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Selective Hydrogenation of Nitriles to Secondary Imines Catalyzed by an Iron Pincer Complex Subrata Chakraborty, and David Milstein ACS Catal., Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017
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ACS Catalysis
Selective Hydrogenation of Nitriles to Secondary Imines Catalyzed by an Iron Pincer Complex Subrata Chakraborty and David Milstein* Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, 76100, Israel ABSTRACT: Selective hydrogenation of nitriles to secondary imines catalyzed by an iron complex, the pincer complex (iPr-‐PNP)Fe(H)Br(CO), in the presence of catalytic base, is reported. A wide range of (hetero)aromatic and aliphatic ni-‐ triles are hydrogenated to the corresponding secondary imines under mild conditions. KEYWORDS: Iron, nitriles, secondary imines, hydrogenation, pincer.
Catalytic hydrogenation of nitriles constitutes an important methodology in industry and academia. Imines and amines that are generally formed serve as important intermediates and pre-‐ cursors in the synthesis of various natural products, agrochemi-‐ cals, dyes, pigments, polymers and pharmaceuticals.1 However, nitrile hydrogenation often leads to mixtures of primary-‐, sec-‐ ondary-‐ and even tertiary-‐ amines via imine intermediates, pre-‐ senting crucial selectivity issues (Scheme 1).2 Therefore the dis-‐ covery of a catalyst for selective hydrogenation of nitrile to either of these products is desirable.
Berke’s group reported homogeneous hydrogenation of nitriles to form secondary imines as major products using molybdenum and tungsten amides bearing the so-‐called “MACHO” PNP pin-‐ cer ligand.18 These reactions proceed at high temperature and pressure (140 0C, 60 bar H2), and high catalyst loadings (5 mol%), resulting in a mixture of primary amine, intermediate imine and secondary imine (major) products, while selectivity towards secondary imine was obtained only at low conversions. The only hydrogenation of nitriles to secondary imines catalyzed by a base-‐metal complex was reported by García using a nickel cata-‐ lyst.19 That system requires high temperatures (140-‐180 0C) and is limited to mono-‐ and dicyano-‐ benzene derivatives. To our knowledge, homogeneous hydrogenation of nitriles to selectively form secondary imines catalyzed by iron was not reported so far.
Scheme 1. Possible products in nitrile hydrogenation.
Conventional methods for reduction of nitriles involve stoichio-‐ metric amounts of metal hydrides, or hydrosilanes.3 Unfortu-‐ nately, these routes are not environmentally benign as they pro-‐ duce copious waste. Heterogeneous catalysts based on Co, Ni and Pd, commonly used for nitrile hydrogenation in industry,4 suffer from low selectivity towards a particular product and low functional group tolerance. Typically, homogeneous catalysts based on precious metals such as Ru, Rh, Ir, and Re are applied in the hydrogenation of nitriles.5-‐7 Replacement of precious metal-‐based catalysts by complexes of earth abundant, low-‐toxicity first row base-‐metals is a topic of much current interest.8 Indeed, recent years have witnessed much progress in the development of homogeneous catalysts based on earth-‐abundant base-‐metals.9-‐12 Of prime interest is the use of iron complexes since iron is generally less toxic than noble metals and it is the most abundant metal on the earth crust. Iron catalyzed hydrogenation of various substrates13 including esters14 and amides15 was reported by a few groups, including ours. Selective catalytic hydrogenation of nitriles to primary amines by iron complexes (Figure 1)16, as well as by Mn10a and Co12i complexes, was also demonstrated. The nitrile group is an important functionality in various natural and synthetic organic compounds, including pharmaceuticals, and it can be further processed by catalytic hydrogenation or hydrolysis.17 In addition to the possibility of its hydrogenation to form primary amines, an interesting but challenging goal is the direct hydrogenation to selectively form secondary imines.
Figure 1. Fe-‐based catalysts for homogeneous nitrile hydrogena-‐ tion/hydrogenative cross-‐coupling with amine. In fact, even reports on precious metal-‐catalyzed selective hy-‐ drogenation of nitriles to secondary imines are rare. Sabo-‐ Etienne observed formation of N-‐benzylidene-‐1-‐ phenylmethaneamine during the hydrogenation of benzonitrile in the absence of a solvent catalyzed by a Ru system.5b Our group, and recently Prechtl and coworkers, reported hydrogena-‐ tion and hydrogenative coupling of nitriles and amines to give secondary self-‐coupled imines and cross-‐imines as major prod-‐ ucts, by Ru-‐PNN20a and Ru-‐PNP20b catalysts, respectively. Very recently we reported the hydrogenative cross-‐coupling of nitriles and amines to form secondary aldimines under mild conditions (10-‐20 bar H2, 60 °C) using the complex Fe(iPr-‐ PNP)(H)Br(CO) (1) in the presence of a catalytic amount of base (Figure 1).21 Herein we employ complex 1 and catalytic base for the selective hydrogenation of nitriles to secondary aldimines. The reaction proceeds under relatively mild conditions (90°C, 30 bar H2).
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Catalytic hydrogenation of benzonitrile using complex 1 to give selectively N-‐benzyl benzaldimine was initially chosen as a mod-‐ el system. The influence of temperature, hydrogen pressure, various additives, and different solvents was examined. Selected optimization experiments are shown in Table 1. Using NaHBEt3 (1 mol%), and complex 1 (1 mol%) under 60 bar H2 at 65 °C in THF, complete conversion of benzonitrile was observed after 38h Table 1. Optimization of the reaction conditions for the hydrogenation of benzonitrile catalyzed by 1.
En-‐ a try
Solvent
1 2 3 4 5 6 7 8 9 10 11 12 c 13 14
THF THF THF THF THF THF THF Dioxane C6H6 EtOH EtOH C6H6 iPrOH C6H6
Additives (mol%)
Time (h)
NaHBEt3 (1) NaHBEt3 (1) NaHBEt3 (1) KHMDS (1) tBuOK(1) tBuOK(1) -‐ tBuOK (1) tBuOK (1) tBuOK (1) tBuOK(1) tBuOK(1) tBuOK (1) tBuOK(1)
38 20 20 10 10 5 10 10 10 10 5 5 5 18
Tem p (°C) 65 90 135 90 90 90 90 90 90 90 90 90 90 90
H2 (ba r) 60 60 60 60 60 60 60 60 60 60 60 60 60 30
b
Conv (%)
>99 >99 >99 84 >99 >99 00 >99 >99 >99 >99 >99 >99 >99
Yield b
(%) 64 98 71 40 97 53 00 96 98 98 97 98 46 93
a
Conditions: benzonitrile (1 mmol), 1 (0.01 mmol) additive (1 equiv. b relative to 1), and solvent (2 mL), heated in an autoclave. yields and conversions determined by GC-‐MS analysis using m-‐xylene as inter-‐ nal standard. Differences in conversions of benzonitrile and yields of N-‐benzylidenebenzylamine indicate formation of benzaldimine and its trimerised N,N-‐di(phenylmethylidene)phenylmethanediamine C product. 53% benzylamine formation was observed.
(Table 1, entry 1) as revealed by GC-‐MS. Surprisingly, no primary benzylamine, or dibenzylamine were detected by GC-‐MS. N-‐ benzyl benzaldimine was obtained in 64% yield. The partially hydrogenated product benzaldimine was also detected by GC-‐ MS, appearing as a broad signal along with its trimerized prod-‐ uct N,N-‐di(phenylmethylidene)phenylmethanediamine (hydro-‐ benzamide) (Scheme 2).22 The hydrobenzamide was character-‐ ized by 1H NMR, showing a typical resonance shift at δ = 5.7 ppm for the methyne proton and at 8.3 ppm for the imine CH proton in a 1:2 ratio. In the 13C{1H} NMR spectra, the methyne carbon resonates at δ = 92.7 ppm and the imine carbon at δ = 160.7 ppm (see Supporting Information (SI)). Scheme 2. Products detected by GC-‐MS and 1H NMR in the hydrogenation of benzonitrile at 65°C catalyzed by 1 (Table 1, entry 1). Significantly, increasing the temperature to 90 °C under analo-‐ gous reaction conditions resulted in 99% consumption of ben-‐ zonitrile, yielding 98% of N-‐benzyl benzaldimine after 20 h (Ta-‐ ble 1, entry 2). Although benzylamine was not detected, it is like-‐ ly formed as an intermediate, followed by its attack on the in-‐ termediate imine to form a gem-‐diamine intermediate which liberates ammonia to yield the desired secondary aldimine, as
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shown in Scheme 1. It is also noteworthy that no hydrogenation of the product imine took place. The selectivity towards the sec-‐ ondary imine depends strongly on the reaction temperature and pressure. At higher temperature (135 °C) the selectivity dropped and a lower yield of N-‐benzylidenebenzylamine (71%) was ob-‐ tained (Table 1, entry 3) under otherwise the same conditions. Exploring the effect of various additives, KHMDS and tBuOK were employed under similar reaction conditions. Using the base KHMDS (1 mol%), a lower yield of N-‐benzyl benzaldimine (40%) was obtained, in comparison to when NaHBEt3 was used (Table 1, entry 4). tBuOK turned out to be the best additive, resulting in 97% yield of N-‐benzyl benzaldimine after just 10 h under analo-‐ gous conditions (Table 1, entry 5). In the absence of base, em-‐ ploying pre-‐catalyst 1 (1 mol%) no hydrogenation of benzonitrile took place (Table 1, entry 7), indicating that one equivalent of Table 2. Hydrogenation of nitriles to secondary imines cat-‐ alyzed by 1. En-‐ a try
Substrate
Product
1
2
c
b
b
Time (h)
Conv (%)
Yield (%)
1 mol % 1
99
97
2
99
93
3
c
1
99
91
4
d
2
18
81
52
5
d
1
24
>99
79
6
d
4
36
>99
56
7
d
4
36
59
28
8
4
14
>99
99
9
4
12
>99
99
10
d
4
16
>99
70
11
d
4
14
80
61
12
8
36
61
61
13
8
36
32
32
14
8
36
69
69
15
8
36
11
11
a
Conditions: benzonitrile (1-‐0.125 mmol), 1 (0.01 mmol), tBuOK (1 equiv. relative to 1), and 2 ml C6H6 (1 mL for 4 mol% and 8 mol% cat b loading), heated in an autoclave at 90 °C under 30 bar H2. yields and 1 conversions determined by GC-‐MS and H NMR analysis using m-‐ c d xylene, or toluene as internal standard. isolated yield. Differences in conversions of nitriles and yields of secondary imines indicate formation of partially hydrogenated intermediate imines and their trimerised products.
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ACS Catalysis base (or NaHBEt3) (relative to 1) is required to generate the ac-‐ tive catalyst. Regarding solvent optimization, dioxane, benzene or EtOH all gave exclusively N-‐benzylidenebenzylamine (98%) in less than 10 h using 1 (1 mol%) and, tBuOK (1 mol%) at 90 °C and 60 bar H2 (Table 1, entries 8-‐12). However, using isopropanol as solvent, 53% of benzylamine formation was observed along with N-‐benzylbenzaldimine (46%) (Table 1, entry 13) after 5 h under otherwise similar conditions. Gratifyingly, lowering the H2 pressure to 30 bar in the presence of tBuOK (1 mol%) and 1 (1 mol%) using benzene as solvent furnished 93% N-‐benzyl ben-‐ zaldimine (Table 1, entry 14) in less than 18 h at 90 °C. Using the optimized reaction conditions (C6H6, 90 °C, 30 bar H2, 1 mol% tBuOK and 1 mol% 1), the generality of this iron-‐ catalysed hydrogenation of nitriles was explored. As shown in Table 2, benzonitriles bearing electron-‐donating substituents at the para positions, including 4-‐methoxybenzonitrile, 4-‐ methylbenzonitrile, 4-‐N,N-‐ dimethylbenzonitrile, and meta-‐ substituted 3-‐methylbenzonitrile were hydrogenated to their corresponding secondary aldimines in excellent conversions with good selectivities (Table 2, entries 1-‐4). Catalytic hydro-‐ genation of benzonitriles bearing electron withdrawing substitu-‐ ents at the para positions, including 4-‐fluorobenzonitrile and 4-‐ chlorobenzonitrile, also afforded selectively the corresponding secondary imines, although higher catalyst loading (4 mol%) was required in the case of the latter. No hydro-‐dehalogenation was observed (Table 2, entries 5 and 8). However, the meta-‐ and ortho-‐ substituted 3-‐fluorobenzonitrle and 2-‐fluorobenzonitrile yielded the corresponding secondary imines in moderate yields (Table 2, entries 6 and 7) along with the formation of 43% and 31% of partially hydrogenated inter-‐ mediate 3-‐fluorobenzaldimine and 2-‐fluorobenzaldimine, re-‐ spectively which appeared as their trimeric products, as shown by 1H NMR. Hydrogenation of p-‐bromobenzonitrile, applying 1 (4 mol %) as pre-‐catalyst and 4 mol% tBuOK in THF resulted in 99% yield of N-‐(4-‐ bromobenzylidene)-‐1-‐(4-‐ bromophenyl)methaneamine (Table 2, entry 9), showing that even bromo substituents are tolerated. Hydrogenation of 2-‐ naphthonitrile resulted in the formation of N-‐(naphthylidene)-‐1-‐ (naphthyl)methanamine in70% yield after 16 h (Table 2, entry 10). The scope of the reaction was further probed by employing the heterocyclic nitrile 3-‐pyridinecarbonitrile, furnishing N-‐(3-‐ pyridinylmethylene)-‐3-‐pyridinemethanamine in 61% yield after 14 h (Table 2, entry 11). Catalytic hydrogenation of aliphatic nitriles bearing alpha hy-‐ drogen atoms is more challenging due to potential base-‐induced condensation side reactions. In addition, aliphatic imines are inherently less stable. Employing the Fe-‐PNP complex 1, catalytic hydrogenation of aliphatic nitriles progressed sluggishly com-‐ pared to aromatic nitriles, and catalyst loading had to be in-‐ creased. Thus, valeronitrile and butyronitrile were hydrogenated to the corresponding secondary imines in 61% and 32% yield, respectively, using 8 mol% catalyst 1 and NaHBEt3 (8 mol%) under 30 bar H2 at 90 °C (Table 2, entries 12 and 13). Hydrogena-‐ tion of the secondary alkyl nitrile cyclohexylcarbonitrile resulted in formation of the corresponding secondary imine in 69% yield (Table 2, entry 14). Using isobutyronitrile under similar condi-‐ tions, only 11% conversion to the corresponding secondary imine was noticed (Table 2, entry 15). The expected lower stability of the generated alkyl imine intermediate might be a reason for the inferior performance. Regarding the mechanism of the nitrile hydrogenation, we have previously reported21 that complex 1 reacted with 1 equiv. of tBuOK in THF at room temperature forming a deprotonated amido complex (iPr-‐PNP)Fe(H)(CO) 2 (Scheme 3) and reaction with 1 equiv. NaHBEt3 gives the cis-‐dihydridocarbonyl complex (iPr-‐PNP)Fe(H)2(CO) 3. The deprotonated amido complex 2 did
not react with benzonitrile at room temperature in C6D6, where-‐ as 3 did react with benzonitrile, regenerating the amido complex 2 and benzaldimine. Based on these previously observed detailed mechanistic results,21 a plausible outer-‐sphere mechanism is depicted in Scheme 3. Initially, complex 2, formed by reaction of base with complex 1, adds dihydrogen by metal–ligand cooperation to generate the cis-‐dihydrido complex 3, as previously reported.21 Complex 3 is likely in equilibrium with the unobserved trans dihydride com-‐ plex 3’, which is believed to be the active species. The imine in-‐ termediate is generated by hydride and proton transfer from 3’ to the nitrile group, either in a concerted or stepwise fashion and 3’ is converted to the amido complex 2. Similarly, in another catalytic cycle H2 is transferred to the imine intermediate to form the primary amine. This step is likely the rate limiting step, otherwise full hydrogenation to the amine would have taken place. As soon as the primary amine is formed, nucleophilic at-‐ tack by the amine on the reactive imine intermediate produces a gem-‐diamine, which liberates NH3 to give the desired secondary imine (Scheme 3).
Scheme 3. Plausible mechanism for the hydrogenation of nitriles to secondary imine catalyzed by iron. In conclusion, we have presented the first iron-‐catalyzed hydro-‐ genation of nitriles to selectively form secondary imines. The reaction proceeds at relatively low temperature and pressure (90 °C, 30 bar H2), under apparently neutral, homogeneous condi-‐ tions, using the pincer complex (iPr-‐PNP)Fe(H)Br(CO) 1 and a base (in an equimolar amount to Fe). No products of full hydro-‐ genation (primary or secondary amines) were observed. In con-‐ trast, nitrile hydrogenation catalyzed by a related “MACHO” ligand-‐based iron system developed by Beller16a afforded selec-‐ tively primary amines, providing another example as to the criti-‐ cal role that ligand modification may have.
AUTHOR INFORMATION Corresponding Author *E-‐mail:
[email protected].
Notes
The authors declare no competing financial interests.
ASSOCIATED CONTENT Experimental details of the catalytic experiments, GC-‐MS data of hydrogenation products, and NMR spectra.
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ACKNOWLEDGMENT This research was supported by the European Research Council (ERC AdG 692775) and by the Kimmel Center for Molecular Design. D.M. holds the Israel Matz Professorial Chair of Organic Chemistry. S.C. thanks the Swiss Friends of the Weizmann Insti-‐ tute of Science for a generous postdoctoral fellowship.
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