Isonicotinate Ester Catalyzed Decarboxylative Borylation of (Hetero)Aryl and Alkenyl Carboxylic Acids through N-Hydroxyphthalimide Esters

Article abstract of DOI:10.1021/acs.orglett.7b01950

Decarboxylative borylation of aryl and alkenyl carboxylic acids with bis(pinacolato)diboron was achieved through N-hydroxyphthalimide esters using tert-butyl isonicotinate as a catalyst under base-free conditions. A variety of aryl carboxylic acids possessing different functional groups and electronic properties can be smoothly converted to aryl boronate esters, including those that are difficult to decarboxylate under transition-metal catalysis, offering a new method enabling use of carboxylic acid as building blocks in organic synthesis. Mechanistic analysis suggests the reaction proceeds through coupling of a transient aryl radical generated by radical decarboxylation with a pyridine-stabilized persistent boryl radical. Activation of redox active esters may proceed via an intramolecular single-electron-transfer (SET) process through a pyridine-diboron-phthalimide adduct and accounts for the base-free reaction conditions.

Full text of DOI:10.1021/acs.orglett.7b01950

Letter
pubs.acs.org/OrgLett
Isonicotinate Ester Catalyzed Decarboxylative Borylation of
(Hetero)Aryl and Alkenyl Carboxylic Acids through
N‑Hydroxyphthalimide Esters
Wan-Min Cheng, Rui Shang,*,† Bin Zhao, Wei-Long Xing, and Yao Fu*
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Urban Pollutant Conversion, Anhui
Province Key Laboratory of Biomass Clean Energy, iChEM, University of Science and Technology of China, Hefei 230026, China
S
* Supporting Information
ABSTRACT: Decarboxylative borylation of aryl and alkenyl
carboxylic acids with bis(pinacolato)diboron was achieved through
N-hydroxyphthalimide esters using tert-butyl isonicotinate as a
catalyst under base-free conditions. A variety of aryl carboxylic
acids possessing different functional groups and electronic properties
can be smoothly converted to aryl boronate esters, including those
that are difficult to decarboxylate under transition-metal catalysis, offering a new method enabling use of carboxylic acid as
building blocks in organic synthesis. Mechanistic analysis suggests the reaction proceeds through coupling of a transient aryl
radical generated by radical decarboxylation with a pyridine-stabilized persistent boryl radical. Activation of redox active esters
may proceed via an intramolecular single-electron-transfer (SET) process through a pyridine−diboron−phthalimide adduct and
accounts for the base-free reaction conditions.
ecarboxylative cross-coupling reactions1 have been exten-
Scheme 1. Decarboxylative Borylation as a Platform To Utilize
Aryl/Alkenyl Carboxylic Acids
D
sively studied in the past decades. Aryl and alkenyl
carboxylic acids and their salts have been applied as carbanion
equivalents invarious transformations.2 However, compared with
transformations of boronic acids and boronates,3 the scope of
decarboxylative transformation is still limited regarding both the
scope of aryl carboxylic acids and the types of transformations.
The limitation is mainly due to the high activation energy for
redox neutral decarboxylation4 in transition-metal catalysis (Cu,
Ag, Pd). Recently, new redox activation modes for decarbox-
ylation have enabled application of various aliphatic carboxylic
acids in cross-coupling reactions5 either through photoredox
catalysis6 or by using redox activation groups (e.g., NHPI ester).7
Though the concept was mainly applied in decarboxylative
coupling of aliphatic carboxylic acids, we wondered whether this
new redox activation mode can achieve a transformation which is
a challenge in a previous decarboxylative coupling of aryl and
alkenyl carboxylates to expand the amenable substrate scope and
transform versatility. With this in mind, a radical type
decarboxylative borylation of C(sp2)−COOH to deliver C-
(sp2)−Bpin as a platform8 for diverse transformation may solve
the problem faced in transition-metal-catalyzed decarboxylative
couplings (Scheme 1a). Enlightened by the recent work by Jiao et
al. depicting a pyridine-catalyzed borylation of aryl halides,9 we
envisioned that aryl carboxylic acid N-hydroxyphthamide esters
may also be suitable to deliver aryl radicals through radical
decarboxylationtoreactwithapyridine-stabilized borylradical, to
enable decarboxylative borylation of aryl carboxylic acids
(Scheme 1b). Decarboxylative borylation of NHPI esters was
recently reported by Baran,10a Glorius,10b Li,10c and Aggarwal10d
groups using visible-light irradiation or nickel catalysis. We
reported herein that isonicotinate tert-butyl ester acts as an
effective catalyst to decarboxylatively borylate aryl carboxylic acid
N-hydroxyphthamide esters in refluxing trifluorotoluene
(TFT).10b The reaction system is extremely simple without any
additive or base. Various aryl carboxylic acids and alkenyl
carboxylic acids were successfully transformed to the correspond-
ing pinacolboronates, including electron-rich and unbiased aryl
carboxylic acids, which are challenging substrates in transition-
metal-catalyzed decarboxylative transformations. The reaction
offers a fully metal-free method to prepare valuable aryl and
alkenyl boronate reagents from readily available, environmentally
benigncarboxylicacidsanddemonstratesthepotentialofutilizing
one electron redox process11 to solve problems faced in
decarboxylative couplings of C(sp2)−COOH.
We demonstrate the optimized reaction conditions in Table 1.
Heating a mixture of 4-phenylbenzoic acid N-hydroxy-
Received: June 26, 2017
© XXXX American Chemical Society
A
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Letter
heteroarenes, such as pyrazine and isoquinoline, failed to act as
catalysts.
Table 1. Investigation of Reactions Parameters
3-Hydroxy-1,2,3-benzotriazin-4(3H)-one ester can also be
used but is less effective. N-Hydroxysuccinimide ester is totally
ineffective. It is interesting tonote that the Lewis acidity and steric
properties of the boronate ester significantly affect the reaction
outcomes. As shown at the bottom of Table 1.
With the optimized reaction conditions in hand, we
investigated thereactionscopewithrespecttodifferentcarboxylic
acids. The scope of this reaction is broad as shown in Scheme 2. A
a
Scheme 2. Reaction Scope
a
Reaction conditions: NHPI ester (0.2 mmol), B2Pin2 (0.4 mmol),
isonicotinate tert-butyl ester (20 mol %), PhCF3 (1.0 mL), 100 °C,
10 h.
phthalimide ester (0.2 mmol), bis(pinacolato)diboron (0.4
mmol), and isonicotinate tert-butyl ester in refluxing TFT in a
sealed Schlenk tube under argon afforded the desired borylation
product in 93% yield (GC). Ethyl acetate can also be used as the
solvent to afford the borylation product in comparable yield
(entry 1). Interestingly, a base is generally necessary to activate
diboronreagentsinvariousborylationreactions, whileadditionof
a base exhibited a deteriorative effect to this reaction. Although
product can be detected under irradiation conditions at room
temperature, the yield is significantly lower (25%).10b Some
representative examples of thecatalyst screen are listed in Table1.
The reaction is highly sensitive to the electronic and steric
properties of N-heteroarenes. When isonicotinate methyl ester
and isonicotinate ethyl ester were used, the product was obtained
in reduced yield. This observation may be due to the instability of
methyl ester and ethyl ester toward SN2 substitution under the
reaction conditions to consume the isonicotinate ester catalyst.
Putting a substituent on the 2-position of isonicotinate tert-butyl
ester entirely removes the catalytic activity. Picolinate ester was
ineffective. Using 1.0 equiv of B2Pin2 caused a reduced yield.
4-Cyanopyridine, which was demonstrated to catalyze
homolysis of bis(pinacolato)diboron,12 works as a less efficient
catalyst. 4-Phenyl-pyridine and 4,4′-bipyridine are less effective
catalysts. Testing electron-rich and simple pyridines as catalysts
entirely failed, showing the requirement of the electron-deficient
property of pyridine to be catalytically effective. Other N-
a
Reaction conditions: NHPI ester (0.2 mmol), B2Pin2 (0.4 mmol),
isonicotinate tert-butyl ester (15 mol %), PhCF3 (1.0 mL), 110 °C, 15
h. Yield determined by H NMR. Reaction time was 24 h.
b
c
1
variety of aryl carboxylic acids can be used regardless of their
electronic nature. It is important to note that both electron-rich
and -deficient aryl carboxylic acids are amenable substrates and
ortho-, meta-, para-substituted benzoic acids are all reactive. In
transition-metal-catalyzed decarboxylative cross-coupling reac-
tions, the reactive substrates required a specific electronic
property13 (e.g., highly electronic-deficient14a or -rich14b with
ortho-substituent14c), andageneralscopeincludingmeta-orpara-
substituted benzoic acids is still challenging. The reaction
possesses excellent functional group compatibility, as demon-
strated in Scheme 2: ether (6, 10, 20, 26), trifluoromethyl (7),
B
DOI: 10.1021/acs.orglett.7b01950
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Letter
cyano (8), aryl chloride (9, 19), aryl bromide (11), ketone (12),
trifluoromethyl sulfide (13), sulfonamide (14), ester (15), and
sulfone (16) are all well tolerated. Naphthalene carboxylic acids
are also reactive (21, 22). For entries of moderate yields (4, 6, 12,
20), starting materials were recovered rather than forming side
products (e.g., decarboxylative protonation, homocoupling).
Heteroarene carboxylic acids such as pyridine-3-carboxylic acid
(23), indole-6-carboxylic acid (25), and quinoline-6-carboxylic
acid (24) are all amenable substrates. Testing pyridine-2-
carboxylic acid was unsuccessful, possibly due to the intrinsic
instability of pyridine-2-boronate.15 Jiao et al. reported aryl
halides, such as aryl bromides, can be borylated using a pyridine
catalyst. However, in our reaction the NHPI ester was
decarboxylatively borylated in preference to aryl bromide (11).
For 3-bromobenzoate, the product of borylation on the aryl
bromide was not detected. The different chemoselectivity may be
attributed to the base-free conditions that distinguish it from the
reported methods.16 Besides monosubstituted benzoic acids,
disubstituted benzoic acids also work well (18, 19, 20). Alkenyl
carboxylicacidsalsoreactundertheoptimizedreactionstodeliver
valuable alkenyl boronates (27, 28, 29, 30).17
carboxylic acids is very much limited; through the platform of
boronate, various types of decarboxylative couplings can be
achieved.
Oneintriguingmechanisticaspectofthereactionsystemisvery
simple without any additive or base. From literature studies we
know that cleavage of the B−B bond in diboron species generally
requires coordination of two Lewis bases with diboron.19 By 11B
NMR analysis of the completed reaction mixture, we detected a
peak at 22 ppm, which suggests the formation of a phthalimide-
coordinated Bpin species.20 It is reasonable that the NHPI redox
ester may form a complex with pyridine-coordinated diboron (B)
to form an adduct (C) (Figure 2). Single electron transfer may
The simple reaction conditions and low cost of the
isonicotinate catalyst make this reaction highly valuable to
produce a boronate reagent on large scale. The reaction can be
easily scaled up to gram scale without reducing the yield (Scheme
3).
Figure 2. Plausible mechanism.
Scheme 3. Gram-Scale Transformation of Aryl Carboxylic
Acid to Ar-Bpin
proceed intramolecularly in C to activate the NHPI ester through
B−B cleavage to deliver a carboxylate radical (D) and a pyridine-
stabilized persistent boryl radical (E).9 The persistent boryl
radical (E) rapidly reacts with a transient aryl radical21 generated
after radical decarboxylation to form the product and regenerate
the isonicotinate ester catalyst (A). Since a three-component,
acid−base adduct C is supposedly formed, it is easy to rationalize
the sensitivity of this reaction toward the steric hindrance and
Lewis acidity of both N-heteroarenes and diboron reagents
observed in the optimization study. The deleterious effect of
additional base can be ascribed to the coordination with diboron
impeding the formation of adduct C (Table 1, entries 2−4). Since
isonicotinate ester is used as a catalyst (15 mol %) in the presence
of a large excess of B2Pin2, formation of a diboron complex
coordinated with two pyridines to induce homolysis of the B−B
bond is less probable. The efficacy of isonicotinate ester as a
catalyst can be rationalized from two aspects. First, it coordinates
to bis(pinacolato)diboron and induces a negative charge on the
boron atom for electron transfer to activate the NHPI ester. The
effect of the para-ester substituent can be explained by its subtle
turningoftherateofsingleelectrontransferincomplexC, making
the rate match that of the decarboxylation step. Second,
isonicotinate ester is crucial for generation of the persistent
boryl radical.22 The ester substituent may also affect the stability
of the persistent radical (E) and enhance its reactivity with the
transient aryl radical, suppressing undesired side reactions.
In summary, we discovered that isonicotinate tert-butyl ester
efficiently catalyzes decarboxylative borylation of a variety of aryl
and alkenyl carboxylic acid redox active esters to form
synthetically useful aryl and alkenyl pinacol boronates. A broad
scope of aryl carboxylic acids possessing various functional
groups, including challenging substrates in metal-catalyzed
decarboxylation, are amenable substrates. The reaction proceeds
under base- and additive-free conditions and features operational
simplicity and easy scale-up ability. The reaction proceeds
through a radical coupling mechanism via formation of a
The decarboxylative borylation method offers a route to
achieve a series of challenging decarboxylative transformations
such as trifluoromethylation,18a sulfenylation,18b amination,18c
and hydroxylation18d (Figure 1). These transformations are
highly valuable in current organic synthesis. Though several
transition-metal-catalyzed methods are applicable, the scope of
Figure 1. Diverse transformation enabled by decarboxylative boryation.
Reactionconditions:(a)CuTc(10mol%),1,10-phenanthroline(10mol
%), Togni’s reagent (100 mol %), LiOH·H2O (200 mol %), 45 °C,
CH2Cl2, 6 h; (b) CuCl (10 mol %), 2,2′-bipyridine (10 mol %), disulfide
(66 mol %), 80 °C, DMSO/H2O, under air, 12 h; (c) Cu2O (10 mol %),
imidazole (150 mol %), 40 °C, MeOH, under air, 12 h; (d)
[Ru(bpy)3Cl2]·6H2O (2 mol %), i-Pr2NEt (200 mol %), 36 W CFL,
DMF, under air, 12 h.
C
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Letter
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radical decarboxylation. Considering the diverse transformations
of organoboronates, the reaction offers a new strategy to achieve
diverse decarboxylative functionalization.
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ASSOCIATED CONTENT
* Supporting Information
■
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Experimental details and characterization data for all
products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: rui@chem.s.u-tokyo.ac.jp.
*E-mail: fuyao@ustc.edu.cn.
ORCID
(11) (a) Radicals in Organic Synthesis; Renaud, P., Sibi, M. P., Eds.;
Wiley-VCH:Weinheim, 2001;pp229−249.(b)Shaw,M. H.;Twilton,J.;
MacMillan, D. W. C. J. Org. Chem. 2016, 81, 6898.
Rui Shang: 0000-0002-2513-2064
Yao Fu: 0000-0003-2282-4839
Present Address
†Department of Chemistry, School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
(12) (a) Wang, G.; Zhang, H.; Zhao, J.; Li, W.; Cao, J.; Zhu, C.; Li, S.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by NSFC (21325208, 21572212),
Ministry of Science and Technology of China (2017YFA0303-
500), CAS (XDB20000000), the Key Technologies R&D
Programme of Anhui Province (1604a0702027), FRFCU, and
PCSIRT.
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