Study on the ArI-catalyzed intramolecularoxy-cyclization of 2-alkenylbenzamides to benzoiminolactones

Article abstract of DOI:10.1039/d0ob00612b

A new intramolecularoxy-cyclization of 2-alkenylbenzamides catalyzed by ArI has been developed. This protocol is highlighted by its metal-free catalytic system and extremely short reaction time, providing efficient and straightforward access to various benzoiminolactones in good to excellent yields. Interestingly, a regioselective transformation occurred when using two different reaction systems. Mechanistic studies suggested thatmCPBA acts as both oxidant and ligand at the I^IIIcenter, and the Lewis acid BF₃accelerated ligand exchange and reductive elimination in the catalytic process.

Full text of DOI:10.1039/d0ob00612b

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DOI: 10.1039/D0OB00612B
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Study on the ArI-catalyzed Intramolecular Oxy-Cyclization of 2-
Alkenylbenzamides to Benzoiminolactones
Received 00th January 20xx,
Accepted 00th January 20xx
Huixia Liu, Xiaojun Deng, Xie Huang, Nan Ji, Wei He*
DOI: 10.1039/x0xx00000x
A new intramolecular oxy-cyclization of 2-alkenylbenzamides
catalyzed by ArI has been developed. This protocol is highlighted
by its metal-free catalytic system and extremely short reaction
time, providing efficient and straightforward access to various
benzoiminolactones in good to excellent yields. Interestingly, a
regioselective transformation occurred when using two different
reaction systems. Mechanistic studies suggested that mCPBA act
as both oxidant and ligand at the IIII center, and the Lewis acid BF3
accelerated ligand exchange and reductive elimination in the
catalytic process.
(a) Liu's work:Selective oxy-cyclization of 2-alkylbenzamides
O
TBAB (0.1 equiv.)
O
N
R
oxone (2.0 equiv.)
K2CO3 (3 equiv.)
N
R
R
H
N
O
THF:H2
80 o
O
TMS
R = H, CH3, F, Br, Cl
C
N-5-exo-dig
O-5-exo-dig
(b) This work: Oxy-cyclization of 2-alkenylbenzamides
ArI (cat)
O
R
R
N
N
[O]
R
N
H
R1
ArI (III)
R1
R1
+
O
O
R3
-2H
R2
R2
sp2 C-O priority
R2 =H, Me;
When: R2 = Et, R3 = Me; R2 = Et, R3 = Me
Functionalized benzoiminolactones are multifunctional
synthetic building blocks for the synthesis of various bioactive
molecules1 and chemical materials2. To date, various methods
for constructing benzoiminolactones have been reported. The
synthesis of iminolactones can be traced back to the [4+1]
cycloaddition reaction in the early 1980s, which employed ß-
unsaturated carbonyl compounds and isocyanides as starting
materials catalyzed by diethylaluminum chloride.3 In the past
few years, the reactions of arynes derived from different
precursors with isocyanide have been extensively exploited.4
Yoshida,4a,4b Stoltz,4c and Nishihara4d,4e groups independently
reported the formation of benzoiminolactones through the
three-component coupling reactions of arynes and isocyanides
with various unsaturated carbonyl compounds, such as
aldehydes/ketones, phenyl acetates, and cyanoformates. In
addition, it was demonstrated that benzoiminolactones can
also be prepared from the palladium-catalyzed insertion of
isocyanides.5 Recently, 2-alkynylbenzamides have been used
to replace these three components through intramolecular
cyclization, which has proven to be a powerful alternative
synthetic method for preparing benzoiminolactones.6
However, the transformations based on benzamides resulted
in the
Scheme 1 Synthesis of Benzoiminolactones.
discrepancy of both N/O nucleophilic selectivity under
different reaction systems, owing to the ambident nature of
the amide group.7 Therefore, this cyclization has four
possibilities through N- or O-attack modes with 5-exo-dig or 6-
endo-dig pathways, sometimes giving poor selectivity and
mixed products. By this protocol but replacing the 2-
alkynylbenzamide
with
N-phenyl
2-
trimethylsilylethynylbenzamide, Liu group reported the
synthesis of benzoiminolactones catalyzed by TBAB through a
radical process at 80 C overnight.6f But the spectral data
indicated that this reaction actually afforded isoindolinones
(N-5-exo-dig
product)8
instead
of
the
reported
benzoiminolactones (O-5-exo-dig product), where cyclization
proceeded on the nitrogen of the amide functionality rather
than the oxygen of the carbonyl group (Scheme 1a, see the
ESI† for full details). Thus, the structures of the products
reported in the literature need to be revised. Herein, an ArI-
catalyzed
intramolecular
oxy-cyclization
of
2-
alkenylbenzamides for the synthesis of benzoiminolactones
with high selectivity in good to excellent yields was firstly
described and correct structural data were provided (Scheme
1b).
a. Department of Chemistry, School of Pharmacy, The Fourth Military Medical
University, 169 Changle West Road, Xi'an 710032, P. R. China.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
We began our investigation by using 1a as a model substrate
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to determine optimal reaction conditions (Table 1; see the ESI† a good yield up to 90%, and was selected asVitehweArtoicpletOimnlinael
for full details). The oxidation process catalyzed by hypervalent conditions (Table 1, entry 17). Under DtOhIe: 10o.1p0t3im9/Dal0OrBe0a0c6ti1o2nB
Table 1 Optimization of reaction conditions.
conditions, the reaction was successfully scaled up to 1.0
mmol, affording 2a in 82% isolated yield (Table 1, entry 18).
Encouraged by these findings, we aimed to explore the
reaction scope. For a number of N-aryl-2-vinylbenzamides, the
outcome was largely insensitive to changes in the electronic
nature of substituents on the N-phenyl group. 2,6-dimethyl
(2b), methyl (2c–2e), 4-methoxyl (2f), and 4-tert-butyl (2g)
groups substituted products were obtained in excellent yields,
ranging from 79% to 90%. Those with halogen substituents
(1h–1l) were also well tolerated. Similarly, the substrates
bearing electron-withdrawing groups CF3 (1i) and NO2 (1j) gave
the corresponding products in 89% and 90% yields,
respectively. When the N-phenyl group was replaced with a 2-
naphthyl group (1o), an 84% yield was achieved. Remarkably,
replacing the N-phenyl group with an N-cyclohexyl group (1p)
or N-aliphatic group, such as neopentyl (1q) and benzyl (1r)
groups, were also applicable, although 2q was obtained in a
slightly lower yield. Furthermore, this method accommodated
various substitution patterns on the benzamide moiety.
Substrates with electron-donating or withdrawing groups at
the 4- or 5-positions of the benzene ring afforded desired
product 2s–2y in comparable
I
I
O
O
O
N
Catalyst
Oxidant
N
H
O
Acid, Solvent
2a
1a
I8
I6
Entrya
1c
Cat.
Oxidant
Lewis acid
CF3COOH
CH3SO3H
CH3COOH
BF3·Et2O
-
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
Solvent
HFIP
HFIP
HFIP
HFIP
Yield %b
15
I6 (10)
I6 (10)
I6 (10)
I6 (10)
I6 (10)
I6 (10)
I6 (10)
I8 (10)
I2 (10)
KI (20)
PIDA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
-
2c
23
3c
4
5
6
7
8
9d
10d
11e
12
13
14d
15d
16d
17f
18g
trace
58
trace
46
HFIP
DCM
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
88
89
78
83
79
85
I6 (5)
-
mCPBA
mCPBA
DTBP
trace
trace
trace
22
90 (95)
82
I6 (10)
I6 (10)
I6 (10)
I6 (10)
I6 (10)
TBHP
Oxone
mCPBA
mCPBA
Table 2 Substrate scope.a,b
a
Reaction conditions: 1a (0.2 mmol), catalyst (Y mol%), oxidant (0.4
O
R1
R1
I6 (10 mol%)
N
6
3
mmol), Lewis acid (0.3 mmol), solvent (2 mL), r.t., air atmosphere, 30–
60 min. b Isolated yield; Yield shown in parentheses was determined by
1H NMR using CH2Br2 (2.5 L) as the internal standard. C Lewis acid (0.4
1
5
mCPBA (1.5 equiv.)
N
R2
H
R2
O
BF3·Et2O (1.5 equiv.)
MeCN (2 mL)
R3
4
2
e
mmol). d Reacted for 3 h. PIDA (0.2 mmol). f mCPBA (0.3 mmol, 1.5
R3
1
2
equiv.). g Performed on 1.0 mmol scale. mCPBA, 3-chloroperoxybenzoic
acid; HFIP, 1,1,1,3,3,3-hexafluoro-2-propanol; PIDA, iodosobenzene
diacetate; DTBP, di-tert-butyl hydroperoxide; TBHP, tert-butyl
hydroperoxide.
N
2f, R = 4-OMe, 78%
2g, R = 4-t-Bu, 79%
2h, R = 4-F, 82%
2i, R = 2-Cl, 83%
2j, R = 3-Cl, 83%
2k, R = 4-Cl, 88%
2l, R = 4-Br, 85%
2m, R = 4-NO2, 89%
2n, R = 4-CF3, 90%
R
N
N
O
O
iodine reagents is often accelerated in the presence of
Brønsted or Lewis acids.9 So our initial studies involved tuning
the addition of Lewis acid to achieve benzoiminolactone
synthesis (Table 1, entries 1–5). Screening of Lewis acids
showed that BF3·Et2O achieved increased efficiency (Table 1,
entry 4). The control experiment proved that trace product
was formed in the absence of Lewis acid (Table 1, entry 5).
Among different solvents, MeCN was best for the formation of
the desired product 2a (Table 1, entries 4, 6 and 7). When
different iodine reagents were examined, both I6 and I8
afforded the product 2a in good yields, and I6 was determined
as the optimal catalyst for its ready availability (entries 7-8).
Moderate to good yields of 2a were also obtained with I2
(78%), KI (83%), and PIDA (79%) (Table 1, entries 9–11).
Decreasing the iodine catalyst loading to 5% afforded the
product in a slightly diminished yield of 85% (Table 1, entry
12). The product obtained in trace amounts without an iodine
catalyst indicated that the direct oxidation of 1a with mCPBA
may occur as background oxidation to give product 2a (Table
1, entry 13). Next, several other oxidants were tested, but no
better results were obtained (Table 1, entries 14-16).
Encouragingly, decreasing mCPBA to 1.5 equivalent resulted in
O
2c, R =2-Me, 79%
2d, R =3-Me, 79%
2e, R =4-Me, 81%
2a, 90%
2b, 83%
R
N
N
N
O
R
O
O
R
2v, R = Me, 81%
2w, R = F, 86%
2x, R = Cl, 89%
2y, R = NO2, 89%
2o, R = 1-naphthyl, 88%
2p, R = cyclohexyl, 78%
2q, R = neo-pentyl, 75%
2r, R = Bn, 80%
2s, R = Me, 80%
2t, R = Cl, 89%
2u, R = NO2, 90%
N
O
N
O
N
O
2z, 88%
2aa, 68%
2ab, 80%
R1
N
N
O
2ac, R1 = H, R2 = H, 60%
2ad, R1 = CH3, R2 = H, 64%
2ae, R1 = Cl, R2 = H, 66%
2af, R1 = H, R2 = CH3, 65%
2ag, R1 = H, R2 = Cl, 70%
O
R2
2ah, not detected
2 | J. Name., 2012, 00, 1-3
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NPh
DOI: 10.1039/D0OB00612B
a Reaction conditions: 1a (0.2 mmol), I6 (10 mol %), mCPBA (0.3 mmol),
BF3·Et2O (0.3 mmol), MeCN (2 mL), r.t., air atmosphere, 30 min.
Isolated yield.
O
NPh
b
Iodine reagent
NHPh
O
O
+
BF3·Et2O (1.5 equiv.)
MeCN (2 mL)
1ak
Entry
2ak
3ak
O
NPh
NPh
Iodine reagent
I6 (10 mol%)
mCPBA (Y equiv.)
Yielda
79%
75%
86%
75%
37%
50%
74%
2ak/3ak
1:2.3
1: 2.0
4.7:1
3.3:1
1:1.5
1:2.0
1:3.7
NHPh
R
Condition A
1
2
3
4
5
6
7
1.5
1.5
O
O
+
I6(OAc)2 (10 mol%)
I6(OAc)2 (1.2 equiv.)
I6(OAc)2 (1.2 equiv.)
I6 (10 mol%)
or Condition B
R
R
none
0.5
1
3
2
Substrates
1ai, R = H
1aj, R = Me
Conditions
Product
2ai, 82%, Z/E = 5.8:1a
2aj, 32%, Z/E = 5.6:1a
2aj, 68%, Z/E = 6.3:1a
Ratio
0.5
A
A
B
A
B
3ai, 0%
I6 (10 mol%)
1.0
I6 (10 mol%)
2.0
3aj, 45%
3aj, 13%
2aj/3aj = 1:1.4
2aj/3aj = 5.2:1
2ak/3ak = 1:2.3
2ak/3ak = 4.7:1
Scheme 3 Effect of mCPBA on the selective transformation of 1ak. a
Total yield of 2ak and 3ak.
1ak, R = Et
2ak, 24%, Z/E = 4.5:1a 3ak, 55%
2ak, 71%, Z/E = 6.7:1a 3ak, 15%
O
O
NPh
Scheme 2 Selective transformation of 1ak–1ai under ArI/mCPBA
H2, Pd/C
6N HCl
a
1
O
O
and I6(OAc)2 systems. Z/E ratio was determined by H NMR.
Conditions A: Substrate (0.2 mmol), I6 (10 mol%), mCPBA (0.3
mmol), BF3·Et2O (0.3 mmol), MeCN (2 mL), rt., air atmosphere,
15–30 min; Conditions B: Substrate (0.2 mmol), I6(OAc)2 (0.24
mmol), BF3·Et2O (0.3 mmol), MeCN (2 mL), r.t., air atmosphere,
30 min.
O
EtOAc, r.t.
THF, r.t.
99%
75%
2ak
5
4
NBP
(Z)-3-butylidenephthalide
Scheme 4 Transformation of product 2ak
yields. Interestingly, installing a methyl group at the 6-position
(1aa) significantly reduced the reaction yield to 68%. However,
shifting the methyl group to the 3-position led to the
generation of 2ab in 80% yield. The stilbene derived amides
(1ac–1ag) can undergo O-attack cyclization smoothly, with
2ac–2ag generated in moderate yields (60%–70%).
regioselective ß-hydride elimination might be involved in the
reaction process.
Next, we focused on explaining this selective elimination by
studying the effects of mCPBA. The findings were surprising,
leading us to speculate that in the catalytic cycle, mCPBA
might not be a mere spectator, but have potential dual roles as
both oxidant and ligand at the I(III) center, with the formation
of I6(OCOAr)2 or I6(OAc)OCOAr (Ar=m-ClC6H4).12 Indeed, using
10 mol% I6(OAc)2 as the catalyst giving the similar results with
I6 (Scheme 3, entries 1 and 2). Accordingly, adding mCPBA
significantly diminished the selectivity profile when using
I6(OAc)2 (Scheme 3, entries 3 and 4). The 2ak/3ak ratio was
profoundly affected by the mCPBA concentration (entries 1
and 5–7). In addition, these products provide synthetic handles
for the further construction of diversely substituted
phthalides. For example, a simple procedure6e,13 starting from
2ak can be used to deliver 3-butylphthalide (NBP), which has a
powerful curative effect on stroke14 and ischemia15 (Scheme
4).
Furthermore, the terminal alkene bearing
a 1-methyl
substituent (1ah) was not reactive and no desired 6-endo-dig
product (2ah) was detected, which suggested a high selectivity
in this reaction.
For internal alkenes bearing aliphatic groups, the reaction
became more complex (Scheme 2). Before process
optimization, we found that using 1ai under the standard
conditions afforded the expected product 2ai in 77% yield as a
5.8:1 mixture of Z/E isomers. Notably, when 1aj was used, an
additional single regioisomer was isolated, which was
identified as 3aj, a species expected on the basis of selective ß-
hydride elimination between the two ortho C–H sites.10
Intriguingly, using equally effective11 stoichiometric I(III)
reagent I6(OAc)2, derived from I6, resulted in the opposite
elimination selectivity, giving in a much higher 2aj/3aj ratio.
The successful transformation of 1ak with a similar change in
2ak/3ak ratio further demonstrated the generality of this
intriguing selectivity, indicating that
To explore the reaction mechanism, a series of control
experiments were conducted (see the ESI† for full details).
Adding TEMPO and BHT did not completely inhibit the
reaction, and no radical intermediate was captured, indicating
that a
(a) Synthesis of byproduct 2a'
O
N
I6 (10 mol%)
N
NHPh
mCPBA (1.5 equiv.)
O
+
O
MeCN (2 mL)
TsOH.H2O (2.0 equiv.)
OH
1a
2a, 9%
2a', 73%
(b) 1H NMR monitored the cyclization process of
1a
2aa
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intramolecular oxy-cyclization of 2-alkenylbenzamides
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catalyzed by I(III) reagents generated in DsiOtuI:.10T.h10e39re/Da0cOtiBo0n06a1ls2oB
proceeded smoothly using stoichiometric I(III) reagents, and
underwent selective transformation when substrates 1aj and
1ak were used. Further investigations into the application of
these products are ongoing in our laboratory.
We are grateful for the financial support from the National
Science and Technology Major Project of China on “Key New
Drug Creation and Development Program” (Project No.
2014ZX09J14104-06C), and the Shaanxi Province Key Research
and Development Program (S2019-YFZDCXL-ZDLSF-0138).
Scheme 5 Control experiments. a I: 1H NMR (400 MHZ) spectra of
reaction mixture of 1a (0.05 mmol), I6 (10 mol%), and mCPBA
Conflicts of interest
(0.075 mmol) in CD3CN (0.5 mL) with CH2Br2 (2.5 L) as There are no conflicts to declare.
internal standard. II: BF3·Et2O (0.075 mmol) was added to the
NMR tube; III: for 12 min; IV: for 22 min; V: for 30 min.
Notes and references
2a
1
(a) R. E. Royer, R. G. Mills, S. A. Young and D. L. V. Jagt,
mCPBA
Pharmacol. Res.,1995, 31, 49; (b) Y. Yu, J. A. Deck, L. A.
Hunsaker, L. M. Deck, R. E. Royer, E. Goldberg and J. D.
Vander, Biochem. Pharmacol., 2001, 62, 81; (c) S. G. Hegde,
R. D. Bryant, L. F. Lee, S. K. Parrish and W. B. Parker, Cyclic
ArI
CBA
m
RCO2H
RO2C
CO2R
Imidate
Derivatives
of
5-Amino-2,6-
I
Ph
H
HN
bis(polyfluoroalkyl)pyridine-3-carboxylates,
Chemical Society: Washington, DC, 1995, p. 60.
American
Ar
O
2
(a) K. Kurita, Y. Suzuki, T. Enari, S. Ishii and S. Nishimura,
Macromolecules, 1995, 28, 1801; (b) C. D. Georgiou, D.
Zisimopoulos, V. Argyropoulou, E. Kalaitzopoulou, P. V.
Ioannou, G. Salachas and T. Grune, Redox Biology, 2018, 17,
236.
Y. Ito, H. Kato and T. Saegusa, J. Org. Chem., 1982, 47, 741.
(a) H. Yoshida, H. Fukushima, J. Ohshita and A. Kunai, Angew.
Chem., Int. Ed., 2004, 43, 3935; (b) H. Yoshida, H. Fukushima,
T. Morishita, J. Ohshita and A. Kunai, Tetrahedron, 2007, 63,
4793; (c) K. M. Allan, C. D. Gilmore and B. M. Stoltz, Angew.
Chem., Int. Ed., 2011, 50, 4488; (d) Y. Nishihara, J. Li, S.
Noyori, M. Iwasaki and K. Nakajima, Heterocycles, 2012, 86,
933; (e) J. Li, S. Noyori, K. Nakajima and Y. Nishihara,
Organometallics, 2014, 33, 3500.
(a) C. G. Saluste, S. Crumpler, M. Furber and R. J. Whitby,
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Tang, Y.-M. Zhu and S.-J. Ji, J. Org. Chem., 2012, 77, 10321.
(a) C. Schlemmer, L. Andernach, D. Schollmeyer, B. F. Straub
and T. Opatz, J. Org. Chem., 2012, 77, 10118; (b) S. Mehta, T.
Yao and R. C. Larock, J. Org. Chem., 2012, 77, 10938; (c) R.
Mancuso, I. Ziccarelli, D. Armentano, N. Marino, S. V. Giofrè
and B. Gabriele, J. Org. Chem., 2014, 79, 3506; (d) X. Bantreil,
A. Bourderioux, P. Mateo, C. E. Hagerman, M. Selkti, E.
Brachet and P. Belmont, Org. Lett., 2016, 18, 4814; (e) D.
Ding, T. Mou, J. Xue and X. Jiang, Chem. Commun., 2017, 53,
5279; (f) Y. Wang, B. Ouyang, G. Qiu, H. Zhou and J. Liu, Org.
Biomol. Chem., 2019, 17, 4335.
BF3
O
1a
I
RCO2H
O
IM2
Ar
R
BF3
Ph
O
HN
BF3.Et2O
3
4
BF3.Et2O
O2CR
I
IM1
Ar
Scheme 6 Plausible reaction mechanism.
radical process was unlikely. Dioxygenation product 2a’ was
obtained in 73% yield by performing the reaction in the
presence of TsOH·H2O (Scheme 5a), which indicated the
5
6
1
important role of BF3·Et2O in this reaction. And dynamic H
NMR study well monitored the reaction progress (Scheme 5b).
Based on these results, a tentative reaction mechanism was
proposed (Scheme 6). Mechanistically, ArI is activated by
mCPBA, generating a free coordination site at the I(III) center.
BF3 then facilitates ligand exchange and I(III) reacts with the
double bond to construct three-membered iodonium ion
intermediate IM1, which is the prerequisite step. Species IM1
is then attacked by amide oxygen as the O–nucleophile to give
intermediate IM2. Finally, IM2 undergoes rapid reductive
elimination to afford oxy-cyclization product 2a and
regenerate active catalyst ArI, completing the catalytic cycle. In
this case, BF3 coordination converts the acyloxy group into a
strong leaving group, which promotes the reductive
elimination step.
7
8
(a) K. Tani and B. M. Stoltz, Nature, 2006, 441, 731; (b) C. R.
Kemnitz and M. J. Loewen, J. Am. Chem. Soc., 2007, 129,
2521.
For N-5-exo-dig products obtained using other methods: (a) J.
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In conclusion, we have presented a useful method for the
9
synthesis
of
various
benzoiminolactones
through
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