Amino alcohols, ligands

Scheme 2.30 Ni-catalysed 1,4-addition of ZnEt2 to chalcone with P-amino disulfide, P-amino thiolate or P-amino alcohol ligands.

Scheme 3.27 D-Cysteine-derived C2-symmetric bis-P-amino alcohol ligands for addition of ZnEt2 to benzaldehyde.

Scheme 3.49 A -Sulfonylated amino alcohol ligand for additions of ZnEt2 to aldehydes.

Novel chiral thiolated amino alcohols have been recently synthesised and then evaluated by Vilaivan el al. as a potential new class of ligands for Cu-catalysed nitroaldol reactions. Amino alcohol ligands bearing Ai-(2-alkyl-thio)benzyl substituents provided only modest enantioselectivities (22-46% ee) while those carrying Al-2-thienylmethyl substituents provided better enantioselectivities of up to 75% ee for the nitroaldol reaction between p-nitro-benzaldehyde and nitromethane. A range of aromatic aldehydes were acceptable substrates giving moderate to high enantioselectivities of up to 88% ee, as shown in Scheme 10.32. 

Scheme 10.32 Cu-catalysed Henry reactions with thiolated amino alcohol ligands.

Scheme 10.41 Silylcyanations with sulfonylated P-amino alcohol ligand.

Similar reactions were undertaken by Choi et al. in the presence of a new family of A -sulfonylated p-amino alcohols possessing two stereocentres as the chiral ligands. In using the chiral sulfonylated p-amino alcohol ligand depicted in Scheme 10.41, the asymmetric addition of McsSiCN to a wide range of aldehydes afforded the corresponding cyanohydrins in both excellent yields and enantioselectivities of up to 96% ee. 

Scheme 10.61 Borane reduction of acetophenone with D-cysteine-derived bis-P-amino alcohol ligands.

The reaction between dialkylzinc and several chiral amino alcohol ligands satisfies these two key factors. Since the discovery by Oguni that various addi-... 

To improve the rate of reduction the amino alcohol ligand of the ruthenium complexes was exchanged for monotosylated 1,2-diamine ligands. For exploratory experiments AT-tosylethane-1,2-diamine was prepared hy monotosylation of ethane-1,2-diamine and attached to the primary face of P-CD yielding 80. With P-CD as the only chiral unit the ruthenium complex of 80 could reduce aromatic and aliphatic standard ketones 63 and 69 in 91% 5deld, 25% ee (S) and 68% 5deld, 58% ee, respectively, within only 4h under standard conditions (Fig. 24). 

Meijer et al. 84) investigated the asymmetric addition of Et2Zn to benzaldehyde catalyzed by PPI dendrimers functionalized with chiral amino alcohol ligands (94-95). 

Dinudear copper complexes with aliphatic tripodal amino alcohol ligands were tested in the oxidation of DTBC (Eq. 17) [221]. It was shown that... 

In summary, the configuration of the desired product is controlled by the planar-chiral imine and ketimine ligand backbone. The selectivity of the reaction depends on both the chiral center and the communication of the side-chain with the ligand backbone. We tuned the side-chain to increase the enantioselectivity up to 90% ee. In the case of the amino alcohol ligands, chiral cooperativity is also observed. However, the influence of the planar chirality is much lower, whereas central chirality is dominant in this instance. In most cases the enantioselectivity is lower than for the ketimines. 

When the -amino alcohol ligand is chiral and optically pure, there are four potentially low-energy TS sriuctures that may lead to products. Several chiral ligands have been shown... 

Figure 5.3 Four alternative TS structures for catalyzed addition of diethylzinc to benzaldehyde. The descriptors refer to the side of the four-membered ring on which the aldehyde carbon is found relative to the alkoxide carbon - same (syn) or opposite (anti) - and the absolute configuration of the new stereogenic center formed following ethyl transfer, R or S. In the absence of chirality in the /3-amino alcohol ligand, indicated by the G group(s), the TS structures at opposite corners would be enantiomeric with one another, and no preference for R over 5 product would...

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