Carbamates: A Directing Group for Selective C-H Amidation and Alkylation under Cp*Co(III) Catalysis

Article abstract of DOI:10.1021/acs.orglett.0c00589

The selective reactivity of carbamate and thiocarbamate toward alkylation and amidation is reported under stable, high-valent, cost-effective cobalt(III) catalysis. This method reveals the wide possibility of designing a different branch of synthetically

Full text of DOI:10.1021/acs.orglett.0c00589

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Letter
Carbamates: A Directing Group for Selective C−H Amidation and
Alkylation under Cp*Co(III) Catalysis
Sourav Sekhar Bera and Modhu Sudan Maji*
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ABSTRACT: The selective reactivity of carbamate and thiocarba-
mate toward alkylation and amidation is reported under stable,
high-valent, cost-effective cobalt(III) catalysis. This method reveals
the wide possibility of designing a different branch of synthetically
challenging yet highly promising asymmetric catalysts based on
BINOL and SPINOL scaffolds. Late-stage C−H functionalization
of L-tyrosine and estrone was also achieved through this approach.
The mechanistic study shows that a base-assisted internal
electrophilic substitution mechanism is operative here.
he ortho functionalization of aromatic phenols is the
ligands.10 In 2013, Chang et al. reported a carbamate-directed
sulfonamide synthesis using tosyl azide under iridium
catalysis.9a Later, Singh et al. reported C−H amidation
through intramolecular cyclization via an outer sphere
pathway.6f Recently, Miura et al. demonstrated a copper-
catalyzed amination reaction on phenols using bidentate
auxiliary.9b To the best of our knowledge, phenol derivatives,
that is, carbamate-directed C−H functionalization, have not
been addressed under the cobalt catalysis. Driven by these facts
and inspired by our continuous effort in cobalt catalyzed C−H
functionalizations,11 we set to develop carbamate-directed
amidation and alkylation protocols under cost-effective, stable,
high-valent cobalt(III) catalysis.12−14
T
linchpin for the design and synthesis of numerous chiral
ligands and catalysts.1 In addition, 2-amino and alkylated
phenols are key building blocks present in several biologically
active heterocycles, drug molecules, and also important
synthetic intermediates (Figure 1).2 The known methods,
In line with the previous report by Ackermann et al.,15 our
attempts to conduct carbamate-directed amidation were also
ineffective. We envisioned that tuning the polarizability of the
coordinating atom by changing the directing group to
thiocarbamate may solve the reactivity issue16 and may also
lead to specific reactivities toward a different set of electro-
philes; however, the possibility of sulfur toxicity to transition
metals is a serious concern.17 This is probably the reason
behind the lesser exploration of thiocarbamates as a directing
group,18 although versatile postsynthetic modifications are
possible through Newman−Kwart,19 anionic Fries,20 and O-
neophyl rearrangements of this directing groups.21 Pleasingly,
Figure 1. 2-Amido-phenol-derived bioactive molecules.
such as the Friedel−Crafts reaction, the Buchwald−Hartwig
coupling, and ortho-lithiation strategies, suffer from a lack of
regioselectivity, requiring prefunctionalized substrates, harsh
reaction conditions, longer reaction sequences, and poor
yields, restricting them from much broader applications.3,4
This motivated us to develop new strategies based on
regioselective 3,3′-functionalizations of indispensable BINOL,
biphenyl, and SPINOL scaffolds through directed C−H bond
activation.
Although various electrophiles have been employed for the
directed ortho-C−H bond functionalization of phenols using
noble metal such as Pd,5 Rh,6 Ru,7 and Ir8 catalysts, amidation
using phenol derivatives has not been well explored.9
Moreover, the amidation of the BINOL and SPINOL systems
is largely not studied despite their potentiality as chiral
Received: February 14, 2020
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© XXXX American Chemical Society
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A

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switching the directing group from carbamate to thiocarbamate
provided us the desired amidation products. On the contrary,
in a similar catalytic system, thiocarbamate-directed alkylation
was unfruitful; rather, it selectively reacted toward carbamates.
Herein we report the first example of selective thiocarbamate-
and carbamate-directed amidation and alkylation strategies
under high-valent Cp*Co(III) catalysis (Scheme 1). These
did not produce encouraging results (entries 9 and 10).
Excitingly, switching of the solvent from DCE to TCE
increased the yield up to 73% (entry 11). Further optimization
of temperature and catalyst loading did not provide any better
results (entries 12 and 13).
After optimizing the reaction conditions (Table 1, entry 11),
first, the generality of the protocol was examined by employing
a wide range of thiocarbamates 1 and amidating agent 2a
(Scheme 2). 2-Fluoro- and methoxy-substituted aryl thio-
Scheme 1. Previous Reports and Our Work
a b
,
Scheme 2. Scope of Aryl Thiocarbamates
methods responded well to various electronically different
carbamates, amidating agents, and alkylating partners, thus
opening an avenue for the direct access to valuable BINOL and
SPINOL derivatives, which are reliable components for
asymmetric catalysis.1,10,22
We initiated our optimization study using DCE as a solvent,
Cp*Co(CO)I2 (10 mol %) as a catalyst, AgSbF6 (20 mol %) as
an additive, 20 mol % of NaOAc as an acetate source, and
phenyl dioxazolone 2a as an amidating agent, and product 3aa
was isolated in 56% yield (Table 1, entry 1). After the
screening of several solvents and acetate sources, the yield was
improved to 62% (entries 2−6). Interestingly, whereas 60 mol
% of KOAc retarded the reaction, 30 mol % of KOAc provided
65% of 3aa (entries 7 and 8). The use of other cobalt catalysts
a
Reaction conditions: thiocarbamates 1 (0.2 mmol), dioxazolone 2a
b
c
(0.4 mmol), TCE (1.5 mL). Isolated yields. 20.0 mol % of
Cp*Co(CO)I2 and 40 mol % AgSbF6 and 40 mol % KOAc were used.
carbamates successfully furnished the desired products 3ba−ca
in 64−74% yield. Methoxy substitution at the three-position
delivered the product 3da in 67% yield, indicating that the
steric effect prevails over the electronic effect.11e The
thiocarbamates bearing methoxy, chloro, and iodo substitu-
tions at the para position were also suitable substrates (3ea−
ga, 65−82% yield), providing an opportunity for further
functionalization. Pleasingly, the electron-withdrawing ester
group was also tolerated under the reaction conditions to
provide 3ha in 58% yield. The reactivity of α-naphthol has also
been accessed to provide 3ia in 62% yield. Doubly protected
2,2′-bisphenol was smoothly converted to diamidation product
3ja in 66% yield. Considering the importance of asymmetric
catalysis, different axially chiral common key building blocks
for the ligand and catalyst were subjected to the reaction
conditions. In this line, we switched our focus to sterically and
electronically unique axially chiral BINOL derivatives.
Thiocarbamate-protected (R)-BINOL 1k readily reacted to
offer amidation product 3ka in an excellent yield of 84%.
Gratifyingly, the MOM protecting group in 3la, widely used in
ligand synthesis, tolerated the reaction conditions. Likewise,
the important octahydro-BINOL 1m also underwent smooth
amidation to afford 3ma in 72% yield, keeping the four
benzylic C−H bonds intact. Next, we switched our focus to
unprecedented and challenging direct bis-C−H amidations of
BINOL, biphenyl, and SPINOL in a regioselective manner,
a b
,
Table 1. Optimization of the Reaction Conditions
entry solvent
catalyst
additive
temp (°C) yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
DCE
TFE
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
CoCl2
[Cp*CoCl2]2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
NaOAc
NaOAc
none
100
100
100
100
100
100
100
100
100
100
100
80
56
none
47
none
62
59
none
65
none
5
73
56
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
TCE
TCE
TCE
Cu(OAc)2
KOAc
PivOH
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
c
d
d
d
d
d
de
,
100
53
a
Reaction conditions: thiocarbamate 1a (0.2 mmol), oxazolone 2a
(0.4 mmol), catalyst (9.5 mg, 10 mol %), AgSbF6 (13.7 mg, 20 mol
b
c
%), additive (0.04 mmol, 20 mol %). Isolated yield. 60 mol % of
d
e
KOAc was used. 30 mol % of KOAc was used. 5.0 mol % of catalyst
was used. DCE, 1,2-dichloroethane; TCE, 1,1,2,2-tetrachloroethane.
B
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which can lead to a path for novel catalyst and ligand design.1
The isolation of diamidation products 3na−oa in 78−82%
yield showed the efficiency and applicability of this method. It
is important to note that the difunctionalization of BINOL and
SPINOL by an ortho-lithiation strategy is often associated with
the monofunctionalization side reaction. To our delight, the
diamidation of SPINOL 3pa was also achieved in 67% yield. It
is noticed that BINOL thiocarbamates are more reactive
toward amidation than simple phenol counterparts.
a b
,
Scheme 4. Scope of Carbamate-Directed Alkylations
Next, the scope of amidating agent was investigated using
thiocarbamate 1a or 1k as a coupling partner (Scheme 3).
a b
,
Scheme 3. Scope of Dioxazolones
a
Reaction conditions: carbamates 4 (0.2 mmol), arcrylates 5 (0.4
b
c
mmol), DCE (1.5 mL). Isolated yields. Dialkylation product has
been reported. 30.0 mol % of Cp*Co(CO)I2, 60 mol % AgSbF6, and
40 mol % KOAc, 80 mol % PivOH were used.
d
products and the fact that the regioselective C3 alkylation of
BINOL derivatives is a stiff task because the C6 position of the
BINOL is more activated toward electrophilic substitution, we
shifted our focus to direct the 3,3′-alkylation of BINOL
dicarbamates. To our delight, BINOL derivatives 4g−i
responded well to accomplish alkylated products 6g−i in
36−60% yield. The challenging dialkylated product 6j was also
synthesized in 48% yield by increasing the catalyst loading. The
SPINOL dicarbamate was also viable to offer 6k in 51% yield.
Hence, this method can trigger a route for the modification of
BINOL- and SPINOL-based chiral ligands.
a
Reaction conditions: Thiocarbamates 1 (0.2 mmol), dioxazolones 2
b
(0.4 mmol), TCE (1.5 mL). Isolated yields.
The late-stage C−H bond functionalization is an efficient
tool for rapid product diversification. Considering this fact, the
tyrosine derivatives 1q were subjected to the amidation
conditions, and the desired product 3qa was obtained in
69% yield (Scheme 5a). In another example, the late-stage
Electron-withdrawing para fluoro-, chloro-, bromo-, and
trifluoromethyl-substituted aryl dioxazolones reacted efficiently
to provide 3ab−ae in good to excellent yield (61−82%).
Methoxy-substituted aryl dioxazolones also underwent smooth
amidation (3af−ag, 63−68%). The reaction of butyl-
dioxazolone was relatively sluggish and furnished 3ah in 50%
yield. Heteroaryl dioxazolone 2i reacted efficiently to provide
3ai in 85% yield. A series of BINOL derivatives 3kd, 3kf, and
3ki were also synthesized in excellent yield (76−90%).
Scheme 5. Utilization of these Developed Methods
After establishing the amidation protocol, we switched our
attention to develop an alkylation strategy under a similar
catalytic system using thiocarbamate as a directing group;
however, our attempts were in vain. Interestingly, upon
replacing the solvent with DCE and changing the directing
group to carbamate, the alkylated product 6a was isolated in
40% yield, along with the formation of alkenylated side
product 7a in 10% yield (Scheme 4). To this point, the use of
TCE did not provide any encouraging result for the alkylation
reactions. Later, we found that the formation of 7a was
successfully suppressed upon the introduction of pivalic acid,
and after the gradual increment, 50 mol % of pivalic acid
furnished 6a almost exclusively (62%, 6a/7a 15:1). In the
absence of KOAc, the use of 50 mol % pivalic acid solely
offered 6a but only in 38% yield. Carbamates bearing electron-
donating methoxy substituents participated in the reaction to
furnish 6b,c in 47−55% yield. Pleasingly, bromo- and iodo-
substituted carbamates were more reactive to afford 6d,e in
58−66% yield along with the formation of 12−18% of
dialkylation products. Biphenyl carbamate 4f also responded
well to produce 6f in 53% yield. Considering the value of the
alkylation of estrone derivative 4l provided 6l in 42% yield
(Scheme 5b). The versatility of the directing group was
demonstrated by converting 3ea to the carbamate 8 by using
(diacetoxyiodo)benzene as an oxidant (Scheme 5c). New-
man−Kwart rearrangement, a key reaction for sulfonic-acid-
based catalyst design,19 has been successfully performed on
3ea to obtain compound 9 in 88% yield (Scheme 5d). A gram-
scale synthesis was executed, and 0.58 g of amide 3ea was
isolated (Scheme 5e). To demonstrate the further utility of the
C
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method, using the reaction conditions of Huang et al.,18a the
biologically important heterocycle benzoxazole 10 has been
synthesized from 3ea.
Scheme 7. Mechanistic Studies
To further validate the selective nature of the directing
groups, 1j and 4m were subjected to amidation and alkylation
conditions. Whereas dithiocarbamate 1j responded promptly
to the amidation conditions to furnish 3ja, it was unreactive
toward alkylation (Scheme 6a). On the contrary, although the
Scheme 6. Proof of Selective Reactivity of the Directing
Groups
acetic acid, suggests that an irreversible C−H activation step is
involved here (Scheme 7b). Finally, the kinetic isotopic effect
(KIE) value of 2.6 from the intermolecular competition
experiment and the KH/KD value of 3.0 from a parallel
experiment between 1a and [D5]-1a indicate the rate-
determining C−H activation step (Scheme 7c).
Relying on our mechanistic studies and previous reports,24
a
plausible catalytic cycle is proposed, where in the presence of
AgSbF6 and KOAc additives, catalyst Cp*Co(Co)I2 is assumed
to transform into active species A (Scheme 8). Cp*Co(III)
Scheme 8. Proposed Mechanism
dicarbamate 4m did not provide any amidation product, it
reacted smoothly to provide alkylated product 6m in 46% yield
(Scheme 6b). This directly demonstrates the specific reactivity
of the carbamate and thiocarbamate directing groups. Later,
substrate 11 bearing both carbamate and thiocarbamate
directing groups was exposed to the amidation conditions,
which produced the amidation product 12, but interestingly,
no alkylation product was obtained under standard alkylation
conditions (Scheme 6c). These results suggest that in a
competition, thiocarbamate wins over the weakly coordinating
carbamate to form the cyclometalated species, which provide
only the amidation product, not the alkylated one. This
conclusion was supported by the unsuccessful alkylation of
compound 13 (Scheme 6d). These reactivity patterns may
have originated from the coordinating ability of electrophiles to
the cyclometalated species B and E (Scheme 8). We believe
that thiocarbamate 1a and electrophiles compete for
coordination to B, where strongly coordinated 2a might win
over 1a, which may not be the case for 5a. Sulfur toxicity
toward the alkylation reaction was further corroborated by the
ineffective alkylation reaction in the presence of compound 1r
(Scheme 6e).
complex A generates a kinetically relevant metallacycle B
through the acetate-assisted C−H activation. After coordina-
tion through the nitrogen center of dioxazolone 2a, species C
is formed, which undergoes migratory insertion through CO2
extrusion to form amido-inserted species D. After that, proto-
demetalation occurs in the presence of AcOH to offer
amidation product 3aa, and the active catalyst A is regenerated,
thus completing the catalytic cycle. In another catalytic cycle,
cobaltacycle E undergoes coordination to acrylate 5a to form
species F. Then, the migratory insertion followed by proto-
demetalation in the presence of pivalic acid provides 6a along
with the active catalyst A.
In conclusion, we developed directing-group-specific C−H
bond amidation and alkylation strategies for phenol derivatives
under cost-effective, stable, high-valent cobalt(III) catalysis.
The successful responses from various carbamates, dioxazo-
lones, and arylates exhibit the versatility of this method.
Amidation and alkylation on BINOL and SPINOL derivatives
disclose a significant route for the further development of these
chiral ligands. The supremacy of this method was further
revealed through the late-stage C−H functionalization of L-
The preferential conversion of 1e over 1f in a 2:1 ratio (3ea/
3fa) suggests that the reaction proceeds through a base-
assisted internal electrophilic substitution (BIES) reaction-type
mechanism (Scheme 7a).23 The lack of significant H/D
scrambling at the ortho positions of 1a, in the presence of d4-
D
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ASSOCIATED CONTENT
* Supporting Information
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sı
The Supporting Information is available free of charge at
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Experimental procedures, spectroscopic data, and NMR
spectra of compounds (PDF)
Accession Codes
CCDC 1978184 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
■
Modhu Sudan Maji − Department of Chemistry, Indian
Institute of Technology Kharagpur, Kharagpur 721302, India;
orcid.org/0000-0001-9647-2683; Email: msm@
chem.iitkgp.ac.in
Author
Sourav Sekhar Bera − Department of Chemistry, Indian
Institute of Technology Kharagpur, Kharagpur 721302, India;
orcid.org/0000-0002-7365-2208
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Fu, Y.; Liu, L. Rhodium-Catalyzed Selective C-H Activation/
Olefination of Phenol Carbamates. Org. Lett. 2011, 13, 3235−3537.
Complete contact information is available at:
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̈
(b) Schroder, N.; Wencel-Delord, J.; Glorius, F. High-Yielding,
Notes
Versatile, and Practical [Rh(III)Cp*]-Catalyzed Ortho Bromination
and Iodination of Arenes. J. Am. Chem. Soc. 2012, 134, 8298−8301.
(c) Singh, R.; Nagesh, K.; Parameshwar, M. Rhodium(II)-Catalyzed
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Z.; Zhang, Y. Rhodium(III)-Catalyzed Sequential Cleavage of Two C-
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.S.M. gratefully acknowledges SERB, Department of Science
and Technology, New Delhi, India (sanction no. CRG/2018/
000317) for funding. S.S.B. acknowledges CSIR, India for the
fellowship.
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