Catalysts enantioselective

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

Agbossou E., Carpentier J. E. Hapiot E., Suisse I., Mortreux A. The Aminophos-phine-Phosphinites and Related Ligands Synthesis, Coordination Chemistry and Enantioselective Catalysis Coord. Chem. Rev. 1998 I78-I80 1615-1645 Keywords stereoselective Diels-Alder reaction catalysts, aminophosphine-phosphinites, enantioselective catalysts... 

Furthermore, the biocatalysts will be even more important with the shift of the raw materials from oil to biomass. Since biomass is a mixture of various multifunctional compounds, chemo-, regio-, and enantioselective catalysts will be... 

Ligands of type 48 were synthesized by the cyclization reaction of diamines with dithioaldehydes. Iron complexes formed with those structures led, however, to active but weakly enantioselective catalysts. The best results were... 

The resulting solids were used as enantioselective catalysts in cyclopropa-nation, Diels-Alder and ene reations. In many cases, the recoverability and reusability of these catalysts were carefully studied and this aspect has been the driving force for new research. 

The reaction was first tested with these substances as ligands but the organic molecule, in the absence of any added metal ion, proved to be the most enantioselective catalyst (library 1 19% ee vs. less than 13% ee for the best metal catalyst). The effects of selective variations of the amino acid nature and of the salicylidene moiety on the diamine structure were investigated for urea and thiourea derivatives via HTS (library 2 48 urea compounds and... 

A complex with the metal atom as the inducing chirality can behave as an enantioselective catalyst... 

Caller M, Hollis TK, Overman LE, Ziller J, Zipp GG (1997) First enantioselective catalyst for the rearrangement of allylic imidates to allylic amides. J Org Chem 62 1449-1456... 

Cohen F, Overman LE (1998) Planar-chiral cyclopalladated ferrocenyl amines and imines as enantioselective catalysts for allylic imidate rearrangements. Tetrahedron Asymmetry 9 3213-3222... 

Jautze S, Seiler P, Peters R (2007) Macrocyclic ferrocenyl-bisimidazoline palladacycle dimers as highly active and enantioselective catalysts for the Aza-Claisen rearrangement of Z-conflgured A-para-methoxyphenyl trifluoroacetimidates. Angew Chem Int Ed 46 ... 

Scheme 2.9 gives some examples of use of enantioselective catalysts. Entries 1 to 4 are cases of the use of the oxazaborolidinone-type of catalyst with silyl enol ethers and silyl ketene acetals. Entries 5 and 6 are examples of the use of BEMOL-titanium catalysts, and Entry 7 illustrates the use of Sn(OTf)2 in conjunction with a chiral amine ligand. The enantioselectivity in each of these cases is determined entirely by the catalyst because there are no stereocenters adjacent to the reaction sites in the reactants. 

Scheme 2.25 shows some examples of additions of enolate equivalents. A range of Lewis acid catalysts has been used in addition to TiCl4 and SnCl4. Entry 1 shows uses of a lanthanide catalyst. Entry 2 employs LiC104 as the catalyst. The reaction in Entry 3 includes a chiral auxiliary that controls the stereoselectivity the chiral auxiliary is released by a cyclization using (V-methylhydroxylamine. Entries 4 and 5 use the triphenylmethyl cation as a catalyst and Entries 6 and 7 use trimethylsilyl triflate and an enantioselective catalyst, respectively. 

Many enantioselective catalysts have been developed for reduction of functional groups, particularly ketones. BINAP complexes of Ru(II)C12 or Ru(II)Br2 give good enantioselectivity in reduction of (3-ketoesters.49 This catalyst system has been shown to be subject to acid catalysis.50 Thus in the presence of 0.1 mol % HC1, reduction proceeds smoothly at 40 psi of H2 at 40° C. 

Other effective enantioselective catalysts include Yb(OTf)3 with BINOL,160 Mg2+-to-oxazolines,161 and oxazaborolidinones.162... 

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

The axially chiral 2,2 -bipyridine E is also an effective enantioselective catalyst for addition of allyltrichlorosilane to aldehydes.109... 

Chapters 1 and 2 focus on enolates and other carbon nucleophiles in synthesis. Chapter 1 discusses enolate formation and alkylation. Chapter 2 broadens the discussion to other carbon nucleophiles in the context of the generalized aldol reaction, which includes the Wittig, Peterson, and Julia olefination reactions. The chapter and considers the stereochemistry of the aldol reaction in some detail, including the use of chiral auxiliaries and enantioselective catalysts. 

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