Copper-Catalyzed Intramolecular Oxidative Amination of Unactivated Internal Alkenes

Article abstract of DOI:10.1002/chem.201600329

A copper-catalyzed oxidative amination of unactivated internal alkenes has been developed. The Wacker-type oxidative alkene amination reaction is traditionally catalyzed by a palladium through a mechanism involving aminopalladation and β-hydride elimination. Replacing the precious and scarce palladium with a cheap and abundant copper for this transformation has been challenging because of the difficulty associated with the aminocupration of internal alkenes. The combination of a simple copper salt, without additional ligand, as the catalyst and Dess-Martin periodinane as the oxidant, promotes efficiently the oxidative amination of allylic carbamates and ureas bearing di- and trisubstituted alkenes leading to oxazolidinones and imidazolidinones. Preliminary mechanistic studies suggested a hybrid radical-organometallic mechanism involving an amidyl radical cyclization to form the key C-N bond.

Full text of DOI:10.1002/chem.201600329

DOI: 10.1002/chem.201600329
Communication
&
Amination
Copper-Catalyzed Intramolecular Oxidative Amination of
Unactivated Internal Alkenes
+
[a]
+ [a]
[a]
[a]
[b]
[a]
Peng Xiong , Fan Xu , Xiang-Yang Qian, Yared Yohannes, Jinshuai Song, Xin Lu,*
[a]
and Hai-Chao Xu*
Abstract: A copper-catalyzed oxidative amination of unac-
tivated internal alkenes has been developed. The Wacker-
type oxidative alkene amination reaction is traditionally
catalyzed by a palladium through a mechanism involving
aminopalladation and b-hydride elimination. Replacing
the precious and scarce palladium with a cheap and abun-
dant copper for this transformation has been challenging
because of the difficulty associated with the aminocupra-
tion of internal alkenes. The combination of a simple
copper salt, without additional ligand, as the catalyst and
Dess–Martin periodinane as the oxidant, promotes effi-
ciently the oxidative amination of allylic carbamates and
ureas bearing di- and trisubstituted alkenes leading to ox-
azolidinones and imidazolidinones. Preliminary mechanis-
tic studies suggested a hybrid radical–organometallic
mechanism involving an amidyl radical cyclization to form
the key CÀN bond.
Scheme 1. Transition-metal-mediated oxidative amination of unactivated al-
kenes.
In this context, Bäckvall reported the palladium-catalyzed cycli-
[
2h]
zation of allylic N-tosyl carbamates. However, this method is
limited by the decarboxylative decomposition of substrates
bearing trans-alkenes.
The palladium-catalyzed aza-Wacker-type cyclization has at-
tracted considerable attention due to its ability to simultane-
ously generate a new CÀN bond and alkene moiety for further
[
7]
The use of copper catalysts for alkene amination has been
extensively studied due to the fact that copper is more abun-
dant as well as less expensive and toxic than noble metals,
[1]
[2]
functionalizations (Scheme 1a). Elegant recent efforts have
led to the discovery of new protocols featuring milder reaction
conditions, as well as higher product formation efficiency and/
[
8]
and can also tolerate a wide range of functional groups. De-
spite these advantages, synthetic applications of copper-pro-
moted alkene amination reactions are generally limited to ter-
[3]
or greater enantioselectivity. However, there is no generally
applicable method for the oxidative cyclization of easily avail-
able allylic carbamates and ureas to synthesize medicinally im-
minal alkenes and very few examples that involve internal
[4]
[7a]
portant oxazolidinones and imidazolidinones, which are also
alkene substrates have been described (Scheme 1b).
For
[
5,6]
common precursors for 1,2-aminoalcohols and 1,2-diamines.
similar reasons, a copper-catalyzed oxidative amination of un-
activated internal alkenes has yet to be reported. Drawing in-
spiration from both the copper-catalyzed radical CÀH amina-
+
+
[
a] P. Xiong, F. Xu, X.-Y. Qian, Y. Yohannes, Prof. Dr. X. Lu, Prof. Dr. H.-C. Xu
Collaborative Innovation Center of Chemistry for Energy Material
State Key Laboratory of Physical Chemistry of Solid Surfaces
Key Laboratory of Chemical Biology of Fujian Province
Xiamen University
[
9a]
[9b]
tion reactions that Chemler and Chiba developed and our
[
10]
amidyl radical cyclization reactions, we decided to explore
a copper-catalyzed oxidative amination reaction that is mecha-
nistically different from the palladium-catalyzed variant and
Xiamen 361005 (P. R. China)
E-mail: xinlu@xmu.edu.cn
haichao.xu@xmu.edu.cn
Homepage: Web:http://chem.xmu.edu.cn/groupweb/hcxu/
[
11]
uses an amidyl radical cyclization to form the key CÀN bond.
Herein, we report the successful development of such an intra-
molecular oxidative amination reaction using a cheap copper
[
b] Dr. J. Song
salt, copper(II) 2-ethylhexanoate [Cu(eh) ], as the catalyst and
2
Fujian Institute of Research on Structure of Matter
Chinese Academy of Sciences
Fuzhou 350002 (P. R. China)
Dess–Martin periodinane (DMP) as the oxidant (Scheme 1c).
The reactions employ allylic carbamates and ureas bearing di-
and trisubstituted alkenes as the substrates to afford function-
alized oxazolidinones and imidazolidinones in high yields and
with excellent diastereoselectivities.
+
[
] These authors contributed equally to this work.
Supporting information and ORCID from the author for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201600329.
Chem. Eur. J. 2016, 22, 4379 – 4383
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Communication
[
a]
[a]
Table 1. Optimization of reaction conditions.
Table 2. Cyclization of allylic carbamates.
[b]
Entry
Catalyst
Oxidant (Equiv)
T [8C]
t [h]
Yield [%]
1
2
3
4
5
6
7
8
9
Cu(eh)
Cu(eh)
Cu(eh)
Cu(eh)
Cu(eh)
Cu(eh)
Cu(OAc)
Cu(TFA)
Cu(OTf)
Cu(acac)
2
2
2
2
2
2
IBX (3)
DMP (3)
90
90
90
90
110
110
110
110
110
110
110
110
110
4.5
1
4.5
4.5
1
1
1
1
1
1
1
1
1
56
65
0
I
2
O
5
(3)
PhI(OAc)
DMP (3)
DMP (2)
DMP (2)
DMP (2)
DMP (2)
DMP (2)
DMP (2)
DMP (2)
DMP (2)
2
(3)
0
70
78
68
59
22
67
54
14
2
2
2
1
0
2
[
c]
1
1
Cu(eh)
Cu(eh)
Cu(eh)
2
2
2
[
d]
1
1
2
3
75 (0.83 g)
II
[
[
a] Reaction conditions: 1 (0.4 mmol), Cu (0.08 mmol), DMSO (8 mL).
b] Isolated yield. [c] DMF as solvent. [d] MeCN as solvent.
We first set out to optimize the reaction conditions for the
cyclization of carbamate 1a bearing a removable 4-methoxy-
phenyl (PMP) group (Table 1). Treating 1a with Cu(eh)
2
[12a]
(
20 mol%) and 2-iodoxybenzoic acid (IBX)
in DMSO at 908C
II
[
a] Reaction conditions: 1 (0.4 mmol), Cu (0.08 mmol), DMP (0.8 mmol),
for 4.5 h led to the formation of the desired alkene 2a in 56%
yield (entry 1). Surprisingly, substitution of DMP for IBX short-
ened the reaction time to 1 h and improved the yield to 65%
without impacting the PMP group. These results are in sharp
1
solvent (8 mL), 110 8C, 1 h. [b] Isolated yield. [c] Determined by H NMR
analysis of the crude reaction mixtures. [d] Reaction using 50 mol% of
2
Cu(eh) . TBDPS=tert-butyldiphenylsilyl. N.A.=not applicable
[12b]
contrast to Nicolaou’s
previously reported results, in which
DMP reacted with the aryl group of N-aryl allylic carbamates to
give quinones and polycycles. Other hypervalent iodine re-
agents, such as I O (entry 3) and PhI(OAc) (entry 4), failed to
ties of the allylic substituent (entries 2–4) nor affected by
common functional groups, such as silyl ether (entry 5) or ester
(entry 6). The structure of 2 f was confirmed by single crystal
2
5
2
[
14]
afford any desired product. Consequently, the subsequent re-
action optimization was based on the use of DMP as oxidant
because of its unique reactivity and high compatibility with
X-ray diffraction studies (Figure S1). Submitting the sterically
demanding trisubstituted alkene 1i (entry 8) to the reaction
system led to the efficient formation of a tertiary CÀN bond,
albeit with the requirement of higher catalyst loading
(50 mol%). Cyclic alkenes could be used to synthesize a host
of cis-fused rings and spirocylic compounds (entries 9–12).
Further studies indicated that allylic ureas, which had not
been tested previously in oxidative amination reactions despite
their wide availability, could also be used as viable substrates
to synthesize various vinyl-substituted imidazolidinones
[
12b]
functional groups.
reaction temperature to 110 8C boosted the yield to 70%
entry 5). A further increase to 78% (entry 6) was achieved by
It was then discovered that raising the
(
reducing the amount of DMP to 2 equiv. The employment of
other copper salts, such as Cu(OAc)2 (entry 7), Cu(TFA)2
(
entry 8), Cu(OTf) (entry 9) and Cu(acac) (entry 10), or of alter-
2 2
native solvents, such as DMF (entry 11) and MeCN (entry 12),
led to less satisfactory results. Under the optimal conditions
[
15]
(Table 3). The reaction exhibited excellent diastereoselectivity
for branched ureas bearing a 1,2-disubstituted acyclic alkenyl
moiety (entries 1–4), including the ones that contained trans-
(
entry 6), the reaction could be carried out on a gram scale
without difficulty (entry 13).
[
10b]
We next investigated a variety of carbamates substituted
with different unactivated alkenyl moieties (Table 2). The
alkene geometry had no significant impact on the cyclization
efficiency as the cis-alkene 1b also cyclized smoothly to give
olefins (entries 3 and 4).
In addition, trisubstituted (entry 5)
and cyclic (entries 6–10) alkenes were also well tolerated. Note
that the transition-metal labile ArÀBr (entry 9) and ArÀI
(entry 10) bonds remained intact. One limitation of the current
cyclic urea synthesis consisted in its inexplicable tendency to
undergo over-oxidation when the linking nitrogen carried an
alkyl group. For example, the N-benzyl (Bn) urea 1x reacted to
give the desired 2x in 58% yield along with 16% of benzimi-
dazol-2-one 2x’ as a result of dehydrogenation of 2x
(entry 11).
[2h,13]
the desired 2b in 74% yield (entry 1).
Furthermore, we
found that the cyclization of cis-alkenes exhibited substantially
higher diastereoselectivity compared to the one that employed
[10b]
a trans-alkene substrate (entries 2–6 versus entry 7).
Impor-
tantly, when branched cis-alkenes were used, the oxidative
amination reaction was neither sensitive to the steric proper-
Chem. Eur. J. 2016, 22, 4379 – 4383
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[
a]
Table 3. Cyclization of allylic ureas.
Scheme 3. Proposed mechanism.
A plausible mechanism of the oxidative amination reaction
is proposed in Scheme 3. The process begins with the oxida-
tion of 1 with the copper catalyst and DMP to give the radical
[
12a,18,19]
III
[9b]
cation A,
or alternatively the NÀCu species B. Both
intermediates (A or B) can convert to the key amidyl radical C,
either through deprotonation (as in A) or homolytic cleavage
of the NÀCu bond (as in B). In the following steps, carbon
radical D is generated from the 5-exo-trig radical cyclization of
C that complexes with the Cu salt to afford organocopper(III)
species E.
III
[9b]
II
[
20]
b-Hydride elimination from E affords the final
I
[20]
II
product 2 and a Cu species. The latter is re-oxidized to Cu
II
[
a] Reaction conditions: 1 (0.4 mmol), Cu (0.08 mmol), DMP (0.8 mmol),
by DMP to complete the catalytic cycle for the copper catalyst.
To verify the proposed mechanism, carbamate 5, bearing
a cyclopropyl group as the radical probe, was reacted under
the standard conditions [Eq. (1)]. The isolation of ring-opening
product 6 in 37% yield offered compelling evidence for
a carbon radical intermediate (for example D; cf. Scheme 3). Al-
though the generation of D can be rationalized by the 5-exo-
trig cyclization of the nitrogen-centered radical C, an alterna-
tive explanation would be a process consisting of an initial
alkene aminocupration followed by homolysis of the resultant
1
solvent (8 mL), 110 8C, 1 h. [b] Isolated yield. [c] Determined by H NMR
analysis of the crude reaction mixtures. [d] Reaction using 50 mol% of
2
Cu(eh) . [e] Reaction for 0.5 h.
The high diastereoselectivity observed for the copper-cata-
lyzed method, in conjunction with the ease of access to enan-
tioenriched allylic alcohols and amines, prompted us to test
whether enantioenriched substrates could be tolerated. To our
delight, the allylic carbamate ( ++ )-1e and allylic urea (+)-1n re-
acted without noticeable racemization (Scheme 2). To further
demonstrate the synthetic versatility of the copper-catalyzed
method, we performed the oxidative cascade cyclization of
[
7a]
CÀCu bond. To test these possibilities, 1a was treated with
2 equiv of Cu(eh) in the absence of DMP [Eq. (2)]. No reaction
2
occurred under these conditions, implying that the aminocup-
ration pathway was very unlikely. On the other hand, a radical
mechanism can be supported by the following results: first,
the reaction of 1a under copper-free conditions using 1,4-cy-
clohexadiene (1,4-CHD) as a H-atom donor afforded the hydro-
[
16]
diene 3 (Scheme 2). The expected tricyclic product 4 was ob-
tained in 64% yield, offering a rare example of cascade aza-
[17]
Wacker-type cyclization of an internal alkene.
[
21]
amidation product 7 in 15% yield [Eq. (3)]. Second, the reac-
tion efficiency strongly depended on the oxidation potential of
the substrate [Eq. (4)], a phenomenon also observed by Nico-
[
12a]
laou
in their IBX-promoted radical hydroamidation reaction.
These results implied that the oxidative amination reaction de-
scribed in this study could also begin with single-electron
transfer leading to the formation of intermediate A or B. Final-
ly, treating 1a with DMP without Cu(eh) for 1 h did not form
2
2
a [Eq. (5)]. Meanwhile, the starting material 1a was recovered
in 21% yield, in contrast to its complete consumption when
Cu(eh) was added (Table 1, entry 6). This observation indicated
2
that the copper catalyst was critical for the formation of the
alkene moiety of the amination product, and while DMP was
capable of oxidizing 1a, the copper salt accelerated the pro-
[
22]
Scheme 2. Oxidative cyclization of enantioenriched 1e and 1n and diene 3.
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In summary, we have developed an unprecedented copper-
catalyzed oxidative amination that can be applied to unactivat-
ed internal alkene substrates, such as allylic carbamates and
ureas bearing a variety of di- and trisubstituted internal olefin
moieties. This novel transformation serves as a much-needed
complement to the palladium-catalyzed aza-Wacker-type cycli-
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boasts excellent diastereoselectivity and is fully compatible
with enantioenriched substrates without racemization.
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[
[
Acknowledgements
The authors acknowledge the financial support of this research
from XMU, NSFC (21402164 and 21273177), the “Thousand
Youth Talents Plan” and Fundamental Research Funds for the
Central Universities.
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Keywords: copper · cyclization · oxidation · radicals · synthetic
methods
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Published online on February 16, 2016
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