Hydrogenation chiral support

Asymmetric hydrogenations catalyzed by supported transition metal complexes have included use of both chiral support materials (poly-imines, polysaccharides, and polyalcohols), and bonded chiral phosphines, although there have been only a few reports in this area. 

Before 1968, attempts to perform enantioselective hydrogenations had either used a chiral auxiliary attached to the substrate [1] or a heterogeneous catalyst that was on a chiral support, usually derived from Nature [2]. Since the disclosure of chiral phosphine ligands to bring about enantioselective induction in a hydrogenation, many systems have been developed, as evidenced in this book. The evolution of these transition-metal catalysts has been discussed in a number of reviews [3-12]. 

Ultimately, the ligand itself can be polymerized and used as both ligand and support in one [110]. Pu and coworkers have prepared the rigid chiral poly-BI-NAP ligand below (Fig. 42.17) and used it after treatment with Rh and Ru in asymmetric hydrogenations. The supported catalysts showed similar activities and enantioselectivities to their homogeneous analogues, with the benefit of... 

The generally low-percent asymmetric synthesis in asymmetric heterogeneous hydrogenations may be due, in part, to a nonuniform distribution of chiral modifying agents over the catalytic surfaces. In the case of silk fibroin, metal clumping on the chiral support or dissociation of the metal from the fibroin may allow some reduction to occur in an achiral local environment. 

A chiral diamine leads to a non-racemic hydrogenation product, supporting the importance of chirality in the diamine activator for selective activation of one enantiomer of the ( )-RuCl2(tolbinap) catalyst (3a) (Runs 2 vs 3). Thus, the asym-... 

The desire to produce enantiomerically pure pharmaceuticals and other fine chemicals has advanced the field of asymmetric catalytic technologies. Since the independent discoveries of Knowles and Homer [1,2] the number of innovative asymmetric catalysis for hydrogenation and other reactions has mushroomed. Initially, nature was the sole provider of enantiomeric and diastereoisomeric compounds these form what is known as the chiral pool. This pool is comprised of relatively inexpensive, readily available, optically active natural products, such as carbohydrates, hydroxy acids, and amino acids, that can be used as starting materials for asymmetric synthesis [3,4]. Before 1968, early attempts to mimic nature s biocatalysis through noble metal asymmetric catalysis primarily focused on a heterogeneous catalyst that used chiral supports [5] such as quartz, natural fibers, and polypeptides. An alternative strategy was hydrogenation of substrates modified by a chiral auxiliary [6]. 

The most important heterogeneous systems for the hydrogenation of C=N groups have been reviewed by Blaser and Muller [8]. Neither the use of soluble modifiers nor of chiral supports led to heterogeneous catalysts with useful enantioselectivities. The interaction between adsorbed substrate, the active site and the chiral auxiliary employed until now was probably not sufficient for a good discrimination. 

This is the earliest strategy for preparing a chiral solid hydrogenation-dehydrogenation catalyst (for reviews see [l-3,5,7,8]). Quartz, silk fibroin, cyclodextrin, and cellulose were applied as chiral supports of natural origin. With a Pd/silk catalyst up to 66 % optical yield was obtained in C=C bond hydrogenation, but subsequently the results proved irreproducible. 

Atropisomerism in 3-aryl-2-thioxo-l,3-thiazolidin-4-ones 413,3-aryl-2-thioxoimidazolidin-4-one 414, 3-aryl-2-thioxo-l,3-oxazolidin-4-one 415 have attracted much interest. Atropisomerism was monitored by NMR of the AB spectra of the hydrogen in position 5. 5-Substituted derivatives yielded diastereomers. In the case of a high barrier, separation was performed by chromatography on a chiral support. 

Metallic surface Chiral support Hydrogenation Dehydrogenation Isomerization... 

An early approach toward chiral heterogeneous catalysts was the deposition of the catalyticaUy active metal or metal oxide particles onto intrinsically chiral supports such as quartz [24], cellulose [25], or synthetic chiral polymers [26-28]. Hydrogenation and dehydration reactions were tested, but enantioselective performance was found to be poor. In a recent review, Mallat et al. [29] attributed this poor enantioselectivity to the fact that only a small fraction of the metal atoms would... 

An interesting example of a chiral biopolymer-derived ligand was demonstrated by employment of pivaloyl functionalised chitosan together with a ruthenium complex for the TH reaction [72]. Although only moderate ees (around 60 %) were observed in hydrogenation with /-PrOH//-PrONa in methanol, the use of natural molecules as chiral supports can be of interest in the development of sustainable technologies for the synthesis of fine chemicals. 

Most of the reported applications of this technique involve the direct resolution of derivatised enantiomers on chiral stationary phases. The earliest successful resolution of yV-trifluoroacetyl amino acid esters on glass capillary columns coated with N-trifluoroacetyl-5-isoleucine lauryl ester (2) was effected in 1966. Superior stationary phases developed quickly, such as (3), in which the additional amide bond may interact via hydrogen bonding. These chiral supports work well for the separation of A -perfluoracylated derivatives of a given amino acid. However, their low thermal stability (190 C maximum) and appreciable volatility have inhibited their use for the simultaneous... 

Podolean 1, Hardacre C, Goodrich P, Brun N, Backov R, Coman SM, Parvulescu VI (2013) Chiral supported ionic liquid phase (CSILP) catalysts for greener asymmetric hydrogenation processes. Catal Today 200 63-73. doi 10.1016/j.cattod.2012.06.020... 

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