Design and Synthesis of 2-Methyl-7-aminobenzoxazole as Auxiliary in the Palladium(II)-Catalyzed Arylation of a beta-Positioned C(sp3)-H Bond

Article abstract of DOI:10.1002/adsc.201500834

A palladium(II)-catalyzed direct arylation of methylene C(sp³)-H bonds by 2-methyl-7-aminobenzoxazole as an effective auxiliary is reported. This process exhibited high beta-site selectivity, broad substrate scope, and compatibility with different functional groups with moderate to high yields up to 89%.

Full text of DOI:10.1002/adsc.201500834

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DOI: 10.1002/adsc.201500834
Design and Synthesis of 2-Methyl-7-aminobenzoxazole as Auxili-
ary in the Palladium(II)-Catalyzed Arylation of a beta-Positioned
3
À
C(sp ) H Bond
Feihua Luo,a,b Jun Yang,a Zhengkai Li,a,* Haifeng Xiang,a and Xiangge Zhoua,*
a
Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University, Chengdu 610064, Peopleꢀs Republic of
China
Fax: (+86)-28-8541-2904, phone: (+86)-28-8541-2026; e-mail: zhengkaili@scu.edu.cn or zhouxiangge@scu.edu.cn
School of Chemistry and Chemical Engineering, Sichuan University of Arts and Science, Dazhou 635000, Peopleꢀs
b
Republic of China
Received: June 29, 2015; Revised: January 6, 2016; Published online: February 17, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500834.
Abstract: A palladium(II)-catalyzed direct arylation
of methylene C(sp ) H bonds by 2-methyl-7-amino-
And several bidentate auxiliaries have been reported
3
3
À
À
to activate methylene C(sp ) H bonds, such as 8-ami-
benzoxazole as an effective auxiliary is reported.
This process exhibited high beta-site selectivity,
broad substrate scope, and compatibility with differ-
ent functional groups with moderate to high yields
up to 89%.
noquinoline (AQ),[4,5] picolinamides(PA),[4b,6] 2-meth-
ylthioaniline (MTA),[4b,7] N-(2-pyridyl)sulfonamide
(PSA),[4c] 2-(pyridin-2-yl)isopropylamine (PIP),[4e,8] 2-
methoxyiminoacetyl (MIA)[4d] and 1,2,3-triazoleamide
(TAM)[4f] (Scheme 1A). These reports have prompted
3
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tremendous progress in C(sp ) H activation and func-
À
Keywords: arylation; benzoxazoles; catalysis; C H
activation; directing group
tionalization, but there still exist many problems and
challenges. For example, some of these auxiliaries are
expensive or difficult to be synthesized and modified.
Therefore, the development of new economic and ver-
3
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satile bidentate directing groups for C(sp ) H activa-
tion is still in high demand.
In recent years, transition metal-catalyzed functionali-
zation of C H bonds has emerged as an efficient a kind of organic compounds, which can be easily syn-
On the other hand, oxazoles and oxazolines are
À
À
À
method for C C and C heteroatom bond forma- thesized and modified, and have already been report-
tion.[1] A wide variety of catalytic functionalizations of ed in the C(sp ) H and C(sp ) H functionalization by
2
3
À
À
C(sp ) H bonds has been developed and exhibited several groups.[9] For example, oxazoline-directed ace-
2
À
significant impact in the field of organic synthesis.[2] toxylation and iodination of C(sp ) H were reported
3
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Compared with the significant progress in the aryla- by Yu and co-workers,[9a,b] and amide-tethered oxazo-
2
À
tion of C(sp ) H bonds, the catalytic functionalization line (Scheme 2) was also reported as an excellent bi-
3
2
À
À
À
of C(sp ) H bonds, especially inactive methylene dentate auxiliary for C(sp ) H activation to form C
3
[10]
À
À
C(sp ) H bonds, remains an important fundamental -C and C N bonds by the same group. Therefore,
challenge. Moreover, because of steric hindrance and these versatile auxiliaries inspired us to examine their
intrinsic inertness, methylene C(sp ) H bonds are sig- potential use in methylene C(sp ) H arylation.
nificantly more difficult to cleave than primary Herein we report the design and synthesis of biden-
3
3
À
À
3
[3]
À
C(sp ) H bonds. And only few examples of the tate auxiliaries for the directed arylation of inactive
3
3
À
À
functionalization of methylene C(sp ) H bonds have primary and methylene C(sp ) H bonds with aryl io-
been reported.[4]
dides. This reaction can tolerate a variety of function-
Meanwhile, the utilization of bidentate directing al groups with good yields and offers a new auxiliary
3
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À
groups in the activation of C H bonds has significant- for inactivated methylene C(sp ) H functionalization
ly grown in popularity since the seminal discovery by (Scheme 1B).
Daugulis and co-workers,[5] and this area appears to
At first, based on the successful examples of Pd-
[4a,b,e]
À
show a high potential for exploring new types of catalyzed C H activation,
we designed and syn-
À
transformations of C H bonds, which cannot be ach- thesized N-[2-(4,5-dihydrooxazol-2-yl)phenyl]butyra-
ieved by conventional monodentate directing groups. mide A as a bidentate directing substrate (Scheme 3).
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Scheme 2. An amide-oxazoline was used as an excellent bi-
2
À
À
dentate auxiliary for C(sp ) -H activation to form C C and
À
C N bonds by the Yu group.
À
on the C H activation (Scheme 3a). Then, N-(ben-
zo[d]oxazol-4-yl)butyramide B with a rigid ring struc-
ture was synthesized and tried in this reaction. How-
ever, the arylation occurred this time on the 2 posi-
tion of the oxazole ring as the main product with
a yield of 58% (Scheme 3b).
2
À
Considering the activity of the C(sp ) H bond of
the 2 position of benzoxazole, a methyl group was in-
troduced to occupy this position, and N-(2-methylben-
zo[d]oxazol-4-yl)-butyramide 1a was synthesized as
substrate. The results are listed in Table 1.
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Scheme 1. Pd-catalyzed direct C(sp ) H activation assisted
by bidentate auxiliaries.
Encouragingly, the reaction of 1a and iodobenzene
in the presence of Pd(OAc)2 (10 mol%) and AgOAc
in toluene at 1008C gave the desired product 3a in
Unfortunately, under standard catalytic conditions,[4a] 25% yield (Table 1, entry 1), and AgOAc was found
the arylation reaction occurred on the a position of to be the best iodide scavenger in further tests (en-
the carbonyl group in a yield of 32%, which indicated tries 2–6). After screening different solvents, we
that free rotation of the s bond between the phenyl found that t-amyl-OH exhibited the best efficacy in
and oxazoline rings might cause a coordination effect an increased yield of 56% (entries 7–15). Other palla-
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Scheme 3. Our preliminary design for arylation C(sp ) H with bidentate directing groups.
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Design and Synthesis of 2-Methyl-7-aminobenzoxazole as Auxiliary
Table 1. Optimization of the reaction conditions.[a]
Entry
Cat.
Additive
Solvent
Temp. [8C]
Yield[b]
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
AgOAc
AgTFA
AgBF4
Ag2CO3
AgSbF6
K2CO3
toluene
toluene
toluene
toluene
toluene
toluene
o-xylene
t-butyl-OH
t-amyl-OH
dioxane
CH3CN
DCE
chlorobenzene
t-amyl-OH/H2O(4:1)
–
t-amyl-OH
t-amyl-OH
t-amyl-OH
t-amyl-OH
t-amyl-OH
t-amyl-OH
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
120
100
100
25%
trace
ND
15%
trace
trace
28%
46%
56%
43%
23%
31%
NR
15%
17%
48%[c]
63%[d]
75%[e]
74%
15%[f]
trace[g]
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
9
10
11
12
13
14
15
16
17
18
19
20
21
[a]
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Pd(OAc)2 (10% mmol), additive(0.4 mmol), solvent (1 mL), 1008C
unless otherwise specified.
Isolated yields.
[b]
[c]
[d]
[e]
[f]
Decrement the amount of Pd(OAc)2 to 5% mmol under the standard conditions.
3 equiv. of AgOAc were used.
1a:2a=1:6, 3 equiv. of AgOAc were used.
Bromobenzene was used instead of 2a.
Chlorobenzene was used instead of 2a.
[g]
dium resources and additives proved to be less effec-
As show in Table 2, iodobenzenes bearing a broad
tive for this transformation and resulted in almost the range of functional groups such as alkyl, aryl, fluoro,
same or lower yields (see the details in the Supporting chloro, bromo, methoxy, trifluoromethyl, acetyl,
Information). A reduced yield of 48% was obtained cyano or methoxycarbonyl were smoothly reacted
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when the dosage of Pd catalyst was reduced from 10 with methylene C(sp ) H bonds, giving the desired b-
to 5 mmol% (entry 16). Further optimization of reac- arylated products (3a–3w) in moderate to good yields
tion conditions showed that an increment of the (61–86%). Both electron-donating (3a–3j) and elec-
amount of AgOAc to 3 equiv. could improve the yield tron-withdrawing groups (3k–3u) were compatible
to 63%. At last, a 75% yield was obtained when with good yields. Meanwhile, the reaction was sensi-
6 equiv. of iodobenzene were used (entries 17 and 18). tive to steric hindrance. For example, 4-iodoanisole
And the efficiency could not be enhanced by simply and 3-iodoanisole were reacted smoothly with 1a,
lengthening the reaction time or raising the tempera- whereas 2-iodoanisole completely failed to give the
ture (entry 19). Bromobenzene and chlorobenzene desired product (3b, 3c and 3d). More importantly,
were also tested in this reaction and resulted in 15% some functional groups including cyano, methoxycar-
and traces of product.[11]
bonyl and bromo survived in the reaction system with
With the optimized catalytic conditions in hand, good yields of up to 85% (3n, 3o, 3r–3t), which of-
a variety of different iodobenzene substrates was ap- fered opportunities for further orthogonal transforma-
plied in this reaction.
tions. The heteroaryl iodides such as those of thio-
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[a,b]
À
Table 2. Pd(II)-catalyzed arylation of methylene C(sp ) H bonds with ArI.
[a]
Reaction conditions: 1a (0.2 mmol), 2a (1.2 mmol), Pd(OAc)2 (10%) and AgOAc(0.6 mmol) in 1.0 mL t-amyl-OH at
1008C for 24 h.
Isolated yields.
[b]
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Design and Synthesis of 2-Methyl-7-aminobenzoxazole as Auxiliary
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phene and pyridine were also successfully arylated, mary sp C H bonds of propionyl and pivaloyl deriva-
albeit with lower yields of 36% and 21%, respectively tives also performed well to give both of mono- and
(3v, 3w).
di-arylated products in ratios near 3:1 and 1:1, respec-
Next, variation of the alkyl substituents of the car- tively (3af and 3ag). When sterically hindered cyclo-
boxamides was then probed. Different aliphatic car- pentyl and benzyl groups were tested, the yields of
boxylic acid derivatives were tested (3x–3ak). As the b-arylated products were reduced to 59% and
shown in Table 3, substrates containing valeryl, hexa- 67% (3aa and 3ac). As the alicyclic carboxylic acids
noyl and heptanoyl groups were smoothly arylated to are very common in natural products[4d,f] and pharma-
give the desired products in yields from 72% to 78%, ceutical agents,[4g–i] we tested the six- and five-mem-
and the length of the chain seemed to have few ef- bered cycloalkyl amides for this arylation. When the
fects on the results (3x–3z).[13] Furthermore, the pri- ratio of amide to aryl iodide was 1:2, only the mono-
3
[a,b]
À
Table 3. Pd-catalyzed direct arylation of C(sp ) H bonds with different aliphatic amides.
[a]
Reaction conditions: 1a (0.2 mmol), 2a (1.2 mmol), Pd(OAc)2 (10%) and AgOAc(0.6 mmol) in 1.0 mL t-amyl-OH at
1008C for 24 h.
Isolated yields.
2 equiv. of aryl iodide were used.
82% of starting material was recovered.
[b]
[c]
[d]
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Feihua Luo et al.
arylated products could be afforded with moderate iodide with 5 forms Pd(IV) complex 6. After subse-
yields of 41% and 63%, respectively (3ah and 3ai). quent the reductive elimination, the desired product
And the amino acids compounds could also be mono- 3a was generated. At the same time, the Pd(II) cata-
selectivity arylated with good yields (3aj and 3ak). lyst is released for the next catalytic cycle.
2
À
But this directing group was ineffective in C(sp ) H
In conclusion, we have developed a Pd(II)-cata-
3
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arylation of an aryl amide derivative and only the cor- lyzed direct arylation of methylene C(sp ) H bonds
responding starting material (1al) was recovered.[12]
with 2-methyl-7-aminobenzoxazole as an effective
Finally, this directing group could be successfully auxiliary. This process exhibited high b-site selectivity,
removed by simple hydrolysis under acidic conditions. broad substrate scope, and high compatibility with
The corresponding acid 4 was obtained in an excellent various important functional groups. Compared with
yield (Scheme 4).
other bidentate directing groups, 7-aminobenzoxazole
On the basis of this work and successful examples has the advantages of being easy to synthesize and
in the literature,[4b,12,14] a plausible reaction pathway is modify, which would provide future potential applica-
3
À
proposed as shown in Scheme 5. The coordination of tions of the effective auxiliary in C(sp ) H functional-
the amide 1a with palladium catalyst formed complex ization. Further studies toward the application of this
À
4, which further underwent C H activation at the reaction to the synthesis of complex targets and the
b position to produce 5. Oxidative addition of aryl elucidation of the mechanistic details of the process
are under way.
Experimental Section
Typical Procedure
A screw-capped vial with stir bar was charged with N-(2-
methylbenzo[d]oxazol-4-yl)-butyramide
(1a)
(43.6 mg,
0.2 mmol), aryl iodide (2a) (134 mLmg, 1.2 mmol),
Pd(OAc)2 (4.5 mg, 0.02 mmol), AgOAc (100 mg, 0.6 mmol),
and t-amyl-OH (1 mL). The reaction vessel was closed, and
the reaction mixture was stirred at 1008C for 24 h. The reac-
tion mixture was then washed with EtOAc. The organic
phase was washed with water, dried over magnesium sulfate,
and concentrated. The crude product was purified with
column chromatography (petroleum/EtOAc 4:1) to give 3a
as a white solid; yield: 44.1 mg (75%); Rf =0.31 (petroleum/
EtOAc 4:1).
Scheme 4. Auxiliary can be easily removed under acidic con-
ditions.
Acknowledgements
We are grateful to the Natural Science Foundation of China
(grant nos. 21272161, 21372163, 21472128, J1103315,
J1310008), the Ministry of Education (no. 20120181110050)
and major project of Sichuan Provincial Education Depart-
ment (no. 12A282) for financial support. We also acknowl-
edge the Key Laboratory of Green Chemistry and Technolo-
gy of Ministry of Education for HR-MS analysis.
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