Ruthenium(II)-catalyzed reductive N–O bond cleavage of N-OR (R = H, alkyl, or acyl) substituted amides and sulfonamides

Abstract

With a commercially available ruthenium(II) catalyst and a mixture of HCOOH/NEt₃ as the hydride source under an air atmosphere, a convenient method for the reductive cleavage of N–O bonds was described. This catalytic system was applicable for a variety of N-oxygen-substituted amides, as well as N-alkoxy sulfonamides, efficiently delivering the corresponding amide or primary sulfonamide products with good functional group tolerance in moderate to good yields.

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Introduction

The development of an efficient strategy to access primary amides is an important topic in synthetic organic chemistry, which has been widely used in the fields of materials, pesticides and pharmaceuticals.¹ Several classical methods such as Schmidt-type reaction,² hydration of nitrile compounds³ and amidation reaction of carboxylic acid derivatives⁴ have been developed over the past few years. On the other hand, transition metal-catalyzed C–H activation of amides represented a powerful methodology to increase the molecular complexity of amide derivatives.⁵ However, these transformations were generally realized by employing N-oxygen-substituted amide functionalities as the directing groups (Fig. 1) owing to the low reactivity of the free amide group in such reactions.⁶ While the N–O moiety in the directing group may act as an internal oxidant in certain systems,⁷ it is still retained in the products in many cases.⁸ Thus, the development of an effective method for removing the activating oxygen-containing substituents would be very useful to produce the value-added primary amide.

Fig. 1 Selected examples of directing groups containing an N-oxygen-substituted amide moiety for C–H activation.

Traditional methods for the reductive cleavage of N–O bonds usually featured the use of stoichiometric amounts of metal reagents including titanium(III),⁹ samarium diiodide,¹⁰ lithium powder,¹¹ sodium amalgam¹² and Cu(II) ions.¹³ In 2008, Murphy and co-workers developed a metal-free reductive cleavage of N–O bonds in Weinreb amides with 1.5 equivalents of an organic neutral super-electron donor, delivering the corresponding secondary amides as the products.¹⁴ Very recently, the Du group disclosed an elemental sulfur-mediated conversion of N-alkoxyamides to primary amides.¹⁵ Although various methods have been reported, the problems associated with the formation of a lot of solid waste and limited substrate scope are yet to be solved. Hence, the development of a novel catalytic process¹⁶ for efficient reductive cleavage of N–O bonds is highly desirable.

Transition metal-catalyzed transfer hydrogenation is regarded as a convenient and powerful method to access target hydrogenated products, and has been widely adopted in recent years.¹⁷ As an attractive alternative protocol to direct hydrogenation with hydrogen gas, transfer hydrogenation has recently become the center of research in hydrogenation science. Using readily available and inexpensive hydrogen donors including alcohols, acids and amines, catalytic transfer hydrogenation and asymmetric transfer hydrogenation reactions of unsaturated compounds, such as ketones,¹⁸ imines,¹⁹ as well as alkenes²⁰ and alkynes²¹ have been well established (Scheme 1a).²² This methodology is also applicable for the reductive cleavage of single polar bonds, and has been employed as an efficient approach for transfer hydro-dehalogenation of organic halides (Scheme 1b).²³ However, little attention has been paid to the reduction of other single polar bonds.²⁴ During the course of our previous investigation on transfer hydro-dehalogenation of organic halides, we found the Ru(II) catalyst to be efficient at transforming C–X bonds (X = Cl, Br, or I) into C–H bonds using the iso-propanol solvent as the hydride source.^23h This encouraged us to extend the transfer hydrogenation catalysis to the other single polar bonds. Herein, we report a Ru-catalyzed reductive cleavage of N–O bonds in N-oxygen-substituted amides and sulfonamides using HCOOH/NEt₃ as the hydride source without any additives under air (Scheme 1c).

Scheme 1 The applications of transition metal-catalyzed transfer hydrogenation.

Results and discussion

We initiated our study by identifying reliable conditions for this Ru-catalyzed reductive cleavage of N–O bonds starting from N-methoxybenzamide (1a). After screening the reaction parameters, we found that 1a could be converted to the desired product 2a in almost quantitative yield (92% isolated yield) using 2.5 mol% of [RuCl₂(p-cymene)]₂ as the catalyst, and H₂O/HCOOH/NEt₃ as a co-solvent and hydride source (Table 1, entry 1). The metal catalyst was proven to be essential (entry 2), and when Ru(PPh₃)₃Cl₂ was used as the catalyst, the yield of 2a decreased to 22% (entry 3). Employing [RhCp*Cl₂]₂ instead of [RuCl₂(p-cymene)]₂ also led to an excellent yield of 2a (entry 4). In the absence of HCOOH/NEt₃ or H₂O, the reaction gave 2a in 6% and 57% yields, respectively (entries 5 and 6). On using MeOH, EtOH or ⁱPrOH instead of the co-solvent mixture, the yield of 2a decreased obviously (entries 7–9). The yield of 2a decreased dramatically when the reaction was run at a relatively lower temperature, indicating that heating at 100 °C is necessary for high yield (entry 10).

Table 1 Screening of the reaction conditions^a

Entry	Deviation of standard conditions^a	Yield^b/%
a Reaction conditions: 1a (0.2 mmol), catalyst (2.5 mol%), HCOOH/Et3N (molar ratio: 1 : 1, 0.5 mL), and H2O (0.5 mL) at 100 °C, 12 h. b Yields were determined by ^1H NMR analysis using mesitylene as an internal standard. c Isolated yield is shown in brackets.
1	None	99 (92)^c
2	Without [RuCl2(p-cymene)]2	0
3	Ru(PPh3)3Cl2 instead of [RuCl2(p-cymene)]2	22
4	[RhCp*Cl2]2 instead of [RuCl2(p-cymene)]2	99
5	H2O instead of a co-solvent mixture	6
6	HCOOH/Et3N (1/1) instead of a co-solvent mixture	57
7	MeOH instead of a co-solvent mixture	16
8	EtOH instead of a co-solvent mixture	8
9	^iPrOH instead of a co-solvent mixture	45
10	At 80 °C	65

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The scope of this catalytic system turned out to be broad. With the optimized conditions, the substrate scope of N-alkoxy amides is illustrated in Table 2. Various electron-donating or -withdrawing groups at the phenyl ring of N-methoxybenzamides, such as methyl, alkoxy, chloro and phenyl groups, were well tolerated, affording the corresponding primary amides in good to excellent yields (1a–1i). A slight decrease in the yield was observed when the substrate contained a free phenolic hydroxyl group (1j). The reactions of N-methoxybenzamides bearing two substituents on the phenyl ring resulted in moderate yields (1k and 1l). N-Methoxy poly- and heterocycle substituted amides, such as 1-naphthyl (1m), 2-naphthyl (1n), 4-pyridinyl (1o), 2-furoyl (1p) and 2-thienyl (1q) amides, were quite suitable for this catalytic transformation, yielding the desired amides in 33–99% yields. Methoxy aliphatic amides (1r–1t) were also compatible with this method, showing a similar reactivity under the standard conditions. N-Alkoxy-benzamides containing ethoxy (1u) and benzyloxy (1v) substituents proceeded smoothly to give benzamides in 91% and 88% yields, respectively. Benzohydroxamic acids (1w–1y) were then investigated, and they afforded the desired products in 78–83% yields. The reactivities were relatively lower in the case of acyloxy benzamides (1aa–1ae). The nitro group could also be well tolerated in this reaction, delivering the desired product in moderate yield (1ac). The N–O bond cleavage catalytic system could be applied to a tertiary amide (1ad), giving the corresponding secondary amide in 64% yield. In addition, N-(benzoyloxy)succinimide (1ae) was also compatible with this catalytic system, producing benzoic acid as the product in excellent yield. We found that the substrates containing alkyl halide, cyano, alkyne, and alkene moieties led to complex mixtures, indicating that these groups are not compatible in the current transformation.

Table 2 Substrate scope of various N-alkoxy amides^a

a Standard conditions: 1 (0.2 mmol), [RuCl₂(p-cymene)]₂ (2.5 mol%), and H₂O (0.5 mL) + HCOOH/Et₃N (molar ratio: 1 : 1, 0.5 mL), 12 h. b Yield of benzyl alcohol. c HCOOH/Et₃N (molar ratio: 5 : 2). d Yield of benzoic acid.

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Next, we extended this N–O bond cleavage methodology to N-alkoxy sulfonamides (Scheme 2). N-Methoxy sulfonamides were quite suitable for this catalytic system, producing the corresponding primary sulfonamides in excellent yields under the standard conditions (4a–4d).

Scheme 2 Ru(II)-catalyzed N–O bond cleavage of N-alkoxy sulfonamides.

To further showcase the synthetic utility, a gram-scale reaction was performed smoothly to give the desired product without compromising the yield [eqn. (1)].

On the basis of previous reports,^23h,25 a plausible mechanism for this Ru(II)-catalyzed N–O bond cleavage reaction is proposed in Scheme 3. First, a ligated ruthenium hydride species (A) is formed by treating [RuCl₂(cymene)]₂ with HCOOH/NEt₃. Then oxidative addition of the N–O bond to the Ru(II) center generates a Ru(IV) complex (B), which would undergo reductive elimination to give the desired product and a Ru(II) alkoxy species (C). Subsequently, a ligand exchange occurred between C and HCOO⁻ to deliver a Ru(II) formate complex (D). Finally, D released a CO₂ molecule to regenerate A.

Scheme 3 Plausible reaction mechanism.

Conclusions

In summary, we have developed a Ru-catalyzed N–O bond cleavage of N-oxygen-substituted amides and sulfonamides employing the transfer hydrogenation methodology using H₂O/HCOOH/NEt₃ as the co-solvent and hydride source. This catalytic system was applicable for a variety of substituted amides and sulfonamides, delivering the corresponding amides and primary sulfonamides in moderate to excellent yields. The reaction could be scaled up to gram scale smoothly under air. Featuring a commercially available catalyst, simple and easy operation, and readily available starting materials with good functional group compatibility, this is a practical method to access valuable primary amides. Further investigations on the reductive cleavage of the other single polar bonds with the transfer hydrogenation methodology are undergoing in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support was provided by NSFC (No. 21801191, 21572163, and 21873074) and Wenzhou Science & Technology Bureau (G20180016).

Notes and references

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18. For selected reviews on transfer hydrogenation of ketones, see: (a) X. Wu and J. Xiao, Aqueous-phase asymmetric transfer hydrogenation of ketones – a greener approach to chiral alcohols, Chem. Commun., 2007, 2449–2466; (b) R. Malacea, R. Poli and E. Manoury, Asymmetric hydrosilylation, transfer hydrogenation and hydrogenation of ketones catalyzed by iridium complexes, Coord. Chem. Rev., 2010, 254, 729–752; (c) Y.-Y. Li, S.-L. Yu, W.-Y. Shen and J.-X. Gao, Iron-, cobalt-, and nickel-catalyzed asymmetric transfer hydrogenation and asymmetric hydrogenation of ketones, Acc. Chem. Res., 2015, 48, 2587–2598; (d) F. Foubelo, C. Najera and M. Yus, Catalytic asymmetric transfer hydrogenation of ketones: recent advances, Tetrahedron: Asymmetry, 2015, 26, 769–790. For selected examples, see: (e) S. Hashiguchi, A. Fujii, J. Takehara, T. Ikariya and R. Noyori, Asymmetric transfer hydrogenation of aromatic ketones catalyzed by chiral ruthenium(II) complexes, J. Am. Chem. Soc., 1995, 117, 7562–7563; (f) X.-D. Du, L.-X. Dai, X.-L. Hou, L.-J. Xia and M.-H. Tang, Highly efficient catalysts derived from planar chiral ferrocenes for asymmetric transfer hydrogenation of ketones, Chin. J. Chem., 1998, 16, 90–93; (g) Y.-X. Chen, Y.-M. Li, K.-H. Lam and A. S. C. Chan, New chiral ligand N-toluenesulfonyl-2,2′-dimethoxy-6,6′-di-aminobiphenyl for catalytic asymmetric transfer hydrogenation of ketones, Chin. J. Chem., 2002, 20, 606–609; (h) W. Zuo, A. J. Lough, Y. F. Li and R. H. Morris, Amine(imine)diphosphine iron catalysts for asymmetric transfer hydrogenation of ketones and imines, Science, 2013, 342, 1080–1083; (i) M. Perez, S. Elangovan, A. Spannenberg, K. Junge and M. Beller, Molecularly defined manganese pincer complexes for selective transfer hydrogenation of ketones, ChemSusChem, 2017, 10, 83–86; (j) Y.-M. Zhang, M.-L. Yuan, W.-P. Liu, J.-H. Xie and Q.-L. Zhou, Iridium-catalyzed asymmetric transfer hydrogenation of alkynyl ketones using sodium formate and ethanol as hydrogen sources, Org. Lett., 2018, 20, 4486–4489; (k) L.-S. Zheng, P. Phansavath and V. Ratovelomanana-Vidal, Ruthenium-catalyzed dynamic kinetic asymmetric transfer hydrogenation: stereoselective access to syn 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives, Org. Chem. Front., 2018, 5, 1366–1370; (l) L. De Luca, A. Passera and A. Mezzetti, Asymmetric transfer hydrogenation with a bifunctional iron(II) hydride: experiment meets computation, J. Am. Chem. Soc., 2019, 141, 2545–2556.
19. For selected examples on transfer hydrogenation of imines, see: (a) N. Uematsu, A. Fujii, S. Hashiguchi, T. Ikariya and R. Noyori, Asymmetric transfer hydrogenation of imines, J. Am. Chem. Soc., 1996, 118, 4916–4917; (b) A. A. Mikhailine, M. I. Maishan and R. H. Morris, Asymmetric transfer hydrogenation of ketimines using well-defined iron(II)-based precatalysts containing a PNNP ligand, Org. Lett., 2012, 14, 4638–4641; (c) Z. Wu, M. Perez, M. Scalone, T. Ayad and V. Ratovelomanana-Vidal, Ruthenium-catalyzed asymmetric transfer hydrogenation of 1-aryl-substituted dihydroisoquinolines: access to valuable chiral 1-aryl-tetrahydroisoquinoline scaffolds, Angew. Chem., Int. Ed., 2013, 52, 4925–4928; (d) G. Zhang and S. K. Hanson, Cobalt-catalyzed transfer hydrogenation of C=O and C=N bonds, Chem. Commun., 2013, 49, 10151–10153; (e) H. Xu, P. Yang, P. Chuanprasit, H. Hirao and J. Zhou, Nickel-catalyzed asymmetric transfer hydrogenation of hydrazones and other ketimines, Angew. Chem., Int. Ed., 2015, 54, 5112–5116; (f) V. S. Shende, S. H. Deshpande, S. K. Shingote, A. Joseph and A. A. Kelkar, Asymmetric transfer hydrogenation of imines in water by varying the ratio of formic acid to triethylamine, Org. Lett., 2015, 17, 2878–2881; (g) M. Wu, T. Cheng, M. Ji and G. Liu, Ru-catalyzed asymmetric transfer hydrogenation of α-trifluoromethylimines, J. Org. Chem., 2015, 80, 3708–3713; (h) H.-J. Pan, Y. Zhang, C. Shan, Z. Yu, Y. Lan and Y. Zhao, Asymmetric transfer hydrogenation of imines using alcohol: efficiency and selectivity are influenced by the hydrogen donor, Angew. Chem., Int. Ed., 2016, 55, 9615–9619; (i) Y. Li, M. Lei, W. Yuan, E. Meggers and L. Gong, An N-heterocyclic carbene iridium catalyst with metal-centered chirality for enantioselective transfer hydrogenation of imines, Chem. Commun., 2017, 53, 8089–8092; (j) E. de Julián, E. Menéndez-Pedregal, M. Claros, M. Vaquero, J. Díez, E. Lastra, P. Gamasa and A. Pizzano, Practical synthesis of enantiopure benzylamines by catalytic hydrogenation or transfer hydrogenation reactions in isopropanol using a Ru-pybox catalyst, Org. Chem. Front., 2018, 5, 841–849.
20. For selected examples on transfer hydrogenation of alkenes, see: (a) O. Soltani, M. A. Ariger and E. M. Carreira, Transfer hydrogenation in water: enantioselective, catalytic reduction of (E)-β,β-disubstituted nitroalkenes, Org. Lett., 2009, 11, 4196–4198; (b) S. Horn and M. Albrecht, Transfer hydrogenation of unfunctionalised alkenes using N-heterocyclic carbeneruthenium catalyst precursors, Chem. Commun., 2011, 47, 8802–8804; (c) G. K. Zielinski, C. Samojlowicz, T. Wdowik and K. Grela, In tandem or alone: a remarkably selective transfer hydrogenation of alkenes catalyzed by ruthenium olefin metathesis catalysts, Org. Biomol. Chem., 2015, 13, 2684–2688; (d) V. H. Mai and G. I. Nikonov, Transfer hydrogenation of nitriles, olefins, and N-heterocycles catalyzed by an N-heterocyclic carbene-supported half-Sandwich complex of ruthenium, Organometallics, 2016, 35, 943–949; (e) Y. Wang, Z. Huang, X. Leng, H. Zhu, G. Liu and Z. Huang, Transfer hydrogenation of alkenes using ethanol catalyzed by a NCP pincer iridium complex: scope and mechanism, J. Am. Chem. Soc., 2018, 140, 4417–4429.
21. For selected examples on transfer hydrogenation of alkynes, see: (a) P. Hauwert, G. Maestri, J. W. Sprengers, M. Catellani and C. J. Elsevier, Transfer semihydrogenation of alkynes catalyzed by a zero-valent palladium N-heterocyclic carbene complex, Angew. Chem., Int. Ed., 2008, 47, 3223–3226; (b) J. Li, R. Hua and T. Liu, Highly chemo- and stereoselective palladium-catalyzed transfer semihydrogenation of internal alkynes affording cis-alkenes, J. Org. Chem., 2010, 75, 2966–2970; (c) P. Hauwert, R. Boerleider, S. Warsink, J. J. Weigand and C. J. Elsevier, Mechanism of Pd(NHC)-catalyzed transfer hydrogenation of alkynes, J. Am. Chem. Soc., 2010, 132, 16900–16910; (d) C. Belger, N. M. Neisius and B. Plietker, A selective Ru-catalyzed semireduction of alkynes to Z olefins under transfer-hydrogenation conditions, Chem. – Eur. J., 2010, 16, 12214–12220; (e) G. Wienhöfer, F. A. Westerhaus, R. V. Jagadeesh, K. Junge, H. Junge and M. Beller, Selective iron-catalyzed transfer hydrogenation of terminal alkynes, Chem. Commun., 2012, 48, 4827–4829; (f) R. M. Drost, T. Bouwens, N. P. van Leest, B. de Bruin and C. J. Elsevier, Convenient transfer semihydrogenation methodology for alkynes using a Pd^II-NHC precatalyst, ACS Catal., 2014, 4, 1349–1357; (g) S. Fu, N.-Y. Chen, X. Liu, Z. Shao, S.-P. Luo and Q. Liu, Ligand-controlled cobalt-catalyzed transfer hydrogenation of alkynes: stereodivergent synthesis of Z- and E-alkenes, J. Am. Chem. Soc., 2016, 138, 8588–8594; (h) J. Yang, C. Wang, Y. Sun, X. Man, J. Li and F. Sun, Ligand-controlled iridium-catalyzed semihydrogenation of alkynes with ethanol: highly stereoselective synthesis of E- and Z-alkenes, Chem. Commun., 2019, 55, 1903–1906.
22. For selected references on transfer hydrogenation of other unsaturated compounds, see: (a) Y.-C. Chen, J.-G. Deng, T.-F. Wu, X. Cui, Y.-Z. Jiang, M. C. K. Choi and A. S. C. Chan, Facile preparation of chiral 1, 3-diols via stereoselective transfer hydrogenation of 1, 3-diones, Chin. J. Chem., 2001, 19, 807–810; (b) C.-Y. Wang, C.-F. Fu, Y.-H. Liu, S.-M. Peng and S.-T. Liu, Synthesis of iridium pyridinyl N-heterocyclic carbene complexes and their catalytic activities on reduction of nitroarenes, Inorg. Chem., 2007, 46, 5779–5786; (c) C. Wang, C. Li, X. Wu, A. Pettman and J. Xiao, pH-Regulated asymmetric transfer hydrogenation of quinolines in water, Angew. Chem., Int. Ed., 2009, 48, 6524–6528; (d) O. Soltani, M. A. Ariger and E. M. Carreira, Transfer hydrogenation in water: enantioselective, catalytic reduction of (E)-β,β-disubstituted nitroalkenes, Org. Lett., 2009, 11, 4196–4198; (e) D. Spasyuk, S. Smith and D. G. Gusev, Replacing phosphorus with sulfur for the efficient hydrogenation of esters, Angew. Chem., Int. Ed., 2013, 52, 2538–2542; (f) P. Yang, H. Xu and J. Zhou, Nickel-catalyzed asymmetric transfer hydrogenation of olefins for the synthesis of α– and β–amino acids, Angew. Chem., Int. Ed., 2014, 53, 12210–12213; (g) M. Liu, F. Zhou, Z. Jia and C.-J. Li, A silver-catalyzed transfer hydrogenation of aldehyde in air and water, Org. Chem. Front., 2014, 1, 161–166; (h) N. Dai, R. Shang, M. Fu and Y. Fu, Transfer hydrogenation of ethyl Levulinate to γ-Valerolactone catalyzed by iron complexes, Chin. J. Chem., 2015, 33, 405–408; (i) Z. Shao, S. Fu, M. Wei, S. Zhou and Q. Liu, Mild and selective cobalt-catalyzed chemodivergent transfer hydrogenation of nitriles, Angew. Chem., Int. Ed., 2016, 55, 14653–14657.
23. For selected examples on transfer hydro-dehalogenation of organic halides, see: (a) A. C. Bényei, S. Lehel and F. Joó, Transfer hydrodehalogenation of organic halides catalyzed by water soluble ruthenium(II) phosphine complexes, J. Mol. Catal. A: Chem., 1997, 116, 349–354; (b) M. E. Cucullu, S. P. Nolan, T. R. Belderrain and R. H. Grubbs, Catalytic dehalogenation of aryl chlorides mediated by ruthenium(II) phosphine complexes, Organometallics, 1999, 18, 1299–1304; (c) K. Fujita, M. Owaki and R. Yamaguchi, Chemoselective transfer hydrodechlorination of aryl chlorides catalyzed by Cp*Rh complexes, Chem. Commun., 2002, 2964–2965; (d) S. Kuhl, R. Schneider and Y. Fort, Catalytic carbon-fluorine bond activation with monocoordinated nickel-carbene complexes: reduction of fluoroarenes, Adv. Synth. Catal., 2003, 345, 341–344; (e) O. Navarro, N. Marion, Y. Oonishi, R. A. Kelly III and S. P. Nolan, Suzuki–Miyaura, α-ketone arylation and dehalogenation reactions catalyzed by a versatile N-heterocyclic carbene–palladacycle complex, J. Org. Chem., 2006, 71, 685–692; (f) J. Yang and M. Brookhart, Iridium-catalyzed reduction of alkyl halides by triethylsilane, J. Am. Chem. Soc., 2007, 129, 12656–12657; (g) J. Chen, Y. Zhang, L. Yang, X. Zhang, J. Liu, L. Li and H. Zhang, A practical palladium catalyzed dehalogenation of aryl halides and α-haloketones, Tetrahedron, 2007, 63, 4266–4270; (h) T. You, Z. Wang, J. Chen and Y. Xia, Transfer hydro-dehalogenation of organic halides catalyzed by ruthenium(II) complex, J. Org. Chem., 2017, 82, 1340–1346; (i) I. Parveen, D. Khan and N. Ahmed, Regioselective hydrodehalogenation of aromatic α– and β–halo carbonyl compounds by CuI in isopropanol, Eur. J. Org. Chem., 2019, 759–764; (j) C. Chen, H. Zuo and K. S. Chan, Catalytic hydrodebromination of aryl bromides by cobalt tetra-butyl porphyrin complexes with EtOH, Tetrahedron, 2019, 75, 510–517.
24. For an example of the reductive cleavage of N-C bonds with transfer hydrogenation catalysis, see: (a) D. Pu, Y. Zhou, F. Yang, G. Shen, Y. Gao, W. Sun, R. Khan and B. Fan, Asymmetric ring-opening reactions of azabenzonorbornadienes through transfer hydrogenation using secondary amines as hydrogen sources: tuning of absolute configuration by acids, Org. Chem. Front., 2018, 5, 3077–3082. For an example of the reductive cleavage of N-O bonds with transfer hydrogenation catalysis, see: (b) H. Fukuzawa, Y. Ura and Y. Kataoka, Ruthenium-catalyzed reduction of N-alkoxy- and N-hydroxyamides, J. Organomet. Chem., 2011, 696, 3643–3648.
25. W. Leitner, J. M. Brown and H. Brunner, Mechanistic Aspects of the Rhodium-Catalyzed Enantioselective Transfer Hydrogenation of α,β-Unsaturated Carboxylic Acids Using Formic Acid/Triethylamine (5:2) as the Hydrogen Source, J. Am. Chem. Soc., 1993, 115, 152–159.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0qo01093f

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