Palladium-catalyzed coupling of propargylic carbonates with N-tosylhydrazones: Highly selective synthesis of substituted propargylic N-sulfonylhydrazones and vinylallenes

Article abstract of DOI:10.1002/chem.201100248

An efficient palladium-catalyzed coupling of propargylic carbonates (1) with N-tosylhydrazones (2, 2′) has been carried out. A wide range of propargylic N-sulfonylhydrazones (3) and multisubstituted vinylallenes (4) can be obtained selectively by using either [PdCl₂(CH₃CN) ₂] or [Pd₂(dba)₃] as catalysts. (dba=trans,trans-dibenzylideneacetone, dppp=propane-l,3- diylbis(diphenylphosphane), Ts=4-toluenesulfonyl.) Copyright

Full text of DOI:10.1002/chem.201100248

DOI: 10.1002/chem.201100248
Palladium-Catalyzed Coupling of Propargylic Carbonates with
N-Tosylhydrazones: Highly Selective Synthesis of Substituted Propargylic
N-Sulfonylhydrazones and Vinylallenes
Zi-Sheng Chen,[a] Xin-Hua Duan,[b] Lu-Yong Wu,[c] Shaukat Ali,[a] Ke-Gong Ji,[a]
Ping-Xin Zhou,[a] Xue-Yuan Liu,[a] and Yong-Min Liang*[a]
The reaction of propargylic compounds with nucleophiles
catalyzed by palladium is one of the most successful and re-
liable methods in organic synthesis.[1] The key step in these
reactions is the formation of an allenyl- or p-propargylpalla-
dium complex (an equilibrium process) by decarboxylation,
which undergoes further reactions with other nucleophiles
to afford different kinds of allene, alkene, alkyne, and enyne
derivatives.[1] In sharp contrast, much less attention has been
paid to the propargylic substitution reactions of propargylic
carbonates with nucleophiles catalyzed by palladium.[2] This
is mainly because most of these reactions give rise to the
corresponding allenic systems and subsequent rearranged
derivatives.[1]
reported in the literature.[5h] On the basis of our study of al-
lenylpalladium chemistry,[6] we envisioned that N-tosylhy-
drazones might serve as nucleophiles and react with propar-
gylic carbonates through propargylpalladium[2a–d] or an allen-
AHCTUNGTRENNUNG
ylpalladium intermediate[1,6] and would lead to substituted
propargylic N-sulfonylhydrazones or vinyl substituted al-
lenes. Herein, we wish to report a novel propargylic substi-
tution of propargylic carbonates with N-tosylhydrazones to
synthesize selectively substituted propargylic N-sulfonylhy-
drazones 3[7] and vinylallenes 4[8–11] (Scheme 1).
Recently, a new type of Pd-catalyzed cross-coupling of
diazo compounds has been developed and it has been
À
proven to be a reliable method for the formation of C C
bonds.[3] An alternative route consists of using N-tosylhydra-
zones as nucleophiles because they are readily available
from the corresponding ketones. Some unstable diazo com-
pounds can also be generated in situ from N-tosylhydra-
zones through the Bamford-Stevens-Shapiro reaction.[4] This
method, initially developed by Barluenga et al., has been
proved to be useful for the preparation of substituted ole-
fins.[5] However, the synthesis of substituted allenes using N-
tosylhydrazones as nucleophiles is much less developed. To
the best of our knowledge, there is only one recent method
Scheme 1. Pd-catalyzed synthesis of substituted propargylic N-sulfonylhy-
drazones and vinylallenes. dba=trans,trans-dibenzylideneacetone, dppp=
propane-l,3-diylbis(diphenylphosphane), Ts=4-toluenesulfonyl).
Initially, the treatment of propargylic carbonate 1a with
N-tosylhydrazone 2a in the presence of [Pd2ACTHNUTRGNEU(GN dba)3] (5
[a] Dr. Z.-S. Chen, S. Ali, K.-G. Ji, P.-X. Zhou, X.-Y. Liu,
Prof. Dr. Y.-M. Liang
mol%) resulted in the desired product vinylallene 4aa in
16% yield (Table 1, entry 1). Surprisingly, when Xphos
(10 mol%) was used as a ligand, the propargylic substitution
product N-sulfonylhydrazones 3aa was isolated in 11%
yield (Table 1, entry 2). The formation of the propargylic
substitution products such as 3aa from propargyl/allenyl pal-
ladium complexes has rarely been observed.[1,2a–d] After a
few trails, we found that the combination of [PdCl2-
State Key Laboratory of Applied Organic Chemistry
Lanzhou University
Lanzhou 730000 (China)
Fax : (+86) 931-8912582
E-mail: liangym@lzu.edu.cn
[b] Dr. X.-H. Duan
Department of Applied Chemistry, Faculty of Science
Xi’an Jiaotong University
Xi’an 710049 (China)
[c] Dr. L.-Y. Wu
AHCTUNGTRENN(GUN CH3CN)2] and dppp gave the best result (Table 1,
entry 5).[12] In addition, the molecular structure of 3aa was
unambiguously confirmed through X-ray crystallography.[13]
To improve the yield of vinylallene 4aa, a series of differ-
ent catalytic systems have been investigated.[12] In all cases,
vinylallene 4aa was obtained in low yields. It has been well
Key Laboratory of Tropical Medicinal Plant Chemistry
of Ministry of Education
Hainan Normal University
Hainan 571158 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100248.
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Chem. Eur. J. 2011, 17, 6918 – 6921

COMMUNICATION
Table 1. Optimization of the reaction conditions.[a]
ring of secondary carbonates
were also investigated (Table 3,
entries 5–9). Electron-donating
groups in the meta or para posi-
tion of the aryl group usually
led to moderate to good yields
of the corresponding vinylal-
lenes (Table 3, entries 5–7). On
the other hand, with an elec-
Pd catalyst (c [mol%])
L
Base
t
Yield [%][b]
3aa
[h]
4aa
1
2
3
4
5
6
7
8
9
10
11
12
13
A
G
–
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
–
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
–
10
6
12
7
trace
11
trace
trace
75
68
0
0
0
0
0
0
0
16
R
Xphos
Xphos
dppe
dppp
dppf
dppp
–
–
–
–
–
–
trace tron-withdrawing group the
E
trace
trace
0
0
0
0
product was only formed in
trace amount (Table 3, entry 9).
Treatment of secondary carbo-
nates 1b and 1k with different
substituted N-tosylhydrazone
salts 2’b-2’c also produced the
corresponding products in 20–
68% yields (Table 3, entries 11–
13). Tertiary propargylic carbo-
nates 1l and 1m were also ef-
fective and moderate yields
were obtained (Table 3, en-
N
N
3
G
1.5
12
12
10
3
10
10
3
C
N
A
U
E
48[c]
56[c]
42[c]
30[c]
33[c]
A
U
R
A
U
G
A
E
U
A
U
E
[a] Reactions conditions: 1a (0.25 mmol), 2a (0.38 mmol), Cs2CO3 (0.55 mmol), and the ligand (10 mol%) in
dioxane (2.5 mL). [b] Yield of isolated product. [c] PTC (0.25 mmol) and N-tosylhydrazone salt 2’a
(0.38 mmol) were used.
Table 2. Pd-catalyzed synthesis of propargylic N-sulfonylhydrazones.[a]
documented that unstable diazo compounds also can be gen-
erated in situ from N-tosylhydrazone salts through the Bam-
ford-Stevens-Shapiro reaction in the presence of a phase
transfer catalyst (PTC).[14a] Keeping this in mind, we then
found that the addition of BnEt3NCl, and the use of N-tosyl-
hydrazone salt 2’a instead of 2a, the yield of 4aa was dra-
matically improved to 48% (Table 1, entry 9). No propargyl-
ic substitution product 3aa was detected in this reaction. N-
Bu4NBr and nBu4NI were proved to be less effective
(Table 1, entries 11 and 12). Finally, we found that a higher
loading of catalyst was required to achieve a good yield and
to shorten the reaction time (Table 1, entry 10).
1, Ar1, R1, R2
2, Ar2
t
Yield
ACHTUNGTRENNUNG
[3,%][b]
[h]
1
2
3
4
5
1a, C6H5, Et, H
1b, C6H5, iPr, H
1d, C6H5, n-hexyl, H
1 f, p-MeC6H4, Et, H
1g, p-MeOC6H4, Et, H
2a, C6H5
2a, C6H5
2a, C6H5
2a, C6H5
2a, C6H5
3
9
4
4
4
3aa, 75
3ba, 17
3da, 75
3 fa, 71
3ga, 67
Next, we examined the scope of this Pd-catalyzed substi-
tution reaction. A series of propargylic N-sulfonylhydra-
zones have been successfully synthesized (Table 2). Propar-
gylic secondary carbonates usually gave moderate to good
yields (Table 2, entries 1–7 and 10–12), although a lower
yield was observed for secondary carbonates possessing an
isopropyl group at the propargylic position (Table 2 entry 2).
Unfortunately, when tertiary carbonate 1m was used, no de-
sired product was obtained (Table 2, entry 8). This result in-
dicated that the steric hindrance has significantly retarded
the reaction. In addition, primary carbonate 1n was also ef-
fective, but a lower yield was obtained (Table 2, entry 9).
Under the optimal conditions (Table 1, entry 10), a series
of multisubstituted vinylallenes have been prepared in mod-
erate yields (Table 3). Compared with propargylic carbo-
nates 1a, the corresponding propargylic acetate gave a
lower yield of 2,3-diphenylhepta-1,3,4-triene (4aa; Table 3,
entry 1). Various secondary carbonates possessing aliphalic
or aromatic substituents at the propargylic position worked
well and afforded the corresponding products in 39–64%
yields (Table 3, entries 2–13). Substituents on the phenyl
6
1h, o-ClC6H4, Et, H
2c,
4
3hc, 76
7
8
9
10
11
12
1i, p-MeCOC6H4, Et, H
1m, C6H5, -(CH2)5-
1n, C6H5, H, H
1a, C6H4, Et, H
1a, C6H5, Et, H
2a, C6H5
2a, C6H5
2a, C6H5
2b, p-PhC6H4
2c
4
5
5
4
4
4
3ia, 66
3ma, 0
3na, 9
3ab, 85
3ac, 70
3ad, 65
1a, C6H5, Et, H
2d, p-MeOC6H4
[a] Reaction conditions: propargylic carbonate (0.25 mmol, 1.0 equiv), N-
tosylhydrazone (0.38 mmol, 1.5 equiv), dppp (0.025 mmol, 10 mol%),
Cs2CO3 (0.55 mmol, 2.2 equiv), and [PdCl2ACTHNURGTNEG(UN CH3CN)2] (5 mol%) in diox-
ane (2.5 mL) at 808C. [b] Yield of isolated product.
tries 14 and 15). Unfortunately, when primary carbonate 1n
was used, only traces of the desire product was detected
(Table 3, entry 16).
Although the precise mechanism of this substitution reac-
tion remains unclear at this moment, we assume that the re-
action may involve two different routes. As shown in
Scheme 2, the palladium catalyst initially promotes decar-
boxylation of propargylic carbonate 1 to generate an allenyl-
Chem. Eur. J. 2011, 17, 6918 – 6921
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www.chemeurj.org
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Y. -M. Liang et al.
Table 3. Pd-catalyzed synthesis of vinylallenes.[a]
product 4 (Route I), the allenylpalladium complex A reacts
with diazo compound D generated in situ from N-tosylhy-
drazone salt C in the presence of base,[14] to afford allenyl-
palladium carbene intermediate E.[3,5] Then a new migratory
insertion of the allenyl group takes place to give the alkyl
palladium complex F,[3,5] and subsequent elimination of a b-
hydrogen atom affords the vinylallene 4 and regenerates the
1, Ar1, R1, R2
2’, Ar2
t
Yield
ACHTUNGTRENNUNG
[4,%][b]
[h]
Pd0 catalyst. The propargylic substitution product
3
1
2
3
4
5
6
7
8
9
1a, C6H5, Et, H
1b, C6H5, iPr, H
1c, C6H5, n-Pr, H
1d, C6H5, n-hexyl, H
1e, m-MeC6H4, Et, H
1 f, p-MeC6H4, Et, H
1g, p-MeOC6H4, Et, H
1h, o-ClC6H4, Et, H
2’a, C6H5
2’a, C6H5
2’a, C6H5
2’a, C6H5
2’a, C6H5
2’a, C6H5
2’a, C6H5
2’a, C6H5
3
3
3
2
3
3
2
4
5
4
3
4aa, 56(39)[c]
4ba, 64
4ca, 51
4da, 61
4ea, 51
4 fa, 53
4ga, 70
4ha, 54
trace
(Route II) may be produced through direct nucleophilic sub-
stitution from the allenylpalladium complex A or propargyl-
palladium intermediate B.[2a,b] However, the cationic-type
complexes in the catalytic process cannot be completely ex-
cluded.[2c]
In summary, we have discovered that propargylic carbo-
nates react with N-tosylhydrazones under palladium-cata-
lyzed conditions through a direct propargylic substitution.
This is an unusual example in which the nucleophilic substi-
tution at the propargylic position is not an allenyl moiety.
Furthermore, a novel palladium-catalyzed coupling of prop-
argylic carbonates with N-tosylhydrazones to prepare multi-
substituted vinylallenes is also developed. This diverse pro-
cess allows the conversion of simple propargylic carbonates
and N-tosylhydrazones into propargyl N-sulfonylhydrazones
or multisubstituted vinylallenes depending on the proper
choice of catalytic system.
1i, p-MeCOC6H4, Et, H 2’a, C6H5
10 1j, C6H4, C6H4, H
11 1b, C6H5, iPr, H
2’a, C6H5
2’b, p-PhC6H4
4ja, 39
4bb, 44
12 1b, C6H5, iPr, H
2’c,
3
4bc, 68[d]
13 1k, C6H5, cyclohexyl, H 2’c
3
4
4kc, 53[d]
4la, 40
14 1l, C6H5, Me, Me
15 1m, C6H5, -(CH2)5-
16 1n, C6H5, H, H
2’a, C6H5
2’a, C6H5
2’a, C6H5
10 4ma, 45[e]
4
trace
[a] Reaction conditions: propargylic carbonate (0.25 mmol, 1.0 equiv), N-
tosylhydrazone salt (0.38 mmol, 1.5 equiv), BnEt3NCl (0.25 mmol,
1.0 equiv), Cs2CO3 (0.55 mmol, 2.2 equiv), and [Pd2ACTHNUGTRNEU(GN dba)3] (10 mol%) in
dioxane (2.5 mL) at 808C. [b] Yield of isolated product. [c] The yields in
parenthesis refer to the yield of corresponding propargylic acetates as a
reaction partner. [d] 2.5 equiv of 2’c was used. [e] The reaction was car-
ried out at 708C.
Experimental Section
Typical procedure for the preparation of propargyl N-sulfonylhydra-
zones:
(0.38 mmol, 1.5 equiv), Cs2CO3 (0.55 mmol, 2.2 equiv), dppp
(0.025 mmol, 10 mol%), and [PdCl2A(CH3CN)2] (0.0125 mmol, 5 mol%)
were added to a Schlenk tube under an argon atmosphere, and then diox-
ane (2.5 mL) was introduced through a syringe. The mixture was stirred
Propargylic
carbonates
(0.25 mmol),
N-tosylhydrazones
CTHUNGTRENNUNG
palladium complex A, which would be in equilibrium with
propargylpalladium intermediate B.[1,2a–c] For vinylallenes
Scheme 2. Proposed mechanism of the reaction
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Synthesis of Substituted Propargylic N-Sulfonylhydrazones and Vinylallenes
COMMUNICATION
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was purified by chromatography on silica gel.
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Acknowledgements
We thank the National Science Foundation (NSF-20732002, NSF-
20090443, and NSF-20872052) and the National Basic Research Program
of China (973 Program, 2010CB833203) for financial support.
Keywords: allenes · cross-coupling · N-tosylhydrazones ·
palladium · propargylic substitution
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[12] See the Supporting Information.
[13] The molecular structure of the product 3aa was determined by
means of X-ray crystallographic studies. CCDC-803965 (3aa) con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Received: January 23, 2011
Revised: March 15, 2011
Published online: May 12, 2011
Chem. Eur. J. 2011, 17, 6918 – 6921
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6921