Hydrogen-bonding directed reversal of enantioselectivity.

The preparation of both enantiomers of a chiral compound is still a vital task in pharmaceutical and bioorganic chemistry.1 Basically, it can be achieved with the use of both enantiomers of a chiral ligand, respectively, in a catalytic asymmetric process. However, the two enantiomers of a chiral ligand are not always easily available from natural chiral sources, such as amino acids, carbohydrates, and alkaloids. Alternatively, the enantioselectivity can be reversed by using different metals2 in the presence of the same ligands, including additives,3 or changing the reaction parameters (temperature4 and solvents5) or other methods.6 Although the use of hydrogen bonding to accelerate or catalyze certain reactions has been well documented recently,7 reversal of enantioselectivity directed by hydrogen bonding is still rare.8 We envisage that the formation of hydrogen bonds may reverse the enantioselectivity if ligands were properly designed. The reversal could be caused by the variation of reaction transition states (Figure 1), in which transition state A is common. Both substrates SM1 and SM2 coordinate with the central metal M, but in transition state B, SM2 can coordinate with the central metal M, while the other substrate SM1 has an interaction with NH2 of the ligand through hydrogen bonding. Thus, different enantiofacial attack was afforded, leading to the reversal of enantioselectivity. In this Communication, we present a hydrogen-bond-directed reversal of enantioselectivity in AgOAc-catalyzed [3+2] cycloaddition of azomethine ylides. In our initial study, we chose the cycloaddition of iminoester 2a with dimethyl maleate as the model reaction, which is a useful way to form highly substituted pyrrolidine rings9 (Table 1). Ligands 1a-e could be conveniently synthesized according to the known literature method.10 The reaction proceeded smoothly in the presence of AgOAc/1a in Et2O, and only endo cycloaddition product was detected with 76% ee. To our delight, the opposite absolute configuration of adduct 3a was obtained with 83% ee when ligand 1b was used, whose substituents on the N atom are H atoms. To achieve better enantioselectivity, we replaced phenyl groups in 1a/ 1b with 3,5-dimethylphenyl (1c/1d). Both enantiomers of adduct 3a were obtained with 84% and -84% ees, respectively (entries 3 and 4). The ee was improved to 92% using 1c when the reaction was run at -25 °C. Adduct 3a with 92% ee and opposite absolute configuration was obtained using ligand 1d at -25 °C. Thus, both enantiomers of adduct 3a can be obtained with excellent enantioselectivity using AgOAc/1c/-25 °C and AgOAc/1d/-25 °C, respectively. To explore the scope of the hydrogen bond-directed reversal of enantioselectivity in cycloaddition of azomethine ylides, a variety of iminoester substrates derived from aldehydes with different steric and electronic properties were examined (Table 2). All reactions went to completion within 4 h in high isolated yields (91-98%). Reversal of the absolute configuration was realized in the reactions of various iminoesters 2a-f and dimethyl maleate regardless of the steric hindrance and electronic properties of the benzene ring of iminoester 2. Other dipolarophiles such as tert-butylacrylate and N-phenylmaleimide were also tested; successful reversal of the absolute configuration was observed (entries 11-16). To further understand the role of hydrogen bonding, a computational study was performed. As shown in Figure 2, the complexes formed by 2b and Ag-1a/1b have four possible types of structures (C1 to C4). Eight structures for 1a (R ) Me) and 1b (R ) H) of C1 to C4 type are fully optimized by B3LYP11 method with the Gaussian 03 program.12 For C, H and N, the 6-31G* basis set was used; for Ag, the Lanl2DZ basis set with effective core potential (ECP)13 was used. The results show that the most stable complexes for both 1a and 1b are C2 type (Figure 3). Figure 3 shows that there is large space at both sides of the iminoester in C2-1b. The two carbonyl groups of the dimethyl maleate can coordinate with the Ag center. Furthermore, they may form two hydrogen bonds with the NH2 group. This interaction can stabilize the negatively charged oxygen atom in the possible zwitterionic intermediate and the transition state.9h This indicates that it is favorable for C2-1b to be attacked from the top face. While in C2-1a, the dimethylamino group cannot form hydrogen bonds, and the methyl groups will cause steric repulsion. Therefore the dimethyl maleate will attack from the bottom face of C2-1a, † Dalian Institute of Chemical Physics. ‡ Shanghai Institute of Organic Chemistry. Figure 1. Proposed variation of transition states.