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reaction is highly reproducible and not very sensitive to ex- ternal factors such as oxygen or moisture and requires only 1-2 mol % of the catalyst. The yields are good, although the enantioselectivites at this stage of our studies are moder- ate (usually 50-79% ee). For active aldehydes such as glyox- ylates and glyoxals, the allylation works well under ambient conditions even with no solvents and on large scale. Allyla- tion of simple aromatic and aliphatic aldehydes requires application of a high-pressure technique.

Figure 5 reaction is highly reproducible and not very sensitive to ex- ternal factors such as oxygen or moisture and requires only 1-2 mol % of the catalyst. The yields are good, although the enantioselectivites at this stage of our studies are moder- ate (usually 50-79% ee). For active aldehydes such as glyox- ylates and glyoxals, the allylation works well under ambient conditions even with no solvents and on large scale. Allyla- tion of simple aromatic and aliphatic aldehydes requires application of a high-pressure technique.

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Table 1. Screening of the metallosalen complexes of type 1 with a classic salen ligand in the reaction of 2a with 3"
Table 1. Screening of the metallosalen complexes of type 1 with a classic salen ligand in the reaction of 2a with 3"
Chromium complexes with modified salen ligand The metallosalen complexes were chosen as candidate chiral Lewis acids. In many cases, they are easily prepared and handled, and stable in the presence of moisture and oxygen. The model reaction was the allylation of n-butyl glyoxylate (2a) with allyltributyltin (3) leading to n-butyl 2-hydroxy- pent-4-enoate (4a) (Scheme 2). Subsequent to the prelimi- nary screening of the chiral salen complexes of type 1 (Fig. 1) of the following metals: Ti7/V), VO(IV), Crd), Mn), Fel), CoD) and (ID, Nii) and (IM), CudI) and AI(II), it transpired that the only enantioselectively efficient catalysts were the (salen)chromium(III) complexes la—c (Table 1, entries 1-3). Although the remaining com- plexes 1d—m (entries 4-13) did catalyze the allylation, the Figure 1. The metallosalen complexes used in this work. We also tested the applicability of other chromium com- plexes with modified salen ligands. We studied the effect of the ligand structure with respect to the substituted enantiomeric excess was 10% at best. The commercially available (salen)CrCl complex 1a (2 mol %) provides the re- action at moderate enantioselectivity (54% ee) and in good yield. However, higher activity and slightly better enantio- selectivity (over 60% ee) were observed for the chromium complexes with less coordinating counterions such as BFz (1b) and ClO; (1c) (entries 2 and 3, respectively) both easily prepared from 1a.°
Chromium complexes with modified salen ligand The metallosalen complexes were chosen as candidate chiral Lewis acids. In many cases, they are easily prepared and handled, and stable in the presence of moisture and oxygen. The model reaction was the allylation of n-butyl glyoxylate (2a) with allyltributyltin (3) leading to n-butyl 2-hydroxy- pent-4-enoate (4a) (Scheme 2). Subsequent to the prelimi- nary screening of the chiral salen complexes of type 1 (Fig. 1) of the following metals: Ti7/V), VO(IV), Crd), Mn), Fel), CoD) and (ID, Nii) and (IM), CudI) and AI(II), it transpired that the only enantioselectively efficient catalysts were the (salen)chromium(III) complexes la—c (Table 1, entries 1-3). Although the remaining com- plexes 1d—m (entries 4-13) did catalyze the allylation, the Figure 1. The metallosalen complexes used in this work. We also tested the applicability of other chromium com- plexes with modified salen ligands. We studied the effect of the ligand structure with respect to the substituted enantiomeric excess was 10% at best. The commercially available (salen)CrCl complex 1a (2 mol %) provides the re- action at moderate enantioselectivity (54% ee) and in good yield. However, higher activity and slightly better enantio- selectivity (over 60% ee) were observed for the chromium complexes with less coordinating counterions such as BFz (1b) and ClO; (1c) (entries 2 and 3, respectively) both easily prepared from 1a.°
e.g., 1 (Scheme 1). Moreover, we decided to extend the investigation to other active aldehydes such as glyoxals, as well as to other allyltin reagents such as crotylstannanes.
e.g., 1 (Scheme 1). Moreover, we decided to extend the investigation to other active aldehydes such as glyoxals, as well as to other allyltin reagents such as crotylstannanes.
Table 2. The reaction of 2a with 3 catalyzed by chromium(III) complexes with modified salen ligands*
Table 2. The reaction of 2a with 3 catalyzed by chromium(III) complexes with modified salen ligands*
Figure 2. Absence of nonlinear effects in the reaction of 2a with 3 catalyzed by 1b.
Figure 2. Absence of nonlinear effects in the reaction of 2a with 3 catalyzed by 1b.
Table 3. Results of the enantioselective reaction of n-butyl glyoxylate (2a) with allyltributyltin catalyzed by the complex 1b under various reaction conditions’ * The reactions were carried out using 1 mmol of n-butyl glyoxylate (2a) and 1.2 mmol of allyltributylti1 > The yield was determined by GC. © The enantiomeric excess was determined by GC on a capillary chiral B-dex 120 column.
Table 3. Results of the enantioselective reaction of n-butyl glyoxylate (2a) with allyltributyltin catalyzed by the complex 1b under various reaction conditions’ * The reactions were carried out using 1 mmol of n-butyl glyoxylate (2a) and 1.2 mmol of allyltributylti1 > The yield was determined by GC. © The enantiomeric excess was determined by GC on a capillary chiral B-dex 120 column.
Table 4. Enantioselective allylation of 2-oxoaldehydes catalyzed by 1b*
Table 4. Enantioselective allylation of 2-oxoaldehydes catalyzed by 1b*
aocdcs High-pressure reactions were carried out in 2 ml Teflon ampoule using 1.1 equiv of allyltributyltin at 20 °C The yield was determined by GC. The enantiomeric excess was determined by GC on capillary chiral B-dex 120 column. Instead of allyltributyltin, 1.1 equiv of allyltrimethyltin was used. Table 5. Results of the model reaction of furfural (10a) with allyltributyltin*
aocdcs High-pressure reactions were carried out in 2 ml Teflon ampoule using 1.1 equiv of allyltributyltin at 20 °C The yield was determined by GC. The enantiomeric excess was determined by GC on capillary chiral B-dex 120 column. Instead of allyltributyltin, 1.1 equiv of allyltrimethyltin was used. Table 5. Results of the model reaction of furfural (10a) with allyltributyltin*
“ The reactions were carried out using 1 mmol of aldehyde, 2 mol % of 1b, and 1.5 mmol of 12 or 13, in 1 ml of solvent, at 20 °C for 3-4 h under 1 bar and 24 h under 10 kbar. > Isolated yield. © The enantiomeric excesses were determined by GC on a capillary chiral B-dex 120 column.
“ The reactions were carried out using 1 mmol of aldehyde, 2 mol % of 1b, and 1.5 mmol of 12 or 13, in 1 ml of solvent, at 20 °C for 3-4 h under 1 bar and 24 h under 10 kbar. > Isolated yield. © The enantiomeric excesses were determined by GC on a capillary chiral B-dex 120 column.
Table 6. High-pressure enantioselective allylation of aryl and alkyl alde hydes catalyzed by (1R,2R)-1b*
Table 6. High-pressure enantioselective allylation of aryl and alkyl alde hydes catalyzed by (1R,2R)-1b*
Scheme 4. Equilibrium of conformational isomers of trans-(salen)Cr(ID complexes. Scheme 3. The stereochemical model.
Scheme 4. Equilibrium of conformational isomers of trans-(salen)Cr(ID complexes. Scheme 3. The stereochemical model.
Table 8. Chromatographic parameters of enantioseparation of homoallylic alcohols 11a—] * Analyzed as a isopropylidene derivative of pent-4-ene-1,2-diol.
Table 8. Chromatographic parameters of enantioseparation of homoallylic alcohols 11a—] * Analyzed as a isopropylidene derivative of pent-4-ene-1,2-diol.
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Organic ChemistryAsymmetric catalysis
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