Chiral ligands derived from abrine. Part 7: Effect of O, S, N in aromatic ring substituents at C-1 on enantioselectivity induced by tetrahydro-β-carboline ligands in diethylzinc addition to aldehydes (original) (raw)
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The Journal of Organic Chemistry, 2005
Three structurally similar series of 1,2,3,4-tetrahydro--carboline ligands, 4a-d, 6a-d and 7ad, and two series of chiral oxazolidines, 8a-d and 9a-g, were synthesized and used as chiral catalysts in the addition of diethylzinc to benzaldehyde. The enantioselectivities of the resulting 1-phenyl-1-propanol were obtained in each case, and these ee values were, in most cases, related to the conformational populations of the free ligand as expressed by the calculated differences in the energies of the ligand conformations formed by inversion at nitrogen. This suggested the possible existence of a linear free energy relationship. The effect on enantioselectivity of the carbon chain length of the R group located (1) on the C-3 substituent of 4a-d, 6a-d, and 7a-d or (2) at C-5 in 8a-d and 9a-g was studied in detail. On the basis of the correlations observed and the ligands' structural characterization, a structure was proposed for the transition state during ethyl group transfer when using ligands 8a-d. Furthermore, the change in enantioselectivity was successfully predicted when diastereomeric ligands 11 and 12 were compared in this chiral addition.
Tetrahedron: Asymmetry, 2006
The synthesis of chiral ligands 4-18 derived from N-[(S)-a-phenylethyl]-trans-b-aminocyclohexanols (S,S,S)-1a and (R,R,S)-2 is described. Addition of diethylzinc to benzaldehyde catalyzed by ligands 4-18 (6 mol %) proceeds in fair to good yield (45-86%), and low to good enantioselectivities (1-76% ee). Highest enantioselectivities were induced by ligands (S,S,S)-4 and (S,S,S,S,R,R)-18 (76% and 68% ee, respectively). The configuration of the major enantiomer of carbinol 3 is (R) in both cases.
Applied Organometallic Chemistry, 2014
Enantiopure C 2-symmetric diol bidentate ligands have been synthesized in a straightforward manner through a three-step reaction with good yields. The synthesized C 2-symmetric diol bidentate ligands were used in the addition of diethylzinc to various aromatic aldehydes, a general catalytic benchmark reaction, in order to assess their enantioselective induction properties. The enantioselective addition of diethylzinc to 1-naphthaldehyde and 3-chlorobenzaldehyde was achieved with an enantiomeric excess (ee) of up to 98%. All synthesized ligands were also evaluated in the addition of diethyzinc to aromatic aldehydes including an extra metal such as Ti(IV) (up to 99% ee).
Tetrahedron, 1992
Chiral amino pyridines derived from a-aminoacids were demonstrated to be enantioselective catalysts in the addition of diethylzinc to aldehydes: optically active secondary alcohols in up to 98% ee were obtained in high yields. The temperature dependence of the stereochemical course of the reaction was found to be strictly related to the alkyl structure of the chiral catalyst. The nature of the possible catalytic complex and the origin of temperature effect was thus discussed on the basis of NMR data too. A catalytic enantioselective process is now recognised as one of the most important problems in organic synthesis.2 It is known that chiral amines and amino alcohols, mainly p-amino alcohols as ligands, may accelerate and direct the stereochemical outcome of the asymmetric alkylation of aldehydes by zinc dialkyls, sometimes achieving high stereoselectivity. Recently, in our laboratory, we found that also amino pyridines act as chiral catalysts in the enantioselective addition of diethylzinc to aldehydes. In fact, we have prepared and used 2-[(2S)-2-pyrrolidinyl]pyridine (1): ees up to 100% were observed. 4 On this basis, we have extended the investigation to other optically active amino pyridines 2-7. which have a greater conformational mobility than 1.
Polyhedron, 2012
Series of functionalized organolithium compounds were prepared and added to chiral bicyclic ketones (1R-(+)-camphor analogue 2 and 1R-(À)-fenchone 3), resulting in the preparation of a small ''library'' of chiral aminoalcohols able to serve as ligands in metal mediated asymmetric synthesis. The configuration of the chiral ligands was approved by applying advanced NMR experiments. The absolute configurations of 1,2-disubstituted planar chiral ferrocene-based aminoalcohols 15, 18 and 19 were determined by means of NMR experiments and confirmed by X-ray crystallography. The new chiral ligands were tested as pre-catalysts for the addition of diethyl zinc to benzaldehyde. The reactions proceeded with excellent conversions and a moderate degree of enantioselectivity.