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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Henry Reaction Revisited. Crucial Role of Water in an Asymmetric Henry Reaction Catalyzed by Chiral NNO-Type Copper(II) Complexes Vladimir A. Larionov,*,†,‡ Lidiya V. Yashkina,† Michael G. Medvedev,†,§,∥ Alexander F. Smol’yakov,†,⊥ Alexander S. Peregudov,† Alexander A. Pavlov,† Dmitry B. Eremin,§,¶ Tat’yana F. Savel’yeva,† Victor I. Maleev,† and Yuri N. Belokon*,†

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†

A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Street 28, 119991 Moscow, Russian Federation ‡ Department of Inorganic Chemistry, Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklaya Street 6, 117198 Moscow, Russian Federation § N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, 119991 Moscow, Russian Federation ∥ National Research University Higher School of Economics, Myasnitskaya Street 20, 101000 Moscow, Russian Federation ⊥ Plekhanov Russian University of Economics, Stremyanny per. 36, 117997 Moscow, Russian Federation ¶ The Bridge@USC, University of Southern California, 1002 Childs Way, Los Angeles, California 90089-3502, United States S Supporting Information *

ABSTRACT: Chiral copper(II) and cobalt(III) complexes (1−5 and 6, respectively) derived from Schiff bases of (S)-2(aminomethyl)pyrrolidine and salicylaldehyde derivatives were employed in a mechanistic study of the Henry reaction-type condensation of nitromethane and o-nitrobenzaldehyde in CH2Cl2 (CD2Cl2), containing different amounts of water. The reaction kinetics was monitored by 1 H and 13C NMR. The addition of water had a different influence on the activity of the two types of complexes, ranging from a crucial positive effect in the case of the copper(II) complex 2 to insignificant in the case of the stereochemically inert cobalt(III) complex 6. No experimental support was found by 1H NMR studies for the classical Lewis acid complexation of the carbonyl group of the aldehyde by the central copper(II) ion, and, moreover, density functional theory (DFT) calculations support the absence of such coordination. On the other hand, a very significant complexation was found for water, and it was supported by DFT calculations. In fact, we suggest that it is the Brønsted acidity of the water molecule coordinated to the metal ion that triggers the aldehyde activation. The rate-limiting step of the reaction was the removal of an α-proton from the nitromethane molecule, as supported by the observed kinetic isotope effect equaling 6.3 in the case of the copper complex 2. It was found by high-resolution mass spectrometry with electrospray ionization that the copper(II) complex 2 existed in CH2Cl2 in a dimeric form. The reaction had a second-order dependence on the catalyst concentration, which implicated two dimeric forms of the copper(II) complex 2 in the rate-limiting step. Furthermore, DFT calculations help to generate a plausible structure of the stereodetermining transition step of the condensation.

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INTRODUCTION

norepinephrine and (R)-salbutamol, the HIV protease inhibitor Amprenavir (Vertex 478), the anthracycline class of antibiotics L-acosamine, and others. Naturally, because the pharmaceutically active compounds should mostly be used as enantiomerically pure substances, the need for the asymmetric version of the Henry reaction was and still is in a great demand. Expectedly, it is a catalytic version that is really required.

1

The nitroaldol (Henry) reaction for the formation of C−C bonds is a powerful method for the preparation of functionalized compounds.2 The reaction involves a combination of aldehydes or ketones and nitroalkanes to form β-nitro alcohols. The resulting β-nitro alcohols either are in demand themselves or can simply be converted into a variety of α-nitroketones,3 ketones,3 nitroalkenes,4 and β-amino alcohols.5 Some very important drug substances can be easily obtained from the corresponding β-nitro alcohols, as depicted in Figure 1:6 for example, β-blocker (S)-propranolol, β-receptor agonists (R)© XXXX American Chemical Society

Received: May 28, 2019

A

DOI: 10.1021/acs.inorgchem.9b01574 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Some very important drug substances that can be derived from nitro alcohols.6

The first successful asymmetric version of the Henry reaction was implemented by Shibasaki in 1992, by applying rare earth−alkali metal binaphthoxide complexes as catalysts.7 Since then, a lot of catalytic systems have been applied for this reaction8 including C2-symmetry copper complexes based on the BOX-type,9 tetrahydrosalen (salan),10 diamine,11 and aminosulfonamide ligands.12 Copper complexes based on chiral C1-symmetry ligands have also been shown to be effective in the Henry reaction.13 Some very impressive asymmetric inductions were reported in the literature.9a,10a,12a,13a,b The conceptionally simple assumed reaction mechanism includes base-catalyzed ionization of a nitro compound, followed by attack of the generated carbanion on the aldehyde (or ketone) carbonyl group, which is activated by the metal center of the Lewis acid (Scheme 1).14

performances of both components. Additionally, the asymmetric inductions in some condensations were the best in highly polar solvents such as alcohols,9a,b,10a,b,d,11,12,13a−d,f,n and the chemical yields were greatly improved when reactions were conducted on water15,16 or, at least, in the presence of some water.7 For example, the Pombeiro and Punniyamurthy groups independently used dinuclear and polymeric copper complexes as catalysts for the Henry reaction on water; however, there was no clear explanation of the role of water in the reaction.15c−j Evidently, according to the accepted mechanism, water should be expected to coordinate to the Lewis acid center,17 serving as a competitive inhibitor and slowing the reaction instead of increasing its efficiency. Accordingly, the most effective catalytic complex systems include both metal-ion Lewis and Brønsted acidic sites of the activated ligand such as NH or OH groups, coordinated with the metal center.10b,18 The relative importance of both Lewis and Brønsted acidity on the reaction efficiency was never assessed. Unfortunately, there were no detailed studies of the mechanism of activation of an aldehyde carbonyl group by a copper complex or Lewis acid promoted Henry reactions and no quantitative assessment of the effects of water addition on the catalytic performance of the system. We assign ourselves a task of partially filling this knowledge gap by dwelling on alternative scenarios of protic solvent involvements: (1) According to a generally held concept (Scheme 1), the formation of a basic alcoholate of the intermediate nitro alcohol would expectedly be strongly coordinated to the catalytic center of the copper ion. Consequently, it should be removed before the next catalytic step occurred. The function of water (or alcohol) could be protonation of the intermediate alcoholate. Then the recovery of the catalyst might become the rate-limiting step of the process, and the positive influence of protic solvents might be attributed to this effect. (2) Was the copper complex catalysis of the Henry reactions (Scheme 1) a typical Lewis acid promoted type (generally assumed) or did the copper ion serve only as the activator of the water molecules (or alcohols) coordinated with the copper ion? Such coordination might make water (or alcohol) acidic enough to activate the aldehyde groups of the substrates via

Scheme 1. Generally Accepted Mechanism of the Lewis Acid Promoted Henry Reaction

However, at a closer look, the mechanism is difficult to apply for the rationalization of some puzzling features of the Lewis acid catalyzed Henry reactions. For example, the mutual coexistence of a base and a Lewis acidic center in the same system would inevitably cause the mutual interaction of the acid and base types, potentially inhibiting the catalytic B


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hydrogen bonding.19,20 In other words, catalysis was not of the Lewis acid type but of Brønsted acid type. There were some literature precedents; for example, Carmona and co-workers demonstrated that the protons of the coordinated water molecule in the iridium(I) complex, being a Brønsted acid, activated the carbonyl group of 3,3,3-trifluoropyruvates in an asymmetric Friedel−Crafts hydroxyalkylation of indoles.20 Here, we report the application of chiral copper(II)21 and cobalt(III)22 complexes of (S)-2-(aminomethyl)pyrrolidine and salicylaldehyde derivatives (both complexes were previously developed by some of us and are based on Schiff bases) in a mechanistic study of the condensation of nitromethane (MeNO2) and o-nitrobenzaldehyde 7 (Scheme 2). The choice of the catalyst was based on the following

RESULTS AND DISCUSSION The synthesis of copper(II) complexes 1−5 was achieved as reported by us earlier according to Scheme 3, as illustrated for (S)-2 and (S)-5.21 The cobalt(III) complex Λ(S)-6 was available from our previous work.22b,c The structures of (S)-2 and (S)-5 have previously been determined by single-crystal Xray crystallographic analysis.21 Accordingly, the complex (S)-2 formed polymeric chains with the repeating units of LCuOAc (L = ligand) monomers organized in a perpendicular manner by the bridging acetate units. The central copper(II) atoms are six-coordinated by the Schiff base phenolic oxygen, imine nitrogen, and pyrrolidine nitrogen atoms and, in addition, by three oxygen atoms from the acetate bridges. As a result, (S)-2 has a distorted octahedral geometry. The nitrogen atoms are neutral donors, while the phenolic oxygen atom and the one oxygen atom of the acetate bridges are anionic donors to keep the electroneutrality of the complex and compensate for the two positive charges of the copper ion.21 In contrast, complex (S)-5 has an island-type structure, with two symmetric independent molecules bound together through N−H···Cl hydrogen bonds. The copper(II) ions are four-coordinated by the NNO donor set of the Schiff base and on a chloride anion, and as a result, (S)-5 has a square-planar geometry.21 Obviously, both the Lewis acidic metal ion and the Brønsted acidic NH sites of the copper(II) complexes were capable of interacting with Lewis bases. Expectedly, all of the catalysts accelerated the benchmark Henry reaction of o-nitrobenzaldehyde and nitromethane in a mixture of CH2Cl2/tetrahydrofuran (THF; 1:1) and added sodium acetate (NaOAc; Table 1). Surprisingly, when the copper complex (S)-2 was used in a ratio of 1:1 to the initial aldehyde 7, the reaction did not occur even after 12 h. Evidently, at a high concentration of the catalyst, its polymeric or oligomeric structure in solution is retained and its catalytic activity in this form is very low. As the concentration of the complex decreases, polymer dissociation occurs and its catalytic activity recovers. Table 1 displays the enantioselectivity of the benchmark Henry reaction promoted by the catalysts 1−6 and added basic NaOAc. The most salient feature of these results was the completely different stereochemical outcome of the copper(II) and cobalt(III) catalysts (Table 1; compare entries 1−5 and 6). All of the (S)-ligand-derived copper(II) catalysts generated the product of the S configuration (Table 1, entries 1−3 and 5), and (R)-4, expectedly, produced R-nitro alcohol (Table 1, entry 4). Cobalt(III)-derived catalyst Λ(S)-6 gave a very low ee of the product of the R configuration (Table 1, entry 6). Evidently, stereochemically labile and stereochemically inert

Scheme 2. Henry Reaction Catalyzed by Chiral Copper(II) and Cobalt(III) Complexes

assumptions. First, the catalysts (S)-1 and (S)-6 differ in the function of the central ion. In the case of cobalt(III), the metal ion does not participate in the reaction, serving only the purpose of increasing the acidity of the NH functions, being a “Brønsted acid in disguise”.22c,23 Second, the copper(II) complex, in addition to the Brønsted acidity of the ligand, has a free coordination site at the metal center, possessing some Lewis acidity. A comparison of the catalytic efficiency of both complexes would provide valuable information on the relative importance of both factors in catalysis. In addition, the copper center can coordinate the water molecule or molecules to provide another Brønsted acidic site.20 The opportunity is evidently missing in the case of the cobalt(III) complex.

Scheme 3. Synthesis of the Chiral Copper(II) Complexes (S)-2 and (S)-5a

Reagents and conditions: (a) benzene, reflux, 4 h; (b) Cu(OAc)2·H2O/MeOH, reflux, 5 h; (c) aqueous KCl/CH2Cl2.

a

C


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Inorganic Chemistry Table 1. Enantioselective Henry Reaction of the oNitrobenzaldehyde 7 with the Nitromethane Catalyzed by Chiral Complexes 1−6a

entry

catalyst

convn (%)b

ee (%)c

1 2 3 4 5 6

(S)-1 (S)-2 (S)-3 (R)-4 (S)-5 Λ(S)-6

>99 >99 >99 >99 >99 >99

70 (S) 78 (S) 45 (S) 25 (R) 77 (S) 2 (R)

gave identical kinetic curves in the same experiment, testifying to the absence of any side reactions in the condensation. The kinetic profile (conversion vs time plot) for the Henry reaction catalyzed by (S)-2 without and with the addition of 1 equiv of NaOAc (relative to the catalyst) is presented in Figure 2.

a Reaction conditions: o-nitrobenzaldehyde 7 (0.15 mmol), nitromethane (10 eq., 1.5 mmol), catalyst (10 mol % relative to the aldehyde), and NaOAc (10 mol %, 0.015 mmol) in 0.5 mL of the mixture CH2Cl2/THF (1:1) were stirred for 24 h at room temperature. bConversion determined by 1H NMR. cDetermined by chiral HPLC analysis. The configuration of the product 8 is provided in parentheses.

Figure 2. Kinetic profiles for the condensation promoted by (S)-2 without and with the addition of 1 equiv of NaOAc in CD2Cl2.

complexes, formed by the same ligand, had different structures of the transition state of the C−C bond formation. The best asymmetric induction (78% ee; Table 1, entry 2) was achieved by (S)-2, containing tert-butyl groups in the aromatic ring, but, surprisingly, (S)-1, not containing the groups in the aromatic ring, gave a very close value of the asymmetric induction (70% ee; Table 1, entry 1). This observation differs from the usually large difference found in the asymmetric-inducing properties of ortho-substituted and unsubstituted salen-complex-based catalysts.24 Substitution of the protons at the NH and CHN moieties of the copper(II) complexes resulted in a significant decrease of the asymmetric-inducing properties of the catalysts (S)-3 and (R)-4 (45% and 25% ee, respectively; Table 1, entries 3 and 4). A significant decline in the asymmetric induction was particularly evident in the case of NH conversion into N-Bn (Table 1; compare entries 2 and 4). The asymmetric efficiency of (S)-5 was the same as that of (S)-2 (Table 1; compare entries 2 and 5). Most likely, the catalytic particle originating from both (S)-2 and (S)-5 was the same for substitution of the chlorine anion in (S)-5 and that for an acetate ion from NaOAc, effectively converting 5 into 2. However, the observation raises the question, how many acetate molecules per one catalyst molecule were needed to catalyze the reaction? The catalyst derived from (S)-5 had only one molecule at the base per one copper ion. Was the second molecule of NaOAc in the case of (S)-2 catalysis really necessary or was its addition superfluous? Could (S)-2 show catalytic activity in the absence of extra acetate ions, due to the already present acetate moiety functioning as a viable basic site? In order to take a closer look at the mechanism of the reaction, 1H NMR kinetic studies of the condensation were undertaken in CD2Cl2. For this purpose, the appearance of the product 8 resonances at 7.6 and 4.6 ppm and the disappearance of the resonance of the CHO group of aldehyde 7 were monitored at different ratios of catalyst/substrate and a 10:1 molar ratio of MeNO2/7. Both types of measurements

The kinetic curves are similar for both catalytic systems (with CD2Cl2 derived from the same batch of the solvent in both cases) during the reaction in spite of the additional acetate molecule added to the catalyst. It means that the acetate ion coordinated to the copper ion served as the competent base in the reaction. The observation matched that of Evans et al. on the chiral copper acetate catalyzed Henry reaction.9a The subject will be discussed in more detail later in the text. In the case of the kinetic experiments, the results were found to be dependent on the CD2Cl2 batches and the fluctuation was tentatively attributed to the different amounts of water present in the solvent samples. To prove this point, the reaction was explored in CD2Cl2 containing different amounts of water, in polar solvents such as methanol (MeOH), and in a two-phase system: neat aldehyde 7, MeNO2, water. The results are summarized in Table 2. Evidently, the addition of water has a significant effect on the catalytic properties of (S)-2 (Table 2, entries 1−3; see also Figure 3A). The reaction in CD2Cl2 containing 1 equiv of water (relative to the catalyst) goes to 78% conversion within 24 h with 77% enantioselectivity, whereas in CD2Cl2 saturated with water or almost without water (10 or 0.2 equiv of water correspondingly), the conversions were only 47% and 37% after 24 h, respectively, with some loss of ee (68% and 61%, respectively). The reaction proceeds efficiently in MeOH, but no asymmetric induction was observed in the condensation (Table 2, entry 3). It is highly likely that in this case the real catalytic particle in MeOH had a structure different from that in CH2Cl2 (vide infra). The data indicate that the function of water is to increase the catalytic property of the catalyst and not speed up the background reaction by changing the media polarity. Surprisingly, the reaction performed in water (twophase reaction) already goes to completion within 1 h, although with a slightly decreased enantioselectivity (ee 61%; Table 2, entry 5). On the other hand, the reaction performed in neat nitromethane (without any solvent and water) goes D


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Inorganic Chemistry

copper(II) complex (Table 2; compare entries 6 and 8); however, the promoting effect of water was much less pronounced than that in the case of the copper(II) complex (S)-2 (Table 2; compare entries 5 and 6). The comparison indicates that the Brønsted acidity of the cobalt(III) complex is sufficient to promote the catalytic activity of the complex. Without water added, the additional Lewis acidity of the copper(II) complex is a questionable advantage, while its catalytic efficiency is concerning because of the opening polymerization of the catalyst. In other words, the results hint at the Lewis acidity of the copper(II) complex linked to its catalytic performance only in the presence of water molecules. Figure 3A illustrates the influence of the amount (equivalents) of added water on the reaction performance, as monitored by 1H NMR in CD2Cl2. Evidently, both the shortage of water (0.2 equiv) and the excess of water (10 equiv) were not productive, whereas 1 equiv of water was the best choice. The results rule out the mechanistic scheme, involving protonation of the intermediate-coordinated alcoholate (Scheme 1) as the rate-limiting step of the reaction. In the latter case, the reaction rate would be expected to be linearly dependent on the water concentration. On the other hand, the observation can be rationalized by assuming water coordination to the copper ion, 15d,17 decomposing the polymeric structure of the initial (S)-221 and, highly likely, forming the Lewis acid-assisted Brønsted acid catalyst (Scheme 4). The growing amount of data

Table 2. Reaction of Aldehyde 7 with MeNO2 Promoted by the Catalyst (S)-2 or Λ(S)-6a entry

catalyst

solvent

time (h)

convn (%)b

ee (%)c

1d 2e 3f 4 5 6 7 8

(S)-2 (S)-2 (S)-2 (S)-2 (S)-2 (S)-2 Λ(S)-6 Λ(S)-6

CD2Cl2 CD2Cl2 CD2Cl2 MeOH H2O g H2O g

24 24 24 24 1 1 1 1

78 47 37 >99 >99 28 84 52

77 (S) 68 (S) 61 (S) racemate 61 (S) 73 (S) 6 (R) 9 (R)

a

Reaction conditions: o-nitrobenzaldehyde 7 (0.15 mmol), MeNO2 (10 equiv, 1.5 mmol), and catalyst (10 mol %) in 0.5 mL of solvent were stirred at room temperature. bConversion determined by 1H NMR. cDetermined by chiral HPLC analysis. Configuration of the product 8 provided in parentheses. d1 equiv of water relative to the catalyst. e10 equiv of water relative to the catalyst. f0.2 equiv of water relative to the catalyst. gWithout any solvent in a neat mixture of 7 and MeNO2.

Scheme 4. Simplified Hypothetical Formation of the Lewis Acid Assisted Brønsted Acid Catalyst from the Initial Polymeric Structure of (S)-2 and Water in CD2Cl2

supports the notion of previously assumed cases of Lewis acid catalysis as being, in fact, Brønsted acid25 or Lewis acid assisted Brønsted acid catalysis, as was formulated by Yamomoto et al.26 The additional water molecules, probably, functioned as competitive inhibitors of the reaction, preventing substrate activation by the coordinated water molecule.20 An additional proof of the concept came from quantumchemical calculations (see the Supporting Information, SI) at the PBE0-D3/LANL2TZp (on Cu atoms)|6-31+G** (all other atoms)/PCM(CH2Cl2) level of theory (the PBE0 functional was known to provide accurate results for the organometallic chemistry calculations27 and was recently shown to be well-grounded in theory28,29) and was performed on a dimeric structure of (S)-2 (the justification for choosing the complex structure will be given below). The results have shown that benzaldehyde does not form a coordinated bond with the copper atom of the catalyst (Figure 3B) because of the required loss of conjugation between the carbonyl group and benzene ring. The energy minimum of the Cu and OC group system is situated at a distance of 4 Å, which effectively precludes any Lewis acid type activation of the aldehyde

Figure 3. (A) Influence of the water content in the solvent on the conversion of o-nitrobenzaldehyde 7 catalyzed by the complex (S)-2 (blue, ratio of catalyst/water = 1:1; red, ratio of catalyst/water = 1:10; green, ratio of catalyst/water = 1:0.2). (B) Potential energy surface relaxed scan of the model dimeric (S)-2 complex with benzaldehyde along the Cu···OC distance. Computed points along the scan coordinate are shown as bullets, and they are connected with splines. Hydrogen atoms attached to carbon atoms are omitted for clarity.

much slower compared with the reaction in water, with the yield of 8 being only 28% (Table 2, entry 6). In contrast, the cobalt(III) complex Λ(S)-6 catalyzed the reaction better in neat nitromethane compared to the E


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Inorganic Chemistry

Figure 4. Variation of the 1H NMR spectrum of the initial aldehyde 7 with the addition of different amounts of (S)-2 (the concentration of 7 was 0.2 M in CD2Cl2).

Effects of the same type were observed in the case of a 2naphthaldehyde solution in CD2Cl2, containing some water. The addition of (S)-2 led to the selective line broadening of the water present in solution, whereas the resonances of the aldehyde protons did not change until the concentration of (S)-2 reached 0.06 M (the range of the ratios of aldehyde/ copper varied from 100:1 to 3:1; see Figure S1). No visible changes in the 13C NMR spectra of 2-naphthaldehyde were observed at the same aldehyde/copper ion ratios (see Figure S2). The behavior pattern of nitromethane was similar to that of 7, with very little if any broadening of its resonances up to 0.6 equiv of the added complex (S)-2 (Figure 5). The signals of the water protons were greatly broadened even at 0.02 equiv of the added copper complex. Thus, both substrates were not able to compete with water for the copper Lewis acid center of (S)2. In addition, an electrospray ionization mass spectrometry (ESI-MS) study (see below) did not detect any complexes of (S)-2 with either MeNO2 or 7. However, the situation was different for product 8 (Figure 6). The resonances of product 8 became broadened even at 0.02 equiv of the added copper(II) complex (S)-2, whereas the dichloromethane signal remained unperturbed (Figure 6). Expectedly, it was the signals of the α and β protons and OH groups that were broadened first. Obviously, 8 was a much stronger complexing agent toward (S)-2 relative to the initial substrates and the water molecules present in the solvent. A

carbonyl group. Water, on the other hand, according to the calculations, does form a strong coordinate bond (length 2.6 Å; vide infra Figure 10), which justifies its role as a Lewis acid assisted Brønsted acid catalytic center. The proof of a much greater ability of the water molecule to coordinate to the copper ion of (S)-2, compared to both 7 and MeNO2, may be found by analyzing the resonance line broadening of the initial substrates, water and product 8, in the presence of the copper(II) catalyst (S)-2. Coordination of a diamagnetic compound with a paramagnetic center leads to a reduction in the relaxation times T1 and T2 of the former. In the case of systems with total electron spin number S = 1/2 [copper(II) complexes], the Solomon−Bloembergen−Morgan theory is applicable.30 In other words, the line broadenings of the proton resonances of the substrates in the presence of copper(II) complexes should indicate the extent and even the geometry of the substrates/copper ligation. The line broadening of the proton resonances of aldehyde 7, nitromethane, and nitro alcohol 8 effected by (S)-2 is shown in Figures 4−6. Expectedly, there was very little broadening of the resonances of 7 with up to 0.6 equiv of added copper(II) complex (S)-2 (Figure 4). Even then a similar broadening of the CH2Cl2 admixture signal at 5.3 ppm indicated that there were no specific interactions of 7 and the copper ion. On the other hand, the water admixture signal at 1.65 ppm was already broadened to a great extent at 0.02 equiv of the complex (S)-2. F


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Inorganic Chemistry

Figure 5. Variation of the 1H NMR spectrum of the initial nitromethane with the addition of different amounts of (S)-2 (the concentration of nitromethane was 0.2 M in CD2Cl2).

To ascertain the rate-limiting step of the reaction and the number of the catalyst molecules in the transition state of the reaction, the order with respect to the substrates and catalyst had to be quantified. For this purpose, the reactions were carried out at different concentrations of each substrate/ component while the other component concentration was kept constant. The initial zero-order rates of the reactions were determined, and the coefficients x and y in eq 1 were found to be 1.04 and 0.16, respectively (see Figures S5 and S7). Thus, the concentration of aldehyde 7 was of no importance to the reaction kinetics, while the rate of the reaction directly depended on the nitromethane concentration and one molecule of the substrate entered the rate-limiting step of the reaction.

similar behavior was observed in the case of the addition of some amounts of isopropyl alcohol to the complex (S)-2 (see Figure S3). The intermolecular distances between the nuclei of the copper(II) ion of (S)-2 and 8 can be estimated through the effect of paramagnetic relaxation enhancement (see details in the SI).31,32 The proton−copper internuclear distances calculated from the equation (see details in the SI, part 4) are presented in Table 3. The nearest nuclei to the copper ion are H5 and H6, which reliably show that 8 primarily binds to the copper ion of the complex (S)-2 by its OH group in CD2Cl2 solution. Certainly, other types of interactions could be taken into account that additionally stabilize the complex formation, including NH hydrogen-bond formation with the NO2 group of product 8. Certainly, 8 should serve as a competing inhibitor of the Henry reaction, and its formation at the latter stages slows the reaction. In the reaction catalyzed by 10 mol % of the catalyst after 10% conversion, the inhibition process commences, as illustrated in Figure S10. The results additionally implied that the generally assumed intermediate formation of a much more basic alcoholate of the nitro alcohol, according to the generally held concept (Scheme 1), would completely block the catalytic center of the copper ion and the recovery of the catalyst might become the ratelimiting step of the process, which is not the case (see above). Thus, the generally accepted mechanism of the Henry reaction (Scheme 1) is not operational, at least, in our case.

V=

[8] = k[MeNO2 ]x [7] y [(S )‐ 2]z dt [8]0

(1)

The observation could be rationalized by assuming that the rate-limiting step of the reaction was α-proton removal from the nitromethane molecule, whereas the attack of the generated carbanion on the carbonyl group of 7 (C−C bond formation) and other stages of the catalytic cycle were much faster. In order to provide some support for the mechanism, a sample of CD3NO2 was employed in the reaction under the standard conditions. The condensation was slowed and the observed kinetic isotope effect equaled 6.3 (see Figure S9), proving the nature of the rate-limiting step of the reaction was proton removal from the nitromethane molecule and not G


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Inorganic Chemistry

Figure 6. Variation of 1H NMR spectrum of the nitro alcohol 8 with the addition of different amounts of (S)-2 (the concentration of 8 was 0.2 M in CD2Cl2).

proton removal from the nitromethane molecule indirectly implied very high electrophilicity of the carbonyl group of the aldehyde facilitated by the coordinated water molecule, as depicted in Scheme 4. The change from H2O to D2O in the saturated CD2Cl2 had very little, if any, influence on the observed kinetics of the reaction (see Figure S11). Evidently, no rate-limiting steps of proton or deuteron transfer to anion particles were taking place in the reaction media.33 In other words, the result additionally proves that the proton-driven slow decomposition of the copper alcoholate intermediate can be ruled out in the mechanistic scheme. Also, the mechanism, implicating some water molecules in proton removal from the nitromethane molecule, looks unlikely. The switch from H2O to D2O would inevitably have some sizable effects on the reaction kinetics, which was not observed. Evidently, it was only a coordinated acetyl group that removed a proton from the nitromethane molecule in the transition state. The observation is a puzzling one. The acidity of nitromethane in dimethyl sulfoxide is 17.2, whereas that of acetic acid in dimethyl sulfoxide is 12.3.34 The difference constitutes 5 orders of magnitude. Additionally, the acetate is coordinated to the copper ion and its basicity is additionally decreased. Thus, the nitromethane should be significantly activated before its proton can be removed by the acetate. Such activators could be other copper complex molecules, and the

Table 3. Distances (Å) between the Protons of 8 and the Copper Ion of (S)-2

N2/N1

H1

H2

H3

H4

H5

H6

H7

H8

0.02 0.05 0.1 0.3

6.47 6.55 6.44 6.46

6.33 6.10 6.16 6.32

4.70 a a 5.97

4.26 4.18 a 5.05

3.79 4.09 4.23 4.23

2.93 3.08 3.18 a

4.67 4.97 5.00 5.31

4.02 4.60 4.36 5.24

a Determination of the relaxation rate is difficult because of signal overlap. N2/N1 = ratio of the concentrations of complex (S)-2 and product 8.

catalyst recovery from the alcoholate-coordinated catalyst intermediate depicted in Scheme 1. In order to understand if deuterium would be incorporated in the final product, we conducted experiments in deuterium oxide (D2O). When the reaction was catalyzed by either the copper(II) complex (S)-2 or the cobalt(III) complex Λ(S)-6, we did not observe the incorporation of deuterium in the nitro alcohol 8 at the H7 and H8 positions (for clarity, see Table 3). These results additionally confirmed that the stage of C−C bond formation was irreversible. The rate-limiting step of H


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Inorganic Chemistry

Another question is, how many (S)-2 molecules constitute the “monomer”? For the purpose of answering the question, we investigated the system at a molecular level by using ESI-MS. The mass spectrum of (S)-2 measured in CH2Cl2 clearly showed the presence of dimeric species at m/z 815.3592 as [2M − OAc]+, which appeared to ionize significantly more efficiently compared to the monomeric species at m/z 378.1727 ([M − OAc]+) with a ratio of over 20:1 (Figure 8). Variation of the (S)-2 concentrations from 0.01 to 1 mg mL−1 confirmed that it is not an artifact of the spraying process (Figure 8). A similar result was observed for (R)-4 with a −Bn substituent instead of −H (see Figure S13). It is worth noting that measuring the spectra in acetonitrile and MeOH immediately resulted in monomeric species as the most abundant ions (see Figure S14). On the other hand, the addition of water to the solution of (S)-2 in CH2Cl2 in the range from 0.05 to 0.5 vol % did not affect the ratio between [2M − OAc]+ and [M − OAc]+ ions (see Figure S15). Furthermore, the addition of 100 equiv of MeNO2 or 10 equiv of 7 shifted the ratio dimer/monomer to 100:1 in solution. The same effect was observed when both 100 equiv of MeNO2 or 10 equiv of 7 had been added to the solution (see Figure S16). Evidently, the equilibrium monomer/dimer could not be responsible for the experimentally observed second order in the catalyst because (S)-2 were already predominantly present as the dimer. However, the seeming contradiction could be rationalized by assuming the formation of two dimeric particles in the transition state of condensation (four copper ions together). The aggregate exists in very small amounts and could not be found by ESI-MS. Thus, the so-called “monomer” of (S)-2 is a dimeric particle, and two molecules of the dimer (four molecules of (S)-2) constitute the catalytically active particle. The observation implies that the real catalytic structures of the catalytic entities of (S)-2 in MeOH and CH2Cl2/water had different structures,

order in which the catalyst entered the kinetic expression should differ from one unit.35 To determine the order with respect to the catalyst concentration, the reaction was carried out in duplicate at four different catalyst concentrations (5, 10, 15, and 20 mol %; for the original kinetic plots, see Figure S8). The initial zeroorder rate constants (V) were used to draw a plot of log V against log[(S)-2]. The resulting plot is shown in Figure 7, and

Figure 7. Plot of log V versus log[(S)-2] for the Henry reaction of onitrobenzaldehyde 7 and nitromethane in CD2Cl2. Reaction conditions: aldehyde 7 (0.15 mmol), nitromethane (1.5 mmol), catalyst (S)-2 (5, 10, 15, and 20 mol %), 1 equiv of water relative to the catalyst, and CD2Cl2 (0.5 mL).

the best-fit line through the data points has a slope of 2.1, suggesting that the reaction has a second-order dependence on the catalyst concentration (z = 2 in eq 1).35 This indicates that the catalytically active species are dimeric and exist in equilibrium with the catalytically inert monomers.

Figure 8. Calculated and experimental ESI-MS spectra of (S)-2 in CH2Cl2 at two concentrations (0.01 and 1 mg mL−1), expanded to monomeric (left) and dimeric (right) species. I


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Inorganic Chemistry

Figure 9. Structures and relative energies of modeled dimeric copper complexes. Top: Feasible water-free complexes. Nonpolar hydrogen atoms from optimized geometries are omitted for clarity. Bottom: Feasible water-containing complexes. Water is shown in blue. Red numbers designate bond lengths in angstroms.

which resulted in different catalytic performances of (S)-2 in the solvents (Table 2; compare entries 1 and 4). The initial catalyst dimer can maintain the original crystal geometry21 with one bridging acetyl group between the copper atoms (CD1; Figure 9) or adopt a bis(μ2-Ac) structure (CD2; Figure 9), which is a common feature for acylcopper(II) complexes according to the Cambridge Structural Database,36 or apply a bonding pattern similar to that in the crystal of (S)521 (CD3; Figure 9). Quantum-chemical calculations of (S)-2 catalyst dimers at the PBE0-D3/LANL2TZp (on Cu atoms)|631+G** (all other atoms)/PCM(CH2Cl2) level of theory (same as above; see the computational details in the SI) have shown that these structures have remarkably close energies, with CD1 being the most stable. Water molecule coordination with the copper atom can lead to any of the three adducts shown in the bottom of Figure 9 (all of them were preoptimized with the Cu−O distance fixed at 2.5 Å, but only three maintained this bond in optimization without constraints). According to quantum-chemical modeling using the same level of theory, the adducts CD2+Wtop and CD3+Wtop have the lowest energy, and thus one of them can be expected to be the key catalytic entity. Notably, in all adducts, at least one of the NH groups is engaged in a strong intramolecular hydrogen bond, which may be crucial for stereochemical recognition during condensation. This notion is supported by a low enantiocontrol exerted by (R)-4 bearing a benzyl group on the nitrogen atom of the ligand (see Table 1, entry 4). Dimer formation is, evidently, needed to beef up the catalytic properties of the copper complex. One hypothesis would envisage the nitromethane CH acidity promotion by a dimeric molecule of the catalyst via hydrogen-bond formation with the coordinated water. The second molecule of the dimer (Figure 10) functions as a base in the rate-determining step of proton removal from the activated nitromethane molecule by the coordinated acetyl group. The bridged structure is well suited for minimizing the energy of the charge-transfer changes

Figure 10. Plausible transition state of nitromethane ionization promoted by two dimeric molecules of (S)-2.

in the process by encompassing the four copper ions. Noneaccidentally polymetallic clusters are a common motif in many metal enzymes.37 Figure 11 illustrates a plausible structure of the stereodetermining transition step of condensation. It is based on the CD2+Wtop water adduct, which is the second most energetically favorable and also the only adduct containing two acidic OH and NH groups in close proximity. Both acidic hydrogen atoms are expected to bond with a second water molecule (blue on Figure 11A). Then, o-nitrobenzaldehyde binds to the catalyst by means of a stacking interaction with the ligand π system and a hydrogen bond with the second-sphere water molecule. The presence of π stacking between the catalyst and aldehyde is consistent with the lowered catalytic activity of (S)3 (see Table 1, entry 3), where it is prohibited by a phenyl substituent at R2, which stands perpendicular to the catalyst π system. Benzaldehyde double binding ascertains exposure of its Re face to the solution, which leads to the experimentally obtained (S)-product configuration upon nitromethane anion attack (see Figure 11B). Apparently, C−C bond formation induces proton transfer from the second-sphere water to the carbonyl atom, which finalizes the product formation. J


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Inorganic Chemistry

Figure 11. (A) Plausible stereodetermining step of the reaction catalyzed by (S)-1/(S)-2. tert-Butyl groups and ligands of the peripheral fragments are omitted for clarity. (B) Plausible structure of the catalyst−aldehyde complex, which participates in the stereodetermining step. hydrogen atoms attached to carbon atoms are omitted for clarity.

Scheme 5. Hypothetical Catalytic Cycle of the Asymmetric Henry Reaction Catalyzed by Chiral Dimeric Copper Complex (S)2 in CH2Cl2 in the Presence of Water

previously studied chiral copper(II) catalysts was underestimated because the water (or alcohols) content of the reaction was not taken into account and assessed. This work will pave the way for the study of the Henry reaction mechanism and the design of new catalytic systems based on copper complexes.

Still, testing of the nonlinear effect present in our system was performed in a range of enantiomeric purities of the catalyst (see Figure S12). No sizable nonlinear effects were observed. Finally, Scheme 5 shows the plausible catalytic cycle of the asymmetric Henry reaction catalyzed by the chiral dimeric copper complex (S)-2.

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CONCLUSIONS In summary, we have demonstrated that chiral NNO-type copper(II) complexes effectively catalyzed a benchmark reaction of o-nitrobenzaldehyde condensation with nitromethane only in the presence of water. Additionally, the copper(II) ion was not involved in the direct Lewis acid type activation of the aldehyde carbonyl group contrary to the generally held view on the copper complex Henry reaction catalysis. Instead, the Lewis acid center coordinates with water molecules, making them highly efficient Brønsted acid components of the catalysts. In addition, the coordinated water forms hydrogen bonds with the nitromethane molecule, greatly increasing its CH acidity. For the efficient catalytic performance of the system, four initial complexes were needed. The connotation of the results is that the efficiency of the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01574. Crystallographic data (XYZ) Crystallographic data (XYZ) Details of NMR and high-resolution ESI-MS kinetic studies, computational methods, and procedure for the enantioselective Henry reaction (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (V.A.L.). *E-mail: [email protected] (Y.N.B.). K


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2615−2618. (b) Gogoi, N.; Boruwa, J.; Barua, N. C. A Total Synthesis of (−)-Bestatin Using Shibasaki’s Asymmetric Henry Reaction. Tetrahedron Lett. 2005, 46, 7581−7582. (c) Li, H.; Wang, B.; Deng, L. Enantioselective Nitroaldol Reaction of α-Ketoesters Catalyzed by Cinchona Alkaloids. J. Am. Chem. Soc. 2006, 128, 732− 733. (d) Blay, G.; Hernández-Olmos, V.; Pedro, J. R. Synthesis of (S)(+)-Sotalol and (R)-(−)-Isoproterenol via a Catalytic Enantioselective Henry Reaction. Tetrahedron: Asymmetry 2010, 21, 578−581. (e) Guo, Z.-L.; Deng, Y.-Q.; Zhong, S.; Lu, G. Enantioselective Synthesis of (R)-Salmeterol Employing an Asymmetric Henry Reaction as the Key Step. Tetrahedron: Asymmetry 2011, 22, 1395− 1399. (7) Sasai, H.; Suzuki, T.; Arai, S.; Arai, T.; Shibasaki, M. Basic Character of Rare Earth Metal Alkoxides. Utilization in Catalytic Carbon-Carbon Bond-Forming Reactions and Catalytic Asymmetric Nitroaldol Reactions. J. Am. Chem. Soc. 1992, 114, 4418−4420. (8) For reviews on the asymmetric Henry reaction, see: (a) Luzzio, F. A. The Henry Reaction: Recent Examples. Tetrahedron 2001, 57, 915−945. (b) Murugavel, G.; Sadhu, P.; Punniyamurthy, T. Copper(II)-Catalyzed Nitroaldol (Henry) Reactions: Recent Developments. Chem. Rec. 2016, 16, 1906−1917. (c) Zhang, S.; Li, Y.; Xu, Y.; Wang, Z. Recent Progress in Copper Catalyzed Asymmetric Henry Reaction. Chin. Chem. Lett. 2018, 29, 873−883. (9) For selected examples about the application of copper complexes based on chiral BOX-type ligands in the Henry reaction, see: (a) Evans, D. A.; Seidel, D.; Rueping, M.; Lam, H. W.; Shaw, J. T.; Downey, C. W. A New Copper Acetate-Bis(oxazoline)-Catalyzed, Enantioselective Henry Reaction. J. Am. Chem. Soc. 2003, 125, 12692−12693. (b) Ginotra, S. K.; Singh, V. K. Enantioselective Henry Reaction Catalyzed by a C2-Symmetric Bis(oxazoline)−Cu(OAc)2· H2O Complex. Org. Biomol. Chem. 2007, 5, 3932−3937. (c) Cruz, H.; Aguirre, G.; Madrigal, D.; Chávez, D.; Somanathan, R. Enantioselective Nitromethane Addition to Brominated and Fluorinated Benzaldehydes (Henry Reaction) Catalyzed by Chiral Bisoxazoline− Copper(II) Complexes. Tetrahedron: Asymmetry 2016, 27, 1217− 1221. (10) For selected examples about the application of copper complexes based on chiral tetrahydrosalen (salan) ligands in the Henry reaction, see: (a) Xiong, Y.; Wang, F.; Huang, X.; Wen, Y.; Feng, X. A New Copper(I)−Tetrahydrosalen-Catalyzed Asymmetric Henry Reaction and Its Extension to the Synthesis of (S)Norphenylephrine. Chem. - Eur. J. 2007, 13, 829−833. (b) Kureshy, R. I.; Das, A.; Khan, N. H.; Abdi, S. H. R.; Bajaj, H. C. Cu(II)Macrocylic [H4]Salen Catalyzed Asymmetric Nitroaldol Reaction and Its Application in the Synthesis of α1-Adrenergic Receptor Agonist (R)-Phenylephrine. ACS Catal. 2011, 1, 1529−1535. (c) White, J. D.; Shaw, S. A New Catalyst for the Asymmetric Henry Reaction: Synthesis of β-Nitroethanols in High Enantiomeric Excess. Org. Lett. 2012, 14, 6270−6273. (d) Kannan, M.; Punniyamurthy, T. Effect of Ligand N,N-Substituents on the Reactivity of Chiral Copper(II) Salalen, Salan, and Salalan Complexes Toward Asymmetric Nitroaldol Reactions. Tetrahedron: Asymmetry 2014, 25, 1331−1339. (e) Fan, Y.; Ren, Y.; Li, J.; Yue, C.; Jiang, H. Enhanced Activity and Enantioselectivity of Henry Reaction by the Postsynthetic Reduction Modification for a Chiral Cu(salen)-Based Metal−Organic Framework. Inorg. Chem. 2018, 57, 11986−11994. (11) For selected examples about the application of copper complexes based on chiral diamines in the Henry reaction, see: (a) Kowalczyk, R.; Sidorowicz, L.; Skarzewski, J. Asymmetric Henry Reaction Catalyzed by Chiral Secondary Diamine-Copper(II) Complexes. Tetrahedron: Asymmetry 2008, 19, 2310−2315. (b) Sanjeevakumar, N.; Periasamy, M. Highly Enantioselective Henry Reaction Catalyzed by a New Chiral C2-Symmetric N,N′-Bis(isobornyl)ethylenediamine-Copper Complex. Tetrahedron: Asymmetry 2009, 20, 1842−1847. (c) Chunhong, Z.; Liu, F.; Gou, S. Application of Chiral N,N′-Dialkyl-1,2-Cyclohexanediamine Derivatives in Asymmetric Copper(II)-Catalyzed Henry Reactions. Tetrahedron: Asymmetry 2014, 25, 278−283. (d) Malhotra, S. V.; Brown, H. C. C2-Symmetric N,N′-Bis(terpenyl)ethylenediamines − Synthesis

Vladimir A. Larionov: 0000-0003-3535-1292 Michael G. Medvedev: 0000-0001-7070-4052 Alexander A. Pavlov: 0000-0002-4612-0169 Dmitry B. Eremin: 0000-0003-2946-5293 Victor I. Maleev: 0000-0002-8096-4197 Yuri N. Belokon: 0000-0002-9859-2512 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Russian Science Foundation (Grant 15-13-00039). ESI-MS measurements were performed with financial support from the Ministry of Science and Higher Education of the Russian Federation. V.A.L. thanks the Council of the President of the Russian Federation (Grant for Young Scientists no. MK-3343.2019.3) for financial support. The publication has been prepared with support of the RUDN University Program 5-100 (the kinetic investigations). The Siberian Supercomputer Center, Siberian Branch of the Russian Academy of Sciences, is gratefully acknowledged for providing supercomputer facilities. This work has been carried out using computing resources of the Federal Collective Usage Center Complex for Simulation and Data Processing for Mega-science Facilities at NRC “Kurchatov Institute” (http://ckp.nrcki.ru/). The research was carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University.38 We greatly appreciate the reviewer’s comments and suggestions, which led to significant improvement of the manuscript.

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DEDICATION Dedicated to Prof. Hisashi Yamamoto on the occasion of his 75th birthday. REFERENCES

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Inorganic Chemistry

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