphine catalytic system, avoiding metallic or metalloid allylating agents 54 The imines can be conveniently formed in situ. 

As for C-C bond formation via an addition of organometallic reagent to aldehyde, ketone or imine or allylation, supported-complexes of prolinol, ephedrine and oxazoline derivated constitute the best catalytic systems. 

A series of tetradentate pyridyl-imine terminated Schiff-bases, bis(pyridyl-imine) terminated siloxane and other related polymers, can be used as ligands to host copper(II) ions. These CuBr2/polyL/TEMPO catalytic systems (polyL stands for polydimethylsiloxane derived pyridyl-imine terminated ligand) are effective for aerobic oxidations of primary and secondary alcohols under aqueous conditions. Chiral N,0-Hgands, e.g., inexpensive L-proline, can also be used to prepare copper catalysts that are particularly effective for the oxidation of sterically hindered, allylic or heterocyclic alcohols such as l-(3-pyridyl)ethanol, l-(2-furfuryl)ethanol. 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

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. 

In an innovative approach, Yamamoto developed the dimeric pinene-derived palladium catalyst 263 for catalytic enantioselective allylation of imi-nes (Equation 25) [170], A catalyst loading of 5 mol% promoted the enantioselective allylation of a host of imines including 261 in up to 82% ee. In subsequent investigations of the reaction conditions, the beneficial effect of addition of one equivalent of water was observed, with increases both in the reactivity of the system and in the enantioselectivity (up to 91 % ee)... 

Kobayashi recently developed a catalytic allylation reaction that employs /3-substituted allylsilanes (cf 266) in combination with a chiral copper complex derived from diamine ligand 267 (Equation 26) [172]. Several classes of imines were observed to undergo highly enantioselective allylations in the presence of this catalyst system. As an example, the addition reaction between 265 and allylsilane 266 provided access to optically active allylglycine 268 in 88 % ee and 99 % yield. 

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