Palladium-catalyzed oxidative cross-coupling of N-tosylhydrazones or diazoesters with terminal alkynes: A route to conjugated enynes

Article abstract of DOI:10.1002/anie.201007224

Coming at them from another angle: In a fresh approach to the synthesis of conjugated alkynes, the palladium-catalyzed cross-coupling of N-tosylhydrazones or diazoesters with terminal alkynes provided the desired enyne products with excellent stereoselectivity (see scheme; Ts=p-toluenesulfonyl). The reaction is proposed to involve an unprecedented alkynyl migratory insertion of a palladium carbene complex. Copyright

Full text of DOI:10.1002/anie.201007224

Communications
DOI: 10.1002/anie.201007224
À
C C Coupling
Palladium-Catalyzed Oxidative Cross-Coupling of N-Tosylhydrazones
or Diazoesters with Terminal Alkynes: A Route to Conjugated
Enynes**
Lei Zhou, Fei Ye, Jiachen Ma, Yan Zhang, and Jianbo Wang*
Conjugated enynes have attracted considerable attention
owing to their biological importance, their occurrence in
various natural products,[1] and their versatility as building
blocks for the synthesis of organic conducting polymers.[2] As
a result, enormous effort has been devoted to the regio- and
stereoselective synthesis of conjugated enynes. Among the
various methods developed in the past decades for the
synthesis of enynes, the metal-catalyzed dimerization of
terminal alkynes is a straightforward and atom-efficient
approach (Scheme 1a, R = R’’, R’ = H).[3] However, the
complicated regio- and stereoselectivity of the dimerization
process hampers the wide application of this method. More-
over, only in limited cases is the selective cross-coupling of
two different alkynes possible.[3a,b] A solution to overcome
this setback is the coupling of alkynes with structurally
defined organometallic alkenes (Scheme 1b).[4] However,
organometallic substrates are usually difficult to handle and
are toxic in general.
The coupling of terminal alkynes with vinyl halides under
the catalysis of palladium complexes and copper(I) iodide,
namely, the Sonogashira reaction, is another option for the
synthesis of conjugated enynes (Scheme 1c).[5] This elegant
process does not require a stoichiometric amount of an
organometallic reagent and can produce enynes in a stereo-
selective manner. The major limitation of this method is that
multiple steps are generally required for the preparation of
the vinyl halide. In view of the limitations and shortcomings of
these established methods, we conceived that it would be
desirable to further develop a new type of coupling reaction
that may circumvent these drawbacks.[6]
Palladium-catalyzed cross-coupling reactions involving
diazo compounds as coupling partners have emerged as a
[7,9–15]
=
new type of C C bond-forming reaction.
In pioneering
studies, Van Vranken and co-workers demonstrated palla-
dium-catalyzed coupling reactions with trimethylsilyldiazo-
methane.[7,8] Barluenga et al. first discovered a palladium-
catalyzed coupling reaction of N-tosylhydrazones with aryl
bromides.[9] We have also reported a series of palladium-
catalyzed cross-coupling reactions with a-diazocarbonyl
compounds or N-tosylhydrazones as coupling part-
ners.[10a,e,f,11c,13,14] Migratory insertion involving a palladium
carbene is proposed to account for these cross-coupling
reactions.[15] Aryl,[9,10] benzyl,[11] vinyl,[12] allyl,[13] and acyl
groups[14] have so far been used as the migratory group. As a
continuation of our interest in palladium-catalyzed migra-
tory-insertion reactions, we report the palladium-catalyzed
cross-coupling of terminal alkynes with N-tosylhydrazones or
diazoesters. The reaction involves an unprecedented alkynyl
migratory insertion of a palladium carbene[16] to afford
Scheme 1. Synthesis of conjugated enynes. EWG=electron-withdraw-
ing group, Ts=p-toluenesulfonyl.
[*] Dr. L. Zhou, F. Ye, J. Ma, Dr. Y. Zhang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of the Ministry of Education
conjugated enynes in
(Scheme 1d).
a highly stereoselective manner
College of Chemistry, Peking University, Beijing, 100871 (China)
Fax: (+86)10-6275-1708
E-mail: wangjb@pku.edu.cn
Homepage: http://www.chem.pku.edu.cn/physicalorganic/
home.htm
Initially, we chose phenylbromoethyne (2a) as the cross-
coupling partner and examined its reaction with the
N-tosylhydrazone 1a in the presence of various palladium
catalytic systems. However, none of the desired product was
obtained. We then decided to explore an oxidative coupling
process with a terminal alkyne. Li and co-workers reported a
[**] This project is supported by the NSFC (Grant No. 20902005,
20832002, 21072009, 20821062), the 973 Program (No.
2009CB825300), GSK R&D China, and the China Postdoctoral
Science Foundation.
=
palladium-catalyzed 1,4-addition of terminal alkynes to C C
À
bonds in which the reactivity of the alkynyl C Pd bond could
be increased by an electron-rich ligand.[17] Enlightened by this
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007224.
report, we attempted the coupling of phenylacetylene (2b)
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Table 1: Palladium-catalyzed cross-coupling of phenylacetylene (2b)
with N-tosylhydrazone 1a.[a]
Table 2: Palladium-catalyzed cross-coupling of terminal alkynes with N-
tosylhydrazones.[a]
Entry
Catalyst (mol%)
Oxidant
(equiv)
Base
Yield
[%][b]
Entry
1
1
2
3
Yield [%][b]
65
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
[Pd(allyl)(IPr)Cl] (3)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
BQ (2)
DDQ (2)
AQ (2)
BQ (1)
BQ (3)
BQ (2)
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
K2CO3
Cs2CO3
NaOEt
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuOLi
15
20
5
29
13
67
85
0
21
3
10
0
Pd(OAc)2 (5)/PMe3 (10)
Pd(OAc)2(5)/Xphos (10)
Pd(OAc)2 (5)/PCy3 (10)
Pd(OAc)2 (5)/PPh3 (10)
Pd(OAc)2 (5)/P(2-furyl)3 (10)
Pd(OAc)2 (5)/P(2-furyl)3 (20)
Pd(OAc)2 (5)/P(2-furyl)3 (20)
Pd(OAc)2(5)/P(2-furyl)3(20)
Pd(OAc)2(5)/P(2-furyl)3(20)
Pd(OAc)2 (5)/P(2-furyl)3 (20)
Pd(OAc)2 (5)/P(2-furyl)3 (20)
Pd(OAc)2 (5)/P(2-furyl)3 (20)
Pd(OAc)2 (5)/P(2-furyl)3 (20)
–
2b
2
3
2c
2c
77
74
4[c]
2d
42
42
60
0
5
6
2c
2c
73
71
[a] Reaction conditions: 1a (0.25 mmol), 2b (1.5 equiv), base
(3.5 equiv), 1,4-dioxane (2 mL), BQ (1.5 equiv), catalyst, 908C, 4 h.
[b] The yield was determined by GC with dodecane as an internal
standard. AQ=anthraquinone, BQ=benzoquinone, Cy=cyclohexyl,
DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, IPr=1,3-bis(2,6-dii-
sopropylphenyl)imidazole,
triisopropylbiphenyl.
Xphos=2-dicyclohexylphosphanyl-2’,4’,6’-
7
2c
53
with 1a in the presence of [Pd(allyl)(IPr)Cl] and benzoqui-
none (BQ) at 908C. To our delight, we obtained the desired
enyne 3a, albeit in low yield (Table 1, entry 1). We tested
different ligands and found that the weak electron-rich ligand
tri(2-furyl)phosphane provided the optimal result and
afforded the desired product 3a in 67% yield (Table 1,
entries 2–6). When the ratio of P(2-furyl)3 to Pd(OAc)2 was
increased from 2:1 to 4:1, the reaction improved further
(Table 1, entry 7).
Next, we investigated the effect of the base. A number of
bases, including K2CO3, Cs2CO3, and NaOEt, were examined;
however, they were all less efficient than tBuOLi (Table 1,
entries 7–10). We also studied the effect of the oxidant and
found that oxidants other than BQ, such as 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) or anthraquinone (AQ),
were not as effective (Table 1, entries 11 and 12). The reaction
resulted in diminished yields when the loading of BQ was less
or more than 2 equivalents (Table 1, entries 13 and 14).
Having established the optimal reaction conditions
(Table 1, entry 7), we next explored the scope of the reaction
with a variety of terminal alkynes and N-tosylhydrazones
(Tables 2 and 3). The palladium-catalyzed oxidative cross-
coupling of N-tosylhydrazones with terminal alkynes led to
the formation of the corresponding enynes (Table 2). The
reaction occurred smoothly with ortho-, para-, and meta-
substituted aromatic N-tosylhydrazones to afford the desired
enynes in moderate to good yields (Table 2, entries 2 and 5–
8). Both electron-donating and electron-withdrawing groups
were tolerated on the aromatic ring (Table 2, entries 6–8).
8
9
2e
2c
2b
68
69
65
10
[a] Reaction conditions: 1 (0.25 mmol), 2 (1.5 equiv), tBuOLi (3.5 equiv),
BQ (2 equiv), Pd(OAc)2 (5 mol%), P(2-furyl)3 (20 mol%), 1,4-dioxane
(2 mL), 908C, 4 h. [b] Yield of the isolated product. [c] 45% of alkyne 2d
was recovered.
The naphthyl N-tosylhydrazone 1g was also a suitable
substrate for this reaction (Table 2, entry 9). Notably, cyclic
N-tosylhydrazone 1h, which was readily derived from natu-
rally occurring 4-chromanone, also underwent a smooth
reaction to afford the corresponding enyne 3j in 65% yield
(Table 2, entry 10).
Next, we used N-tosylhydrazone 1i to examine the cross-
coupling reaction with a series of terminal alkynes (Table 3).
The reaction was found to tolerate a variety of functional
groups in the alkyne substrate, including phenyl, alkyl, and
hydroxy groups. Alkyne substrates containing an aromatic
ring with a functional group at the para position or a
heterocyclic or phenylethyl substituent were all converted
into the corresponding enynes in good yields (Table 3,
entries 1–5). The reaction of an alkyne bearing a SiMe3
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Table 3: Palladium-catalyzed cross-coupling of N-tosylhydrazone 1i with
various terminal alkynes.[a]
Table 4: Palladium-catalyzed cross-coupling of terminal alkynes with
diazo compounds 4.[a]
Entry
R
R’
R“
Product
Yield [%][b]
Entry
1
Alkyne 2
Enyne 3
Yield [%][b]
76
Z/E[c]
1
2
3
4
5
6
PhCH2CH2
CH2CH3
n-C4H9
CO2Et
CH2CH3
CH2CH3
Me
Me
Bn
Et
Me
Me
Ph
Ph
Ph
Ph
TMS
3-thienyl
5a
5b
5c
5d
5e
5 f
64
71
58
27
75
52
>20:1
2
3
4
81
70
72
>20:1
>20:1
>20:1
[a] Reaction conditions: 4 (0.25 mmol), the alkyne (1.5 equiv), iPr2NH
(4 equiv), toluene (2 mL), BQ (1.5 equiv), Pd(OAc)2 (5 mol%), P(2-
furyl)3 (20 mol%), 808C, 4 h. [b] Yield of the isolated product. In all cases
only one isomer was formed. The configuration of the product was
1
determined by H NMR spectroscopy.
5
6
7
8
75
83
66
61
>20:1
>20:1
>20:1
>20:1
Scheme 2. Palladium-catalyzed tandem cross-coupling/cyclopropana-
tion reaction. Bn=benzyl.
catalyzed cyclopropanation of olefins occurs readily when the
C C bond bears an electron-withdrawing substituent.[18] Thus,
=
the initially formed enyne products react further with a
second equivalent of the a-diazoester under conditions of
palladium catalysis to afford the cyclopropane derivatives as
the final products.
[a] Reactions were carried out under the conditions described in Table 2.
TMS=trimethylsilyl. [b] Yield of the isolated product. [c] The Z/E
selectivity was determined by 1H NMR spectroscopy. For product 3p,
the configuration was further confirmed by NOESY and COSY spectra.
A plausible mechanism for the cross-coupling reaction is
proposed in Scheme 3. The electron-rich ligand promotes the
formation of the alkynyl palladium intermediate A,[3b,17,19]
which reacts with diazoesters or the diazo substrate B
generated in situ to afford a palladium carbene complex C.
Migratory insertion of the alkynyl group into the palladium
carbene at the carbenic carbon atom leads to the intermediate
D, b-H elimination from which gives enynes 3. Finally, Pd0 is
oxidized by benzoquinone to regenerate the PdII species. The
high stereoselectivity for the formation of enynes 3 can be
explained on the basis of the transition state of the syn b-H
elimination from intermediate D: the alkyl group R’ prefers
to eclipse with the less bulky linear alkyne moiety, and this
arrangement leads to enynes 3 as the main products.[9,11b]
In conclusion, we have developed a novel method for the
synthesis of conjugated enynes through the cross-coupling of
N-tosylhydrazones with terminal alkynes. This reaction,
which involves an unprecedented alkynyl migratory insertion
of a palladium carbene, was used to obtain a variety of
conjugated enynes in a stereoselective manner. Unlike
established methods for enyne synthesis, the current method-
ology requires no additional organometallic reagent or halide.
Moreover, the N-tosylhydrazones used are readily available
group resulted in the enyne product 3p in 83% yield;
desilylation did not occur under the reaction conditions
(Table 3, entry 6). We were also pleased to find that alkynes
containing a primary or secondary hydroxy group reacted
smoothly with N-tosylhydrazones to afford the corresponding
enynes in good yields without the need for functional-group
protection (Table 3, entries 7 and 8). In all reactions in
Table 3, the Z isomer was obtained as the sole product.
To further expand the scope of the reaction, we used a-
diazocarbonyl compounds 4a–d as substrates under slightly
modified conditions (Table 4). The treatment of terminal
alkynes with this series of a-diazocarbonyl compounds in the
presence of Pd(OAc)2 and P(2-furyl)3 at 808C furnished the
corresponding products 5a–f in moderate to good yields with
complete stereoselectivity.
Interestingly, when a-diazocarbonyl compounds 6a–c
were treated with terminal alkynes under similar conditions,
cyclopropanes 7a–c were obtained as the only diastereoiso-
mers according to NMR spectra (Scheme 2). This result is
consistent with our previous observation that the palladium-
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Experimental Section
Representative procedure: Pd(OAc)2 (5 mol%), tri(2-furyl)phos-
phane (20 mol%), tBuOLi (3.5 equiv), benzoquinone (2 equiv), and
N-tosylhydrazone 1a (75 mg, 0.25 mmol) were suspended in 1,4-
dioxane (2 mL) in a 5 mL Schlenk tube under nitrogen. Phenyl-
acetylene (2b; 1.5 equiv) was then added, and the resulting solution
was stirred at 908C for 4 h. The mixture was then cooled to room
temperature and filtered through a short path of silica gel with hexane
and CH2Cl2 as eluents. The volatile compounds were removed
in vacuo, and the crude residue was purified by column chromatog-
raphy (silica gel, hexane) to give 3a (35 mg, 65%) as a light-yellow oil.
Received: November 17, 2010
Published online: March 7, 2011
Keywords: alkynes · cross-coupling · enynes ·
.
migratory insertion · palladium
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