N-Bu4NI/TBHP-catalyzed direct amination of allylic and benzylic C(sp3)-H with anilines under metal-free conditions

Article abstract of DOI:10.1039/c4cc01189a

A novel and efficient n-Bu₄NI/TBHP-catalyzed direct amination of allylic and benzylic C(sp³)-H with anilines to form N-substituted anilines under metal-free conditions has been developed. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01189a

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n-Bu4NI/TBHP-catalyzed direct amination
of allylic and benzylic C(sp3)–H with anilines
Cite this:DOI: 10.1039/c4cc01189a
under metal-free conditions†
Xusheng Zhang,a Min Wang,*a Pinhua Lia and Lei Wang*ab
Received 14th February 2014,
Accepted 3rd June 2014
DOI: 10.1039/c4cc01189a
www.rsc.org/chemcomm
A novel and efficient n-Bu4NI/TBHP-catalyzed direct amination of in situ in the presence of a transition metal catalyst, Rh,9
allylic and benzylic C(sp3)–H with anilines to form N-substituted Ru,10 Cu,11 Fe,12 Co,13 or Mn.14 On the other hand, alternative
anilines under metal-free conditions has been developed.
nitrene derivatives, chloramines-T,15 bromamine-T,16 tosyloxy-
carbamates,17 and azides18 can also be used as the nitrogen
The amino moiety, as a key skeletal framework structure in natural sources. In addition, secondary amines, MeOC(O)NHSO2R,19
products, agrochemicals, polymers, and pharmaceuticals, plays a MeNHSO2Ph,20 and N-fluorobis(phenylsulfonyl)imide (NFSI)21
pivotal role in both chemistry and biology.1 It has therefore stimu- have been used in the C(sp3)–H amination to afford tertiary
lated chemists over several decades to develop efficient approaches sulfonylamines. However, to our knowledge, most achieve-
for the C–N bond formation. A general method is employing pre- ments have been made by using nitrogen sources, which have
functionalized starting materials, most representatively (hetero)aryl to be attached to a very strong electron-withdrawing group on
(pseudo)halides to react with amines or amides.2 Of particular the nitrogen atom with a few exceptions.22
importance is the Buchwald–Hartwig C–N cross-coupling reaction.3
Recently, aromatic amines as the nitrogen sources in the
In recent years, the celebrated strategy involving direct C–H bond C(sp3)–H bond amination have been developed,23 for example,
activation followed by C–N bond formation of arenes and hetero- Pd-catalyzed and C–H amination of unactivated C(sp3)–H bonds23a
arenes has received more attention.4–7 Among these transforma- with anilines, and Cu-catalyzed C(sp3)–H amination of cyclohexene
tions, transition-metal-catalyzed direct oxidative amination by C–H/ with anilines,23b oxidative amination and allylic amination of
N–H double activation represents an extremely attractive approach alkenes,23c and intermolecular C–H amination of unactivated
for inducing nitrogen functional groups, which circumvents the sp3 carbons.23d However, a transition-metal is essential in the
need for pre-functionalization of the coupling partners and is highly above reactions,23 and metal-free oxidation is the best choice in
efficient to target molecules from the simplest starting materials.5–8 the pharmaceutical industry due to the avoidance of metal
Compared with the direct C(sp2)–H bond amination of arenes and contamination.7c In 2011, Lee described hypoiodite-mediated
heteroarenes, the direct C(sp3)–H bond amination of saturated saturated C–H amination for the synthesis of an oxazaspiroketal-
hydrocarbons to accomplish the regioselective C(sp3)–H to C–N containing cephalostatin analog under metal-free conditions.24a In
transformation is still a challenge and is complementary to the this general direction, an elegant preparation of imidazole and
synthesis of valuable nitrogen-containing compounds. Over the purine nucleoside derivatives was developed recently by Zhu using
last few decades, nitrene-induced C(sp3)–H aminations have n-Bu4NI-catalyzed benzylic C–H bond amination.24b In our ongoing
been developed from the corresponding secondary amines as interest in the construction of nitrogen-containing compounds and
typical nitrogen sources bearing electron-withdrawing groups, metal-free organic reactions,25 we report herein a novel n-Bu4NI/
such as H2NSO2R or H2NC(O)OR and PhI(OAc)2 as an oxidant, TBHP-catalyzed oxidative C(sp3)–H amination of cyclohexene, cyclo-
providing the corresponding iminoiodinane intermediates pentene and benzylic substrates with anilines, which illustrated
a practical straightforward route to N-substituted anilines from
a Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000,
P. R. China. E-mail: leiwang@chnu.edu.cn; Fax: +86-561-309-0518;
Tel: +86-561-380-2069
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
† Electronic supplementary information (ESI) available. CCDC 1005251. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc01189a
simple and readily available starting materials under metal-free
conditions (Scheme 1).
We first chose 4-aminobenzonitrile (1a) and cyclohexene (2a)
as model substrates to optimize the reaction conditions. The
results are summarized in Table 1. When the model reaction was
performed in the presence of n-Bu4NI/TBHP (tert-butyl hydro-
peroxide) at 90 1C, 80% yield of product 3a was obtained under
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Scheme 1 Direct amination of allylic C(sp3)–H with anilines.
neat conditions (Table 1, entry 1). However, as the analogues of
n-Bu4NI, n-Bu4NBr and n-Bu4NCl were ineffective (Table 1, entries
2 and 3), it is reasonable to speculate that the iodide anion is an
efficient activator of the oxidative amination of cyclohexene (allylic
type Csp3–H) owing to a good candidate as the iodide anion of
n-Bu4NI. However, KI or I2 as a direct or indirect iodide anion source
could not catalyze the reaction (Table 1, entries 4 and 5). Only TBHP
exhibited excellent activity to the transformation, but, DTBP (di-tert-
butyl peroxide), TBPB (tert-butyl perbenzoate), BQ (1,4-benzoquinone),
DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone), K2S2O8, and
(NH4)2S2O8 completely failed (Table 1, entries 1 vs. 6–11). It did
not occur when the reaction was carried out in EtOAc, CH2Cl2,
1,2-dichloroethane (DCE), 1,4-dioxane, CH3CN, DMF, or DMSO
(Table 1, entries 12–18). With respect to n-Bu4NI and TBHP
loading, 0.20 equiv. of n-Bu4NI and 3.0 equiv. of TBHP were
found to be optimal. It should be noted that no 3a was detected
when the model reaction was performed in the absence of
n-Bu4NI or TBHP. When the reaction temperature was decreased
to 70 1C or increased to 110 1C, the product yield was slightly
lower than that obtained at 90 1C.
Scheme 2 Scope of the substrates [reaction conditions: aniline (0.50 mmol),
cyclohexene or cyclopentene (10 mmol), n-Bu4NI (0.10 mmol), TBHP
(1.5 mmol), at 90 1C for 10 h, isolated yields].
After optimization of the reaction conditions, the reaction
scope of C(sp3)–H amination of cyclohexene (2a) with respect to
the aryl amine component was evaluated. A variety of anilines
with substituents on the benzene rings were examined, and the
results are shown in Scheme 2. The direct reactions proceeded
well for the reactions of 2a with anilines bearing electron-
withdrawing groups, such as cyano, nitro, acetyl, halogen, and
trifluoromethyl on the benzene rings, and generated the corre-
sponding C–H amination products in 50–80% yields (3a–g). It is
important to note that the reaction of 4-nitroaniline with 2a
afforded the desired product 3b in 62% yield under the present
metal-free conditions, however, the reaction of the same sub-
strates failed in the Cu(I)–tBuOOtBu system.23b The reaction of
2a with aniline without a substituent gave 3h in 41% yield.
However, the present reaction conditions were not suitable for the
anilines bearing electron-donating substituents on the benzene rings.
It should be noted that 2-aminopyridine and 3-aminopyridine
underwent the reaction with 2a to generate the desired amina-
tion products 3i and 3j in 68 and 51% yields, respectively. The
reactions of cyclopentene (2b) with various anilines were also
successful under the optimized reaction conditions, but the
corresponding product yields are lower than that of cyclohexene
(2a) with the same anilines (3a vs. 3k, 3b vs. 3l, 3c vs. 3m, 3d vs. 3n,
and 3i vs. 3q). An ortho-position effect of anilines was not observed
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst
Oxidant
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
n-Bu4NI
n-Bu4NBr
n-Bu4NCl
KI
TBHP
TBHP
TBHP
TBHP
TBHP
DTBP
TBPB
BQ
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
EtOAc
CH2Cl2
DCE
Dioxane
CH3CN
DMF
DMSO
80
Trace
Trace
NR
Trace
Trace
Trace
NR
I2
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
9
DDQ
NR
NR
NR
10
11
12
13
14
15
16
17
18
K2S2O8
(NH4)2S2O8
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
Tracec
Tracec
Tracec
Tracec
NRc
NRc
NRc
a
Reaction conditions: 4-aminobenzonitrile (1a, 0.50 mmol), cyclohexene in the reactions of 4-, 3- and 2-chloroaniline with cyclopentene
(2a, 10 mmol, excess, as well as solvent), catalyst (0.20 equiv., 0.10 mmol),
(3n vs. 3o, vs. 3p). 4-Cyano-N-methylaniline (a secondary aro-
oxidant (3.0 equiv., 1.5 mmol), at 90 1C for 10 h. b Isolated yields. c Additional
matic amine) reacted with 2a under the optimized reaction
solvent (2.0 mL) was added and 1.0 mmol cyclohexene (2a) was
added.
conditions, providing the desired product 3r in 56% yield.
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Scheme 3 The reactions of 3-methylcyclohex-1-ene with anilines [reaction
conditions: aniline (0.50 mmol), 3-methylcyclohex-1-ene (10 mmol), n-Bu4NI
(0.10 mmol), TBHP (1.5 mmol), at 90 1C for 10 h, isolated yields].
No product was detected for the reaction of N-acetylaniline with 2a,
and the complicated oxidation products were observed for the
reaction of N-methylaniline with 2a under the present reaction
conditions. However, electron-rich aniline, such as 4-methylaniline
or 4-methoxyaniline, was used in the reaction with cyclohexene under
the standard reaction conditions, an oxidative homodimerization
of aniline, 4,40-dimethylazobenzene or 4,40-dimethoxyazobenzene
was isolated in 56% and 51% yield, respectively.
It should be noted that the reactions of 3-methylcyclohex-1-ene
with 4-acetylaniline and 4-cyanoaniline under the present reaction
conditions afforded the rearrangement amination products26
3s and 3t with 78% and 72% yield, respectively, and good
regioselectivity (Scheme 3). In addition, the structure of 3s was
confirmed by single crystal X-ray crystallography.27
Scheme 5 The proposed mechanism.
When 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO) was added
to 0.2 equiv. of aniline (equals to n-Bu4NI), 3a was isolated in
75% yield. While 2.0 equiv. of TEMPO (10 times to n-Bu4NI) was
added to the reaction, 3a was obtained in 43% yield. However,
the reaction was completely shut down with the addition of
4.0 equiv. of TEMPO to the system. It is suggested that TEMPO
acts as a radical scavenger and the reaction involves a radical
process (see ESI† for details). A possible reaction mechanism is
proposed in Scheme 5 based on our results and the literature.22b,24,28
Initially, the active iodine species ammonium hypoiodite I or iodite II
was formed via the oxidation of n-Bu4NI with TBHP. The obtained I
or/and II then reacted with cyclohexene (2a) to generate the allylic
radical III, which was further oxidized by II or/and I to afford allylic
cation IV. Finally, the desired product 3a was formed through a
nucleophilic reaction of amine 1a to IV. The proposed mechanism
was further investigated through the trapping of the free radical
intermediate with TEMPO by the HPLC-HRMS probe. The coupling
product of allylic radical III with TEMPO was confirmed by HRMS
(see ESI† for details). Moreover, the stability of the free radical and
carbonium ion of 3-methylcyclohex-1-ene, and the steric effect (allylic
rearrangement process) in the reaction also supported the regio-
selective formation of 3s and 3t (Scheme 6).
Because of the similar property of benzylic C(sp3)–H to
allylic C(sp3)–H, one of the substrates was extended to benzylic
C(sp3)–H. The reactions of ethylbenzene and its derivatives with
4-aminobenzonitrile (1a) as the nitrogen source were examined
under the aforementioned conditions. As can be seen from
Scheme 4, 52% yield of the amination product 4a was obtained
from the reaction of ethylbenzene and 1a. Similarly, the amination
of ethylbenzene with 4-acetylaniline also proceeded to generate the
corresponding product 4b in 48% yield. To our delight, amination
of other benzylic compounds, such as propylbenzene, cumene,
p-cymene with 4-acetylaniline, were accomplished to afford the
corresponding products, 4c in 46% yield, 4d in 55% yield, and 4e
in 48% yield, respectively. Importantly, in the case of p-cymene,
the amination occurred at the 31 benzylic C(sp3)–H bond com-
pletely to give the corresponding product 4e, but not at the 11
benzylic C(sp3)–H bond.
In conclusion, we have developed an efficient n-Bu4NI/TBHP-
catalyzed direct amination of cyclohexene, cyclopentene, and
benzylic compounds with anilines. This new method is comple-
mentary to existing C–H amination reactions for expanding the
scope and practicality. This amination reaction was realized under
metal-free conditions and employed aromatic amines as the
nitrogen sources which illustrated a practical straightforward
Scheme 4 Scope of the substrates [reaction conditions: aniline
(0.50 mmol), benzylic hydrocarbon (10 mmol), n-Bu4NI (0.10 mmol), TBHP
(1.5 mmol), at 90 1C for 10 h, isolated yields].
Scheme 6 The free radical rearrangement of 3-methylcyclohex-1-ene.
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This work was financially supported by the National
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