Nitrogen ligands prochiral

A prerequisite for effective asymmetric hydrogenation is that the prochiral olefin is bound stereoselectively to metal at the rate-determining transition-state (Scheme 1). It is therefore of interest to consider stable metal-olefin complexes which may exist as diastereomers by virtue of alternative modes of prochiral olefin complexation. Most work has been done with comparatively simple asymmetric sulfur or nitrogen ligands, and selectivity is usually low. With simple olefins this is not surprising, since discrimination depends on rather small differences in steric bulk in the absence of polar interactions. 

Extensive studies of the asymmetric hydrosiiyiation of prochiral ketones have been performed using rhodium(I) complexes with chiral phosphine ligands and nitrogen ligands " , although the first studies of asymmetric hydrosiiyiation of ketones used chiral phosphine-platinum complexes . ... 

Williams group observed low enantioselectivities for the Michael addition of a prochiral nucleophile, ethyl 2-cyanopropionate 623, to methyl vinyl ketone 624 catalyzed by chiral platinum complexes (Scheme 8.196)." The NMR analysis indicated that these cationic Pt complexes act as Lewis acids toward nitriles. The X-ray crystal structure as well NMR analysis showed that the solvent ligand that is readily displaced by an organic substrate is situated cis to the nitrogen donor in the Pt complex and, therefore, is in a chiral pocket created by the oxazoline ring. 

Both 1- and 1 -A -alkylated ethylenediamine ligands are prochiral and coordination completes the asymmetry at the nitrogen atom. Under favorable conditions, such complexes can be resolved and the absolute configuration determined by X-ray or other methods. Structures (17)-(22) show some representative configurations. 

Note that in 9.33 hydride abstraction generates quaternary nitrogen containing conjugated allyl amine ligand. This prochiral molecule coordinates to the... 

In asymmetric catalysis a prochiral substrate binds to an enantiomerically pure catalyst to generate a pair of diastereomeric intermediates. The energy difference and the rate of exchange between them controls the optical yield (e.e.) of the final product. In the case of a-aminocinnamic acid derivatives, the acyl auxiliary on the nitrogen is required to enable the substrate to form a chelate complex with rhodium.12 The mechanism of this reaction is shown in Fig. 22-3 the ligand in this case is DIPAMP (22-XV). 

Asymmetric addition of organolithium reagents to prochiral diarylimines 6.44 has been performed by Tomioka, Koga and their coworkers with 2.40 as chiral ligand at -100°C in toluene [636,637,638], In these reactions, the presence of a 4-methoxy group and a 2-substituerrt on the nitrogen-substituted aryl ring increase... 

With the formation of 2, two interdependent stereocentres are formed, the bridgehead iron and carbon atoms. The imine carbon atoms in 1 are prochiral, and the alkyne can approach either of them from either the re- or the si-face. With chiral substituents at the nitrogen atoms, the approach of one face may be favoured over the other. This would lead to diastereoselectivity which has been investigated with different types of diazadiene ligands and a series of chiral N-substituents. Depending on both the type of diazadiene (C2 and non-C2 symmetric) and the chiral N-substituents, diastereoselectivities from 0 to > 99% have been observed. 

Concerning the nature of the nitrogen-containing ligand itself, immobilized derivatives of DPEN revealed to be the most efficient in the reduction of prochiral ketones by catalytic hydrogenation in the presence of BINAP or by HTR and CBS-supported ligands for the enantioselective hydride reduction. 

Aminophosphines and aminophosphites with the chirality in the nitrogen part were prepared by Pkmies et al. (Figure 2.39) [23]. The ligands were screened in the hydroformylation of styrene, styrene derivatives, dihydrofurans, and prochiral dioxepines. 

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