Novel bifunctional chiral squaramide-amine catalysts for highly enantioselective addition of mono-and diketones to nitroalkenes

Novel bifunctional chiral squaramide–amine organocatalysts have been developed by rational combination of pyrrolidine and a cinchona alkaloid. The catalysts promoted the enantioselective Michael addition of both mono-and diketones to a broad range of nitroalkenes providing the corresponding products in moderate to high yields with excellent enantioselectivities and diastereoselectivities (up to 96% yield, 96% ee, 98:2 dr ) under mild conditions. These results demonstrate that the assembly of two chiral privileged skeletons, pyrrolidine and cinchonine with a squaramide linker is a useful strategy to reach a wider substrate scope, high reaction efﬁciency and enantioselectivity. The match of the chiralities between two backbones embedded in the catalysts is also critical for improving enantioselectivity.


Introduction
In the past decade, the organocatalytic asymmetric Michael reaction has emerged as one of the most important carbon-carbon bond forming reactions in organic chemistry. 1In particular, the conjugate addition of ketones to nitroolefins has received extensive attention since the resulting nitroalkanes are versatile intermediates, in which the nitro group can be transformed into various useful functional groups. 2Stimulated by the seminal works of List, Barbas and co-workers, 3 several research groups focused on the development of more efficient primary, 4 secondary, 5 amine-based catalysts and also thioureas, in order to both improve the selectivity and substrate scope. 6The development of multifunctional chiral organocatalysts is of great importance. 7In 2006, the Jacobsen group successfully applied bifunctional thiourea catalysts to Michael addition and achieved good results. 8Recently, the Xiao group developed new thiourea-amine organocatalysts by combining two privileged backbones into one, in the generation of highly efficient catalysts for Michael reaction. 9Despite the progress in this area, the existing catalytic systems are still limited.Therefore, new strategies to design more efficient chiral catalysts for this type of reaction are highly desirable.
In addition, squaramides have been studied in molecular recognition, 10 supramolecular assemblies, 11 and chiral organocatalysts for asymmetric transformations. 12It is known that these molecules also exhibit a dual donor-acceptor hydrogen bonding ability.Due to its unique structure feature and wide application, squaramide still has great potential to explore in asymmetric catalysis.To the best of our knowledge, there is no report on the development of new squaramide-amine bifunctional catalysts by combination of two backbones, which are connected by a squaramide motif.In continuation of our interest in developing new squaramides that are of potential in asymmetric catalysis, 13 we envisioned that the assembly of two privileged frameworks with a squaramide linker would constitute a new class of bifunctional organocatalysts, and the well-defined, rigid, bifunctional squaramide would serve as an efficient stereocontrol element for asymmetric Michael reaction.Herein, we describe the synthesis of unprecedented squaramide-amine bifunctional catalysts by rational incorporation of structurally privileged pyrrolidine, 14 and cinchona alkaloids, 15 into one molecule.
The pyrrolidine structural unit can also be replaced with appropriate functionalities, such as 3,5-bis(trifluoromethyl)benzylamine, 12 2-amino-1,2-diphenyl-ethanol and prolinol, 16 as illustrated in Scheme 1.These squaramide bifunctional catalysts proved to be highly efficient for Michael addition of both mono-and diketones to a broad range of nitroalkenes, affording the products in high yields with excellent enatioselectivities and diastereoselectivities.

Scheme 1. New class of bifunctional chiral squaramide-amine catalysts 1-3 and previously reported catalyst 4.
A series of squaramide-based catalysts (Scheme 1) were prepared and applied to enantioselective Michael addition reactions.The catalysts were easily synthesized by sequential additionelimination reactions of two amines with diethyl squarate 5 (3,4-diethoxy-3-cyclobutene-1,2dione).The synthetic procedure is illustrated in Scheme 2. Treatment of (S)-2-aminomethyl-1-N-Boc-pyrrolidine 6 with one equivalent of diethyl squarate 5 in the presence of triethylamine in ethanol at room temperature gave the corresponding squaric monoamide monoester 7 in 87% yield.Subsequently, the reaction of 7 with the cinchonine-derived amine 8 under the same reaction conditions afforded the Boc-protected squaramide-amine 9 (73%), which was then deprotected directly with HCl to give the desired organocatalyst 1a in 88% yield.Other squaramide-amine catalysts 1b-d, 2 and 3 were synthesized by a similar procedure.The structures of the catalysts 1-3 were fully characterized (see the Supporting Information).

Scheme 2. Synthetic route for catalyst 1a.
Initially, the Michael addition of a monoketone was investigated.A brief survey of reaction conditions was carried out for the reaction of cyclohexanone 10 with trans-nitrostyrene 11a using 1a as catalyst (Table 1).We found that the solvent, catalyst loading, and acid additive are critical determinants of the reaction efficiency.Ultimately, we found that the use of 10 mol% 1a and 10 mol% of PhCO2H in THF is the preferred catalyst system for achieving good yield and high enantioselectivity (Table 1, entry 4).Reducing or enhancing the catalyst loading did not benefit the enantioselectivity at all.It is noteworthy that the reaction proceeded much more slowly and only trace amount of the product was observed in the absence of PhCO2H (Table 1, entry 11).Other substituted benzoic acids such as o-nitrobenzoic acid, p-bromobenzoic acid, p-methylbenzoic acid, and TFA gave the product in lower diastereoselectivity or enantioselectivity.The result indicated that the nitrogen atom of the cinchonine as well as the benzoic acid may orientate the nitro group through a hydrogen bonding interaction, so that the cyclohexanone pyrrolidine enamine attacks the nitrooalkene as a nucleophile from a specific face to give the high enantio-and diastereoselectivity.So in all the reactions investigated, 10 mol% of PhCO2H was used as additive.
The effectiveness of several related chiral squaramides was also evaluated.As shown in Table 1, the combinations of the pyrrolidine with a cinchona alkaloid do indeed play an important role in the stereocontrol in this Michael addition.For chiral squaramide-amine 1a, derived from L-proline and cinchonine, in the addition of cyclohexanone 10 to nitrostyrene 11a, always displayed higher selectivity.However, when cinchonidine was incorporated into the catalyst backbone (catalyst 1b), both the yield and enantioselectivity of Michael adduct decreased (Table 1, entries 1 vs 12).For catalyst 1c, which was prepared from D-proline and cinchonine, inferior results were observed (entry 13).The catalyst 1d, derived from the D-proline and cinchonidine, induced higher enantioselectivity than those of 1b-c, the opposite enantiomeric product was generated in 52% yield and 68% ee, respectively (entry 14).In contrast, catalysts 2 and 3, which possess 2-amino-1,2-diphenylethanol and prolinol moieties, gave no product in this reaction.Surprisingly, the known catalyst 4, 12a which exhibits excellent enantioselectivity in catalyzing Michael reaction of diketones and nitroalkenes, showed no activity in conjugate addition of cyclohexanone 10 to nitrostyrene 11a (Table 1, entry 18).These results demonstrated that the presence of two chiral privileged skeletons, pyrrolidine and cinchonine, with a squaramide linker improve the performance of the catalyst, which is a useful strategy to reach a wider substrate scope, high reaction efficiency and enantioselectivity, justifying the design concept.Having optimized the reaction conditions, we next investigated the scope of chiral squaramide catalyzed enantioselective Michael reaction of cyclohexanone with a variety of nitroolefins and the results are shown in Table 2.In addition to cyclohexanone 10, various nitroolefin derivatives with different substitution patterns on the aromatic ring in the presence of 10 mol% of 1a with 10 mol% of PhCO2H as the cocatalyst, gave the corresponding Michael adducts 12a-j with excellent diastereoselectivities (up to 98/2 dr) and enantioselectivities (up to 93% ee) (Table 2, entries 1-10).However, the yields vared dramatically and no clear trend was observed with regard to the substituent on the aromatic ring.When aryl nitroolefins containing electron-withdrawing or electron-donating substituents on the aromatic ring (i.e.11b-d, 11f, 11i and 11j) were used for the reaction with 10, the reactions proceeded smoothly resulting in the formation of products in good yields (72-84%, Table 2, entries 2-4, 6, 9 and 10).However, aryl nitroolefins 11e, 11g, and 11h, bearing 2-chloro, 4-trifluoro and 3-nitro groups, respectively, were exceptional, giving the producs in only low to moderate yields (41-60%, Table 2, entries 5, 7 and 8).Phenyl nitroolefin 11a without a substituent afforded the product in 60% yield (Table 2, entry 1).Heteroaromatic nitroolefins, such as 2-thienyl nitroolefin 11l, gave a 75% of ee (Table 2, entry 12).2-Furyl nitroolefin 11k, gave the product in 70% yield with 80/20 dr and 81% ee (Table 2, entry 11).Other catalysts, such as 1d, proved to be effective providing the products with opposite configuration in reasonable yields and good ee values (entries 13 and 14).Aliphatic nitroolefins displayed much less reactivity in this reaction, for example, when (E)-1nitropent-1-ene 11n was used, only trace amount of product was observed (Table 2, entry 15).Besides monoketone 10, the catalytic effect of the bifunctional squaramides catalysts in Michael addition of diketones was also evaluated.After a brief optimization of reaction conditions, we found that CH2Cl2 was preferred as solvent with 1 mol% catalyst 1a (Table 3, entry 7).Other solvents, e.g.toluene, THF, n-hexane, methanol, etc were also investigated, however, none of them gave better results (Table 3, entries 2-5).Neither reducing the catalyst loading from 1 to 0.5 mol% (Table 3, entry 6) nor enhancing the catalyst loading to 2, 5, even 10 mol% (Table 3, entries 1-8 and 9) benefited the enantioselectivity.The optimized conditions were then used to assess the activity of other chiral squaramides 1b-d, 2 and 3.As in the Michael additions of a monoketone, the combination of the pyrrolidine with cinchona alkaloid proved critical for this Michael addition.Again the chiral squaramideamine 1a, derived from L-proline and cinchonine, displayed higher selectivity; while the catalyst 1b, possessing a cinchonidine backbone, was less effective, giving the product with opposite configuration in 86% yield and 82% ee, respectively (Table 3, entry 10).A lower yield but good enantioselectivity was observed when 1c was used in this reaction, which prepared from Dproline and cinchonine (entry 11).Furthermore, catalyst 1d, derived from D-proline and cinchonidine, induced higher enantioselectivity than 1b-c, giving the desired product in 93% yield and 87% ee (entry 12).These results demonstrated the significance of matching the chiralities of the two backbones embedded in the catalysts on the reactivity and enantioselectivity.Lastly, two catalysts having the core structure of cinchonine-substituted squaramide but differing in the pyrrolidine moiety were examined, for example 2 and 3, and all were found to provide the product in good yield but lower ee value (Table 3, entries 13-15).
With the optimal reaction conditions in hand, we next examined other nitroolefins and 1,3dicarbonyl compounds to expand the substrate scope of catalyst 1a, and the results are shown in Table 4. Various electron-poor and electron-rich nitroolefins with various substitution patterns on the aromatic ring reacted smoothly with 2,4-pentanedione 13a in the presence of 1 mol% of 1a, giving the corresponding Michael adducts 14a-h in high to excellent yields (88-96%), with high enantioselectivities (81-96% ee) (Table 4, entries 1-8), including the heteroaromatic nitroolefin, such as 11l, afforded the product in 83% ee (Table 4, entry 7).When comes to other 1,3-dicarbonyl compounds 13b-h, in the reaction with 11a, all reactions proceeded in good to excellent yields and, generally, good enantioselectivities (Table 4, entries 9-15).Although several of the reactions proceeded with lower diastereoselectivity, each of the diastereomers was formed in very good enantioselectivity.As expected, employment of catalyst 1d led to high selectivity and enantioselectivity and opposite configuration was observed (Table 4, entries 16 and 17).
In addition, we also investigated the use of these nitroalkanes as starting materials for further transformation.For example, as shown in Scheme 3, in the presence of zinc in acetic acid, 14j was smoothly reduced to corresponding amine 15 in 86% yield.Scheme 3. Further transformation of the Michael adduct.

Conclusions
In conclusion, we have designed a new class of bifunctional chiral squaramide-amine catalysts by a rational combination of readily available pyrrolidines with cinchona alkaloids.The catalyst 1a exhibited the best performance in the asymmetric addition of both mono-and diketones to a broad range of nitroalkenes.The corresponding products can be obtained in moderate to high yields with excellent enantioselectivities and diastereoselectivities under mild conditions.The match of the chiralities between two backbones embedded in catalysts is also critical for improving enantioselectivity.

Experimental
General.Tetrahydrofuran, diethyl ether and toluene were dried over Na/benzophenone, dichloromethane was dried over CaH2 and distilled prior to use.Reaction progress was monitored using analytical thin-layer chromatography (TLC) on 0.25 mm Merck F-254 silica gel glass plates.Visualization was achieved by UV light (254 nm).Flash chromatography was performed with silica gel (Merck, 230-400 mesh).Unless otherwise noted, all NMR spectra were recorded using CDCl3 as the solvent with reference to residual CHCl3 ( 1 H at 7.24 ppm and, 13 C at 77.0 ppm).Optical rotations were measured at room temperature on a Perkin-Elmer 241MC automatic polarimeter (concentration in g/100 mL).Melting points were obtained on a micromelting apparatus and the data were uncorrected.Determination of% ee was achieved using a chiral HPLC equipped with a chiralpak AD or OD column with n-hexanes: 2-propanol as the mobile phase at a flow rate of 1 mL/min.

Table 1 .
Optimization and catalyst screening for Michael addition of cyclohexanone 10 with nitrostyrene 11a a

Table 1 .
Continued a Unless otherwise noted, the reactions were carried out with 0.2 mmol of 11a, 2.0 mmol of cyclohexanone 10 in the presence of 10 mol% of catalyst 1a and 10 mol% of PhCO2H, in 1.0 mL of solvent indicated, for 48 h, b Isolated yield for both diastereomers, c Numbers in parenthesis correspond to the syn/anti ratio determined by 1 H NMR spectroscopy, d Enantiomeric excess of major diasteroisomer, determined by HPLC on AD-H chiral column, e Reaction conducted using 5 mol% of 1a, f Reaction conducted in the absence of PhCO2H.g Opposite configuration was observed.

Table 2 .
Evaluation of the scope of the Michael addition of cyclohexanone 10 and nitroalken 11 a .
e Opposite configuration was observed.

Table 3 .
Optimization of reaction conditions of the Michael addition of 2, 4-pentanedione 13a with nitrostyrene 11a a d Opposite configuration was observed.
Unless otherwise noted, the reactions were carried out with 0.2 mmol of 11, 0.4 mmol of 13 in 1.0 mL of CH2Cl2 in the presence of 1 mol% of catalyst 1a for 12-36h.b Isolated yield; c Numbers in parenthesis correspond to the syn/anti ratio determined by 1 H NMR. d Values are listed for major and minor diastereomers, respectively, and are determined by HPLC.e Opposite configuration was observed.