Diboron-Catalyzed Dehydrative Amidation of Aromatic Carboxylic Acids with Amines

Article abstract of DOI:10.1021/acs.orglett.8b01480

Tetrakis(dimethylamido)diboron and tetrahydroxydiboron are herein reported as new catalysts for the synthesis of aryl amides by catalytic condensation of aromatic carboxylic acids with amines. The developed protocol is both simple and highly efficient over a broad range of substrates. This method thus represents an attractive approach for the use of diboron catalysts in the synthesis of amides without having to resort to stoichiometric or additional dehydrating agents.

Full text of DOI:10.1021/acs.orglett.8b01480

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Diboron-Catalyzed Dehydrative Amidation of Aromatic Carboxylic
Acids with Amines
Dinesh N. Sawant,§,† Dattatraya B. Bagal,† Saeko Ogawa,‡ Kaliyamoorthy Selvam,‡
and Susumu Saito*,‡
‡
†Research Center for Materials Science and Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: Tetrakis(dimethylamido)diboron and tetrahy-
droxydiboron are herein reported as new catalysts for the
synthesis of aryl amides by catalytic condensation of aromatic
carboxylic acids with amines. The developed protocol is both
simple and highly efficient over a broad range of substrates. This
method thus represents an attractive approach for the use of
diboron catalysts in the synthesis of amides without having to
resort to stoichiometric or additional dehydrating agents.
mide moieties are ubiquitous in, e.g., pharmacologically
for the amidation of aromatic carboxylic acids.7 However, the
Aactive compounds, agrochemicals, as well as synthetic and use of coupling reagents has its own limitations, as, e.g., the
natural polymers.1 Several methods have been developed for
formation of stoichiometric amounts of waste is inevitable,
which decreases the atom economy.13
the construction of amide C−N bonds, which include
carbonylative amidations,2 transamidations,3 the hydrolysis of
A major reason for the lower number of examples reported
nitriles,4 and the condensation of carboxylic acids with
on the ideal one-step condensation of aromatic carboxylic acids
amines.5 Among these, the direct condensation of carboxylic
with amines is their higher acidity relative to that of the
aliphatic analogues. In reality, this reaction ends with the
formation of an ammonium salt of the aryl carboxylate, which
renders the synthesis of the amide kinetically less favora-
ble.7,9e,14 Likewise, this synthetic obstacle can be easily
demonstrated by simple comparison experiments, i.e., cata-
lyst-free thermal reactions of the same amine with an aliphatic
or aromatic carboxylic acid, which affords different products
acids with amines is one of the simplest ways to prepare
amides, and several homogeneous and heterogeneous tran-
sition metal-based catalysts have been reported to promote the
formation of the amide bond.6,7 Subsequently, the use of
Lewis-acidic boron catalysts for the direct amidation between
carboxylic acids and amines has been discovered as a promising
greener alternative. In 1996, Yamamoto et al. reported the first
example of the dehydrative amide condensation catalyzed by
substituted phenylboronic acids under azeotropic conditions
and an atmosphere of inert gas.8a Since then, much progress
has been accomplished on such organoboron compounds via,
e.g., tuning the electronic nature of boron by modulating their
substituents, which may result in the formation of hydrogen-
acceptor bonds. Seminal studies by Ishihara,8 Whiting,9
Blanchet,10 and Hall11 have shown that bespoke boronic/
borinic acids can serve as more effective catalysts. It should be
noted, however, that most of the reported boronic/borinic
acid-catalyzed amidation protocols aim at the synthesis of
aliphatic amide derivatives. In contrast, examples for aromatic
carboxylic acids such as benzoic acid derivatives remain scarce.
Very recently, Kumagai and Shibasaki et al. disclosed that a
B3NO2 ring system based on 1,3-dioxa-5-aza-2,4,6-triborinane
(DATB) efficiently promotes this reaction at low temperature
(50 °C−reflux in toluene).12 However, in this study, molecular
sieves (4 Å) were used as a dehydrating agent, and systematic
studies on a viable catalyst system that exhibits increased
compatibility with various combinations of aromatic carboxylic
acids and amines are still elusive. A practical alternative that is
widely encountered in industry is the use of coupling reagents
(eqs 1 and 2). At 90 °C in air, the aliphatic acid was fully
converted to the corresponding amide within 24 h.
Given our continuous interest in developing new methods
for the catalytic addition of H−H and O−H bonds to the C=O
bonds of amides15 and carboxylic acids,16 as well as the
removal of OH groups from alcohols in the form of H2O by SN
reactions in N-alkylations,17 we report herein the use of
commercially available tetrakis(dimethylamido)diboron (DB-
1) and tetrahydroxydiboron (DB-3) as efficient dehydrative
Received: May 10, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01480
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Various Combinations in the Amidation between
a b
,
1 and 2 to give 3
Figure 1. Working hypothesis for this study: commercially available
diboron reagents of the type (X2B−BX2) (DB-1: X = NMe2; DB-3: X
= OH) as potential catalysts for the amidation of aromatic carboxylic
acids.
Scheme 1. Initial Screening of Different Diboron (DB) or
a b
,
Boron (B) Catalysts for the Amidation of 1a with 2a
a
Reaction conditions: 1a (0.5 mmol) and 2a (0.5 mmol) in toluene
b
(10 mL). GC-MS yield of 3aa under N2 or in air (in parentheses)
using chlorobenzene as the internal standard.
Figure 2. Control experiments: effects of the addition order of DB (2
mol %) and substrates on the yield of 3aa (eqs 3 and 4; [1a]0 = [2a]0
= 100 mM) and proposed structure for IDB•1a, which was detected by
ESI-HRMS.
catalysts for the formation of C−N bonds (N−H addition to
the C=O bond of HOC=O and subsequent removal of the OH
group) between aromatic carboxylic acids and amines (Figure
1). Considering that these diboron reagents are inexpensive
and easy to handle, it is very surprising that they have not yet
been used for such dehydrative amidations.
a
Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol) and DB-1 (2 mol
%), DB-3 (2 mol %) or B-1 (4 mol %) in anhydrous toluene (5 mL).
b
NMR yield based on 1,1,2,2-tetrachloroethane as the internal
c
standard with isolated yields in parentheses. In refluxing
d
chlorobenzene instead of toluene. Using DB-1 (10 mol %).
e
f
Prior to starting this project, our working hypothesis (Figure
1) was based on two critical aspects: (i) retrieving the free
PhCO(NMe)(CH2)2NHCOPh (5%) was a by-product. X =
CO2R (3ak: R = PhCH2; 3al−3ao: R = Me).
B
DOI: 10.1021/acs.orglett.8b01480
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
amine from the corresponding ammonium salt and (ii)
selectively activating the carboxylate by a bidentate coordina-
tion to the B−B moiety. Steps (i) and (ii) should be facilitated
by the cooperative role of the two boron centers and a
secondary coordination (ligand) sphere. Because the molecular
framework of IDB•1 contains four X groups that are
electronically equivalent, the liberated proton should be
distributed with equal probability over the four X groups,
which would enhance the charge separation where the free
amine and H+ would be spatially separated under a potentially
smooth concomitant protonation of the C=O group.
The dehydrative amidation was initially examined using
benzoic acid (1a) and benzylamine (2a) as the representative
substrates in the presence of 5 mol % of DB-1 (X = NMe2 in
Figure 1) in anhydrous toluene under N2 and azeotropic reflux
conditions (Scheme 1). Within 6 h, amide 3aa was formed in
81% yield. Gratifyingly, the azeotropic reflux in air afforded 3aa
more efficiently. For example, when the reaction was carried
out in air for 6 h with a decreased catalyst loading of DB-1 (1−
2 mol %) or at higher concentration ([1a]0 = 100 mM; [DB-
1]0 = 5 mM), 3aa was obtained in 86−88% and 98% yields,
respectively. Upon decreasing the temperature to 80 °C, only a
negligible amount of 3aa was produced. In contrast, the use of
bis(pinacolato)diboron (DB-2, X = O; 5 mol %, Scheme 1)
resulted in a substantial decline of the reaction rate (3aa: 32−
34%).
Encouraged by these initial studies, we conducted a series of
control experiments to examine potential further advantages of
using a reduced amount of DB-1 (2 mol %) compared to DB-2
(2 mol %) under azeotropic reflux conditions in air (Figure 2).
The addition order of 1a, 2a, and DB-1 did not affect the final
yield of 3aa (Figure 2, eqs 3 and 4), whereas the reaction with
DB-2 was consistently slow. The initial interaction of 1a with
DB-1 afforded 3aa quantitatively (Figure 2, eq 3), which
supports our working hypothesis (Figure 1). The formation of
catalyst intermediate IDB•1 (Figure 1), which contains a B−B
structure, was confirmed by ESI-HRMS analysis (Figure 2 and
Figure S19). A chemical entity, presumably IDB•1a (m/z =
320.2555; calcd: 320.2555) in which DB-1 and 1a interact
datively and/or ionically, was detected. Such tetra-coordinated
boron center(s), obtained from the more preferable interaction
of the B−B moiety with the carboxylate than the amine, was
confirmed by a 11B NMR spectroscopic analysis (external
standard: BF3·OEt2 at 0 ppm) of a 1:1:1 molar mixture of DB-
1 (δ 35.0, bs), 1a, and pyrrolidine (2f), which showed 11B
signals (δ 21.1, bs and 2.7, s) similar to those of a 1:1 mixture
of DB-1 and 1a (δ 20.7, bs and 2.7, s) (Figure S2). In any
event, selective “1a−DB-1 interaction” that initially occurs is
primarily important at the early stage during the course of
amidation. Simple reflux conditions (where a reflux condenser
without a water trap was used) gave higher yields of 3aa when
using DB-1 (68%) than when using DB-2 (<5%). These results
highlight practical advantages over previous methods that
frequently use molecular sieves as a dehydrating agent and/or
azeotropic reflux conditions, which require cooling water and
inert gas.
possibility of the formation of B-1 from DB-1 and DB-3
followed by in situ generation of boron (tri)carboxylate as the
catalytic species could be ruled out. Bis(neopentylglycolato)-
diboron (DB-4), bis(hexylene glycolato)diboron (DB-5), and
bis(catecholato)diboron (DB-6) consistently furnished much
poorer yields of 3aa. The low yield of 3aa using DB-6 may be
due to the lower basicity of the substituents on boron (in this
case, a lone pair on oxygen) that are delocalized over the Ph
groups of the catecholates.
Under the optimized conditions (azeotropic reflux in air),
the substrate scope of the amidation of several aromatic
carboxylic acids with 2a using a catalytic amount of DB-1 (2
mol %) was explored, and the results were compared with the
amidation using DB-3, B-1, and that without using any
catalysts (Scheme 2). Both electron-donating and electron-
withdrawing groups at the para, meta, and ortho position of 1a
were mostly compatible, furnishing the corresponding
amidation products (3aa−3ha and 3ka−3oa). Functional
thioether, aryl iodide, (benzo)furan, thiophene, and pyridine
groups were also tolerated (3ea, 3ga, and 3pa−3sa). While
employing 4-nitrobenzoic acid (1i) and 4-(dimethylamino)-
benzoic acid (1j), the corresponding carboxylates were
considerably less soluble in toluene. Hence, the reactions
were carried out in chlorobenzene at higher temperature,
which resulted in the formation of 3ia in 98% yield (for a
solvent screening, see Table S2). The amidation of 1j
proceeded sluggishly to furnish 3ja in low yield, as the
solubility of the corresponding amine salt of 1j remains low.
Acid 1a was also tested in the reaction with different amines
(2b−2g) (Scheme 2), where primary (1°, 2b and 2c) and
secondary (2°, 2e−2g) aliphatic amines underwent effective
amidations using catalytic DB-1 or DB-3. The less basic aniline
2d showed a lower reactivity (3ad: 37%), although DB-1 still
proved to be the best catalyst among those tested. Sterically
bulky ArCO2H (1t and 1u) and 2° amines (2h and 2i) are also
compatible with reaction conditions, giving 3ta, 3ua, 3ah, and
3ai. The 1° amine of 2j underwent faster amidation over the
interior 2° amine, giving 3aj. Amidation of 1a or other
carboxylic acids with chiral amines such as C-protected α-
amino acids with different functionalized side chains (Scheme
2, 3ak−3ao and Table S3) were amenable to the catalyst
system, although H2N-Thr-OMe underwent poor amidation
under azeotropic reflux of toluene. Slight to moderate
racemization was observed, giving scalemic 3ak and 3al
(Figures S13 and S14) derived from 1a. In contrast,
epimerization at the stereogenic carbon center of the
amidation products Boc-Gly-Val-OBn and Boc-Gly-Phe-
NHBn was negligible (Figures S16 and S17). Scaling up the
amidation of 1a with 2a to 40 mmol worked as well (Figure
S6).
In summary, we have demonstrated that the diboron
compounds can serve as viable catalyst alternatives for the
promotion of one of the most difficult amidation reactions
involving aromatic carboxylic acids. Further extensions of this
method to amidation that involve a series of other N- and C-
protected amino acids with chiral centers are currently in
progress.
Different catalytic activities among various diboron reagents
and a boric acid (B-1)18 were also compared in the dehydrative
amidation of 1a with 2a (Scheme 1; for detailed optimization
studies, see Table S1). Notably, tetrahydroxydiboron (DB-3)
in which characteristic multiple OH groups similar to that of B-
1 are incorporated afforded an appreciable yield of 3aa (81%).
Because B-1 (10 mol %) barely promoted the reaction, the
C
DOI: 10.1021/acs.orglett.8b01480
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01480.
■
S
Experimental details and spectroscopic data for all
intermediates, reactants, and products (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: saito.susumu@f.mbox.nagoya-u.ac.jp.
ORCID
Dinesh N. Sawant: 0000-0003-2064-1692
Dattatraya B. Bagal: 0000-0002-4755-5893
Kaliyamoorthy Selvam: 0000-0001-7539-871X
Susumu Saito: 0000-0003-0749-2020
Present Address
§D.N.S.: Leibniz Institute for Catalysis (LIKAT Rostock),
Albert-Einstein-Straße, 29a, 18059 Rostock, Germany.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by JSPS KAKENHI (No.
JP16H01012 in Precisely Designed Catalysts with Customized
Scaffolding) and partially by the Asahi Glass Foundation (to
S.S.). D.N.S. and D.B.B. are supported by IRCCS, Nagoya
University, and S.K. acknowledges a JSPS postdoctoral
fellowship (standard) for research in Japan.
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