Abstract
Three new chiral Mn macrocycle catalysts containing 20 or 40 atoms in the macrocycle were synthetized and tested in the enantioselective epoxidation of cis-β-ethyl-styrene and 1,2-dihydronathalene. The effect of the presence of a binaphtol (BINOL) compound in the catalyst backbone has been evaluated, including by Density Functional Theory (DFT) calculations.
Highlights
Asymmetric epoxidation of unfunctionalized prochiral olefins catalyzed by chiralMn(III) complexes has proven to be one of the most useful reactions in organic synthesis since it generates chiral epoxides containing two new stereocenters, which can be transformed into a large variety of compounds useful in industrial, biological, pharmaceutical and agricultural fields [1]
To better understand the importance of the approach direction as well as the importance of bulkiness in the 3,3 -positions, we report here three new chiral Mn macrocycle catalysts based on the classic Jacobsen’s salen catalyst bearing an (R)-(+)-BINOL unit in the 3,3 -position of the salen backbone
The solvent effects (CH3OH) were considered by single-point calculations at the same level as above using the self-consistent reaction field (SCRF) method based on the polarizable continuum solvent model (PCM) [30,31,32]
Summary
Asymmetric epoxidation of unfunctionalized prochiral olefins catalyzed by chiral (salen)Mn(III) complexes has proven to be one of the most useful reactions in organic synthesis since it generates chiral epoxides containing two new stereocenters, which can be transformed into a large variety of compounds useful in industrial, biological, pharmaceutical and agricultural fields [1]. These macrocycle catalysts possess different chiral diimine bridges (diphenyl or cyclohexyl) and, different steric hindrances to the Mn-oxo group along approach directions A and B (reported in Figure 1), the enantiomeric excess values were similar. Considering methanol as a solvent, the energy barriers resulted in 10.62 and 11.02 kcal/mol for the transition states related to the (2S,3R) and (2R,3S) epoxides, respectively, with a ratio between the two enantiomers of 66:34 and a theoretical EE value of 32%, which was in line with the experimental one (Table 1, entry 2) Taking into account these results and considerations, substituents in the 3,3 -positions play a crucial role in the determination of the enantioselectivity values. Their fundamental importance is probably ascribed to the stabilizing/destabilizing effect of the transition state, as showed in Figures 2a and 4A
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