Direct oxidative coupling of enamines and electron-deficient amines: TBAI/TBHP-mediated synthesis of substituted diaminoalkenes under metal-free conditions

Article abstract of DOI:10.1021/ol5026525

A metal-free cross-coupling of enamines and electron-de fi cient amines through oxidative C(sp²)-N bond formation has been realized by using TBAI as catalyst and TBHP as oxidant. This novel strategy allows for an efficient organocatalytic synthesis of the synthetically useful diaminoalkene derivatives and is highlighted by appealing features such as readily available of the starting materials, wide substrate scope and transition-metal-free characteristics. (Chemical Equation Presented).

Full text of DOI:10.1021/ol5026525

Letter
pubs.acs.org/OrgLett
Direct Oxidative Coupling of Enamines and Electron-Deficient
Amines: TBAI/TBHP-Mediated Synthesis of Substituted
Diaminoalkenes under Metal-Free Conditions
Yucheng Yuan,† Wenjuan Hou,† Daisy Zhang-Negrerie,† Kang Zhao,† and Yunfei Du*,†,‡
†School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, China
‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
S
* Supporting Information
ABSTRACT: A metal-free cross-coupling of enamines and
electron-deficient amines through oxidative C(sp2)−N bond
formation has been realized by using TBAI as catalyst and
TBHP as oxidant. This novel strategy allows for an efficient
organocatalytic synthesis of the synthetically useful diaminoal-
kene derivatives and is highlighted by appealing features such
as readily available of the starting materials, wide substrate
scope and transition-metal-free characteristics.
Scheme 1. Intermolecular C(sp2)−N Cross-Coupling
ross-coupling reactions between an sp2 carbon of an arene
or alkene and an N-moiety represent the most adopted
Reactions
C
approach in building N-containing compounds in modern
organic chemistry.1 Fascinated by this powerful synthetic
strategy, organic chemists have devoted a great deal of efforts
to the development of notable C(sp2)−N cross-coupling
reactions. To date, the existing approaches generally fall into
one of the following types. Type I is the cross-coupling reaction
between aryl/vinyl halides and N-containing amines or amides,
either in the presence of transition metals such as Cu,2 Pd,3 Co,4
Rh,5 Ni,6 etc., including the well-known Goldberg reaction and
Buchwald−Hartwig amination, or through the assistance of
Brønsted acid/[DBU][HOAc] under metal-free conditions
(Scheme 1, type I).7 The second type involves aryl boronates
reacting with aminating agents under basic conditions which give
the cross-coupled products through C(sp2)−N bond formation
(Scheme 1, type II).8 Significant progress in recent years has led
to success in direct oxidative amination of olefins with amines
catalyzed by Pd or Au (Scheme 1, type III).9 It is evident that in
these three types, prefunctionalization of the C(sp2) of the
arene/alkene and/or the presence of transition metals are/is
indispensable for the coupling to occur. To our knowledge, there
are few reports describing direct oxidative amination of an
unfunctionalized arene/alkene under metal-free conditions. A
literature search shows that the only existing such examples
involve a hypervalent iodine-mediated intermolecular oxidative
C(sp2)−N coupling reaction between an arene and phthalimide
or saccharine (Scheme 1, type IV),10 but none with the more
general alkene functionality. In this paper, we disclose an
unprecedented cross-coupling reaction between the alkene
moiety of enamines and electron-deficient amines through
TBAI/TBHP-mediated oxidative C(sp2)−N bond formation
(Scheme 1, type V).
system in various oxidative coupling reactions.11 For example, it
has been applied to the syntheses of amide compounds from
cross-couplings of aryl methyl ketones,12a alcohols,12b,c or
aldehydes12c−e with dialkylformamides, amines, or ammonium
salts. Meanwhile, TBAI/TBHP could also be used for the
construction of heterocyclic imidazo[1,2-α]pyridines13a or
oxazoles13b from β-keto esters with 2-aminopyridines or
benzylamine, respectively, via C(sp3)−H functionalization.
The combined use of TBAI as catalyst and TBHP as the
terminal oxidant has been widely used as a powerful oxidation
Received: September 6, 2014
Published: October 6, 2014
© 2014 American Chemical Society
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Furthermore, the TBAI/TBHP oxidative system has been
successfully applied to the amination of heteroarenes,14a benzylic
C−H bonds,14b and ketones.14c However, to our knowledge, it
has never been used for the oxidative amination of enamine
compounds. It is worth noting that such a direct amination of
enamines through C(sp2)−N bond formation provides a
straightforward synthesis of a series of synthetically useful
diaminoalkenes, which are not readily accessed through any of
the known approaches.15
In our previous work, direct β-acyloxylation of enamine
compounds was achieved via PhIO-mediated intermolecular
oxidative C−O bond formation.16 Inspired by this conversion,
we anticipated the C−N cross-coupling between enamine and
phthalimide using PhIO as oxidant.17 Accordingly, enamine 1a
and phthalimide 2a were used to test the feasibility of the
transformation. However, the reaction of enamine 1a with
phthalimide 2a under our previously optimized conditions for
the coupling of enamines with carboxylic acids provided no
desired product at all (Table 1, entry 1). Other hypervalent
iodine reagents including PIDA and PIFA were also tested but
found to be ineffective for the anticipated transformation (Table
1, entries 2−3).
shortened the reaction time but was accompanied by decreased
yield (48%, not shown) due to the formation of more
byproducts. On the other hand, operating the reaction at 80
°C required increased reaction time without improving the yield
(70%, not shown). On the basis of these results, the best
conditions were concluded to be 5.0 equiv of TBHP with 0.3
equiv of TBAI in DMF at 100 °C for 1 h.19
To explore the generality and scope of the novel reaction,
various enamines were examined as substrates to react with
phthalimide (2a) under the optimized conditions. The aryl
enamine substrates bearing either electron-donating or electron-
withdrawing substituents on the aromatic ring were all converted
to the corresponding products in satisfactory to good yields
(Figure 1, 3a−e). For substrates containing other heterocyclic
rings in place of the aromatic ring, i.e., pyridyl and furyl, the
reaction also conveniently afforded the desired products 3f and
3g, respectively, albeit in relatively lower yields. For substrates
where the phenyl group was replaced by a methyl group or a
more stericly hindered tert-butyl group, imidation products 3h
and 3i were produced in 65% and 58% yield, respectively (Figure
1). These results indicated that the phenyl substituent in
enamine 1a was not indispensible for this transformation. The
methoxycarbonyl group in the substrate could also be altered to
the electron-withdrawing acyl group. Remarkably, when the
methoxycarbonyl group in 3h was displaced by an acetyl group,
the reaction was completed within 10 min and the highest yield
of 95% was achieved for the desired product 3j. In the case where
the electron-withdrawing group was a benzoyl group, 3k was
obtained with a 56% yield, possibly as a result of the hindrance of
the phenyl groups. Further experiments showed that this method
worked equally well for a broad range of enaminones containing
a variety of R1 and electron-withdrawing groups (E) (Figure 1,
3l−q). To our delight, the method was also applicable to the N-
substituted enamines. The N-substituted substrates 1r−u were
all converted to the corresponding coupled products to afford
3r−u in moderate to good yields, respectively. To our
disappointment, when the E substituent was a cyano group,
the corresponding substrate failed to couple with 2a (not
shown).
To further probe the scope of the reaction, other nitrogenous
compounds were examined. As illustrated in Figure 2, both
succinimide and 4-nitrophthalimide successfully reacted with 1a
to afford 4a and 4b in moderate yields. In addition, 1H-
benzimidazole and 1,2,4-1H-triazole also coupled with 1a to give
products 4c−d in satisfactory yields. When benzotriazole was
used, the reaction provided a mixture of 4e and its regioisomeric
compound 4e′, with the former being the predominant product.
Unfortunately, with saccharine or simple amines such as
diphenylamine, dibenzylamine, or aniline as nitrogenous
substrates, no desired product resulted in each case (not shown).
One useful application of the obtained diaminoalkene involves
conversion to the 4-aminoisoxazole compound. Treating 3m
with PIDA in DCE at rt provided isoxazole 5 in 60% yield,
together with the separable 2H-azirine 6 in 35% yield.20 Upon
treatment with FeCl2 in 1,4-dioxane at reflux temperature, 2H-
azirine 6 was converted smoothly to isoxazole 5 in an excellent
95% yield (Scheme 2).21
a
Table 1. Optimization of the Reaction Conditions
b
c
entry oxidant
catalyst
solvent
temp (°C) time (h) yield (%)
d
e
1
PhIO
DCE
rt
rt
0.5
0.5
0.5
2
ND
ND
ND
20
d
e
e
2
PIDA
DCE
d
3
PIFA
DCE
rt
4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
NaI
DCE
100
100
100
100
100
100
100
100
100
5
EtOAc
EtOH
DMF
DMSO
MeCN
toluene
DMF
DMF
1
40
6
2
13
7
1
80
8
1
65
9
0.7
2
60
10
11
12
55
2
50
KI
2
53
a
Reaction conditions: 1a (1.0 mmol), oxidant (5.0 mmol), and catalyst
b
(0.3 mmol) in solvent (5.0 mL), unless otherwise stated. TBHP (70%
in water) was extracted with petroleum ether and was evaporated for
use. Isolated yields. 1.2 equiv of oxidant was used. No desired
product.
c
d
e
Fortunately, when TBHP was used as the oxidant and TBAI as
the catalyst, the desired cross-coupling product 3a was obtained
in 20% yield (Table 1, entry 4). Employing other oxidants such as
K2S2O8, O2 and H2O2 (30% aq),18 however, yielded no product
in this reaction (not shown). Solvent screening showed that
EtOAc, EtOH, DMF, DMSO, MeCN, and toluene were also
effective for the reaction (Table 1, entries 5−10), with DMF
proven the best choice, as the corresponding reaction gave the
product in the highest observed yield of 80%. Our further control
experiments indicated that TBAI was indispensable for the
conversion since no reaction occurred if TBAI was not used (not
shown). Instead of TBAI, the use of other catalysts, such as NaI
and KI, significantly decreased the yield of 3a (Table 1, entries
11−12), while TBAB, TBAC, I2, and NIS resulted in no desired
product (not shown). Increasing the temperature to 120 °C
Our control experiment showed that the addition of TEMPO
did not hamper the reaction, which indicated that the
transformation may not involve a radical pathway. On the basis
of this result and prior publications,22 a plausible mechanism has
been formulated and is presented in Scheme 3. Initially, tetra-n-
butylammonium hypoiodite A was formed catalytically by the
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Figure 1. TBAI-catalyzed imidation of enamines with phthalimide. General conditions: substrate 1 (1.0 mmol), phthalimide 2a (1.2 mmol), TBHP (5.0
mmol), and TBAI (0.3 mmol) in DMF (5 mL) at 100 °C.
Scheme 3. Proposed Reaction Mechanism
the TBAI/TBHP-mediated intermolecular oxidative C(sp2)−N
bond formation. This novel method provides a convenient
approach for the synthesis of diaminoalkene derivatives, which
Figure 2. Oxidative C(sp2)−N coupling of enamine 1a with other
are not only synthetically useful but can be further converted to
nitrogenous compounds. General conditions: substrate 1a (1.0 mmol),
4-aminoisoxazoles. The methodology features readily available
2 (1.2 mmol), TBHP (5.0 mmol), and TBAI (0.3 mmol) in DMF (5
starting materials, environmentally benign oxidant, and metal-
mL) at 100 °C.
free characteristics. Further studies on the reaction mechanism as
well as its application are still in progress in our laboratory.
Scheme 2. Conversion of 3m to 5
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectral data for all new compounds,
and X-ray structural data of 3d. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
peroxide t-BuOOH and was further oxidized in the same fashion
Corresponding Author
to form tetra-n-butylammonium iodite B. Electrophilic addition
*E-mail: duyunfeier@tju.edu.cn.
Notes
reaction of enamine 1 by iodite B afforded intermediate C, which
was then nucleophilically attacked by 2a to give imine 3′, while
the released hypoiodite A was reoxidized by t-BuOOH to give B.
Finally, imine 3′ isomerized into the more conjugated (therefore
thermodynamically more stable) title product 3.
In summary, we have disclosed an amination coupling reaction
of enamines with electron-deficient amines including phthali-
mides, succinimide, and other nitrogenous compounds through
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the National Natural Science Foundation of
China (No. 21072148) for financial support.
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Letter
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