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Switching the Selectivity of Cobalt-Catalyzed Hydrogenation of Nitriles Huiguang Dai, and Hairong Guan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02645 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
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ACS Catalysis
Switching the Selectivity of Cobalt-Catalyzed Hydrogenation of Nitriles Huiguang Dai and Hairong Guan* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States ABSTRACT: Previous studies of base metals for catalytic hydrogenation of nitriles to primary amines or secondary aldimines focus on designing complexes with elaborate structures. Herein, we report “twin” catalytic systems where the selectivity of nitrile hydrogenation can be tuned by including or omitting the ligand HN(CH2CH2PiPr2)2 (iPrPNHP). Simply treating CoBr2 with NaHBEt3 generates cobalt particles, which can catalyze the hydrogenation of nitriles to primary amines with high selectivity and broad functional group tolerance. Ligating CoBr2 with iPrPNHP followed by the addition of NaHBEt3, however, forms a homogeneous catalyst favoring secondary aldimines for both hydrogenation and hydrogenative coupling of benzonitrile. KEYWORDS: nitriles, primary amines, secondary aldimines, cobalt catalysts, hydrogenation, hydrogenative coupling Hydrogenation of nitriles is an important process for the production of aliphatic amines that are indispensable to the plastics, textile and coating industries.1 The best-known examples are the hydrogenation of adiponitrile to hexamethylenediamine2 for making nylon 66 and the hydrogenation of fatty nitriles to fatty amines,3 which are surfactant precursors. Catalysts employed for nitrile hydrogenation are often heterogeneous, and the most common ones are PtO2, Pd-C, Raney® Ni and Raney® Co.4 Although the latter two are less expensive, hydrogenation conditions with these skeletal catalysts are more demanding, and in some cases, temperatures up to 180 °C and H2 pressures as high as 250 bar are required.1 A more challenging issue is how to control the selectivity. As illustrated in Scheme 1, the primary aldimine resulting from the first hydrogenation event can be intercepted by various nucleophiles generated during the hydrogenation reaction, leading to a non-selective process. To improve the selectivity for the primary amine (often the most desirable product), NH3, MOH (M = Na, K or Li) or HCl has been routinely added to suppress the side reactions.4
(A) nitriles to primary amines E2 P
CO Fe
PE2
H H H3B (E = iPr or Cy) 1 mol% [Fe] H2 (30 bar), 70-130 oC Beller (2014 & 2016)
Co
NCl N H Cl
Cl 2 mol% [Co] 2 mol% NaHBEt3 4.4 mol% NaOEt H2 (30 bar), 135 oC Milstein (2015)
py
Cy2P
N Cl
PCy2
P
L= PCy2
2 mol% [Co] 4 mol% NaHBEt 3 6 mol% KOtBu H2 (4 bar), 115 oC Fout (2017)
[Ni] or [Fe] R N R - NH3 [Fe] R' RCN + R'NH2 + H2 N - NH3 R
(C) switchable selectivity (this work)
4 mol% Co(acac)3 + 4.4 mol% L
N Co
(B) nitriles to secondary aldimines 2RCN + 3H2
H Br 3 mol% [Mn] 10 mol% NaOtBu H2 (50 bar), 120 oC Beller (2016)
1-5 mol% [Fe] 1-5 mol% NaHBEt 3 3-15 mol% KHMDS H2 (60 bar), 140 oC Milstein (2016)
N
RCH2NH2
(iPr)2 CO CO P Mn P(iPr)2 N
H
N 2P
[Fe], [Mn] or [Co]
(iPr)2 Br P Fe Br P(iPr)2 N
H
N
(tBu)
RCN + 2H2
10 mol% KOtBu H2 (30 bar), 80-140 oC Beller (2017)
(iPr)2 H P CO Fe Ni P(iPr)2 N H H P i Br ( Pr)2 1-8 mol% [Fe] 0.015-0.5 mol% [Ni] 1-8 mol% KOtBu o H2 (4 bar), 140-180 C H2 (10-30 bar), 60-90 oC García (2009) Milstein (2017) (iPr)2 P Ni P i ( Pr)2
H
(iPr)2 P
2 mol% CoBr2 + iPrPNHP 6 mol% NaHBEt 3 R R N P(iPr)2 110 oC homogeneous iPrPN HP = NH RCN + H2 (20-40 bar) 2 mol% CoBr2 6 mol% NaHBEt3 P(iPr)2 RCH2NH2 110-130 oC heterogeneous
Scheme 2. Examples for Nitrile Hydrogenation Catalyzed by a Base Metal A major breakthrough in this research area was made by Beller, who reported (EPNHP)FeH(CO)(BH4) as selective hydrogenation catalysts, converting nitriles to primary amines with excellent functional group tolerance (Scheme 2A).7 A four-coordinate iron bromide complex and a PNP pincer ligated manganese complex developed by Milstein8 and Beller,9 respectively, also exhibited high selectivity towards primary amines, albeit operating under harsher conditions. Similarly, the use of cobalt dichloride complexes supported by a PNN10 or CCC pincer ligand11 led to primary amines with high selectivity and functional group compatibility. In particular, the precatalyst developed by Fout was found to be bench stable and efficient under a remarkably low H2 pressure (4 bar).11 In a recent study, Beller reported a much simpler catalytic recipe that involved the mixing of Co(acac)3 with P(CH2CH2PCy2)3 and KOtBu, which led to the formation of primary amines.12
Scheme 1. Potential Products from Catalytic Hydrogenation of Nitriles Homogeneous catalysts potentially operate under milder conditions and at the same time address the selectivity issue. Numerous transition metal complexes have thus been studied for catalytic hydrogenation of nitriles,4b,5 although most catalysts are derived from precious metals such as Ru and Rh. It is not until very recently that complexes of base metals have been demonstrated as efficient and selective catalysts for nitrile hydrogenation (Scheme 2).6
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Developing catalysts specifically for the hydrogenation of nitriles to secondary aldimines has been less frequently pursued,13 likely due to the fact that aldimines are readily accessible from condensation of aldehydes with amines.14 However, cases can be made for the need to develop such process when the nitriles are more available than the corresponding aldehydes.15 In 2009, García reported that {(dippe)Ni(µ-H)}2 (Scheme 2B) catalyzed the hydrogenation of PhCN to PhCH=NCH2Ph with only a negligible amount of byproducts.16 In a more recent study, Milstein revealed that in the presence of KOtBu, a PNP pincer iron complex was effective for catalytic hydrogenation of nitriles to secondary aldimines with the general formula of RCH=NCH2R.17 Adding an external amine R'NH2 to the catalytic system led to the isolation of RCH=NR', which resulted from the trapping of RCH=NH by R'NH2 before RCH2NH2 could be produced.18 In this work, we demonstrate that selectivity of nitrile hydrogenation can be modulated through the addition or omission of a tridentate ligand iPrPNHP (Scheme 2C). Other ingredients of the catalytic mixture are CoBr2 and NaHBEt3, which are commercially available and inexpensive. The catalytic system with iPrPNHP added in the beginning is homogeneous, favoring the formation of secondary aldimines. However, without iPrPNHP, the catalytic system becomes heterogeneous, leading to primary amines as the major products. Recent studies of (iPrPNHP)CoCl2 (Co-1), (iPrPNHP)CoBr2 (Co-2) and their derivatives for catalytic hydrogenation of esters19 and transfer hydrogenation of alkynes20 and nitriles21 (with H3N•BH3) prompted us to examine their abilities to serve as precatalysts for nitrile hydrogenation.22,23 Utilizing these specific complexes is attractive because iPrPNHP is commercially available, and (iPrPNHP)CoX2 can be readily generated by mixing iPrPNHP with CoX2.19-21,24 Given Milstein’s success with the four-coordinate iron bromide complex (Scheme 2A), we were also curious about the reactivity of (iPrPNHP)FeBr2 (Fe-1),25 which was synthesized according to the literature procedure.26 We surmised that a base could tune the selectivity but might not be needed to make the cobalt or iron halide complexes catalytically active, as long as a sufficient amount of NaHBEt3 is present to convert them to hydride species. Guided by this hypothesis, we treated Co-1, Co-2 and Fe-1 with 3 equiv of NaHBEt3 (1.0 M solution in THF) and used the resulting mixtures to catalyze the hydrogenation of PhCN (eq 1). With Co1, after 4 h of reaction, 61% of PhCN was converted to PhCH=NCH2Ph. During the hydrogenation process, we did not observe any primary, secondary or tertiary amines or the diimine PhCH(N=CHPh)2. Under the same conditions, Co-2 was more active, converting 92% of PhCN to PhCH=NCH2Ph. In contrast, Fe-1 was completely inactive, resulting in a full recovery of PhCN. Hydrogenation of an aliphatic nitrile
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PhCH2CN was also tested with precatalyst Co-2; however, the desired secondary imine was obtained in only 28% NMR yield.
The high selectivity for PhCH=NCH2Ph indicates that PhCH=NH reacts with PhCH2NH2 at a rate much faster than its hydrogenation to PhCH2NH2. To test if an externally added amine could efficiently trap PhCH=NH, an equimolar mixture of cyclohexylamine and PhCN was subjected to similar hydrogenation conditions (Table 1). Using Co-2 as the precatalyst, a hydrogenative coupling product, Nbenzylidenecyclohexylamine, was identified as the major product (entry 1). PhCH=NCH2Ph was present in a minute quantity while PhCH2NH2 was not even detected, suggesting that hydrogenation of PhCH=NH was comparatively slow. In an attempt to simplify the procedure, Co-2 was replaced by a 1 : 1 mixture of CoBr2 and iPrPNHP. The in-situ generated precatalyst proved to be as effective and selective as the premade Co-2 (entry 2). Interestingly, changing the addition sequence by mixing CoBr2 with NaHBEt3 first before adding iPr PNHP generated a catalytic mixture selective for PhCH2NH2 (entry 3). Despite the presence of CyNH2, PhCH=NCy was not found from the hydrogenation products. PhCH=NCH2Ph was identified as a minor product, implying that the newly formed PhCH2NH2 was more reactive than CyNH2 towards PhCH=NH, likely due to an inner-sphere mechanism.27 While the first two reactions in Table 1 appeared to be homogeneous, the last one was clearly heterogeneous. Treatment of CoBr2 with NaHBEt3 immediately produced a black precipitate, which was found to be ferromagnetic as evidenced by its attraction to the stopped stir bar. This phenomenon indicated that CoBr2 was reduced to cobalt particles. Jacobi von Wangelin et al. recently used LiHBEt3 to accomplish a fast reduction of CoBr2 to cobalt particles, although they focused on the strategy of adding alkenes to stabilize the particles for catalytic hydrogenation of alkenes, ketones, imines and heteroarenes.28 Of particular interest was that added benzonitrile inhibited the alkene hydrogenation. In our work, control experiments showed that when iPrPNHP was left out, the resulting cobalt particles remained efficient for the hydrogenation of PhCN to PhCH2NH2 without compromising the selectivity.
Table 1. Hydrogenative Coupling of Benzonitrile with Cyclohexylaminea
entry
[Co]
1
Co-2
2
b
iPr
H
CoBr2/ PN P
conversion of PhCN (%)d
conversion of CyNH2 (%)d
yield for PhCH=NCH2Ph (%)d
yield for PhCH=NCy (%)d
yield for PhCH2NH2 (%)d
92
93
1
91
0
88
90
1
87
0
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ACS Catalysis 3c
CoBr2/iPrPNHP
>99
0
3
0
97
a
Reaction conditions: PhCN (1.0 mmol), CyNH2 (1.0 mmol), [Co] catalyst (0.020 mmol), NaHBEt3 (0.040 mmol, 1.0 M in THF) and tridecane (50 µL, GC internal standard) in THF (1.0 mL), 20 bar of H2, 100 oC, 20 h. bCoBr2 and iPrPNHP (1 : 1) were mixed first before adding NaHBEt3. cCoBr2 and NaHBEt3 were mixed first before adding iPrPNHP. dDetermined by GC.
the one catalyzed by particles generated from CoBr2. Under the same conditions, the commercially available cobalt particles (2 µm) and cobalt boride were completely inactive. Evaluation of other reducible metal salts showed an incomplete hydrogenation of PhCN with FeBr2 (11%) and NiBr2 (28%) while no reaction with MnCl2 and CuBr2. Further optimizations showed that hydrogenation of PhCN could be carried out in EtOH or under neat conditions, although the selectivity for PhCH2NH2 dropped to 70% or 88%, respectively.29 Hydrogenation in toluene was slower and less selective for PhCH2NH2 (88%). As expected, lowering the hydrogen pressure from 20 to 10 bar slowed down the hydrogenation and decreased the selectivity. The temperature could be lowered to 90 ºC; however, a longer reaction time (8 h) was needed. Beller recently developed a heterogeneous cobalt catalyst on an α-Al2O3 support for the hydrogenation of nitriles to primary amines.30 Preparation of the catalyst involved pyrolysis of Co(OAc)2•4H2O, 1,10-phenanthroline and αAl2O3 at 800 °C, and an aqueous ammonia solution must be added to achieve high selectivity. Considering our method
Based on the stoichiometry, 2 equiv of NaHBEt3 is needed to fully reduce Co(II) to Co(0). Decreasing the amount of NaHBEt3 from 4 mol% to 3 mol% under otherwise the same conditions resulted in a less selective catalyst, giving a mixture of PhCH2NH2 (73%), PhCH=NCH2Ph (23%) and (PhCH2)2NH (4%).29 To avoid any unintended partial reduction of CoBr2 that would erode the selectivity, we employed 3 equiv of NaHBEt3 in all our subsequent studies. The use of NaHBEt3 proved to be critical to achieving both high efficiency and high selectivity. Replacing it with a less powerful reductant (e.g., NaBH4 or H3N•BH3) resulted in 99
83
9
6
2 3
Particles B
–
>99
60
14
26
Particles C
–
>99
36
11
52
4
Particles B
6 mol% NaBr
>99
79
11
9
5
Particles B
6 mol% BEt3
>99
56
37
6
a Reaction conditions: PhCN (1.0 mmol), [Co] catalyst (0.020 mmol) and tridecane (50 µL, GC internal standard) in THF (1.0 mL), 20 bar of H2, 110 oC, 4 h. bDetermined by GC.
ACKNOWLEDGMENT
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(13) (a) Srimani, D.; Feller, M.; Ben-David, Y.; Milstein, D. Catalytic Coupling of Nitriles with Amines to Selectively Form Imines Under Mild Hydrogen Pressure. Chem. Commun. 2012, 48, 11853-11855. (b) Chakraborty, S.; Berke, H. Homogeneous Hydrogenation of Nitriles Catalyzed by Molybdenum and Tungsten Amides. ACS Catal. 2014, 4, 2191-2194. (c) Choi, J.-H.; Prechtl, M. H. G. Tuneable Hydrogenation of Nitriles into Imines or Amines with a Ruthenium Pincer Complex Under Mild Conditions. ChemCatChem 2015, 7, 1023-1028. (14) Belowich, M. E.; Stoddart, J, F. Dynamic Imine Chemistry. Chem. Soc. Rev. 2012, 41, 2003-2024. (15) Rabinovitz, M. Reduction of the Cyano Group. In The Chemistry of the Cyano Group; Rappoport, Z., Ed.; John Wiley & Sons: New York, 1970; Chapter 7, pp 307-340. (16) Zerecero-Silva, P.; Jimenez-Solar, I.; Crestani, M. G.; Arévalo, A.; Barrios-Francisco, R.; García, J. J. Catalytic Hydrogenation of Aromatic Nitriles and Dinitriles with Nickel Compounds. Appl. Catal., A 2009, 363, 230-234. (17) Chakraborty, S.; Milstein, D. Selective Hydrogenation of Nitriles to Secondary Imines Catalyzed by an Iron Pincer Complex. ACS Catal. 2017, 7, 3968-3972. (18) Chakraborty, S.; Leitus, G.; Milstein, D. Iron-Catalyzed Mild and Selective Hydrogenative Cross-Coupling of Nitriles and Amines to Form Secondary Aldimines. Angew. Chem. Int. Ed. 2017, 56, 2074-2078. (19) Junge, K.; Wendt, B.; Cingolani, A.; Spannenberg, A.; Wei, Z.; Jiao, H.; Beller, M. Cobalt Pincer Complexes for Catalytic Reduction of Carboxylic Acid Esters. Chem. Eur. J. 2018, 24, 10461052. (20) Fu, S.; Chen, N.-Y.; Liu, X.; Shao, Z.; Luo, S.-P.; Liu, Q. Ligand-Controlled Cobalt-Catalyzed Transfer Hydrogenation of Alkynes: Stereodivergent Synthesis of Z- and E-Alkenes. J. Am. Chem. Soc. 2016, 138, 8588-8594. (21) While (RPNHP)CoCl2 (R = Cy, iPr, tBu) catalyze the reduction of PhCN with H3N•BH3, more efficient cobalt catalysts bear a PNN pincer ligand with a nitrogen-based sidearm. For details, see: Shao, Z.; Fu, S.; Wei, M.; Zhou, S.; Liu, Q. Mild and Selective Cobalt-Catalyzed Chemodivergent Transfer Hydrogenation of Nitriles. Angew. Chem. Int. Ed. 2016, 55, 14653-14657. (22) Complex [(CyPNHP)CoCH2SiMe3]BArF4 (CyPNHP = HN(CH2CH2PCy2)2) was reported to catalyze the hydrogenation of alkenes, aldehydes, ketones and imines. For details, see: (a) Zhang, G.; Scott, B. L.; Hanson, S. K. Mild and Homogeneous CobaltCatalyzed Hydrogenation of C=C, C=O, and C=N Bonds. Angew. Chem. Ind. Ed. 2012, 51, 12102-12106. (b) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions. J. Am. Chem. Soc. 2013, 135, 8668-8681. (23) Complex [(iPrPNMeP)Co(CO)2]Cl (iPrPNMeP = MeN(CH2CH2PiPr2)2) and its derivatives were reported to catalyze the hydrogenation of CO2 to formate in the presence of DBU (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) and LiOTf. For details, see: (a) Spentzos, A. Z.; Barnes, C. L.; Bernskoetter, W. H. Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO2 Hydrogenation. Inorg. Chem. 2016, 55, 8225-8233. (b) Mills, M. R.; Barnes, C. L.; Bernskoetter, W. H. Influences of Bifunctional PNP-Pincer Ligands on Low Valent Cobalt Complexes Relevant to CO2 Hydrogenation. Inorg. Chem. 2018, 57, 1590-1597. (24) (a) Rozenel, S. S.; Kerr, J. B.; Arnold, J. Metal Complexes of Co, Ni and Cu with the Pincer Ligand HN(CH2CH2PiPr2)2: Preparation, Characterization and Electrochemistry. Dalton Trans. 2011, 40, 10397-10405. (b) Lagaditis, P. O.; Schluschaß, B.; Demeshko, S.; Würtele, C.; Schneider, S. Square-Planar Cobalt(III) Pincer Complex. Inorg. Chem. 2016, 55, 4529-4536. (25) Nguyen, D. H.; Morin, Y.; Zhang, L.; Trivelli, X.; Capet, F.; Paul, S.; Desset, S.; Dumeignil, F.; Gauvin, R. M. Oxidative Transformations of Biosourced Alcohols Catalyzed by EarthAbundant Transition Metals. ChemCatChem 2017, 9, 2652-2660. (26) Fillman, K. L.; Bielinski, E. A.; Schmeier, T. J.; Nesvet, J. C.; Woodruff, T. M.; Pan, C. J.; Takase, M. K.; Hazari, N.; Neidig,
The authors acknowledge the National Science Foundation (CHE-1464734) and the University of Cincinnati (Doctoral Enhancement Research Fellowship to H.D.) for support of this research.
REFERENCES (1) Roose, P.; Eller, K.; Henkes, E.; Rossbacher, R.; Höke, H. Amines, Aliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weiheim, Germany, 2015. DOI: 10.1002/14356007.a02_001.pub2. (2) Herzog, B. D.; Smiley, R. A. Hexamethylenediamine. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weiheim, Germany, 2012. DOI: 10.1002/14356007.a12_629.pub2. (3) (a) Barrault, J.; Pouilloux, Y. Synthesis of Fatty Amines. Selectivity Control in Presence of Multifunctional Catalysts. Catal. Today 1997, 37, 137-153. (b) Foley, P.; Kermanshahi pour, A.; Beach, E. S.; Zimmerman, J. B. Derivation and Synthesis of Renewable Surfactants. Chem. Soc. Rev. 2012, 41, 1499-1518. (4) (a) Volf, J.; Pašek, J. Hydrogenation of Nitriles. In Studies in Surface Science and Catalysis; Červený, L., Ed.; Elsevier: Amsterdam, 1986; Vol. 27, pp 105-144. (b) De Bellefon, C.; Fouilloux, P. Homogeneous and Heterogeneous Hydrogenation of Nitriles in a Liquid Phase: Chemical, Mechanistic, and Catalytic Aspects. Catal. Rev.-Sci. Eng. 1994, 36, 459-506. (c) Nishimura, S. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis; John Wiley & Sons: New York, 2001; pp 254-285. (d) Gomez, S.; Peters, J. A.; Maschmeyer, T. The Reductive Amination of Aldehydes and Ketones and the Hydrogenation of Nitriles: Mechanistic Aspects and Selectivity Control. Adv. Synth. Catal. 2002, 344, 1037-1057. (e) Banwell, M. G.; Jones, M. T.; Reekie, T. A.; Schwartz, B. D.; Tan, S. H.; White, L. V. RANEY® Cobalt – an Underutilised Reagent for the Selective Cleavage of C–X and N–O Bonds. Org. Biomol. Chem. 2014, 12, 7433-7444. (5) Bagal, D. B.; Bhanage, B. M. Recent Advances in Transition Metal-Catalyzed Hydrogenation of Nitriles. Adv. Synth. Catal. 2015, 357, 883-900. (6) Filonenko, G. A.; van Putten, R.; Hensen, E. J. M.; Pidko, E. A. Catalytic (De)Hydrogenation Promoted by Non-Precious Metals – Co, Fe and Mn: Recent Advances in an Emerging Field. Chem. Soc. Rev. 2018, 47, 1459-1483. (7) (a) Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.; Alberico, E.; Baumann, W.; Junge, H.; Junge, K.; Beller, M. Mild and Selective Hydrogenation of Aromatic and Aliphatic (Di)Nitriles with a Well-Defined Iron Pincer Complex. Nat. Commun. 2014, 5, 4111. (b) Lange, S.; Elangovan, S.; Cordes, C.; Spannenberg, A.; Jiao, H.; Junge, H.; Bachmann, S.; Scalone, M.; Topf, C.; Junge, K.; Beller, M. Selective Catalytic Hydrogenation of Nitriles to Primary Amines Using Iron Pincer Complexes. Catal. Sci. Technol. 2016, 6, 4768-4772. (8) Chakraborty, S.; Leitus, G.; Milstein, D. Selective Hydrogenation of Nitriles to Primary Amines Catalyzed by a Novel Iron Complex. Chem. Commun. 2016, 52, 1812-1815. (9) Elangovan, S.; Topf, C.; Fischer, S.; Jiao, H.; Spannenberg, A.; Baumann, W.; Ludwig, R.; Junge, K.; Beller, M. Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by WellDefined Manganese Pincer Complexes. J. Am. Chem. Soc. 2016, 138, 8809-8814. (10) Mukherjee, A.; Srimani, D.; Chakraborty, S.; Ben-David, Y.; Milstein, D. Selective Hydrogenation of Nitriles to Primary Amines Catalyzed by a Cobalt Pincer Complex. J. Am. Chem. Soc. 2015, 137, 8888-8891. (11) Tokmic, K.; Jackson, B. J.; Salazar, A.; Woods, T. J.; Fout, A. R. Cobalt-Catalyzed and Lewis Acid-Assisted Nitrile Hydrogenation to Primary Amines: A Combined Effort. J. Am. Chem. Soc. 2017, 139, 13554-13561. (12) Adam, R.; Bheeter, C. B.; Cabrero-Antonino, J. R.; Junge, K.; Jackstell, R.; Beller, M. Selective Hydrogenation of Nitriles to Primary Amines by Using a Cobalt Phosphine Catalyst. ChemSusChem 2017, 10, 842-846.
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ACS Catalysis M. L. Flexible Binding of PNP Pincer Ligands to Monomeric Iron Complexes. Inorg. Chem. 2014, 53, 6066-6072. (27) PhCH2NH2 may be intrinsically more nucleophilic than CyNH2; however, at the early stage of the reaction, the concentration of CyNH2 should be overwhelmingly higher than that of PhCH2NH2. We suspect that secondary aldimine formation takes place on cobalt and PhCH2NH2 “born” in the coordination sphere is more competitive. For nucleophilicity of primary amines measured using benzhydrylium ions Ar2CH+ as electrophiles, see: Kanzian, T.; Nigst, T. A.; Maier, A.; Pichl, S.; Mayr, H. Nucleophilic Reactivities of Primary and Secondary Amines in Acetonitrile. Eur. J. Org. Chem. 2009, 2009, 6379-6385.
(28) Sandl, S.; Schwarzhuber, F.; Pöllath, S.; Zweck, J.; Jacobi von Wangelin, A. Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations. Chem. Eur. J. 2018, 24, 34033407. (29) See Supporting Information. (30) Chen, F.; Topf, C.; Radnik, J.; Kreyenschulte, C.; Lund, H.; Schneider, M.; Surkus, A.-E.; He, L.; Junge, K.; Beller, M. Stable and Inert Cobalt Catalysts for Highly Selective and Practical Hydrogenation of C≡N and C=O Bonds. J. Am. Chem. Soc. 2016, 138, 8781-8788.
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