FeCl2-promoted cleavage of the unactivated C-C bond of alkylarenes and polystyrene: Direct synthesis of arylamines

Article abstract of DOI:10.1002/anie.201202464

Ironing it out: An efficient and convenient nitrogenation strategy involving C-C bond cleavage for the straightforward synthesis of versatile arylamines is presented. Various alkyl azides and alkylarenes, including the common industrial by-product cumene, react using this protocol. Moreover, this method provides a potential strategy for the degradation of polystyrene. Copyright

Full text of DOI:10.1002/anie.201202464

Angewandte
Chemie
DOI: 10.1002/anie.201202464
À
C C Cleavage
À
FeCl2-Promoted Cleavage of the Unactivated C C Bond of
Alkylarenes and Polystyrene: Direct Synthesis of Arylamines**
Chong Qin, Tao Shen, Conghui Tang, and Ning Jiao*
À
Selective metal-promoted C C bond cleavage under mild
reaction conditions presents one of the most challenging and
attractive processes[1] because: 1) it enables the straightfor-
ward utilization of hydrocarbons in organic synthesis;
2) polymers, which are not biodegradable and produce
a large amount of environmental pollution, may be degraded
nitrogen source may provide a novel approach for diverse
arylamine derivatives.
Herein, we describe the discovery of a FeCl2-catalyzed
À
cleavage of unactivated C C bonds for the straightforward
synthesis of various arylamines [Eq. (1)]. This transformation
À
and reused by chemical C C bond cleavage; 3) the liquefac-
À
tion of biomass and coal through selective C C bond cleavage
may alleviate the energy crisis we are facing. As a result of the
À
À
inertness of C C bonds and the competition of C H
activation,[2] examples of C C bond activation are much less
has a remarkably broad substrate scope. A variety of diaryl-
methanes and alkylarenes are compatible with this protocol.
Significantly, the readily available and industrial feedstock
cumene, which is usually used for industrial preparation of
phenol (O2, sulphuric acid, 1808C),[11] could be converted into
the corresponding economically important N-alkylanilines
and propanone. Notably, even a mixture of alkylarenes could
be employed as substrates, thus validating the potential
application for industrial crude materials. Furthermore, the
diversity of the aminating agent enables the wider application
of the industrial chemical cumene. Significantly, polystyrene,
which is an important commodity thermoplastic in our daily
life does not biodegrade for hundreds of years and is resistant
to photolysis, thus generating huge amounts of environmental
pollution,[12] could be degraded by this strategy. This may
provide a novel concept for polystyrene degradation and
reuse. Furthermore, an inexpensive and environmentally
benign iron salt is employed as the catalyst, thus making
this strategy green and practical.
À
[3]
À
common, with the exception of strained C C bonds,
functionalized substrates having functional fragments as the
leaving group such as carbonyl,[4] cyano,[5] and carboxylic
acids.[6] However, the metal-catalyzed cleavage of unactivated
C C bonds under mild reaction conditions is still undevel-
oped. Therefore, the discovery of a novel process for selective
À
À
C C bond cleavage is undoubtedly attractive and formidably
challenging.
Arylamines are common and fundamental industrial
feedstocks. For instance, N-alkylanilines are widely used in
the synthesis of important dyes, polymers, herbicides, insecti-
cides, pharmaceuticals, plant-growth agents, and antiknock
agents for gasoline engines.[7] On behalf of green and
sustainable chemistry, the direct intermolecular amination
À
À
strategy of simple arenes through C H or C C bond cleavage
has attracted considerable attention, but is very challeng-
ing[8–10] and has not yet been realized. In the case of direct C
À
H amination of non-preactivated arenes, the control of
chemoselectivity, prior oxidation of the parent nucleophilic
amines, and secondary amination when using primary amines
as a coupling partner are the challenging issues. Alternatively,
To test the possibility of the proposed direct arylamine
À
synthesis through C C bond cleavage, we chose as a model
the reaction of diphenylmethane (1a) with 1-azidononane
(2a; alkyl azides with long chain are stable; Table 1). The
À
a selective C C bond cleavage strategy with a suitable
initial screening indicated that a Brønsted acid additive could
[13]
À
assist the C C bond cleavage, thus generating the desired
nitrogenation product N-nonylaniline (3a; 11–39%) by
employing FeCl2 as catalyst in the presence of 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) as the oxidant in 1,2-
dichloroethane (DCE; Table 1, entries 2–4; for additional
screening results see Table S1 in the supporting Information).
When trifluoroacetic acid (TFA) was used as the solvent, 3a
was obtained in 75% yield upon isolation, with the generation
72% (GC) benzaldehyde (4a; entry 6). Nickel, cobalt,
manganese, and copper salts also catalyze this transformation,
but with low yields (entries 7–11). Although the reaction
under metal-free conditions worked, with an average yield
(40%, entry 12), the iron catalyst is essential in for relatively
unactivated alkylarene substrates. Other oxidants such as
ceric ammonium nitrate (CAN) and PhI(OAc)2 gave low
yields (entries 13 and 14).
[*] C. Qin, T. Shen, C. Tang, Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
Xue Yuan Rd. 38, Beijing 100191 (China)
E-mail: jiaoning@bjmu.edu.cn
Homepage: http://sklnbd.bjmu.edu.cn/nj
Dr. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
[**] Financial support from the National Basic Research Program of
China (973 Program) (Grant No. 2009CB825300) and the National
Science Foundation of China (Nos. 20872003, 21172006) are greatly
appreciated. We thank Yijin Su and Yuepeng Yan in our group for
reproducing the results of polystyrene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202464.
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Table 1: Optimization for the reaction of 1a with 2a.[a]
benzaldehydes were simultaneously generated through the
À
cleavage of benzyl C C bonds of diarylmethanes. The
nitrogenation process can also be extended to inert benzyl
alkanes, thus producing the desired N-alkylaniline in moder-
ate to good yields (entries 7–18). Aryl-derived N-alkylanilines
À
were accessed through the iron-catalyzed unactivated C C
Entry Solvent Additives
Catalyst
Oxidant
3a
Yield [%]
bond cleavage of the corresponding alkylbenzenes
(entries 13–18). The tolerance for halogens on the aromatic
ring in this transformation (3g and 3h) offers an opportunity
for subsequent cross-coupling, thereby facilitating expedient
synthesis of complex aniline molecules. The alkyl naphtha-
lene 1p performed more efficiently, even in the mixed solvent
DCE/TFA (5:1), and led to N-nonylnaphthalen-2-amine (3i)
in 93% yield (entry 16). It is noteworthy that the regioselec-
tivity is very high in the case of 1,2,4-trimethylbenzene (1q)
because 3,4-dimethyl-N-nonylaniline (3j), as determined by
1H-1H nuclear overhauser enhancement (NOE) experiments,
was the sole product, possibly because of a steric effect
(entry 17). To investigate the effect of electronic factors in
regioselectivity, the unsymmetrical diarylmethane substrate
1-benzyl-4-methoxybenzene (1t) was employed, and the two
anilines, 3b and 3a, were obtained in 45% and 21% yields,
respectively [Eq. (2)]. This result indicates that electron-rich
arenes preferentially migrate to the nitrogen atom in the
rearrangement process.
1
2
3
4
DCE
DCE
DCE
DCE
DCE
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
–
FeCl2
FeCl2
FeCl2
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
n.d.
11
24
TsOH
TfOH
benzoic acid FeCl2
TFA
–
–
–
–
–
–
15
5
FeCl2
FeCl2
CoCl2
NiCl2
Mn(OAc)3 DDQ
CuI
FeCl3
–
FeCl2
FeCl2
(39)
75 (72)
65
57
47
53
63
40
32
6[b]
7
8
9
10
11
12
13
14
DDQ
DDQ
DDQ
CAN
–
–
–
PhI(OAc)2 47
[a] Reaction conditions: 1a (0.5 mmol) and 2a (1.0 mmol), catalyst
(0.05 mmol), oxidant (1.1 mmol), H2O (1.0 mmol), solvent (2 mL),
608C. Yields determined by gas chromatography using n-octadecane as
an internal standard. The yield of the isolated product is given within the
parentheses. [b] 72% GC yield of benzaldehyde was detected. n.d. =not
determined, Ts=4-toluenesulfonyl, Tf=trifluoromethanesulfonyl.
Under the optimized reaction conditions, the substrate
scope of benzyl hydrocarbons was investigated (Table 2). A
variety of diarylmethanes were found to be compatible with
this protocol (entries 2–6). Substituted N-alkylanilines and
Table 2: The reaction of various benzyl hydrocarbons 1 to aniline 3.[a]
Entry
Substrate
Product
Yield [%]
Entry
10
Substrate
Product
3a
Yield [%]
3a
4a
3b
4b
75
(72)[b]
74
1
2
1a
1b
1c
1d
1j
70
65
11
1k
1l
3a
68
3c
4c
94
85
3
4
5
12
13
14
3a
3b
3g
65
43
35
3d
4d
69
62
1m
3e
4e
63
43
1e
1 f
1n
1o
3 f
4 f
53
6
15
3h
41
(51)[b]
7
8
9
1g
1h
1i
3a
3a
3a
62
66
76
16
17
18
1p
1q
1r
3i
3j
93
35
71
3k
[a] Reaction conditions: 1 (0.5 mmol), 2a (1.0 mmol), FeCl2 (0.05 mmol), H2O (1.0 mmol), DDQ (1.1 mmol) in TFA (2 mL) at 608C. Yields of isolated
products. [b] Numbers in parentheses are yields as determined by GC. [c] Using DCE and TFA (10:1, 2 mL) as the solvent.
2
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Importantly, by using common the industrial chemical
cumene (1j) as the substrate N-nonylaniline (3a) could be
obtained in 70% yield under these reaction conditions
(Table 2, entry 10), and may provide an alternative approach
to various N-alkylanilines just by changing the alkyl azide. To
our delight, a large-scale reaction (24 g) proceeded well with
similar efficiency (61%) by distillation and with the recovery
of the TFA solvent (93%), which enhances the potential
application in organic synthesis [Eq. (3)].
[Eq. (7)]. This may provide a novel concept for polystyrene
degradation and reuse.
Moreover, the nature of the alkyl azides used in the
reaction can be varied, therefore extending the utility of this
transformation (Table 3). The length of the alkyl chain did not
Furthermore, the mixture of alkyl benzenes 1l,j,k,g under
these reaction conditions produced 3a in 67% yield [Eq. (4)],
Table 3: Scope of organic azide 2 in this transformation.[a]
3m 78%
4b 74%
3n 69%
4b 66%
3o 71%
4b 69%
which demonstrates the potential for applying this oxidative
À
cleavage of unactivated C C bonds to the conversion of
a crude mixture of benzyl hydrocarbons from the oil and coal
industry into a single N-alkylaniline product. Moreover, the
present strategy was also applicable to ring-opening reactions.
The doubly functionalized product 2-(2-(propylamino)phe-
nethyl)benzaldehyde (3l) can be easily obtained from diben-
zosuberane (1s) in excellent yield (92%) by this direct
3p 65%
4b 51%
3q 64%
4b 53%
3r 93%
4b 87%
3s 75%
4b 67%
3t 46%
4b 44%
3u 67%
4b 62%
À
oxidative C C cleavage [Eq. (5)].
[a] Reaction conditions: 1b (0.5 mmol) and 2 (1.0 mmol), FeCl2
(0.05 mmol), H2O (1.0 mmol), DDQ (1.1 mmol) in TFA (2 mL) at 608C.
Yields of the isolated products.
affect the efficiencies (3m,n). In addition, secondary azides
remained intact in the reaction (3s,t). Alkyl azides having
functional groups such as phenyl, alkenyl, and ethylene glycol
units on the alkyl chain were also compatible (3o,p,r,u).
Particularly noteworthy is that the alkyl azides, acting as
aminating reagents in this transformation, can be rapidly
obtained from the corresponding alkyl halides by reaction
with NaN3 without the traditionally redundant work-up
involving distillation or chromatography (see the Supporting
Information).
Polystyrene is an important commodity thermoplastic in
our daily life and occupies about 20–22% of the total market
based on worldwide consumption.[12] However, like other
polymers, polystyrene degrades very slowly (more than 500
years for a single cup), and therefore generates large amounts
of waste which accumulates in the environment. The waste
polystyrene has been considered a serious environmental
problem by the EPA (Environmental Protection Agency).
When considering it an aromatic hydrocarbon, we envisioned
The mechanism and possible key intermediates have been
investigated. One possible pathway starts with the nitrene,
À
that our selective C C bond cleavage could be realized.
À
Significantly, the desired N-nonylaniline (3a) was obtained in
17% yield when polystyrene (purchased from Alfa Aesar)
was used as the substrate [Eq. (6)]. Moreover, the waste
polystyrene foam performed well, giving 3a in 12% yield
generated from the azide, inserting into the benzyl C H bond.
However, no benzylamine was detected in the reaction of 1b
under similar reaction conditions in the absence of DDQ (see
the Supporting Information). Although a trace amount of
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benzophenone can be observed as a by-product in some cases,
4,4’-dimethoxybenzophenone (5) cannot be converted into 3b
and 4b under standard reaction conditions [Eq. (8)], thus
Subsequent isomerization of the cation 8 leads to an iminium
cation, which may exist as the iminium trifluoroacetic acid salt
9 in the reaction mixture. Finally, N-alkylanilines and
benzaldehydes are generated by hydrolysis of 9.
À
The application of the selective C C bond cleavage
strategy in organic synthesis presents an attractive and
challenging goal. From the results of the present nitrogena-
tion reaction and the proposed mechanism, it should be
possible to develop some novel chemical transformations
À
through C C bond cleavage. Versatile substrates including
suggesting that the reaction does not undergo oxidation of
diphenylmethane to benzophenone. Furthermore, the deu-
terated substrate [D2]-1b was subjected to the reaction and
produced the deuterated product 4-methoxybenzaldehyde
([D1]-4b) in 62% yield [Eq. (9)], thus possibly excluding the
pathway involving the initial oxidation to a ketone or the
industrially common cumene and aminating partners for this
method may enable the potential applications for the syn-
thesis of N-alkylarylamines. With the selective cleavage of
À
unactivated Csp2 Csp3 bonds, the method may provide a strat-
egy for degradation of polystyrene under mild reaction
conditions, and therefore prove important for laboratory
and industrial purposes. Additional studies to better under-
stand the reaction mechanism and discover synthetic appli-
cations are ongoing in our group.
Received: March 29, 2012
Published online: && &&, &&&&
À
Keywords: amines · arenes · C C activation · iron ·
rearrangements
.
formation of an imine intermediate through the nitrene
before the aryl shift. Furthermore, the intra- and intermolec-
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6
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unactivated alkylarenes.
4
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Communications
À
C C Cleavage
C. Qin, T. Shen, C. Tang,
N. Jiao*
&&&& — &&&&
Ironing it out: An efficient and convenient
arenes, including the common industrial
by-product cumene, react using this pro-
tocol. Moreover, this method provides
a potential strategy for the degradation of
polystyrene.
À
FeCl2-Promoted Cleavage of the
nitrogenation strategy involving C C
À
Unactivated C C Bond of Alkylarenes and
bond cleavage for the straightforward
synthesis of versatile arylamines is pre-
sented. Various alkyl azides and alkyl-
Polystyrene: Direct Synthesis of
Arylamines
6
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