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Reduction of imines catalysed by NHC substituted group 6 metal carbonyls
Inorganica Chimica Acta 486 (2019) 119–128
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Reduction of imines catalysed by NHC substituted group 6 metal carbonyls Noor U Din Reshi, Lakshay Kathuria, Ashoka G. Samuelson
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T
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
ARTICLE INFO
ABSTRACT
Keywords: Reduction Hydrosilylation Silyl hydrides Group 6 NHC complexes Imines
The catalytic activity of a series of metal carbonyls [M(CO)6], and the corresponding NHC substituted [M (CO)5(NHC)], (M = Cr, Mo, W) complexes was examined in the reduction of N-benzylideneaniline and acetophenone using silyl hydrides and isopropanol/KOH as reductants. The use of various additives and ultraviolet irradiation to promote the reduction of imines using silyl hydrides as reductants was explored. From a comparison of the reactivity of [Mo(CO)6], [Mo(CO)5(NHC)], and [Mo(CO)4(bis NHC)] it was inferred that electron density on the metal centre plays a key role in the catalysis. Four of the best catalysts were then tested in the reduction of a variety of imines with different electronic and steric properties.
1. Introduction Group 6 carbonyl complexes play an important role as catalysts or precatalysts in many organic transformations [1–8]. During the last two decades, a wide range of these carbonyl complexes with various ancillary ligands have been examined as precursors to M(VI) catalysts for the epoxidation of olefins [9–13], the cis-dihydroxylation of olefins [14], and the oxidation of amines [15], alcohols [16,17], and sulfides [18,19]. Surprisingly, there are not too many examples of the use of group 6 metal complexes in general and group 6 metal carbonyl complexes in particular for catalysing reduction reactions such as transfer hydrogenation [20–22], hydrogenation [23–26], and hydrosilylation [27,28]. This is in-spite of the fact that group 6 metal-based complexes are attractive hydride transfer agents in view of their low cost and high coordination number enabling great flexibility in the design of the ligand sphere. Moreover, most of these complexes are readily available from the corresponding hexacarbonyl complexes [29]. Among the few reduction reactions catalysed by such metal carbonyls, fewer still utilize hydrosilanes as reducing agents. The conjugate reduction of various Michael acceptors, including α,β-unsaturated ketones, carboxylic acids, carboxylic esters, amides, and nitriles with phenylsilane is catalysed rather sluggishly by M(CO)6 (M = Cr, Mo, W) in refluxing tetrahydrofuran [30]. Hydrosilylation of 1,3-dienes catalysed by M(CO)6 is also known [31,32]. Tinnis and Adolfsson et al. reported Mo(CO)6 (5 mol%) catalysed chemoselective hydrosilylation of α, β-unsaturated amides leading to allylamines (yield 51–95%) using 1,1,3,3-tetramethyldisiloxane (TMDS) (1.5 equivalents) for 24 h at 65 °C [33]. [Mo(CO)2(oxadiene)2] (oxadiene = pulegone, pinocarvone and (E)-5-methyl-3-hexen-2-one), showed better catalytic activity in the
⁎
hydrosilylation of unsaturated ketones and aldehydes with phenylsilane. All aldehydes underwent 1, 2-addition exclusively whereas the ketones gave both 1,2-addition and 1,4-addition products [34]. Anionic µ-hydride complexes of group 6 metals of the type [(CO)5M(µ-H)M (CO)5][NEt4]+ were reported to catalyse the hydrosilylation of aldehydes and ketones under rather harsh conditions using HSiEt3, while silyl enol ethers were obtained using H2SiPh2 [35]. A tungsten-based catalyst, [CpW(CO)2(IMes)]+[B(C6F5)4]- (where Cp = cyclopentadienyl, IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, has been reported to catalyse the solvent-free hydrosilylation of ketones and an ester under mild conditions [36]. The catalytic activity of isocyanide complexes of group 6 metals in the hydrosilylation of olefins is also known [37]. Surprisingly group 6 carbonyl complexes have not been tested for the hydrosilylation of imines to give the corresponding amines. Although the alkylation of ammonia using halo alkanes or alcohols is a simple way to prepare amines, the degree of alkylation is difficult to control with alkyl iodides and bromides [38]. The synthesis of amines by reduction of imines, amides, nitriles, and nitroarenes using stoichiometric amount of alkali hydrides, such as lithium aluminium hydride and boron hydrides is limited by the air and moisture sensitivity of these reductants, lack of tolerance to various functional groups and formation of metal salts as by-products [39]. The requirement of high temperatures and high pressures, and lack of chemoselectivity makes hydrogenation of imines using dihydrogen gas unsuitable for some substrates [40–43]. Reductive amination of carbonyl derivatives in the presence of primary amines, through sequential condensation/catalytic reduction often requires drastic reaction conditions [44–50]. Due to these reasons the hydrosilylation of imines is an attractive alternative
Corresponding author. E-mail address: [email protected] (A.G. Samuelson).
https://doi.org/10.1016/j.ica.2018.10.026 Received 5 July 2018; Received in revised form 17 October 2018; Accepted 17 October 2018 0020-1693/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. NHC complexes studied in this work.
for the synthesis of corresponding amines. In this paper we wish to report our studies on the use of NHC substituted group 6 metal carbonyl complexes of the type [M(CO)5(NHC)], (M = Cr, Mo, W) for catalysing the reduction reactions of imines using silyl hydrides and isopropanol/ KOH as the hydride source. We also compared their catalytic activity with the activity of [Mo(CO)6] and [Mo(CO)4(bis-NHC)] complexes.
NMR, IR-spectroscopy, and elemental analysis. 2.2. Catalytic studies The preliminary catalytic tests were carried out using complex 1 as a catalyst and N-benzylideneaniline and acetophenone as test substrates and phenylsilane and isopropanol/KOH as reductants. There was no catalytic reduction of N-benzylideneaniline in isopropanol in presence of KOH. Similarly, acetophenone was not reduced under these conditions. The reduction of N-benzylideneaniline using PhSiH3 in THF and toluene were also unsuccessful (Entries 1 and 2, Table 1). In dichloromethane and chloroform, a modest conversion of nearly 30% of N-benzylideneaniline to the corresponding amine (N-benzylaniline) was observed using 5 mol% of complex 1 at room temperature (Entries 3 and 4, Table 1). At room temperature, the best yield was obtained in ethanol where 45% of N-benzylideneaniline converted to the corresponding amine in 48 h using 5 mol% of complex 1 (Entry 5, Table 1). The rate of the reaction increased significantly with higher catalyst
2. Results and discussion 2.1. Synthesis and characterization Ag-NHC complexes were synthesized from the corresponding benzimidazolium salts using a reported procedure [51]. The complexes 1–11 (Fig. 1) were synthesized by transferring the carbene from the corresponding Ag-NHC complexes to [M(CO)5THF] (Scheme 1) [10,5254]. Complex 12, on the other hand, was prepared by a reported procedure using di(1-ethylimidazolium)methane diiodide and [Mo (CO)4(pip)2] [55]. The complexes were characterized by 1H NMR, 13C 120
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Scheme 1. Synthesis of complexes 1–10.
methoxybenzylidene)aniline which was reduced to 4-Acetyl-N-(4methoxybenzyl)aniline and no reduction of the keto group to give 1-(4((4-methoxybenzylidene)amino)phenyl)ethan-1-ol was observed. However, using a mixture of N-benzylidineaniline and benzaldehyde as substrate, nearly 50% of benzaldehyde was reduced to benzyl alcohol (Entry 2, Table 2).
Table 1 Optimization of reaction conditions for reduction using PhSiH3 as reductant.
Entrya
Solvent
Temperature
Time (h)
Conversion (%)
1 2 3 4 5 6b 7 8 9c 10d
Toluene Tetrahydrofuran Dichloromethane Chloroform EtOH EtOH EtOH EtOH EtOH EtOH
r.t r.t r.t r.t r.t r.t 50 (°C) reflux reflux reflux
48 48 48 48 48 48 48 24 24 24
0 0 30 31 45 90 90 92 80 87
2.3. Effect of peroxides, Me3NO, and ultraviolet irradiation The catalytic activity of M(CO)6 (M = Group 6 metal) in the hydrosilylation of 1,3-dienes is enhanced in the presence of organic oxidants such as peroxides [32]. This enhancement is presumably due to the ability of peroxides to oxidize coordinated carbon monoxide to weakly coordinating carbon dioxide [57] resulting in a vacant site. Taking a cue from this observation, the reduction of N-benzylideneaniline using complex 1 as the catalyst was carried out in the presence of 1 equivalent of tert-butyl hydroperoxide (with respect to complex 1). Surprisingly, no rate enhancement was observed (Entry 2, Table 3). Me3NO is also known to facilitate the loss of CO ligand [57]. Hence the reaction was carried out in presence of 1 equivalent of Me3NO (with respect to complex 1). In this case also the rate of the reaction did not improve (Entry 3, Table 3). Group 6 hexacarbonyls have been reported to be active photocatalysts for the reduction of 1,3-dienes [58]. Their catalytic activity upon irradiation has been attributed to the formation of the species [Mo (CO)5] (M = Cr, Mo, W). Hence the reduction of N-benzylideneaniline using complex 1 as a catalyst was carried out in refluxing ethanol while being irradiated by ultraviolet radiation (400 W, high-pressure Hg vapor lamp, λ max = 365 nm). However, contrary to expectations, no rate enhancement was observed (Entry 4, Table 3). Since no rate enhancement is observed in the presence of Me3NO, tert-butyl hydroperoxide, and ultraviolet irradiation, the dissociation of CO to form a coordinatively unsaturated species is probably not the rate-determining step.
a Substrate (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and complex 1 (5 mol%), solvent (5 mL). In case of solvents other than ethanol a desilylation step (treating the reaction mixture with methanol and 2 M aqueous NaOH) was also carried out before 1H NMR was recorded [56]; conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy. b 10 mol% of complex 1 used. c HPLC grade ethanol used (not dried). d Reaction carried out in air.
loading (Entry 6, Table 1). Since only modest conversion was observed at room temperature, the reaction was carried out at higher temperatures with 5% catalyst loading. When the reaction temperature was increased to 50 °C, 90% conversion of N-benzylideneaniline to the corresponding amine was observed in 48 h (Entry 7, Table 1). Whereas, 92% conversion of Nbenzylideneaniline to the corresponding amine was observed in 24 h when the reaction was carried out in refluxing ethanol (Entry 8, Table 1). The conversion of N-benzylideneaniline to the corresponding amine decreased by nearly 12% when HPLC grade ethanol was used as a solvent as compared to the case when ethanol was dried and distilled before use as a solvent for the reaction (Entry 9, Table 1). The conversion also decreased when the reaction was not carried under an atmosphere of dinitrogen (Entry 10, Table 1). Based on the results of the optimization studies, the reduction of imines was carried out in refluxing ethanol which was dried and freshly distilled, under an atmosphere of dry nitrogen and a catalyst loading of 5% with respect to the substrate. Interestingly, under the optimized conditions for the reduction of imines there was no significant reduction of acetophenone. The reduction of a mixture of N-benzylidineaniline (100 mol%) and acetophenone (100 mol%) was attempted using PhSiH3 (400 mol%) and catalysts 1, 11-W and 12-Mo (10 mol%) in refluxing ethanol. In all the three cases N-benzylidineaniline was selectively reduced whereas no significant reduction of acetophenone was observed (Entry 1, Table 2). Similar selectivity was observed in the case of 4-Acetyl-N-(4-
2.4. Varying the nature and amount of silane The effect of varying the nature and amount of silane was studied in the reduction of a test substrate N-benzylideneaniline using a representative complex (1) as the catalyst. Since each molecule of PhSiH3 has three reducing hydrides, 34 mol% of PhSiH3 should be sufficient to reduce the imine completely. When the amount of phenylsilane was decreased accordingly, conversion of N-benzylideneaniline to the corresponding amine decreased drastically to 14% (Entry 2, Table 4). This shows that not all H atoms on phenylsilane are equally available for the reduction of the imine. When a secondary silane (diphenylsilane) was used instead of phenylsilane, little effect on the conversion and the rate of reaction was observed (Entry 3, Table 4). When only 50 mol% of diphenylsilane was used, 13% conversion of N-benzylideneaniline to the corresponding amine was observed (Entry 4, Table 4). The use of a tertiary silane (triphenylsilane) resulted in a further decrease in the yield compared to diphenylsilane and phenylsilane. Only 5% of N-benzylideneaniline 121
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Table 2 Reduction of a mixture of N-benzylideneaniline and acetophenone/benzaldehyde. Entry
Substrate
Conversion Catalyst 1
1 2 3
N-benzylideneaniline + acetophenone a,c N-benzylideneaniline + benzaldehyde a,d 4-Acetyl-N-(4-methoxybenzylidene)aniline
b,e
Catalyst 11-W
Catalyst 12
% of Amine
% of Alcohol
% of Amine
% of Alcohol
% of Amine
% of Alcohol
92 91 83
4 49 0
88 87 78
4 45 0
99 100 94
7 52 0
a N-benzylideneaniline (0.25 mmol (100 mol%), acetophenone/benzaldehyde (0.25 mmol (100 mol%), PhSiH3 (400 mol%), and catalyst (10 mol%), solvent (5 mL). b 4-Acetyl-N-(4-methoxybenzylidene)aniline (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and catalyst (5 mol%), solvent (5 mL). c Conversion of N-benzylidene to N-benzylaniline, and acetophenone to 1-phenylethanol by 1H NMR spectroscopy. d Conversion of N-benzylidene to N-benzylaniline, and benzaldehyde to benzyl alcohol by 1H NMR spectroscopy. e Conversion of 4-Acetyl-N-(4-methoxybenzylidene)aniline to 4-Acetyl-N-(4-methoxybenzyl)aniline and 1-(4-((4-methoxybenzylidene)amino)phenyl)ethan-1-ol by 1H NMR spectroscopy.
Table 3 Effect of addition of Me3NO, tert-Butyl hydroperoxide, and ultraviolet irradiation.
Entrya
Remarks
Conversion (%)b
1 2 3 4
Standard conditions tert-butyl hydroperoxide added (1 eq.) Me3NO added (1 eq.) Ultraviolet irradiation
12 h 76 72 74 74
24 h 92 85 88 86
a N-benzylideneaniline (0.25 mmol (100 mol%)), PhSiH3 (200 mol%), and complex 1 (5 mol%), ethanol (5 mL), reflux. b Conversion by 1H NMR spectroscopy.
Table 4 Effect of varying the nature and amount of silane.
Entrya
Silane
Mol% of silane
Conversionb
1 2 3 4 5
PhSiH3 PhSiH3 Ph2SiH2 Ph2SiH2 Ph3SiH
200 34 200 50 100
92 14 88 13 5
a
N-benzylideneaniline (0.25 mmol (100 mol%), silane, and complex 1 (5 mol%), ethanol (5 mL), 24 h, reflux. b Conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy.
converted to the corresponding amine (Entry 5, Table 4) under conditions where almost complete conversion was achieved with phenylsilane.
2.5. Reduction of N-benzylideneaniline using the catalysts The activity of all the complexes (shown in Fig. 1) was compared in the reduction of a test substrate (N-benzylideneaniline) under 122
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Table 5 Reduction of N-benzylidineaniline using complexes group 6 complexes studied.
Entrya
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Conversion (%)b
Catalyst
1 2 3 4 5 6 7 8 9 10 11-W 11-Mo 11-Cr 12-Mo W(CO)6 M(CO)6 Cr(CO)6
12 h
24 h
76 67 72 75 70 71 73 68 72 73 72 73 71 90 – – –
92 94 95 94 92 94 93 92 95 93 87 86 83 98 25 28 26
a
N-benzylideneaniline (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and catalyst 1 (5 mol%), ethanol (5 mL), reflux. b Conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy.
optimized conditions. The data is given in Table 5. The change of the substituents on the aromatic ring (attached to N) did not have a significant effect on the rate of the reaction (Entries 1–10, Table 5). This is not surprising as the aromatic ring is separated by a CH2 bridge from the N of NHC ring. The comparison of the catalytic activity of 11-W, 11Mo, and 11-Cr which are prepared from same imidazolium salt but contain different metal atoms (W, Mo, and Cr respectively) indicated that the nature of Group 6 metal does not affect the rate of the reaction (Entries 11, 12, and 13, Table 5). Interestingly, the biscarbene complex 12-Mo exhibited higher activity for the reduction of N-benzylideneaniline. Thus, there was 90% conversion of N-benzylideneaniline to the corresponding amine in 12 h using 5 mol% of complex 12-Mo in refluxing ethanol. Under similar
Entry
Metal complex
ν (CO) (cm−1)
Refs.
conditions, complex 11-Mo gave 73% conversion of N-benzylideneaniline to the corresponding amine in 12 h (Entry 14, Table 5). The activity of hexacarbonyl complexes was less than that of NHC complexes (Entries 15, 16, and 17, Table 5). So, the overall trend in the catalytic activity was M(CO)6 < [M (CO)5(NHC)] < [M(CO)4(NHC)2]. The decrease in the number of strongly π accepting CO ligands and increase in the number of strongly sigma-donating NHC ligand on going from M(CO)6 to [M(CO)5(NHC)] to [M(CO)4(NHC)2] results in an increase in the electron density on the metal centre in the same order. This change in the electron density on the metal centre is reflected in the change in the vibrational frequencies of coordinated CO in these complexes as shown in (Table 6). The increase in the electron density is likely to make the oxidative addition of silane to the metal centre more facile, which in turn increases the rate of the reaction. Since the substitution with electron donating or electron withdrawing groups on the aromatic ring does not significantly affect the effective electron density on the metal, the catalytic activity of complexes 1–10 is similar.
Hexacarbonyls
Cr(CO)6
2118.7 (A1g) 2026.7 (Eg) 2000.4 (F1u) 2120.7 (A1g) 2024.8 (Eg) 2000.3 (F1u) 2126.2 (A1g) 2021.1 (Eg) 1997.6 (F1u) 2056, 1925 2064, 1930 2062, 1924 2058, 1961, 1884 1987, 1876, 1818, 1799
[59]
2.6. Substrate scope
[54]
The four representative complexes 1, 11-W, 11-Mo and 12-Mo were tested for the reduction of substrates with varying electronic and steric factors. Complex 1 has a carbene ligand based on a bisimidazolium system whereas complexes 11-W and 11-Mo have carbene ligands based on an imidazolium ring and complex 12-Mo contains a biscarbene ligand. The data is given in Table 7. This reaction tolerated functional groups like -Cl, –CN, –OH, –NO2, –NH2, -R (alkyl). In the presence of electron withdrawing groups (nitrile and nitro), the reduction was more difficult to perform. Using 10 mol% of complex 1, 70% conversion of N-(4-cyanobenzyl)-aniline to the corresponding amine was observed in 48 h (Entry 4, Table 7). Under these conditions, 50% conversion of N-(4-Nitro)-aniline to the corresponding amine was
Table 6 Vibrational Frequencies of CO ligand in the complexes studied.
Mo(CO)6 W(CO)6 Monocarbenes with an imidazole NHC Monocarbene with a benzimidazole NHC Biscarbene
11-Cr 11-Mo 11 1 12
This work [55]
123
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Table 7 (continued)
Table 7 Reduction of various aldimines catalysed by 1, 11-W, 11-Mo and 12-Mo.
Entrya
Substrates
12
Entrya
Substrates
Conversion (%)b 1
11-W
11-Mo
12-Mo
0
0
0
0
Conversion (%)b 1
11-W
11-Mo
12-Mo
1
89
89
91
94
2
91
86
87
94
3c
50
47
51
48
4c
70
68
66
68
5
90
84
86
93
6
92
89
90
92
7
85
84
83
93
a Substrate (0.25 mmol, PhSiH3 (0.5 mmol), and catalyst (0.0125 mmol), ethanol (5 mL), reflux, 24 h unless mentioned otherwise. b Conversion by 1H NMR spectroscopy. c 0.025 mmol of catalyst (10 mol%) used and conversion after 48 h.
observed (Entry 3, Table 7). Ortho substitution by –CH3 on N-aryl substituent also made the reaction slower (Entry 7, Table 7). There was no reduction of aldimines derived from pyridine carboxylaldehydes (Entries 11 and 12 Table 7). 2.7. Mechanistic considerations A probable mechanistic scheme is given in (Scheme 2). The dissociation of a CO ligand can be followed by the oxidative addition of a molecule of silane to form a hydride complex (NHC)W(H)(SiHPh2) (CO)4. Subsequent hydride transfer can take place via many possible pathways. Ojima pathway can be ruled out as it does not explain the observed trend in reactivity with different silanes which is PhSiH3 ≅ Ph2SiH2 > > Ph3SiH [60-65]. To detect the possible presence of a metal silylene intermediate a trapping experiment was carried using complexes 1 and 12-Mo in a way similar to that reported by Kuhn et al [64,66]. These experiments did not indicate the presence of a metal silylene species. Although the absence cannot be considered to be a conclusive evidence against the silylene pathway, it may be noted that Rh and Ru silylene intermediates were detected easily using this method [64,67]. It is possible that the silylene intermediate formed in this reaction does not react with the added silanol to form disiloxane. 3. Conclusions
8
87
83
81
89
9c
48
42
45
45
10
90
88
87
81
11
0
0
0
0
The homoleptic carbonyls of group 6 metals show poor catalytic activity in the reduction of imines using silanes. The addition of reagents like tert-butylhydroperoxide, Me3NO, and ultraviolet irradiation to aid in the generation of a vacant coordination site did not improve the rate of the reaction which indicates that the dissociation of CO is not involved in the rate determining step. It is shown that these unreactive metal complexes can be made better catalysts for the reduction of imines by substitution of a CO with an NHC ligand. A series of 14 Group 6 NHC complexes including 10 new complexes were studied as catalysts in the reduction of imines using silanes. The catalytic activity improved when two carbonyls were substituted with a biscrabene ligand as compared to complexes with a single NHC ligand. This trend is probably due to the increased electron density on the metal centre. The rate of the reaction was similar with a dihydrosilane and trihydrosilane. However, almost no reaction was observed using a monohydrosilane. This rate enhancement with dihydrosilanes and trihydrosilanes as compared to monohydrosilanes suggests that the reaction proceeds either via ZC or GH pathways. The trapping experiment using a silanol did not give any evidence for the formation of a silylene intermediate. Although the possibility of other mechanisms is not completely ruled out, a plausible mechanism similar to ZC pathway is proposed which is consistent with all the observations made. This system could provide an avenue to asymmetric reductions through the use of chiral NHC ligands on the [Mo(CO)n] core. 124
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Scheme 2. The plausible mechanism for the reduction of imines catalysed by group 6 NHC complexes.
4. Experimental section 4.1. General remarks All reactions and manipulations were carried out under a nitrogen atmosphere by using standard Schlenk techniques in oven-dried glassware. The solvents were purified by standard methods under dry nitrogen before use. Aldimines were prepared similar to the reported procedure, by refluxing an equivalent mixture of amine and aldehyde in dry ethanol in the presence of a catalytic amount of acetic acid and molecular sieves [68]. NMR spectra were recorded on a Bruker AMX 400 spectrometer that operates at 400 MHz for 1H and 100 MHz for 13C. Chemical shifts for 1H NMR spectra are reported as δ in ppm relative to the residual solvent peak. Electrospray ionization mass spectrometry (ESI-MS) experiments were done on Agilent 6538 Ultra High Definition (UHD) Accurate-Mass Q-TOF using standard spectroscopic grade solvents. Elemental analysis was carried out on a vario MICRO CUBE CHNS instrument. FT-IR spectra were recorded using Bruker ALPHA FTIR spectrometer.
Yellow solid, yield: 132 mg. (80%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.29–7.42 (m, 5H, Ar), 7.02–7.17 (m, 4H, Ar), 5.86 (s, 2H, CH2), 4.18 (3H, NCH3).13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 200.96 (CO), 197.87 (4 CO), 194.54 (carbene C), 136.16, 135.95, 134.71, 129.67, 129.34, 128.86, 128.35, 127.95, 126.72, 123.96, 111.93, 110.63 (Ar), 54.66 (CH2), 37.70 (N-CH3). IR: νCO, 2061, 1971, 1888. Elemental analysis for C20H14N2O5W, calcd. (%) C, 43.98; H, 2.58; N, 5.13; Found: C, 43.95; H, 2.60; N, 5.09. Complex 2
Yellow solid, yield: 150 mg. (85%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.34 (d, J = 8.0 Hz, 1H, Ar), 7.17–7.21 (m, 1H, Ar), 6.89–6.94 (m, 3H, Ar), 6.34 (d, J = 8.0 Hz, 1H, Ar), 5.79 (s, 2H, CH2), 4.14 (s, 3H, NCH3), 2.31 (s, 3H, p-CH3), 2.16 (s, 6H, o-CH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.18 (CO), 198.42 (4 CO), 194.84 (carbene C), 138.89, 138.13, 136.10, 134.68, 130.38, 128.67, 123.62, 123.53, 112.03, 110.40 (Ar), 53.51 (CH2), 37.82 (N-CH3), 21.47 (p-Me), 20.72 (o-Me); IR: νCO, 2058, 1961, 1884. Elemental analysis for C23H20N2O5W, calcd. (%) C, 46.96; H, 3.43; N, 4.76; Found: C, 46.91; H, 3.47; N, 4.69. Complex 3
4.2. General procedure for the synthesis of complexes A mixture of N-benzyl-N/-methylbenzimidazolium halide (0.30 mmol) and Ag2O (0.42 mg, 0.18 mmol) in CH2Cl2 (10 mL) was stirred for 6 h at room temperature in the absence of light. After that, the reaction mixture was filtered. The filtrate was concentrated to dryness and a solution of [M(CO)5THF] was added to it [10]. The reaction mixture was stirred at room temperature overnight. The solvent was removed, and the residue was dissolved in CHCl3. The resultant mixture was filtered through a short column of celite. The filtrate was layered with petroleum ether and the yellow crystals of the pure compound formed overnight. The complexes were characterized by 1H NMR, 13C NMR, and elemental analysis. The synthesis of 11, 11-Mo and 11-Cr using free carbene method is known in the literature [54,69]. However, in the present study, these complexes were synthesized by the transfer of the carbene ligand from the Ag-NHC complex to [M (CO)5THF]. The corresponding Ag-NHC complex was synthesized using N,N/-dimethylimidazolium iodide, and Ag2O [70]. Complex 12-Mo was prepared by a reported procedure using di(1-ethylimidazolium)methane diiodide and [Mo(CO)4(pip)2] [55].
Yellow solid, yield: 144 mg. (80%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.30–7.41 (m, 4H, Ar), 7.0–7.18 (m, 4H, Ar), 5.81 (s, 2H, CH2), 4.17 (3H, NCH3), 1.27 (s, 9H, tert-Bu). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.02 (CO), 197.91 (4 CO), 194.49 (carbene C), 151.35, 136.13, 134.83, 132.82, 126.50, 126.19, 123.87, 112.05, 110.52 (Ar), 54.43 (CH2), 37.67 (N-CH3), 34.97 (-CMe3), 31.72 (Me3). IR: νCO, 2062, 1966, 1886. Elemental analysis for C24H22N2O5W, calcd. (%) C, 47.86; H, 3.68; N, 4.65; Found: C, 47.78; H, 3.61; N, 4.68.
4.3. Characterization data for complexes Complex 1 125
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Complex 4
Complex 8
Yellow solid, yield: 150 mg. (70%). 1H NMR (400 MHz, CDCl3 (sparingly soluble), 298 K): δ (ppm) = 7.29–7.42 (m, 2H, Ar), 7.14–7.24 (m, 2H, Ar), 6.98.0 (s, 2H, Ar), 5.76 (s, 2H, CH2), 4.18 (s, 3H, NCH3), 2.31 (s, 3H, COCH3), 1.23 (s, 18H, tert-But). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.95 (CO), 194.63 (4 CO), 171.27 (C]O), 143.50, 135.95, 134.89, 132.37, 124.81, 124.04, 123.96, 111.97, 110.50 (Ar), 54.42 (CH2), 37.73 (N-CH3), 35.86 (C(CH3)3), 31.81, 31.70 ((CH3)3), 23.06 (CH3(CO)). IR: νCO, 2059, 1967, 1894. Elemental analysis for C30H32N2O7W, calcd. (%) C, 50.30; H, 4.50; N, 3.91; Found: C, 50.25; H, 4.55; N, 3.85. Complex 9
Yellow solid, yield: 135 mg. (78%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.28–7.41 (m, 2H, Ar), 7.23 (t, J = 8 Hz, 1H, Ar), 7.17 (t, J = 8 Hz, 1H, Ar), 7.07 (d, J = 8 Hz, 1H, Ar), 6.81 (d, J = 8.7 Hz, 1H, Ar), 6.68 (d, J = 8 Hz, 1H, Ar), 6.62 (s, 1H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 3.75 (s, 3H, OCH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 200.96 (CO), 197.89 (4 CO), 194.55, 160.50, 137.59, 136.14, 134.74, 130.41, 123.97, 119.15, 113.31, 112.99, 111.92, 110.61 (Ar), 55.69 (O-CH3), 54.58 (CH2), 37.69 (N-CH3). IR: νCO, 2061, 1960, 1877. Elemental analysis for C21H16N2 O6W, calcd. (%) C, 43.77; H, 2.80; N, 4.86; Found: C, 43.70; H, 2.85; N, 4.81. Complex 5
Yellow solid, yield: 140 mg. (81%). 1H NMR (400 MHz, CDCl3 (sparingly soluble), 298 K): δ (ppm) = 7.28–7.42 (m, 2H, Ar), 7.14–7.17 (m, 1H, Ar), 7.08–7.06 (s, 1H, Ar), 6. 90 (s, 1H, Ar), 6.70 (s, 2H, Ar), 5.76 (s, 2H, CH2), 4.18 (s, 3H, NCH3), 2.25 (s, 6H, m-CH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.96 (4 CO), 138.88, 136.16, 135.79, 130.04, 124.61, 123.87, 112.08, 110.54 (Ar), 54.76 (CH2), 37.69 (N-CH3), 21.74 (m-CH3). IR: νCO, 2064, 1968, 1881. Elemental analysis for C21H18N2O5W, calcd. (%) C, 46.02; H, 3.16; N, 4.88; Found: C, 46.08; H, 3.21; N, 4.81. Complex 10
Yellow solid, yield: 137 mg. (78%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.28–7.40 (m, 2H, Ar), 6.99–7.19 (m, 6H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 2.88 (septet, J = 8 Hz, 1H, i-Pr), 1.21 (d, J = 8 Hz, 6H, i-Pr). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.02 (CO), 197.91 (4 CO), 194.46 (carbene C), 149.05, 136.15, 134.81, 133.19, 127.35, 126.75, 123.87, 112.05, 110.54 (Ar), 54.54 (CH2), 37.67 (N-CH3), 34.17 (–CH(Me2), 24.32 (Me2). IR: νCO, 2062, 1965, 1886. Elemental analysis for C23H20N2O5W, calcd. (%) C, 46.96; H, 3.43; N, 4.76; Found: C, 46.90; H, 3.45; N, 4.70. Complex 6
Yellow solid, yield: 144 mg. (81%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 8.20 (d, J = 8.7 Hz, 2H, Ar), 7.46 (d, J = 8.76 Hz, 1H, Ar), 7.36 (t, J = 8.0 Hz, 1H, Ar), 7.18–7.25 (m, 3H, Ar), 6.97 (d, J = 8.0 Hz, 1H, Ar), 5.97 (s, 2H, CH2), 4.20 (3H, NCH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 200.48 (CO), 197.69 (4 CO), 195.21 (carbene C), 148.17, 143.39, 136.18, 134.35, 127.53, 124.67, 124.49, 124.39, 111.28, 111.04 (Ar), 53.90 (CH2), 37.83 (N-CH3). IR: νCO, 2063, 1968, 1868. Elemental analysis for C20H13N3O7W, calcd. (%) C, 40.63; H, 2.22; N, 7.11; Found: C, 40.70; H, 2.30; N, 7.06.
Yellow solid, yield: 133 mg. (77%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.27–7.40 (m, 2H, Ar), 7.12–7.16 (t, J = 8 Hz, 1H, Ar), 7.04 (t, J = 7.2 Hz, 3H, Ar), 6.84 (d, J = 8.0 Hz, 2H, Ar), 5.79 (s, 2H, CH2), 4.16 (s, 3H, NCH3), 3.77 (s, 3H, OCH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.01 (CO), 197.91 (4 CO), 194.28 (carbene C), 159.71, 136.20, 134.67, 128.03, 127.89, 123.86, 114.75, 112.06, 110.56 (Ar), 55.69 (OCH3), 54.34 (CH2), 37.66 (N-CH3). IR: νCO, 2060, 1975, 1879. Elemental analysis for C21H16N2O6W, calcd. (%) C, 43.77; H, 2.80; N, 4.86; Found: C, 43.71; H, 2.85; N, 4.79. Complex 7
4.4. General procedure for reduction The catalyst (0.0125 mmol) was dissolved in 5 mL of freshly dried and distilled ethanol. 0.25 mmol of imine was added to this solution. This was followed by the addition of PhSiH3 (0.5 mmol). Then the reaction mixture was refluxed. Small portions of the reaction mixture were withdrawn at specified intervals. The volatiles were removed by the rotary evaporation. Then 1HNMR spectra were recorded in CDCl3. The conversion was calculated from the intensity ratios of the peaks arising from the reactant and product. The amines were purified by preparative thin layer chromatography using a mixture of hexane/ethyl acetate (2–5%) as the eluent.
Yellow solid, yield: 134 mg. (80%). 1H NMR (400 MHz, CDCl3 (sparingly soluble, 298 K): δ (ppm) = 7.28–7.41 (m, 2H, Ar), 7.13–7.19 (m, 2H, Ar), 7.03–7.08 (m, 2H, Ar), 6.95 (s, 1H, Ar), 6.83 (d, J = 8 Hz, 1H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 2.30 (s, 3H, m-CH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.92 (4 CO), 139.06, 136.16, 135.87, 134.77, 129.14, 127.48, 123.91, 123.80, 112.01, 110.57 (Ar), 54.72 (CH2), 37.69 (N-CH3), 21.86 (m-CH3). IR: νCO, 2062, 1965, 1885. Elemental analysis for C21H16N2O5W, calcd. (%) C, 45.02; H, 2.88; N, 5.00; Found: C, 45.08; H, 2.91; N, 4.92. 126
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Reduction of imines catalysed by NHC substituted group 6 metal carbonyls Noor U Din Reshi, Lakshay Kathuria, Ashoka G. Samuelson
⁎
T
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
ARTICLE INFO
ABSTRACT
Keywords: Reduction Hydrosilylation Silyl hydrides Group 6 NHC complexes Imines
The catalytic activity of a series of metal carbonyls [M(CO)6], and the corresponding NHC substituted [M (CO)5(NHC)], (M = Cr, Mo, W) complexes was examined in the reduction of N-benzylideneaniline and acetophenone using silyl hydrides and isopropanol/KOH as reductants. The use of various additives and ultraviolet irradiation to promote the reduction of imines using silyl hydrides as reductants was explored. From a comparison of the reactivity of [Mo(CO)6], [Mo(CO)5(NHC)], and [Mo(CO)4(bis NHC)] it was inferred that electron density on the metal centre plays a key role in the catalysis. Four of the best catalysts were then tested in the reduction of a variety of imines with different electronic and steric properties.
1. Introduction Group 6 carbonyl complexes play an important role as catalysts or precatalysts in many organic transformations [1–8]. During the last two decades, a wide range of these carbonyl complexes with various ancillary ligands have been examined as precursors to M(VI) catalysts for the epoxidation of olefins [9–13], the cis-dihydroxylation of olefins [14], and the oxidation of amines [15], alcohols [16,17], and sulfides [18,19]. Surprisingly, there are not too many examples of the use of group 6 metal complexes in general and group 6 metal carbonyl complexes in particular for catalysing reduction reactions such as transfer hydrogenation [20–22], hydrogenation [23–26], and hydrosilylation [27,28]. This is in-spite of the fact that group 6 metal-based complexes are attractive hydride transfer agents in view of their low cost and high coordination number enabling great flexibility in the design of the ligand sphere. Moreover, most of these complexes are readily available from the corresponding hexacarbonyl complexes [29]. Among the few reduction reactions catalysed by such metal carbonyls, fewer still utilize hydrosilanes as reducing agents. The conjugate reduction of various Michael acceptors, including α,β-unsaturated ketones, carboxylic acids, carboxylic esters, amides, and nitriles with phenylsilane is catalysed rather sluggishly by M(CO)6 (M = Cr, Mo, W) in refluxing tetrahydrofuran [30]. Hydrosilylation of 1,3-dienes catalysed by M(CO)6 is also known [31,32]. Tinnis and Adolfsson et al. reported Mo(CO)6 (5 mol%) catalysed chemoselective hydrosilylation of α, β-unsaturated amides leading to allylamines (yield 51–95%) using 1,1,3,3-tetramethyldisiloxane (TMDS) (1.5 equivalents) for 24 h at 65 °C [33]. [Mo(CO)2(oxadiene)2] (oxadiene = pulegone, pinocarvone and (E)-5-methyl-3-hexen-2-one), showed better catalytic activity in the
⁎
hydrosilylation of unsaturated ketones and aldehydes with phenylsilane. All aldehydes underwent 1, 2-addition exclusively whereas the ketones gave both 1,2-addition and 1,4-addition products [34]. Anionic µ-hydride complexes of group 6 metals of the type [(CO)5M(µ-H)M (CO)5][NEt4]+ were reported to catalyse the hydrosilylation of aldehydes and ketones under rather harsh conditions using HSiEt3, while silyl enol ethers were obtained using H2SiPh2 [35]. A tungsten-based catalyst, [CpW(CO)2(IMes)]+[B(C6F5)4]- (where Cp = cyclopentadienyl, IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, has been reported to catalyse the solvent-free hydrosilylation of ketones and an ester under mild conditions [36]. The catalytic activity of isocyanide complexes of group 6 metals in the hydrosilylation of olefins is also known [37]. Surprisingly group 6 carbonyl complexes have not been tested for the hydrosilylation of imines to give the corresponding amines. Although the alkylation of ammonia using halo alkanes or alcohols is a simple way to prepare amines, the degree of alkylation is difficult to control with alkyl iodides and bromides [38]. The synthesis of amines by reduction of imines, amides, nitriles, and nitroarenes using stoichiometric amount of alkali hydrides, such as lithium aluminium hydride and boron hydrides is limited by the air and moisture sensitivity of these reductants, lack of tolerance to various functional groups and formation of metal salts as by-products [39]. The requirement of high temperatures and high pressures, and lack of chemoselectivity makes hydrogenation of imines using dihydrogen gas unsuitable for some substrates [40–43]. Reductive amination of carbonyl derivatives in the presence of primary amines, through sequential condensation/catalytic reduction often requires drastic reaction conditions [44–50]. Due to these reasons the hydrosilylation of imines is an attractive alternative
Corresponding author. E-mail address: [email protected] (A.G. Samuelson).
https://doi.org/10.1016/j.ica.2018.10.026 Received 5 July 2018; Received in revised form 17 October 2018; Accepted 17 October 2018 0020-1693/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. NHC complexes studied in this work.
for the synthesis of corresponding amines. In this paper we wish to report our studies on the use of NHC substituted group 6 metal carbonyl complexes of the type [M(CO)5(NHC)], (M = Cr, Mo, W) for catalysing the reduction reactions of imines using silyl hydrides and isopropanol/ KOH as the hydride source. We also compared their catalytic activity with the activity of [Mo(CO)6] and [Mo(CO)4(bis-NHC)] complexes.
NMR, IR-spectroscopy, and elemental analysis. 2.2. Catalytic studies The preliminary catalytic tests were carried out using complex 1 as a catalyst and N-benzylideneaniline and acetophenone as test substrates and phenylsilane and isopropanol/KOH as reductants. There was no catalytic reduction of N-benzylideneaniline in isopropanol in presence of KOH. Similarly, acetophenone was not reduced under these conditions. The reduction of N-benzylideneaniline using PhSiH3 in THF and toluene were also unsuccessful (Entries 1 and 2, Table 1). In dichloromethane and chloroform, a modest conversion of nearly 30% of N-benzylideneaniline to the corresponding amine (N-benzylaniline) was observed using 5 mol% of complex 1 at room temperature (Entries 3 and 4, Table 1). At room temperature, the best yield was obtained in ethanol where 45% of N-benzylideneaniline converted to the corresponding amine in 48 h using 5 mol% of complex 1 (Entry 5, Table 1). The rate of the reaction increased significantly with higher catalyst
2. Results and discussion 2.1. Synthesis and characterization Ag-NHC complexes were synthesized from the corresponding benzimidazolium salts using a reported procedure [51]. The complexes 1–11 (Fig. 1) were synthesized by transferring the carbene from the corresponding Ag-NHC complexes to [M(CO)5THF] (Scheme 1) [10,5254]. Complex 12, on the other hand, was prepared by a reported procedure using di(1-ethylimidazolium)methane diiodide and [Mo (CO)4(pip)2] [55]. The complexes were characterized by 1H NMR, 13C 120
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Scheme 1. Synthesis of complexes 1–10.
methoxybenzylidene)aniline which was reduced to 4-Acetyl-N-(4methoxybenzyl)aniline and no reduction of the keto group to give 1-(4((4-methoxybenzylidene)amino)phenyl)ethan-1-ol was observed. However, using a mixture of N-benzylidineaniline and benzaldehyde as substrate, nearly 50% of benzaldehyde was reduced to benzyl alcohol (Entry 2, Table 2).
Table 1 Optimization of reaction conditions for reduction using PhSiH3 as reductant.
Entrya
Solvent
Temperature
Time (h)
Conversion (%)
1 2 3 4 5 6b 7 8 9c 10d
Toluene Tetrahydrofuran Dichloromethane Chloroform EtOH EtOH EtOH EtOH EtOH EtOH
r.t r.t r.t r.t r.t r.t 50 (°C) reflux reflux reflux
48 48 48 48 48 48 48 24 24 24
0 0 30 31 45 90 90 92 80 87
2.3. Effect of peroxides, Me3NO, and ultraviolet irradiation The catalytic activity of M(CO)6 (M = Group 6 metal) in the hydrosilylation of 1,3-dienes is enhanced in the presence of organic oxidants such as peroxides [32]. This enhancement is presumably due to the ability of peroxides to oxidize coordinated carbon monoxide to weakly coordinating carbon dioxide [57] resulting in a vacant site. Taking a cue from this observation, the reduction of N-benzylideneaniline using complex 1 as the catalyst was carried out in the presence of 1 equivalent of tert-butyl hydroperoxide (with respect to complex 1). Surprisingly, no rate enhancement was observed (Entry 2, Table 3). Me3NO is also known to facilitate the loss of CO ligand [57]. Hence the reaction was carried out in presence of 1 equivalent of Me3NO (with respect to complex 1). In this case also the rate of the reaction did not improve (Entry 3, Table 3). Group 6 hexacarbonyls have been reported to be active photocatalysts for the reduction of 1,3-dienes [58]. Their catalytic activity upon irradiation has been attributed to the formation of the species [Mo (CO)5] (M = Cr, Mo, W). Hence the reduction of N-benzylideneaniline using complex 1 as a catalyst was carried out in refluxing ethanol while being irradiated by ultraviolet radiation (400 W, high-pressure Hg vapor lamp, λ max = 365 nm). However, contrary to expectations, no rate enhancement was observed (Entry 4, Table 3). Since no rate enhancement is observed in the presence of Me3NO, tert-butyl hydroperoxide, and ultraviolet irradiation, the dissociation of CO to form a coordinatively unsaturated species is probably not the rate-determining step.
a Substrate (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and complex 1 (5 mol%), solvent (5 mL). In case of solvents other than ethanol a desilylation step (treating the reaction mixture with methanol and 2 M aqueous NaOH) was also carried out before 1H NMR was recorded [56]; conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy. b 10 mol% of complex 1 used. c HPLC grade ethanol used (not dried). d Reaction carried out in air.
loading (Entry 6, Table 1). Since only modest conversion was observed at room temperature, the reaction was carried out at higher temperatures with 5% catalyst loading. When the reaction temperature was increased to 50 °C, 90% conversion of N-benzylideneaniline to the corresponding amine was observed in 48 h (Entry 7, Table 1). Whereas, 92% conversion of Nbenzylideneaniline to the corresponding amine was observed in 24 h when the reaction was carried out in refluxing ethanol (Entry 8, Table 1). The conversion of N-benzylideneaniline to the corresponding amine decreased by nearly 12% when HPLC grade ethanol was used as a solvent as compared to the case when ethanol was dried and distilled before use as a solvent for the reaction (Entry 9, Table 1). The conversion also decreased when the reaction was not carried under an atmosphere of dinitrogen (Entry 10, Table 1). Based on the results of the optimization studies, the reduction of imines was carried out in refluxing ethanol which was dried and freshly distilled, under an atmosphere of dry nitrogen and a catalyst loading of 5% with respect to the substrate. Interestingly, under the optimized conditions for the reduction of imines there was no significant reduction of acetophenone. The reduction of a mixture of N-benzylidineaniline (100 mol%) and acetophenone (100 mol%) was attempted using PhSiH3 (400 mol%) and catalysts 1, 11-W and 12-Mo (10 mol%) in refluxing ethanol. In all the three cases N-benzylidineaniline was selectively reduced whereas no significant reduction of acetophenone was observed (Entry 1, Table 2). Similar selectivity was observed in the case of 4-Acetyl-N-(4-
2.4. Varying the nature and amount of silane The effect of varying the nature and amount of silane was studied in the reduction of a test substrate N-benzylideneaniline using a representative complex (1) as the catalyst. Since each molecule of PhSiH3 has three reducing hydrides, 34 mol% of PhSiH3 should be sufficient to reduce the imine completely. When the amount of phenylsilane was decreased accordingly, conversion of N-benzylideneaniline to the corresponding amine decreased drastically to 14% (Entry 2, Table 4). This shows that not all H atoms on phenylsilane are equally available for the reduction of the imine. When a secondary silane (diphenylsilane) was used instead of phenylsilane, little effect on the conversion and the rate of reaction was observed (Entry 3, Table 4). When only 50 mol% of diphenylsilane was used, 13% conversion of N-benzylideneaniline to the corresponding amine was observed (Entry 4, Table 4). The use of a tertiary silane (triphenylsilane) resulted in a further decrease in the yield compared to diphenylsilane and phenylsilane. Only 5% of N-benzylideneaniline 121
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Table 2 Reduction of a mixture of N-benzylideneaniline and acetophenone/benzaldehyde. Entry
Substrate
Conversion Catalyst 1
1 2 3
N-benzylideneaniline + acetophenone a,c N-benzylideneaniline + benzaldehyde a,d 4-Acetyl-N-(4-methoxybenzylidene)aniline
b,e
Catalyst 11-W
Catalyst 12
% of Amine
% of Alcohol
% of Amine
% of Alcohol
% of Amine
% of Alcohol
92 91 83
4 49 0
88 87 78
4 45 0
99 100 94
7 52 0
a N-benzylideneaniline (0.25 mmol (100 mol%), acetophenone/benzaldehyde (0.25 mmol (100 mol%), PhSiH3 (400 mol%), and catalyst (10 mol%), solvent (5 mL). b 4-Acetyl-N-(4-methoxybenzylidene)aniline (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and catalyst (5 mol%), solvent (5 mL). c Conversion of N-benzylidene to N-benzylaniline, and acetophenone to 1-phenylethanol by 1H NMR spectroscopy. d Conversion of N-benzylidene to N-benzylaniline, and benzaldehyde to benzyl alcohol by 1H NMR spectroscopy. e Conversion of 4-Acetyl-N-(4-methoxybenzylidene)aniline to 4-Acetyl-N-(4-methoxybenzyl)aniline and 1-(4-((4-methoxybenzylidene)amino)phenyl)ethan-1-ol by 1H NMR spectroscopy.
Table 3 Effect of addition of Me3NO, tert-Butyl hydroperoxide, and ultraviolet irradiation.
Entrya
Remarks
Conversion (%)b
1 2 3 4
Standard conditions tert-butyl hydroperoxide added (1 eq.) Me3NO added (1 eq.) Ultraviolet irradiation
12 h 76 72 74 74
24 h 92 85 88 86
a N-benzylideneaniline (0.25 mmol (100 mol%)), PhSiH3 (200 mol%), and complex 1 (5 mol%), ethanol (5 mL), reflux. b Conversion by 1H NMR spectroscopy.
Table 4 Effect of varying the nature and amount of silane.
Entrya
Silane
Mol% of silane
Conversionb
1 2 3 4 5
PhSiH3 PhSiH3 Ph2SiH2 Ph2SiH2 Ph3SiH
200 34 200 50 100
92 14 88 13 5
a
N-benzylideneaniline (0.25 mmol (100 mol%), silane, and complex 1 (5 mol%), ethanol (5 mL), 24 h, reflux. b Conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy.
converted to the corresponding amine (Entry 5, Table 4) under conditions where almost complete conversion was achieved with phenylsilane.
2.5. Reduction of N-benzylideneaniline using the catalysts The activity of all the complexes (shown in Fig. 1) was compared in the reduction of a test substrate (N-benzylideneaniline) under 122
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Table 5 Reduction of N-benzylidineaniline using complexes group 6 complexes studied.
Entrya
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Conversion (%)b
Catalyst
1 2 3 4 5 6 7 8 9 10 11-W 11-Mo 11-Cr 12-Mo W(CO)6 M(CO)6 Cr(CO)6
12 h
24 h
76 67 72 75 70 71 73 68 72 73 72 73 71 90 – – –
92 94 95 94 92 94 93 92 95 93 87 86 83 98 25 28 26
a
N-benzylideneaniline (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and catalyst 1 (5 mol%), ethanol (5 mL), reflux. b Conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy.
optimized conditions. The data is given in Table 5. The change of the substituents on the aromatic ring (attached to N) did not have a significant effect on the rate of the reaction (Entries 1–10, Table 5). This is not surprising as the aromatic ring is separated by a CH2 bridge from the N of NHC ring. The comparison of the catalytic activity of 11-W, 11Mo, and 11-Cr which are prepared from same imidazolium salt but contain different metal atoms (W, Mo, and Cr respectively) indicated that the nature of Group 6 metal does not affect the rate of the reaction (Entries 11, 12, and 13, Table 5). Interestingly, the biscarbene complex 12-Mo exhibited higher activity for the reduction of N-benzylideneaniline. Thus, there was 90% conversion of N-benzylideneaniline to the corresponding amine in 12 h using 5 mol% of complex 12-Mo in refluxing ethanol. Under similar
Entry
Metal complex
ν (CO) (cm−1)
Refs.
conditions, complex 11-Mo gave 73% conversion of N-benzylideneaniline to the corresponding amine in 12 h (Entry 14, Table 5). The activity of hexacarbonyl complexes was less than that of NHC complexes (Entries 15, 16, and 17, Table 5). So, the overall trend in the catalytic activity was M(CO)6 < [M (CO)5(NHC)] < [M(CO)4(NHC)2]. The decrease in the number of strongly π accepting CO ligands and increase in the number of strongly sigma-donating NHC ligand on going from M(CO)6 to [M(CO)5(NHC)] to [M(CO)4(NHC)2] results in an increase in the electron density on the metal centre in the same order. This change in the electron density on the metal centre is reflected in the change in the vibrational frequencies of coordinated CO in these complexes as shown in (Table 6). The increase in the electron density is likely to make the oxidative addition of silane to the metal centre more facile, which in turn increases the rate of the reaction. Since the substitution with electron donating or electron withdrawing groups on the aromatic ring does not significantly affect the effective electron density on the metal, the catalytic activity of complexes 1–10 is similar.
Hexacarbonyls
Cr(CO)6
2118.7 (A1g) 2026.7 (Eg) 2000.4 (F1u) 2120.7 (A1g) 2024.8 (Eg) 2000.3 (F1u) 2126.2 (A1g) 2021.1 (Eg) 1997.6 (F1u) 2056, 1925 2064, 1930 2062, 1924 2058, 1961, 1884 1987, 1876, 1818, 1799
[59]
2.6. Substrate scope
[54]
The four representative complexes 1, 11-W, 11-Mo and 12-Mo were tested for the reduction of substrates with varying electronic and steric factors. Complex 1 has a carbene ligand based on a bisimidazolium system whereas complexes 11-W and 11-Mo have carbene ligands based on an imidazolium ring and complex 12-Mo contains a biscarbene ligand. The data is given in Table 7. This reaction tolerated functional groups like -Cl, –CN, –OH, –NO2, –NH2, -R (alkyl). In the presence of electron withdrawing groups (nitrile and nitro), the reduction was more difficult to perform. Using 10 mol% of complex 1, 70% conversion of N-(4-cyanobenzyl)-aniline to the corresponding amine was observed in 48 h (Entry 4, Table 7). Under these conditions, 50% conversion of N-(4-Nitro)-aniline to the corresponding amine was
Table 6 Vibrational Frequencies of CO ligand in the complexes studied.
Mo(CO)6 W(CO)6 Monocarbenes with an imidazole NHC Monocarbene with a benzimidazole NHC Biscarbene
11-Cr 11-Mo 11 1 12
This work [55]
123
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Table 7 (continued)
Table 7 Reduction of various aldimines catalysed by 1, 11-W, 11-Mo and 12-Mo.
Entrya
Substrates
12
Entrya
Substrates
Conversion (%)b 1
11-W
11-Mo
12-Mo
0
0
0
0
Conversion (%)b 1
11-W
11-Mo
12-Mo
1
89
89
91
94
2
91
86
87
94
3c
50
47
51
48
4c
70
68
66
68
5
90
84
86
93
6
92
89
90
92
7
85
84
83
93
a Substrate (0.25 mmol, PhSiH3 (0.5 mmol), and catalyst (0.0125 mmol), ethanol (5 mL), reflux, 24 h unless mentioned otherwise. b Conversion by 1H NMR spectroscopy. c 0.025 mmol of catalyst (10 mol%) used and conversion after 48 h.
observed (Entry 3, Table 7). Ortho substitution by –CH3 on N-aryl substituent also made the reaction slower (Entry 7, Table 7). There was no reduction of aldimines derived from pyridine carboxylaldehydes (Entries 11 and 12 Table 7). 2.7. Mechanistic considerations A probable mechanistic scheme is given in (Scheme 2). The dissociation of a CO ligand can be followed by the oxidative addition of a molecule of silane to form a hydride complex (NHC)W(H)(SiHPh2) (CO)4. Subsequent hydride transfer can take place via many possible pathways. Ojima pathway can be ruled out as it does not explain the observed trend in reactivity with different silanes which is PhSiH3 ≅ Ph2SiH2 > > Ph3SiH [60-65]. To detect the possible presence of a metal silylene intermediate a trapping experiment was carried using complexes 1 and 12-Mo in a way similar to that reported by Kuhn et al [64,66]. These experiments did not indicate the presence of a metal silylene species. Although the absence cannot be considered to be a conclusive evidence against the silylene pathway, it may be noted that Rh and Ru silylene intermediates were detected easily using this method [64,67]. It is possible that the silylene intermediate formed in this reaction does not react with the added silanol to form disiloxane. 3. Conclusions
8
87
83
81
89
9c
48
42
45
45
10
90
88
87
81
11
0
0
0
0
The homoleptic carbonyls of group 6 metals show poor catalytic activity in the reduction of imines using silanes. The addition of reagents like tert-butylhydroperoxide, Me3NO, and ultraviolet irradiation to aid in the generation of a vacant coordination site did not improve the rate of the reaction which indicates that the dissociation of CO is not involved in the rate determining step. It is shown that these unreactive metal complexes can be made better catalysts for the reduction of imines by substitution of a CO with an NHC ligand. A series of 14 Group 6 NHC complexes including 10 new complexes were studied as catalysts in the reduction of imines using silanes. The catalytic activity improved when two carbonyls were substituted with a biscrabene ligand as compared to complexes with a single NHC ligand. This trend is probably due to the increased electron density on the metal centre. The rate of the reaction was similar with a dihydrosilane and trihydrosilane. However, almost no reaction was observed using a monohydrosilane. This rate enhancement with dihydrosilanes and trihydrosilanes as compared to monohydrosilanes suggests that the reaction proceeds either via ZC or GH pathways. The trapping experiment using a silanol did not give any evidence for the formation of a silylene intermediate. Although the possibility of other mechanisms is not completely ruled out, a plausible mechanism similar to ZC pathway is proposed which is consistent with all the observations made. This system could provide an avenue to asymmetric reductions through the use of chiral NHC ligands on the [Mo(CO)n] core. 124
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Scheme 2. The plausible mechanism for the reduction of imines catalysed by group 6 NHC complexes.
4. Experimental section 4.1. General remarks All reactions and manipulations were carried out under a nitrogen atmosphere by using standard Schlenk techniques in oven-dried glassware. The solvents were purified by standard methods under dry nitrogen before use. Aldimines were prepared similar to the reported procedure, by refluxing an equivalent mixture of amine and aldehyde in dry ethanol in the presence of a catalytic amount of acetic acid and molecular sieves [68]. NMR spectra were recorded on a Bruker AMX 400 spectrometer that operates at 400 MHz for 1H and 100 MHz for 13C. Chemical shifts for 1H NMR spectra are reported as δ in ppm relative to the residual solvent peak. Electrospray ionization mass spectrometry (ESI-MS) experiments were done on Agilent 6538 Ultra High Definition (UHD) Accurate-Mass Q-TOF using standard spectroscopic grade solvents. Elemental analysis was carried out on a vario MICRO CUBE CHNS instrument. FT-IR spectra were recorded using Bruker ALPHA FTIR spectrometer.
Yellow solid, yield: 132 mg. (80%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.29–7.42 (m, 5H, Ar), 7.02–7.17 (m, 4H, Ar), 5.86 (s, 2H, CH2), 4.18 (3H, NCH3).13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 200.96 (CO), 197.87 (4 CO), 194.54 (carbene C), 136.16, 135.95, 134.71, 129.67, 129.34, 128.86, 128.35, 127.95, 126.72, 123.96, 111.93, 110.63 (Ar), 54.66 (CH2), 37.70 (N-CH3). IR: νCO, 2061, 1971, 1888. Elemental analysis for C20H14N2O5W, calcd. (%) C, 43.98; H, 2.58; N, 5.13; Found: C, 43.95; H, 2.60; N, 5.09. Complex 2
Yellow solid, yield: 150 mg. (85%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.34 (d, J = 8.0 Hz, 1H, Ar), 7.17–7.21 (m, 1H, Ar), 6.89–6.94 (m, 3H, Ar), 6.34 (d, J = 8.0 Hz, 1H, Ar), 5.79 (s, 2H, CH2), 4.14 (s, 3H, NCH3), 2.31 (s, 3H, p-CH3), 2.16 (s, 6H, o-CH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.18 (CO), 198.42 (4 CO), 194.84 (carbene C), 138.89, 138.13, 136.10, 134.68, 130.38, 128.67, 123.62, 123.53, 112.03, 110.40 (Ar), 53.51 (CH2), 37.82 (N-CH3), 21.47 (p-Me), 20.72 (o-Me); IR: νCO, 2058, 1961, 1884. Elemental analysis for C23H20N2O5W, calcd. (%) C, 46.96; H, 3.43; N, 4.76; Found: C, 46.91; H, 3.47; N, 4.69. Complex 3
4.2. General procedure for the synthesis of complexes A mixture of N-benzyl-N/-methylbenzimidazolium halide (0.30 mmol) and Ag2O (0.42 mg, 0.18 mmol) in CH2Cl2 (10 mL) was stirred for 6 h at room temperature in the absence of light. After that, the reaction mixture was filtered. The filtrate was concentrated to dryness and a solution of [M(CO)5THF] was added to it [10]. The reaction mixture was stirred at room temperature overnight. The solvent was removed, and the residue was dissolved in CHCl3. The resultant mixture was filtered through a short column of celite. The filtrate was layered with petroleum ether and the yellow crystals of the pure compound formed overnight. The complexes were characterized by 1H NMR, 13C NMR, and elemental analysis. The synthesis of 11, 11-Mo and 11-Cr using free carbene method is known in the literature [54,69]. However, in the present study, these complexes were synthesized by the transfer of the carbene ligand from the Ag-NHC complex to [M (CO)5THF]. The corresponding Ag-NHC complex was synthesized using N,N/-dimethylimidazolium iodide, and Ag2O [70]. Complex 12-Mo was prepared by a reported procedure using di(1-ethylimidazolium)methane diiodide and [Mo(CO)4(pip)2] [55].
Yellow solid, yield: 144 mg. (80%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.30–7.41 (m, 4H, Ar), 7.0–7.18 (m, 4H, Ar), 5.81 (s, 2H, CH2), 4.17 (3H, NCH3), 1.27 (s, 9H, tert-Bu). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.02 (CO), 197.91 (4 CO), 194.49 (carbene C), 151.35, 136.13, 134.83, 132.82, 126.50, 126.19, 123.87, 112.05, 110.52 (Ar), 54.43 (CH2), 37.67 (N-CH3), 34.97 (-CMe3), 31.72 (Me3). IR: νCO, 2062, 1966, 1886. Elemental analysis for C24H22N2O5W, calcd. (%) C, 47.86; H, 3.68; N, 4.65; Found: C, 47.78; H, 3.61; N, 4.68.
4.3. Characterization data for complexes Complex 1 125
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Complex 4
Complex 8
Yellow solid, yield: 150 mg. (70%). 1H NMR (400 MHz, CDCl3 (sparingly soluble), 298 K): δ (ppm) = 7.29–7.42 (m, 2H, Ar), 7.14–7.24 (m, 2H, Ar), 6.98.0 (s, 2H, Ar), 5.76 (s, 2H, CH2), 4.18 (s, 3H, NCH3), 2.31 (s, 3H, COCH3), 1.23 (s, 18H, tert-But). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.95 (CO), 194.63 (4 CO), 171.27 (C]O), 143.50, 135.95, 134.89, 132.37, 124.81, 124.04, 123.96, 111.97, 110.50 (Ar), 54.42 (CH2), 37.73 (N-CH3), 35.86 (C(CH3)3), 31.81, 31.70 ((CH3)3), 23.06 (CH3(CO)). IR: νCO, 2059, 1967, 1894. Elemental analysis for C30H32N2O7W, calcd. (%) C, 50.30; H, 4.50; N, 3.91; Found: C, 50.25; H, 4.55; N, 3.85. Complex 9
Yellow solid, yield: 135 mg. (78%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.28–7.41 (m, 2H, Ar), 7.23 (t, J = 8 Hz, 1H, Ar), 7.17 (t, J = 8 Hz, 1H, Ar), 7.07 (d, J = 8 Hz, 1H, Ar), 6.81 (d, J = 8.7 Hz, 1H, Ar), 6.68 (d, J = 8 Hz, 1H, Ar), 6.62 (s, 1H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 3.75 (s, 3H, OCH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 200.96 (CO), 197.89 (4 CO), 194.55, 160.50, 137.59, 136.14, 134.74, 130.41, 123.97, 119.15, 113.31, 112.99, 111.92, 110.61 (Ar), 55.69 (O-CH3), 54.58 (CH2), 37.69 (N-CH3). IR: νCO, 2061, 1960, 1877. Elemental analysis for C21H16N2 O6W, calcd. (%) C, 43.77; H, 2.80; N, 4.86; Found: C, 43.70; H, 2.85; N, 4.81. Complex 5
Yellow solid, yield: 140 mg. (81%). 1H NMR (400 MHz, CDCl3 (sparingly soluble), 298 K): δ (ppm) = 7.28–7.42 (m, 2H, Ar), 7.14–7.17 (m, 1H, Ar), 7.08–7.06 (s, 1H, Ar), 6. 90 (s, 1H, Ar), 6.70 (s, 2H, Ar), 5.76 (s, 2H, CH2), 4.18 (s, 3H, NCH3), 2.25 (s, 6H, m-CH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.96 (4 CO), 138.88, 136.16, 135.79, 130.04, 124.61, 123.87, 112.08, 110.54 (Ar), 54.76 (CH2), 37.69 (N-CH3), 21.74 (m-CH3). IR: νCO, 2064, 1968, 1881. Elemental analysis for C21H18N2O5W, calcd. (%) C, 46.02; H, 3.16; N, 4.88; Found: C, 46.08; H, 3.21; N, 4.81. Complex 10
Yellow solid, yield: 137 mg. (78%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.28–7.40 (m, 2H, Ar), 6.99–7.19 (m, 6H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 2.88 (septet, J = 8 Hz, 1H, i-Pr), 1.21 (d, J = 8 Hz, 6H, i-Pr). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.02 (CO), 197.91 (4 CO), 194.46 (carbene C), 149.05, 136.15, 134.81, 133.19, 127.35, 126.75, 123.87, 112.05, 110.54 (Ar), 54.54 (CH2), 37.67 (N-CH3), 34.17 (–CH(Me2), 24.32 (Me2). IR: νCO, 2062, 1965, 1886. Elemental analysis for C23H20N2O5W, calcd. (%) C, 46.96; H, 3.43; N, 4.76; Found: C, 46.90; H, 3.45; N, 4.70. Complex 6
Yellow solid, yield: 144 mg. (81%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 8.20 (d, J = 8.7 Hz, 2H, Ar), 7.46 (d, J = 8.76 Hz, 1H, Ar), 7.36 (t, J = 8.0 Hz, 1H, Ar), 7.18–7.25 (m, 3H, Ar), 6.97 (d, J = 8.0 Hz, 1H, Ar), 5.97 (s, 2H, CH2), 4.20 (3H, NCH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 200.48 (CO), 197.69 (4 CO), 195.21 (carbene C), 148.17, 143.39, 136.18, 134.35, 127.53, 124.67, 124.49, 124.39, 111.28, 111.04 (Ar), 53.90 (CH2), 37.83 (N-CH3). IR: νCO, 2063, 1968, 1868. Elemental analysis for C20H13N3O7W, calcd. (%) C, 40.63; H, 2.22; N, 7.11; Found: C, 40.70; H, 2.30; N, 7.06.
Yellow solid, yield: 133 mg. (77%). 1H NMR (400 MHz, CDCl3, 298 K): δ (ppm) = 7.27–7.40 (m, 2H, Ar), 7.12–7.16 (t, J = 8 Hz, 1H, Ar), 7.04 (t, J = 7.2 Hz, 3H, Ar), 6.84 (d, J = 8.0 Hz, 2H, Ar), 5.79 (s, 2H, CH2), 4.16 (s, 3H, NCH3), 3.77 (s, 3H, OCH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 201.01 (CO), 197.91 (4 CO), 194.28 (carbene C), 159.71, 136.20, 134.67, 128.03, 127.89, 123.86, 114.75, 112.06, 110.56 (Ar), 55.69 (OCH3), 54.34 (CH2), 37.66 (N-CH3). IR: νCO, 2060, 1975, 1879. Elemental analysis for C21H16N2O6W, calcd. (%) C, 43.77; H, 2.80; N, 4.86; Found: C, 43.71; H, 2.85; N, 4.79. Complex 7
4.4. General procedure for reduction The catalyst (0.0125 mmol) was dissolved in 5 mL of freshly dried and distilled ethanol. 0.25 mmol of imine was added to this solution. This was followed by the addition of PhSiH3 (0.5 mmol). Then the reaction mixture was refluxed. Small portions of the reaction mixture were withdrawn at specified intervals. The volatiles were removed by the rotary evaporation. Then 1HNMR spectra were recorded in CDCl3. The conversion was calculated from the intensity ratios of the peaks arising from the reactant and product. The amines were purified by preparative thin layer chromatography using a mixture of hexane/ethyl acetate (2–5%) as the eluent.
Yellow solid, yield: 134 mg. (80%). 1H NMR (400 MHz, CDCl3 (sparingly soluble, 298 K): δ (ppm) = 7.28–7.41 (m, 2H, Ar), 7.13–7.19 (m, 2H, Ar), 7.03–7.08 (m, 2H, Ar), 6.95 (s, 1H, Ar), 6.83 (d, J = 8 Hz, 1H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 2.30 (s, 3H, m-CH3). 13C NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.92 (4 CO), 139.06, 136.16, 135.87, 134.77, 129.14, 127.48, 123.91, 123.80, 112.01, 110.57 (Ar), 54.72 (CH2), 37.69 (N-CH3), 21.86 (m-CH3). IR: νCO, 2062, 1965, 1885. Elemental analysis for C21H16N2O5W, calcd. (%) C, 45.02; H, 2.88; N, 5.00; Found: C, 45.08; H, 2.91; N, 4.92. 126
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4.5. Trapping experiment
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25 μL (0.15 mmol) of t-BuMe2SiOH was reacted with 50 μL (50.1 mg, 0.26 mmol) of Ph2SiH2 in presence of 0.015 mmol of 1 in refluxing ethanol. The solvent was removed after 24 h and CDCl3 was added to the residue. The 1H NMR and 29Si NMR were recorded which showed the presence of small amount of t-BuMe2SiOH and some other species that could not be identified. No signals corresponding to the disiloxane were detected in the 1H NMR and 29Si NMR. Similarly, a trapping experiment carried out using the biscarbene complex 12-Mo also, did not indicate the formation of the disiloxane. Acknowledgments N.U.R thanks Council of Scientific and Industrial Research (CSIR), New Delhi for senior. research fellowship. L.K thanks University Grants Commission (UGC), New Delhi for senior. research fellowship. Authors acknowledge Department of Science and Technology (DST), New Delhi for research grant (DST-EMR/2015/001853). Conflict of Interest There are no conflicts to declare. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2018.10.026. References [1] R.J. Newland, A. Smith, D.M. Smith, N. Fey, M.J. Hanton, S.M. Mansell, Accessing alkyl- and alkenylcyclopentanes from Cr-catalyzed ethylene oligomerization using 2-phosphinophosphinine ligands, Organometallics 37 (2018) 1062–1073, https:// doi.org/10.1021/acs.organomet.8b00063. [2] M. Torrent, M. Solà, G. Frenking, Theoretical studies of some transition-metalmediated reactions of industrial and synthetic importance, Chem. Rev. 100 (2000) 439–494, https://doi.org/10.1021/cr980452i. [3] Q. Xu, Metal carbonyl cations: generation, characterization and catalytic application, Coord. Chem. Rev. 231 (2002) 83–108, https://doi.org/10.1016/S00108545(02)00115-7. [4] A.J. Esswein, D.G. Nocera, Hydrogen production by molecular photocatalysis, Chem. Rev. 107 (2007) 4022–4047, https://doi.org/10.1021/cr050193e. [5] C. Freund, M. Abrantes, F.E. Kühn, Monomeric cyclopentadiene molybdenum oxides and their carbonyl precursors as epoxidation catalysts, J. Organomet. Chem. 691 (2006) 3718–3729, https://doi.org/10.1016/j.jorganchem.2006.05.007. [6] R.M. Reich, M. Kaposi, A. Pöthig, F.E. Kühn, Kinetic studies of fluorinated aryl molybdenum(II) tricarbonyl precursors in epoxidation catalysis, Catal. Sci. Technol. 6 (2016) 4970–4977, https://doi.org/10.1039/c5cy02220g. [7] J. Santamaría, E. Aguilar, Beyond Fischer and Schrock carbenes: non-heteroatomstabilized group 6 metal carbene complexes – a general overview, Org. Chem. Front. 3 (2016) 1561–1588, https://doi.org/10.1039/C6QO00206D. [8] N. Iwasawa, Group 6 Metal Vinylidenes in Catalysis (Cr, Mo, W), in: C. Bruneau, P. Dixneuf (Eds.), Met. Vinylidenes Allenylidenes Catal. From React. to Appl. Synth, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2008pp. 159–191. doi:10.1002/9783527622870.ch5. [9] S. Figueiredo, A.C. Gomes, J.A. Fernandes, F.A.A. Paz, A.D. Lopes, J.P. Lourenço, M. Pillinger, I.S. Gonçalves, Bis(pyrazolyl)methanetetracarbonyl-molybdenum(0) as precursor to a molybdenum(VI) catalyst for olefin epoxidation, J. Organomet. Chem. 723 (2013) 56–64, https://doi.org/10.1016/j.jorganchem.2012.09.019. [10] C.-H. Cheng, R.-Y. Guo, Q. Cui, H.-B. Song, L.-F. Tang, Synthesis and catalytic activity of N-heterocyclic carbene metal carbonyl complexes based on 1-[2-(pyrazol1-yl)phenyl]imidazole, Transit. Met. Chem. 40 (2015) 297–304, https://doi.org/10. 1007/s11243-015-9917-2. [11] A.C. Gomes, P. Neves, S. Figueiredo, J.A. Fernandes, A.A. Valente, F.A. Almeida Paz, M. Pillinger, A.D. Lopes, I.S. Gonçalves, Tris(pyrazolyl)methane molybdenum tricarbonyl complexes as catalyst precursors for olefin epoxidation, J. Mol. Catal. A Chem. 370 (2013) 64–76, https://doi.org/10.1016/j.molcata.2012.12.010. [12] Z. Wang, S.W.B. Ng, L. Jiang, W.J. Leong, J. Zhao, T.S.A. Hor, Cyclopentadienyl molybdenum(II) N, C-chelating benzothiazole-carbene complexes: Synthesis, structure, and application in cyclooctene epoxidation catalysis, Organometallics 33 (2014) 2457–2466, https://doi.org/10.1021/om401128z. [13] V.V. Krishna Mohan Kandepi, J.M.S. Cardoso, B. Royo, N-heterocyclic carbenebased molybdenum and tungsten complexes as efficient epoxidation catalysts with H2O2 and tert-butyl hydroperoxide, Catal. Lett. 136 (2010) 222–227, https://doi. org/10.1007/s10562-010-0332-1.
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