Intramolecular metal-free oxidative aryl-aryl coupling: An unusual hypervalent-iodine-mediated rearrangement of 2-substituted n-phenylbenzamides

Article abstract of DOI:10.1002/anie.201402925

Hypervalent-iodine-mediated oxidative coupling of the two aryl groups in either 2-acylamino-N-phenyl-benzamides or 2-hydroxy-N-phenylbenzamides, with concomitant insertion of the ortho-substituted N or O atom into the tether, has been described for the first time. This unusual metal-free rearrangement reaction involves an oxidative C(sp²)?C(sp²) aryl-aryl bond formation, cleavage of a C(sp²)?C(O) bond, and a lactamization/lactonization. Furthermore, unsymmetrical diaryl compounds can be easily obtained by removing the tether within the cyclized product.

Full text of DOI:10.1002/anie.201402925

Angewandte
Chemie
DOI: 10.1002/anie.201402925
Heterocycle Synthesis
Hot Paper
Intramolecular Metal-Free Oxidative Aryl–Aryl Coupling: An Unusual
Hypervalent-Iodine-Mediated Rearrangement of 2-Substituted N-
Phenylbenzamides**
Siyun Shang, Daisy Zhang-Negrerie, Yunfei Du,* and Kang Zhao*
Abstract: Hypervalent-iodine-mediated oxidative coupling of
the two aryl groups in either 2-acylamino-N-phenyl-benza-
mides or 2-hydroxy-N-phenylbenzamides, with concomitant
insertion of the ortho-substituted N or O atom into the tether,
has been described for the first time. This unusual metal-free
2
2
À
rearrangement reaction involves an oxidative C(sp ) C(sp )
2
À
aryl–aryl bond formation, cleavage of a C(sp ) C(O) bond,
and a lactamization/lactonization. Furthermore, unsymmetri-
cal diaryl compounds can be easily obtained by removing the
tether within the cyclized product.
O
xidative coupling of two aryl groups[1] has been intensively
studied and has become an exceedingly hot research topic in
modern organic synthesis. The direct oxidative coupling
reaction between two unfunctionalized aromatic substrates,
also known as cross-dehydrogenative coupling (CDC),[2] has
been gradually recognized as the most attractive and ideal
approach because of its simplicity. Unfortunately, most
existing methods require the participation of a transition
metal as the catalyst.[3] The metal-free parallels are, however,
much less reported, but such explorations have begun to
receive attention from organic chemists.[4] In view of the cost
as well as environmental concerns of heavy metal involve-
ment, the development of alternative metal-free oxidative
coupling systems, especially those that can permit special
structural types of coupling, are pressingly worthwhile.
Scheme 1. Aryl–aryl oxidative coupling mediated by hypervalent iodine
reagents.
by a tether, thereby affording polycyclic compounds (Sche-
me 1b),[7] and 3) the spirocyclization of a tethered phenol
derivative involving an ipso attack from the side-chain arene
during the completion of the oxidative carbon–carbon bond
formation (Scheme 1c).[8] It is worth noting that the phenol
ring was converted into a cyclic dienone in the process, and
also that in the last two strategies, the tether was kept intact
throughout the reaction. Herein, we report the discovery of
a novel iodine(III)-mediated rearrangement reaction of the 2-
acylamino-N-phenylbenzamides 1 and 2-hydroxy-N-phenyl-
Hypervalent iodine reagents, which are efficient and
environmentally benign nonmetal oxidants,[5] have been
successfully applied to the oxidative coupling reactions
between two arene moieties.[4b,6] A careful literature survey
showed that the existing iodine(III)-mediated oxidative
coupling reactions can be classified into three types: 1) the
intermolecular cross-coupling of two arenes (Scheme 1a),[4b,6]
2) the intramolecular oxidative coupling of two arenes joined
benzamides 3, a process which involves an unusual cleavage
2
À
of a C(sp ) C(O) bond and an intramolecular condensation
reaction, after an intramolecular oxidative aryl–aryl bond
formation. To our knowledge, this is an unprecedented metal-
free protocol for the oxidative coupling of two aryl groups
with concomitant insertion of an ortho-substituted N or O
moiety into the tether (Scheme 1d).
Initially, we were interested in synthesizing dibenzo[b,e]-
[1,4] diazepin-11-one derivatives from 1 through an iodine-
2
À
(III)-mediated intramolecular aromatic C(sp ) N bond for-
[*] S. Shang, Dr. D. Zhang-Negrerie, Prof. Dr. Y. Du, Prof. Dr. K. Zhao
Tianjin Key Laboratory for Modern Drug Delivery & High-Efficiency,
School of Pharmaceutical Science and Technology
Tianjin University, Tianjin 300072 (China)
and
mation.[9] But to our surprise, the reaction of 1 equivalent of
1a with 1.5 equivalents of PIDA in acetonitrile at reflux for
24 hours afforded a 41% yield of the rearranged product 2a
(Table 1, entry 1), the structure of which was unambiguously
confirmed by X-ray crystallography.[15] The discovery of this
new rearrangement reaction opened the door to a new
approach for the construction of the structurally interesting
and pharmaceutically[10] important dibenzodihydro-1,3-diaze-
pin-2-one skeletons.[11] The compound 1a was then used as
a model substrate to screen for the optimal reaction
conditions. A series of experimental variables, including the
amount of the oxidant, the solvent, temperature, additive, and
type of hypervalent iodine oxidant were systematically
Collaborative Innovation Center of Chemical Science and Engi-
neering (Tianjin), Tianjin 300072 (China)
E-mail: duyunfeier@tju.edu.cn
kangzhao@tju.edu.cn
[**] Y.D. acknowledges the National Natural Science Foundation of
China (no 21072148) for financial support. We also thank Prof.
Richard P. Hsung (University of Wisconsin at Madison) for helpful
discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402925.
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Table 1: Optimization of the reaction conditions.[a]
Table 2: Scope of PIDA-mediated dibenzodihydro-1,3-diazepin-2-ones
synthesis.[a]
Entry
Oxidant (equiv)
Solvent
T [8C]
Yield [%][b]
1
PIDA (1.5)
PIDA (3.0)
PIDA (3.0)
PIDA (3.0)
PIDA (3.0)
PIFA (1.5)
PhIO (1.5)
IBX (1.5)
MeCN
MeCN
DCE
TFE
HFIP
MeCN
MeCN
MeCN
reflux
reflux
reflux
reflux
reflux
RT
41
83
35
23
15
2[c]
3
4
5
6
7
n.d.[d]
n.r.
n.r.
reflux
reflux
8
[a] All reactions were carried out with 1a (0.5 mmol), PIDA (3.0 equiv),
MeCN (10 mL), reflux for 24 h, unless otherwise stated. [b] Yield of
isolated product. [c] 14 h. [d] No desired product was detected.
DCE=1,2-dichloroethane, HFIP=1,1,1,3,3,3-hexafluoro-2-propanol,
IBX=ortho-iodoxybenzoic acid, n.r.=no reaction, PIDA=phenyliodine-
(III) diacetate, TFE=trifluoroethanol, PIFA= phenyliodine(III) bis(tri-
fluoroacetate).
examined (Table 1; see the Supporting Information for
details). After several experiments, we found the best
conditions to be 3.0 equivalents of PIDA in MeCN at reflux
for 14 hours (Table 1, entry 2).[12]
Under the optimal reaction conditions, we examined
a wide range of substrates (1; Table 2). Results show: 1) The
electronic effect of R1 at the meta-position has an effect, that
is, electron-donating groups enhance the reaction, with the
highest yields obtained from 1a and 1c. Moderate electron-
withdrawing groups (F or Br) negatively affected the reaction
yield (2g–h), and strongly electron-withdrawing groups
(ester, nitro, or trifluoromethyl) completely impeded the
reaction (not shown). The observed R1 effect concurs with an
electrophilic mechanism, wherein the aniline moiety serves as
a nucleophile. 2) Steric effects are substantial as high
regioselectivity was demonstrated in the formation of
a single product from substrates bearing an unsymmetrically
substituted aniline moiety, namely, 2a, 2c, 2g, and 2h. 3) The
reaction yield is insensitive to the electronic nature of R2 (2i–
m). 4) Other than N-methyl, R3 can also be an N-isopropyl, N-
benzyl, or even N-phenyl group (2n–p). 5) The acetyl group
(R4) can be replaced with a tert-butoxy or phenyl group
(2q,r). 6) The same method afforded a class of novel
tetracyclic heterocyclic compounds (2s–u) in moderate
yields when starting with N-aryltetrahydroquinoline sub-
strates.
Encouraged by the above findings, we tested out the
method on 3a, where the entire acylamino group in 1e was
replaced by an OH group. Disappointingly, subjecting 3a to
the identical reaction conditions afforded only a complex
mixture, with no trace of the expected rearranged product.
New studies led us to discover the formation of the
spirolactam 4 upon treating 3a with PIFA (1.5 equiv) in
CH2Cl2 (0.05m) at 08C for 0.5 hours (Scheme 2; see the
Supporting Information for details). To our delight, 4 could be
conveniently transformed into the expected dibenzo[d,f][1,3]
[a] All reactions were carried out with 1 (1 mmol) and PIDA (3.0 equiv)in
MeCN (20 mL) at reflux. [b] Yield of isolated product. [c] PIDA
(4.0 equiv). [d] PIDA (2.0 equiv).
Scheme 2. The PIFA/BF3·Et2O-mediated spirocyclization and rearrange-
ment of 2-hydroxy-N-methyl-N-phenylbenzamide (3a).
oxazepin-6(7H)-one (5a) after mixing with BF3·Et2O
(0.3 equiv) at room temperature. Considering the fact that
both steps could be performed in CH2Cl2, a one-pot protocol
was envisaged. The most efficient reaction conditions for this
reaction were determined to be PIFA (1.3 equiv) in CH2Cl2
(0.05m) at 08C for 0.5 hours, followed by introduction of
BF3·Et2O (0.3 equiv) and stirring at room temperature for an
additional 0.5 h.[13]
.
Similarly positive results show that this PIFA/BF3 OEt2
one-pot method can be applied to substrates bearing a variety
of R1 groups (Table 3; 5a–i), halogens (R2; 5j,k), and a range
of R3 groups including phenyl, benzyl, and isopropyl (5l–n).
2
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Table 3: Scope of one-pot synthesis of dibenzo[d,f][1,3]oxazepin-6(7H)-
iodide and acetic acid. The presence of the electron-with-
ones mediated by PIFA/BF3·Et2O.[a]
drawing acyl group on the imine N and the lone pair on the
amide N then facilitate a rapid ring opening of 7 to give the
intermediate 8. Finally, lactamization of 8 provides the title
compound 2e. Although 7 was not directly observed during
the reaction, the isolation of 4 in the two-step reaction of 3a
provides convincing evidence for the reaction sequence
proposed.
This particular rearrangement reaction was also applied to
the synthesis of biaryls. Treating 2j with KOH (10 equiv) in
DMSO/H2O (v/v, 1:1) at 1208C for 5 hours provided the
diaryl 9 in a satisfactory 74% yield (Scheme 4). Similarly, the
biaryl 11 could be formed through the cleavage of the
carbamate group in 5m (Scheme 5). The transformation
involved a known debenzylation procedure[14] and subsequent
hydrolysis conditions.
Scheme 4. Conversion of 2j into 9 through base-promoted hydrolysis.
DMSO=dimethylsulfoxide.
[a] All reactions were carried out with 3 (1 mmol) and PIFA (1.3 equiv) in
CH2Cl2 (20 mL) at 08C for 0.5 h. BF3·OEt2 (0.3 equiv) was then added
was added at RT. [b] Yield of isolated product. [c] 1.5 equiv of PIFA was
added and the reaction mixture stirred at À358C for 2.5 h, then BF3·OEt2
(0.3 equiv) was added at RT.
Scheme 5. Conversion of 5m into 11 through debenzylation and
hydrolysis. AIBN=2,2’-azobis(2-methylpropionitrile), NBS=N-bromo-
succinimide.
Two regioisomeric products were obtained in the cases of 3 f
and 3g because they contain a substituent at the meta posi-
tion. An unsubstituted amino moiety resulted in none of
desired product (5p).
A plausible mechanism has been proposed (Scheme 3).
Take the reaction of 1e as an example. The intermediate 6,
initially formed from the reaction of 1e and PIDA and loss of
one molecule of acetic acid, gives rise to the spiro-intermedi-
ate 7 after a nucleophilic ipso attack of the aniline ring on the
electron-deficient aromatic carbon atom of the anthranilic
ring in 6, accompanied by the loss of one molecule of phenyl
In conclusion, we have discovered an unprecedented
iodine(III)-mediated tandem reaction involving the dehydro-
genative coupling of two aryl groups, the cleavage of a C-
2
À
(sp ) C(O) bond, and final intramolecular cyclization. This
novel protocol provides easy access to a series of diversely
functionalized dibenzodihydro-1,3-diazepin-2-ones (2) and
dibenzo[d,f][1,3]oxazepin-6(7H)-ones (5), which are not read-
ily accessible by any known approaches. Furthermore, unsym-
metrical diaryl compounds can be easily obtained by remov-
ing the tether within the cyclized product.
Received: March 2, 2014
Revised: March 31, 2014
Published online: && &&, &&&&
Keywords: cross-coupling · heterocycles ·
.
hypervalent iodine reagents · rearrangement
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Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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[13] We also performed the reaction by using a combination of 20%
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[15] http:///CCDC 973387 (2a), formula: C18H18N2O4, unit cell
parameters: a = 8.6936(17), b = 9.901(2), c = 10.245(2) ꢁ, P-1.
4
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Heterocycle Synthesis
S. Shang, D. Zhang-Negrerie, Y. Du,*
K. Zhao*
&&&& — &&&&
Intramolecular Metal-Free Oxidative Aryl–
Aryl Coupling: An Unusual Hypervalent-
Iodine-Mediated Rearrangement of 2-
Substituted N-Phenylbenzamides
Making (re)arrangements: Hypervalent-
iodine-mediated oxidative coupling of the
two aryl groups in either 2-acylamino-N-
phenyl-benzamides or 2-hydroxy-N-phe-
nylbenzamides with concomitant inser-
tion of the ortho-substituted N or O atom
into the tether is described. This unusual
metal-free rearrangement reaction
2
2
À
involves an oxidative C(sp ) C(sp ) aryl–
aryl bond formation, cleavage of a C-
2
À
(sp ) C(O) bond, and a lactamization/
lactonization.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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These are not the final page numbers!