Synthesis and unusual [2 + 2] cycloaddition reactions of 1-trifluoroacetyl-2-haloacetylenes

Full text of DOI:10.1007/s10631-005-0065-7

Doklady Chemistry, Vol. 404, Part 1, 2005, pp. 174–177. Translated from Doklady Akademii Nauk, Vol. 404, No. 2, 2005, pp. 201–204.
Original Russian Text Copyright © 2005 by Tsvetkov, Koldobskii, Kalinin.
CHEMISTRY
Synthesis and Unusual [2 + 2] Cycloaddition Reactions
of 1-Trifluoroacetyl-2-haloacetylenes
N. P. Tsvetkov, A. B. Koldobskii, and V. N. Kalinin
Presented by Academician Yu.N. Bubnov May 16, 2005
Received May 17, 2005
Acetylenic dienophiles and enophiles containing ments not only confirmed this assumption but also gave
two activating substituents, one of which is a halogen, striking results: compound 2b was found to react with
are of great interest not only for theory but also as sub- alkenes to form [2 + 2] cycloaddition products 3 with-
strates in the synthesis of biologically active com- out illumination and in the absence of a catalyst. The
pounds [1, 2]. Previously, we obtained certain trifluoro- reaction of 1,1-disubstituted alkenes is exothermic even
acetylacetylenes and studied their Diels–Alder reac- at 20°C, while the addition of 1,2-disubstituted alkenes
tions with various 1,3-dienes [3, 4]. The consideration requires heating to 70–80°C. Ene reaction products 4
of the structure of 1-trifluoroacetyl-2-haloacetylenes are formed simultaneously, and their quantity depends
allowed one to suppose that these compounds should on the nature of the alkene.
exhibit high reactivity in different addition reactions;
As yet, we have studied chloride 2a only in its reac-
therefore, we attempted to develop a general method
tion with isobutene. Although the addition reaction pro-
for the synthesis of these acetylenes.
ceeds somewhat more slowly than in the case of bro-
Readily available [5] bis(trimethylstannyl)acetylene
mide 2b, the corresponding cyclobutene 3a is formed
was found to react exothermically with trifluoroacetic
more selectively.
anhydride to form 1-trifluoroacetyl-2-(trimethylstan-
We failed to separate cyclobutenes and their iso-
mers, ene adducts, by rectification or chromatography.
However, we found that the ene adducts readily under-
went bromine addition across the nonconjugated C=C
bond, whereas cyclobutenes 3 did not react even at
20°C. Thus, after vacuum distillation of the isomer
mixture (Table 1) and the determination of the isomer
ratio therein by 1H NMR, the mixture was treated with
bromine in an amount equivalent to that of the ene
adduct and then redistilled. This technique enabled us
to isolate pure cyclobutenes 3 in high yields except the
adduct with methylenecyclopentane 3d, which gave
rise to the decomposition of the mixture in the course of
distillation:
nyl)acetylene (1), which is a stable compound distil-
lable in a vacuum:
Me3Sn–C≡ C–SnMe3 + (CF3CO)2O
Me2Sn–C≡ C–COCF3 + CF3CO2SnMe3.
1
The halogenation of acetylene 1 readily proceeds
even in the cold to give the corresponding 1-trifluoro-
acetyl-2-haloacetylenes (2a–2c) in high yields:
from –30 to 0°C
1 + X2
X–C≡ C–COCF3 + Me3SnX,
X = Cl (2a), Br (2b), I (2c).
Chloroacetylene 2a and bromoacetylene 2b are sta-
ble compounds distillable without decomposition even
at atmospheric pressure, whereas iodoacetylene 2c
gradually becomes dark and undergoes resinification in
storage.
Br2
–4
3 + 4
3.
Concerted thermal [2 + 2] cycloaddition with the
participation of two alkene molecules or alkene and
acetylene is known to be symmetry-forbidden accord-
ing to the Woodward–Hoffmann rules [6]. Notice that
[2 + 2] cycloaddition for ketenes, ketiminium salts, and
other heterocumulenes is not forbidden, and such reac-
tions are well known [7]. Alkenes containing strong
electron-donating substituents (NR2, OR) at the C=C
bond react with electron-withdrawing alkenes and acet-
ylenes to form cyclobutanes and cyclobutenes [8, 9];
however, these reactions proceed with a considerable
asymmetry of the transition state and cannot be consid-
ered as concerted processes.
1
In the course of H NMR study of the reaction of
bromide 2b with isoprene, which yielded a complex
mixture of products, we noticed that one product
resulted not from the Diels–Alder reaction. Taking into
account this observation, we supposed that bromide 2b
could react with simple alkenes. Even the first experi-
Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, ul. Vavilova 28,
Moscow, 119991 Russia
0012-5008/05/0009-0174 © 2005 Pleiades Publishing, Inc.

SYNTHESIS AND UNUSUAL [2 + 2] CYCLOADDITION REACTIONS
175
Table 1
Compound
Alkene,
conditions
[2 + 2] Cycloaddition
Ene reaction
product 4
Total yield, %;
product ratio
Tb of mixture, °C;
index
product 3
pressure, mmHg
O
O
60
80 : 20
41–60
8
a
CF3
CF3
20°C
72 h
Cl
O
Cl
O
81
55–70
7
b
c
CF3
CF3
CF3
65 : 35
20°C
36 h
Br
O
Br
O
81
75 : 25
45–50
1
CF3
Br
20°C
Br
24 h
O
O
70
53–60
1
d
e
f
CF3
CF3
Br
35 : 65
20°C
Br
O
18 h
O
79
90 : 10
53–57
1
CF3
CF3
Br
20°C
Br
18 h
O
O
66
95 : 5
40–44
1
CF3
CF3
Br
78°C
Br
O
18 h
O
CF3
78
95 : 5
55–56
1
g
CF3
Br
O
78°C
Br
24 h
C4H9
O
CF3
C4H9
CF3
Br
O
50
90 : 10
52–63
1
h
80°C
Br
45 h
CF3
Br
C4H9
O
O
79
95 : 5
64–67
10
CF3
i
CF3
Br
80°C
Br
24 h
DOKLADY CHEMISTRY Vol. 404 Part 1 2005

176
TSVETKOV et al.
Table 2
EXPERIMENTAL
All manipulations with trimethylstannylacetylenes
were carried out in an argon atmosphere with the use of
anhydrous solvents. Trifluoroacetic anhydride was dis-
tilled over ê2é5 prior to use. The structure of the com-
pounds obtained was confirmed by elemental analysis
Tb, °C;
Cyclobutene 3
Yield, %
pressure, mmHg
41–42/8
50–51/7
45–46/1
–
3a
3b
3c
3d
3e
3f
3g
3h
3i
45
50
60
0
1
data, ç and 13C NMR spectra, and IR spectroscopy.
The yields and boiling points of isomer mixtures and
pure cyclobutenes 3 are given in Tables 1 and 2.
55–56/1
40/1
61
60
65
41
67
Trimethylstannyltrifluoroacetylacetylene (1). Tri-
fluoroacetic anhydride (42.4 mL, 63 g, 0.30 mol) was
added in one portion with stirring to a solution of 100 g
(0.28 mol) of bis(trimethylstannyl)acetylene in 200 mL
of tetrahydrofuran using a powerful magnetic stirrer;
after an exothermic reaction was completed, the reac-
tion mixture was allowed to stand overnight. The major
portion of the tetrahydrofuran was distilled off in the
vacuum of a water-jet pump over a water bath at 20°C.
The reflux condenser was replaced with a distilling col-
umn fitted with a descending condenser and a receiver
adapter equipped with a trap cooled with dry ice. In a
vacuum of 10–15 mmHg, the water bath temperature
was gradually increased to 90–95°C while the reaction
mixture was vigorously stirred until the distillation of a
product within 45–65°C was completed (the distillation
residue, accumulating trimethylstannyl trifluoroace-
tate, quickly solidified and partly sublimed, and the stir-
ring became difficult; however, the distillation was con-
tinued). An additional amount of the product was dis-
tilled off from the reaction mixture in the vacuum of an
oil pump at 1–2 mmHg after replacement of the
receiver. Repeated distillation afforded 72.7 g (90%) of
acetylene 1, bp 53–56°C (8 mmHg).
1-Trifluoroacetyl-2-chloroacetylene (2a) and
1-trifluoroacetyl-2-bromoacetylene (2b). A solution
of 4.0 g (0.056 mol) of chlorine (for preparing com-
pound 2a) or 8.4 g (0.052 mol) of bromine (for prepar-
ing compound 2b) in 10 mL of ethyl benzoate was
added dropwise with stirring and cooling to –20°C to a
solution of 15 g (0.052 mol) of acetylene 1 in 10 mL of
ethyl benzoate. The solution was stirred for 5 min at
20°C, and then the halogenation product was short-path
distilled on gradual heating in a vacuum of 15–
20 mmHg into a trap cooled with dry ice. The distilla-
tion was terminated after the vapor temperature rose to
40°C, and the distillate was redistilled to give 6.9 g
(85%) of chloroacetylene 2a, bp 64–66°C, or 9.2 g
(87%) of bromoacetylene 2b, bp 89–91°C.
55/1
63/1
66–67/10
In certain cases, the addition of acetylenes to al-
kenes is catalyzed by aluminum chloride and ethylalu-
minum dichloride [10, 11], as well as by transition
metal complexes [12, 13]. Polyhalogenated alkenes
were also reported to participate in [2 + 2] cycloaddi-
tion reactions, which occurred under severe conditions
via a radical mechanism [14].
The unusualness of the revealed addition reaction is
that it falls into none of the above cases. The reaction
requires neither illumination nor a catalyst. It was also
found that the addition of radical scavengers has no
effect on the rate or direction of the reaction.
Polar solvents are known to accelerate considerably
the rate of [2 + 2] dipolar addition reactions; however,
the addition of acetonitrile provided no discernible
acceleration of the studied reactions. The formation of
two regioisomers in a 1 : 1 ratio in the reaction of acet-
ylene 2b with hexene-1 is also contradictory to both the
polar and the radical mechanisms.
The above circumstances allow us to suppose that acet-
ylenes 2a and 2b undergo addition to alkenes by a con-
certed mechanism, although it is symmetry-forbidden.
To overcome this contradiction, we supposed that
the double bond index of the terminal carbon atom and
the halogen atom in the molecules of compounds 2a
and 2b is sharply increased owing to the strong negative
mesomeric effect of the trifluoroacetyl group. The res-
onance structure describing such a redistribution of
electron density has a ketenium structure, which
enables concerted [2 + 2] cycloaddition:
–
1-Trifluoroacetyl-2-iodoacetylene (2c). Crystalline
iodine (3.3 g, 0.013 mol) was added in small portions to
a solution of 2.5 mL (3.72 g, 0.013 mol) of acetylene 1
in 10 mL of methylene chloride. The solution was
stirred for 10 min at 20°C, concentrated in a vacuum,
and distilled to give 1.6 g (50%) of iodoacetylene 2c,
bp 40–45°C (15 mmHg).
O
+
Br–C≡ C–COCF3
Br=C=C=C
.
CF3
According to the literature data, electron-withdraw-
ing acetylenes such as acetylenedicarboxylic esters,
hexafluorobutyne-2, 1,1,1-trifluoropropyne, and pro-
piolic esters, whose molecules are devoid of a meso-
meric electron-donating substituent at the ë≡C bond,
General procedure for the synthesis of
react with simple alkenes under severe conditions to cyclobutenes 3a–3i.Acetylene 2a or 2b (0.01 mol) was
give only ene addition products [15].
mixed with a 30 mol % excess of alkene (the reaction
DOKLADY CHEMISTRY Vol. 404 Part 1 2005

SYNTHESIS AND UNUSUAL [2 + 2] CYCLOADDITION REACTIONS
177
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was carried out in a sealed ampoule in the case of gas-
eous alkenes or when heating was necessary). The reac-
tion mixture was kept for the required time at the pre-
scribed temperature and then distilled in a vacuum col-
lecting a fraction within the specified temperature
range (Table 1). The distillate was dissolved in 5 mL of
methylene chloride and a solution of bromine in an
amount equivalent to the ene adduct in 2 mL of methyl-
ene chloride was added dropwise with stirring and
cooling to –20°C (Table 1). After warming to 0°C, the
solution was concentrated in a vacuum and redistilled
to give pure cyclobutenes 3a–3i (Table 2).
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