Capture of in situ generated diazo compounds or copper carbenoids by triphenylphosphine: Selective synthesis of trans-alkenes and unsymmetric azines via reaction of aldehydes with ketone-derived N-tosylhydrazones

Article abstract of DOI:10.1002/adsc.201300302

Copper(II) acetylacetonate-catalyzed Wittig olefination reactions of aldehydes with ketone-derived N-tosylhydrazones are reported. A series of tosylhydrazones was investigated and our results showed that the carbon number of the alkyl chain influences the E-selecivity of the alkenes greatly. Alkenes could be obtained in moderate yields and excellent E-selectivity when the carbon numbers were up to two. Under metal-free conditions, triphenylphosphine was able to capture the in situ generated diazo compounds and the corresponding unsymmetrical azines were formed in good yields.

Full text of DOI:10.1002/adsc.201300302

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DOI: 10.1002/adsc.201300302
Capture of In Situ Generated Diazo Compounds or Copper
Carbenoids by Triphenylphosphine: Selective Synthesis of trans-
Alkenes and Unsymmetric Azines via Reaction of Aldehydes
with Ketone-Derived N-Tosylhydrazones
Qiang Sha,a Yifei Ling,a Wenyong Wang,a and Yunyang Weia,*
a
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Peopleꢀs Republic of
China
Fax : +(86)-25-8431-7078; e-mail: ywei@mail.njust.edu.cn
Received: April 12, 2013; Revised: June 15, 2013; Published online: August 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300302.
Abstract: Copper(II) acetylacetonate-catalyzed
Wittig olefination reactions of aldehydes with
ketone-derived N-tosylhydrazones are reported. A
series of tosylhydrazones was investigated and our
results showed that the carbon number of the alkyl
chain influences the E-selecivity of the alkenes
greatly. Alkenes could be obtained in moderate
yields and excellent E-selectivity when the carbon
numbers were up to two. Under metal-free condi-
tions, triphenylphosphine was able to capture the in
situ generated diazo compounds and the corre-
sponding unsymmetrical azines were formed in
good yields.
Mo,[3] Re,[4] Fe,[5] Rh,[6] Ru,[7] Co,[8] Cu,[9] Ir.[10] It was
also reported that in the absence of a metal catalyst,
phosphazine B would be formed as an intermediate
which can react with aldehydes to form azines (Sche-
me 1).[3,4d,7g]
However diazo compounds without an electron-
withdrawing group are usually unstable and thus diffi-
cult to handle, which significantly limits the scope of
this type of reaction. N-Tosylhydrazones, which can
be utilized as safe diazo precursors, have been used in
organic synthesis for a long time.[11] In the past several
years, rapid progress has been made in tosylhydra-
À
zone-participated cross-coupling reactions and X H
(X=C, O, S, N, P) insertion reactions.[12]
Keywords: alkenes; diazo compounds; N-tosylhy-
drazones; unsymmetrical azines; Wittig reactions
The method of generating diazo compounds in situ
from tosylhydrazones looks as if it is an ideal way to
make up for the shortcomings of the diazo com-
pounds without an electron-withdrawing group. But
less attention has been paid to the Wittig olefination
using tosylhydrazones. To the best of our knowledge,
Diazo compounds have a long history and have at- only two examples were reported. In 2003, Aggarwal
tracted much attention due to their wide applications and co-workers[13] reported
a ClFeTPP-catalyzed
in organic synthesis.[1] Among the various important Wittig reaction of aldehyde-derived tosylhydrazones
applications, Wittig olefination plays an important with aldehydes (Scheme 2, I). The alkenes were
role. The generation of free ylide A (Scheme 1), formed in high yields and high E-selecivity. In 2004,
which could subsequently react with aldehydes or ke- Zhu and co-workers[14] described a straightforward
tones to form the corresponding alkenes, has been ex- method for the synthesis of trans-pentafluorophenyl-
tensively studied.[2] Many kinds of metal catalysts containing alkenes from aldehydes in moderate to
have been developed to catalyze this reaction, such as good yields using Rh
ACHTUNGTRENNUNG
Scheme 1. The generation of ylide A and phosphazine B.
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Scheme 2. Wittig reaction of in situ generated diazo compounds with aldehydes.
However, there are several problems remaining un- which may be due to the steric hindrance of the sub-
solved in this field. There was no report about Wittig strates. Therefore appropriate reaction conditions
olefination using ketone-derived tosylhydrazones need be found to make this reaction work in reality.
Table 1. Reaction results of 1a with 2a in different reaction conditions.[a]
Entry
Catalyst
CuI
Base
Solvent
T [oC]
t [h]
Product
Yield [%][b]
E:Z[c]
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
dioxane
dioxane
dioxane
dioxane
dioxane
DMF
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
50
50
90
90
90
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
24
24
10
10
14
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
3a+3a’
4a
49
68
34
28
24
64
51
39
<5
52
<5
61
80
<5
51
75
33
36
85:15
84:16
87:13
82:18
92:8
84:16
89:11
82:18
–
83:17
–
83:17
80:20
–
82:18
89:11
81:19
88:12
89:11
–
Cu
Cu
U
CuCl2
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
–
N
toluene
CH2ClCH2Cl
DMSO
CH3CN
dioxane
dioxane
dioxane
dioxane
dioxane
toluene
dioxane
toluene
toluene
toluene
dioxane
9
K2CO3
K2CO3
NaOH
10
11
12
13
14
15
16
17
18
19
20[f]
21[f]
Cs2CO3
t-BuOLi
t-BuONa
t-BuOK
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
t-BuOLi
75 (65[d]
)
80
–
4a
91 (81[e])
—
[a]
Reaction conditions: 1a (0.6 mmol), 2a (0.5 mmol), catalyst (10 mol%), base (0.7 mmol), solvent (5 mL), PPh3 (0.7 mmol),
under an N2 atmosphere.
Determined by GC or GC-MS, yield of Z+E.
Determined by GC or GC-MS.
Isolated yield of product 3a based on aldehyde 2a.
Isolated yield based on hydrazone 1a.
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), base (0.6 mmol), solvent (5 mL), PPh3 (0.6 mmol), under an N2 atmos-
[b]
[c]
[d]
[e]
[f]
phere.
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Capture of In Situ Generated Diazo Compounds or Copper Carbenoids by Triphenylphosphine
Table 2. Olefinations with a range of aldehydes and N-tosylhydrazones.[a]
Entry
R1
R2
R3
Product
Yield of 3 [%][b]
E/Z (3:3’)[c]
1
2
3
4
5
6
7
8
Ph
4-ClC6H4
2-thienyl
4-NCC6H4
4-MeCO2C6H4
3-MeCONHC6H4
Ph
Ph
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
Et
Et
Et
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
65
51
55
36[d]
0[e]
0[e]
71
52
43
49
50
61
62
53
23[d]
60
57
55
45
60
50
51
55
89:11
78:22
83:17
76:24
–
–
99:1
90:10
89:11
60:40
64:36
74:26
97:3
97:3
99:1
99:1
99:1
98:2
99:1
99:1
96:4
99:1
94:6
9
Ph
Ph
Ph
4-CF3C6H4
n-butyl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
n-dodecyl
4-O2NC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
Ph
4-CF3C6H4
2-thienyl
2-furyl
1-naphthyl
4-ClC6H4
4-CF3C6H4
4-ClC6H4
4-MeOC6H4
4-ClC6H4
4-MeC6H4
4-NCC6H4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3s
3t
3u
3v
3w
n-propyl
n-propyl
n-butyl
[a]
Reaction conditions: 1a (0.6 mmol), 2a (0.5 mmol), Cu
(0.7 mmol), under an N2 atmosphere, 908C, 10 h.
Isolated yield of 3 based on aldehydes.
Determined by GC-MS or H NMR.
The yield was determined by GC-MS.
ACHTUNGRTENN(NUG acac)2 (10 mol%), t-BuOLi (0.7 mmol), toluene (5 mL), PPh3
[b]
[c]
[d]
[e]
1
Messy products were obtained and the desired products were not detected by GC-MS.
Since copper(I) salts can catalyze the olefination of tion. Optimization of the copper catalysts (Table 1,
aldehydes with diazo reagents,[9] we think that copper entries 2–5) revealed that Cu
(acac)2 was the best
AHCTUNGTRENNUNG
catalysts may also catalyze the olefination reaction of which resulted in an increase in yield to 68%. A varie-
aldehydes with tosylhydrazones.[15] On the other hand, ty of solvents (Table 1, entry 6–10) and bases (Table 1,
PPh3 was reported to be able to capture the diazo entries 11–16) were then screened. Among the bases
compounds forming phosphazine B (Scheme 1). If the tested, t-BuOLi gave the best result. As to the sol-
in situ generated diazo compounds could also be cap- vent, dioxane or toluene are superior to other sol-
tured by PPh3 successfully, a simple and efficient way vents tested. When we decreased the temperature to
to synthesize unsymmetrical azines would be possible, 508C, the yield dramatically decreased to only 33%
which are not easy to synthesize by literature meth- and 36% (Table 1, entries 17 and 18). Under opti-
ods.[16] In a continuation of our work on the develop- mized conditions, 10 h were needed to complete the
ment of new synthetic applications based on tosylhy- reaction (Table 1, entry 19). When using PACTHUNGTRENNNUG
drazones,[17] here we report the results of our attempt
in achieving these goals (Scheme 2, III).
P
N
ACHTUNGTRENNUNG
Our study started with the CuI-mediated reaction
of acetophenone-derived tosylhydrazone (1a) with 4- lighted to find that PPh3 was able to capture the diazo
chlorobenzaldehyde (2a) and PPh3 using K2CO3 as compound generated in situ from acetophenone-de-
base at 908C in dioxane. With 0.1 equiv. of CuI, the rived tosylhydrazone (1a), and the desired product 4a
corresponding alkenes (3a+3a’) were formed in 49% was formed in 80% yield (Table 1, entry 20). After
yield with an E/Z ratio of 85:15 (Table 1, entry 1). In- optimization of the solvent and reaction time, the
spired by this result, we decided to optimize this reac-
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Qiang Sha et al.
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yield increased to 91% and the product was isolated phenylpentan-1-one (the carbon number of the alkyl
in 81% yield (Table 1, entry 21). chain was up to 4) were used, good yields and high E-
With the optimized Wittig olefination reaction con- selectivity were also obtained (Table 2, entries 21–23).
ditions in hand, scope of the reaction was then stud- Finally, tosylhydrazones derived from aliphatic ke-
ied. By using tosylhydrazones derived from 1-(4- tones (cyclohexanone, cyclopentanone and 3-pentan-
chloro
G
ACHTUNGTRENoNUNG ne) were used, but no desired product was obtained
4-acetylbenzonitrile, moderate yields were obtained, which shows that the new method needs to be devel-
the E-selectivity depends on the aldehydes used. oped further to carry out this reaction.
Using 4-chlorobenzaldehyde, benzaldehyde or 4-(tri-
Without the use of a copper catalyst, the reaction
fluoromethyl)benzaldehyde as the substrate, a good of tosylhydrazones, aldehydes and PPh3 affords un-
E-selectivity was obtained (Table 2, entries 1–4, 8 and symmetrical azines in good yield. This method pro-
9). When tosylhydrazones derived from 4-acetylphen- vides an easy and efficient way to synthesize a variety
yl acetate and N-(3-acetylphenyl)acetamide were of unsymmetrical azines. A variety of electron-rich,
used, the desired product alkenes were not detected electron-netural, and electron-deficient aryl tosylhy-
by GC-MS (Table 2, entries 5 and 6). When aliphatic drazones or aldehydes underwent the reaction
aldehydes such as butyraldehyde and dodecanal were smoothly to afford the desired products in good yields
used, moderate yields were obtained but the E-selec- (Table 3, entries 1–18). When tosylhydrazones derived
tivity was low (Table 2, entries 10 and 11). When pro- from aliphatic ketones (cyclohexanone, cyclopenta-
piophenone-derived tosylhydrazone was used, we none and 3-pentanone) were used, no desired product
were delighted to find that the E-selectivity increased was obtained.
greatly and the E/Z ratio was up to 99/1 (Table 2,
A possible mechanism for the above reported reac-
tions is depicted in Scheme 3. Initially, diazo com-
entry7).[18,19]
Inspired by this phenomenon, we studied the reac- pounds were formed in situ from the corresponding
tions of propiophenone-derived tosylhydrazone with tosylhydrazones 1 under basic conditions. In the pres-
different aldehydes, they all gave moderate yields and ence of a copper catalyst, the diazo compounds were
high E-selectivity (Table 2, entries 13–20). Finally, to- converted to copper carbenoids which subsequently
sylhydrazones derived from 1-phenylbutan-1-one (the react with PPh3 to form the free ylide A. Reaction of
carbon number of the alkyl chain was up to 3) and 1- free ylide A with aldehydes gave olefination products.
Table 3. . Preparation of unsymmetric azines 4 from the reaction of 1, 2, and PPh3.[a]
Entry
R1
R2
R3
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Ph
4-ClC6H4
4-MeOC6H4
4-O2NC6H4
4-NCC6H4
4-MeCO2C6H4
3-MeCONHC6H4
Ph
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Ph
Me
Me
Me
Me
Me
Me
Me
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
Ph
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
81
73
83
62
75
77
79
71
68
77
72
70
82
67
61
85
80
86
9
4-MeC6H4
4-NCC6H4
Ph
10
11
12
13
14
15
16
17
18
1-naphthyl
Ph
Ph
Ph
Ph
Ph
Ph
4-MeOC6H4
4-Me2NC6H4
4-O2NC6H4
2-thienyl
2-O2NC6H4
[a]
Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), t-BuOLi (0.6 mmol), dioxane (5 mL), PPh3 (0.6 mmol), under an N2 at-
mosphere.
Isolated yield based on tosylhydrazones.
[b]
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Capture of In Situ Generated Diazo Compounds or Copper Carbenoids by Triphenylphosphine
Scheme 3. Proposed pathway for the reactions of N-tosylhydrazones with aldehydes in the presence of PPh3.
Metal-Free Reaction of N-Tosylhydrazones with
Aldehydes toward Unsymmetrical Azines (4a as an
Example)
Under metal-free conditions, the diazo compounds
react with PPh3 to form the intermediate B which
reacts with aldehydes to form the corresponding un-
symmetrical azines 4.
A Schlenk tube with a magnetic stir bar was charged with
hydrazone 1a (144.0 mg, 0.5 mmol), 2a (84.0 mg, 0.6 mmol),
t-BuOLi (48.0 mg, 0.6 mmol), PPh3 (157.2 mg, 0.6 mmol)
and dioxane (5 mL). The system was heated at 908C with
stirring for 14 h, The reaction mixture was then allowed to
cool to ambient temperature, it was diluted with 10 mL of
ethyl acetate, and washed with brine (30 mL), water
(30 mL), after which the organic layer was dried over
Na2SO4. After concentration under vacuum, the crude prod-
uct was purified by column chromatography.
In conclusion, we have developed an efficient cop-
per(II) acetylacetonate-catalyzed Wittig olefination of
aldehydes with ketone-derived N-tosylhydrazones. It
was found that the alkyl chain of the tosylhydrazones
influenced the E-selecivity of the alkenes greatly.
When the carbon numbers of the alkyl chain were up
to two, high E-selectivity could be obtained. We have
also developed an efficient one-pot method for the
synthesis of the unsymmetrical azines. Studies on
futher applications of this methodology are now in
progress in our laboratory.
Acknowledgements
Experimental Section
We are grateful to Nanjing University of Science and Tech-
nology for financial support.
CuACHTUNGTRENNUNG(acac)2-Catalyzed Wittig Olefination Reaction of
Ketone-Derived N-Tosylhydrazones with Aldehydes
(3a as an Example)
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