Letter Cite This: Org. Lett. 2019, 21, 287−291
pubs.acs.org/OrgLett
Hydrosilane Reduction of Nitriles to Primary Amines by Cobalt− Isocyanide Catalysts Atsushi Sanagawa and Hideo Nagashima* Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
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S Supporting Information *
ABSTRACT: Reduction of nitriles to silylated primary amines was achieved by combination of 1,1,3,3-tetramethyldisiloxane (TMDS) as the hydrosilane and a catalytic amount of Co(OPiv)2 (Piv = COtBu) associated with isocyanide ligands. The resulting silylated amines were subjected to acid hydrolysis or treatment with acid chlorides to give the corresponding primary amines or imides in good yields. One-pot synthesis of primary amides to primary amines with hydrosilanes was also achieved by iron−cobalt dual catalyst systems.
P
limited to benzonitrile.12 The Co2(CO)8-catalyzed reactions of nitriles with Me3SiH to form disilylamines, reported by Murai and co-workers, successfully achieved conversion of aromatic nitriles13a and several aliphatic nitriles to the corresponding amines.13b However, drawbacks included low to moderate product yields, relatively high catalyst loadings (4 mol %, TON < 25), instability of Co2(CO)8, and reaction conditions that require a CO atmosphere. Thus, previous reports of the syntheses have problems realizing the following goals: easily available and easy-to-handle catalysts, wide substrate scope of nitriles, selective formation of disilylamines, and high catalyst efficiency. A solution is required to establish practical synthetic protocols to convert nitriles to amines by hydrosilane reduction, which should be provided by development of efficient new catalysts. This report describes an efficient catalyst composed of a mixture of cobalt carboxylate and isocyanide that could convert nitriles to silyl amines, followed by hydrolysis to furnish an easy preparation of primary amines (Scheme 1). The new
rimary amines are an important class of compounds in organic synthesis, and a number of studies have been reported for effective syntheses.1 Reduction of nitriles is one method to access primary amines, and hydrogenation2 and hydride reduction3 are often used in the reaction protocols. Since nitriles are relatively less reactive than other carbonyl groups toward the addition of metal-hydrides, highly reactive but water-sensitive LiAlH4, BH3, AlH3, and LiBEt3H were used as the reducing reagents.3 In this context, the reduction of nitriles with stable borohydrides4 and hydrosilanes5 recently has received considerable attention, for which use of catalysts is important to accelerate the reaction. Several successful examples for reduction of nitriles to primary amines with hydrosilanes have been reported by development of appropriate transition metal catalysts or Lewis acids.6−13 Several precious metal complexes, such those including rhodium,6 ruthenium,7 rhenium,8 and iridium,9 catalyzed the reduction of nitriles to primary amines; however, a limited number of nitriles, which were often aromatic nitriles, were included in the studies. A notable report from Nikonov and co-workers involved cationic ruthenium-complex-catalyzed reactions of nitriles, which were tolerant toward reducible groups such as ketone and nitro groups;7 however, most of the examples produced silylimines as a single product; only a few nitriles were included in successful disilylamine syntheses. Recent interest in environmentally friendly chemical processes has prompted the development of nonprecious metal catalysts.14 Chang and co-workers investigated B(C6F5)3 as a catalyst, which promoted hydrosilylation of nitriles to disilylamines.10 A number of nitriles were successfully reduced, but polar functional groups are not compatible with the catalysts. Iron-complex-catalyzed hydrosilane reduction of nitriles was reported by Nakazawa,11 which used nitrile amounts in excess of the hydrosilane amount and was not useful for practical synthesis of amines. Nitrile reduction was briefly mentioned in a study of hydrosilylation of carbonyl compounds by a molybdenum catalyst, but the substrate was © 2018 American Chemical Society
Scheme 1. This Work
catalysts provided the ability to achieve the goals described above; the catalysts were active enough to promote the reaction with a 1 mol % catalyst loading that could be generated by simply mixing stable cobalt carboxylates and isocyanides, and primary amines were obtained as a single product from both aromatic and aliphatic nitriles. Of particular Received: November 21, 2018 Published: December 26, 2018 287
DOI: 10.1021/acs.orglett.8b03736 Org. Lett. 2019, 21, 287−291
Letter
Organic Letters
realized nitrile reduction in high yields. 1,1,1,3,3-Pentamethydisiloxane (PMDS) showed low activity, whereas no reaction took place with other primary, secondary, and tertiary hydrosilanes, including PhSiH3 commonly useful in recent hydrosilane reduction studies. Polymethylhydrosiloxane (PMHS) showed no reactivity at 50 °C, but promoted the reaction at 80 °C. As shown in Supporting Information (SI), Table S1, the reaction was accompanied by gelation, and EtNH3Cl was obtained in around 50% yield after the reaction for 24 h and subsequent treatment of the crude product with HCl in Et2O. The product yield was lower than that obtained by the reaction using TMDS. All of the reactions were conducted without solvent. As summarized in SI, Table S4, addition of toluene, THF, dioxane, and benzene inhibited the reaction. Catalysis of the Co(OPiv)2/CNtBu system realized reduction of a variety of aromatic and aliphatic nitriles with TMDS to form the corresponding disilylamines. The resulting disilylamines were subjected to treatment with HCl in Et2O, and the primary amines were obtained as their HCl salts in good yields. Scheme 2 shows results for reaction of several aliphatic and aromatic nitriles at 80 °C for 24 h. Under the standard reaction conditions using 1 mol % of the catalyst and 1.3 equiv of TMDS, aliphatic nitriles 1a, 1b, 1d, and 1e and aromatic nitriles, 1i, 1j, and 1n were converted to the corresponding HCl salts of primary amines in high yields. Both of the cyano groups in α,ω-dicyanoalkanes, 1g and 1h, underwent reduction to give diammonium salts 3g and 3h in approximately 95% yield. Additional amounts of either the catalyst or TMDS or both of them increased product yields in the reduction of 1k−1m, 1o, and 1q−1s. Functional group compatibility toward other reducible functional groups was interesting. While cyanoester 1s underwent reaction with the ester group remaining intact, both the oxo and cyano groups were reduced in the reaction of cyanoaldehyde 1t and cyanoketone 1s. Halogens in the substrate complicated the reaction. The Cl group in aliphatic nitrile 1c inhibited the reaction, whereas that in aromatic nitrile 1q did not disrupt the reaction. p-Fluorobenzonitrile 1r smoothly reacted with TMDS to give 3r in high yield, but no reaction took place for p-bromobenzonitrile 1p. Reaction of PhCH2CN (1f) was unusual and gives a mixture of disilylamine (24%) and disilylenamine (10%) with recovery of 1f (49%) under standard conditions. An attempt to improve the yield of 2f by changing the amount of the catalyst and TMDS was not successful. A solution was obtained from the experiments using Co 2(CNR)8 . In a typical example, Co2(CNtBu)8 (1 mol % Co) acted as a better catalyst than the Co(OPiv)2/CNtBu system, promoting the reaction at 50 °C and giving product 2f in high yield (vide infra, and SI, Table S6). It is worthwhile to mention that Co2(CNR)8 was generally a better catalyst than the Co(OPiv)2/CNR system for hydrosilane reduction of nitriles to primary amines. As reported previously,16b treatment of Co(OPiv)2 with hydrosilanes in the presence of CNR afforded R3SiCo(CNR)4. Co2(CNR)8, which proved to be a catalyst possessing higher activity than either the Co(OPiv)2/CNR system or R3SiCo(CNR)4 for catalytic hydrosilylation of alkenes, was prepared separately. The catalytic activity of three catalysts, Co2(CNR)8, the Co(OPiv)2/CNR system, and R3SiCo(CNR)4, toward the reaction of MeCN with TMDS was compared. As shown in SI, Table S6, Co2(CNR)8 behaved as a catalyst, which promoted the reaction at a temperature lower than that used in the case
importance is 1,1,3,3-tetramethyldisiloxane (TMDS), which is an inexpensive and safe hydrosilane that can be applied to industrial reduction processes.15 The reaction was accomplished without solvents. Previous papers reported a 1:3 mixture of cobalt carboxylate and isocyanide catalyzed hydrosilylation of alkenes with hydrosiloxanes.16 This same catalyst system was found to be useful for reduction of nitriles to disilylamines using TMDS as the hydrosilane. Table 1 shows optimization of the reaction Table 1. Screening of the Catalyst, Ligand, and Silane
entry
Co precursor
ligand RNC
Si−H
yield [conv of 1a]a (%)
1 2 3 4 5 6 7 8 9b 10c 11 12 13 14 15 16 17 18
Co(OPiv)2 Co(OAc)2 Co(OiPr)2 Co(stealate)2 Co(acac)3 Co(acac)2 CoCl2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2 Co(OPiv)2
AdNC AdNC AdNC AdNC AdNC AdNC AdNC t BuNC t BuNC t BuNC MesNC AdNC AdNC AdNC AdNC AdNC AdNC AdNC
TMDS TMDS TMDS TMDS TMDS TMDS TMDS TMDS TMDS TMDS TMDS PMDS PhMe2SiH Et3SiH (EtO)3SiH PhSiH3 Ph2SiH2 BDSBe
53 [53] 37 [37] 7 [7] 46 [46] 15 [15] 18 [18] NDd [99 [>99] >99 [>99] NDd [
