ortho-Amino group functionalized 2,2′-bipyridine based Ru(II) complex catalysed alkylation of secondary alcohols, nitriles and amines using alcohols

Bivas Chandra Roy, Subhankar Debnath, Kaushik Chakrabarti, Bhaskar Paul, Milan Maji and Sabuj Kundu*
Department of Chemistry, IIT Kanpur, Kanpur 208016, UP, India. E-mail: sabuj@iitk.ac.in

Received 28th November 2017 , Accepted 3rd January 2018

First published on 3rd January 2018


Various Ru(II) complexes bearing functionalized 2,2′-bipyridine ligands were synthesized and fully characterized. Among them, a new N6,N6′-dimethyl-2,2′-bipyridine-6,6′-diamine ligand was found to be the most electron-rich ligand as its corresponding Ru(II) complex (1a) displayed the lowest νco value and the highest efficiency in the β-alkylation of secondary alcohols with primary alcohols (TON = 98[thin space (1/6-em)]860). Complex 1a also exhibited a greater reactivity in the monoalkylation of acetonitrile, α-alkylation as well as α-methylation of arylacetonitriles. Compared to the other reported systems, in α-methylation of nitriles complex 1a presented superior catalytic activity. The potential of complex 1a was extended further in N-methylation of amines using methanol as a green methylating agent.


Introduction

Ligand design plays a critical role in homogeneous transition-metal catalysis.1–6 Because ligands control the stability and compatibility of the corresponding metal complexes, they can directly influence the product selectivity. Among the various types of ligands, electron-rich ancillary ligands based on phosphines have played a central role in various chemical transformations such as C–H bond activation, olefin metathesis, hydroformylation, cross-coupling reactions and many more.7–14 However, most of these ligands are expensive and sensitive to air and moisture.

Functionalized pyridine ligands are one of the most promising classes of ligands in coordination chemistry and homogeneous catalysis as they can substitute for phosphines. They have received attention due to their easy functionalizability and tunability and also due to their significantly greater stability towards oxygen and moisture.15–17 This encouraged us to explore the catalytic activity of metal complexes having electron-rich functionalized pyridine ligands.

Recently, substituted bipyridines containing Ru and Ir complexes were probed by various groups for water oxidation, hydrogenation of formic acid, esters and carboxylic acids, CO2 reduction, alcohol dehydrogenation and many more (Fig. 1).18–24 For example, the electron-donating dimethylamine group containing bipyridine ligands played a crucial role in the hydrogenation of biomass derived compounds.25 Borylation of benzylic and aromatic C−H bonds were reported with Ir complexes comprising both phenyl substituted phenanthroline and 2,2′-bipyridine based ligands.26–28 Szymczak's group disclosed that the introduction of hydroxyl and NHMes groups in the NNN pincer ligand platform considerably improved the catalytic activity in transfer hydrogenation and alcohol oxidation.29,30 Alkyl groups in the ortho-position of the bipyridine and phenanthroline ligands dramatically enhanced the catalytic activity in Ni-catalyzed carboxylation and reductive amidation reactions as conveyed by Martin and co-workers.31–34


image file: c7qo01061c-f1.tif
Fig. 1 Example of recently reported functionalized bipyridine based metal complexes.

Inspired by this, we hypothesized that the introduction of amine groups in the ortho-positions of the 2,2′-bipyridine would also produce more active catalysts. However, Schlaf et al. observed that the introduction of NH2 groups to the ortho-position of bipyridine did not enhance the catalytic activity of their Ru complexes in the deoxygenation of terminal-diols.35 Hence, we were interested to incorporate –NHR (R = Me, Ph) groups into the ortho-position of the 2,2′-bipyridine framework which would increase the electron-density over the metal center and we further wanted to test how this could influence the catalytic activity.

As the fossil feed-stock reserve as well as crude oil are decreasing rapidly, the search for an alternative pathway for the production of fine chemicals and fuels becomes inevitably significant.36,37 In this regard, the efficient transformation of biomass feedstock to sustainable chemicals is one of the major focuses which lately received noteworthy attention.38,39 Recently, the conversion of complex biomass molecules including lignocellulose to alcohols has been reported.40 Hence, development of sustainable methodologies for the transformation of alcohols to fine chemicals is highly essential.

α-Alkylated nitriles, monoalkylated nitriles and N-methylated amines are versatile building blocks to manufacture various fine chemicals such as carboxylic acids, amines, amides, esters, dyes, surfactants, agrochemicals, and biologically active compounds.41–48 Similarly, β-alkylation of secondary alcohols is also significantly important for the synthesis of long chain alcohols and biofuels.49–52 Conventionally, the excess of expensive alkyl halides, strong bases and highly toxic cyanating reagents such as KCN or NaCN were essential to produce these compounds which generated stoichiometric amounts of toxic salt waste.53–58 Hence, the synthesis of these valuable molecules by transforming alcohols to alkylating agents is considered as cleaner and greener methodologies.59 Water is the only waste generated in this environment friendly process. In the last decade, several catalytic systems were reported to achieve these conversion;54,60,61 however, many of them suffered from either low activity, higher catalyst loading, more than the stoichiometric amount of bases, or harsh reaction conditions.

Herein, we reveal the synthesis and catalytic activities of –NHR (R = Me, Ph) groups containing 2,2′-bipyridine-based ruthenium complexes and report a remarkably efficient and sustainable catalytic system for the β-alkylation of secondary alcohols, α-alkylation of arylacetonitriles, monoalkylation of acetonitrile and N-methylation of amines.

Results and discussion

N6,N6′-Dimethyl-2,2′-bipyridine-6,6′-diamine (6DNHMeBP) was synthesized in 50% yield by reacting 6,6′-dibromo-2,2′-bipyridine and aqueous methylamine solution in the presence of copper(I) oxide. N6,N6′-Diphenyl-2,2′-bipyridine-6,6′-diamine (6DNHPhBP) was prepared by a neat reaction of 6,6′-dibromo-2,2′-bipyridine and aniline following the modified literature report62 and 6,6′-dimethyl-2,2′-bipyridine (6DMeBP) and 6,6′-dimethoxy-2,2′-bipyridine (6DMeOBP) were synthesized according to the reported literature procedure.63,64 Treatment of 6DNHMeBP with equimolar RuHCl(CO)(PPh3)3 in DCM at room temperature afforded complex 1a with 80% yield. Similarly, complexes 2, 3 and 5 were prepared in good yields by reacting corresponding bipyridine ligands with RuHCl(CO)(PPh3)3 in DCM. However, for complex 4, 6DMeBP and RuHCl(CO)(PPh3)3 were heated in DCM–EtOH mixture (2[thin space (1/6-em)]:[thin space (1/6-em)]1) at 70 °C for 24 hours. Likewise, the reaction of RuCl2(PPh3)3 with 6DNHMeBP in DCM at room temperature resulted in complex 1b in 75% yield. Complex 1c was synthesized by treating equimolar amounts of the 6DNHMeBP ligand with [Ru(p-cymene)Cl2]2 in refluxing methanol followed by anion metathesis with NH4PF6. Due to the hygroscopic nature of complex 1c-Cl, the corresponding PF6 exchange complex (1c) was synthesized (Scheme 1).
image file: c7qo01061c-s1.tif
Scheme 1 Synthesis of new amine based bipyridine ligands and general synthesis of bipyridine-based Ru(II) complexes.

In the 1H NMR spectrum, the hydride signal of the complexes 1a, 2, 4 and 5 appeared as a triplet at δ = −12.17 ppm (JHP = 19.2 Hz), −12.16 ppm (JHP = 17.9 Hz), −11.69 ppm (JHP = 19.1 Hz) and −11.11 ppm (JHP = 19.9 Hz), respectively. The appearance of 31P NMR resonances of 1a, 2, 4 and 5 at δ = 42.75, 41.80, 43.61 and 46.64 ppm, respectively, as a singlet indicated the presence of one kind of phosphorus environment around the Ru(II) centres. In the infrared spectra the νco of complexes 1a, 2, 3, 4, and 5 appeared at 1930, 1939, 1940, 1935 and 1938 cm−1, respectively. The lowest νco value of complex 1a compared to others clearly indicated much higher electron density over the ruthenium centre in 1a. This also specified that among all the bipyridine based ligands discussed here the 6DNHMeBP ligand was more electron rich.

The molecular structures of complexes 1a, 1b and 1c were determined by X-ray crystallography (Fig. 2). The solid state structure of complexes 1a and 1b showed that the P–Ru–P bond angles were 173.91° and 176.11°, respectively, suggesting the trans arrangement of the two PPh3. The Ru–H bond distance in complex 1a was 1.551 Å. The solid state structure of complex 1b revealed the H-bonding interaction between the hydrogen atom associated with the NHMe moiety and the –Cl attached to the Ru centre which exhibited asymmetric H–Cl distances of 2.120 Å and 2.061 Å.


image file: c7qo01061c-f2.tif
Fig. 2 Molecular structure of [(6DNHMeBP)Ru(H)(CO)(PPh3)2]Cl (1a), (6DNHMeBP)RuCl2(PPh3)2 (1b) and [(6DNHMeBP)RuCl(p-cymene)]PF6 (1c) (30% thermal ellipsoids; counter Cl and PF6 anion of complex 1a and 1c were omitted for clarity, respectively).

To evaluate the catalytic activities of these Ru(II) complexes, β-alkylation of secondary alcohols with primary alcohol was chosen which was recently explored by several groups.55,65–82 Among them Ir and Ru metal based complexes are the most common ones. For the synthesis of long chain alcohols, this hydrogen borrowing methodology became attractive due to atom economical and greener nature of this reaction.

Initially, to investigate the catalytic activity of these Ru(II) complexes, 1-phenylethanol and benzyl alcohol were selected as model substrates and the reaction was carried in toluene at 130 °C for 45 minutes using Ru(II) complexes (0.1 mol%) (Table 1). Compared to the Ru(II) precursors such as RuHCl(CO)(PPh3)3 and RuCl2(PPh3)3 with the isolated metal complexes both the conversion and selectivity were improved which indicated the ligand effect (Table 1, entries 1–9). Among all ruthenium hydrochloride carbon monoxide complexes, 1a delivered the best results, showing 63% conversion of 1-phenylethanol with 93% selectivity of 1,3-diphenylpropanol after 45 minutes (Table 1, entry 1). Moreover, among the three types of Ru(II) complexes bearing the 6DNHMeBP ligand (1a, 1b and 1c), 1a offered better catalytic activity in this reaction (Table 1, entries 1–3).

Table 1 β-Alkylation of 1-phenylethanol with benzyl alcohol using different Ru-complexesa

image file: c7qo01061c-u1.tif

Entry Complex Conv.b (%) A/B ratioc
a Reaction conditions: Complex (0.1 mol%), 1-phenylethanol (1.1 mmol), benzyl alcohol (1.1 mmol) and KOtBu (0.55 mmol) in 2 mL toluene at 130 °C for 45 min, closed argon conditions in the Schlenk tube.b Conversion of 1-phenylethanol was determined by GC using mesitylene as an internal standard.c Determined by 1H NMR using 1,3,5-trimethoxybenzene as the internal standard.
1 1a 63 93[thin space (1/6-em)]:[thin space (1/6-em)]7
2 1b 34 94[thin space (1/6-em)]:[thin space (1/6-em)]6
3 1c 25 92[thin space (1/6-em)]:[thin space (1/6-em)]8
4 2 47 90[thin space (1/6-em)]:[thin space (1/6-em)]10
5 3 48 86[thin space (1/6-em)]:[thin space (1/6-em)]14
6 4 54 89[thin space (1/6-em)]:[thin space (1/6-em)]11
7 5 49 86[thin space (1/6-em)]:[thin space (1/6-em)]14
8 6 45 76[thin space (1/6-em)]:[thin space (1/6-em)]24
9 7 29 58[thin space (1/6-em)]:[thin space (1/6-em)]42
image file: c7qo01061c-u2.tif


Next, in the presence of complex 1a, different bases were tested to optimize the reaction conditions. Carbonate bases under the standard reaction conditions performed very poorly (Table 2, entries 1–3) whereas NaOH and KOH gave 31% and 78% conversion of 1-phenylethanol respectively after 1 hour (Table 2, entries 4 and 5). On the other hand, KOtBu acted most efficiently with 82% conversion (Table 2, entry 6). Subsequently, different amounts of KOtBu were tested (Table 2, entries 7–9) and 0.5 equivalent of KOtBu was found to be ideal for this reaction (Table 2, entry 6).

Table 2 β-Alkylation of 1-phenylethanol with benzyl alcohol in the presence of different basesa

image file: c7qo01061c-u3.tif

Entry Base (equiv.) Conv.b (%) A/B ratioc
a Reaction conditions: Cat. 1a (0.1 mol%), 1-phenylethanol (1.1 mmol), benzyl alcohol (1.1 mmol) and KOtBu (0.55 mmol) in 2 mL toluene at 130 °C for 1 h; closed argon conditions in the Schlenk tube.b Conversion of 1-phenylethanol was determined by GC using mesitylene as an internal standard.c Determined by 1H NMR using 1,3,5-trimethoxybenzene as the internal standard.d No catalyst.
1 Cs2CO3 (0.5) <2
2 K2CO3 (0.5) <1
3 Na2CO3 (0.5) 0
4 NaOH (0.5) 31 95[thin space (1/6-em)]:[thin space (1/6-em)]5
5 KOH (0.5) 78 95[thin space (1/6-em)]:[thin space (1/6-em)]5
6 KOtBu (0.5) 82 95[thin space (1/6-em)]:[thin space (1/6-em)]5
7 KOtBu (0.7) 85 96[thin space (1/6-em)]:[thin space (1/6-em)]4
8 KOtBu (0.4) 70 89[thin space (1/6-em)]:[thin space (1/6-em)]11
9 KOtBu (0.3) 44 78[thin space (1/6-em)]:[thin space (1/6-em)]22
10d KOtBu (0.5) 0


After optimizing the reaction conditions, β-alkylation of a wide range of alcohols was conducted to explore the generality of the present catalytic system which is summarized in Schemes 2 and 3. The reaction of benzyl alcohol with different para-substituted 1-phenylethanols with both electron-donating groups (–Me, –OMe) and electron withdrawing atoms (–F, –Cl and –Br) afforded the desired β-alkylated secondary alcohols in excellent yields with high selectivity (2b–2f). 1-(Naphthalen-2-yl)ethanol, 1-tetralinol and 1-phenyl-1-propanol were also smoothly coupled with benzyl alcohol with moderate to good yields (2g–2i). Following this protocol the reaction of benzyl alcohol with 1-cyclopropylethanol and 3-methylbutan-2-ol also produced the corresponding β-alkylated alcohols in excellent yields after 5 hours (2j–2k).


image file: c7qo01061c-s2.tif
Scheme 2 β-Alkylation of different secondary alcohols with benzyl alcohol; Reaction conditions: Cat. 1a (0.1 mol%), secondary alcohol (1.1 mmol), benzyl alcohol (1.1 mmol) and KOtBu (0.55 mmol) in 2 mL toluene at 130 °C for 1.5 h; closed argon conditions in a Schlenk tube, isolated yield. a[thin space (1/6-em)]2 h heating. b[thin space (1/6-em)]5 h heating. c[thin space (1/6-em)]2.2 mmol secondary alcohol.

image file: c7qo01061c-s3.tif
Scheme 3 β-Alkylation of 1-phenylethanol with different primary alcohols; Reaction conditions: Cat. 1a (0.1 mol%), 1-phenylethanol (1.1 mmol), primary alcohol (1.1 mmol) and KOtBu (0.55 mmol) in 2 mL toluene at 130 °C for 1.5 h; closed argon conditions in a Schlenk tube, isolated yield. a[thin space (1/6-em)]5 h heating. b[thin space (1/6-em)]7 h heating.

Afterward, the β-alkylation of 1-phenylethanol with a range of primary alcohols was carried out to magnify the scope of this catalytic system. The reaction of 1-phenylethanol with benzyl alcohols having both electron withdrawing atoms and electron donating groups was carried out efficiently (3a–3e). ortho and meta-substituted benzyl alcohols also delivered the expected products in good to moderate yields (3f–3h). With naphthalen-1-ylmethanol and cyclohexylmethanol, longer heating was required for the complete conversion of 1-phenylethanol (3i–3j). Furthermore, following this protocol, β-alkylation of 1-phenylethanol using challenging long chain alcohols also proceeded smoothly (3k–3l). Moreover, for the β-alkylation of 1-phenylethanol with benzyl alcohol, the present system (4.4 × 10−4 mol% cat. 1a) displayed a high TON of 98[thin space (1/6-em)]860 after 30 hours (conversion = 43.5%) which revealed the excellent catalytic activity of complex 1a.

Next, the α-alkylation of arylacetonitriles with primary alcohols was investigated to expand the potential of the complex 1a. The synthesis of alkylated nitrile from alcohol is an attractive greener protocol from both economical and environmental points of view over the conventional processes.59,83–88 Initially, the coupling of phenylacetonitrile with benzyl alcohol was carried out in the presence of 0.5 equiv. KOtBu and 1a (0.5 mol%) in refluxing dioxane under an argon atmosphere for 1 hour which afforded 44% yield of the desired 2,3-diphenylpropanenitrile (Table 3, entry 4). By switching to KOH as the base, the yield of 2,3-diphenylpropanenitrile was slightly improved as compared to other bases (Table 3, entries 1–6). After 2 hours, it delivered 99% yield for the desired product. The yield of 2,3-diphenylpropanenitrile in toluene was poor and it also dropped when the amount of benzyl alcohol was decreased (Table 3, entry 9).

Table 3 α-Alkylation of phenylacetonitrile in the presence of different basesa

image file: c7qo01061c-u4.tif

Entry Base Yieldb (%)
a Reaction conditions: Cat. 1a (0.5 mol%), phenylacetonitrile (0.5 mmol), benzyl alcohol (2.5 mmol) and base (0.25 mmol) in 2 mL dioxane at 115 °C for 1 h; closed argon conditions in the Schlenk tube.b Determined by GC using mesitylene as an internal standard.c Toluene as the solvent.d 2 h heating.e Benzyl alcohol (1.5 mmol).
1 Na2CO3 0
2 K2CO3 0
3 K3PO4 0
4 KOtBu 44
5 NaOH 7
6 KOH 48
7c KOH 31
8d KOH >99
9d,e KOH 88


To evaluate the scope of the present catalytic system, α-alkylation of various arylacetonitriles was carried out under the optimized conditions which are listed in Scheme 4. The reaction of benzyl alcohol with both the electron-donating and electron-withdrawing substituted phenylacetonitriles worked smoothly with excellent yields (4b–4d). 3-Pyridylacetonitrile provided the corresponding desired product after 3 hours (4e). Different substituted (o, m and p) benzyl alcohols afforded the desired products in excellent yields (4f–4h). The reaction of naphthalen-1-ylmethanol was completed within 2 hours (4i). Aliphatic alcohol like n-butanol was converted to 2-phenylhexanenitrile after 5 hours of heating (4j). Notably, following this protocol 3-phenylquinolin-2-amine was efficiently synthesized by coupling 2-aminobenzyl alcohol with phenylacetonitrile (4k).89


image file: c7qo01061c-s4.tif
Scheme 4 Substrate scope of α-alkylation of arylacetonitriles with primary alcohols; Reaction conditions: Cat. 1a (0.5 mol%), arylacetonitrile (0.5 mmol), alcohol (2.5 mmol) and KOH (0.25 mmol) refluxed in 2 mL dioxane at 115 °C in necessary time; closed argon conditions in a Schlenk tube, isolated yield. a[thin space (1/6-em)]1 mol% cat. 1a. b[thin space (1/6-em)]Heated at 130 °C. c[thin space (1/6-em)]1 equiv. KOH.

To further expand the scope of the alcohols in this reaction, methylation of nitrile using methanol as a green methylating agent was investigated which is challenging compared to the long chain alcohols due to the higher activation energy barrier.90–92 Under the optimized conditions complex 1a smoothly transformed phenylacetonitrile to 2-phenylpropanenitrile in 88% yield after 16 hours (5a). Notably, for the α-methylation of nitriles compared to the previously reported systems, this protocol was found to be more efficient.59,93,94 Afterward, different arylacetonitriles were effectively methylated in excellent yields using methanol (5b–5d). α-Methylation of 3-pyridylacetonitrile also proceeded smoothly to afford the desired product in 76% yield (5e) (Scheme 5).


image file: c7qo01061c-s5.tif
Scheme 5 Substrate scope for the α-methylation of arylacetonitriles using methanol; Reaction conditions: Cat. 1a (1 mol%), arylacetonitrile (0.6 mmol) and NaOMe (0.6 mmol) in MeOH (1.0 mL) at 135 °C for 16 h; closed argon conditions in the pressure tube, isolated yield.

The preparation of monoalkylated acetonitrile following the conventional processes is hazardous as it mostly required highly poisonous cyanide reagents such as KCN or NaCN together with toxic alkyl halide.57,58 Hence, to synthesize them following an atom economical and eco-friendly process using alcohols and acetonitrile is undoubtedly attractive.95–97 Motivated by this, we investigated the catalytic activity of complex 1a in the monoalkylation of acetonitrile using benzyl alcohol as the model substrate. Initially, the reaction was performed in the presence of complex 1a (2.5 mol%), benzyl alcohol (1 equiv.), acetonitrile (125 equiv.) and KOtBu (1.0 equiv.) in toluene at 120 °C for 24 hours which afforded only 36% of 3-phenylpropanenitrile (Table 4, entry 1). To improve the yield of the desired product, the catalyst loading was increased to 5 mol% and the yield of 3-phenylpropanenitrile was increased up to 59% (Table 4, entry 2). The yield of the expected product was enhanced by changing the base from KOtBu to NaOH, (Table 4, entries 3 and 4) and further incrementing the amount of the base (2 equiv.) led to 94% yield of 3-phenylpropanenitrile (Table 4, entry 5).

Table 4 Monoalkylation of acetonitrile by benzyl alcohol under different conditionsa

image file: c7qo01061c-u5.tif

Entry Cat. 1a (mol%) Base (equiv.) Yieldb (%)
a Reaction conditions: Cat. 1a (x mol%), benzyl alcohol (0.5 mmol), acetonitrile (62.5 mmol) and base in 1 mL toluene at 120 °C for 24 h; closed argon conditions in the Schlenk tube.b Determined by GC using mesitylene as an internal standard.c Dioxane as the solvent.
1 2.5 KOtBu (1) 36
2 5.0 KOtBu (1) 59
3 5.0 KOH (1) 49
4 5.0 NaOH (1) 63
5 5.0 NaOH (2) 94
6c 5.0 NaOH (2) 54


With the established reaction conditions, monoalkylation of acetonitrile with different alcohols was conducted and the results are summarized in Scheme 6. The reaction with para-substituted benzyl alcohol (–Me, –OMe and –F) proceeded smoothly with good to excellent yields (6b–6d). ortho-Methoxybenzyl alcohol was also converted to the desired product with moderate yield (6e). Piperonyl alcohol and 1-naphthalenemethanol were transformed to the corresponding nitrile compounds with 82% and 78% yields, respectively (6f–6g). Notably, this catalytic system was also able to convert aliphatic and long chain alcohols to the corresponding nitriles smoothly (6h–6i).


image file: c7qo01061c-s6.tif
Scheme 6 Monoalkylation of acetonitrile using different alcohols; Reaction conditions: Cat. 1a (5 mol%), alcohol (0.5 mmol), NaOH (1.0 mmol) and CH3CN (62.5 mmol) heated in 1 mL toluene at 120 °C for 24 h; closed argon conditions in the Schlenk tube, isolated yield.

With the success of monoalkylation of acetonitrile and α-methylation of nitriles we subsequently focused on complex 1a mediated N-methylation of primary amines using methanol as a methylating agent.98–102 The reaction was carried with 1a (1 mol%) and NaOMe (1 equiv.) in MeOH at 110 °C for 12 hours which afforded 86% yield of N-methylaniline (see ESI, Table S9). To our delight, the yield of N-methylaniline reached 99% within 15 hours of heating (see ESI, Table S9). To establish the generality of this catalytic system, N-methylation of different amines was performed which is listed in Scheme 7. Aromatic amines with both electron donating and electron withdrawing groups were efficiently transformed to the corresponding N-methylated anilines in excellent yields (7b–7e). Under the optimized conditions, 1-naphthalene amine produced the N-methylnaphthalen-1-amine in good yield (7f). Heteroatom containing pyridin-3-amine was also transformed smoothly (7g). Notably, long chain hexyl amine produced the corresponding N,N-dimethyl amine selectively probably due to greater nucleophilicity compared to the aromatic amines (7h).101


image file: c7qo01061c-s7.tif
Scheme 7 N-Methylation of amines using methanol; Reaction conditions: Cat. 1a (1 mol%), amine (0.8 mmol), NaOMe (0.8 mmol) in methanol (1.0 mL) refluxed at 110 °C for 15 h; closed argon conditions in the pressure tube, isolated yield. a[thin space (1/6-em)]24 h.

To establish the practical applicability and efficiency of the complex 1a, preparative scale synthesis of several compounds was carried out following the optimized reaction conditions which furnished the respective products in good yields (Scheme 8).


image file: c7qo01061c-s8.tif
Scheme 8 Preparative scale synthesis of different compounds using cat. 1a.

The general schematic diagram for the complex 1a catalysed C–C and C–N bond formation by alcohol activation is shown in Scheme 9. Initially, in the presence of a base and alcohol, an alkoxoruthenium complex would form which via β-hydride elimination would produce the corresponding aldehyde. The resulting aldehyde next engaged in base promoted condensation with ketone (which was formed by a similar way from secondary alcohol), or with arylacetonitrile, acetonitrile and amine to form the unsaturated (C[double bond, length as m-dash]C) ketone, nitrile and imine intermediates respectively which was subsequently hydrogenated by the ruthenium hydride to produce the desired products.


image file: c7qo01061c-s9.tif
Scheme 9 General schematic diagram for the cat. 1a catalysed C–C and C–N bond formation.

To check which unsaturated bond in the α,β-unsaturated ketone (C[double bond, length as m-dash]C or C[double bond, length as m-dash]O) would be hydrogenated first, the reactions of 1-phenylethanol with benzaldehyde and acetophenone with benzyl alcohol were carried out under the standard catalytic conditions for 30 min (see ESI). In both cases, the selective C[double bond, length as m-dash]C bond reduction product of chalcone was observed as major and no trace amount of only C[double bond, length as m-dash]O bond reduction product was detected which indicated that the hydrogenation of the C[double bond, length as m-dash]C bond of the α,β-unsaturated ketone was much faster than the hydrogenation of the C[double bond, length as m-dash]O bond.70,75 In the time-dependent product distribution of β-alkylation of 1-phenylethanol with benzyl alcohol, it was noticed that the selectivity for the 1,3-diphenylpropan-1-ol gradually increased with time and reached the maximum at the end of the reaction (see ESI).

To investigate whether the methyl group of α-methylated arylacetonitrile and N-methylated amine were coming from methanol, two experiments were carried using methanol-d4 (Scheme 10). These studies revealed the clean formation of deuterated 2-phenylpropanenitrile and N-methylaniline which confirmed that complex 1a mediated the transformation of methanol to a methylating agent.


image file: c7qo01061c-s10.tif
Scheme 10 Mechanistic studies with methanol-d4.

Conclusions

In summary, new amine groups containing electron-rich functionalized 2,2′-bipyridine ligands and their corresponding Ru(II) complexes were synthesized and fully characterized. The lowest νco value of complex 1a compared to others indicated a higher electron density over the ruthenium centre in 1a which indicated that the 6DNHMeBP ligand was more electron rich among all the ligands discussed in this report. Among these Ru(II) compounds, complex 1a was found to be highly efficient in β-alkylation of secondary alcohols with primary alcohols (TON = 98[thin space (1/6-em)]860) and following this greener protocol, a variety of β-alkylated secondary alcohols were synthesized. Complex 1a also displayed superior reactivity in α-alkylation of arylacetonitriles and large sets of arylacetonitriles were α-alkylated. Particularly, for the α-methylation of nitriles compared to the previously reported systems, 1a acted more effectively and various arylacetonitriles were successfully methylated in excellent yields using methanol. Additionally, this catalyst was also effective in monoalkylation of acetonitrile. The potential of complex 1a was extended further in N-methylation of primary amines using methanol. Following this methodology, a variety of amines were efficiently converted to the corresponding N-methylated anilines in good to excellent yields. Finally, the electron-rich 6DNHMeBP ligand played a vital role in various sustainable C–C and C–N bond formation reactions using alcohols which can be extended to gram scale synthesis.

Experimental section

General procedures and materials

All reactions were carried out under inert atmosphere using standard Schlenk line techniques. Glassware was dried in a 100 °C oven overnight before use. Solvents were dried by distillation under argon conditions according to standard literature methods. RuCl3·nH2O (39% Ru) was purchased from Arora Matthey, India. All the chemicals were purchased from Sigma-Aldrich, Alfa Aesar, SDFCL and Spectrochem. 6,6′-Dihydroxy-2,2′-bipyridine, 6,6′-dimethoxy-2,2′-bipyridine, RuHCl(CO)(PPh3)3, RuCl2(PPh3)3, [(2,2′-bipyridine)Ru(H)(CO)(PPh3)2]Cl (3) and [(6,6′-dimethoxy-2,2′-bipyridine)Ru(H)(CO)(PPh3)2]Cl (5) were synthesized according to previously reported literature procedures.63,82,103–105 For complexes 3 and 5, the FT-IR (νCO) was found to be 1940 and 1938 cm−1. 1H, 13C, and 31P NMR spectra were recorded on a JEOL 400 and 500 MHz spectrometer. FT-IR spectra were recorded using a PerkinElmer FT-IR spectrometer. Elemental analysis was performed on a Thermoquest EA1110 CHN analyser. The crystallized compounds were powdered, washed several times with dry diethyl ether and dried under vacuum for at least 24 hours prior to elemental analyses. ESI-MS were recorded on a Waters Micromass Quattro Micro triple-quadrupole mass spectrometer. All the GC analyses were performed using a PerkinElmer Clarus 600 Gas Chromatograph and GC-MS spectra were recorded using an Agilent 7890 A Gas Chromatograph equipped with an Agilent 5890 triple-quadrupole mass system.

Synthesis of N6,N6′-dimethyl-2,2′-bipyridine-6,6′-diamine (6DNHMeBP)

6,6′-Dibromo-2,2′-bipyridine (250 mg, 0.796 mmol), copper(I) oxide (45.6 mg, 0.319 mmol) and 12 mL methylamine solution (40 wt% in H2O) were added into a 25 mL hydrothermal autoclave reactor with 10 mL ethylene glycol. Then, the autoclave was sealed and heated at 130 °C for 3 days. At the end of the reaction the autoclave was cooled to room temperature and the excess pressure was released. The resultant dark red solution was extracted with DCM after adding water into it. The organic part was then evaporated under reduced pressure and was purified through silica gel column chromatography using ethyl acetate and hexane as eluents to afford the desired yellow coloured compound. Yield: 85 mg (50%). 1H NMR (400 MHz, CDCl3): δ = 7.59 (d, J = 7.6 Hz, 2H), 7.53 (t, J = 8.1 Hz, 2H), 6.38 (d, J = 8.2 Hz, 2H), 4.61 (bs, 2H), 2.95 (d, J = 5.0 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ = 159.25, 159.09, 138.26, 110.32, 106.10, 29.35.

Synthesis of N6,N6′-diphenyl-2,2′-bipyridine-6,6′-diamine (6DNHPhBP)

A mixture of 6,6′-dibromo-2,2′-bipyridine (160 mg, 0.509 mmol) and aniline (9.3 mL) was taken in a 15 mL hydrothermal autoclave reactor under argon conditions and heated at 130 °C for 2 days. At the end of the reaction the autoclave was cooled to room temperature and the excess pressure was released. The mixture was purified by recrystallization from methanol, to give N6,N6′-diphenyl-2,2′-bipyridine-6,6′-diamine as an off-white solid. Yield: 91.6 mg (53%). 1H NMR (400 MHz, CDCl3 + DMSO-d6): δ = 8.09 (bs, 2H), 7.09–7.04 (m, 6H), 6.92 (t, J = 8.0 Hz, 2H), 6.58 (t, J = 8. Hz, 4H), 6.20 (t, J = 7.4 Hz, 2H), 6.15 (d, J = 8.3 Hz, 2H).13C NMR (100 MHz, CDCl3 + DMSO-d6): δ = 153.89, 153.81, 152.53, 140.36, 140.28, 136.41, 127.21, 119.10, 116.82, 116.72, 109.78, 109.62. ESI-MS: m/z 339.1613 ([M + H]+, predicted: 339.1610).

Synthesis of [(6DNHMeBP)Ru(H)(CO)(PPh3)2]Cl (1a)

A mixture of N6,N6′-dimethyl-2,2′-bipyridine-6,6′-diamine (6DNHMeBP) (55 mg, 0.256 mmol) and RuHCl(CO)(PPh3)3 (244 mg, 0.256 mmol) was stirred in dry DCM (10 mL) under an argon atmosphere at room temperature for 24 hours. Then, the solution was concentrated under reduced pressure and diethyl ether was added to precipitate the bright yellow product. The precipitate was further washed multiple times with diethyl ether and hexane to remove the free triphenylphosphine. Yield: 186 mg (80%); 1H NMR (400 MHz, CD3CN): δ = 7.47 (t, J = 8.3 Hz, 1H), 7.32–7.16 (m, 30H), 7.12 (t, J = 8.3 Hz, 1H), 7.01 (d, J = 7.8 Hz, 1H), 6.62 (d, J = 4.2 Hz, 1H), 6.27 (d, J = 8.8 Hz, 1H), 5.56 (d, J = 8.5 Hz, 1H), 2.38 (d, J = 5.1 Hz, 3H), 2.17 (d, J = 5.0 Hz, 3H), −12.17 (t, J = 19.2 Hz, 1H). 13C NMR (100 MHz, CD3CN): δ = 204.66 (t, J = 15.45), 159.32, 156.87, 155.00, 154.87, 138.80, 138.67, 133.00 (t, J = 5.8 Hz), 132.38, 132.16, 131.94, 130.22, 128.46 (t, J = 4.68 Hz), 117.71, 112.01, 111.87, 107.03, 106.68, 54.40, 29.32, 28.57. 31P{1H} NMR (202 MHz, CDCl3): δ = 42.75 ppm. ESI-MS: m/z 869.2133 ([M − Cl]+, predicted: 869.2112). Elemental analysis calcd (%) for C49H45ClN4OP2Ru: C 65.08, H 5.02, N 6.20; found: C 64.88, H 4.92, N 5.94. FT-IR (νCO): 1930 cm−1.

Synthesis of [(6DNHMeBP)RuCl2(PPh3)2 (1b)

A mixture of N6,N6′-dimethyl-2,2′-bipyridine-6,6′-diamine (6DNHMeBP) (100 mg, 0.466 mmol) and RuCl2(PPh3)3 (447 mg, 0.466 mmol) was stirred in dry DCM (8 mL) under an argon atmosphere at room temperature for 24 hours. Then the solvent was evaporated under reduced pressure and the orange product was washed many times with diethyl ether and hexane to remove the free triphenylphosphine. Yield: 320 mg (75%). Due to the poor solubility of this complex in most of the common solvent, NMR characterization was not possible to record. Elemental analysis calcd (%) for C48H44Cl2N4P2Ru: C 63.30, H 4.87, N 6.15; found: C 63.05, H 4.70, N 5.92.

Synthesis of [(6DNHMeBP)RuCl(p-cymene)]PF6 (1c)

A mixture of N6,N6′-dimethyl-2,2′-bipyridine-6,6′-diamine (6DNHMeBP) (50 mg, 0.233 mmol) and [RuCl2(p-cymene)]2 (71.5 mg, 0.116 mmol) was refluxed in dry methanol (6 mL) under an argon atmosphere at 70 °C for 24 hours. After cooling down the mixture, ammonium hexafluorophosphate (76 mg, 4.67 mmol) was added and the mixture was stirred at room temperature overnight. The solution was then evaporated under reduced pressure and DCM was added to the solid mixture. The DCM soluble part was taken after filtrating off the excess NH4PF6 and the solvent was evaporated and washed with diethyl ether and hexane which afforded a yellow product. Yield: 109 mg (74%). 1H NMR (400 MHz, CD3CN): δ = 7.77 (t, J = 8.0 Hz, 2H), 7.42 (d, J = 6.8 Hz, 2H), 6.77 (d, J = 8.7 Hz, 2H), 6.13 (d, J = 4.8 Hz, 2H), 5.72 (d, J = 6.2 Hz, 2H), 5.65 (d, J = 6.2 Hz, 2H), 3.01 (d, J = 8.7 Hz, 6H), 2.12 (s, 3H), 2.08 (sept, J = 2.6 Hz, 1H), 0.89 (d, J = 7.0 Hz, 6H). 13C NMR (100 MHz, CD3CN): δ = 162.22, 154.29, 140.12, 117.15, 112.02, 108.23, 104.81, 104.10, 85.26, 84.21, 30.35, 29.56, 29.43, 21.06, 17.59. ESI-MS: m/z 485.1049 ([M − PF6]+, predicted: 485.1046). Elemental analysis calcd (%) for C22H28ClF6N4PRu: C 41.94, H 4.48, N 8.89; found: C 41.78, H 4.32, N 8.70.

Synthesis of [(6NHPhBP)Ru(H)(CO)(PPh3)2]Cl (2)

A mixture of N6,N6′-diphenyl-2,2′-bipyridine-6,6′-diamine (6DNHPhBP) (25 mg, 0.0738 mmol) and RuHCl(CO)(PPh3)3 (70.4 mg, 0.0738 mmol) was stirred in dry DCM (8 mL) under an argon atmosphere at room temperature for 36 hours. Then the solution was evaporated under reduced pressure and the solid was washed with hexane and diethyl ether multiple times which gave the yellowish product. Yield: 50 mg (66%). 1H NMR (500 MHz, CDCl3): δ = 8.63 (s, 1H), 7.80 (d, J = 6.85 Hz, 1H), 7.71 (t, J = 8.4 Hz, 1H), 7.66 (d, J = 6.8 Hz, 2H), 7.44–7.09 (m, 35H), 6.88–6.85 (m, 2H), 6.70 (d, J = 7.2 Hz, 2H), 6.30 (d, J = 8.1 Hz, 2H), 6.17 (d, J = 8.7 Hz, 1H), −12.16 (t, J = 17.9 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ = 204.25 (t, J = 15.0 Hz), 155.98, 155.56, 155.49, 155.18, 154.85, 154.61, 141.23, 139.94, 138.35, 137.81, 137.27, 134.64, 132.95 (t, J = 5.58 Hz), 132.21, 132.11, 132.02, 131.69, 131.47, 130.59, 129.66 (d, J = 3.89), 129.09, 128.80 (t, J = 3.56 Hz), 128.64, 128.52, 125.79, 125.30, 123.00, 121.96, 119.74, 116.10, 115.48, 112.26, 109.84, 109.76, 108.26, 96.19. 31P{1H} NMR (202 MHz, CDCl3): δ = 41.80 ppm. ESI-MS: m/z 993.2422 ([M − Cl]+, predicted: 993.2425). Elemental analysis calcd (%) for C59H50ClN4P2Ru: C 68.83, H 4.90, N 5.44; found: C 68.58, H 4.72, N 5.20. FT-IR (νCO): 1939 cm−1.

Synthesis of [(6DMeBP)Ru(H)(CO)(PPh3)2]Cl (4)

A mixture of 6,6′-dimethyl-2,2′-bipyridine (6DMeBP) (25 mg, 0.135 mmol) and RuHCl(CO)(PPh3)3 (129.2 mg, 0.135 mmol) was heated at 70 °C in a 10 mL mixture of DCM and EtOH (2[thin space (1/6-em)]:[thin space (1/6-em)]1) under an argon atmosphere for 24 hours. After that the whole solution was evaporated under reduced pressure and the solid was washed with hexane and diethyl ether multiple times which gave a yellow solid product. Yield: 71 mg (60%). 1H NMR (500 MHz, CDCl3): δ = 8.87 (d, J = 7.80 Hz, 1H), 8.67 (d, J = 7.90 Hz, 1H), 8.04 (t, J = 7.2 Hz, 1H), 7.65 (t, J = 4.8 Hz, 1H), 7.30–7.11 (m, 30H), 7.06 (d, J = 7.0 Hz, 1H), 6.42 (d, J = 6.8 Hz, 1H), 2.00 (s, 3H), 1.76 (s, 3H), −11.69 (t, J = 17.9 Hz, 1H). 13C NMR (100 MHz, CD3CN): δ = 204.94 (t, J = 15.43 Hz), 162.44, 160.03, 157.19, 155.97, 139.34, 139.25, 133.04 (t, J = 5.5 Hz), 132.55, 132.33, 132.12, 130.27, 128.55 (t, J = 4.28 Hz), 127.45, 126.76, 123.52, 123.28, 31.52, 27.64. 31P{1H} NMR (202 MHz, CDCl3): δ = 43.61 ppm. ESI-MS: m/z 839.1890 ([M − Cl]+, predicted: 839.1894). Elemental analysis calcd (%) for C49H43ClN2OP2Ru: C 67.31, H 4.96, N 3.20; found: C 67.08, H 4.70, N 2.99. FT-IR (νCO): 1935 cm−1.

General procedure for β-alkylation of secondary alcohols with primary alcohols

The catalytic β-alkylation of secondary alcohols was carried out in a Schlenk tube under closed argon conditions. Initially, catalyst 1a (0.1 mol%) was taken in a Schlenk tube from a stock solution of catalyst 1a in acetonitrile and the acetonitrile was evaporated under reduced pressure. Then, KOtBu (0.55 mmol), secondary alcohol (1.1 mmol), primary alcohol (1.1 mmol) and toluene (2 mL) were added under argon conditions and the resulting mixture was heated at 130 °C (oil bath temperature) for 90 min. After immediate cooling, 10 μL solution was syringed out for GC analysis. The remaining portion of the reaction mixture was concentrated under reduced pressure and the crude residue was subjected for 1H NMR. The desired product was isolated through silica gel column chromatography using hexane/ethyl acetate as the eluent. Preparative scale synthesis was carried out with 1 g of secondary alcohol using 8 mL toluene in a 100 mL Schlenk flask following the standard procedure.

General procedure for α-alkylation of arylacetonitriles with primary alcohols

The catalytic α-alkylation of arylacetonitrile was carried out in a Schlenk tube under closed argon conditions. Initially, catalyst 1a (0.5 mol%) was taken in a Schlenk tube from the stock solution of catalyst 1a in acetonitrile and the acetonitrile was evaporated under reduced pressure. Then, KOH (0.25 mmol), arylacetonitrile (0.5 mmol), alcohol (2.5 mmol) and dioxane (2 mL) were added under argon conditions and the reaction mixture was heated at 115 °C (oil bath temperature) for 2 hours. After it was cooled to room temperature, the crude reaction mixture was analysed by GC. Furthermore the solvent was evaporated under reduced pressure and the resulting mixture was purified through silica gel column chromatography using ethyl acetate and hexane as eluents to afford the desired product. Preparative scale synthesis was carried out with 1 g of arylacetonitrile using 18 mL dioxane in a 100 mL Schlenk flask following the standard procedure.

General procedure for α-methylation of arylacetonitriles

Catalyst 1a (1 mol%), NaOMe (0.6 mmol), arylacetonitrile (0.6 mmol) and methanol (1.0 mL) were added into a pressure tube under argon conditions and the mixture was heated at 135 °C for 16 hours. After that it was cooled to room temperature and the crude reaction mixture was analysed by GC. Further, the solvent was evaporated under reduced pressure and the resulting mixture was purified through silica gel column chromatography using ethyl acetate and hexane as eluents to afford the desire product. Preparative scale synthesis was carried out with 1 g arylacetonitrile using 14 mL methanol in the pressure tube following the standard procedure.

General procedure for monoalkylation of acetonitrile with alcohols

The monoalkylation of acetonitrile with alcohols was carried out in a Schlenk tube under closed argon conditions. Initially, catalyst 1a (5.0 mol%) and NaOH (1.0 mmol) were taken as solids and then under argon conditions alcohol (0.5 mmol), acetonitrile (62.5 mmol) and toluene (1 mL) were added and the resulting mixture was heated at 120 °C (oil bath temperature) for 24 hours. After it was cooled to room temperature, the crude reaction mixture was analysed by GC. Further, the solvent was evaporated under reduced pressure and the resulting mixture was purified through a silica gel column using ethyl acetate and hexane as eluents to afford the desired product. Preparative scale synthesis was carried out with 1 g of alcohol using 8 mL toluene in a pressure tube following the standard procedure.

General procedure for N-methylation of amines

Catalyst 1a (1 mol%), NaOMe (0.8 mmol), amine (0.8 mmol) and methanol (1.0 mL) was added into a pressure tube under argon conditions and the mixture was heated at 110 °C for 15 hours. After that it was cooled to room temperature and the crude reaction mixture was analysed by GC. Afterward, the solvent was evaporated under reduced pressure and the resulting mixture was purified through a silica gel column using ethyl acetate and hexane as eluents to afford the desired product. Preparative scale synthesis was carried out with 1 g of amine using 10 mL methanol in the pressure tube following the standard procedure.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to Science and Engineering Research Board (SERB), India, Council of Scientific and Industrial Research (CSIR), India and IIT Kanpur for financial support. B. C. R. and M. M. thank CSIR; K. C. and B. P. thank UGC, India, for their fellowships. The authors also sincerely thank Dr Sourav Biswas for his help in solving the crystal structures.

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Footnote

Electronic supplementary information (ESI) available. CCDC 1568230–1568232. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qo01061c

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