Highly regioselective radical amination of allenes: Direct synthesis of allenamides and tetrasubstituted alkenes

Article abstract of DOI:10.1002/anie.201506066

The first controllable, regioselective radical amination of allenes with N-fluoroarylsulfonimide is described to proceed under very mild reaction conditions. With this methodology, a general and straightforward route for the synthesis of both allenamides and fluorinated tetrasubstituted alkenes was realized from a wide range of terminal and internal allenes. Radical highway: By employing N-fluoroarylsulfonimide as a nitrogen source, a facile, mild, and highly regioselective copper-catalyzed oxidative radical amination of various allenes was developed. In addition, a protocol involving nitrogen radical addition/fluorine-atom transfer affords fluorinated tetrasubstituted alkenes by using silver as catalyst.

Full text of DOI:10.1002/anie.201506066

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506066
German Edition: DOI: 10.1002/ange.201506066
Allenes
Highly Regioselective Radical Amination of Allenes: Direct Synthesis
of Allenamides and Tetrasubstituted Alkenes
Ge Zhang, Tao Xiong, Zining Wang, Guoxing Xu, Xuedan Wang, and Qian Zhang*
Abstract: The first controllable,
regioselective radical amination of
allenes with N-fluoroarylsulfon-
imide is described to proceed
under very mild reaction condi-
tions. With this methodology, a gen-
eral and straightforward route for
the synthesis of both allenamides
and fluorinated tetrasubstituted
alkenes was realized from a wide
range of terminal and internal
allenes.
T
he allenamide motif represents
one of the most versatile synthetic
building blocks in a variety of
synthetic transformations and is
excellent precursor in natural
product synthesis.[1] It is also dis-
played in some anti-HIV-1 and
Scheme 1. Direct arylation, oxidative amination, and aminofluorination of allenes. Cp*=C5Me5,
TMS=trimethylsilyl.
anti-hepatitis-B
virus
agents
within the pharmaceutical field.[2] Accordingly, the synthesis
of these versatile units has attracted intensive research
interest, thus resulting in a variety of strategies towards its
construction, including base-induced isomerization of prop-
argyl amides,[3] sigmatropic rearrangements,[4] elimination
reactions of enol triflates, and propargyl phosphonium
ether.[5] In addition, the transition-metal-catalyzed intramo-
lecular amino-cyclizations[6] and Trost–Hsung N-allenyla-
tion[7] methods have also been developed in recent years.
Nevertheless, most of the current methods suffer from at least
one of the following drawbacks: harsh reaction conditions
(such as use of strong bases, high temperature), the require-
ment of prefunctionalization of substrates, or employment
noble metals. Hence, a general, mild, and efficient approach
to the synthesis of allenamides is highly desired.
Table 1: Optimization of reaction conditions.[a]
Entry
Catalyst
(mol%)
Ligand
(mol%)
Solvent
Yield
[%]
1
CuCl (5)
none
CH3CN
CH3CN
CH3CN
CH3CN
DCE
31
21
47
25
32
28
trace
67
66
2
CuTc (5)
none
3
4
5
6
7
8
9
10
11
12
13
CuBr2 (5)
Cu(OTf)2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
CuBr2 (5)
none
none
none
none
À
Transition-metal-catalyzed C H amination has emerged
toluene
THF
as a straightforward and economical protocol in chemical
synthesis. Accordingly, enormous efforts have been devoted
to this reaction and impressive progress has been achieved in
none
Phen (6)
BC (6)
NC (6)
BPy (6)
NC (6)
NC (6)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
recent years.[8] However, direct C H amination protocols of
72
À
highly reactive allenes[9] to afford amino-substituted allenes
40
82[b]
78[c]
[*] G. Zhang,[+] Prof. T. Xiong,[+] Z. Wang, G. Xu, X. Wang, Prof. Q. Zhang
Department of Chemistry, Northeast Normal University
Changchun, 130024 (China)
E-mail: Zhangq651@nenu.edu.cn
[+] These authors contributed equally to this work.
[a] Reaction was performed with 1.3 equiv of 1a, 0.2 mmol NFSI,
5 mol% catalyst, and 6 mol% ligand in 2 mL of solvent at 508C under N2
for 3 h unless otherwise stated. [b] 0.2 mmol 1a, 1.3 equiv of NFSI were
employed. [c] Reaction was performed at room temperature for 5 h.
BC=bathocuproine, BPy=2,2’-bipyridine, DCE=1,2-dichloroethane,
Phen=1,10-phenanthroline, NC=neocuproine, Tf =trifluoromethane-
sulfonyl, THF=tetrahydrofuran.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506066.
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12649

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Angewandte
Communications
species could be generated under mild reac-
tion conditions.[12] Herein, by employing an
inexpensive copper catalyst, we report the first
À
example of the highly regioselective C H
amination of allenes by a radical process
which generates various allenamides from
both terminal and internal allenes. In addition,
with silver as the catalyst, aminofluorination
of allenes for fluorinated polysubstituted
alkenes was also realized (Scheme 1b).
To test our hypothesis on the amination of
allenes, we initially employed copper salts as
the catalysts and NFSI as the nitrogen
source.[12,13] We were pleased to find that
treatment of 1.3 equivalents of the 1,1-disub-
sitituted allene 1a with 5 mol% CuCl and
NFSI in CH3CN at 5088C for 3 hours under
a N2 atmosphere gave amination product 2a in
31% yield with high regioselectivity (Table 1,
entry 1). Other copper(I) and copper(II) cat-
alysts were also investigated and CuBr2 was
identified as the most efficient catalyst for this
transformation (entries 2–4). Among the sol-
vents screened, CH3CN was found to be
optimal (entries 5–7). To further improve the
yield, we turned our attention to screening
ligands.[14] Thus, various mono-, bi-, and tri-
dentate nitrogen-containing ligands were
investigated, and the best yield (72%) was
afforded with neocuproine (NC) as the ligand
(entries 8–11). Moreover, by increasing the
ratio of NFSI to allene, the yield was further
improved (entry 12). To our delight, the
desired amination product was isolated in
78% yield when the reaction was run at room
temperature (entry 13), and the reaction con-
2
À
Scheme 2. Direct oxidative coupling of the C(sp ) H bond of allenes with N-fluoroaryl-
sulfonimides. Reaction was performed with 0.2 mmol of 1, 1.3 equiv N-fluoroarylsulfon-
imide, 5 mol% catalyst, and 6 mol% ligand in 2 mL solvent at RT under N2, unless
otherwise stated. Yield is that of the isolated product. [a] The reaction was performed at
À158C. [b] The reaction was performed at 508C.
remains elusive. Challenges mainly result from: 1) control of
ditions were thus defined as the standard reaction conditions
for subsequent investigations. In the absence of the copper
catalyst, no amination product was observed. A comprehen-
sive list of screening data is presented in the Supporting
Information.
À
the regioselectivity for C N bond formation at either the
terminal or central allenic carbon atom; 2) thermally disfa-
vored processes for the formation of higher-energy substi-
tuted allene products as compared to aminodifunctionlization
of allenes. Recently, Ma and co-workers developed a mild
With the optimized reaction conditions (Table 1,
entry 13), we next explored the scope and limitations of the
process with respect to the allenes and nitrogen sources, and
the results are summarized in Scheme 2. Gratifyingly, the
allenes 1b–k, bearing either electron-donating or electron-
withdrawing groups on arenes, participated in this highly
regioselective amination reaction to provide 2b–k in moder-
ate to excellent yields. The substrates with aryl groups
displaying electron-withdrawing substituents (1g–k) resulted
in lower product yields, and bromoamination and amino-
fluorination byproducts of allenes, as well as NH(SO2Ph)2,
resulting from the decomposition of NFSI, were observed. In
addition, ortho-, meta-, and para-substituents on the aromatic
ring of the allenes 1g–i were viable substrates for these
transformations, thus affording the desired amination prod-
ucts in moderate yields. When the ethyl-substituted substrate
1l was applied, 2l was obtained in 43% yield. Next, we
investigated the linear and branched alkyl-substituted sub-
À
rhodium(III)-catalyzed C(aryl) C(allenyl) bond-forming
reaction of geminal-disubstituted allenes with N-methoxy-
benzamides through a Heck-type process, and the arylation of
allenes was realized by utilizing the bulk of the silyl group to
promote b-hyride elimination (Scheme 1a).[10]
A number of efficient transformations based on allenes
have been successfully realized,[9] however, mild and regio-
selective radical reactions of allenes are far less developed.[11]
More recently, we realized a general radical cascade reaction
of an alkyne with N-fluoroarylsulfonimide and an alcohol,
during which a key nitrogen radical addition to the alkyne, for
the formation of vinyl radical intermediate, was involved.[12a]
The vinyl radical intermediate A (Scheme 1b) led us to test
the challenging regioselective radical amination reaction of
allenes with N-fluoroarylsulfonimide. It has been demon-
strated that starting from N-fluorobenzenesulfonimide
(NFSI), a sterically hindered metal-stabilized nitrogen radical
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strates 1m and 1n, and results showed that 1n, having a more
bulky substituent, is more efficient.[10] The cyclopentyl
substrate 1o and naphthyl-substituted allene 1p are also
effective. It is worthy to note that the synthetically more
attractive silyl motif, which is sensitive to oxidants and
fluorides, also survived the reaction conditions and the
desired amination products 2q, 2r, and 2s were afforded in
50–67% yields. The more challenging substrate 1t, bearing
a strongly coordinating pyridine moiety,[15] also worked well
and delivered 2t in 55% yield. Remarkably, the unsym-
metrical 1,3-disubstituted allenes were found to be suitable
for the amination reaction and delivered the corresponding
allenamides 2u–w in acceptable yields, with the nitrogen atom
being attached to the less sterically hindered terminal allenic
carbon atom. The structure of 2v was further confirmed by
a NOESY experiment. The monosubstituted allene 1x and
geminal-dialkyl-substituted allene 1y were not suitable sub-
strates under the optimal reaction conditions, and thus
provided complex mixtures. Additionally, the allene 1z,
bearing an electron-withdrawing ester group, was intact in
this transformation. Finally, various N-F reagents bearing
either strong electron-withdrawing groups (F, -CF3, -NO2) or
electron-rich groups on aromatic rings also furnished allena-
mides (2aa–ae) in 53–81% yields under this mild catalytic
reaction. The potential mechanism of this direct amination of
allene is discussed in the Supporting Information.
The unique properties of the
tuted alkenes by a highly regioselective radical aminofluori-
nation process of allenes. To our knowledge, this radical
transformation still remains a significant challenge and only
one example has been disclosed for the intermolecular radical
aminofluorination process of allene with N2F4 in the gas
phase.[20] Additionally, it is difficult to synthesize tetrasub-
stituted alkenes bearing four different carbon-linked groups
because of the congested nature of the unsaturated bond.[21]
Therefore, we set out to explore our hypothesis. Indeed, when
the reaction was carried out in dichloromethane at the 508C
with AgF as the catalyst, we were delighted to see that the
expected aminofluorination product 4a was observed in 64%
yield as isolated as the major product with an 8:1 stereose-
lectivity (Scheme 3).[22] The data presented shows the scope of
the developed radical aminofluorination of allenes for the
synthesis of a wide range of fluorinated polysubstituted
alkenes. In general, various of 1,1-disubstituted allenes
reacted smoothly and provided the desired products in 43–
93% yields. Allenes with an electron-donating groups on the
aromatic ring (4a–f) were more efficient than substrates
bearing electron-withdrawing groups (4g–i), and NH(SO2Ph)2
was the major byproduct obtained in some cases. For
example, 17% and 31% NH(SO2Ph)2 was isolated during
the transformations of 1g into 4g and 1i into 4i, respectively,
under standard reaction conditions. Additionally, the stereo-
chemistry profile of our method was significantly influenced
fluorine atom render fluorine-con-
taining compounds useful for ap-
plications in agrochemistry, the
pharmaceutical industry, materials
science, and as radiotracers for
positron emission tomography
(PET).[16] Also, a limited number
of structurally simple fluorine-con-
taining compounds exist as natural
products.[17] These facts stimulated
the development of efficient, selec-
tive, and general methodologies for
À
the formation of C F bonds.
Among them, both nucleophilic
and electrophilic fluorinations
have made considerable advances
in recent years. However, radical
fluorination under benign reaction
conditions is far less explored,
especially for the construction of
[18]
À
C(vinyl) F bonds. Based on the
above experimental results for the
facile formation of the vinyl inter-
mediate A (Scheme 1b), and our
previous strategy,[12a] as well as the
well-established
silver-catalyzed
radical fluorination by fluorine-
atom-transfer protocols,[19] we envi-
sioned that
a
combination of
Scheme 3. Silver-catalyzed aminofluorination of allenes. Reaction was performed with 2.0 equiv of 1,
0.3 mmol N-fluoroarylsulfonimide, 10 mol% catalyst in 3 mL solvent at RT under N2, unless
otherwise stated. Yield is that of the isolated product. The E/Z ratio was determined by 1H NMR
spectroscopy.
À
a silver catalyst, an N F reagent,
and allenes might be utilized to
access the fluorinated polysubsti-
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e) B. C. N. M. Jones, J. V. Silverton, C. Simons, S. Megati, H.
Nishimura, Y. Maeda, H. Mitsuya, J. Zemlicka, J. Med. Chem.
1995, 38, 1397 – 1405; f) S. Megati, Z. Goren, J. V. Silverton, J.
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by the sterics of the substituents. Bulky alkyl-substituted
allenes exhibited exclusive stereoselectivity (4j, 4k), while
methyl- and ethyl-substituted allenes resulted in low stereo-
selectivity (4l, 4m). Notably, the allene bearing either
a thiophene or pyridine moiety was well tolerated and
delivered the targets 4n (70%) and 4o (43%), respectively.
Furthermore, monosubstituted allenes were suitable sub-
strates for these transformations (4p, 4q). In addition,
geminal-dialkyl-substituted allenes were successfully trans-
formed into the corresponding aminofluorination products
4r–t with acceptable yields. The electron-deficient allenes 1u–
w did not react with NFSI under the optimal reaction
conditions, even when the reaction was run at 1008C. Finally,
various N-F reagents showed good reactivity and afforded the
desired products in 66%-88% yields (4x–4aa). Some addi-
tional experimental results for preliminary mechanistic inves-
tigations and the possible mechanism for this aminofluorina-
tion process are provided in the Supporting Information.
In summary, the first example of the highly regioselective
oxidative amination of allenes with N-fluoroarylsulfonimides
has been developed. In addition, by tuning the metal catalyst,
aminofluorination products were also be realized. As a result,
general and efficient approaches for the synthesis of the
versatile allenamides and fluorinated polysubstituted alkenes
are provided. These transformations proceed at very mild
conditions and show good tolerance of many synthetically
attractive functionalities. Additional investigations to expand
the scope with respect to the substrates, as well as to clarify
the reaction mechanism, are currently underway in our
laboratory.
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Acknowledgements
Financial support of this research from the NNSFC
(21202014,
21372041,
21172033)
and
SRFDP
(20120043120009) are greatly acknowledged.
Keywords: allenes · copper · fluorine · radicals ·
transition metals
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Received: July 2, 2015
Revised: August 3, 2015
Published online: September 1, 2015
Angew. Chem. Int. Ed. 2015, 54, 12649 –12653
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