Oxidative Coupling between Methylarenes and Ammonia: A Direct Approach to Aromatic Primary Amides

Article abstract of DOI:10.1002/adsc.201500310

A direct oxidative amidation between methylarenes and aqueous ammonia using a tert-butyl hydroperoxide and tetrabutylammonium iodide (TBHP/TBAI) oxidation system with co-catalysis of iron(III) chloride has been developed. Both coupling partners were used in their native form to render prior functionalization unnecessary and afford a facile approach to aromatic primary amides.

Full text of DOI:10.1002/adsc.201500310

UPDATES
DOI: 10.1002/adsc.201500310
Oxidative Coupling between Methylarenes and Ammonia:
A Direct Approach to Aromatic Primary Amides
Zhenguang Zhao,b Tao Wang,a,b,* Lin Yuan,b Xu Hu,b Fei Xiong,b
and Junfeng Zhaoa,b,*
a
National Research Center for Carbohydrate Synthesis and Key Laboratory of Chemical Biology, Jiangxi Province,
Nanchang 330022, Jiangxi, Peopleꢀs Republic of China
College of Chemistry & Chemical Engineering, Jiangxi Normal University, 99, Ziyang Road, Nanchang 330022, Jiangxi,
b
Peopleꢀs Republic of China
E-mail: wangtao@jxnu.edu.cn or zhaojf@jxnu.edu.cn
Received: March 28, 2015; Revised: April 23, 2015; Published online: August 3, 2015
Abstract: A direct oxidative amidation between
methylarenes and aqueous ammonia using a tert-
butyl hydroperoxide and tetrabutylammonium
iodide (TBHP/TBAI) oxidation system with co-cat-
alysis of iron(III) chloride has been developed.
Both coupling partners were used in their native
form to render prior functionalization unnecessary
and afford a facile approach to aromatic primary
amides.
atom economy reagents” has been identified as one
of the top challenges for organic chemistry in 2005.[3]
Thus, amide bond formation processes beyond the
traditional coupling strategy have emerged as attrac-
tive alternatives in recent years.[4] However, most of
these efforts focus on secondary and tertiary amide
synthesis instead of the primary amide synthesis. Pri-
mary amides are important structural motifs in many
drugs, materials, and versatile intermediates for other
fine chemicals (Figure 1).[5] Current synthetic strat-
egies for primary amides include the hydration of ni-
triles,[6] aminocarbonylation of aryl halides[7] and phe-
Keywords: amides; tert-butyl hydroperoxide; iron;
methylarenes; oxidative coupling
nols,[8] rearrangement of aldoximes,[9] cleavage of C
À
C bonds,[10] and oxidation of primary amines.[11] Most
of these methods use noble transition metal catalysts,
harsh reaction conditions, or have a limited substrate
The amide unit is one of the most prevalent function- scope. Given the important applications of primary
al groups widely found in proteins, pharmaceuticals, amides, highly efficient, economic, and environmen-
functional materials, and other versatile fine synthetic
chemicals.[1] Traditional amide chemical synthesis gen-
erally uses prior activated carboxylic derivatives or
large excesses of coupling reagents under harsh reac-
tion conditions.[2] “Amide formation that avoids poor
Scheme 1. Amide synthesis by the direct oxidative amida-
tion of methylarenes.
Figure 1. Selected examples of amide drugs.
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Oxidative Coupling between Methylarenes and Ammonia
Table 1. Optimization of the reaction conditions.[a]
tally benign synthetic methods for primary amide syn-
thesis are now in high demand.
A breakthrough was made by Mizuno et al. in this
context by employing methylarenes and urea as cou-
pling partners for primary amide synthesis under
5 atm of O2 at 1508C (Scheme 1a).[12] Although an
excess amount of the promoter MnO2 (ca. 4 equiv.)
was essential and the reaction conditions were slightly
harsh, this pioneering work opened a new scenario
for amide synthesis because the raw hydrocarbon
chemical methylarenes instead of the pre-functional-
ized fine chemicals such as aldehydes[13] or alcohols[14]
were employed as the acyl donor. On the other hand,
the combination of tert-butyl hydroperoxide (TBHP)
and iodide reagents[15] in recent years has been recog-
nized as a green oxidation system because the general
side products are environmentally benign tert-butyl al-
cohol and water. This oxidation system has recently
been extended to the oxidative amidation of methyl-
arenes.[16] However, all of these protocols are limited
to the preparation of secondary and tertiary amides
because activated amine surrogates were necessary.
The synthesis of primary amides by the oxidative ami-
dation of methylarenes and ammonia is still a chal-
lenging topic. Here we demonstrate that the aromatic
primary amide can be prepared efficiently from oxi-
dative amidation between methylarenes and aqueous
ammonia using TBHP as the environmentally benign
oxidant for the first time.
Entry
Amine
Additive
Yield[b]
1
2
3
4
5
6
7
8
NH3·H2O
NH3·H2O
NH3·H2O
NH3·H2O
NH3·H2O
NH4Cl
NH4HCO3
HCO2NH4
(NH4)2CO3
CH3CO2NH4
urea
–
44
31
44
63
93
88
83
86
75
47
trace
91 (88)
CuBr2
AgNO3
FeBr2
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
9
10
11
12[c]
NH3·H2O
[a]
Reaction conditions: methylarene 1 (5 mmol), aqueous
ammonia 5 (25% in water, 0.24 mmol), TBHP (70% in
water,
2.4 mmol),
tetrabutylammonium
(TBAI)
(20 mol%), FeCl3·6H2O (15 mol%), 808C, 18 h.
GC yield, isolated yield is indicated in parentheses.
1.5 mmol TBHP (70% in water) was used.
[b]
[c]
Table 2. Direct oxidative amidation of methylarenes.[a]
Our group recently developed a direct oxidative
amidation of methylarenes for the synthesis of secon-
dary and tertiary amides by employing free amines as
nitrogen source in the TBHP/TBAI oxidation system
with the co-catalysis of FeCl3 (Scheme 1b).[17] We hy-
pothesized that this strategy can also be amenable to
the synthesis of primary amides. It is initiated from
testing the TBHP/TBAI oxidation system using tolu-
ene and aqueous ammonia (25 wt%) as model reac-
tants. Benzamide was indeed obtained with 44% yield
in the presence of TBHP (10 equiv.) and TBAI
(30 mol%) after the reaction mixture had been stirred
at 808C for 18 h (Table 1, entry 1). Only a trace
amount of amide was detected when 30% H2O2 solu-
tion, di-tert-butyl peroxide (DTBP), or meta-chloro-
perbenzoic acid (m-CPBA) was employed as an oxi-
dant (see the Supporting Information). Other iodine-
containing catalysts, such as KI, NaI, NIS, and I2,
proved to be less effective (see the Supporting Infor-
mation). Several additives were evaluated, and iron
salts demonstrated a beneficial effect on this reac-
tion.[18] Interestingly, the reaction efficiency was in-
creased significantly and up to 93% yield was attained
with the co-catalysis of 15 mol% of FeCl3·6H2O
(Table 1, entry 5). Except for urea, the best nitrogen
donors in Mizunoꢀs case, other ammonium salts, in-
[a]
Reaction conditions: methylarene 1 (5 mmol), aqueous
ammonia (25% in water, 0.24 mmol), TBHP (70% in
water, 1.5 mmol), TBAI (20 mol%), FeCl3·6H2O
(15 mol%), 808C, 18 h.
cluding
NH4Cl,
NH4HCO3,
HCO2NH4,
and (Table 1, entries 6–11). Further optimization reveals
(NH4)2CO3 were also effective nitrogen sources that the loading of TBHP and TBAI can be decreased
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UPDATES
Zhenguang Zhao et al.
Scheme 3. Proposed mechanism.
smoothly to provide the corresponding primary
amides in good yields (2n–2p). However, the strong
electron-withdrawing group has a detrimental effect
on the reaction.
Benzyl alcohol, benzaldehyde, benzoic acid, benzo-
nitrile, and benzyl benzoate were detected (Scheme 2)
as side products in our studies. Control experiments
with these side products as the acyl donor were con-
ducted to obtain insights about the reaction mecha-
nism. Both benzyl alcohol and benzaldehyde offered
benzamide in satisfactory yields under the standard
reaction conditions and suggest that both could be in-
termediates for this oxidative amidation reaction
(Scheme 2b and c). In addition, control experiments
indicate that FeCl3 plays an important role in the
transformation of toluene to benzyl alcohol
(Scheme 2a, b, and c).[19] No benzamide was detected
when benzoic acid was subjected to the reaction con-
ditions (Scheme 2d) as expected. Unlike the oxidative
amidation of toluene reported by Mizuno et al.,[12] in
which the formation of primary amides was proposed
from the hydration of nitriles, benzonitrile 6 cannot
be hydrated to release benzamide under our reaction
Scheme 2. Several control experiments.
to 6 equiv. and 20 mol%, respectively, without affect- conditions (Scheme 2e). These outcomes hinted that
ing the reaction efficiency (Table 1, entry 12).
different mechanisms can be applied in these two
With these optimized reaction conditions in hand, studies.[20] The oxidative amidation of toluene and
we then evaluated the substrate scope of this reac- benzyl alcohol was completely suppressed in the pres-
tion.
ence of 2 equiv. of TEMPO, whereas no obvious in-
Different methylarenes were examined, the results hibition was observed when benzaldehyde was em-
of which are shown in Table 2. Both electron-donating ployed as the acyl donor (Scheme 2f). This result sug-
and weak electron-withdrawing groups are compatible gests that a radical process might be involved in the
for this reaction. The sterically demanding ortho- oxidation of toluene and benzyl alcohol to benzalde-
xylene works well for this transformation (2d) albeit hyde.
a slightly decreased yield was observed. Excellent
A possible mechanism was proposed (Scheme 3) on
chemoselectivity was observed, and no bi- or tria- the basis of our experimental results and litera-
mides were detected when ortho-, para-, meta-xylene, ture.[4e,h,l] The sequential oxidation of toluene by hy-
or 1,3,5-trimethylbenzene was used as the acyl donor poiodite (IOÀ) formed in situ from TBAI and
(2b–2e). Halogen substituents such as Cl, Br, and I TBHP[21] with the assistance of FeCl3·6H2O can yield
were compatible and offered the possibility of further benzaldehyde, which is the key intermediate for this
functionalization of these amides (2k–2m). Notably, transformation. The reaction between benzaldehyde
the reaction of methylarenes that contain heterocy- and ammonia furnished the hemiaminal intermediate,
cles, such as thiophene and pyridine, also proceeded which can be further oxidized to benzamide.
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Oxidative Coupling between Methylarenes and Ammonia
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with the synergistic catalysis of TBAI and FeCl3. Dif-
ferent methylarenes can be transformed into primary
amides in good to excellent yields. This reaction real-
izes the transformation of cheap and easily available
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chemicals to high-value products by employing direct
C(sp ) H functionalization of aromatic hydrocarbons.
3
À
In addition, we determined that FeCl3 has a significant
effect on the TBHP/TBAI oxidation system. This
study will spur the application of TBHP/TBAI oxida-
tion systems in the future. Featured with the advan-
tages including (i) both methylarenes and aqueous
ammonia are cheap raw chemical materials; (ii)
TBHP is a “green” oxidant; (iii) TBAI and FeCl3 are
cheap, safe, and environmentally benign catalysts;
and (iv) the reaction can be conducted easily under
mild reaction conditions; this method should find po-
tentially broad applications in pharmaceutical and
other fine chemical syntheses.
Experimental Section
General Experimental Procedure for Primary Amides
Synthesis
To a mixture of TBAI (17.7 mg, 0.048 mmol, 20 mol%) and
FeCl3·6H2O (9.7 mg, 0.036 mmol, 15 mol%) were added
methylarene (4.8 mmol, 20 equiv.), aqueous ammonia
(0.24 mmol), and TBHP (70 wt% in water, 0.21 mL,
1.44 mmol, 6 equiv.). The reaction mixture was stirred at
808C inside a sealed tube for 18 h and then cooled to room
temperature, quenched with saturated Na2S2O3 solution (5
mL) and extracted with ethyl acetate (3 x 10 mL). The com-
bined organic phase was washed with brine, dried over an-
hydrous Na2SO4, filtered and evaporated under reduced
pressure to afford the crude product which was further puri-
fied by silica gel column chromatography with petroleum
ether/ethyl acetate as eluent.
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Acknowledgements
This work is supported by the National Natural Science
Foundation of China (21462023, 21262018) and the Natural
Science Foundation of Jiangxi Province (20143ACB20007,
20133ACB20008).
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