Mechanistic Investigation of DBU-Based Ionic Liquids for Aza-Michael Reaction: Mass Spectrometry and DFT Studies of Catalyst Role

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)-based ionic liquids (ILs) has exhibited a high catalytic activity in the aza-Michael reactions compared to conventional catalysts and with imidazole-based ILs. In the present work DBU-based ILs showed high catalytic potential for aza-Michael addition of aromatic amines to 2-cyclohexen-1-one under solvent-free condition. Electrospray ionization-mass spectrometry (ESI-MS) and density functional theory studies have been carried out to provide an effective activation mode of DBU-based ILs in aza-Michael addition. Our results show that both the presence of the acid hydrogen in the IL and the ability of the anion to carry out a hydrogen bond with the −NH2 group of the arylamine are fundamental for the reaction catalysis. The catalytic model proposed can be used for the rational development of new ILs with excellent catalytic properties.


Introduction
The aza-Michael additions can be used for C−N bond formation by the reaction of α,β-unsaturated carbonyl compounds with amines. The products β-aminocarbonylic are important intermediates for the synthesis of β-aminoalcohol, β-amino acid derivatives, and β-aminocarbamates, that are bioactive compounds. [1][2][3][4] It was usually catalyzed by strong base or a Lewis acid, ultrasound, heterogeneous solid acid, ionic amino acids, etc. [5][6][7][8][9][10][11] However, there are still many deficiencies of these catalysts mentioned above, such as the requirement of long reaction time, harsh reaction conditions, many side reactions and mainly most of the reported methods [12][13][14][15] are successful only with aliphatic amines and lead to low conversions for aromatic amines.
In this work, we evaluated the catalytic efficiency of DBU-based ILs (Scheme 1) by mass spectrometry (MS) and density functional theory (DFT) studies to provide a catalytic model in aza-Michael additions. For a better mechanistic understanding of aza-Michael IL-catalyzed reaction, a few questions need to be addressed: (i) the real role of the protic cation in the title reaction, DBU-based ILs with aprotic cation can also activate the substrate?; (ii) what is the effective function of the anion in the reaction? Accordingly, our results will provide important basis into the DBU-based IL activation mode and can be used to rational design of new organocatalysts. 1 H and 13 C nuclear magnetic resonance (NMR) were recorded on a Bruker Avance III HD spectrometer operating at 500 MHz for 1 H and 125 MHz for 13  DBU (1.5 mL, 10 mmol) and acetonitrile (3 mL) were charged into a 25 mL round-bottom flask. Then, the mixture was taken at 0 °C and 98% H 2 SO 4 (0.6 mL, 10 mmol) or 48% HBr (1.1 mL, 20 mmol) was added dropwise keeping the temperature at 0-5 °C. After addition, the mixture was stirred for 2 h at room temperature. The solution was washed repeatedly with ether (3 × 5 mL) to remove non-ionic residues and the oil residues was dried in vacuum at 60 °C for 12 h to afford desired ionic liquids as light yellow viscous liquids. [HDBU][Br] (1.4 g, 6 mmol) and acetonitrile (5 mL) were charged into a 25 mL round-bottom flask. Then, NaBF 4 (0.7 g, 6 mmol) was added and the mixture was stirred for 24 h at 80 °C. The solution was filtred to remove NaBr and the solid was washed repeatedly with dichloromethane. The solvent was removed under reduced pressure and oil residues was dried in vacuum at 60 °C for 12 h to afford a light yellow viscous liquid. 1  [BDBU][Br] (1.4 g, 6 mmol) and acetonitrile (3 mL) were charged into a 25 mL round-bottom flask. Then, NaBF 4 (0.7 g, 6 mmol) was added and was stirred for 24 h at 60 °C. The solution was filtred to remove NaBr and the solid was washed repeatedly with dichloromethane. The solvent was removed under reduced pressure and oil residues was dried in vacuum at 60 °C for 12 h to afford a dark yellow viscous liquid. 1  General procedure for aza-Michael reaction of aromatic amines with cycloexen-2-one To a mixture of the aromatic amine (1 mmol) and cycloexen-2-one (1 mmol) in 25 mL flask equipped with a magnetic stirrer, ionic liquid was added (0.3 equiv.). The reaction mixture was stirred at room temperature for the desired time. Upon completion of the reaction, the mixture was diluted with water (H 2 O, 3 mL) and extracted with ethyl acetate (3 × 5 mL). The combined organic phase was concentrated through vacuum evaporation and the resulting crude product was analyzed by 1 H NMR. The ionic liquid after extraction was dried in vacuo at 60 °C for 5 h. The recovered ionic liquid was then reused in subsequent reactions.      General procedure for ion-fishing of supramolecular adduct using ESI(+)-MS A mixture of aniline (0.24 mL, 2.6 mmol) and cycloexen-2-one (0.25 mL, 1.0 equiv, 2.6 mmol) was stirred magnetically at room temperature in the presence of ionic liquid (0.3 equiv.). After 15 min, an aliquot portion (10 μL) of the reaction mixture was taken out by micro pipette and dissolved in acetonitrile (1 mL) and when necessary, formic acid (1%). From the resultant solution an aliquot amount (50 μL) was subjected to ESI(+)-MS. Sampling was performed every 15 min for 120 min of reaction.

Experimental
The ion-fishing study was performed on a highresolution mass spectrometer (Impact II, Bruker Daltonics Corporation, Germany), equipped with an electrospray ionization source. The capillary voltage was operated in positive ionization mode, set at 4000 V and with an end plate potential of −500 V. The dry gas parameters were set to 10 μL min −1 at 180 °C with a nebulization gas pressure of 0.4 bar. Data were collected from m/z 50-500 with an acquisition rate of 5 spectra per s, and the ions of interest were selected to MS/MS fragmentation.

Computational methods
Theoretical calculations were performed with the Gaussian 09 program package, revision B.01. 29 The PyMOL program 30 was used to visualize structures and surfaces. The structures of supramolecular complexes were optimized using the density functional method M06-2X 31 coupled with the 6-31++G(d,p) basis set function. 32 Frequency calculations were performed to characterize the structure as a minimum or as a transition state (TS) and to obtain the zero-point energy (ZPE) 33 and thermal correction to Gibbs free energy. Corrections due to basis set superposition error (BSSE) 34 were also calculated at the same level of theory for the complexes. In order to confirm that the transitional state connects the desired reaction, intrinsic reaction coordinate (IRC) calculations were performed at the same level of theory of the optimizations.  16,19 All ILs were obtained in satisfactory yields (90-96%) and were characterized by NMR analysis (see, Supplementary Information (SI) section).

Results and Discussion
Firstly, the catalytic potential of the ILs was evaluated for the aza-Michael addition of aniline to 2-cyclohexen-1-one (Table 1). For comparison, some imidazolium ILs were also used in the model reaction. A summary of the results obtained is provided in Table 1 With the efficient catalytic system in hand, we examine the utility and generality of a wide range of aromatic amines for the [HDBU][HSO 4 ]-aza-Michael reactions using cycloexen-2-one as substrate. All results summarized in Table 2 shown good to excellent conversion rates (entries 1-11), including aromatic amines with high electron-withdrawing group at benzene ring (entries, 1-3). Four novel adducts (3c, 3d, 3g and 3m) were obtained (

Supramolecular assembly by ESI-MS study
Electrospray ionization mass spectrometry (ESI-MS) study was performed to investigate effective catalyst role in the aza-Michael addition for the reaction between 2-cyclohexen-1-one (1a) and aniline (2a) using different DBU-based ionic liquids. The catalytic role of DBU-based IL with the substrates is envisaged through the formation of the non-covalent complex (NCC) by electrophile nucleophile dual activation (Scheme 2) similar to related in literature for [Bmin][MeSO 4 ]. 35 The ESI-MS analysis was also performed for the reaction between 2-cyclohexen-1-one (1a) and cyclohexylamine (2b) with [HDBU] [HSO 4 ] to evaluate if aza-Michael addition of aliphatic amines occurs by the same mechanism proposed for the aromatic amines. For this reaction, an equivalent mass pattern for the non-covalent complex (m/z 445.2610) between the reactants and IL was not detected in the TIC ( Figure S33, SI section). Aliphatic amines are not good donor of N−H hydrogen to form the hydrogen bond with the IL anion, which is determinant for the formation of the non-covalent complex. This result confirms that the nucleophilic activation by the IL anion had crucial role in aza-Michael addition of aromatic amines.

Theoretical investigation of mechanism reaction
The purpose of including the theoretical study here was to corroborate the catalytic role proposed from experimental analysis. For the reaction with [HDBU] [HSO 4 ] were optimized: a transition state for each reaction step (TS1 and TS2); an initial complex involving reagents and the IL; a complex involving the intermediate and the IL; and a complex involving the Michael products and the IL. The results obtained from the mass spectrometry experiments were used for the rational construction of the complex involving the reagents and also for the TS1 transition state.
The optimized structures and the energy diagram for the aza-Michael addition are shown in Figure 2. The dotted lines indicate for transitional states a normal vibration coordinate. The transition state for the first step of the reaction (TS1), involving the addition of aniline to β-carbon of 2-cyclohexen-1-one was characterized by IRC calculations. In TS1 is observed the approximation of the nitrogen of 2a to the β carbon of 1a, one of the hydrogen of the amino group of 2a toward HSO 4 − anion and two interactions toward carbonyl oxygen of 1a, one of the acidic hydrogen of the cation and other of the anion hydrogen. This shows that both the cation and the counter ion play a key role in the catalysis of the reaction. These results are in agreement with the data obtained experimentally. When the IL is no protic, it is not possible to form the non-covalent complex and then this catalytic efficiency decreases, as verified for [

Conclusions
The detailed mechanism of aza-Michael adducts formation from 2-cycloexen-1-one with aromatic amines promoted by DBU-based ILs has been examined using ESI-MS experiments and DFT calculations. The cations and anions of ionic liquids are found to synergistically promote the addition reaction by nucleophilic activation and proton transfer and simultaneously stabilizing transition states by hydrogen bonding interaction. The non-covalent complex formed between reagents and ILs have been identified and characterized by ESI-MS. The mechanistic model proposed can be used as basis of rational design and selection of organocatalysts.

Supplementary Information
Supplementary information is available free of charge at http://jbcs.sbq.org.br as PDF file.
supervision, writing original draft, review and editing; Augusto A. Cândido for the investigation, methodology, validation, data curation and writing original draft; Thiago C. Rozada for the investigation, methodology, data curation, visualization and software of theoretical calculations; Andrew M. F. Rozada for investigation, methodology, formal analysis and writing original draft; João R. B. Souza for the investigation, methodology, data curation, visualization and ESI-MS experiments; Eduardo J. Pilau for the supervision, writing orignal draft and software of ESI-MS experiments; Fernanda A. Rosa for the conceptualization, supervision, writing review and editing; Ernani A. Basso for the project administration, supervision, writing review and editing.