( 2 S , 5 R )-2-Methylaminomethyl-1-methyl-5-phenylpyrrolidine , a chiral diamine ligand for copper ( II )-catalysed Henry reactions with superb enantiocontrol †

The Henry (or nitroaldol) reaction is a powerful tool for C–C bond formation, because it permits rapid access to valuable synthetic intermediates such as 1,2-amino alcohols and a-hydroxy acids. Tremendous advances have been made over the last two decades in the development of enantioselective versions of this reaction. Among the many highly efficient systems based on heterobimetal, transition metal, organo and enzyme catalysis, chirally modified copper complexes have received particular attention due to the wide structural variability of the ligands (diamines, amino alcohols, amino imines, amino pyridines, imino pyridines, Schiff bases, box-type ligands, salen-type ligands, and others), the ease of preparation and the, in part, high levels of stereocontrol reached. Several of these catalysts permit 99% ee in the addition of nitromethane to some of the aldehydes tested, but none is capable of providing 99% ee for the majority of substrates. Herein we present the first copper catalyst that fulfils this demand, giving, for the addition of nitromethane to a broad range of aldehydes, the corresponding ß-nitro alcohols in high yield and excellent 99% ee. In the course of our studies on bicyclic diamines we became interested in 2-aminomethylpyrrolidines of general type 1 (Fig. 1), which carry an additional cis-aryl group in 5-position, as compared to proline derived diamines. Chelation of a metal with 1 will lead to a rigid bicyclic system, in which the aryl substituent is forced into an endo-position directly on top of the active metal site. As illustrated by complex 2, such a shielding might be of particular importance in asymmetric transition metal catalysts preferring Jahn–Teller distorted octahedral geometries, because it selectively blocks one apical position and thereby reduces the number of possible transition states. The equatorial coordination sites Leq and L 2 eq are still differentiated by the intrinsic steric and electronic properties of the C1-symmetric diamine 1, which might offer another advantage over C2-symmetric ligands. Copper(II)-catalysed Henry reactions, which are supposed to proceed via such a pentacoordinate intermediate, might provide an ideal test system to probe the potential of the diamines 1. After investigating some derivatives, we quickly identified the simple compound 3 as the ligand of choice for these reactions. Diamine 3 is easily accessible from commercially available methyl Boc-L-pyroglutamate (4, Scheme 1). Treatment of 4 with phenylmagnesium chloride and re-cyclisation of the resulting, ring-opened ketone delivered the diastereomerically pure pyrrolidine 5 after crystallization. Exhaustive reduction followed by OH/NHMe exchange afforded the target molecule 3 in overall seven simple steps and 40% yield. The enantioselective Henry reactions between the aromatic aldehydes 6a–u and nitromethane (11 equivalents) were performed on a 1 mmol scale in THF at 25 1C (Table 1, entries 1–21). Fig. 1 cis-2-Aminomethyl-5-arylpyrrolidines 1 and 3 and a square-pyramidal metal complex of 1, 2.

The Henry (or nitroaldol) reaction is a powerful tool for C-C bond formation, because it permits rapid access to valuable synthetic intermediates such as 1,2-amino alcohols and a-hydroxy acids. 1 Tremendous advances have been made over the last two decades in the development of enantioselective versions of this reaction. 2Among the many highly efficient systems based on heterobimetal, 3 transition metal, 4-6 organo 7 and enzyme 8 catalysis, chirally modified copper complexes have received particular attention due to the wide structural variability of the ligands (diamines, amino alcohols, amino imines, amino pyridines, imino pyridines, Schiff bases, box-type ligands, salen-type ligands, and others), 5,6 the ease of preparation and the, in part, high levels of stereocontrol reached.Several of these catalysts permit 99% ee in the addition of nitromethane to some of the aldehydes tested, 5 but none is capable of providing 99% ee for the majority of substrates.Herein we present the first copper catalyst that fulfils this demand, giving, for the addition of nitromethane to a broad range of aldehydes, the corresponding ß-nitro alcohols in high yield and excellent 99% ee.
In the course of our studies on bicyclic diamines 9 we became interested in 2-aminomethylpyrrolidines of general type 1 (Fig. 1), which carry an additional cis-aryl group in 5-position, as compared to proline derived diamines.Chelation of a metal with 1 will lead to a rigid bicyclic system, in which the aryl substituent is forced into an endo-position directly on top of the active metal site.
As illustrated by complex 2, such a shielding might be of particular importance in asymmetric transition metal catalysts preferring Jahn-Teller distorted octahedral geometries, because it selectively blocks one apical position and thereby reduces the number of possible transition states.The equatorial coordination sites L 1 eq and L 2 eq are still differentiated by the intrinsic steric and electronic properties of the C 1 -symmetric diamine 1, which might offer another advantage over C 2 -symmetric ligands.
Copper(II)-catalysed Henry reactions, which are supposed to proceed via such a pentacoordinate intermediate, 10 might provide an ideal test system to probe the potential of the diamines 1. 11 After investigating some derivatives, we quickly identified the simple compound 3 as the ligand of choice for these reactions. 12iamine 3 is easily accessible from commercially available methyl Boc-L-pyroglutamate (4, Scheme 1).Treatment of 4 with phenylmagnesium chloride and re-cyclisation of the resulting, ring-opened ketone delivered the diastereomerically pure pyrrolidine 5 after crystallization. 13Exhaustive reduction followed by OH/NHMe exchange afforded the target molecule 3 in overall seven simple steps and 40% yield.
The enantioselective Henry reactions between the aromatic aldehydes 6a-u and nitromethane (11 equivalents) were performed on a 1 mmol scale in THF at À25 1C (Table 1, entries 1-21).The copper(II) complex [3ÁCuBr 2 ], prepared prior to use by stirring CuBr 2 with a slight excess of pyrrolidine 3 in THF, was used as the chiral catalyst and NEt 3 (1.5 mol%) as the base.Under these conditions 12 and in the presence of just 2 mol% [3ÁCuBr 2 ], the Henry products 7a-u were formed within 18 to 67 h in excellent 92-99% yield.Outstanding 99% ee, in several cases even more than 99.5% ee, were obtained with electronically more or less neutral (6a-g), electron-deficient (6h-o) and electron-rich (6p-u) aromatic aldehydes, carrying substituents in ortho-, meta-, or para-position.
The a,b-unsaturated aldehydes 10a and 10b solely afforded the 1,2-addition products 11a and 11b.The latter one is the only compound tested within this context that delivered less than 99.0% ee, namely 98.7%.
In all cases, the re-face of the aldehyde was attacked by the nitronate; the, in part, opposite absolute stereo descriptors in the products are a formal consequence of the CIP-notation.
Aliphatic aldehydes 12 provided significantly lower enantioselectivities and yields under these conditions.Nonanal (12b), for example, delivered the Henry product 13b in unsatisfying 53% yield and 94.5% ee after 40 h.In order to compensate the lower reactivity, we raised the amount of catalyst to 8 mol% and the temperature to À20 1C, which afforded 13b in good 86% yield, but low 92.1% ee.Finally, a significant increase in the level of chirality transfer was observed by changing the copper salt from CuBr 2 to CuCl 2 . 14Under these modified conditions, both, linear (12a-c) and a-branched (12d-g) aliphatic aldehydes provided the Henry products 13a-g in excellent 98.5-99.5% ee and 495% yield (Table 2).
The stereochemical outcome of the Henry reactions can be explained via the transition state 14 (Fig. 2).As mentioned earlier, the aryl group of the chiral ligand 3 blocks the upper apical position at the Cu(II) ion, thus leaving three open coordination sites, two equatorial ones and one apical one.Based on the known model, 10 the nitronate should bind apically for maximum activation, since its negative charge is less stabilised in this position by the copper ion.Of the two higher Lewis-acidic equatorial sites, the aldehyde should coordinate to the one next to the pyrrolidine moiety for two reasons: (i) this allows the sterically more demanding counter ion X to occupy the less congested position next to the aminomethyl group 9b and (ii) with the weaker electron donating secondary amine opposite, the electrophilicity of the carbonyl group is increased thus facilitating a nucleophilic attack.Furthermore, the aldehyde must be oriented inwards in order to avoid severe steric repulsions with the chiral backbone.The C-C bond formation will presumably proceed via a six-membered,   a Performed on a 1 mmol scale in THF (600 mL) and MeNO 2 (600 mL E chair-shaped transition state, thus obviating repulsions between the nitronate-oxygen and the pyrrolidine N-methyl group. 15It might be possible that this arrangement receives some further stabilisation and rigidness by an intramolecular hydrogen bridge between the nitronate oxygen and the NH-proton of the chiral ligand.Thus, the steric and electronic properties of the diamine ligand apparently create close to perfect preconditions for the experimentally observed, almost exclusive re-face attack of the nitronate on the aldehyde carbonyl group.In summary, the cis-5-phenyl substituted 2-aminomethylpyrrolidine 3, which is accessible in just a few steps from methyl Boc-L-pyroglutamate (4), was successfully utilized as the chiral ligand in CuBr 2 -and CuCl 2 -catalysed Henry reactions.Excellent isolated yields (490%) and superb enantioselectivities (98.5-99.6%ee) were obtained with a wide variety of aromatic, heteroaromatic, vinylic and aliphatic aldehydes (36 examples).Further studies are ongoing. 16inancial support of the German research foundation (DFG) is gratefully acknowledged.
E)-1-Penten-1-yl 90 97 98.7 (S) d a Performed on a 1 mmol scale in THF (600 mL) and MeNO 2 (600 mL E 11 eq.).b Isolated yield.c Determined by HPLC analysis on a chiral phase; absolute configurations were established by comparison with literature data.d Absolute configuration was assigned based on a re-face attack on the aldehyde.

Table 1
Aromatic, heteroaromatic and vinylic aldehyde scope a

Table 2
Aliphatic aldehyde scope a