Diboronic Acid Anhydrides as Effective Catalysts for the Hydroxy

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Diboronic Acid Anhydrides as Effective Catalysts for the HydroxyDirected Dehydrative Amidation of Carboxylic Acids Naoyuki Shimada,* Mai Hirata, Masayoshi Koshizuka, Naoki Ohse, Ryoto Kaito, and Kazuishi Makino Laboratory of Organic Chemistry for Drug Development and Medical Research Laboratories, Department of Pharmaceutical Sciences, Kitasato University, Tokyo 108-8641, Japan

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S Supporting Information *

ABSTRACT: The direct catalytic dehydrative amidation of βhydroxycarboxylic acids with amines is described. A biphenylbased diboronic acid anhydride with a B−O−B skeleton is shown to be an exceptionally effective catalyst for the reaction, providing β-hydroxycarboxylic amides in high to excellent yields with a low catalyst loading (minimum of 0.01 mol %, TON up to 7,500). This hydroxy-directed amidation shows excellent chemoselectivity and is applicable to gram-scale drug synthesis.

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Scheme 1. Catalytic Dehydrative Amidations

mide bonds are important chemical linkages constituting many natural and unnatural products, such as peptides, proteins, pharmaceuticals, agrichemicals, and polymer materials.1 Accordingly, synthetic chemists have expended considerable effort toward the development of efficient methodologies for building amide bonds. The catalytic direct dehydrative condensation of readily available carboxylic acids with amines is one of the most attractive approaches because water is the only byproduct of these transformations.2 In 1996, Yamamoto et al. reported the first catalytic dehydrative amide condensation using electron-withdrawing group substituted aromatic boronic acids as catalysts (Scheme 1a).3,4 Subsequently, various modified aromatic boronic acids catalysts were developed by Ishihara et al.5 and Whiting et al.6 As a notable breakthrough in this area, Hall et al.7 found that orthohalogen-substituted phenylboronic acids are effective catalysts for direct amide condensation at room temperature.8,9 Recently, apart from boronic acids, new types of organoboron amidation catalysts were developed; an example of these catalysts is the borate ester B(OCH2CF3)3, which catalyzes the direct amidation of a wide range of substrates.10 More recently, direct amidation using multiboron catalysts was also reported. Kumagai and Shibasaki et al. developed 1,3-dioxa-5-aza-2,4,6triborinane (DATB) compounds constituted of a sixmembered B3NO2 heterocyclic ring as a highly efficient amide condensation catalyst.11 Diboron catalysts for the direct amidation of aromatic carboxylic acids have been reported.12 Hydroxyamides are widely present in natural products and pharmaceuticals and constitute a precursor of medicinally important compounds such as β-lactam and azetidines.13 However, catalytic amidation for synthesizing hydroxyamides is still quite limited. Ishihara reported alkyl boronic acid catalyzed direct dehydrative amidation of α-hydroxycarboxylic © XXXX American Chemical Society

acids with amines to prepare α-hydroxyamides (Scheme 1b).14 More recently, Yamamoto reported the tantalum alkoxide catalyzed amidation of β-hydroxycarboxylic esters with amines (Scheme 1c).15 Although this elaborate substrate-directed catalysis16 shows excellent chemoselectivities toward βhydroxycarboxylic esters over simple esters, the catalytic direct dehydrative amidation of β-hydroxycarboxylic acids has not Received: April 28, 2019

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DOI: 10.1021/acs.orglett.9b01484 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Optimization of Reaction Conditionsa

been sufficiently explored to date.17 Herein, we report the development of diboronic acid anhydride catalysts for the direct amide condensation of α- and β-hydroxycarboxylic acids with amines (Scheme 1d) with high efficiency and high functional group tolerance. As the starting point of our research, we envisaged that diboronic acid anhydrides with a preorganized B−O−B motif,18 which is consistent with the active intermediate in the recently reported revised amidation mechanism by Whiting,19 are active catalysts in the amidation reaction of hydroxycarboxylic acids. This expectation was based on the recognition of the hydroxy group at the β-position of the carboxylic acid by one of the two boron atoms to form a covalent B−O bond (Scheme 2). Our hypothesis was that the Scheme 2. Our Working Hypothesis

entry

catalyst

catalyst loading [mol %]

yield [%]

1 2 3 4 5 6 7b 8c 9d 10

1a none 1a 1a 1b 1c 1c 1c 1c 1c

5.0 − 2.0 1.0 1.0 1.0 1.0 1.0 5.0 0.2

88 0 83 70 87 >99 95 >99 98 98

a The reactions were carried out in the presence of acid 2a (0.38 mmol), amine 3a (0.38 mmol), and catalyst 1 in toluene (1.9 mL) at 110 °C (bath temp). bPerformed in toluene/H2O (19:1, 0.1 M). c Performed at a 1.0 g scale (8 h). dPerformed at 60 °C (bath temp).

two boron atoms arranged on a biphenyl scaffold would bind the carboxylic acid in a bidentate form, thereby prompting the formation of a favorable bicyclic reactive intermediate. Initially, we explored the direct dehydrative amidation of an equimolar mixture of 3-hydroxy-3-phenylpropanoic acid (2a) and N-benzyl-N-methylamine (3a) in the presence of 5 mol % of diboronic acid anhydride 1a20 in toluene (Table 1, entry 1). The reaction proceeded smoothly at 110 °C within 4 h without any azeotropic dehydration protocol or dehydrating agent to give β-hydroxyamide 4a in 88% yield; on the other hand, no reaction was observed in the absence of the catalyst (entry 2).21 High yields were maintained with a lower catalyst loading (entries 3 and 4). Encouraged by these results, we aimed to enhance the activity by introducing an ortho-substituent7,22 on the catalyst. For this purpose, we evaluated the performance of the newly developed diboronic acid anhydride catalysts 1b and 1c. The use of 1.0 mol % of halogenated catalyst 1b, which incorporates bromine atoms at both ortho-positions of the benzene rings, improved the yield to 87% (entry 5). The product yield was further enhanced to a quantitative yield when using 1.0 mol % of 1c, which bears additional bromine substituents at the para-positions (entry 6). We found that an excellent product yield of 95% was maintained when the reaction was performed in a mixture of toluene/water (19:1), which is comparable to 30 equiv of water for the substrates (entry 7). Note that the catalyst system described here is less vulnerable to inhibition by water, presumably because of the preorganized cyclic B−O−B structure of the catalyst. In addition, high yields can be obtained regardless of the order of addition of the substrates and catalyst,7b,23 indicating the operational advantages of this catalytic reaction. Moreover, amides are obtained at a satisfactory level of purity without purification protocols because 1c can be removed by washing with a basic aqueous solution. The present protocol could be performed in 1.0 g scale (entry 8). The amidation reaction also

proceeded smoothly at 60 °C with a 98% yield when 5.0 mol % of 1c was used (entry 9). Moreover, the catalyst loading of 1c further decreased to 0.2 mol %, affording amide 4a in 98% yield (entry 10).24 Next, we examined the importance of the β-hydroxy group for this reaction. When a mixture of 1 equiv each of βhydroxycarboxylic acid 2a, β-methoxycarboxylic acid 5, and Nbenzylamine (3b) was reacted in the presence of a catalytic amount of 1c, β-hydroxyamide 6a was obtained as a sole product in high yield (Scheme 3a), demonstrating the extremely high chemoselectivity for β-hydroxycarboxylic acids. Compared to β-hydroxycarboxylic acid, the present catalytic system was less effective for simple carboxylic acid, affording a trance amount of simple amide 7 (Scheme 3b).25 To gain insight into the reaction intermediate, we attempted to detect the intermediate species by mass spectrometry (Scheme 3c). Negative ESI-MS measurement of a solution containing diboronic acid anhydride 1a, β-hydroxycarboxylic acid 2a, and amine 3b in toluene showed a clear m/z peak at 371.1259. This peak corresponds to cyclic acyloxydiboronate I ([M]−: calcd m/z = 371.1262), a reaction intermediate with the B− O−B motif.23 To demonstrate the substrate generality of the reaction, the reactions for various β-hydroxycarboxylic acids were examined in the presence of 0.5 mol % of 1c (Scheme 4). We found that β-aryl substrates possessing either electron-donating or electron-withdrawing groups provided the corresponding amides 6a−6f in excellent yields above 81%. In particular, the catalyst loading could be reduced to 0.01 mol % in the case of 6b, and the turnover number reached 7,500, which, to the best of our knowledge, is the highest ever reported for organoboron-catalyzed amidations. In addition, the reaction proceeded smoothly even in the case of 60 °C (6a−6c). Alkyl substituents at the β-position, such as propyl or bulky tertB


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Organic Letters

Scheme 4. Substrate Scope for Catalytic Amidationsa

Scheme 3. Effectiveness of β-Hydroxy Group for the Catalytic Amidation

butyl, were well tolerated (6g and 6h). In addition, more hindered α,α- or β,β-disubstituted substrates were also applicable, and they provided the corresponding amides 6i and 6j in 74% and 97% yields, respectively. Salicylic acid (2hydroxybenzoic acid) was not suitable as a substrate in the present catalysis. We also explored the amine scope of the present method using 1c. We found that not only simple amines but also α-branched and α-tertiary amines could be employed. Products 6k−6q were obtained in high to excellent yields (73%−99%), though 2.0 mol % of 1c and elevated temperature were required in the case of sterically demanding amides 6p and 6q. Amines functionalized with alkyl chloride, silyl ether, amide, alkyne, olefin, or ester groups were compatible with this system, giving the corresponding amides 6r−6w. Amines with heterocycles, including indole, furan, and thiophene, were also applicable, and products 6x−6z were obtained (88%−99%). In the case of an alkylamine bearing a free aromatic amine, the chemoselective formation of alkyl amide 6aa was observed. The reaction proceeded even with less reactive aniline derivatives, and products 6ab−6ae were obtained in high to excellent yields (76% to quantitative yields). In particular, the reaction with 4-hydroxyaniline shows that the amide formation proceeds selectively over ester formation; this observation reveals the compatibility of the phenolic hydroxy group. These results establish the broad functional group tolerance of the present catalysis. Amidation using 0.5 mol % of diboronic acid anhydride 1c as a catalyst was successfully applied to the generally difficult synthesis of tertiary amides. This was achieved by reacting the β-hydroxycarboxylic acids with secondary amines (Scheme 5). Not only acyclic dialkyl amines but also cyclic amines such as piperidine or morpholine could be applied as substrates. The reactions were completed within 24 h in all cases, and the

a

The reactions were performed with an equimolar amout of acids to amines (1:1 ratio) in the presence of 0.5 mol % of 1c at 110 °C (bath temp). Percentages in parentheses are the yields in the absence of catalyst determined by 1H NMR using 1,1,2,2,-tetrachloroethane as an internal standard. bPerformed with 0.01 mol % of 1c at 110 °C. c Performed with 3.0 mol % of 1c at 60 °C (bath temp). dPerformed with 2.0 mol % of 1c. ePerformed at 130 °C (bath temp). C


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Organic Letters

without any racemization of the α-position. This result is noteworthy because catalytic activity of the previous boronic acid catalysts5g,7d was lost by the coordination of the carbamate-protected amino acids. Finally, to demonstrate the synthetic utility of our methodology, we synthesized tropicamide, an antimuscarinic drug that dilates the eye pupil (Scheme 6c). The dehydrative amide condensation between tropic acid (2k) and 4-(ethylaminomethyl)pyridine (12) proceeded smoothly within 5 h in the presence of 0.5 mol % of 1c, affording tropicamide (13) in 87% yield and indicating the tolerance of this system toward the Lewis basic pyridine ring. The synthesis of 13 could also be carried out on a gram scale, albeit at elevated temperature (130 °C) and with prolonged reaction time. In summary, we successfully demonstrated that biphenylbased diboronic acid anhydride 1c is a highly efficient catalyst for the dehydrative direct amidation of a wide range of hydroxycarboxylic acids with primary and secondary aliphatic, heterocyclic, and aromatic amines. The present catalytic reaction proceeds with high to excellent yields, broad functional group tolerance, characteristic chemoselectivity, and low catalyst loading (0.01 mol %−5.0 mol %). To the best of our knowledge, diboronic acid anhydride-catalyzed amidation of carboxylic acids with amines has not been reported earlier, and thus, this research paves the way for the design of organoboron catalysts for such a reaction. Further investigation of diboronic acid anhydride catalysts is currently underway in our laboratories.

Scheme 5. Catalytic Amidations for Tertiary Amide Synthesis

a

Percentages in parentheses are the yields in the absence of catalyst determined by 1H NMR using 1,1,2,2,-tetrachloroethane as an internal standard. bPerformed with 1 mol % of 1c. cPerformed at 130 °C (bath temp).

corresponding tertiary β-hydroxyamides 4b−4g were obtained with high to excellent yields (75%−96%). This substrate-directed reaction was preliminarily examined for the catalytic amidation of α-hydroxycarboxylic acids (Scheme 6a). The corresponding α-hydroxy amides 9a−9c

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 6. Application of Catalytic Amidation

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01484. Preparation of catalysts, experimental procedures, and characterization data including 1H and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.S.). ORCID

Naoyuki Shimada: 0000-0002-0143-7867 Kazuishi Makino: 0000-0001-8518-6593 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by JSPS KAKENHI Grant Number 16K18850 (N.S.) for Young Scientists (B), 19K07000 (N.S.) for Scientific Research (C), 17K08218 (K.M.) for Scientific Research (C), and a Kitasato University Research Grant for Young Researchers. We thank Dr. K. Nagai and Ms. M. Sato at Kitasato University for instrumental analyses.

were obtained in high yields (83%−86%) regardless of the electronic factor of the benzene ring at the para-substitution of mandelic acid derivatives. In addition, to expand the scope of this catalytic process, we applied serine derivatives as a βhydroxycarboxylic acid substrate (Scheme 6b). We found that the reaction of N-Cbz-protected serine 10 with amine 3b in the presence of 5.0 mol % of 1c proceeded smoothly within 4 h at 90 °C to afford corresponding amide 11 in 95% yield

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REFERENCES

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Organic Letters (12) Sawant, D. N.; Bagal, D. B.; Ogawa, S.; Selvam, K.; Saito, S. Diboron-Catalyzed Dehydrative Amidation of Aromatic Carboxylic Acids with Amines. Org. Lett. 2018, 20, 4397−4400. (13) For selected examples of natural or biologically active compounds including the β-hydroxyamide structure, see: (a) Nakao, Y.; Fujita, M.; Warabi, K.; Matsunaga, S.; Fusetani, N. Miraziridine A, a Novel Cysteine Protease Inhibitor from the Marine Sponge Theonella aff. mirabilis. J. Am. Chem. Soc. 2000, 122, 10462−10463. (b) Martín, M. J.; Rodríguez-Acebes, R.; García-Ramos, Y.; Martínez, V.; Murcia, C.; Digón, I.; Marco, I.; Pelay-Gimeno, M.; Fernández, R.; Reyes, F.; Francesch, A. M.; Munt, S.; Tulla-Puche, J.; Albericio, F.; Cuevas, C. Stellatolides, a New Cyclodepsipeptide Family from the Sponge Ecionemia acervus: Isolation, Solid-Phase Total Synthesis, and Full Structural Assignment of Stellatolide A. J. Am. Chem. Soc. 2014, 136, 6754−6762. For selected reports on using β-hydroxyamide as a precursor of β-lactams or azetidines, see: (c) Hughes, D. L. The Mitsunobu Reaction. Org. Reac. 1992, 42, 335−656. (d) Gaudelli, N. M.; Townsend, C. A. Stereocontrolled Syntheses of Peptide Thioesters Containing Modified Sery Residues as Probes of Antibiotic Biosynthesis. J. Org. Chem. 2013, 78, 6412−6426. (e) Kern, N.; Hoffmann, M.; Weibel, J.-M.; Pale, P.; Blanc, A. Short and Efficient Route Toward α-Substituted N-Arylazetidines From Acetanilides via Mitsunobu Reaction. Tetrahedron 2014, 70, 5519−5531. (f) Yin, H.; Kumke, J. J.; Domino, K.; Skrydstrup, T. Palladium Catalyzed Carbonylative Coupling of Alkyl Boron Reagents with Bromodifluoroacetamides. ACS Catal. 2018, 8, 3853−3858. (14) Yamashita, R.; Sakakura, A.; Ishihara, K. Primary Alkylboronic Acids as Highly Active Catalysts for the Dehydrative Amide Condensation of α-Hydroxycarboxylic Acids. Org. Lett. 2013, 15, 3654−3657. (15) Tsuji, H.; Yamamoto, H. Hydroxy-Directed Amidation of Carboxylic Acid Esters Using a Tantalum Alkoxide Catalyst. J. Am. Chem. Soc. 2016, 138, 14218−14221. (16) For selected reviews, see: (a) Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Substrate-Directable Chemical Reactions. Chem. Rev. 1993, 93, 1307−1370. (b) Bhadra, S.; Yamamoto, H. Substrate Directed Asymmetric Reactions. Chem. Rev. 2018, 118, 3391−3446. (c) Sawano, T.; Yamamoto, H. Substrate-Directed Catalytic Selective Chemical Reactions. J. Org. Chem. 2018, 83, 4889−4904. (17) Regarding the catalytic dehydrative amidation of β-hydroxycarboxylic acid, there are only a few examples limited to threonine/ serine derivatives and 3-hydroxypropionic acid, and systematic studies have not been conducted to date. See refs 10 and 11d. (18) For selected reports on the B−O−B motif, see: (a) Hayter, R. G.; Laubengayer, A. W.; Thompson, P. G. Tetraacetyl Diborate and So-called “Boron Acetate. J. Am. Chem. Soc. 1957, 79, 4243−4244. (b) Letsinger, R. L.; Smith, J. M.; Gilpin, J.; MacLean, D. B. Organoboron Compounds. XX. Chemistry of Some 1-Naphthaleneboronic Acids with Substituents in the 8-Position. J. Org. Chem. 1965, 30, 807−812. (c) Schäfer, A.; Saak, W.; Haase, D.; Müller, T. Silyl Cation Mediated Conversion of CO2 into Benzoic Acid, Formic Acid, and Methanol. Angew. Chem., Int. Ed. 2012, 51, 2981−2984. (d) Durka, K.; Jarzembska, K. N.; Kamiński, R.; Luliński, S.; Serwatowski, J.; Woźniak, K. Nanotubular Hydrogen-Bonded Organic Framework Architecture of 1,2-Phenylenediboronic Acid Hosting Ice Clusters. Cryst. Growth Des. 2013, 13, 4181−4185. (e) Bloomfield, A. J.; Matula, A. J.; Mercado, B. Q.; Batista, V. S.; Crabtree, R. H. Organometallic Iridium Complex Containing a Dianionic, Tridentate, Mixed Organic−Inorganic Ligand. Inorg. Chem. 2016, 55, 8121− 8129. (f) Zhang, P.; Kriegel, R. M.; Frost, J. W. B−O−B Catalyzed Cycloadditions of Acrylic Acids. ACS Sustainable Chem. Eng. 2016, 4, 6991−6995. (19) Arkhipenko, S.; Sabatini, M. T.; Batsanov, A. S.; Karaluka, V.; Sheppard, T. D.; Rzepa, H. S.; Whiting, A. Mechanistic insights into boron-catalysed direct amidation reactions. Chem. Sci. 2018, 9, 1058− 1072. (20) Das, Hübner, Weber, Bolte, Lerner, and Wagner reported the synthesis of 1a. The structure of 1a was confirmed by single X-ray crystallography as diboronic acid anhydride containing a B−O−B

motif. Das, A.; Hübner, A.; Weber, M.; Bolte, M.; Lerner, H.-W. Wagner, M. 9-H-9-Borafluorene dimethyl sulfide adduct: a product of a unique ring-contraction reaction and a useful hydroboration reagent. Chem. Commun. 2011, 47, 11339−11341. (21) For selected reports on thermal amidations, see: (a) Cossy, J.; Pale-Grosdemange, C. A Convenient Synthesis of Amides From Carboxylic Acids and Primary Amines. Tetrahedron Lett. 1989, 30, 2771−2774. (b) Gooβen, L. J.; Ohlmann, D. M.; Lange, P. P. The Thermal Amidation of Carboxylic Acids Revisited. Synthesis 2009, 2009, 160−164. (c) Charville, H.; Jackson, D. A.; Hodges, G.; Whiting, A.; Wilson, M. R. The Uncatalyzed Direct Amide Formation Reaction − Mechanism Studies and the Key Role of Carboxylic Acid H-Bonding. Eur. J. Org. Chem. 2011, 2011, 5981−5990. See also ref 6a. (22) It was also mentioned that a halogen atom at the ortho-position stabilizes the transition state at the rate-determining step by halogen− hydrogen bond or orbital interactions between the halogen and boron atom; see: (a) Marcelli, T. Mechanistic Insights into Direct Amide Bond Formation Catalyzed by Boronic Acids: Halogens as Lewis Bases. Angew. Chem., Int. Ed. 2010, 49, 6840−6843. (b) Wang, C.; Yu, H.-Z.; Fu, Y.; Guo, Q.-X. Mechanism of arylboronic acid-catalyzed amidation reaction between carboxylic acids and amines. Org. Biomol. Chem. 2013, 11, 2140−2146. See also ref 7b. (23) See the Supporting Information for details. (24) In order to compare the catalytic efficiency with 1c, we examined the reaction using several organoboron and metal catalysts. As a result, it became clear that 1c exhibits higher catalytic activity compared to previous catalysts in the same conditions. See SI-Table 5 in the Supporting Information for details. (25) Amide 7 can be obtained in 68% yield by prolonging the reaction time (18 h) and using a large amount of catalyst 1c (5 mol %) under heating conditions (110 °C).

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Diboronic Acid Anhydrides as Effective Catalysts for the Hydroxy

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