Ruthenium-Catalyzed Deoxygenative Hydroboration of Carboxylic Acids

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Letter Cite This: ACS Catal. 2018, 8, 4772−4776

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Ruthenium-Catalyzed Deoxygenative Hydroboration of Carboxylic Acids Sesha Kisan,† Varadhan Krishnakumar,† and Chidambaram Gunanathan* School of Chemical Sciences, National Institute of Science Education and Research, HBNI, Bhubaneswar-752050, India S Supporting Information *

ABSTRACT: An efficient deoxygenative hydroboration of carboxylic acids to alkyl boronate esters under mild reaction condition is reported. Both aromatic and aliphatic carboxylic acids exhibited excellent reactivities with minimal catalyst load of 0.1 mol % and reactions occurred under neat conditions. This catalytic transformation selectively provides alkyl boronate esters, which can be conveniently hydrolyzed to obtain the corresponding alcohols. Remarkably, this reduction reaction proceeds with the liberation of molecular hydrogen. KEYWORDS: hydroboration, deoxygenation, boronate esters, carboxylic acid, catalysis

A

Scheme 1. Methods for the Reduction of Carboxylic Acids to Alcohols: (a) Reduction by Metal Hydrides;4 (b) Reduction by SmI2−H2O−Et3N Reagents;5 (c) Catalytic Hydrogenation;6 and (d) Catalytic Hydrosilylation7,8

lcohols are ubiquitous in nature and important building blocks in many synthetic transformations, pharmaceuticals, and material syntheses. Among various synthetic maneuvers to obtain the alcohols, reduction of carboxylic acids is an attractive method, because of the stability, ready availability, inexpensive nature, and natural abundance (e.g., fatty acids) of carboxylic acids.1 Reduction of carboxylic acids to alkylboronate esters is also one of the important organic transformation as boronate esters are found to be key synthetic surrogates in organic synthesis.2 Moreover, boronate esters are valuable, attractive, known for their stability and nontoxicity, and possess excellent nucleophilicity; as a result, they are preferred over the other organometallic reagents.3 Conventionally, highly reactive hydride reagents such as LiAlH4 and NaBH4 are employed for the reduction of carboxylic acids (see Scheme 1a). However, safety concerns, poor selectivity, requirement of activators (iodine, catecholTFA, cyanuric halides) and generation of stoichiometric amount of inorganic waste associated with these methods necessitated the development of alternative protocols.4 Electron transfer reduction of carboxylic acid to alcohol via radical pathway using SmI2−H2O−Et3N system is also reported, which required the use of a large excess of reagents (6 equiv of SmI2 and 18 equiv of water and triethylamine relative to substrates; see Scheme 1b).5 Many homogeneous and heterogeneous catalyzed hydrogenations of carboxylic acids were developed with various transition metals. However, invariably, these methods employ high temperature and a high pressure of molecular hydrogen and often applicable for the reduction of aryl carboxylic acids rather than aliphatic carboxylic acids (Scheme 1c).6 Hydrosilylation of carboxylic acids to alcohols is a facile and efficient process (Scheme 1d). However, electronrich aromatic acids generally over-reduced to provide the corresponding alkane derivatives.7 While manganese-catalyzed hydrosilylation of carboxylic acid and further workup provided aldehydes selectively, similar transformation catalyzed by © XXXX American Chemical Society

iridium showed substrate dependence and resulted in either alcohol or aldehyde.8 We have developed the ruthenium-catalyzed efficient and selective hydroboration of carbonyl compounds,9 imines and nitriles,10 pyridines,11 and olefins.12 In continuation of our Received: March 6, 2018 Revised: April 18, 2018

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DOI: 10.1021/acscatal.8b00900 ACS Catal. 2018, 8, 4772−4776

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ACS Catalysis interest in ruthenium-catalyzed hydroelementation reactions,13 herein, we describe a selective, highly efficient, and unprecedented deoxygenative hydroboration of carboxylic acids to alkyl boronate esters. At the outset, the catalytic deoxygenative hydroboration with pinacolborane was investigated using benzoic acid. Reaction of benzoic acid (1 mmol) with pinacolborane (3 mmol, HBpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolane) and complex [Ru(pcymene)Cl2]2 1 (0.1 mol %) at room temperature for 24 h resulted in 65% conversion of benzoic acid, as confirmed by gas chromatography (GC) analysis (entry 1 in Table 1) and the

Table 2. Ruthenium-Catalyzed Deoxygenative Hydroboration of Carboxylic Acids to Alkyl Boronate Estersa

Table 1. Optimization of Deoxygenative Hydroboration of Carboxylic Acidsa

entry

HBpin (equiv)

catalyst (mol %)

temperature (°C)

conversionb (%)

1 2 3 4 5 6

3 4 4 4 3 4

0.1 0.1 0.1 0.05 0.1

rt rt 60 60 60 60

65 76 >99 80 95 trace

a

Conditions: carboxylic acid (0.5 mmol), HBpin (2 mmol), and catalyst 1 (0.1 mol %) were stirred at 60 °C for 24 h; Reported values are conversion of carboxylic acid, based on GC analysis. bHBpin (3.5 mmol) was used. cHBpin (2.5 mmol) was used.

a

Conditions: benzoic acid (1 mmol), catalyst 1 were taken in a scintillation vial. bConversion of benzoic acid is based on GC analysis.

higher yield of the corresponding product (83%). The results imply that the electron-rich carboxylic acids were less effective in the shorter reaction time. In contrast to the reduction of carboxylic acid with SmI2−H2O−Et3N reagent, aryl bromides were survived under this catalytic deoxygenative hydroboration. Interestingly, the aliphatic carboxylic acids were also successfully transformed to the corresponding alkyl boronate esters in excellent conversions. Low- to medium-chain aliphatic carboxylic acids are biorelevant substrates and analogues of these alcohols are widely used as solvents and also employed in polymer industry and in the production of surfactants.14 Treatment of acetic acid with pinacolborane proceeds smoothly to afford the ethylboronate ester in a quantitative conversion. Similarly, other aliphatic carboxylic acids, such as pentanoic acid, hexanoic acid, heptanoic acid, 2-cyclohexyl acetic acid, and pivalic acid, have furnished the respective boronate esters in 96% to quantitative conversion as analyzed by GC. When the phenyl ring containing aliphatic acids and dicarboxylic acids (such as homophthalic acid and adipic acid) were subjected to deoxygenative hydroboration with pinacolborane, the corresponding boronate esters were obtained in excellent conversions. Generally, hydrogenation of diacids has a drawback of either low yield or providing mixture of lactone and diol products.6a Thus, this efficient deoxygenative hydroboration, leading to the diboronate esters and subsequently, to the corresponding diol can provide alternative protocol for the efficient synthesis of diols. Notably, heteroaromatic indole-3acetic acid was reduced to the corresponding boronate ester in 95% conversion (see Table 2). Representatively, selected boronate esters were hydrolyzed to provide corresponding alcohols. As summarized in Table 3, excellent conversions of carboxylic acids to alkylboronate esters and, further, to the alcohols were obtained. Remarkably, sterically hindered tert-butyl carboxylic acid provided 99% conversion to boronate ester and, upon hydrolysis, the

NMR analyses of the reaction mixture, which indicated the selective formation of benzyl boronate ester and diboryl ether. The use of 4 equiv of pinacolborane provided the 76% conversion of benzoic acid (entry 2 in Table 1). Notably, upon increase of reaction temperature to 60 °C with 4 equiv of HBpin, quantitative conversion of benzoic acid was observed (entry 3 in Table 1). However, decreasing the catalyst load further to 0.05 mol % diminished the conversion of benzoic acid (entry 4 in Table 1). Similarly, reducing the amount of pinacolborane (3 mmol) lowered the conversion (entry 5 in Table 1). A control experiment that was performed without catalyst confirmed that the role of catalyst is necessary for this transformation (see entry 6 in Table 1). With these optimization studies, an assortment of aromatic and aliphatic carboxylic acids was subjected to deoxygenative hydroboration (Table 2), which exhibited efficient hydroboration upon reaction with 1 (0.1 mol %), irrespective of steric and electronic nature of the substrate. Interestingly, the reaction required no solvent. Electron-rich carboxylic acids such as 4-tert-butyl benzoic acid, 2-methoxy benzoic acid, and 4ethoxy benzoic acid were efficiently converted to the corresponding boronate esters. Notably, iron-catalyzed hydrosilylation of electron-rich carboxylic acid led to over-reduction, resulting in alkane formation.7b To test the possible formation of over-reduced product of benzoic acid to toluene, benzoic acid was reacted with increased catalyst load (1, 1 mol %) and pinacolborane (6 equiv) at slightly elevated temperature (80 °C) in which the selective formation of benzyl boronate ester was observed as an exclusive product. Benzoic acids containing electron-withdrawing functionalities also provided the boronate esters in 94%−98% conversion. However, when intervened earlier (8 h), the electron-donating carboxylic acid (4-tert-butyl benzoic acid) provided 70% yield, whereas electron-withdrawing carboxylic acid (4-fluoro benzoic acid) resulted in 4773

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RuCl}2(μ-H-μ-Cl)] (1b), as observed in the hydroboration of carbonyl compounds, nitriles, imines, and olefin functionalities.9,10,12 When isolated intermediate 1b was employed in catalysis, excellent conversion of benzoic acid to boronate ester was observed, similar to that of 1, indicating the potential intermediacy of 1b in catalysis (Scheme 3).9,10,12

Table 3. Catalytic Synthesis of Boronate Esters and their Hydrolysis to Alcoholsa

Scheme 3. Catalytic Deoxygenative Hydroboration of Benzoic Acid by an Isolated Intermediate 1b

The progress of the catalytic hydroboration of phenylacetic acid with pinacolborane was also monitored by 1H NMR, which confirmed the disappearance of boryl ester (that was derived from carboxylic acid and pinacolborane from a noncatalytic reaction) over the time and formation of phenethylboronate ester (characteristic singlet signal appeared at δ 3.42 ppm corresponds to methylene protons of 4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl 2-phenylacetate disappeared upon the appearance and rise of two new triplets at δ 2.82 and 4.14 ppm responsible for the corresponding phenethyl boronate ester).15 Furthermore, a mercury-poisoning experiment was performed to examine the involvement of any metal nanoparticles formed from complex 1. When the hydroboration of benzoic acid was carried out in the presence of mercury (10 equiv, relative to substrate), complete conversion of benzoic acid to benzyl boronate ester was obtained (see Scheme 4a),

a

Conditions: carboxylic acid (0.5 mmol), HBpin (2 mmol), and catalyst 1 (0.1 mol %) were stirred at 60 °C for 24 h. bConversion is based on GC analysis. cYields correspond to isolated alcohols after column chromatography.

Scheme 4. Mercury Poisoning Test and Stoichiometric Experiments

corresponding tert-butyl methanol 3e was obtained in 93% yield, which demonstrates the efficiency and applicability of this process in reduction of diverse carboxylic acids to alcohols. Overall, this process demonstrates the mild reduction of carboxylic acids to alcohols. Next, the chemoselective hydroboration of carboxylic acid with esters was explored. The competitive intermolecular catalytic hydroboration of benzoic acid with methyl benzoate and benzyl benzoate resulted in exclusive formation of deoxygenated boronate ester from carboxylic acid. Both methyl benzoate and benzyl benzoate remained unreacted (1H NMR, see the Supporting Information) and recovered quantitatively by column chromatography (see Scheme 2). The in situ 1H NMR monitoring of the stoichiometric experiment of benzoic acid, pinacolborane, and [Ru(pcymene)Cl2]2 1 indicates the immediate formation of the dinuclear monohydride bridged intermediate [{(η6-p-cymene)-

implying that the reaction proceeds through molecular intermediates. In a stoichiometric experiment, when complex 1 was reacted with benzoyl boronate ester, no formation of a new compound was observed via 1H NMR (see Scheme 4b). However, upon reaction of complex 1b with boryl ester, partial formation of benzaldehyde was observed in 1H NMR (Scheme 4c). Similarly, stoichiometric reaction of complexes 1 or 1b with benzoic acid and pinacolborane (4 equiv) indicate the presence of 1b, and minor amounts of benzaldehyde and dihydride Ru(IV) intermediate 1c (1H NMR = δ −13.45 ppm, Scheme 4d). Intermediate 1c was earlier observed in situ upon the hydroboration of carbonyl compounds.9,16 Notably, the aldehyde C−H signal of benzaldehyde resonated at δ 10.81

Scheme 2. Chemoselective Hydroboration of Carboxylic Acid with Esters

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condition favors the reductive hydroboration of carboxylic acid, which further substantiates the involvement of a dihydride Ru(IV) reaction pathway. Furthermore, to confirm the involvement of mononuclear ruthenium species in the catalytic cycle, phosphine-ligated halfsandwich ruthenium complexes [Ru(p-cymene)Cl2(PPh3)] (4a) and [Ru(p-cymene)Cl2(PCy3)] (4b) were prepared and employed as catalysts10,12 in the deoxygenative hydroboration of benzoic acid and phenylacetic acid. Complex 4a provided slightly diminished yield (94%) of benzyl boronate ester (2a), whereas complex 4b with benzoic acid and 4a or 4b with phenylacetic acid provided the quantitative yields of corresponding boronate esters (see Scheme 6).

ppm on both experiments, against the appearance of free benzaldehyde C−H signal at δ 10.03 ppm, perhaps due to the coordination of aldehyde motif to the metal center. On the basis of these experimental observations, the reaction mechanism for deoxygenative hydroboration of carboxylic acid functionality is delineated in Scheme 5. Complex 1 reacts with Scheme 5. Proposed Mechanism for the RutheniumCatalyzed Deoxygenative Hydroboration of Carboxylic Acids

Scheme 6. Mononuclear Ruthenium-Catalyzed Deoxygenative Hydroboration of Carboxylic Acid

In summary, a facile and highly efficient deoxygenative hydroboration of carboxylic acid is reported. Remarkably, the reactions require no solvent and the commercially available simple ruthenium catalyst was employed with a low catalyst load of 0.1 mol %. This hydroboration method is highly selective and an assortment of both aromatic and aliphatic carboxylic acid was transformed to the respective alkyl boronate esters. Representatively, selected substrates were further subjected to the hydrolysis to provide the corresponding alcohols in excellent yields. Catalytic hydroboration proceed via intermediacy of [{(η6-p-cymene)RuCl}2(μ-H-μ-Cl)] (1b) and further studies also indicated the involvement of mononuclear ruthenium intermediates in the catalytic cycle. An intramolecular 1,3-hydride transfer leading to the reduction of carbonyl motif is proposed.

pinacolborane to provide the ruthenium monohydride bridged intermediate [{(η6-p-cymene)RuCl}2(μ-H-μ-Cl)] 1b.13,9,10 The reaction of complex 1b with pinacolborane results in splitting into two monomeric intermediate complexes [Ru(p-cymene)HCl] and [Ru(p-cymene)(Cl)2]9,10,12 to provide Ru(II) intermediate I, which may involve the intermediacy of Ru(0) species and activation of the B−H bond.17 Notably, the in situ formation of intermediate I was observed on selective antiMarkovnikov hydroboration of terminal olefins.12 Boryl ester formation proceed with liberation of dihydrogen from a noncatalytic reaction of carboxylic acids with pinacolborane (Scheme 5).18 The boryl ester coordinates to the metal center to provide the intermediate II. Reduction of the carbonyl motif occurs via an intramolecular 1,3-transfer of a “hydride” ligand from Ru to carbonyl carbon to generate III. Further oxidative addition of pinacolborane to intermediate III results in the formation of a dihydride Ru(IV) intermediate 1c and benzaldehyde (when R = Ph, both observed by 1H NMR) with a concomitant formation of diboryl ether. The intermediate 1c reacts with aldehyde that has been formed in situ, which generates the boronate esters and regenerates intermediate I to continue the catalytic cycles. However, an alternative reaction pathway involving the formation of Lewis adduct from the intermediate III and pinacolborane with subsequent B−H activation, leading to the formation of aldehyde and the liberation of diboryl ether, cannot be ruled out.19 When the catalytic hydroboration of benzoic was performed under open and closed conditions, after 5 h, 38% and 63% formation of benzyl boronate ester were observed, respectively, as analyzed by 1H NMR. These observations indicate that the reaction is sluggish under open conditions, whereas the presence of hydrogen atmosphere in the closed

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b00900. Experimental procedures, spectral data, and copies of 1H, 13 C NMR spectra of the products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Varadhan Krishnakumar: 0000-0001-9400-3429 Chidambaram Gunanathan: 0000-0002-9458-5198 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest. 4775

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ACKNOWLEDGMENTS We thank SERB New Delhi (No. EMR/2016/002517), DAE and NISER for financial support. S.K. thanks DST for INSPIRE fellowship. V.K. thanks SERB for National Postdoctoral Fellowship.

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DOI: 10.1021/acscatal.8b00900 ACS Catal. 2018, 8, 4772−4776