DDQ-promoted direct transformation of benzyl hydrocarbons to amides via tandem reaction of the CDC reaction and Beckmann rearrangement

Article abstract of DOI:10.1039/c3ob41218k

An atom-efficient and transition metal-free approach to amides from the corresponding benzyl hydrocarbons through C-H and C-C bond cleavage has been developed. Mechanistic studies have shown that a DDQ-promoted cross-dehydrogenative coupling (CDC) reaction with subsequent oxidation and rearrangement are involved in this transformation. The Royal Society of Chemistry.

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DDQ-Promoted direct Transformation of Benzyl Hydrocarbons to
Amides via a tandem reaction of CDC reaction and Backmann
rearrangement
Jun Qiu a and Ronghua Zhang*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
45 hydrocarbons.16 In 2011, Their group also reported a FeCl2ꢀ
A
atom-efficient and transition metal-free approach to
catalyzed transformation of diarylmethane reacted with azides
into corresponding amides in the presence of DDQ.17
However, the atomꢀefficient and transition metalꢀfree method
for amide formation from corresponding benzyl hydrocarbons
50 have been studied sporadically. Herein, we demonstrate a
novel and direct transformation of benzyl hydrocarbons into
corresponding amides via a tandem reaction of CDC reaction
and Backmann rearrangement in the presence of
hydroxylamine hydrochloride and DDQ (Scheme 1).
amides from corresponding benzyl hydrocarbons through C-
H and C-C bond cleavage has been developed. Mechanistic
10 studies have shown that
a
DDQ-Promoted cross-
dehydrogenative-coupling (CDC) reaction and the subsequent
oxidation and rearrangement are involved in this
transformation.
The atomꢀefficient and green methodologies for the
15 construction of amide bonds is a longꢀstanding interest and a
challenge in both research and industrial chemistry due to the
prevalence of this group in biologically active molecules,
agrochemicals and pharmaceuticals.1 In the past several
decades, several general strategies toward the synthesis of
20 amide bonds have been explored. The most prevalent route for
amide bond formation relies heavily upon the condensation of
an amine with activated carboxylic acid derivatives.2 Other
methodologies to access amides include oxidative amidation
of aldehydes,3 ketones,4 alcohols,5 amines,6 Beckmann
25 rearrangement,7 Schmidt rearrangement,8 aminocarbonylation
of haloarenes, alkenes, and alkynes,9 Staudinger ligation.10
Recently, the CꢀH functionalization has also been employed
for amide synthesis, which has attracted considerable attention
and been the focus of a significant number of studies.11
30 However, the development of novel methods for the
preparation of amides through direct CꢀH or CꢀC bond
activation (cleavage) is still an extremely attractive yet
challenging task.
It is worthy of note that only a few approaches to nitrogenꢀ
35 containing compounds from hydrocarbon molecules have been
reported.12ꢀ17 In 2005, Chang’s group13 and Fokin’s group14
have reported copper(I) ions efficiently catalyze the direct
formation of Nꢀacylsulfonamides from alkynes and sulfonyl
azides in the presence of water. In 2006, Che’s group
40 demonstrated that manganese porphyrins were efficient
catalysts for the oxidation of alkynes to amides in aqueous
medium.15 Recently, Jiao Ning’s group have developed a
FeCl2ꢀcatalyzed cleavage of CꢀH or CꢀC bond cleavage for
the direct synthesis of various arylamines from benzyl
Scheme 1. DDQꢀpromoted transformation of benzyl hydrocarbons into
amides
55
To begin our study, 1, 3ꢀdiphenylpropene 1a and
hydroxylamine hydrochloride were chosen as the model
substrates, DDQ was used as the oxidant to examine suitable
reaction conditions (Table 1). A number of solvents including
CH3CN, CH3CN/HCOOH (1:1), DCE, 1, 4ꢀdioxane, DMF,
60 THF and CH3NO2 were screened in the presence of PPA at
80°C (Table 1, entries 1ꢀ9). Moderate yields of the desired
product 2a were obtained using Toluene, CH3CN, DCE, 1, 4ꢀ
dioxane and CH3NO2 as a solvent (Table 1, entries 2ꢀ5).
Moreover, considerable amounts of undesired products were
65 formed and resulted in lower yields using DMF and THF,
(Table 1, entries 6 and 7). High yield of the desired product
2a was obtained using CH3CN as a solvent (Table 1, entry 8).
Dramatically, when CH3CN/HCOOH (1:1) instead of CH3CN
was used as the solvent, 2a was obtained in the highest yield
70 of 83% (Table 1, entry 9). To establish the solvent condition
that improve the reactivity, Several acids such as pꢀTSA,
TsCl, H2SO4, CF3COOH, MeSO3H, ZnCl2 were tested in
CH3CN at 80°C (Table 1, entries 10ꢀ15), the desired product
could be obtained in 51%ꢀ74% yield. Only 43% desired
75 product was
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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DOI: 10.1039/C3OB41218K
Table 1. Screening of reaction conditions.a
smoothly to afford the desired products with moderate to high
yields (64ꢀ89%) (Table 2, entries 2, 7ꢀ8 and 13). Surprisingly,
when the substrates bearing paraꢀmethoxyl groups on the
25 aromatic ring were employed, no acrylamides products were
generated and pꢀAnisonitrile was detected (Table 2, entries 3
and 9). We speculated that pꢀAnisonitrile may be generated by
abnormal Beckmann Rearrangement of the intermediate
oximes (Scheme 3).18 Notably electronꢀwithdrawing
30 substituents at different positions of the aromatic rings (paraꢀ,
metaꢀ and orthoꢀ) were also compatible with the process and
did not affect the efficiency of the reaction (Table 2, entries 4ꢀ
6, 10ꢀ12 and 14ꢀ15). However, as for the unsymmetrically
substituted substrates with two different aromatic rings, the
35 corresponding products consisting of regioisomers were
formed (Table 2, entries 7ꢀ15). The regioselectivity is more
likely to originate from the allylic cation rearrangement
between the αꢀ and γꢀpositions under the reaction conditions,
and the incoming nucleophile attacked at the original allylic
40 or γꢀposition to form the isomerized products.19
Yield
(%)e
Entry
1
Acid
PPA
Solvent
ꢀc
Oxide
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
48
52
45
40
62
30
26
78
83
43
74
73
54
71
51
61
2
PPA
DCE
3
PPA
Toluene
4
PPA
1,4ꢀDioxane
CH3NO2
5
PPA
6
PPA
THF
7
PPA
DMF
8
PPA
MeCN
9
PPA
MeCN/HCOOH
MeCN/HCOOH
MeCN/HCOOH
MeCN/HCOOH
MeCN/HCOOH
MeCN/HCOOH
MeCN/HCOOH
MeCN/HCOOH
Encouraged by the above results, we further investigated
the reactions between diarylmethane and hydroxylamine hydrꢀ
10
11
12
13
14
15
16
ꢀb
Table 2. Direct transformation of 1, 3ꢀdiarylpropenes 1 into the acrylamiꢀ
des 2 and 2’.a
PꢀTSA
TsCl
H2SO4
MeSO3H
ZnCl2
PPA
Yield
entry
Ar1, Ar2
product
2a
(%)b
83
DDQ
(2equiv)
DDQ
1
2
C6H5, C6H5 1a
4ꢀCH3C6H4,
4ꢀCH3C6H4 1b
17
18
PPA
PPA
MeCN/HCOOH
MeCN/HCOOH
23
2b
89
(1equiv)
ꢀd
N.D.
4ꢀCH3OC6H4,
4ꢀCH3OC6H4 1c
3
4
5
2c
2d
2e
ꢀc
a
Reaction conditions: 1a (0.5 mmol), NH2OH.HCl (1.5 mmol),
4ꢀFC6H4, 4ꢀFC6H4 1d
4ꢀClC6H4, 4ꢀClC6H4 1e
78
85
DDQ(1.5 mmol), acid(0.15 mmol), solvent (1.5 mL), coꢀsolvent(1.5
mL), stirred at 80 °C over 12 h. b No addition of acid. c No addition of
d
e
solvent. No addition of DDQ. Isolated yield. DDQ=2,3ꢀdichloroꢀ
5,6ꢀdicyanoꢀ1,4ꢀbenzoquinone, PPA= Polyphosphoric acid.
6
4ꢀBrC6H4, 4ꢀBrC6H4 1f
4ꢀCH3C6H4, C6H5 1g
2ꢀCH3C6H4, C6H5 1h
4ꢀCH3OC6H4, C6H5 1i
3ꢀCF3C6H4, C6H5 1j
3ꢀBrC6H4, C6H5 1k
4ꢀBrC6H4, C6H5 1L
C6H5, 4ꢀCH3C6H4 1m
C6H5, 4ꢀFC6H4 1n
2f
82
isolated in the absence of acid (Table 1, entry 10). PPA as the
acid showed relative higher efficiency compared with other
acids and thus was chosen as the acid for further optimization.
The ratios of 1, 3ꢀdiphenylpropene to DDQ were also been
examined. It was found that decreasing the amount of DDQ
resulted in reduced yields (Table 1, entries 9, 16ꢀ18). In
addition, 2a was not observed in the absence of DDQ (Table 1,
7
2g, 2g’
2h, 2h’
2i
86 (2:1)
64 (1.6:1)
ꢀc
8
5
9
10
11
12
13
14
15
2j, 2j’
2k, 2k’
2l, 2l’
2g, 2g’
2n, 2n’
2l, 2l’
75 (1.4:1)
68 (3.3:1)
83 (1.1:1)
88 (4.4:1)
76 (1.1:1)
85 (1.3:1)
entry 18).
The result was better when 1, 3ꢀ
10 diphenylpropene/DDQ = 1:3 (Table 1, entry 9). Therefore, we
chose 1, 3ꢀdiphenylpropene together with hydroxylamine
hydrochloride (3 equiv) and DDQ (3 equiv) in
CH3CN/HCOOH (1:1) at 80 °C for 12 h as our optimized
reaction conditions.
Based on the above study, various substrates were subjected
to the reaction under the optimized conditions (Table 2). To
our delight, the reaction can serve as a really general 1, 3ꢀ
diarylpropene to the syntheses of various substituted
acrylamides, affording moderate to excellent yields. For 1, 3ꢀ
15
C6H5, 4ꢀBrC6H4 1o
a
Reaction conditions: 1 (0.5 mmol), NH2OH.HCl (1.5 mmol), DDQ(1.5
mmol), PPA(0.15 mmol), HCOOH(1.5 mL), MeCN(1.5 mL) stirred at 80
20 diarylpropenes bearing electronꢀdonating substituents like
methyl group on the aromatic ring, the reactions proceeded
°C for 12h. b Yield of isolated product. c pꢀAnisonitrile was detected.
2
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C3OB41218K
ochloride. A variety of electronꢀrich aromatic substrates could
successfully afford the corresponding amides in moderate to
good yields (Table 3). Electronic effect played an important
role in the reaction. The diphenylmethane substrates bearing
electronꢀdonating groups on the phenyl ring gave the desired
products with good yields, especially excellent yields were
obtained when the two phenyl rings of the substrate had
methyl or/ and methoxyl groups respectively (Table 3, 4aꢀ4c).
Electronꢀwithdrawing groups on the diphenylmethane
5
10 decreased the yields considerably (Table 3, 4eꢀ4g),
presumably because they destabilize the proposed methylenyl
cation intermediate generated by oxidation. When
unsymmetric diphenylmethanes were employed, two
regioisomers were obtained with the regioselectivities (Table
15 3, 4cꢀ4g). It was found that the stronger the electronꢀ
withdrawing effect of the substituent was, the higher the ratio
of the isomerized products would be.
Scheme 2. Investigation of the mechanism and key intermediates.
To elucidate the mechanism and gain insight into this novel
reaction, several control experiments were also conducted
20 (Scheme 2). When the direct CDC reaction of 1, 3ꢀ
diphenylpropene 1a (0.5 mmol) and NH2ꢀOH.HCl (3 equiv)
and DDQ (1 equiv) was carried out in room temperature, the
25 desired C–N bond coupling product 5a was produced in 97%
yield and the C–O bond coupling product was not observed
(Eq. 1). A small amount of 1, 3ꢀdiphenylpropene dimer was
simultaneously detected, which implied the coupling reaction
Table 3. Direct transformation of the diarylmethane 3 into the amides 4
and 4’.a
may proceed through
a
singleꢀelectronꢀtransfer (SET)
30 process.20 In addition, when the reaction of 5a (0.5 mmol) and
DDQ (2 equiv) was carried out at 80 °C, the corresponding
oxime 6a was produced in 98% yield (Eq. 2). Then the
chalcone oxime 6a could be cleanly converted to 2a in 91%
yield at 80 °C by the Beckmann Rearrangement (Eq. 3). All of
35 these results suggest that the pathway for the formation of
acrylamide is likely to undergo a CDC reaction process with a
subsequent tandem oxidation and Beckmann rearrangement
rather than the oxidation of 1, 3ꢀdiphenylpropene to chalcone.
On the basis of the above experimental observations, a
40 plausible mechanism for this transformation is hypothesized
(Scheme 3).19ꢀ21 Initially, the reaction is a singleꢀelectron
transfer (SET) process between substrates 1 and DDQ to form
the benzyl cation B. The cation B with a hydroxylamine anion
gave rise to the CꢀN bond coupling products C and C’ by a
OCH3
OCH3
O
O
N
N
H
H
F
H3CO
4e
4a, 96%
+
F
O
CH3
O
N
H
N
H
4e'
H3CO
61%, 4e: 4e'=2.6:1
4b, 89%
H3C
OCH3
OCH3
O
O
N
H
N
H
O
4c
NC
NC
Cl
Cl
4f
+
Cl
H3C
+
O
CH3
Cl
O
NH2-OH
DDQ-2H
DDQ
SET
Ar1
Ar2
Ar1
Ar2
N
H
N
H
OH
1
B
4c'
H3CO
H3CO
4f'
NH-OH
93%, 4c: 4c'=1.1:1
73%, 4f: 4f'=1.4:1
CDC
OH
OCH3
OH
OCH3
O
N
O
HN
H
2
F3C
Ar1
Ar2
N
H
Ar1
DDQ
Ar2
N
H
C
D
4d
4g
+
O
+
O
DDQ-2H
+
HO
HO
N
NH
N
H
N
H
CF3
2'
H
Ar1
Ar2
Ar1
Ar2
4d'
87%, 4d: 4d'=1.3:1
H3CO
4g'
H3CO
D'
C'
38%, 4g: 4g'=3.5:1
substrates
1c and 1i
H
a Reaction conditions: 3 (1.5 mmol), NH2OH.HCl (0.5 mmol), DDQ(1.5
mmol), AlCl3(0.15 mmol), HCOOH (1.5 mL), MeCN (1.5 mL) stirred at
80 °C for 12h. Yield of isolated product.
p-Anisonitrile
Scheme 3. Plausible mechanism for the formation of 2 and 2’.
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65
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the ketoxime D and D’ by the DDQ oxidative system.
Subsequently, the target amides 2 and 2’ as an equilibrating
mixture could be generated from the ketoxime through the
Beckmann rearrangement in the presence of acid. When the
substrates 1c and 1i were employed, pꢀAnisonitrile may be
generated through the abnormal Beckmann rearrangement.
In summary, we have developed an atomꢀefficient and
transition metalꢀfree reaction between benzyl hydrocarbons
10 and hydroxylamine hydrochloride using DDQ as a promoter to
generate corresponding amides through sp3 CꢀH and CꢀC bond
cleavage. The mechanistic study shows that the transformation
is a new process involving a heteroꢀCDC reaction with a
subsequent tandem oxidation and Beckmann rearrangement.
15 To the best of our knowledge, this is the first direct
conversion of benzyl hydrocarbons to amides without a
transition metal catalyst. Further investigation of the detailed
mechanism and application of this chemistry is currently
underway in our lab.
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