Amines Arene chromium complexes

The same chiral auxiliary has also been used for the stereoselective synthesis of arene-chromium complexes treatment of an aromatic aminal with chromium hexacarbonyl gives the corresponding complex with high diastereomeric excess. This protocol was recently applied in a total synthesis of (—)-lasubine (eq 4). A successful application of 1,2-diaminocyclohexane (as its IR,2R enantiomer) as a chiral auxiliary is illustrated by the di-astereoselective alkylation of the potassium enolate of bis-amide (3) with electrophiles such as benzyl bromide to give bis-alkylated products with excellent diastereoselectivity (eq 5). Lower levels... 

Two other groups investigated P,N ligands for amination chemistry with secondary amines. Uemura and coworkers have prepared ligands similar to those of Kumada, but based on arene chromium complexes rather than ferrocenes64. Catalysts containing ligand... 

Similarly, ketene generated from acid chloride by treatment with triethyl-amine reacted with tricarbonylchromium-complexed benzaldimines to afford P-lactam derivatives via [2+2] cycloaddition with high diastereoselectivity. Thus, the cycloaddition of benzaldimine chromium complexes with ketenes generated from acid chloride at 0 °C in the presence of triethylamine afforded czs-p-lactam as a single diastereomer 27 (Eq. 20) [19]. Remote positioned imine having the planar chiral arene chromium complex was also reacted with ketene to afford p-lactam complex as diastereomeric mixture (Eq. 21) [19]. 

Scheme 3.9 Stereoselective synthesis of (ri -arene) chromium complexes possessing chiral amine and hydroxy groups.

A 1,2 or 1,3 unsymmetrically disubstituted arene is prochi-ral and therefore the corresponding chromium tricarbonyl compounds are chiral. (Substituted arene) complexes with amine, carboxyl, and formyl groups at the ortho position are resolved into optically active chromium complexes through corresponding diastereomeric adducts (eq 25). Biocatalysts also perform the kinetic resolution of racemic chromium complexes (eq 26). The optically active chromium complexes can be prepared by di-astereoselective ortho lithiation of the chiral benzaldehyde or acetophenone acetal complexes, and diastereoselective chromium complexation of the chiral ort/io-substituted benzaldehyde am-inals (eq 27). Catalytic asymmetric cross-coupling of meso (1,2-haloarene)chromium complex produces chiral monosubstituted complexes. The chiral (arene)chromium complexes can be used as ligands in asymmetric reactions. ... 

Chiral benzylic amines have been synthesized from an optically pure arene tricarbonyl chromium complex via a stereoselective deprotonation/alkylation step (Scheme 35). 

Whereas nucleophilic addition of alkyl-lithium compounds to the optically pure arene(tricarbonyl)chromium complex (8) proceeds without asymmetric induction, the chelates (9) react to give amines (10), after hydrolysis, with optical purity of up to 94%." Replacement of the phenyl groups on the azomethine function by alkyl groups should provide an efficient route to a large number of chiral amines. 

Alkyllithium addition to the (ti -arene) dicarbonylchromium imine chelates has been examined. Treatment of optically pure chelate (41) with methyllithium provides amine (42) with an enantiomeric excess of 94% (Scheme 5). No diastereoselectivity was reported for alkyllithium additions to (arene)tri-carbonylchromium complex (40). In the absence of additional examples, the generality of this chromium chelate methodology for asymmetric amine synthesis cannot be assessed. 

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