Copper-catalyzed stereoselective alkylhydrazination of alkynes

Article abstract of DOI:10.1039/c9cc07998j

An unprecedented Cu-catalyzed stereoselective alkylhydrazination reaction involving terminal alkynes, azocarboxylic esters as a nitrogen source, and dimethyl 2,2′-azobis(2-methylpropionate) and its analogues as a carbon source is presented here. This protocol provides direct access to tri-substituted (E)-alkenyl-hydrazines with good regio- and stereoselectivity under mild conditions. The transformation proceeds without an external oxidant or additives and shows good functional group tolerance. The alkenylhydrazine products could be easily converted into valuable 1,4-dicarbonyl and allyl carboxylic derivatives.

Full text of DOI:10.1039/c9cc07998j

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Copper-catalyzed stereoselective
alkylhydrazination of alkynes†
Cite this:DOI: 10.1039/c9cc07998j
Jian Lei, *a Huaxin Gao,a Miaoling Huang,a Xiao Liu,b Yangfan Maoa and
Xiaolan Xie*a
Received 12th October 2019,
Accepted 10th December 2019
DOI: 10.1039/c9cc07998j
rsc.li/chemcomm
An unprecedented Cu-catalyzed stereoselective alkylhydrazination Pd- or Cu-catalyzed cross-coupling reactions.8 Examples include
reaction involving terminal alkynes, azocarboxylic esters as a nitro- nucleophilic alkenyl metal species derived from the reaction of
gen source, and dimethyl 2,20-azobis(2-methylpropionate) and its stoichiometric metal reagents (e.g., Al and Ni) with alkynes, which
analogues as a carbon source is presented here. This protocol then react with electrophilic azocarboxylic esters.9 Therefore, a new
provides direct access to tri-substituted (E)-alkenyl-hydrazines with synthetic method for the synthesis of alkenylhydrazines starting
good regio- and stereoselectivity under mild conditions. The trans- from simple alkynes in a ‘one-pot’, regio-/stereoselective, and
formation proceeds without an external oxidant or additives catalytic manner is highly required.
and shows good functional group tolerance. The alkenylhydrazine
On the other hand, with the development of transition metal
products could be easily converted into valuable 1,4-dicarbonyl catalysis, difunctionalization of C–C triple bonds has drawn
and allyl carboxylic derivatives.
considerable attention for its efficient construction of complex
molecules from simple, commercially available compounds.10
Hydrazines are versatile structures in organic chemistry that Among them, tremendous progress in the incorporation of alkyl
can participate in the synthesis of N-containing heterocycles, moieties into triple bonds has been achieved. A major synthetic
such as indoles, pyrazoles, and cinnolines;1 be transformed target for such transformations, including hydroalkylation,11
into amines through N–N bond cleavage;2 or be converted into haloalkylation,12 oxyalkylation13 and carboalkylation,14 is the
other functional groups. Hence, many protocols for the synthesis of incorporation of an alkyl radical across the C–C triple bonds to
organic hydrazines, especially alkyl-, aryl- and acyl-substituted ones, build useful multi-substituted alkenes or ketones. However, alkyl-
have been documented.3–5 However, alkenyl-substituted hydrazines amination of alkynes is still challenging. This is probably due to
have been reported less frequently. It is worth noting that some the vinyl radical intermediate, which is reactive and unstable,
natural alkenylhydrazines possess diverse bioactivities (Scheme 1), readily undergoes H-atom abstraction rather than be captured by
such as hydrazidomycin A (antiproliferative activity), geralcin B the nitrogen source.15 To solve this problem, electron-
(antineoplastic activity), and caribbazoin A (hypotensive activity in withdrawing group radicals were incorporated into the alkynes
rats).6 Previous protocols for the preparation of alkenylhydrazines to stabilize the formed vinyl radicals. Liu, Liang and Bao
mainly include ketone-type Mitsunobu reactions,7a–c addition developed three-component carboazidation of alkynes using
of allenes or enamides to azodicarboxylates,7d–f and aza-Baylis– TMSN3 as an amine source with further transformation to
Hillman reactions from alkyl vinyl ketones and azodicarboxylates.7g,h achieve fluoroalkyl substituted azirines.16 A variety of fluoroalkyl
In addition, the synthesis of alkenylhydrazines from alkynes is a radical sources have been reported in the literature. However, the
promising route because of the availability and flexibility of alkynes, reaction was sluggish when alkyl iodine was used as the radical
which can be divided into two paths (Scheme 2, route A). One path is source.16c Hence, the spread of these protocols to more common
the transformation of alkynes to stereo-fixed alkenyl halides or alkyl radicals is significantly urgent. With this in mind, we
boronic acids, which then react with substituted hydrazines via
a College of Chemical Engineering and Materials Science, Quanzhou Normal
University, Quanzhou 362000, P. R. China. E-mail: leijian0902@hotmail.com,
xxl_qztc@163.com
b Dongguan Institute for Food and Drug Control, Dongguan 523808, P. R. China
† Electronic supplementary information (ESI) available. CCDC 1948643. For ESI
Scheme 1 Biologically active compounds containing alkenylhydrazine
moieties.
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9cc07998j
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worked well for this transformation (entry 6). The solvent has a
significant influence on the reaction. Replacement of the
solvent with DMSO, toluene, CHCl3 or MeOH led to a lower
reaction yield (entries 8–11). The by-product of the reaction of
AIBME and DIAD reminds us to increase the concentration of
alkynes. Although a 95% yield can be obtained by increasing
the alkyne equivalent to 4-fold, excess alkyne resulted in only a
6% increase in the yield. We still choose 3 equiv. alkyne as the
standard condition. Overall, the optimum conditions consist of
heating a solution of 1a in MeCN in the presence of CuBr
(20 mol%), AIBME (1.0 equiv.), and DIAD (1.0 equiv.) at 80 1C.
Under optimal conditions, 4a was isolated in 77% yield.
With the optimal reaction conditions in hand, we set to
examine the alkyne substrate scope for the three-component
alkylhydrazination reactions. As shown in Scheme 3, aryl
alkynes substituted with MeO, alkyl, halides, NCCH2, and F3C
functional groups are tolerated under the optimized condi-
tions, with modest to good yields. The structure of 4c
(CCDC 1948643†) was confirmed by a single-crystal X-ray diffraction
experiment, and the stereochemistry of the trisubstituted alkenyl-
hydrazine is the E configuration. Electron-donating aryl groups
resulted in better yields than the electron-withdrawing examples.
ortho-Substituents on phenyl led to lower efficiency (4a vs. 4j).
2-Ethyl-6-methoxynaphthalene and 5-ethynyl-benzo[1,3]dioxole
afforded the corresponding products 4l and 4m in 56% and
77% yield, respectively. Heteroaromatics such as pyridine, thio-
phene, and (iso)quinoline were highly compatible with the reac-
tion conditions. Unfortunately, aliphatic alkynes failed to react
under our standard conditions.
Scheme 2 Strategies for the synthesis of alkenylhydrazines.
consider whether vinyl radicals created by the addition of alkyl
radicals to simple alkynes could add to azocarboxylic esters, an
amine source and a readily available radical trap,3b,c,17 to generate
alkenylhydrazines. Herein, we report a novel Cu-catalyzed stereo-
selective alkylhydrazination of terminal alkynes with azocarboxylic
esters and dimethyl 2,20-azobis(2-methylpropionate) (AIBME) to
afford trisubstituted E-alkenylhydrazines.
At the onset of our investigation, we chose 4-ethynylanisole
(1a), AIBME (2a), and diisopropyl azodicarboxylate (DIAD) (3a)
as model reagents to survey the reaction conditions. All of these
compounds are commercially available. As shown in Table 1, in
the absence of a Cu catalyst, the reaction affords the desired
alkylhydrazine product in only 43% yield using MeCN as the
solvent at 80 1C. Side products SP-1 and SP-2 were detected.
These side products resulted from the intermolecular radical
coupling between 2a and 3a or the self-coupling of substrate 1a.
Adding CuBr (20 mol%) to the reaction mixture increased the
yield of 4a to 89%. Other Cu catalysts did not show better
reactivity (entries 3–7). It is worth noting that using CuBr2 also
Next, we examined the substrate scope for other azocar-
boxylic esters and AIBME analogues under these reaction
conditions. In addition to DIAD, other azocarboxylic esters,
such as diethyl azodicarboxylate (DEAD) 3b, di-tert-butyl azodi-
carboxylate (DBAD) 3c, dibenzyl azodicarboxylate 3d, and di-2-
methoxyethyl azodicarboxylate 3e, were also compatible with
standard conditions, leading to modest yield (4s–4v). Replacement
Table 1 Optimization of the reaction conditionsa
Run
Equiv. of 1a
Catalyst
Solvent
Yieldb,c (%)
1
2
3
4
5
6
7
8
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
4.0
—
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMSO
MeOH
Toluene
CHCl3
MeCN
43
89 (77)
81
79
80
86
48
Trace
nd
26
CuBr
Cu(TFA)2
Cu(NO3)2
CuI
CuBr2
Cu2O
CuBr
CuBr
CuBr
CuBr
CuBr
9
10
11
12
36
95
a
Reaction conditions: alkyne (1a), AIBME (2a, 0.2 mmol, 1.0 equiv.),
DIAD (3a, 0.2 mmol, 1.0 equiv.), catalyst (20 mol%), solvent (2.5 mL),
Scheme 3 Scope of aryl alkynes.a Isolated yield. b Preparative thin-layer
chromatography was conducted to remove SP-1 from the products owing
to their very similar polarities. c Yield in parentheses was determined by
1H NMR using an internal standard.
b
80 1C, 8 h, under Ar. Yield of 4a was determined by crude 1H NMR
c
with 4-nitroacetophenone as an internal standard. Isolated yield in
parentheses. nd = not detected.
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Scheme 4 Scope of azocarboxylic esters and AIBN analogues, a isolated
yield. b Preparative thin-layer chromatography was conducted.
Scheme 6 Preliminary mechanistic studies.
of AIBME with other azo compounds with various a-cyano tertiary
alkyl groups (e.g., 2,2-azobis(2-methylpropionitrile) (AIBN) 2b, 2,2-
azodi(2-methylbutyro-nitrile) (AMBN) 2c, 2,20-azobis(2,4-dimethyl)-
valeronitrile 2d and 1,10-azobis(cyclohexanecarbonitrile) (ACCN) 2e)
resulted in smooth reactions under the standard conditions to give
allylic cyano-substituted alkenylhydrazines (4w–4z) in 43–76% yield.
To confirm the stereochemistry of products with a-cyano tertiary
alkyl groups, 4x was selected for the NOESY experiment, and the
configuration is as drawn (Scheme 4).
In the following, we hope that more complex molecules can
be modified with this protocol. Erlotinib, a tyrosine kinase
inhibitor, was chosen in the late-stage diversification experi-
ment. As expected, the desired product 4aa was isolated in 61%
yield under standard conditions. Moreover, alkenylhydrazine
product 4a contains multiple functionalities, including ester,
alkene, and hydrazine, which could be transformed into useful
molecules. For example, site-selective hydrolysis of 4a into
1,4-dicarbonyl compound 5 or alkenylhydrazine 6 with an allyl
carboxyl could be realized under acidic or basic hydrolysis
conditions. Reduction of esters to primary alcohols 7 using
DIBAL-H, installation of alkyl-/alkenyl- onto nitrogen to afford 8
and 9 under mild conditions, and acid promoted isomerization
of 4c-E to 4c-Z could also be furnished in modest to good yields
(Scheme 5, see the ESI† for more details).
To explore the reaction mechanism of the alkylhydrazina-
tion process, several control experiments were designed under
optimized conditions (Scheme 6). First, the reaction was
completely suppressed when the radical scavenger 2,2,6,6-tetra-
methyl-1-piperidinyloxy (TEMPO) was added to the reaction
system, and no product was detected by 1H NMR. The alkylated
TEMPO product 10 could be observed by HRMS analysis. These
results indicated that the reaction involves a radical-mediated
pathway. Second, when possible intermediate 11 was used as
the starting material in the absence of 2a, no desired 4a was
observed. Next, when alkenylhydrazine 12 reacted with AIBME
in the presence of CuBr, 4a with 16% yield was detected by
NMR. 12 was reported to show activity towards phosphorus-
centred radicals to give b-ketophosphine oxides in the presence
of cat. Cu and oxygen as stoichiometric oxidants.18 However, we
did not detect 12 under the standard reaction conditions. It is
unlikely that 12 was the possible intermediate. Replacement of
3c with commercially available bis-Boc-hydrazine 3f leads to no
4t, which suggests that the NQN bond is necessary for alkyl-
hydrazination reactions. Finally, using phenylethynyl copper 13
as the starting material, target 4e was not detected under
standard conditions, which means that ethynylcopper species
are not likely to be involved.
Based on the above mechanistic experimental results and
previous reports,4d,11b,19 a plausible mechanism is proposed
involving the decomposition of AIBME by heating to generate
an a-ester tertiary radical, which adds to aryl alkynes to afford
the corresponding vinyl radical B. Further oxidation by Cun to
vinyl–Cun+1 species occurs (C-trans). The E configuration of the
newly formed alkene is probably due to the steric hindrance
of vinyl Cu with an a-ester tertiary alkyl group.12b,14a In the
following, addition of 3 gave rise to Cun+1 species D.4d,11b,c
Afterward, reduction of Cun+1 by excess alkynes provided an
alkenyl-hydrazine product and regenerated the Cun catalyst.11b
SP-1 was also isolated in the reaction system, which was derived
from the trap of radical A by 3 (Scheme 7).
In summary, we have developed an unprecedented copper-
catalyzed stereoselective alkylhydrazination of alkynes with
commercially available carbon and nitrogen sources, providing
the desired E-alkenylhydrazines in modest to good yields. This
methodology possibly involves a radical pathway without
Scheme 5 The application of the alkylhydrazination reaction and further
transformation of products.
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external oxidants or additives. Note that this reaction enhances
the chemistry of azocarboxylic esters towards vinyl radicals,
which may arise from various radical sources. Further investi-
gations into the application of this strategy are still underway in
our laboratory.
Financial support was received from the National Natural
Science Foundation of China (21702120), the Natural Science
Foundation of Fujian Province (2018J01443, 2018Y0073, and
2018J01512), the Fujian Educational Committee (JAT170490),
the Program for the Cultivation of Outstanding Young Scientific
Talents in Fujian Province University, the High-Level Talents in
Quanzhou City (2017Z030), and the PhD Research Startup Foun-
dation of Quanzhou Normal University (G16056).
Conflicts of interest
There are no conflicts to declare.
´
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