Chirality 1.2- diamines

As with hydrogenation, hydrogen transfer of imines is a poorly developed field.126-130 However, recent arene-Ru11 systems bearing chiral 1,2-diamine co-ligands have been found to be excellent catalysts for asymmetric reduction of imines with formic acid as donor.31,131-134... 

In addition to standard cyclic and acyclic enamides, the effective hydrogenation of several more unusual enamides has been reported (Fig. 24.13). A concise method for the synthesis of chiral yS-amino alcohols, amino oximes and chiral 1,2-diamines has been described by Burk et al. via the enantioselective hydroge-... 

The product of this preparation is the most enantioselective catalyst developed to date for asymmetric epoxidation of a broad range of unfunctionalized olefins.6 The procedure includes a highly efficient resolution of trans-1,2-diaminocyclohexane as well as a convenient analytical method for the determination of its enantiomeric purity. This method is general for the analysis of chiral 1,2-diamines. The Duff formylation described in Step B is a highly effective method for the preparation of 3,5-di-tert-... 

Metal salen complexes can adopt non-planar conformations as a result of the conformations of the ethane-1,2-diyl bridge. The conformations may have Cs or C2 symmetry, but the mixtures are racemic. Replacement of the ethylenediamine linker by chiral 1,2-diamines leads to chiral distortions and a C2 chiral symmetry of the complex due to the half-chair conformation of the 5-membered ring of the chelate. Depending on substitution at the axial positions of the salen complex, the symmetry may be reduced to Q, but as we have seen before in diphosphine complexes of rhodium (Chapter 4) and bisindenyl complexes of Group 4 metals (Chapter 10) substitution at either side leads to the same chiral complex. Figure 14.10 sketches the view from above the complex and a front view. 

A-Boc-A -isopropylimidazolidine (175) is deprotonated by the (—)-sparteine method and alkylated to form imidazolidines 176 (equation 41). The yields remain below 50%, since only a part of 175 exists in the shown conformation, which is required for the directed deprotonation. At low temperature, the interconversion between the s-trans- and i-c -conformation is too slow. Ring cleavage furnishes synthetically useful chiral 1,2-diamines 177. 

The addition reaction of 1,2-bisimine 226, prepared from glyoxal and chiral a-phenyl-ethylamine, gives diastereomerically pure product 227, which was converted to the chiral 1,2-diamine 228 (equation 153) . Decreasing the temperature below 50 °C leads to a sharp drop of the stereoselectivity. 

The number of chiral ligands designed for bidentate attachment to copper has grown rapidly during the 1990s, but only three have shown substantial promise Katsuki s chiral bipyrid-ines 13 [48], Fu s C2-symmetric bisazaferrocene 14 [49], and chiral 1,2-diamine 15 [50]. 

Corey collected further evidence against a [2+2] mechanism by investigating the rate acceleration in stoichiometric dihydroxylations with chiral 1,2-diamines [36]. The X-ray structure of a highly reactive heptacoordinate—formally 20-electron bisamine-0s04 com-... 

The excellent enantioselectivity and wide scope of the CBS reduction have motivated researchers to make new chiral auxiliaries [3]. Figure 1 depicts examples of in situ prepared and preformed catalyst systems reported since 1997. Most of these amino-alcohol-derived catalysts were used for the reduction of a-halogenated ketones and/or for the double reduction of diketones [16-28]. Sulfonamides [29,30], phosphinamides [31], phosphoramides [32], and amine oxides [33] derived from chiral amino alcohols were also applied. The reduction of aromatic ketones with a chiral 1,2-diamine [34] and an a-hydroxythiol [35] gave good optical yields. Acetophenone was reduced with borane-THF in the presence of a chiral phosphoramidite with an optical yield of 96% [36]. 

Interestingly, all other diols, tried as a protecting group on the acetal, led to lower diastereoselectivity of the addition. This synthetic strategy was used in the synthesis of indolizidine alkaloids,279 bis(l-arylethyl)amines,280 a protease inhibitor.281 The addition reaction of 1,2-bisimines, easily available from glyoxal and chiral cr-phenylethy-lamine, gives under optimized conditions a diastereomerically pure product, that was converted to a chiral 1,2-diamine. Interestingly, decrease of the temperature below the optimum (50 °C) leads to a sharp drop of the stereoselectivity (Scheme 96).282... 

Whereas 5 only reacts with bifunctional substrates, a new modification enables the catalytic hydrogenation of simple ketones 13. The ternary catalyst consisting of [Ru(II)-BINAP] and a chiral 1,2-diamine in an alcoholic solution of KOH exhibits an activity more than 1000 times that of [Ru(II)-BINAP] alone. The products have been obtained with optical purities greater than 99 % ee by choosing a suitable chiral diamine... 

Chiral Auxiliary. Chiral 1,2-diamines have often been used as chiral auxiliaries in various carbon-carbon bond-forming reactions. The reaction of a diamine with an aldehyde gives a chiral aminal which can undergo stereoselective reactions. This was applied in the synthesis of enantiomerically pure a-hydrazino aldehydes by stereoselective addition of carbon nucleophiles onto the aminal of glyoxal monohydrazone (eqs 2 and 3). In this reaction, the use of 1,2-diaminocyclohexane gave lower diastereomeric excesses than with the related 1,2-diphenyl ethylenediamine. 

In the synthesis of chiral 1,2-diamines, selective hydrolysis of the aminal function group of 653 was achieved with malonic acid to give the BOC-protected diamine 654. In the presence of TFA, both aminal and BOC were cleaved to give 655 (Scheme 157) <20010L3799>. 

Reaction of dimethylformamide dimethyl acetal or formamidine with chiral 1,2-diamines provides optically active imidazolines <1997JOC3586, 1997JMC2931, 20040L43> (Scheme 311). In these examples, the 2-position in the imidazolines is unsubstituted. 

Kise and coworkers [492] reported electroreductive intramolecular hydrocoupling of diimines derives from chiral 1,2-diamines giving enantiomerically pure piperazines. They also prepared enantioselectively 3,4-diphenyladipate by electroreductive intermolecular hydrocoupling of chiral A -tra/w-cinnamoyl-2-oxazolidones [493]. 

Like BINOL, salicylaldehyde imines have become very important in asymmetric catalysis and a variety of polydentate ligands prepared from chiral monoamines and diamines are employed in oxidation reactions, carbenoid reactions and Lewis acid catalyzed reactions. As in the previous section, this section emphasizes the effect of the phenol moiety on the asymmetric catalysis. An imine derived from a chiral 1-phenethylamine and salicylaldehyde was employed in the copper catalyzed asymmetric cyclopropanation by Nozaki, Noyori and coworkers in 1966, which is the first example of the asymmetric catalysis in a homogeneous system . Salicylaldehyde imines with ethylenediamine (salen) have been studied extensively by Jacobsen and Katsuki and their coworkers since 1990 in asymmetric catalysis. Jacobsen and coworkers employed the ligands prepared from chiral 1,2-diamines and Katsuki and coworkers sophisticated ligands possess chirality not only at the diamine moiety but also at the 3,3 -positions. 

The enantioselective discrimination of one of the hydroxyl groups of meso-diols can give chiral monoprotected diols, which serve as versatile intermediates for asymmetric organic synthesis. In addition to the enzymatic methods, a number of chemical approaches have been reported using chiral 1,2-diamine catalysts, chiral phospholane-based catalysts, planar chiral DMAP derivatives, and oligopeptide-based catalysts [2,28], Surprisingly, however, relatively a few publications are devoted to this reaction with cinchona-based organocatalysts. 

In contrast, the chiral 1,2-diamine monomer was copolymerized with styrene to produce 184 (Scheme 3.58) [119]. As the chiral monomer possessed two polymerizable groups, the crosslinking polymers were obtained via a copolymerization. This flexible crosslinking structure, when compared to DVB crosslinkage, gave 184 with high reactivity and satisfactory enantioselectivities in the asymmetric hydrogena-... 

Table I. Structures and Abbreviations of Chiral 1,2-Diamines and Ketones...

R)-2-Aminomethylpipelidine (R-ampi), another 11-subs tituted chiral 1,2-diamine, has a very similar structure to the S-ampr, but the N-H bond for the coordinated R-ampi should preferably have an axial direction. It is possible to suppose a coordination structure having an equatorial N-H bond, as illustrated in Figure 7. However, such a structure will be less stable than that with the axial N-H bond. 

Figure 7. Conformations of coordinated N-suhstituted chiral 1,2-diamines...

Catalysts containing chiral 1,2-diamine been developed for the hydrogenation of alcohols. ... 

Catalysts containing chiral 1,2-diamine ligands in addition to (modified) BINAP have been developed for the hydrogenation of a-amino ketones to furnish chiral 1,2-amino alcohols. ... 

Itsuno et al. [21] synthesized a cross-linked polymer support with a chiral 1,2-diamine for enantioselective ruthenium transfer hydrogenation catalysis of aromatic ketones. 

In addition, a chiral 1,2-diamine derived from L-proUne was investigated as a catalyst for the KR of primary alcohols with acyl chlorides by Oriyama et al. [50], providing the highest selectivity factors of up to 16 in the case of glycerol derivatives as substrates. Chiral Ph-BOX-Cu(II) complex 17 has also been successfully... 

Very recently. Steward and Johnson developed the DKR of jS-silyloxy-a-keto esters, previously prepared through a cyanide-catalysed benzoin-type reaction between silyl glyoxylates and aldehydes. The asymmetric transfer hydrogenation was performed in the presence of TEA as the base, formic acid as the hydride source, and a ruthenium catalyst bearing a chiral 1,2-diamine, providing the corresponding enantioenriched monoprotected diols with moderate diastereo- and enantioselectivities of up to 66% de and 52% ee, respectively. 

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