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Communication

Pd@[nBu4][Br] as a Simple Catalytic System for N-Alkylation Reactions with Alcohols

by
Bastien Cacciuttolo
1,
Oana Pascu
2,
Cyril Aymonier
2,* and
Mathieu Pucheault
1,*
1
CNRS, Université de Bordeaux, ISM, UMR 5255, ISM, 351 cours de la libération, 33405 Talence Cedex, France
2
CNRS, Université de Bordeaux, ICMCB, UPR 9048, ICMCB-CNRS, 87 avenue du Dr Albert Schweitzer, 33608 Pessac Cedex, France
*
Authors to whom correspondence should be addressed.
Molecules 2016, 21(8), 1042; https://doi.org/10.3390/molecules21081042
Submission received: 1 July 2016 / Revised: 28 July 2016 / Accepted: 29 July 2016 / Published: 10 August 2016
(This article belongs to the Special Issue Cascade Catalysis)
Browse Figures
Versions Notes

Abstract

:
Palladium nanoparticles, simply and briefly generated in commercial and cheap onium salts using supercritical carbon dioxide, have been found to be an effective catalytic system for additive free N-alkylation reaction using alcohols via cascade oxidation/condensation/reduction steps.

Graphical Abstract

1. Introduction

The amine function plays a major role in many industrial fields such as materials, pharmaceuticals and agrochemicals [1,2,3]. This function is present not only in bioactive structures, such as amino acids and alkaloids, but also in numerous intermediates for polymers and dyes synthesis. N-alkylated amines are mainly obtained either by reaction between an amine and an alkyl halide [4] in the presence of stoichiometric amount of a base or by reductive amination [5] of ketone or aldehyde starting materials. Alternatively, products can be obtained from alcohols, avoiding the presence of potentially mutagenic alkylating reagents or carbonyl compounds as unstable starting materials. Several methods using direct nucleophilic substitution on the hydroxyl group can be considered. However, the main developed methodology relies on a “self-hydrogen transfer” reaction, the so called “hydrogen-borrowing” method (Scheme 1).
In a one pot synthesis, three distinct steps are carried out: first the oxidation step of the starting alcohol takes place, followed by formation of an imine which will finally be reduced to the N-alkyl amine, the hydrogen transfer being performed by the catalyst during oxidation and reduction steps. Water is the only by-product generated and the reaction could theoretically occur in the absence of hydrogen, thereby enhancing the system safety.
By using noble (Pd [6], Ru [7,8,9,10,11,12], Ir [13,14,15,16], Rh [17,18]) and non-noble metals (Cu [19,20,21], Fe [22,23]) as homogenous catalysts for the N-alkylation reaction, several efficient methods have been developed. The improvement of the reaction system allowing easy recycling of the catalyst has led to the emergence of numerous studies advocating the use of heterogeneous catalysts (Pd [24,25,26,27,28], Ru [29,30,31,32], Pt [33,34], Au [35,36,37,38,39,40], Ag [41,42], Ni [43,44,45], Mn [46], Cu [47,48,49], Fe [50,51]) with more or less efficiency.
The synthesis of nanoscaled materials has allowed for the evolution of a new field of catalysis. Indeed, at nanosize, the physicochemical properties of materials can change dramatically, due to a high specific surface area but also to a higher number of metal low valence favouring radical formations. It is known that noble metal nanoparticles are effective in many catalytic processes for both reduction and oxidation reactions and the trend is the preparation of hybrid noble metal nanocatalysts, as simple as can be. Ionic liquids (ILs), as solvents, stabilizers and reducing agents [52,53] are very attractive for an easy and clean synthesis of noble metal-based nanocatalysts [54]. Bringing together the properties of ILs (low surface tension, the presence of charges able to create an electrostatic shell around the metal core, a myriad of structures and so on) [55] with the advantages of supercritical fluids technology (homogeneous single phase reaction media, properties of both liquids—density and solvation capabilities—and gases—high diffusivity, low viscosity and zero surface tension, increased overall reaction rate) highly crystalline metal NPs embedded in a solid matrix and additionally free from any undesired organic parts can be prepared [56]. Among supercritical fluids, supercritical carbon dioxide (scCO2) is especially attractive due to its low critical coordinates of 31 °C and 7.38 MPa and its accessibility, being abundant, cheap, inoffensive, and environmentally friendly, with also no liquid waste generation [57,58].
The present study is focused on the use of palladium NPs as hybrid nanocatalysts for the N-alkylation reaction. The Pd NPs are generated in onium salts (OSs) with the assistance of scCO2 in the absence of molecular solvent. The resulting easy-to-handle powdered OS embedded Pd NPs are stable towards air and moisture and display good catalytic activities in the green synthesis of N-alkylated anilines.

2. Results

2.1. Synthesis and Characterization of the Catalyst

A high pressure/high temperature stirred stainless steel batch reactor (Vreactor = 60 mL) was used to synthesize for the first time the Pd NPs embedded in the solid matrix of quaternary ammonium salts: (1) tetrabutylammonium bromide, [nBu4N][Br]; (2) tetraethylammonium bromide, [Et4N][Br]; (3) benzyltrimethylammonium bromide, [BnNMe3][Br]; (4) cetyltrimethylammonium bromide, [C16NMe3][Br]; (5) cetyltrimethylammonium bis(trifluoromethylsulfonyl)imide [C16NMe3][NTf2] and (6) cetyltrimethylammonium hexafluorophosphate, [C16NMe3][PF6]. Three bar of H2 as reducing agent were first loaded into the reactor containing the metal precursor (Pd(OAc)2) in powder form mixed with the solid OS to give 1 wt % metal loading. The synthesis reaction took place in a scCO2 media, pressurized at 25 MPa, during 60 min at 100 °C, followed by fast depressurization and CO2 venting through the reactor (a few minutes more) at the end of the reaction in order to dry and clean the final material. A more detailed description of the synthesis approach is reported elsewhere [58]. The prepared final nanoparticles embedded in the solid matrix of ammonium salt, Pd@OS were recovered as dark powder (Figure 1a—inset) and used directly in the reaction of N-alkylation.
Microscopy images of the prepared nanocatalyst, Pd@[nBu4N][Br], are presented in Figure 1. Using the scCO2 approach the prepared Pd NPs are small but deceivingly more agglomerated than expected. An explanation could be due to the high reactivity of primary Pd nuclei obtained in supercritical media, already observed by our group [59,60]. Although [nBu4N][Br], due to its structure, can act as electrostatic stabilizer preventing NPs growth and agglomeration, in the case of Pd, the electrostatic protection is not strong enough to overcome the agglomeration of the highly reactive NPs.
Very small and monodispersed NPs can also be obtained, as we have observed experimentally for other metals such as Ru, Ir, Pt [58]. The Pd NPs are however monocrystalline (Figure 1b,c) with a mean size of about 4 nm (Figure 1d). If this bulkier cation containing four symmetric alkyl chains is replaced by BnNMe3, less symmetrical and containing an aromatic ring, the Pd NPs size, morphology and chemical properties are affected. The obtained NPs are less aggregated, larger in size and less spherical (Figure 2a) leading eventually to different catalytic activity. It was assumed that the use of an OS with a long alkyl chain could stabilize the NPs more efficiently thus avoiding aggregation. With cetyltrimethylammonium bromide, slight improvement in monodispersity and an increase in size for Pd NPs were obtained (Figure 2b,c) with potential repercussions on NPs catalytic activity.
These Pd@OSs systems were then tested in alkylation of anilines using alcohols as alkylating agents. Noticeably, all obtained systems displayed interesting catalytic activities owing to the absence of NP surface modification during the material synthesis process. Indeed, unlike most liquid phase preparation of M(0) NP, surface partial oxidation or residual molecular solvent binding is avoided by the use of scCO2. In all cases, OSs form a protective shell around the NPs allowing for shelf storage at room temperature in standard vials.

2.2. Optimisation of the N-Alkylation Reaction Conditions

A first screening of the catalytic system and reaction conditions was done on a model reaction between benzyl alcohol (1a) and aniline (2a). Two different simple palladium precursors were tested and only the nanoparticles generated from Pd(OAc)2 precursor in tetrabutylammonium bromide, labelled as Pd@[nBu4N][Br], with 0.2 mol %, gave product 3aa in 48% yield (Table 1, Entry 1). NPs generated from PdCl2 led to less active species, probably due to the passivation of the Pd surface by remaining chlorides.
By increasing the catalyst amount to 0.5 and 1 mol %, better yields of 71% and 78% (Table 1, Entries 2–3) were achieved. In situ nanoparticle generation provided the desired product in 71% yield, slightly lower than with the ex situ formed catalytic system (Table 1, Entry 4). Studying the ratio between benzyl alcohol and aniline, the results showed that the use of excess alcohol can be beneficial, leading to 86% yield. A 5 to 1 ratio gave only 85% yield (Table 1, Entries 5–6). In contrast, amine excess reduced the yield to 60%.
Many solvents were evaluated. Among those, ethylene glycol and diglyme had a negative effect, with 44% and 46% yields, respectively (Table 1, Entries 8–9). In the case of ethylene glycol, no competitive N-alkylation with solvent was observed. In the presence of toluene, an excellent yield of 97% was observed (Table 1, Entry 10). Mesitylene was less effective with 78% yield (Table 1, Entry 11). Using a more polar aromatic solvent, such as anisole, compound 3aa was isolated in quantitative yield. The reaction could also take place in an aqueous medium, although leading to a mere 33% yield (Table 1, Entry 12).
Various Pd@OS, OS being chosen from the alkyl ammonium compound class, have been tested. With tetraethylammonium bromide [Et4N][Br] and benzyltrimethylammonium bromide [BnNMe3][Br], the generated nanoparticles seemed less active (28% and 43% yield, respectively, Table 1, Entries 13–14). In addition, the influence of the long alkyl chains of the cationic part and the counter-anion has been tested. Pd@[C16NMe3][Br], Pd@[C16NMe3][NTf2] and Pd@[C16NMe3][PF6] gave less promising results than those obtained with Pd@[nBu4N][Br] (Table 1, Entries 16–18). Bromide and hexafluorophosphate are more effective than triflimidate counter-ions, mostly due to the different Pd NPs size observed. Noticeably, the catalytic properties could arise from the cumulative contribution of the structure and electronic properties of OS with the physicochemical properties (size, morphology, surface chemistry, organization) of Pd NPs.

2.3. N-Alkylation of Amines with Benzyl Alcohol Derivatives

Subsequent screening and optimization, Pd@[nBu4N][Br] NPs system was found to be the best nanocatalyst. Therefore, this catalyst was further used to extend the scope of the “hydrogen-borrowing” methodology. Different amines and alcohols derivatives were tested in the previously optimised conditions (Table 2). Model reaction between benzyl alcohol (1a) and aniline (2a) provides N-benzylaniline (3aa) in 96% isolated yield (Table 2, Entry 1). Aniline para substitution with an alkyl group, R2 = Me or tBu provides the desired amines 3ac and 3ab in good yields (95% and 91%, respectively; Table 2, Entries 2–3). In the presence of an electron donating group such as methoxy group, compound 3ad is isolated in 91% yield (Table 2, Entry 4). An electron withdrawing group such as chlorine afforded product 3af in good yield (88%, Table 2, Entry 5). Different aniline substitutions with fluorine in para and meta positions, also led to good yields of 3af and 3ag (Table 2, Entries 6–7). In the case of ortho substitution, no conversion took place, a phenomenon observed for all substituted products at this position. Polysubstituted anilines led to excellent yields. 3ah, 3ai, 3aj are respectively isolated in 94%, 87% and 97% yields (Table 2, Entries 8–10). A strong electron withdrawing group slightly decreased the yield of target 3ak to 76% (Table 2, Entry 11). The reaction is not limited to anilines. With linear primary amines such as benzylamine (2m) and hexylamine (3an), the reaction was effective, leading to 3am and 3an in 93% and 88% yields, respectively (Table 2, Entries 12–13). Secondary amines, such as morpholine (2l), provided compound 3al in 52% yield (Table 2, Entry 14). Further heating at 160 °C improves the yield to 65%, but decomposition of the onium salt and morpholine became non-negligible.
Several benzylic alcohols were tested next (See NMR in Supplementary Materials). In the presence of electron donor groups such as 4-Me 1b and 4-OMe 1c, the reaction was effective with respective yields of 92% and 97% (Table 2, Entries 15–16). In the case of electron withdrawing groups, only the 4-Cl group provided the desired compound 3da (50%, Table 2, Entry 17). In other cases, deactivated benzyl alcohol derivatives and aliphatic alcohols, no conversion was observed showing that the oxidation step is the limiting step. Furfuryl alcohol 1e provides the N-alkylation 3ea with a yield of 54% (Table 2, Entry 18).

2.4. Heterogeneous Catalysts for Cascade Oxidation/Imine Foration/Hydrogenation Leading to Benzylamine

In an attempt to provide an easily accessible heterogeneous catalyst, we prepared coated SiC and C with the previously described Pd@TBAB. The formation of the Pd NPs was directly performed in the presence of the solid material leading to Pd@TBAB@SiC and Pd@TBAB@C respectively (Figure 3). Depending on the TBAB/Support ratio, the material is obtained as beads coated with TBAB or a grey solid of Pd NP and matrix embedded in TBAB.
In short, silicon carbide turned out to be more efficient than the carbon black particle. This could be attributed to a better inertness of the support not interfering with the low palladium content. This is corroborated by the poor conversion obtained when a large excess of C is used (Table 3, Entry 4). The effect is more limited with silicon carbide, and with a same excess of support, a decent 59% yield is obtained after 24 h in anisole. Nonetheless, the best results were obtained with a Pd:TBAB:SiC ratio of 1:250:500 (Table 3, Entry 8) where both the activity (99% yield) and the form of the catalyst (grey powder) were appealing. When lowering the catalytic charge on the TBAB:SiC mixture, yield remained correct (Table 3, Entries 9–12), only with a 1:250:250 ratio were obtained quantitative yield. However in that case, catalyst is obtained as Pd NPs and SiC particles embedded in TBAB, and practically inconvenient for recycling tests.
Recycling experiments were performed using the carbon support, and unsurprisingly, the catalytic activity in the first cycle was deceiving. In that case, a complete loss of activity was observed after recovering the catalyst by filtration (Table 4, Entry 2).
This phenomenon, due to PdNP release in the reaction mixture was also observed using the 1:250/500 SiC catalysts but only after five cycles (Table 4, Entries 3–7). This is due to the relatively high solubility of TBAB in anisole, leading to leaching of Pd NPs into the reaction mixtures. In an attempt to limit this phenomenon, the solvent was switched to CO2 under supercritical conditions. TBAB is insoluble in scCO2 but scCO2 is soluble in the TBAB phase, favouring reagent/product exchange between scCO2 phase and TBAB phase. While the reaction is inconclusive at 100 °C 80 bar, (Table 4, Entry 8), harsher reaction conditions led to a decent yield, which was maintained for three cycles owing to the insolubility of the catalytic active phase (Pd@TBAB) in the bulk solvent (scCO2) (Table 4, Entries 9–11).

3. Experimental Section

3.1. General Information

GC-MS analysis were performed with a Agilent 7890A GC system (Agilent, Santa Clara, CA, USA) equipped with a J & W Scientific (Agilent) DB-1701 capillary column, a 5975C VL MSD with triple axis detector (EI) using the following program: 70 °C for 1 min then 20 °C·min−1 until 230 °C then 6 min at 230 °C. 1H-, 13C-NMR were recorded on 300 MHz Avance I spectrometers (Bruker, Billerica, MA, USA). The chemical shifts (δ) and coupling constants (J) are expressed in ppm and Hertz respectively. The following abbreviations were used to explain the multiplicities: s = singlet, d =doublet, m = multiplet.

3.2. Chemicals

Amines and alcohols were used without further purification, except for furfuryl alcohol and morpholine which were distillated over CaH2. All catalytic reactions were carried out under argon atmosphere. All chemicals were stored under argon. Silica gel (230–400 mesh) purchased from Merck (Merck KGaA, Darmstadt, Germany) was used for flash chromatography. Analytical TLC silica gel 60 F254 was used.

3.3. General Procedure

Pd@TBAB (1 mol % Pd in 250 mg of TBAB synthetized in scCO2), alcohol (2 mmol, 2 eq), amine (1 mmol, 1 eq) and anisole (1 mL) were introduced into a sealed tube under argon. The reaction was then heated at 140 °C for 12 h. After cooling the reaction at room temperature, the mixture was filtrated on Celite and washed with Et2O and the filtrate was concentrated under vacuum. The crude oil was purified by column chromatography.

3.4. Analyses

N-Phenylbenzenemethanamine (3aa). CAS: 103-32-2; white solid; Yield: 96%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.42–7.27 (m, 5H), 7.24–7.14 (m, 2H), 6.78–6.69 (m, 1H), 6.66 (dd, J = 8.6, 1.0 Hz, 2H), 4.35 (s, 2H), 4.04 (bs, 1H) 13C-NMR (75 MHz, CDCl3, 25 °C) δ 148.0, 139.4, 129.4 (2C), 128.8 (2C), 127.8 (2C), 127.4, 118.0, 113.2 (2C), 48.6 MS (m/z) 183 (78), 106 (18), 91 (100), 77 (20), 65 (14), 51 (8).
N-(4-Methylphenyl)-benzenemethanamine (3ab). CAS: 5405-15-2; Pale yellow oil; Yield: 95%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.42–7.27 (m, 5H), 6.99 (d, J = 7.0 Hz, 2H), 6.57 (d, J = 7.0 Hz, 2H), 4.31 (s, 2H), 3.90 (bs, 1H), 2.24 (s, 3H) 13C-NMR (75 MHz, CDCl3, 25 °C) δ 145.1, 139.1, 130.2, 129.9 (2C), 128.7 (2C), 127.9 (2C), 127.4, 113.9 (2C), 49.3, 20.6 MS (m/z) 197 (74), 120 (21), 91 (100), 77 (10), 65 (14), 51 (4).
N-(4-tert-Butylphenyl)-benzenemethanamine (3ac). CAS : 255835-93-9; White solid; Yield: 91%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.49–7.37 (m, 4H), 7.38–7.31 (m, 2H), 7.19–7.10 (m, 1H), 6.80–6.68 (m, 2H), 4.45 (s, 2H), 4.36 (s, 1H), 1.48 (s, 9H) 13C-NMR (75 MHz, CDCl3, 25 °C) δ 165.8, 162.6, 149.9, 138.8, 130.5, 130.4, 128.9 (2C), 127.6 (2C), 127.5, 108.9, 104.4, 104.1, 99.9, 99.6, 48.4 MS (m/z) 239 (46), 224 (28), 196 (5), 132 (9), 120 (8), 106 (8), 91 (100), 77 (6), 56 (8), 51 (2).
N-(4-Methoxyphenyl)-benzenemethanamine (3ad). CAS: 17377-95-6; Pale yellow oil; Yield: 91%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.40 (d, J = 7.3 Hz, 2H), 7.30 (t, J = 6.3 Hz, 2H), 7.00 (dd, J = 5.3, 2.5 Hz, 2H), 6.83 (d, J = 5.7 Hz, 1H), 6.74 (d, J = 7.6 Hz, 2H), 4.35 (s, 2H), 4.02 (bs, 1H), 3.90 (s, 3H) 13C-NMR (75 MHz, CDCl3, 25 °C) δ 158.9, 148.3, 131.5, 129.3 (2C), 129.3 (2C), 117.6, 114.1 (2C), 112.9 (2C), 55.4, 47.9 MS (m/z) 213 (100), 198 (9), 168 (5), 136 (9), 122 (97), 91 (89), 77 (6), 65 (14), 51 (4).
N-(4-Chlorophenyl)-benzenemethanamine (3ae). CAS: 2948-37-0; Pale yellow oil; Yield: 88%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.35 (d, J = 4.5 Hz, 2H), 7.34–7.25 (m, 3H), 7.11 (d, J = 8.9 Hz, 2H), 6.55 (d, J = 8.9 Hz, 2H), 4.31 (s, 2H), 4.07 (bs, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) 145.3, 138.0, 129.28 (2C), 128.9 (2C), 127.9 (2C), 127.6, 123.7, 115.2 (2C), 49.3. MS (m/z) 217 (40), 140 (7), 111 (6), 91 (100), 65 (9), 51 (3).
N-(4-Fluorophenyl)-benzenemethanamine (3af). CAS: 370-77-4; Pale yellow oil; Yield: 92 %. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.43–7.27 (m, 5H), 6.97–6.84 (m, 2H), 6.63–6.53 (m, 2H), 4.31 (s, 2H), 3.94 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 157.8 (s), 154.7 (s), 143.8 (s), 138.7 (s), 128.7 (s), 127.6 (JCF = 18.1 Hz), 115.9 (s), 115.6 (s), 114.3 (JCF = 7.5 Hz), 77.5 (s), 77.1 (s), 76.7 (s), 49.3 (s). MS (m/z) 201 (53), 124 (9), 91 (100), 65 (10), 51 (3).
N-(3-Fluorophenyl)-benzenemethanamine (3ag). CAS: 123330-53-0; Pale yellow oil; Yield: 87%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.37–7.36 (m, 4H), 7.32–7.28 (m, 1H), 7.12–7.08 (m, 1H), 6.41–6.39 (m, 2H), 6.34–6.31(m, 1H), 4.32 (s, 2H), 4.18 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 164.4 (JCF = 241 Hz), 150.1 (JCF = 11Hz), 139.0, 130.5 (JCF = 10 Hz), 129.0, 127.7, 127.6, 109.0, 104.2 (JCF = 21 Hz), 99.7 (JCF = 25 Hz), 48.4. MS (m/z) 201 (52), 124 (8), 91 (100), 65 (16), 51 (5).
N-(3,4-Dichlorophenyl)-benzenemethanamine (3ah). CAS: 51597-75-2; Pale yellow oil; Yield: 94%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.44–7.26 (m, 1H), 7.18 (d, J = 8.7 Hz, 1H), 6.71 (d, J = 2.7 Hz, 1H), 6.46 (dd, J = 8.7, 2.8 Hz, 1H), 4.29 (s, 1H), 4.23 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) 147.6, 138.4, 132.9, 130.7, 128.9 (2C), 127.7, 127.5 (2C), 120.2, 114.1, 112.7, 48.3. MS (m/z) 251 (27), 174 (4), 145 (4), 91 (100), 65 (8), 51 (2).
N-(3,5-Dichlorophenyl)-benzenemethanamine (3ai). CAS: 65089-00-1; Pale yellow oil; Yield: 87%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.47 – 7.32 (m, 5H), 6.72 (t, J = 1.7 Hz, 1H), 6.53 (d, J = 1.7 Hz, 2H), 4.35 (s, 1H), 4.33 (s, 2H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 140.8, 136.1, 129.6, 128.6 (2C), 127.6, 126.9 (2C), 123.2, 120.2, 113.4, 48.2. MS (m/z) 251 (27), 174 (4), 145 (4), 91 (100), 65 (8), 51 (2).
N-(3-Chloro-4-fluorophenyl)-benzenemethanamine (3aj). CAS: 131776-32-4; Pale yellow oil; Yield: 97%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.36–7.26 (m, 4H), 7.24–7.20 (m, 1H), 7.06 (t, J = 9.2 Hz,1H), 6.63 (dd, J = 2.8, 6.4 Hz, 1H), 6.51 (dt, J = 3.2, 9.2 Hz, 1H), 6.43 (t, J = 5.8 Hz, 1H), 4.22 (s, 2H, J = 6.0 Hz). 13C-NMR (75 MHz, CDCl3, 25 °C) 153.2, 146.2, 146.1, 138.7, 128.9, 128.7, 127.8, 127.6, 127.6, 117.0, 113.9, 112.1, 48.0. MS (m/z) 235 (32), 158 (5), 129 (6), 91 (100), 65 (8), 51 (2).
N-(4-(Trifluoromethyl)phenyl)-benzenemethanamine (3ak). CAS: 405-81-2; Pale yellow oil; Yield: 76%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.49–7.16 (m, 6H), 7.02–6.90 (m, 1H), 6.86 (m, 1H), 6.76 (dd, J = 8.2, 2.3 Hz, 1H), 4.35 (s, 2H), 1.54 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 150.6, 138.6, 128.9, 127.6, 127.5, 126.8, 123.7, 119.2 (q, JCF = 32.5 Hz), 48.0. MS (m/z) 251 (42), 174 (7), 145 (11), 91 (100), 65 (8), 51 (2).
N-(Phenylmethyl)-benzenemethanamine (3am). CAS: 103-49-1; Pale yellow oil; Yield: 93%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.61–7.29 (m, 10H), 3.93 (s, 4H), 1.76 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 140.4 (2C), 128.4 (4C), 128.2 (4C), 127.0 (2C), 53.2 (2C). MS (m/z) 196 (14), 120 (8), 106 (57), 91 (100), 77 (6), 65 (14), 51 (5).
N-Hexylbenzenemethanamine (3an). CAS: 25468-44-4; Pale yellow oil; Yield: 88%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.47–7.22 (m, 5H), 3.84 (s, 2H), 2.67 (t, J = 9.3 Hz, 2H), 1.64–1.29 (m, 6H), 0.94 (t, J = 9.3 Hz, 2H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 140.7, 128.5 (2C), 128.2 (2C), 126.9, 54.2, 49.7, 31.9, 30.2, 27.2, 22.7, 14.2. MS (m/z) 191(2), 120 (65), 106 (8), 91 (100), 65 (6).
4-(Phenylmethyl)-morpholine (3al). CAS: 10316-00-4; yellow oil; Yield: 52%. 1H-NMR (300 MHz, CDCl3, 25 °C), 7.45–7.28 (m, 5H), 4.68 (s, 2H), 2.1–2.0 (m, 8H). 13C-NMR (75 MHz, CDCl3, 25 °C), 140.5, 129.4, 128.1, 126.9, 66.3, 64.9, 60.4. MS (m/z) 177 (30), 146 (31), 91 (100), 86 (17), 65 (11), 56 (6).
4-Methyl-N-phenylbenzenemethanamine (3ba). CAS: 15818-64-1; Pale yellow oil; Yield: 92%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.31 (d, J = 7.9 Hz, 1H), 7.27–7.15 (m, 2H), 6.82–6.73 (m, 1H), 6.73–6.65 (m, 1H), 4.33 (s, 1H), 4.11 (s, 1H), 2.40 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 148.2, 136.9, 136.4, 129.3 (2C), 129.2 (2C), 127.6 (2C), 117.5, 113.9, 112.9, 48.1, 21.2. MS (m/z) 197 (48), 105 (100), 77 (18), 65 (4), 51 (4).
4-Methoxy-N-phenylbenzenemethanamine (3ca). CAS: 3526-43-0; Pale yellow oil; Yield: 97%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.40 (d, J = 7.3 Hz, 2H), 7.30 (t, J = 6.3 Hz, 2H), 7.00 (dd, J = 5.3, 2.5 Hz, 2H), 6.83 (d, J = 5.7 Hz, 1H), 6.74 (d, J = 7.6 Hz, 2H), 4.35 (s, 2H), 4.02 (s, 1H), 3.90 (s, 3H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 159.0, 148.3, 131.5, 129.3 (2C), 128.9 (2C), 117.6, 114.1 (2C), 112.9 (2C), 55.4, 47.9. MS (m/z) 213 (20), 121 (100), 77 (11), 51 (3).
4-Chloro-N-phenylbenzenemethanamine (3da). CAS: 4750-61-2; Pale yellow oil; Yield : 50% 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.54–7.31 (m, 4H), 7.31–7.17 (m, 2H), 6.80 (td, J = 7.3, 1.0 Hz, 1H), 6.67 (dd, J = 7.8, 0.7 Hz, 2H), 4.37 (s, 2H), 4.12 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 147.9, 138.1, 133.0, 129.4 (2C), 128.9 (2C), 128.8 (2C), 117.9, 113.0 (2C), 47.7. MS (m/z) 214 (100), 104 (14), 89 (10), 77 (63), 51 (18).
N-Phenyl-2-furanmethanamine (3ea). CAS: 4439-56-9; Pale yellow oil; Yield: 54%. 1H-NMR (300 MHz, CDCl3, 25 °C) δ 7.48–7.44 (m, 1H), 7.34–7.22 (m, 2H), 6.83 (td, J = 7.3, 1.0 Hz, 1H), 6.76 (dd, J = 7.7, 0.9 Hz, 2H), 6.40 (dd, J = 3.0, 1.7 Hz, 1H), 6.37–6.28 (m, 1H), 4.39 (s, 2H), 4.08 (s, 1H). 13C-NMR (75 MHz, CDCl3, 25 °C) δ 152.8, 148.7, 142.0, 129.3 (2C), 118.1, 113.3 (2C), 110.4, 107.0, 41.5. MS (m/z) 173 (48), 81 (100), 77 (12), 65 (6), 53 (7).

4. Conclusions

Owing to the key combination of onium salts and scCO2, a flexible, robust, solvent-free preparation of Pd NPs has been done. In contrast with many other methods, it provided a ready-to-use, stable, storable powder containing Pd NPs with unmodified surface leading to high catalytic activity. The N-alkylation of amines with benzylic alcohols was optimized in the absence of base or other additives classically used in this case, and generated only water as by-product in the presence of nanocatalysts. Recycling of Pd NP embedded in OS coated on SiC has yet to be improved to achieve a high conversion while ensuring a low leaching level. Supercitical millifluidics is currently under study to obtain a relevant continuous production system for benzylation of amines.

Supplementary Materials

Supplementary materials can be accessed at: https://www.mdpi.com/1420-3049/21/8/1042/s1.

Acknowledgments

All authors acknowledge the financial support of the Aquitaine Region and the ANR-12-CDII-010-NANOCAUSYS.

Author Contributions

M.P. and C.A. conceived and designed the experiments; B.C. and O.P. performed the experiments; B.C. and O.P. analyzed the data; M.P. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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  • Sample Availability: Samples of the compounds are available from the authors.
Molecules 21 01042 sch001 550
Scheme 1. Amination of alcohols by borrowing-hydrogen method, cascade oxidation/imine formation/reduction.
Scheme 1. Amination of alcohols by borrowing-hydrogen method, cascade oxidation/imine formation/reduction.
Molecules 21 01042 sch001
Molecules 21 01042 g001 550
Figure 1. (a,b) TEM and HRTEM images of Pd@[nBu4N][Br] NPs are presented; (c) High NPs crystallinity revealed by selected area diffraction pattern (SADP); (d) LogNormal size distribution of the NPs.
Figure 1. (a,b) TEM and HRTEM images of Pd@[nBu4N][Br] NPs are presented; (c) High NPs crystallinity revealed by selected area diffraction pattern (SADP); (d) LogNormal size distribution of the NPs.
Molecules 21 01042 g001
Molecules 21 01042 g002 550
Figure 2. TEM images of Pd@OS NPs: (a) Pd@[BnNMe3][Br]; (b) Pd@[C16NMe3][Br] and (c) Pd@[C16NMe3][NTf2].
Figure 2. TEM images of Pd@OS NPs: (a) Pd@[BnNMe3][Br]; (b) Pd@[C16NMe3][Br] and (c) Pd@[C16NMe3][NTf2].
Molecules 21 01042 g002
Molecules 21 01042 g003 550
Figure 3. Preparation of Pd@TBAB@(Si)C.
Figure 3. Preparation of Pd@TBAB@(Si)C.
Molecules 21 01042 g003
Table
Table 1. Optimization of N-alkylation of aniline with benzyl alcohol 1.
Table 1. Optimization of N-alkylation of aniline with benzyl alcohol 1.
Molecules 21 01042 i001
EntryRatioOSPd@ILSolventYield (%) 2
1a:2a(mol %)
11:1[nBu4N][Br]0.2-48
21:1[nBu4N][Br]0.5-71
31:1[nBu4N][Br]1-78
41:1[nBu4N][Br]1-71
52:1[nBu4N][Br]1-86
65:1[nBu4N][Br]1-85
71:2[nBu4N][Br]1-60
82:1[nBu4N][Br]1EG 344
92:1[nBu4N][Br]1diglyme46
102:1[nBu4N][Br]1toluene97
112:1[nBu4N][Br]1mesitylene78
122:1[nBu4N][Br]1anisole>99
132:1[nBu4N][Br]1water33
142:1[Et4N][Br]1toluene28
152:1[BnNMe3][Br]1toluene43
162:1[C16NMe3][Br]1toluene63
172:1[C16NMe3][NTf2]1toluene51
182:1[C16NMe3][PF6]1toluene62
1 General conditions: sealed tube, Ar, limiting reagent (1 mmol), solvent (1 mL), 140 °C, 24 h. 2 GC yields determined by reference to an internal standard. 3 EG: ethylene glycol.
Table
Table 2. N-alkylation of amines with alcohols 1.
Table 2. N-alkylation of amines with alcohols 1.
Molecules 21 01042 i002
EntryAlcohol 1Amine 2ProductYield (%) 2
Molecules 21 01042 i003 Molecules 21 01042 i004 Molecules 21 01042 i005
11a (R1 = H)2a (R2 = H)3aa96
21a2b (R2 = 4-Me)3ab95
31a2c (R2 = 4-tBu)3ac91
41a2d (R2 = 4-OMe)3ad91
51a2e (R2 = 4-Cl)3ae 88
61a2f (R2 = 4-F)3af92
71a2g (R2 = 3-F)3ag87
81a2h (R2 = 3,4-Cl)3ah94
91a2i (R2 = 3,5-Cl)3ai87
101a2j (R2 = 3-Cl,4-F)3aj97
111a2k (R2 = 4-CF3)3ak76
121a Molecules 21 01042 i006
2m		3am93
131a Molecules 21 01042 i007
2n		3an88
141a Molecules 21 01042 i008
2l		3al52
151b (R1 = 4-Me)2a3ba92
161c (R1 = 4-OMe)2a3ca97
171d (R1 = 4-Cl)2a3da50
18 Molecules 21 01042 i009
1e		2a3ea54
1 Pd@[nBu4N][Br] (1% in Pd), 1 (2 mmol), 2 (1 mmol), anisole (1 mL), 140 °C, 24 h. 2 isolated yield.
Table
Table 3. N-Alkylation of amines with alcohols using a heterogeneous catalyst 1.
Table 3. N-Alkylation of amines with alcohols using a heterogeneous catalyst 1.
Molecules 21 01042 i010
EntryPd:TBAB:Support RatioSupportYield (%) 2
11:250:250C60
21:250:500C35
31:250:1000C36
41:250:2000C9
51:500:1000C39
61:1000:1000C46
71:250:250SiC99
81:250:500SiC99
91:250:1000SiC73
101:250:2000SiC59
111:500:1000SiC77
121:1000:1000SiC80
1 Pd@[nBu4N][Br]@M (1% in Pd), 1 (2 mmol), 2 (1 mmol), anisole (1 mL), 140 °C, 24 h. 2 isolated yield.
Table
Table 4. Recycling of heterogeneous catalyst 1.
Table 4. Recycling of heterogeneous catalyst 1.
Molecules 21 01042 i011
EntryRunSupport TimeYield (%) 2
11C (1:250:250)anisole 34 h60
22C (1:250:250)anisole 34 h0
31SiC (1:250:500)anisole 34 h99
42SiC (1:250:500)anisole 34 h98
53SiC (1:250:500)anisole 34 h85
64SiC (1:250:500)anisole 34 h60
75SiC (1:250:500)anisole 34 h0
81SiC (1:250:500)scCO2 44 h<5
91SiC (1:250:500)scCO2 56 h51
102SiC (1:250:500)scCO2 56 h50
113SiC (1:250:500)scCO2 56 h50
1 Pd@[nBu4N][Br]@M (1% in Pd), 1 (2mmol), 2 (1 mmol); 2 isolated yield; 3 anisole (1 mL), 140 °C; 4 CO2, 100 °C, 80 bar; 5 CO2, 140 °C, 100 bar.

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Cacciuttolo, B.; Pascu, O.; Aymonier, C.; Pucheault, M. Pd@[nBu4][Br] as a Simple Catalytic System for N-Alkylation Reactions with Alcohols. Molecules 2016, 21, 1042. https://doi.org/10.3390/molecules21081042

AMA Style

Cacciuttolo B, Pascu O, Aymonier C, Pucheault M. Pd@[nBu4][Br] as a Simple Catalytic System for N-Alkylation Reactions with Alcohols. Molecules. 2016; 21(8):1042. https://doi.org/10.3390/molecules21081042

Chicago/Turabian Style

Cacciuttolo, Bastien, Oana Pascu, Cyril Aymonier, and Mathieu Pucheault. 2016. "Pd@[nBu4][Br] as a Simple Catalytic System for N-Alkylation Reactions with Alcohols" Molecules 21, no. 8: 1042. https://doi.org/10.3390/molecules21081042

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