Rhodium(II)-Catalyzed Undirected and Selective C(sp2)-H Amination en Route to Benzoxazolones

Article abstract of DOI:10.1021/acscatal.6b02237

Rhodium(II) can effectively promote the activation and cyclization of arylcarbamate substrates to yield benzoxazolones via an intramolecular nitrene C-H insertion reaction. Investigation of the substrate scope shows that the reaction undergoes selective a

Full text of DOI:10.1021/acscatal.6b02237

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Letter
Rhodium (II)-Catalyzed Undirected and Selective
C(sp2)-H Amination en Route to Benzoxazolones
Ritesh Singh, Kommu Nagesh, and Matam Parameshwar
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02237 • Publication Date (Web): 29 Aug 2016
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ACS Catalysis
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Rhodium(II)-Catalyzed Undirected and Selective C(sp2)-H Amination
en Route to Benzoxazolones
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Ritesh Singh*a, Kommu Nagesha, Matam Parameshwara
aMedicinal Chemistry and Pharmacology Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, Tel-
angana-500007, INDIA
Supporting Information
arylhydroxylamines14 o-hydroxyl acetophenone15a to benzox-
ABSTRACT: Rhodium (II) can effectively promote the activa-
tion and cyclization of arylcarbamate substrates to yield ben-
zoxazolones via an intramolecular nitrene C—H insertion
azolone core. Besides, Lie et al have also reported Cu cata-
lyzed air oxidation of benzoxazole to benzoxazolone.15b How-
ever; all of the aforementioned methods suffer from various
drawbacks, such as air sensitivity of o-aminophenols, toxicity
of phosgene, high temperatures and pressures, along with
undesirable side products. More so, they offer little diversity
because each of them requires o-substituted (generally an
amine or nitro group) phenols or their derivatives and their
availability is often hampered by their multistep synthesis
reaction. Investigation of the substrate scope show that the
reaction undergoes selective aromatic C(sp2)—H amination
over more labile o-C(sp3)—H bonds. Observation of inverse
secondary KIE (PH/PD = 0.42±0.03) indicates involvement of
aromatic electrophilic substitution mechanism for this aryl
C—H amidation transformation.
and non-selective transformations (non-regioselectivity and
Direct functionalization of unactivated C—H bonds into
oversubstitution of the aromatic ring).16Therefore, to devise a
C—N bonds are of immense interest and ideal organic trans-
straightforward synthesis of substituted benzoxazolones via
formations for the construction of N-containing heterocycles
direct C-H bond functionalization would be highly desirable
from cheap, and readily available raw materials.1 An effective
because of the improved synthetic efficiency.
strategy for such transformation involves transition metal
catalyzed direct C—H bond nitrene/ metal nitrenoid inser-
tion, where relatively weaker C(sp3)—H bonds (3o>2o>1o) are
preferentially activated via outer sphere C-H insertion mech-
anism.2 In stark contrast, aryl C(sp2)—H amination via metal
nitrenoid insertion garnered little attention due to lack of
chemo/regioselectivity.3 Notwithstanding, the recent devel-
opments of nitrene and metal nitrenoid insertion in aromatic
C—H bonds via inner sphere mechanism which requires
catalyst directing(chelating) groups),4 aryl C(sp2)—H amina-
tion via “outer sphere mechanism” remains underdevel-
oped.5-7 Moreover, the greatest challenge lies in the selective
(chemo/regio) C—H bond functionalization. Among several
catalytic systems developed till date for C—H bond
Figure 1. Selective C(sp2)—H amination approach to access
substituted benzoxazolones
nitrene/metal-nitrenoid insertion, only few reports describe
preferential aromatic C(sp2)—H amidation in unactivated
system over more labile and available ortho-C(sp3)—H bonds
Our studies commenced with the treatment of O-phenyl
carbamate 2a, which was quantitatively obtained from corre-
with poor chemo/regioselectivity, and very limited substrate
scope. Notable ones include Au(III),8a Cu(I),8b,c and Fe(II)
sponding phenol,17 with different catalytic systems like Ag,18
Pd,4e Cu,19 Fe20a and Rh2a known to promote C-H nitrenoid
insertion from amide precursors. Gratifyingly, after prelimi-
nary survey, Du Bois catalytic amination reaction condition
Rh2(OAc)4, PhI(OAc)2 with MgO and 4 Ao MS in anhydrous
dichloromethane (DCM) at 40oC, resulted in little product
conversion 3a(~15% yield). Further solvent screening at vary-
ing temperatures showed toluene to be the best solvent giv-
ing 100% conversion at 100oC with 85% yield in 6 h (see Table
S1&S2 in ESI and entry 1, Table 1). Electronic nature of the
carboxylate ligands displayed noteworthy effect on the activi-
ty of Rh(II) catalysis, where relatively electron rich carbox-
ylates were able to promote aryl C—H amination quite effi-
complex8d catalyzed intermolecular C-H amidation. Herein,
we wish to report Rh2(II)-catalyzed arylcarbamate system (2)
which undergoes selective intramolecular aromatic C(sp2)—
H amination/nitrenoid insertion en route to substituted ben-
zoxazolones starting from ubiquitous phenols(1)(Figure 1).
Benzoxazolone moiety is an important privileged
scaffold in medicinal chemistry which is endowed with broad
spectrum of biological properties.9 Besides, they also find
usage as achiral template for enantioselective reaction.10 Tra-
ditional methods to synthesize benzoxazolone nucleus in-
cludes cyclocarbonylation of 2-aminophenols with phosgene11
and other carbonyl group donors.12 Recent reports includes
rearrangement of N-aryl-O-acylhydroxylamine,13 N-alkyl-N-
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ciently (entries 7, 10 and 11, Table 1) employing electron defi-
alkyl, aryl and methoxy groups were well tolerated under the
1
2
3
4
5
6
7
8
cient carboxylate provided dismal yield of desired product
(entry 8, Table 1).This is in contrast to their higher activity
demonstrated for conversion of azide based nitrene sub-
strates5a and aryl C(sp2)-H carbenoid insertion.20b,c Rh2(oct)4
performed slightly better than Rh2(OAc)4, perhaps due to
better solubility at higher temperature. Decreasing the
Rh2(OAc)4 catalyst loading (3 mol%) led to unconsumed
starting material (entry 9, Table 1), moreover, increase in
catalytic loading didn’t improve the yield either(entry 2, Ta-
ble 1). Inorganic oxidants screened (K2S2O8, Oxone) (entries 3
and 4, Table 1) showed no product formation whereas other
iodine based oxidants (PhI(O2CCF3)2, PhI(OPiv)2) (entries 5
and 6, Table 1) tested under otherwise similar conditions
provided inferior product formation as compared to
PhI(OAc)2. Control reactions suggested the requirement of
both oxidant (PhI(OAc)2) and catalyst (Rh2(II)), under inert
atmosphere to promote this intramolecular aryl C—H ami-
dation (entries 13 and 14, Table 1). Interestingly, there was no
reaction between 2a and PhI(OAc)2 without catalyst and
both the starting materials remained intact even after ~24 h
at 100oC in toluene, although with slight decomposition. This
suggested the unique role of Rh2(II) catalyst in not only ef-
fecting the reaction between starting materials but also pro-
moting intramolecular C—H amidation. Notably no C—H
amidation of toluene was detected.21a
reaction condition. Significant electronic effect was observed
on 4-substituted carbamates, where substrates with electron
donating substitutents (alkoxy, Me, t-butyl) cyclized effi-
ciently to provide desired products in good to excellent
yields (entries 3b-3d and 3l), while electron withdrawing
substituted (F, Cl, Br) substrates provided diminished yields
of
their
respective
benzoxazolones
(entries
3e-
3g).Halogenated benzoxazolones thus obtained provides
additional handle for further diversification thereof. Howev-
er, only trace amount of desired product was observed from
substrate with strong electron withdrawing group (entry 3i).
Employment of other catalysts was unsuccessful in promot-
ing the C-H bond amidation on this substrate (data not
shown). Subjecting dialkyl substituted carbamates under
optimized reaction condition furnished product 3m and 3o
in very good yields.21b Further, to observe the steric effect on
the C-H bond amination mediated cyclization process, m-
substituted carbamates were tested.
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Table 2. Scope of Rh2(II) catalyzed C(sp2)—H amination
on substituted arylcarbamatesa
Table 1. Optimization of aromatic C—H amidationa
En-
try
Catalyst
(mol%)
Oxidant
(equiv)
Solvent
Temp(
oC)/T
ime(h)
Yieldb
(%)
1
Rh2(OAc)4(5)
Rh2(OAc)4(10)
Rh2(OAc)4(5)
Rh2(OAc)4(5)
Rh2(OAc)4(5)
Rh2(OAc)4(5)
Rh2(oct)4(5)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
K2S2O8(5)
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
100/6
85(81)c
84
2
100/6
3
100/48
100/48
100/24
100/24
100/12
100/48
100/18
100/6
nr
4
Oxone(5)
nr
aReactions were conducted in a Schlenk tube under argon at 0.31 mmol
scale in Toluene with Rh2(OAc)4(5 mol%), PhI(OAc)2(1.2 eq), MgO (2.2 eq), 4
Ao MS(200wt%).bIsolated yields.c Observed from crude NMR. dRegioselectivity
determined from NMR spectroscopy.
5
PhI(O2CCF3)2(1.2)
PhI(OPiv)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(1.2)
PhI(OAc)2(0)
PhI(OAc)2(1.2)
14
6
10
7
86
8
Rh2(O2CCF3)4(5)
Rh2(OAc)2(3)
Rh2(esp)2(5)
10
61d
Moderate regioselectivity (1:2) was observed on m-methyl
substituted carbamate with major C—H amination product
obtained from sterically more congested C-H site in good
yields (entry 3h). Substrates with m-ethyl and 2-naphthol
derived carbamate furnished the regioisomeric products 3n
and 3k respectively in 1:1 ratio. Surprisingly, only single iso-
mer (from sterically less encumbered C-H site) was isolated
from the reaction of m-bromo substituted carbamate (entry
3j).Excellent regioselectivity was demonstrated on o-phenyl
substituted substrate, providing the cyclized product 3p, in
very good yield. Subsequently, in order to probe the
chemoselective potential of this transformation we subjected
various o-alkyl substituted carbamates (Table 3) with 1o, 2o,
and 3o C—H bonds under cyclization condition. Remarkably,
only the o-aryl C—H amidation products were obtained in
9
10
11
12
13
14
15e
72
Rh2(SDOSP)4(5)
Rh2(tpa)4(10)
Rh2(OAc)4(0)
Rh2(OAc)4(5)
Rh2(OAc)4(5)
100/6
77
100/48
100/24
100/24
100/6
79
nr
nr
(78)c
aReactions were conducted in a Schlenk tube under argon at 0.31 mmol scale with
MgO (2.2 eq), 4 Ao MS(200 wt%), solvent (2.0 mL)(see ESI for details).bNMR based
yields (see ESI for details).cIsolated yield in parentheses.d20% starting material
recovered.e7 mmol scale. nr= no reaction.
Scale up of this reaction led to no significant change in iso-
lated yield (entry 15, Table 1).With optimal reaction condi-
tions in hand we investigated the scope of substrates. To test
the steric and electronic influences on the C—H bond func-
tionalization process and demonstrate the potential of this
approach to provide benzoxazolones with substitution pat-
terns that would be less readily accessible by traditional
routes, various substituted carbamates were synthesized and
subjected to optimized reaction condition. As indicated from
results obtained in Table 2, varied substituents like halogens,
excellent yields(entries 5a-5c).This is in contrast to
a
previous report where no aryl C—H amination product was
observed from o-alkyl substituted sulfamates with Rh2(II)
catalyst under similar reaction conditions, instead exclusive
formation of C(sp3)—H aminated product was observed.22a
When o-allyl substituted carbamate, which potentially has
three sites for N insertion, was subjected to oxidative
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reaction condition, it selectively gave only five membered
see ESI for details), indicating beneficial effect of electron
benzoxazolone product (entry 5f) with very good yield.22b
The product 5f having tethered double bond thus provides
additional site for further synthetic manipulations.
Furthermore, differently substituted di-alkyl carbamates gave
good to excellent yields of their respective benzoxazolones
(entries 5d, 5e, 5g and 5h).Thus,these examples represents
one of the rare systems with preferential aromatic C(sp2)—H
bond nitrenoid insertion over more labile o-substitutent
C(sp3)—H bond. Note worthily, during the course of
substrate investigation we observed that carbamate with
electron donating substituents reacted more efficiently and
provided higher yields than electron withdrawing substituted
carbamates. Also, substrates with bulkier alkyl groups (t-
butyl) provided higher yields than substrates with smaller
alkyl groups.
donating groups on the current intramolecular C—H bond
amination system and putative involvement of aromatic
electrophilic mechanism. Further, to probe the electronic
nature of the nitrene intermediate, a Hammett analysis for
the relative rate of aromatic C—H nitrenoid insertion on
different p-substituted (2b, 2c, 2e, 2f) versus unsubstituted
carbamate (2a) was performed under standard reaction con-
dition. Obtention of near linear correlation of corresponding
log(kAr/kPh) against Hammet constant(σp) with slight nega-
tive slope value (ρ = -0.20)(see ESI for details), indicated in-
volvement of positive transition state and thus favoring aro-
matic electrophilic substitution mechanism (see Figure S3,
ESI).25b
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Table 3. Scope of Rh2(II) catalyzed C(sp2)—H amination on
o-alkyl arylcarbamatesa
Scheme 1. Putative mechanism for aryl C(sp2)—H amidation
aReactions were conducted in Schlenk tube under argon at 0.31 mmol scale in
Toluene with Rh2(OAc)4(5 mol%), PhI(OAc)2(1.2 eq), MgO (2.2 eq), 4Ao
MS(200 wt%). bIsolated yields.
It is pertinent to mention here, that the high regio- and
chemoselectivity of these Rh2(II) promoted C—H bond
amidations on substrates capable of forming benzoxazinone
as well as aziridine derivatives (entries 5a-5c and 5f) provides
possible evidence against free nitrene intermediates. This is
in contrast to the indiscriminate free nitrene insertion of
formylnitrenes observed upon photolytic or thermolytic
decomposition of substituted phenylazidoformates.23,24a In
addition, no product formation from N-methyl O-phenyl
carbamate, 10 under similar reaction conditions, further
Scheme 2. Mechanistic investigation experiments
In order to assess whether cleavage of C—H bond is a rate
limiting step in this proposed model, we subjected the
deuterium probe (2a-d1) under standard cyclization
condition. Observation of inverse secondary kinetic isotope
effect (PH/PD = 0.42±0.03) (eq 3, Scheme 2, see ESI for
details), indicated that this intramolecular C—H amidation
is occuring via stepwise mechanism and rules out
involvement of C—H bond cleavage in rate determining step.
Furthermore, no o-H/D scrambling was observed upon D2O
quench of reaction with N,N-dimethyl O-phenyl carbamate
11, (eq 2, Scheme 2, see ESI for details), which clearly rules
out inner sphere activation mechanism, and suggests that
the excellent chemoselectivity observed on o-alkyl
substituted substrates toward exclusive aromatic C-H
amination could possibly be the result of more
conformationally favourable 5-membered benzoxazolone
ring. Finally, we demonstrated the potential of this direct
C(sp2) —H amidation on a natural scaffold, estrone derived
indicated
involvement
of
potential
Rh-nitrenoid
intermediate for aromatic C—H bond amidation, as 10 is
incapable of generating nitrene intermediate.4e,25a Based on
our observation in this report and previous literature
reports,5,20b,25b,c we believe that initial co-ordination of Rh2(II)
with in situ generated iminoiodinane, 6 would lead to Rh–
nitrenoid intermediate species 7,which could potentially
undergo o-C—H bond nitrenoid insertion either via
concerted asynchronous pathway through 8,or by stepwise24b
electrophilic substitution pathway through arenium ion
intermediate 9 (Scheme 1).
To gain some mechanistic insight of this Rh2(II) catalyzed
intramolecular aryl C-H bond amination,
a one pot
competition reaction between an electron rich system (2b)
and electron neutral system (2a) provided their respective
benzoxazolone products in the ratio of 2.9:1(eq 2, Scheme 2;
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Baek, Y.; Chang, S. Angew. Chem., Int. Ed. 2013, 52, 8031-8036.(d)
carbamate 2q, which gave the regioisomeic products 3q in
1.4:1 ratio with 68% yield. Thus setting up the platform for
utilization of this methodology for late stage C(sp2) —H
bond amidation(entry 3q, Table 2).
Thu, H.-Y.; Yu, W.-Y.; Che, C.-M. J. Am. Chem. Soc. 2006, 128, 9048-
9049. (e) Ng, K.-H.; Chan, A. S. C.; Yu, W.-Y. J. Am. Chem. Soc. 2010,
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M. S. Organometallics 2007, 26, 1365-1370.
1
2
3
4
5
6
7
8
9
In conclusion, we have developed an highly efficient, two
step strategy to access diverse privileged benzoxazolone
scaffolds with practical yields via C-H bond activation using
inexpensive Rh2(II) catalyst.This study also provides first
time report of excellent chemoselective aryl C(sp2)—H bond
amidation via nitrenoid insertion over more readily available
and accessible o-C(sp3)—H bonds using non-directed
approach. Further, the current system leverages the chemical
tool box of Rh2(II) catalyzed transformations with the
potential to be utilized in other analogous systems to achieve
different biologically relevant scaffolds besides utilization of
this strategy to access benzoxazole embedded natural
products, which is currently underway in our laboratory.
(5)(a) Stokes, B. J.; Dong, H.; Leslie, B. E.; Pumphrey, A. L.; Driver,
T. G. J. Am. Chem. Soc. 2007, 129, 7500-7501. (b) Chiba, S.; Zhang, L.;
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ca, C.; De Vivo, M.; Summa, M.; Ribeiro, A.; Piomelli, D. Angew.
Chem., Int. Ed. 2015, 54, 485-489.(c) Bach, A.; Pizzirani, D.; Realini,
N.; Vozella, V.; Russo, D.; Penna, I.; Melzig, L.; Scarpelli, R.; Piomelli,
D. J. Med. Chem. 2015, 58, 9258-9272 and references cited therein.
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ASSOCIATED CONTENT
Supporting Information
Supporting Information is available free of charge via the
Internet at http://pubs.acs.org.
(11) Ceccarelli, S. M.; Jaeschke, G.; Buettelmann, B.; Huwyler, J.;
Kolczewski, S.; Peters, J.-U.; Prinssen, E.; Porter, R.; Spooren, W.;
Vieira, E. Bioorg. Med. Chem. Lett. 2007, 17, 1302-1306.
AUTHOR INFORMATION
Corresponding Author
(12)Selected examples only: (a) Shankaran, K.; Donnelly, K. L.;
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Walker, G. R.; Mudhar, H. S.; Flynn, S. T.; Wyman, P. A.; Smith, P.
W.; Murkitt, G. S.; Lucas, A. J.; Mookherjee, C. R.; Watson, J. M.;
Gartlon, J. E.; Bradford, A. M.; Brown, F. Bioorg. Med. Chem. Lett.
2010, 20, 5434-5438.(c) Fu, Y.; Baba, T.; Ono, Y. J. Catal. 2001, 197, 91-
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ritesh@iict.res.in; ritesh.cdri@gmail.com
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the DST INSPIRE fund GAP0548
and CSIR fund CSC0301. RS thanks DST (Department of Sci-
ence and Technology) for INSPIRE faculty award. We are
grateful to Dr Ajay Kumar Srivastava for helpful discussions.
Dr UVR Vijay Saradhi, CSIR-IICT is highly acknowledged for
providing HRMS data. Director, IICT is gratefully acknowl-
edged for his constant motivation and providing necessary
infrastructure.
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o
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KEYWORDS: C-H amination, O-phenyl carbamates, benzoxazolone, chemoselective, nitrene insertion
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