Ruthenium-Catalyzed Deoxygenative Hydroboration of Carboxylic Acids

Article abstract of DOI:10.1021/acscatal.8b00900

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.

Full text of DOI:10.1021/acscatal.8b00900

Letter
Cite This: ACS Catal. 2018, 8, 4772−4776
pubs.acs.org/acscatalysis
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
lcohols are ubiquitous in nature and important building
blocks in many synthetic transformations, pharmaceut-
Scheme 1. Methods for the Reduction of Carboxylic Acids to
4
A
Alcohols: (a) Reduction by Metal Hydrides; (b) Reduction
5
icals, 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.,
by SmI
2
−H
2
O−Et
3
N Reagents; (c) Catalytic
6
7,8
Hydrogenation; and (d) Catalytic Hydrosilylation
1
fatty acids) of carboxylic acids. Reduction of carboxylic acids to
alkylboronate esters is also one of the important organic
transformation as boronate esters are found to be key synthetic
2
surrogates in organic synthesis. Moreover, boronate esters are
valuable, attractive, known for their stability and nontoxicity,
and possess excellent nucleophilicity; as a result, they are
3
preferred over the other organometallic reagents.
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, catechol-
TFA, cyanuric halides) and generation of stoichiometric
amount of inorganic waste associated with these methods
4
necessitated the development of alternative protocols. Electron
transfer reduction of carboxylic acid to alcohol via radical
pathway using SmI −H O−Et N system is also reported, which
2
2
3
required the use of a large excess of reagents (6 equiv of SmI2
and 18 equiv of water and triethylamine relative to substrates;
5
see Scheme 1b). 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
iridium showed substrate dependence and resulted in either
alcohol or aldehyde.
8
aryl carboxylic acids rather than aliphatic carboxylic acids
We have developed the ruthenium-catalyzed efficient and
6
9
(Scheme 1c). Hydrosilylation of carboxylic acids to alcohols is
selective hydroboration of carbonyl compounds, imines and
10
11
12
a facile and efficient process (Scheme 1d). However, electron-
nitriles, pyridines, and olefins. In continuation of our
rich aromatic acids generally over-reduced to provide the
7
corresponding alkane derivatives. While manganese-catalyzed
Received: March 6, 2018
Revised: April 18, 2018
hydrosilylation of carboxylic acid and further workup provided
aldehydes selectively, similar transformation catalyzed by
©
XXXX American Chemical Society
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ACS Catalysis
Letter
13
interest in ruthenium-catalyzed hydroelementation reactions,
Table 2. Ruthenium-Catalyzed Deoxygenative
herein, we describe a selective, highly efficient, and
unprecedented deoxygenative hydroboration of carboxylic
acids to alkyl boronate esters.
Hydroboration of Carboxylic Acids to Alkyl Boronate
Esters
a
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(p-
cymene)Cl ] 1 (0.1 mol %) at room temperature for 24 h
2
2
resulted in 65% conversion of benzoic acid, as confirmed by gas
chromatography (GC) analysis (entry 1 in Table 1) and the
Table 1. Optimization of Deoxygenative Hydroboration of
a
Carboxylic Acids
b
HBpin
(equiv)
catalyst
(mol %)
temperature
conversion
(%)
entry
(°C)
1
2
3
4
5
6
3
4
4
4
3
4
0.1
0.1
0.1
0.05
0.1
rt
65
76
rt
60
60
60
60
>99
80
a
Conditions: carboxylic acid (0.5 mmol), HBpin (2 mmol), and
95
catalyst 1 (0.1 mol %) were stirred at 60 °C for 24 h; Reported values
are conversion of carboxylic acid, based on GC analysis. HBpin (3.5
mmol) was used. HBpin (2.5 mmol) was used.
b
trace
a
c
Conditions: benzoic acid (1 mmol), catalyst 1 were taken in a
scintillation vial. Conversion of benzoic acid is based on GC analysis.
b
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 SmI −H O−Et N reagent, aryl bromides
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
2
2
3
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
(
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).
14
polymer industry and in the production of surfactants.
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 corre-
sponding boronate esters were obtained in excellent con-
versions. Generally, hydrogenation of diacids has a drawback of
either low yield or providing mixture of lactone and diol
With these optimization studies, an assortment of aromatic
and aliphatic carboxylic acids was subjected to deoxygenative
hydroboration (Table 2), which exhibited efficient hydro-
boration 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 4-
ethoxy benzoic acid were efficiently converted to the
corresponding boronate esters. Notably, iron-catalyzed hydro-
silylation of electron-rich carboxylic acid led to over-reduction,
6a
products. 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-3-
acetic acid was reduced to the corresponding boronate ester in
95% conversion (see Table 2).
7b
resulting in alkane formation. 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-with-
drawing carboxylic acid (4-fluoro benzoic acid) resulted in
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
4
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Table 3. Catalytic Synthesis of Boronate Esters and their
Hydrolysis to Alcohols
RuCl} (μ-H-μ-Cl)] (1b), as observed in the hydroboration of
2
a
carbonyl compounds, nitriles, imines, and olefin function-
9
,10,12
alities.
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
9
,10,12
intermediacy of 1b in catalysis (Scheme 3).
Scheme 3. Catalytic Deoxygenative Hydroboration of
Benzoic Acid by an Isolated Intermediate 1b
The progress of the catalytic hydroboration of phenylacetic
1
acid with pinacolborane was also monitored by H 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 4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl 2-phenylacetate disap-
peared upon the appearance and rise of two new triplets at δ
2
.82 and 4.14 ppm responsible for the corresponding phenethyl
15
boronate ester). Furthermore, a mercury-poisoning experi-
ment 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
b
catalyst 1 (0.1 mol %) were stirred at 60 °C for 24 h. Conversion is
based on GC analysis. Yields correspond to isolated alcohols after
c
Scheme 4. Mercury Poisoning Test and Stoichiometric
Experiments
column chromatography.
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
1
benzoate and benzyl benzoate remained unreacted ( H NMR,
see the Supporting Information) and recovered quantitatively
by column chromatography (see Scheme 2).
1
The in situ H NMR monitoring of the stoichiometric
experiment of benzoic acid, pinacolborane, and [Ru(p-
cymene)Cl ] 1 indicates the immediate formation of the
dinuclear monohydride bridged intermediate [{(η -p-cymene)-
2
2
6
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
Scheme 2. Chemoselective Hydroboration of Carboxylic
Acid with Esters
1
new compound was observed via H NMR (see Scheme 4b).
However, upon reaction of complex 1b with boryl ester, partial
formation of benzaldehyde was observed in H NMR (Scheme
1
4
c). 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
1
dihydride Ru(IV) intermediate 1c ( H NMR = δ −13.45 ppm,
Scheme 4d). Intermediate 1c was earlier observed in situ upon
9
,16
the hydroboration of carbonyl compounds.
Notably, the
aldehyde C−H signal of benzaldehyde resonated at δ 10.81
4
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ACS Catalysis
Letter
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.
condition favors the reductive hydroboration of carboxylic acid,
which further substantiates the involvement of a dihydride
Ru(IV) reaction pathway.
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
Furthermore, to confirm the involvement of mononuclear
ruthenium species in the catalytic cycle, phosphine-ligated half-
sandwich ruthenium complexes [Ru(p-cymene)Cl (PPh )]
2
3
(
4a) and [Ru(p-cymene)Cl (PCy )] (4b) were prepared and
2 3
10,12
Scheme 5. Proposed Mechanism for the Ruthenium-
employed as catalysts
in the deoxygenative hydroboration
Catalyzed Deoxygenative Hydroboration of Carboxylic Acids
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 corre-
sponding boronate esters (see Scheme 6).
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
pinacolborane to provide the ruthenium monohydride bridged
6
13,9,10
intermediate [{(η -p-cymene)RuCl} (μ-H-μ-Cl)] 1b.
The
2
reaction of complex 1b with pinacolborane results in splitting
into two monomeric intermediate complexes [Ru(p-cymene)-
9
,10,12
HCl] and [Ru(p-cymene)(Cl)2]
to provide Ru(II)
alcohols in excellent yields. Catalytic hydroboration proceed via
intermediate I, which may involve the intermediacy of Ru(0)
6
17
intermediacy of [{(η -p-cymene)RuCl} (μ-H-μ-Cl)] (1b) and
2
species and activation of the B−H bond. Notably, the in situ
formation of intermediate I was observed on selective anti-
further studies also indicated the involvement of mononuclear
ruthenium intermediates in the catalytic cycle. An intra-
molecular 1,3-hydride transfer leading to the reduction of
carbonyl motif is proposed.
12
Markovnikov hydroboration of terminal olefins. Boryl ester
formation proceed with liberation of dihydrogen from a
noncatalytic reaction of carboxylic acids with pinacolborane
1
8
(
Scheme 5). The boryl ester coordinates to the metal center
ASSOCIATED CONTENT
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
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.8b00900.
1
1
Experimental procedures, spectral data, and copies of H,
benzaldehyde (when R = Ph, both observed by H NMR)
13C NMR spectra of the products (PDF)
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
AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: gunanathan@niser.ac.in.
ORCID
Varadhan Krishnakumar: 0000-0001-9400-3429
Chidambaram Gunanathan: 0000-0002-9458-5198
1
9
out. 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,
Author Contributions
†
1
These authors contributed equally to this work.
respectively, as analyzed by H NMR. These observations
indicate that the reaction is sluggish under open conditions,
whereas the presence of hydrogen atmosphere in the closed
Notes
The authors declare no competing financial interest.
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Letter
Homogeneous Hydrogenation of Biogenic Carboxylic Acids with
ACKNOWLEDGMENTS
■
+
[
Ru(TriPhos)H] : A Mechanistic Study. J. Am. Chem. Soc. 2011, 133,
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and NISER for financial support. S.K. thanks DST for INSPIRE
fellowship. V.K. thanks SERB for National Postdoctoral
Fellowship.
1
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DOI: 10.1021/acscatal.8b00900
ACS Catal. 2018, 8, 4772−4776