Catalytic Reduction of Nitriles by Polymethylhydrosiloxane Using a

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Catalytic Reduction of Nitriles by Polymethylhydrosiloxane Using a Phenalenyl-Based Iron(III) Complex Shyamal Das,† Hari Sankar Das,† Bhagat Singh, Rahul Koottanil Haridasan, Arpan Das, and Swadhin K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and ResearchKolkata, Mohanpur 741246, India

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S Supporting Information *

and toxic metals with earth-abundant, benign, and cheaper metals such as Fe and Co for hydrosilylation reaction of nitriles has drawn considerable attention in recent years but these studies used expensive silanes such as HSiEt3, HSiMe3, (OEt2)MeSiH, etc. (Scheme 1).56−62 Boron-catalyzed reduction of nitriles was also reported using expensive silane Et2SiH2.63

ABSTRACT: The reduction of nitriles to primary amines using an inexpensive silane such as polymethylhydrosiloxane (PMHS) is an industrially important reaction. Herein we report the synthesis of an earth-abundant Fe(III) complex bearing a phenalenyl-based ligand that was characterized by mass spectroscopy, elemental analysis, cyclic voltammetry, and single-crystal X-ray diffraction. The complex showed excellent catalytic activity toward reduction of aromatic, heteroaromatic, aliphatic, and sterically crowded nitriles to produce primary amines using polymethylhydrosiloxane (PMHS).

Scheme 1. (a) Earlier Catalytic Methods of Nitrile Reduction Using Earth-Abundant Metals and Various Expensive Hydrosilanes;56−62 (b) Use of Cost Effective PMHS and an Fe(III)-Based Catalyst for Reduction of Nitriles to Amines

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atalytic reduction of amide, imine, and nitrile via hydrosilylation is a powerful synthetic tool for the synthesis of amine.1−18 There are several natural products, biologically active compounds, agrochemicals, pharmaceuticals, dyes, pigments, and polymers those contain amine molecules in their core.19−28 In this context, the synthesis of primary amines by the reduction of nitriles is a sustainable approach as nitriles are readily available from different natural sources.29 However, nitrile reduction is thermodynamically challenging owing to the high bond dissociation energy of the robust CN bond (179.3 kcal/mol) and moderate electrophilic character of the sp carbon.30 The conventional reductions of nitriles utilize reactive metal hydrides as reductants.31−33 In these approaches, stoichiometric amounts of reagents (LiAlH4, BH3, AlH3, and LiBEt3H) are used, which generate a stoichiometric amount of waste metal salts. This makes the process economically and environmentally less attractive. Previously, several catalytic systems were developed for hydrogenation of nitriles using molecular hydrogen (H2) to produce primary amines.34−46 Alternatively, the catalytic hydrosilylation of nitriles using various silanes as hydride sources has drawn considerable attention because of their stability, low toxicity, and the additional possibility to optimize the reaction by the proper choice of the silanes based on their reactivity. The general order of reactivity of various silanes is PMHS < Me(OEt)2SiH < (EtO)3SiH < Ph3SiH < Ph2SiH2 < PhSiH3. However, the price and availability of different silanes vary substantially.47 PMHS (polymethylhydrosiloxane) not only is economically attractive but also is a byproduct of the silicone industry and environmentally benign. However, most of the catalysts reported for hydrosilylation reaction of nitriles are based on precious metals such as Rh, Re, Ru, and Ir.48−55 In these studies, only a limited number of aromatic nitriles was reduced. Replacing these expensive, rare, © XXXX American Chemical Society

Calas and co-workers first reported catalytic hydrosilylation of nitriles using ZnCl2.56 In 1985, Murai and co-workers demonstrated that conversion of aromatic nitriles and few aliphatic nitriles into their corresponding amines is possible in the presence of Co2(CO)8 as a catalyst.57,58 In 2012, Beller and coworkers successfully achieved the reduction of aromatic and aliphatic nitriles using 10 mol % Fe(OAc)2 and 20 mol % free ligand as the catalyst combination in the presence of an expensive hydrosilane (OEt)2MeSiH at 100 °C.59 Nakazawa’s group reported an iron-complex-catalyzed reduction of nitrile to their corresponding amines requiring an excess amount of nitrile, which is considered as a drawback for scaling up the synthesis of amine industrially.60,61 Recently, Nagashima and co-workers Received: May 11, 2019

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DOI: 10.1021/acs.inorgchem.9b01377 Inorg. Chem. XXXX, XXX, XXX−XXX

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in iron(III) complex of the salen ligand.74 The O−Fe−N bond angles in 1 [O2−Fe1−N2, 86.61(8)°; O1−Fe1−N1, 86.85(8)°] are also similar to the bond angles found in the Fe−salen complex [O2−Fe1−N2, 85.89(8)°; O1−Fe1−N1, 86.54(8)°].74 The cyclic voltammetric study of complex 1 exhibits three reduction waves those are quasi-reversible appearing at −0.85, −1.75, and −2.1 V, respectively, versus the Ag/AgCl as a reference electrode in DMF solution using tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte, as shown in Figure 1c. The first reduction at −0.85 V may be accounted for the Fe(III)/Fe(II) redox couple while reductions at the potentials −1.77 and −2.1 V may be attributed to ligandbased successive reductions leading to L → [L•] → [L••], as also observed in a previously reported Fe(III) phenalenyl-based complex.68 Next, we tested the catalytic activity of complex 1 for the reductive hydrosilylation of nitriles to primary amines using PMHS (4 equiv) as a hydride source, KOtBu (10 mol %), and 1 (5 mol %) at 70 °C in 2 mL of THF for 12 h, where benzonitrile (2) was used as a model substrate. The benzylamine was isolated as its hydrochlorinated salt (2a) on treatment with 1 M methanolic HCl solution in Et2O. It resulted in 90% isolated yield of benzylamine salt (Table S3, entry 2). In the absence of 1, only a trace amount of product was formed (Table S3, entry 5) which confirms the role of 1. Further, when the catalytic reaction was carried out using 5 mol % FeCl3 in the absence of catalyst 1, it formed the desired product only in less than 10% yield (Table S3, entry 11). The use of various other iron precursors gave 5−20% yield (Table S3, entries 12−18), validating the role of catalyst 1. Furthermore, we tested several combinations of the catalyst loading, amount of KOtBu, temperature, time, silane, additive, and solvent (Table S3), resulting in no further improvement in the yield (details in Table S3). Interestingly, PMHS results in almost 90% conversions. As PMHS is an inexpensive silane and it is environment-friendly, we exploited PMHS as the hydrosilane reagent for further substrate scope exploration. Under the optimized reaction conditions (THF, 70 °C, 12 h, 4 equiv of PMHS, 5 mol % precatalyst, and 10 mol % KOtBu), the substrate scope was explored toward both aromatic and aliphatic nitriles (Table 1). As shown in Table 1, aromatic nitriles bearing electron-donating groups, such as 4-methylbenzonitrile, 2methylbenzonitrile, 3-methylbenzonitrile, 4-tert-butylbenzonitrile, and 4-methoxybenzonitrile were reduced to their corresponding primary amine salts, 3a−7a, respectively, in high yields (90−98%, Table 1, entries 3a−7a). Sterically hindered substrates such as 2,6-dimethoxybenzonitrile, 2,4,6-trimethoxybenzonitrile, 2,4,6-trimethylbenzonitrile, and 2,3,4,5,6-pentamethylbenzonitrile were converted to their corresponding substituted primary amine salts in slightly lower yields (63− 90%, Table 1, entries 8a−11a). Benzonitriles substituted with the electron-withdrawing groups including fluoro-, chloro-, bromo-, and trifluoromethyl groups proceeded with moderate yields (55−68%, Table 1, entries 12a−17a). Under the same reaction condition, 1-naphthonitrile was also reduced to 1-naphthylmethylamine (62%, Table 1, entry 18a). The methodology was further tested for the scope of heteroaromatic 3-pyridine carbonitrile and to our delight, 3picolylamine was obtained with an overall conversion of 55% (Table 1, entry 19a). Catalytic reductive hydrosilylation of benzonitrile substrates containing both electron-donating and -withdrawing groups proceeded with excellent yields (80−90%, Table 1, entries 20a−22a). Finally, the reaction was carried out in the presence of more challenging cyclic and acyclic aliphatic

successfully reduced nitriles to primary amines using tetramethyldisiloxane (TMDS) as the hydride source and Co(OPiv)2 (Piv = COtBu) as a catalyst.62 In this context, we have developed an Fe(III) complex bearing a phenalenyl ligand that can successfully convert nitrile into primary amine via hydrosilylation in the presence of a less expensive silane PMHS. To the best of our knowledge, to date, there have been no reports on nitrile reduction using the most economically and environmentally attractive combinations such as PMHS and an Fe-based catalyst. The current work addresses this issue for the first time. As a part of our ongoing interest in developing chemistry of phenalenyl-based molecules,64−72 recently our group has developed a new multidentate phenalenyl-based ligand H2L [9,9′-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1one)],64 bearing two N−H groups, which can form two sixmembered chelate and one five-membered chelate rings around the metal upon coordination. Treatment of H2L with FeCl3 salt in a 1:1 ratio in a mixture of DMF and ethanol (4:1 v/v) under nitrogen atmosphere led to the formation of a pentacoordinated black microcrystalline complex [Fe(L)Cl] (1) in 58% yield (Figure 1a). The complex (1) was characterized by an analytical

Figure 1. (a) Synthetic scheme of [FeIII(L)Cl] (1). (b) ORTEP view of [FeIII(L)Cl]. (c) CV of 1 in DMF acquired with a glassy carbon working electrode, Pt-wire counter electrode, and Ag/AgCl reference electrode.

method including elemental analysis, ESI-MS (Figure S1) and single-crystal X-ray diffraction studies (Figure 1b, Tables S1 and S2). Single crystals of complex 1 suitable for X-ray diffraction were obtained by layer diffusion between the concentrated solution of DMF and diethyl ether at room temperature for one week. An ORTEP representation of the molecular structure of complex 1 is shown in Figure 1b. Complex 1 displays a square pyramidal structure where the Fe(III) ion is coordinated by the tetradentate ligand and one chloride (Cl−) ion. According to the X-ray diffraction crystal data, a = 10.2502(3) Å, b = 11.5898(4) Å, c = 18.3304(7) Å, α = 90°, β = 103.197(4)°, and γ = 90° are indicative of a monoclinic unit cell. The Fe−Cl bond length in complex 1 was determined as 2.249 Å, which can be compared with the literature data reported earlier for a similar Fe(III) complex (Fe−Cl, 2.232 Å) bearing a salen ligand.73 The Fe−N and Fe−O bond lengths observed in 1 [Fe1−N1, 2.026(2) Å; Fe1−N2, 2.028(2) Å] and [Fe1−O1, 1.9113(18) Å; Fe1−O2, 1.9143(18) Å] are comparable with those of the Fe−N and Fe− O bond lengths [Fe1−N1, 2.115(2) Å; Fe1−N2, 2.086(2)Å] and [Fe1−O1, 1.8522(18) Å; Fe1−O2, 1.9035(18) Å] observed B

DOI: 10.1021/acs.inorgchem.9b01377 Inorg. Chem. XXXX, XXX, XXX−XXX

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reacts with KOtBu to generate a reactive Fe−alkoxide complex,79,80 which in turn reacts with PMHS, generating in situ the hypervalent “ate” species (active silane). The active silane serves as the hydride donor, which facilitates the formation of the Fe−H complex.59,78 The intermediate Fe−H, in turn, can reduce the nitrile into an amine following the pathway proposed by Beller and co-workers earlier.59 Despite our repeated attempts, we could not isolate or capture such a paramagnetic Fe(III) hydride complex (d5 ion). The stoichiometric reaction of 1 with KOtBu in the presence of PMHS led to a drastic color change, forming a deep reddish-violet Fe(III) complex, which could not be crystallized. In summary, we have developed an iron(III) complex based on ligand bearing phenalenyl backbone for the catalytic reduction of nitrile via hydrosilylation using economically attractive hydrosilane PMHS. To our delight, aromatic, heteroaromatic, and sterically crowded aromatic and aliphatic nitriles yielded corresponding primary amine salts in good to excellent yield using the most inexpensive silane PMHS. Thus we have developed an affordable strategy, which may be an alternative way to the existing methods for the reduction of nitriles. Further investigations regarding the isolation of catalytically active species to fully understand reaction mechanism are currently underway in our laboratory.

Table 1. Iron(III)-Catalyzed Reduction of Nitriles Using PMHS: Exploring Substrate Scopea

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ASSOCIATED CONTENT

S Supporting Information *

t

Reaction conditions: 1 (5 mol %), KO Bu (10 mol %), nitriles (0.5 mmol), and PMHS (4 equiv). Hydrolysis was performed using NaOH (1 mL, 1 M) in THF (2 mL). Isolated yield of crystallized HCl salt based on substrates. [b] At 80 °C. [c] 0.25 mmol substrate at 80 °C.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01377. General information, preparation details, ESI MS of 1, crystal data, bond distances and angles, general method for nitrile reduction, optimization table of the base, solvent, and silanes for catalytic nitrile reduction, description of tolerance experiments, and NMR spectra (PDF)

nitriles and industrially important adiponitrile, which were reduced to their corresponding primary amine salts 23a (58%), 24a (68%), 25a (76%), 26a (55%), and 27a (48%), respectively (Table 1, entries 23a−27a). In the case of cinnamonitrile, it gave 56% 3-phenylprop-2-en-1-amine salt (28a) under the optimized conditions, indicating that the vinylic double bond remains intact. Our results demonstrate that this methodology is chemoselective toward −OCH3, −Cl, −F, −Br, −CF3, and CC (Table 1, entries, 7a−9a, 12a−17a, and 28a). However, we found that other functional groups such as aldehyde, ketone, and ester are not tolerated under the optimized reaction conditions (Scheme S1). Unfortunately, our method failed to reduce the nitrile substrates containing −NO2, −CONH2, −COOH, and −NH2 functional groups (Table S4, entries 1−4). Further, we performed a catalytic reduction of benzonitrile in the presence of mercury (25 equiv with respect to 1). The addition of mercury could not arrest the reaction, as it yielded 86% expected product, which may suggest that the reaction can proceed through a homogeneous pathway75,76 (Scheme S2). Next, we checked if the reaction follows a radical pathway. When a radical scavenger TEMPO was added (two runs with 1 and 2 equiv of TEMPO) to the reaction mixture, it could not shut down the reaction completely (65% and 60% conversion were observed, respectively), which is indicative of a nonradical pathway (Scheme S3). In 2014, Nikonov and co-workers reported that PMHS can be converted into “active silane” in the presence of KOtBu.77,78 Beller and co-workers have further demonstrated that benzonitrile can be reduced using (OEt)2MeSiH as silane through an Fe−H intermediate.59 On the basis of this information, we propose that the reduction mechanism follows a nonradical pathway and the complex 1

Accession Codes

CCDC 1889620 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Swadhin K. Mandal: 0000-0003-3471-7053 Author Contributions †

S.D. and H.S.D. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the SERB (DST), India (Grant No. EMR/2017/ 000772). S.D. and H.S.D. are thankful to SERB (DST) for NPDF-Fellowships (PDF/2016/000275andPDF/2017/ 001355). B.S., R.K.H., and A.D. are thankful to IISER-Kolkata for their research. C

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DOI: 10.1021/acs.inorgchem.9b01377 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01377 Inorg. Chem. XXXX, XXX, XXX−XXX