Bpe ligands

Fig. 11 (Top) Perspective view of an [Fe]4 rhombus in the [Fe(bpe)2(NCS)2]-CH3OH 2D polymer. (Bottom) Schematic representation of the interpenetration of a layer lying in the plane of the sheet and three orthogonal layers (left). Perspective view of the crossing of two independent net systems defining the rectangular channels (right). Balls and sticks represent iron atoms and bpe ligands, respectively...

The fundamental discovery by Burk et al. that the analogous trans-2,5-disub-stituted phospholanes formed a more rigid steric environment led to the introduction of the DuPhos and BPE ligand classes (Fig. 24.1) [8-13]. Subsequently, these ligands have been successfully employed in numerous enantiomeric catalytic systems [4 a, 5], the most fruitful and prolific being Rh-catalyzed hydrogenations. The reduction of N-substituted a- and /1-debydroarnino acid derivatives,... 

The parent DuPhos and BPE ligands exhibit excellent enantioselectivities routinely in excess of 95% with the majority of model a-dehydroamino acid substrates (Table 24.1) [4a, 8, 12, 13, 20, 90]. High molar SCRs (in the order of >1000 1), as well as TOFs in excess of 1000 h 1, are indicative of the high catalyst activity and productivity typically found with DuPhos and BPE systems with these simple substrates. Burk reported that in the enantiomeric hydrogenation... 

The asymmetric hydrogenation of yS,yS-disubstituted a-dehydroamino acids, in which the yS-substituents are nonequivalent, provides the opportunity to selectively construct two stereogenic centers. The Me-DuPhos or Me-BPE ligands facilitate the rhodium-catalyzed hydrogenation of the E- and Z-isomers of yS,yS-disubstituted a-dehydroamino acid... 

The rhodium complexes of 71 and 80 show high catalytic activities but only moderate enantioselectivities (up to 90% ee) compared to the very high optical yields of analogous phospholane-based DuPHOS and BPE ligands. 

In these hydrogenations, less than 2% of the y,( -double bond was reduced. This feature indicates the ethyl-DuPHOS ligand to be superior in comparison to related DuPHOS/BPE-ligands or... 

It is likely that several results obtained with the homologous ligands of R-DuPHOS (R=Me, Pr) or their opposite enantiomers can be related also to (X5)-ethyl-DuPHOS. Recently, Burk has published a review about the application of phospholane ligands in asymmetric catalysis, which gives a good survey of the use of DuPHOS- and BPE-ligands. ... 

Related Reagents. The homologous derivatives of DuPHOS-and BPE-ligands. RoPHOS " PennPHOS BASPHOS CnrPHOS. - ... 

Recently at ChiroTech, we have implemented a convenient and proficient biocatalytic process that provides facile entry to enantiomerically pure 1,4-diols. By way of example, a dl/meso mixture of 2,5-hexanediol may be enzymatically resolved using the commercially available esterase Chirazyme-L2 in vinyl butyrate. Upon completion, aqueous extraction yields directly (S, 5)-2,5-hexanediol (4) in 17% yield. Further processing of the resultant mixture of butyrate esters allows isolation of antipodal (A.A)-2,5-hexanediol in enantiomerically and diaste-reomerically pure form in 42% overall yield. We have performed this chemistry on up to 200 kg of 2,5-hexanediol, thus providing ample material for production of the Me-DuPHOS and Me-BPE ligands. This development, combined with other process improvements, has rendered a suitable process for manufacture of multikilogram quantities of the DuPHOS and BPE ligands. 

The contributions that the DuPHOS and BPE ligand series have made in the area of asymmetric hydrogenation chemistry are exemplified below. One purpose of this treatise is to underscore the broad scope of the DuPHOS and BPE catalyst systems for the generation of diverse collections of chiral compounds. 

A later optimized synthesis also starts from the 1,4-diols. They are then converted into cyclic sulfates with thionyl chloride on mediation of ruthenium chloride and sodium periodate. The sulfates are transformed with dili-thiumbis(phosphido)ethane into bis(phospho-lano)ethane (BPE) ligands or with dilithium-... 

In 1996 Burk et al. also developed a convenient method to synthesize chiral a-l-arylal-kylamines through enantioselective hydrogenation of various enamides using their Me-DuPHOS and Me-BPE ligands, respectively. They consistently achieved from 94 to 97 % ee. Furthermore their ligands tolerated y9-sub-stituents both in ( )- and (Z)-position. [10]... 

The best enantioselectivities till now have been reached with the Me-BPE ligand. With this ligand a large variety of y5, -disubstituted Af-acetylenamides were hydrogenated with... 

In the case of the tetrasubstituted acrylic acids the rigid DuPHOS ligands block both diagonally oriented quadrants so severly that the larger space requirements of these substrates cannot be accommodated. Here the more flexible Me-BPE ligand with its smaller methyl groups performs better. 

---