Tetraalkynylstannanes in the Stille cross coupling reaction: a new effective approach to arylalkynes

Article abstract of DOI:10.1039/c6nj03905g

The Stille-type cross coupling reaction with tetraalkynylstannanes was studied in detail for the first time. The reaction provides a simple and effective route towards a variety of arylalkynes. The advantages and limitations of the proposed procedure are discussed.

Full text of DOI:10.1039/c6nj03905g

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Tetraalkynylstannanes in the Stille cross coupling reaction: a new
effective approach to arylalkynes
Andrey S. Levashov,*a Dmitrii S. Buryi,a Olga V. Goncharova,a Valeriy V. Konshin,a Victor V.
Dotsenkoa,b and Alexey A. Andreeva (deceased)
scopus
Received 00th January 2016,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The Stille-type cross couling reaction with tetraalkynylstannanes was studied in details for the first time. The reaction
provides simple and effective route towards a variety of arylalkynes. The advantages and limitations of the proposed
procedure are discussed.
www.rsc.org/
tetraalkynyltin compounds (RC≡C)4Sn are far less toxic than
other organotin species having C(sp2)–Sn or C(sp3)–Sn bonds.
Another feature of tetraalkynylstannanes is the high atom
economy. A practical disadvantage of the use of tin mono-
Introduction
For almost forty years after the pioneering research of John K.
Stille, the cross-coupling reaction of organic electrophiles with
organostannanes named after him have been recognized as a
powerful tool for the formation of carbon-carbon bond (for
reviews see1-5). The organotin compounds useful in the Stille
reaction are mild reagents that tolerate a variety of functional
groups and are the reagents of choice for delicate cross-
coupling syntheses of the complex functionalized molecules.5
The Stille reaction has been thoroughly investigated and many
advances have been made to expand both the scope and
utility of this process.5,6 In contrast to other cross-coupling
reactions, the Stille reaction often referred to be effective and
relatively undemanding process allowing for harsher
conditions, as organostannanes are relatively insensitive to
moisture and oxygen.5 On the other hand, the use of
organostannanes such as Bu3SnR raises the problems with
organotin contamination and wastes. Both acute and long-
term toxicities have been reported for many of organotin
reagents,7,8 and the methods designed to limit or avoid the
presence of organotin by-products in reaction products have
been developed.9 In general, the toxicity of alkylstannanes
functional reagents such as R–Sn(alkyl)3 (R is aryl, vinyl or
alkynyl) is that the reactant of a high molecular weight is used
to introduce a hydrocarbon group of a (relatively) low
molecular weight, at the same time producing bulky and highly
toxic triorganotin waste. Since each of four alkynyl fragments
in (RC≡C)4Sn is reactive, tetraalkynyltin compounds may be
compared with sodium acetylides with respect to low
molecular weight and producing only inorganic Sn(IV) waste of
low toxicity. It is noteworthy, that, generally, the reactions
involving organostannanes (e.g., classical Stille coupling or any
other organotin-mediated process) are considered as of a low
atom economy, due to the loss of heavy and toxic tin-
containing moieties.11 In other words, the E-factor12 (which is
defined as the mass ratio of waste to desired product) of the
Stille reaction with R–Sn(alkyl)3 agents is much higher than the
one expected for coupling reactions with tetraalkynyl
stannanes, and the latter reactions could be considered as
more environmentally benign. The advantages of the use of
organotin compounds capable of transferring more than one
organyl group are illustrated by the reactions of tetraallyl-
stannane with electrophilic substrates.13 Tetraallylstannane is
a gentle nucleophile for allylation reactions and easily reacts
with imines,14 aldehydes,15,16 phenacyl bromide,17 other
ketones18 or carbon dioxide19 (Scheme 1). In contrast to
allyltrialkylstannyl reagents which transfer only one organyl
moiety out of four groups on the tin atom, from two to four
allyl residues can be utilized in the case of Sn(CH2CH=CH2)4.
decreases upon size increase of the alkyl groups (Me3SnX
∼
Et3SnX Bu3SnX Octyl3SnX) and upon decrease of the
≫
≫
number of alkyl groups (R3SnX > R2SnX2 > RSnX3);9 R4Sn may
reveal enhanced delayed toxicity due to in vivo
tramsformation into R3SnX.10 However, the toxicity strongly
depends on the nature of the organic group R.10 Due to the
easy hydrolysis of C(sp)–Sn bond, it is generally accepted that
Scheme 1 Some reactions of tetraallylstannane
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Results and discussion
ZnCl2, Et2NH, PhMe, r.t.
R
R
R
SnCl4
Et
+
4
R
CH
We found that tetraalkynylstannanes
1 easily react with a
variety of aryl iodides and bromides under Stille conditions
according to the following scheme (Scheme 4):
1
Sn
ZnCl2, PhMe, 110°C
CH
N
O
Et
Sn
+
4
Ph
R
O
4
R = Ar, n-Bu, octyl, t-BuOCH2,XC(O)OCH2, etc
Scheme 2 Synthesis of tetraalkynylstannanes 1
The alkynyl fragment can be easily introduced onto tin
atom by different ways. Recently we have developed two
convenient and effective methods of synthesis of
Scheme 4 The reaction of tetraalkynylstannanes
1
with aryl halides
3
Tetraorganylstannanes
1
and aryl halides
3
used in the
tetraalkynylstannanes
1; the first is based on the direct
reaction are shown in the Fig. 1. To prevent side reactions such
as hydrolysis of stannanes or oxidative couplings, the
reactions should be conducted in an inert atmosphere (argon)
wherein water and oxygen are excluded. Diaryl diacetylenes
reaction of terminal alkynes with SnCl4 in the presence of
anhydrous ZnCl2 and diethylamine20,21 and the next is based on
the reaction of tin tetra(N,N-diethylcarbamate) with
1
phenylacetylene (Scheme 2).22 Tetraalkynylstannanes
1 are
5
are the by-products probably derived from the Pd-mediated
Glaser-type coupling reaction occurred in the presence of
trace oxygen.
oily or solid compounds that could be easily purified and
isolated in good yields after column chromatography on a
silanized silica. Also they are stable enough to be stored in a
freezer for months.
A number of attempts have been made to optimize the
coupling reaction conditions. We found that a variety of
factors may affect the reaction outcome, such as the nature
and quantity of amine additive used, the nature of Pd catalysts
and solvents, temperature and reaction time. As a model
reaction, we examined the coupling reaction of
tetra(phenylethynyl)tin 1а with p-nitroiodobenzene 3d under
different conditions (Scheme 5). First, we examined the effect
of different solvents and amine additives on the yields of
target aryl acetylene 4ad, using Pd(PPh3)2Cl2 as a catalyst. The
selected results are summarized in Table 1; the complete set
of data is given in Supplementary Table 1.
R1
Et2O
R
C
C
Sn
4
+
R1MgHal
R
Sn
R1
R
R
C
C
Sn R1
3
R
R
BEt2
Et3B, PhMe, -78°C
4 AcCl, Et2O, reflux
Et
R
C
C
Sn
4
Sn
2
Et2B
Et
R
R
O
Ph
C
C
Sn
4
Ph
Me
Scheme 3 The known reactions of tetraalkynylstannanes 1
While much is known about the mono-, di- and trialkynyl tin
compounds, less is known about the chemistry of
tetraalkynyltin compounds. Only a limited number of reactions
are reported encompassing (RC≡C)4Sn as reagents. Thus, the
reactions with Grignard reagents were recognized as
a
convenient method of smooth transmetallation of tetraalkynyl
stannanes with preparation of tri- and dialkynyltins.23,24 The
organoboration of (RC≡C)4Sn with trialkylboranes leads to the
formation of 1,1'-spirobistannoles 2 25-28
Tetra(phenylethynyl)-
.
tin was reported to be an efficient catalyst of ring-opening
polymerization of L-lactide to poly(L-lactide).29 As expected for
tetraalkynylstannanes 1, they may react with acyl chlorides (4
eq.) to afford alkynyl ketones30 (Scheme 3).
To the best of our knowledge, the Stille-type cross coupling
reactions with tetraalkynyl stannanes were not described in
the literature prior to the present work. Recently, French
researchers reported31 the Stille cross-coupling reaction of di-
or trialkynylstannanes with iodovinylic acids/esters, first
introducing a half or a third equivalent of di- or tri-functional
organotin compounds. This is the only report on the Stille cross
coupling with multi-functional C(sp)–Sn organotin compounds.
In this paper, we wish to report the first example of the Stille-
type cross coupling reaction of aryl halides
tetraalkynyltin compounds
3 with
Figure 1 The scope of stannanes 1 and aryl halides 3 used
1.
2 | New J. Chem., 2016, 00, 1-3
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Table 1. The effect of solvents and amines on the yields of aryl acetylene 4ad and the side product, diphenyl diacetylene 5aa
solvent T,°C
Et2NH
Yield of 5a
Et3N
DABCO
yield of 4ad
b
,
,
Time,
Yield of 4ad
b
,
Yield of 5a
b
,
Time,
hc
Yield of 4ad
b
,
Yield of 5a
b
,
Time,
hc
b
%
%
hc
5
–
7
5
3
9
1
2
–
–
%
%
%
%
Et2O
THF
35
80
0
–
0
–
0
3
22
14
13
6
5
71
50
87
63
78
87
87
86
–
6
6
5
20
27
68
44
87
98
85
96
89
5
1
PhMe
MeCN
100
85
55
89
84
91
98
98
–
10
6
5
8
0.5
5
5
6.5
8
dioxane 100
2
2
5
AcOEt
AcOBu
AcOBu
Et3N
80
125
100
80
1
4
2
4
0.5
2
2
2
2
5
2
2
5
5
2
–
4
2.5
2.5
–
–
DMF
100
–
–
9
–
–
–
аThe reaction conditions were as follows: (PhC≡C)4Sn 1a (20 mg, 0.038 mmol), 4-NO2C6H4I 3d (0.153 mmol), Pd(PPh3)2Cl2 (5.4 mg, 0.0077 mmol, 5 mol % vs.
3d), amine (0.15 mmol), and a solvent (2 mL). bYields were determined by GC-MS. cReaction time when the highest yield was achieved.
(the yield of 4ad reached only 66% after 5 h in EtOAc at 80 °C),
and quite rapidly when diethylamine amount is increased. The
use of a large excess of Et2NH under the same conditions
allows one to obtain tolane 4ad in almost quantitative yields
after 2 h (Fig. 2).
Scheme 5. The model reaction of tetra(phenylethynyl)tin 1а with p-
nitroiodobenzene 3d
Table 2. Effects of the different amines and solvents on the yields of 4ad
The yield of aryl acetylene 4ad, %a,b
Amine
As it can be seen, the best results were obtained with BuOAc,
EtOAc, DMF and pure Et3N as solvents, in the temperature
range of 80-125 °C, while the use of less polar (dioxane, PhMe)
and low-boiling (Et2O) solvents resulted in lower yields of aryl
acetylene 4ad with increased amounts of diacetylene by-
product 5a. Though only few examples32-34 were reported for
the amine-promoted Stille reaction, we found that the
presence of an amine additive is strongly required for the
AcOBu
Time,
PhMe
Time,
Dioxane
Time,
hc
hc
hc
Et2NH
Et3N
98
85
97
68
2
55
27
31
87
7
84
44
87
78
3
5
5
2
Bu3N
3
5
3
DABCO
2
0.5
0.5
isophorone
diamine
–
–
–
–
1
54
–
7
–
–
86
54
73
9
1
2
morpholine
N-methyl-
morpholine
pyridine
reaction of tetraalkynylstannanes
1 with aryl halides 3. In the
absence of an amine no reaction occurs, while even trace
amounts give coupling products, albeit in low yields. The
nature of an amine additive as well as its amount has a
dramatic effect on the reaction course, as shown in Table 2.
The best results were obtained with the strong bases such as
Et3N, Bu3N, DABCO, and especially with Et2NH and Pr2NH. The
application of benzylic amines, piperidine, morpholine and N-
12.5
–
–
62
0
–
2
3
5
–
1
–
49
–
–
5
–
5
3
–
0
55
–
3
2
–
5
5
–
piperidine
(CH2)2(NH2)2
PhCH2NH2
(PhCH2)2NH
Pr2NH
41
–
43
11
–
56
26
–
99.5
methylmorpholine lowered significantly the yield of 4ad
,
а
Yields were determined by GC-MS. bThe reaction conditions were as
whereas pyridine and ethylene diamine were found to be
completely inactive. The full set of data on the amines used is
given in Supplementary Table 1. The amine concentration is
also important and strongly influenced the reaction rate. Thus,
the reaction proceeds slower in the presence of 1 eq. Et2NH
follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2 (5 mol % vs 3d),
amine (0.153 mmol) and a solvent (2 mL) at 100 °C. cReaction time when
the highest yield was achieved.
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Table 3. Effects of the amine additive and its amount on the reaction outcomea
100
90
80
70
60
50
40
30
20
10
0
Amine
Volume,
μL
Aryl iodide 3a :
The yieldsb of
4ad, % 5a, %
96.4 1.5
Time,
h
amine ratio
Et2NH
Pr2NH
Pr2NH
Pr2NH
Pr2NH
Pr2NH
Pr2NH
Bu3N
900
21
1 : 57
1 : 1
6
1
1
1
1
1
1
4
1
1
4
4
1
1
1
1
99.5
99.2
98.8
98.9
99.0
98.3
75.7
99.5
99.5
83.7
95.0
97.7
97.2
97.3
94.0
0.5
0.8
1.2
1.1
1.0
1.7
2.0
0.5
0.5
9.2
1.5
2.3
2.8
2.7
6.0
105
210
840
900
1000
36
1 : 5
1 : 10
1 : 40
1 : 43
1 : 47
1 : 1
Bu3N
182
640
900
23
1 : 5
0
1
2
3
4
5
6
7
8
Bu3N
1 : 10
1 : 25
1 : 1
Time, h
Bu3N
V(diethylamine)=15µl
V(diethylamine)=30µl
V(diethylamine)=60µl
V(diethylamine)=140µl
V(diethylamine)=2700µl
TMEDAc
TMEDA
TMEDA
TMEDA
TMEDA
115
229
920
1000
1 : 5
1 : 10
1 : 40
1 : 43
Fig. 2 The kinetics of the reaction 1a
+ 3d → 4ad with the different amounts of
Et2NH. The reaction conditions were as follows: (PhC≡C)4Sn 1a (20 mg, 0.038
mmol), 4-NO2C6H4I 3d (0.153 mmol), Pd(PPh3)2Cl2 as the catalyst (5 mol % with
respect to 3d), AcOEt (2.0 mL), 80 °C.
aUnless otherwise stated, the conditions were as follows: 1a (0.038 mmol), 3d
(0.153 mmol), Pd(PPh3)2Cl2 (5 mol % with respect to 3d), BuOAc, 100°C, total
volume solvent + amine was 1000 μL. bYields were determined by GC-MS.
To optimize the conditions, we studied the effect of
excessive amounts of different amines on the kinetics of the cTMEDA = Me2NCH2CH2NMe2
reaction. It was found that other amines were also as effective
However, when a catalytic amount of CuI was added under the
as Et2NH when they were in high excess. The results are
summarized in Table 3.
same conditions, a trace amount of the coupling product was
detected by GC-MS. It is noteworthy that when (PhC≡C)4Sn 1a
was treated with 4-fold excess of CuBr2 (THF, 0.5 h, 25 °C), the
oxidative Glaser-type coupling product (Ph–C≡C–)2 5a was
formed in a good yield. The first success came with the use of
Pd catalysts, especially Pd(PPh3)2Cl2. To our surprise, the
reaction was completely suppressed by the addition of an
excess of phosphine ligand. Thus, no reaction between
stannane 1a and 1-iodo-4-nitrobenzene 3d occurs in the
presence of Pd(PPh3)2Cl2 and PPh3 (5 mol% and 20 mol% with
respect to 3d, respectively), while Pd(PPh3)2Cl2 with no PPh3
additive gave the best yields. The results of the use of different
Pd catalysts are summarized in Table 4.
We have to admit that the mechanistic picture of the Stille
reaction is rather complex35 and details cannot be specified
with confidence, so the role of an amine and its amount still
remains unclear and requires further investigations. We
suggest that the reaction proceeds by way of formation of
alkynyl-palladium complexes
following scheme (Scheme 6).
6 and 7 according to the
The effect of different catalysts was studied to determine
the best catalytic system. No reaction occurs without a
catalyst: thus, when (PhC≡C)4Sn 1a was reacted with 4-
NO2C6H4I 3d in pure Et3N (80 °C, 2 h), no conversion was
observed.
Ar-X
3
R
C
C
Sn
4
1
or
L
Ar
L
Pd Ln
Pd
R
C
C
n Sn I4-n
X
n R3N
R
C
C
Ar
SnI4
SnI4
n
L is amine or PPh3
4
L
L
L
Ar
L
R
C
C
C
C
Sn I
3
Pd
Pd
or
Ar
C
C
R
Sn I4-(n-1)
C
C
n-1
6
7
R
Scheme 6 A possible mechanism for the reaction of
R
1 with aryl halides 3
4 | New J. Chem., 2016, 00, 1-3
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Next, with an optimized protocol (BuOAc or DMF, 100 °C, 5
mol. % Pd(PPh3)2Cl2, an excess (4-fold or more with respect to
Table 6. The reactivity of different tetraalkynylstannanes
3d under the optimized conditionsa
1 towards 4-NO2C6H4I
stannane
Bu3N) in hands, we studied the reactivity of different
tetraalkynylstannanes and aryl halides . As it was expected,
aryl halides bearing electron-withdrawing groups showed a
better reactivity and gave the highest yields of acetylenes
1) of the amine additive – Et2NH, Pr2NH, DABCO or
Tetraalkynyl stannanes
(PhC≡C)4Sn 1a
1
Time, h
amine
Et2NH
DABCO
Et2NH
Et2NH
DABCO
Yields of
98
4, %
2
1
2
7
5
1
3
(t-BuOCH2C≡C)4Sn 1e
(t-BuOCH2C≡C)4Sn 1e
(n-C8H18CH2C≡C)4Sn 1g
(n-C8H18CH2C≡C)4Sn 1g
74
33
4.
50
The selected results are summarized in Table 5; the complete
set of data is given in Supplementary Table 1.
52
аYields were determined by GC-MS. The reaction conditions were as
follows: (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2 (5 mol % with
respect to 3d), amine (0.153 mmol), BuOAc (2 mL) at 100 °C.
However, when the reaction was carried out using DMF as
a solvent instead of BuOAc, the coupling products with aryl
1
halides 3 bearing electron-donating substituents were
obtained in a good yields. The analogs of (PhC≡C)4Sn 1a,
tetraalkynylstannanes 1b-g, reacted with 4-NO2C6H4I 3d to
form desired acetylenes 4bd-gd, but the yields are generally
lower than with 1a. The selected results are presented in Table
6. Finally, the Stille coupling products
4 were obtained in 40-
93% yields in preparative-scale experiments using conditions
similar to those of the kinetic runs and the optimization
protocols. The results are given in Table 7.
Table 7. The preparative-scale synthesis of acetylenes 4.
Tetraalkynyl
stannane 1
Aryl halide 3
Product
Yield,
a
%
(PhC≡C)4Sn 1a
(PhC≡C)4Sn 1a
(PhC≡C)4Sn 1a
(PhC≡C)4Sn 1a
(PhC≡C)4Sn 1a
Ph–I 3a
4-MeC6H4I 3b
4-BrC6H4I 3c
4-NO2C6H4I 3d
2-Br-4-NO2C6H3I
3f
PhC≡CPh 4aa
4-MeC6H4C≡CPh 4ab
4-BrC6H4C≡CPh 4ac
4-NO2C6H4C≡CPh 4ad
2-Br-4-NO2C6H3C≡CPh
4af
78
81
80
93
40b
Table 4. The effects of the different catalysts on the yields of 4ada
Amine
The yield of 4-NO2C6H4C≡CPh 4ad, %b
PdCl2
Time,
Pd(PhCN)2Cl2
Time,
Pd(PPh3)2Cl2
Time,
hc
7
3
7
2
hc
5
2
5
1
hc
2
7
3
2
Et2NH
Et3N
13
72
60
57
7
98
89
97
68
(PhC≡C)4Sn 1a
(PhC≡C)4Sn 1a
2-NO2C6H4I 3k
2-EtO2CC6H4I 3m
2-NO2C6H4C≡CPh 4ak
2-EtO2CC6H4C≡CPh
4am
88
81
71
50
56
Bu3N
DABCO
(PhC≡C)4Sn 1a
(PhC≡C)4Sn 1a
4-BrC6H4CHO 3n
4-BrC6H4C(O)Me
3o
PhC≡CC6H4CHO 4an
PhC≡CC6H4C(O)Me
4ao
82
65
а The reaction conditions were as follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd
b
catalyst (5 mol % with respect to 3d), amine (0.153 mmol), BuOAc, 100 °C.
Yields were determined by GC-MS. c Reaction time when the maximum yield was
achieved.
(4-MeC6H4C≡C)4Sn
1b
4-NO2C6H4I 3d
4-MeC6H4C≡CC6H4NO2
4bd
91
82
49
47
Table 5. The reactivity of different aryl halides
the optimized conditionsa
3 towards (PhC≡C)4Sn 1a under
(4-ClC6H4C≡C)4Sn 1c
4-NO2C6H4I 3d
4-NO2C6H4I 3d
4-NO2C6H4I 3d
4-ClC6H4C≡CC6H4NO2
4cd
Aryl Halides
3
Solvent
DMF
Time, h
amine
Et3N
Yield of
87
4
(t-BuOCH2C≡C)4Sn
1e
t-BuOCH2C≡CC6H4NO2
4ed
Ph–I 3a
2.5
5
(n-BuC≡C)4Sn 1f
n-BuC≡CC6H4NO2 4fd
4-MeC6H4I 3b
DMF
Et3N
95
4-MeC6H4I 3b BuOAc
4-NO2C6H4I 3d DMF
9
Et2NH
Et3N
54
aIsolated yields are given. bWhen 2-Br-4-NO2C6H3I 3f and (PhC≡C)4Sn 1a were
taken in 2:1 ratio, acetylene 4af was obtained in 81% yield.
2.5
6
89
4-NO2C6H4I 3d BuOAc
4-NO2C6H4I 3d BuOAc
4-NO2C6H4I 3d BuOAc
4-MeC6H4Br 3h BuOAc
4-MeOC6H4Br 3i BuOAc
4-MeOC6H4Br 3i BuOAc
2-NO2C6H4I 3k BuOAc
2-IC6H4CO2H 3l BuOAc
Et2NH
Pr2NH
Bu3N
98
1
99.5
97
3
Conclusions
9
Et2NH
Et2NH
DABCO
Et2NH
Et2NH
Et3N
11
5
7
In conclusion, we have developed the effective synthetic
protocol based on the Stille cross-coupling reaction of easily
available tetraalkynylstannanes with aryl halides. The reported
method provides atom-economical access to aryl acetylenes
and diaryl acetylenes (tolanes) which are valuable reagents for
further transformations. The scope and limitations of the
reaction were studied and the conditions were optimized.
3
15
10
5
83
0
2-IC6H4CO2H 3l
Et3N
4
63
аYields were determined by GC-MS. Unless otherwise stated, the reaction
conditions were as follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2
(5 mol % with respect to 3d), amine (0.153 mmol) and the solvent (DMF or
BuOAc, 2 mL) at 100 °C.
Experimental
Materials and methods
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Solvents and starting reagents were thoroughly dried and the use of the modified silica gel allowed us to remove easily
purified according to common procedures.36 All reactions were the by-product SnI4, which appears to be hard to separate
carried out and the target compounds were isolated in argon when a non-modified silica gel is used as sorbent), eluent –
(99.993%) atmosphere. 1H, 13C, and 119Sn NMR spectra were hexane. Column fractions were analyzed by GC-MS. The eluent
recorded on
a JEOL ECA 400 instrument at operating was evaporated to give 152.3 mg of tolane 4aa as colorless
frequencies 399.78, 100.52 and 149.08 MHz respectively, in crystalline solid (78%, purity by GCMS – 95.3%).
CDCl3 (Aldrich) with reference to TMS or to the residual signals 1H NMR (400 MHz, CDCl3) δ 7.33-7.73 (m, 6H, Ph), 7.52-7.54
of a solvent (with SnMe4 as a standard for 119Sn NMR). (m, 4H, Ph); 13C NMR (100 MHz, CDCl3) δ 89.4, 123.3, 128.2,
Chemical shifts are given in ppm, coupling constants are given 128.3, 131.6. IR (KBr, cm-1) νmax 3063, 2928 (C-H, C-C), 1599
in Hz. IR spectra were recorded on a InfraLUM FT-02 (C=C). MS (m/z, EI, 70 eV) 178 ([M+], 100), 152 ([M-C2H2]+,
instrument in the range of 400-4200 cm-1 (KBr or HCCl3 17.6), 77 ([Ph]+, 3.3).
solution) and on a Bruker Vertex 70 instrument in ATR
4-Methyltolane (1-methyl-4-(phenylethynyl)benzene) (4ab)
(attenuated total reflection) mode. Mass spectra (EI, 70 eV)
were obtained on a Shimadzu GCMS-QP 2010 spectrometer.
The purity of the compounds was checked by TLC (Sorbfil A
plates) with Et2O : hexane (10 : 1), MeOH : HCCl3 (1 : 10) of
HCCl3 : Me2CO (10 : 1) mixtures as eluents. The spots were
visualized with iodine vapors, KMnO4–H2SO4 solution or UV-
light. The starting tetraalkynylstannanes 1a-g were obtained
according to the reported methods;20,21 the detailed
procedures are given in the Supplementary Materials.
A dry 25-mL, two-necked, round-bottomed flask equipped
with argon gas inlet tube and a magnetic stirrer was flushed
with argon and charged with 4-iodotoluene (3b) (227.4 mg,
1.043 mmol), Pd(PPh3)2Cl2 (36.6 mg, 0.052 mmol, 5% mol. vs
3b) and (PhC≡C)4Sn 1a (150 mg, 0.287 mmol). Then Pr2NH
(1.43 mL, 10.43 mmol) and dry BuOAc (6 mL) were added, and
the solution was degassed by freezing in liquid nitrogen and
pumping under vacuum several times, and then flushed with
argon. The reaction mixture was stirred at 100 °C for 5 h, then
allowed to cool and quenched with EtOH (10 mL). The mixture
General procedure for the synthesis of 4-nitrotolane (1-nitro-4-
(phenylethynyl)benzene) (4ad) (the model reaction, Scheme 5,
Tables 1,2,4).
was treated with 0.5
g of silica modified with 3-
aminopropyltriethoxysilane, the solvent was removed on a
rotary evaporator and the traces of BuOAc were removed in
vacuo. The resulting mixture was purified by column
chromatography on the mixture of silica gel (6 g) and silica gel
A 5-mL sealable Wheaton vial was charged with 0.00765 mmol
of Pd catalyst (PdCl2, Pd(PPh3)2Cl2, or Pd(PhCN)2Cl2), 0.153
mmol of the amine additive (Et2NH, Et3N, Bu3N, DABCO,
morpholine, etc.). Then the vial was flushed with a stream of modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol/g
of NH2 groups), eluent - hexane. The yield of 4ab was 81%
(161.8 mg), white crystalline solid. 1H NMR (400 MHz, CDCl3) δ
2.35 (s, 3H, Me), 7.114 (d, 3J = 8.0 Hz, 2H, Ar), 7.29-7.35 (m, 3H,
Ph), 7.42 (d, 3J = 8.0 Hz, 2H, Ar), 7.50-7.53 (m, 2H, Ph); 13C NMR
(100 MHz, CDCl3) δ 21.5, 88.7, 89.6, 120.2, 123.5, 128.1, 128.3,
129.1, 131.6, 132.5, 138.4. IR (KBr, cm-1) νmax 2920.6, 2853.1
(C-H, C-C), 2214.6 (C≡C), 1595.3 (C=C). MS (m/z, EI, 70 eV) 192
([M+], 100), 115 ([M-Ph]+, 8.3), 77 ([Ph]+, 2.6).
dry argon, and a solution of 0.0382 mmol of (PhC≡C)4Sn 1a in a
dry solvent (1 mL) and a solution of 0.153 mmol of 4-NO2C6H4I
in a dry solvent (1 mL) were added subsequently through a
syringe. The mixture was stirred for the indicated time, and
the yields were determined by GC-MS.
Preparative procedure for the synthesis of tolane (diphenyl
acetylene) (4aa) from (PhC≡C)4Sn (1a)
A dry 25-mL, two-necked, round-bottomed flask equipped
with argon gas inlet tube and a magnetic stirrer was flushed
with argon and charged with iodobenzene 3a (212.7 mg, 1.043
mmol), Pd(PPh3)2Cl2 (36.6 mg, 0.052 mmol, 5% mol. vs 3a) and
tetrakis(phenylethynyl)tin 1a (150 mg, 0.287 mmol). Then
Pr2NH (1.43 mL, 10.43 mmol) and dry BuOAc (6 mL) were
added, and the solution was degassed by freezing in liquid
nitrogen and pumping under vacuum several times, and then
flushed with argon. The reaction mixture was stirred at 100 °C
for 5 h, then allowed to cool and quenched with EtOH (10 mL).
The mixture was treated with 0.5 g of silica modified with 3-
aminopropyltriethoxysilane, the solvent was removed on a
rotary evaporator and the traces of BuOAc were removed in
vacuo. The resulting mixture was purified by column
chromatography over silica gel (2 g) with pure dry PhMe (50
mL). The eluent was evaporated, the residue was dissolved in
n-hexane and purified by column chromatography on the
mixture of silica gel (6 g) and silica gel modified with 3-
aminopropyltriethoxysilane (1 g, 1.14 mmol/g of NH2 groups;
4-Bromotolane (1-bromo-4-(phenylethynyl)benzene) (4ac)
4-Bromotolane (4ac) was prepared according to a similar
procedure as for 4ab, using 1-bromo-4-iodobenzene (3c) (295
mg, 1.043 mmol). The yield was 80% (214.4 mg), white
1
crystalline solid. H NMR (400 MHz, CDCl3) δ 7.32-7.35(m, 3H,
3
3
Ar), 7.38 (d, J = 8.7 Hz, 2H, Ar), 7.47 (d, J = 8.7 Hz, 2H, Ar),
7.50-7.53 (m, 2H, Ar); 13C NMR (100 MHz, CDCl3) δ 88.3, 90.5,
122.3, 122.5, 122.9, 128.4, 128.5, 131.60, 131.62, 133.0. IR
(KBr, cm-1) νmax 3049.8, 2924.4, 2855 (C-H, C-C), 2214.6 (C≡C),
1599.2 (C=C). MS (m/z, EI, 70 eV) 258 ([M+ 81Br], 97.2), 256
([M+ 79Br], 100), 177 ([M-Br]+, 13.6), 77 ([Ph]+, 6.8).
4-Nitrotolane (1-nitro-4-(phenylethynyl)benzene) (4ad)
4-Nitrotolane was prepared according to a similar procedure as for
4ab, using 1-iodo-4-nitrobenzene (3d) (259.7 mg, 1.043 mmol). The
reaction time was 1 h 40 min. The yield was 93% (216.5 mg), light
1
yellow crystals. H NMR (400 MHz, CDCl3) δ 7.38-7.39 (m, 3H, Ph);
3
7.54-7.56 (m, 2H, Ph); 7.65 (d, 3J = 8.7 Hz, 2H, Ar), 8.20 (d, J = 8.7
6 | New J. Chem., 2016, 00, 1-3
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Hz, 2H, Ar). 13C NMR (100 MHz, CDCl3) δ 87.6, 94.7, 122.1, 123.6, stirred at 100 °C for 5 h, then allowed to cool and quenched with
128.6, 129.3, 130.3, 131.9, 132.3, 147.0; IR (KBr, cm-1) νmax 2922.5, EtOH (10 mL). The mixture was treated with 0.5 g of silica gel
2851.1 (C-H, C-C), 2216.5 (C≡C), 1591.5 (C=C), 1346.5 (symm NO2). modified with 3-aminopropyltriethoxysilane, the solvent was
MS (m/z, EI, 70 eV) 223 ([M+], 100), 177 ([M-NO2]+, 21.1), 77 ([Ph]+, removed on a rotary evaporator and the traces of BuOAc were
8.8).
removed in vacuo. The resulting mixture was purified by column
chromatography on the mixture of silica gel (6 g) and silica gel
modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol/g of
2-Bromo-4-nitro-1-(phenylethynyl)benzene (4af)
A vial was charged with 2-bromo-1-iodo-4-nitrobenzene (3f) (102.5 NH2 groups), eluated with hexane, then with hexane : PhMe 4 : 1
mg, 0.313 mmol), Pd(PPh3)2Cl2 (11.0 mg, 0.016 mmol, 5% mol. vs 3f) (after diphenyldiacetylene 5a was eluated). Column fractions were
and (PhC≡C)4Sn 1a (90 mg, 0.172 mmol). Then Pr2NH (0.43 mL, 3.13 analyzed by GCMS. The eluent was evaporated to give 243 mg
mmol) and dry BuOAc (1.8 mL) were added, and the solution was (purity by GCMS – 87.3%) of benzoate 4am as yellow solid. The
degassed by freezing in liquid nitrogen and pumping under vacuum product was further purified with flash chromatography (2 g of
1
several times, and then a vial was flushed with argon. The reaction silica gel, hexane). Yield was 81%. H NMR (400 MHz, CDCl3) δ 1.40
3
3
mixture was stirred at 100 °C for 4.5 h, then allowed to cool and (t, J = 7.3 Hz, 3H, OCH2CH3); 4.42 (q, J = 7.3 Hz, 2H, OCH2CH3),
quenched with EtOH (2 mL). The mixture was treated with 0.2 g of 7.34-7.39 (m, 4H, Ar), 7.46-7.50 (m, 1H, Ar), 7.56-7.58 (m, 2H, Ar),
silica gel modified with 3-aminopropyltriethoxysilane, the solvent 7.64 (d, 3J = 8.2 Hz, 1H, Ar), 7.97 (dd, 3J = 7.8 Hz, 4J = 0.9 Hz, 1H, Ar);
was removed on a rotary evaporator and the traces of BuOAc were 13C NMR (100 MHz, CDCl3) δ 14.4, 61.2, 88.3, 94.2, 123.4, 123.6,
removed in vacuo. The resulting mixture was purified by column 127.9, 128.4, 128.5, 130.4, 131.5, 131.7, 132.3, 134.0, 166.4; IR
chromatography on the mixture of silica gel (6 g) and silica gel (KBr, cm-1) νmax 3061.4, 2982.3 (C-H, C-C), 2218.4 (C≡C), 1726.5
modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol/g of (C=O). MS (m/z, EI, 70 eV) 250 ([M+], 94.4), 235 ([M-Me]+, 3.0), 222
NH2 groups), eluated with hexane, then hexane : PhMe 4 : 1. ([M-CO]+, 100), 221 ([M-Et]+, 29.7), 205 ([M-EtO]+, 36.0), 177 ([M-
Column fractions were analyzed by GCMS. The eluent was evapora- COOEt]+, 22.1), 77 ([Ph]+, 13.9).
ted to give 76 mg (81%) of acetylene 4af as yellow crystalline solid.
In addition, the sample can be recrystallized from n-heptane. 1H
4-(Phenylethynyl)benzaldehyde (4an)
NMR (400 MHz, CDCl3) δ 7.38-7.42 (m, 3H, Ph); 7.59-7.61 (m, 2H, Aldehyde 4an was prepared according to a similar procedure
Ph); 7.68 (d, 3J = 8.2 Hz, 1H, H-6 Ar), 8.20 (dd, 3J = 8.2 Hz, 4J = 2.3 Hz,
as for benzoate 4am, using 4-BrC6H4CHO (3n) (193 mg, 1.043
4
1H, H-5 Ar), 8.48 (d, J = 2.3 Hz, 1H, H-3 Ar). 13C NMR (100 MHz,
mmol), Pd(PPh3)2Cl2 (36.6 mg, 0.052 mmol, 5% mol. vs 3n),
(PhC≡C)4Sn 1a (150 mg, 0.287 mmol), TMEDA (1.56 mL, 10.43
mmol) and dry BuOAc (6 mL). The reaction time was 3 h. The
yield of crude aldehyde 4an was 176 mg. For further
purification, the sample was recrystallized from n-heptane.
CDCl3) δ 86.8, 99.5, 121.8, 122.1, 125.9, 127.6, 128.6, 129.7, 132.0,
132.1, 133.3, 146.9; IR (KBr, cm-1) νmax 3094.2, 3074.9 (C-H, C-C),
2218.4 (C≡C), 1583.8 (C=C), 1340.7 (symm NO2). MS (m/z, EI, 70
eV) 303 ([M+ 81Br], 59.4), 301 ([M+ 79Br], 60.2), 257 ([M-NO2)+, 81Br],
0.8), 255 ([M-NO2)+, 79Br], 0.9), 176 ([M-NO2-Br)+, 100), 77 ([Ph]+,
1.6).
1
Yield 82%, beige crystalline solid. H NMR (400 MHz, CDCl3) δ
3
7.36-7.38 (m, 3H, Ph), 7.54-7.57 (m, 2H, Ph), 7.67 (d, J = 8.2
Hz, 2H, Ar), 7.85 (d, 3J = 8.2 Hz, 2H, Ar), 10.00 (s, 1H, CHO); 13
C
NMR (100 MHz, CDCl3) δ 88.5, 93.5, 122.5, 128.5, 129.0,
129.58, 129.60, 131.8, 132.1, 135.4, 191.4; IR (cm-1) νmax
3049.3, 2845.8 (C–C, C–H), 2216.1 (C≡C), 1697.3 (C=O), 1599.9
(C=C); MS (m/z, EI, 70 eV) 206 ([M+], 100), 205 ( [M-H]+, 71.4),
178 ([M-CO]+, 13.3), 77 ([Ph]+, 5.9).
2-Nitrotolane (2-nitro-4-(phenylethynyl)benzene) (4ak)
2-Nitrotolane was prepared according to a similar procedure
as for 4ab, using 1-iodo-2-nitrobenzene (3k) (259.7 mg, 1.043
mmol). The reaction time was 3.5 h. The yield was 88% (205
mg), red oil. 1H NMR (400 MHz, CDCl3) δ 7.35-7.38 (m, 3H, Ph),
7.42-7.46 (m, 1H, Ar), 7.56-7.60 (m, 3H, Ar), 7.70 (d, 3J = 7.8 Hz,
1H, Ar), 8.06 (d, 3J = 8.3 Hz, 1H, Ar). 13C NMR (100 MHz, CDCl3)
δ 84.8, 97.1, 118.7, 122.4, 124.7, 128.45, 128.54, 129.2, 132.0,
132.8, 134.6, 149.6. IR (cm-1, νmax) 3059.9 (C-C, C-H), 2219.0
(C≡C), 1339.5 (symm NO2). MS (m/z, EI, 70 eV) 223 ([M+], 5.2),
177 ([M-NO2]+, 3.8), 77 ([Ph]+, 89.0).
1-[4-(Phenylethynyl)phenyl]ethanone (4ao)
Ketone 4ao was prepared according to a similar procedure as for
benzoate 4am, using 4-BrC6H4C(O)CH3 (3o) (151.4 mg, 0.761 mmol),
Pd(PPh3)2Cl2 (26.7 mg, 0.038 mmol, 5% mol. vs 3o), (PhC≡C)4Sn 1a
(109.4 mg, 0.209 mmol), TMEDA (0.57 mL, 3.8 mmol) and dry
BuOAc (4.5 mL). The product was purified by column
chromatography on a silica gel (7 g), eluent – hexane, then hexane :
PhMe 4 : 1 (after diphenyldiacetylene 5a was eluated). The yield of
crude ketone 4ao was 129.3 mg (purity by GCMS – 84%). For
further purification, sample was recrystallized from n-heptane. Yield
65%, white crystalline solid. 1H NMR (400 MHz, CDCl3) δ 2.61 (s, 3H,
Ethyl 2-(Phenylethynyl)benzoate (4am)
A dry 25-mL, two-necked, round-bottomed flask equipped with
argon gas inlet tube and a magnetic stirrer was flushed with argon
and charged with 2-IC6H4C(O)OEt (3m) (287.9 mg, 1.043 mmol),
Pd(PPh3)2Cl2 (36.6 mg, 0.052 mmol, 5% mol. vs 3m) and (PhC≡C)4Sn
1a (150 mg, 0.287 mmol). Then TMEDA (1.56 mL, 10.43 mmol) and
dry BuOAc (6 mL) were added, and the solution was degassed by
freezing in liquid nitrogen and pumping under vacuum several
times, and then flushed with argon. The reaction mixture was
3
COCH3), 7.36-7.37 (m, 3H, Ph), 7.53-7.56 (m, 2H, Ph), 7.61 (d, J =
3
8.5 Hz, 2H, Ar), 7.93 (d, J = 8.5 Hz, 2H, Ar); 13C NMR (100 MHz,
CDCl3) δ 26.6, 88.6, 92.7, 122.7, 128.2, 128.3, 128.5, 128.8, 131.7,
131.8, 136.2, 197.3; IR (KBr, cm-1) νmax 3061.4, 2997.7 (C-H, C-C),
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2218.4 (C≡C), 1680.2 (C=O), 1603.0 (C=C). MS (m/z, EI, 70 eV) 220 BuOCH2C≡C)4Sn 1e (180 mg, 0.32 mmol), Pr2NH (1.6 mL, 11.64
([M+], 72.5), 205 ( [M-CH3]+, 100), 177 ([M-COCH3]+, 27.8), 77 ([Ph]+, mmol) and BuOAc (6 mL). The reaction time was 5.5 h. The resulting
6.4).
crude product was purified by column chromatography on a
mixture of silica gel (6 g) and silica gel modified with 3-
aminopropyltriethoxysilane (1 g, 1.14 mmol/g of NH2 groups),
1-Methyl-4-[(4-nitrophenyl)ethynyl]benzene (4bd)
Acetylene 4bd was prepared according to a similar procedure as for subsequently eluated with hexane and hexane : HCCl3 9 : 1. The
4ab, using 1-iodo-4-nitrobenzene (3d) (178.3 mg, 0.716 mmol), yield of acetylene 4ed was 132 mg (49%), yellow crystalline solid.
Pd(PPh3)2Cl2 (25.1 mg, 0.036 mmol, 5% mol. vs 3d), (4- 1H NMR (400 MHz, CDCl3) δ 1.30 (s, 9H, tBu), 4.34 (s, 2H, OCH2),
3
3
MeC6H4C≡C)4Sn 1b (114.1 mg, 0.197 mmol), Pr2NH (0.98 mL, 7.16 7.57 (d, J = 9.2 Hz, 2H, Ar), 8.16 (d, J = 9.2 Hz, 2H, Ar); 13C NMR
mmol) and BuOAc (4.5 mL). The reaction time was 3.5 h. The (100 MHz, CDCl3) δ 27.5, 51.0, 74.8, 83.0, 92.7, 123.5, 130.0, 132.4,
resulting crude product was purified by column chromatography on 147.1; IR (KBr, cm-1) νmax 3105.8, 2976.5, 2862.7 (C-H, C-C), 2220.3
the mixture of silica gel (6 g) and silica gel modified with 3- (C≡C), 1593.4 (C=C), 1342.6 (symm NO2). MS (m/z, EI, 70 eV) 233
aminopropyltriethoxysilane (1 g, 1.14 mmol/g of NH2 groups), ([M+], 0.3), 218 ([M-CH3]+, 10.4), 160 ([M-tBuO]+, 100), 57 ([tBu]+,
subsequently eluated with hexane, hexane : EtOAc 9 : 1, hexane : 60.5).
EtOAc 4 : 1 and pure toluene. Recrystallization from toluene yielded
108.6 mg (63.9%) of acetylene 4bd as white crystalline solid. 1-Hex-1-ynyl-4-nitrobenzene (4fd)
Another crop of product (49.1 mg, purity by GC-MS – 95%) was Acetylene 4fd was prepared according to a similar procedure as for
obtained from the mother liquor. Total yield of tolane 4bd was 4ab, using 1-iodo-4-nitrobenzene (3d) (249.0 mg, 1.00 mmol),
1
155.2 mg (91%). H NMR (400 MHz, CDCl3) δ 2.38 (s, 3H, CH3), 7.18 Pd(PPh3)2Cl2 (35.1 mg, 0.05 mmol, 5% mol. vs 3d),
3
3
(d, J = 7.8 Hz, 2H, 4-MeC6H4), 7.44 (d, J = 7.8 Hz, 2H, 4-MeC6H4), (CH3CH2CH2CH2C≡C)4Sn 1f (121.9 mg, 0.275 mmol), Pr2NH (1.37 mL,
3
3
7.63 (d, J = 9.2 Hz, 2H, 4-NO2C6H4), 8.19 (d, J = 9.2 Hz, 2H, 4- 10.0 mmol) and BuOAc (6 mL). The reaction time was 15 h. The
NO2C6H4); 13C NMR (100 MHz, CDCl3) δ 21.6, 87.1, 95.1, 119.1, resulted product was purified by column chromatography over a
123.6, 129.3, 130.5, 131.8, 132.2, 139.7, 146.9; IR (KBr, cm-1) νmax mixture of silica gel (6 g) and silica gel modified with 3-
2922.5, 2847.3 (C-H, C-C), 2212.6 (C≡C), 1589.5 (C=C), 1342.6 aminopropyltriethoxysilane (1 g, 1.14 mmol/g of NH2 groups),
(symm NO2). MS (m/z, EI, 70 eV) 237 ([M+], 100), 191 ([M-NO2]+, eluent – hexane. The column fractions were concentrated, the
13.6), 176 ([M-Me-NO2]+, 17.1).
unreacted 1-iodo-4-nitrobenzene (3d) was filtered off, and the
crude product was again purified under the same conditions as in
the aforesaid column chromatography to give 96.1 mg (47%) of
acetylene 4fd as light yellow oil. 1H NMR (400 MHz, CDCl3) δ 0.96 (t,
3J = 7.3 Hz, 3H, Me), 1.44-1.53 (m, 2H, CH2), 1.58-1.65 (m, 2H, CH2),
1-Chloro-4-[(4-nitrophenyl)ethynyl]benzene (4cd)
Acetylene 4cd was prepared according to a similar procedure
as for 4ab, using 1-iodo-4-nitrobenzene (3d) (249 mg, 1.00
mmol), Pd(PPh3)2Cl2 (35.1 mg, 0.05 mmol, 5% mol. vs 3d), (4-
ClC6H4C≡C)4Sn 1c (181.7 mg, 0.275 mmol), Pr2NH (1.37 mL,
10.0 mmol) and BuOAc (6 mL). The reaction time was 2.5 h.
The resulting crude product was purified by column
chromatography on a mixture of silica gel (6 g) and silica gel
modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol/g
of NH2 groups), subsequently eluated with hexane, hexane :
HCCl3 9 : 1, hexane : HCCl3 3 : 2 and hexane : HCCl3 1 : 1.
Recrystallization from toluene yielded 160.9 mg (62%) of
acetylene 4cd as white crystalline solid. Another crop of
product (49.8 mg) was isolated from the mother liquor. Total
yield was 210.7 mg (82%). 1H NMR (400 MHz, CDCl3) δ 7.36 (d,
3J = 8.2 Hz, 2H, Ar), 7.49 (d, 3J = 8.2 Hz, 2H, Ar), 7.65 (d, 3J = 8.7
3
3
2.45 (t, J = 6.9 Hz, 2H, CH2C
≡), 7.5
1 (d, J = 8.7 Hz, 2H, Ar), 8.15 (d,
3J = 8.7 Hz, 2H, Ar); 13C NMR (100 MHz, CDCl3) δ 13.6, 19.3, 22.0,
30.5, 79.3, 96.8, 123.5, 131.3, 132.3, 146.6; IR (KBr, cm-1) νmax
3107.7, 3080.7, 2957.2, 2932.2, 2872.4 (C-H, C-C), 2230.0 (C≡C),
1592.3 (C=C), 1342.6 (symm NO2). MS (m/z, EI, 70 eV) 203 ([M+],
39.9), 188 ([M-CH3]+, 58.3), 174 ([M-CH2CH3]+, 6.6), 157 ([M-NO2]+,
17.9).
Further details on the experimental procedures and spectra are
given in the Supplementary materials file available at http…
Acknowledgements
3
Hz, 2H, Ar), 8.22 (d, J = 8.7 Hz, 2H, Ar); 13C NMR (100 MHz,
VVD is grateful for financial support by the Russian Ministry of
Education and Science (State Assignment to Higher Education
Institutions, project no. 4.5547.2017/БЧ).
CDCl3) δ 88.4, 93.4, 120.6, 123.7, 129.0, 129.9, 132.3, 133.1,
135.5, 147.2; IR (KBr, cm-1) νmax 3092.3, 2926.4, 2851.1 (C-H, C-
C), 2210.7 (C≡C), 1587.6 (C=C), 1346.5 (symm NO2). MS (m/z,
EI, 70 eV) 259 ([M+] 37Cl, 32.7), 257 ([M+] 35Cl, 100), 213 ([M-
NO2]+ 37Cl, 2.8), 211 ([M-NO2]+ 35Cl, 9.5), 176 ([M-Cl-NO2]+,
82.7).
Notes and references
1
2
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tert-Butyl 3-(4-nitrophenyl)prop-2-ynyl ether (4ed)
Acetylene 4ed was prepared according to a similar procedure as for
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