Methoxycarbonyltrifluoromethylthioketene

Article abstract of DOI:10.1023/A:1024781929253

Anew method was developed for the synthesis of methoxycarbonyltrifluoromethylthioketene by thermal cleavage of tert-butyl 1,3,3,3-tetrafluoro-2-methoxycarbonylpropenyl sulfide in the presence of P ₂O₅. This thioketene was demonstrated to exhibit high reactivity in reactions with nucleophiles as well as in ene and diene syntheses.

Full text of DOI:10.1023/A:1024781929253

1198
Russian Chemical Bulletin, International Edition, Vol. 52, No. 5, pp. 1198—1200, May, 2003
Methoxycarbonyltrifluoromethylthioketene
ꢀ
A. N. Kovregin, A. Yu. Sizov, and A. F. Ermolov
Military University of Radiational, Chemical, and Biological Defense,
13 Brigadirskii per., 107005 Moscow, Russian Federation.
Fax: +7 (095) 267 5107
A new method was developed for the synthesis of methoxycarbonyltrifluoromethylthioketene
by thermal cleavage of tertꢀbutyl 1,3,3,3ꢀtetrafluoroꢀ2ꢀmethoxycarbonylpropenyl sulfide in the
presence of P2O5. This thioketene was demonstrated to exhibit high reactivity in reactions with
nucleophiles as well as in ene and diene syntheses.
Key words: thioketenes, tertꢀbutyl polyfluoroalkenyl sulfides, methoxycarbonyltrifluoroꢀ
methylthioketene, ene reactions, [4+2]ꢀcycloaddition.
Thioketenes exhibit high reactivity and, as a conseꢀ
quence, lability. The first stable representative of this class
of compounds, viz., bis(trifluoromethyl)thioketene, was
synthesized and characterized in 1966.1 Its reactions with
usual nucleophilic reagents proceed with retention of the
thiocarbonyl group, whereas reactions with various unꢀ
saturated compounds either afford products of the ene
reaction or lead to cycloaddition with its participation.2—4
Later on, some other thioketenes were also synthesized.
Their stability is provided primarily by the shielding effect
of bulky substituents (for example, bis(tertꢀbutyl)thioꢀ
ketene).5
accompanied by elimination of isobutene and hydrogen
fluoride (Scheme 1).
Scheme 1
The aim of the present study was to synthesize previꢀ
ously unknown stable fluorineꢀcontaining thioketene, viz.,
methoxycarbonyltrifluoromethylthioketene (1), and study
its properties.
Fractionation of the resulting mixture over silica gel
was demonstrated to substantially decrease the content of
acid fluoride 3. After triple distillation, thioketene 1 was
isolated in the individual form. We cannot unambiguꢀ
ously interpret this fact, which could be a consequence of
either dehydrofluorination of acid fluoride 3 to yield the
target thioketene 1 or its binding on silica gel as a result of
some other transformations. An attempt to use NaF for
this purpose failed, and the composition of the mixture
remained unchanged.
Results and Discussion
It is evident that the known procedure for the syntheꢀ
sis of bis(trifluoromethyl)thioketene2 by pyrolysis of the
corresponding dimer having the 1,3ꢀdithiethane structure
at 650—750 °C is unsuitable for the preparation of thioꢀ
ketene 1 containing the methoxycarbonyl fragment, which
is labile under these temperature conditions. Hence, we
examined a new approach to the synthesis of fluorineꢀ
containing thioketenes based on the cleavage of tertꢀbutyl
polyfluoroalkenyl sulfides with P2O5. The procedure for
the preparation of the latter sulfides has been described
earlier.6
Under analogous conditions, tertꢀbutyl perfluoroꢀ
isobutenyl sulfide (4a) and benzyl perfluoroisobutenyl sulꢀ
fide (4b) generated exclusively αꢀHꢀhexafluorothionoꢀ
isobutyryl fluoride (5) in good yields (Scheme 2).
It appeared that thermal decomposition of tertꢀbutyl
1,3,3,3ꢀtetrafluoroꢀ2ꢀmethoxycarbonylpropenyl sulꢀ
fide (2) in the presence of P2O5 at 180—200 °C afforded a
mixture of thioketene 1 and αꢀHꢀαꢀmethoxycarbonylꢀ
trifluorothionopropionyl fluoride (3) (3 : 2 ratio) and was
Earlier,7 acid fluoride 5 has been prepared by acidic
cleavage of sulfide 4b by HSO3F. It has been postulated
that the reaction proceeded throught the intermediate
formation of enethiol, which was isomerized into a more
stable thiocarbonyl derivative through the 1,3ꢀprototropic
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1134—1136, May, 2003.
1066ꢀ5285/03/5205ꢀ1198 $25.00 © 2003 Plenum Publishing Corporation

Methoxycarbonyltrifluoromethylthioketene
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
1199
Scheme 2
(Х = Y = CF3). In contrast, elimination of the fluoride
ion occurs in the case of the least basic substituent (X =
Y = COOMe). In the intermediate case (X = CF3,
Y = COOMe), both processes occur competitively to give
a mixture of products.
Attempts to synthesize cyanoꢀcontaining thioketenes
under analogous conditions based on tertꢀbutyl 2ꢀcyanoꢀ
1,3,3,3ꢀtetrafluoropropenyl and tertꢀbutyl 2ꢀcyanoꢀ1,2ꢀ
difluorovinyl sulfides were unsuccessful and resulted only
in resinification.
Thioketene 1 is a storageꢀstable brightꢀorange liquid,
which can be distilled in vacuo without decomposition.
Its structure was supported by the 19F NMR spectrum,
which has a singlet of the trifluoromethyl group at the
double bond (δ 20.4), and the IR spectrum, which shows
a signal characteristic of thioketenes (ν = 1780 cm–1). In
addition, the structure of 1 was confirmed by chemical
transformations.
Thus, thioketene 1 readily added methanol at the C=C
bond to yield ester 8. The reactions with cyclohexene and
cyclopentadiene gave rise to the corresponding products
of the ene reaction, viz., sulfide 9 and [4+2]ꢀcycloadꢀ
duct 10 (Scheme 4). The reaction conditions were analoꢀ
gous to those described earlier for bis(trifluoromethyl)thioꢀ
ketene.2—4
R = But (a), Bn (b)
shift. Attempts to transform acid fluoride 5 into bis(triꢀ
fluoromethyl)thioketene by dehydrofluorination failed,
because 5 appeared to be resistant to fractionation both
over NaF and silica gel.
In turn, heating of tertꢀbutyl 1ꢀfluorovinylꢀ2,2ꢀ
bis(methoxycarbonyl) sulfide (6) with P2O5 afforded the
dimer of the corresponding thioketene 7 as the only prodꢀ
uct in low yield (Scheme 3). The reaction was accompaꢀ
nied by destruction resulting in resinification and gas forꢀ
mation (apparently, due to decarboxylation).
Scheme 3
Scheme 4
Taking into account these substantial differences in
the results of the reactions of such close structural analogs
as sulfides 2, 4, and 6, it can be stated that the nature of
βꢀsubstituents in the alkenyl group has a decisive effect on
the direction of the reaction.
The initial step of these reactions may involve either
the insertion of the phosphoric anhydride at the But—S
bond analogously to reactions of this anhydride with
ethers8 or protonation of the S atom followed by the forꢀ
mation of enethiols analogously to the cleavage of alkyl
polyfluoroalkenyl sulfides by protic acids.7 In the latter
case, the process can be promoted by impurities of
polyphosphoric acids and then proceed autocatalytically.
For both possible alternatives, mesomeric anionic interꢀ
mediates should be assumed as direct precursors of thioacyl
fluorides and thioketenes.
The further pathway of stabilization of such anions
(either protonation or elimination of the fluoride ion) is
determined primarily by their basicity, which, in turn,
depends on the electronic and steric effects of the subꢀ
stituents.
To summarize, we synthesized methoxycarbonyltriꢀ
fluoromethylthioketene (1) as the second representative
of stable fluorineꢀcontaining thioketenes. However, this
reaction cannot be used as the general procedure. Under
analogous conditions, other alkyl polyfluoroalkenyl sulꢀ
fides produce either acid fluorides of the corresponding
thionic acids or dimerization and resinification products.
Experimental
Of two possible pathways of stabilization, only protoꢀ
nation takes place in the case of the most basic of these
substituents, viz., of the perfluoroisobutenylthiolate ion
The 19F NMR spectra were recorded on a Bruker ACꢀ200F
1
spectrometer (188.31 MHz). The H NMR spectra were meaꢀ

1200
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
Kovregin et al.
sured on a Bruker ACꢀ300SF instrument (300.13 MHz). The
chemical shifts (δ) are given in the δ scale relative to CF3COOH
(19F, external standard) and Me4Si (1H, internal standard). The
NMR spectra of compounds 1, 3, and 5 were recorded for
samples sealed in glass inserts without a solvent. The NMR
spectra of all other compounds were measured in solutions in
DMSOꢀd6 or CDCl3. The mass spectra (EI, 70 eV) were obꢀ
tained on an HP 5890 gas chromatograph equipped with an HP
5972 massꢀselective detector. The IR spectra were recorded on a
Perkin—Elmer 1720X instrument in CCl4.
Methoxycarbonyltrifluoromethylthioketene (1). A mixture of
tertꢀbutyl 1,3,3,3ꢀtetrafluoroꢀ2ꢀmethoxycarbonylpropenyl sulꢀ
fide (2) (7.8 g, 30 mmol) and P2O5 (12.8 g, 90 mmol) was heated
at 180—200 °C for 10 min. Then volatile products were distilled
off under gradually increased vacuum (to 10 Torr), the temperaꢀ
ture of the reaction mixture being maintained at 180—200 °C.
A mixture of thioketene 1 and αꢀHꢀαꢀmethoxycarbonyltriꢀ
fluorothionopropionyl fluoride (3) was obtained in a yield of
2.3 g (3 : 2 ratio). The 19F NMR spectrum of compound 3, δ:
13.0 (m, 3 F, CF3); 157.0 (m, 1 F, C(S)F). After triple fractionꢀ
ation of the resulting mixture over silica gel (1.0 g), thioketene 1
was obtained in a yield of 1.1 g (20%), b.p. 65—68 °C (30 Torr).
Found (%): C, 32.43; H, 1.60. C5H3F3O2S. Calculated (%):
C, 32.61; H, 1.63. 1H NMR, δ: 3.86 (s, OMe). 19F NMR, δ: 20.4
(s, CF3). IR, ν/cm–1: 1780 (C=C=S); 1710 (C=O).
Cyclohexenꢀ2ꢀyl 3,3,3ꢀtrifluoroꢀ2ꢀmethoxycarbonylpropꢀ1ꢀ
enꢀ1ꢀyl sulfide (9). Thioketene 1 (1.8 g, 10 mmol) was added
dropwise with stirring to cyclohexene (1 g, 12 mmol) at 5—10 °C.
The reaction mixture was kept at 20 °C for 15 h and then fracꢀ
tionated. Sulfide 9 (mixture of the Z and E isomers) was obꢀ
tained in a yield of 1.1 g (42%), b.p. 121—123 °C (3 Torr).
Found (%): C, 49.80; H, 4.95. C11H13F3O2S. Calculated (%):
C, 49.62; H, 4.89. 19F NMR (DMSOꢀd6), δ: 15.7 and 19.3
(both s, CF3, 2 : 3 isomer ratio). 1H NMR (DMSOꢀd6), δ:
1.68—2.13 (m, 6 H, 3 CH2); 3.83 (m, 4 H, OMe + CHS); 5.69
and 6.02 (both m, 1 H each, —CH=CH—); 8.02 and 8.32 (both s,
1 H, SCH=C, 2 : 3 isomer ratio).
3ꢀ(1ꢀMethoxycarbonyltrifluoroethylidene)ꢀ2ꢀthiabicycꢀ
lo[2.2.1]heptꢀ5ꢀene (10). Thioketene 1 (1.8 g, 10 mmol) was
added with stirring to a solution of cyclopentadiene (0.65 g,
10 mmol) in CH2Cl2 (5 mL) at 15—20 °C. The reaction mixture
was kept for 3 h and then fractionated. Heptene 10 (mixture of
the Z and E isomers) was obtained in a yield of 1.2 g (49%), b.p.
87—88 °C (3 Torr). Found (%): C, 48.12; H, 3.63. C10H9F3O2S.
Calculated (%): C, 48.00; H, 3.60. MS, m/z: 250 [M]+. 19F NMR
(CDCl3), δ: 24.3 and 28.8 (both s, CF3, 4 : 5 isomer ratio).
1H NMR (CDCl3), δ: 1.95 (m, 2 H, C(7)H2); 4.40 and 4.48
(both s, 1 H, C(1)H, 5 : 4 isomer ratio); 4.81 and 4.87 (both s,
3 H, OMe, 5 : 4 isomer ratio); 4.70 and 5.05 (both s, 1 H,
C(4)H, 5 : 4 isomer ratio); 6.24 and 6.63 (both m, 1 H each,
C(5)H, C(6)H).
2ꢀHydrohexafluorothionoisobutyryl fluoride (5). A mixture
of tertꢀbutyl 1,3,3,3ꢀtetrafluoroꢀ2ꢀtrifluoromethylpropenyl sulꢀ
fide (4a) (5.4 g, 20 mmol) and P2O5 (14.2 g, 100 mmol) was
heated to 200 °C, and the distilled products were collected into a
cooled receiver. Repeated distillation afforded acid fluoride 5 in
a yield of 2.4 g (56%), b.p. 42—43 °C (cf. lit. data9). 1H NMR, δ:
4.52 (sept, 1 H, CH(CF3)2, J = 7 Hz). 19F NMR, δ: 11.2 (t, 6 F,
2 CF3, J = 7 Hz); 117.5 (m, 1 F, C(S)F).
References
1. M. S. Raasch, Chem. Commun., 1966, 577.
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2,4ꢀBis[bis(methoxycarbonyl)methylidene]ꢀ1,3ꢀdithiethane
(7). A mixture of tertꢀbutyl 1ꢀfluorovinylꢀ2,2ꢀbis(methoxyꢀ
carbonyl) sulfide (6) (7.5 g, 30 mmol) and P2O5 (12.8 g, 90 mmol)
was heated in vacuo (10 Torr) to 200 °C and the distilled prodꢀ
ucts were collected. The distillate crystallized, after which it was
washed with hexane, filtered off, and recrystallized from dioxꢀ
ane. Dithiethane 7 was obtained in a yield of 0.6 g (11.5%), m.p.
224—225 °C. Found (%): C, 41.07; H, 3.32. C12H12O8S2. Calꢀ
4. M. S. Raasch, J. Org. Chem., 1978, 43, 2500.
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1
culated (%): C, 41.38; H, 3.45. MS, m/z: 348 [M]+. H NMR
(DMSOꢀd6), δ: 3.92 (s, OMe).
Methyl 3,3,3ꢀtrifluoroꢀ2ꢀmethoxycarbonylthionopropionate
(8). Thioketene 1 (1.8 g, 10 mmol) was added dropwise with
stirring to MeOH (5 mL). The reaction mixture was kept at
30 min, poured into water, and extracted with Et2O. The orꢀ
ganic layer was separated, dried over Na2SO4, and fractionated.
Ester 8 was obtained in a yield of 1.5 g (71%), b.p. 39—40 °C
(3 Torr). Found (%): C, 33.47; H, 3.20. C6H7F3O3S. Calcuꢀ
lated (%): C, 33.33; H, 3.24. MS, m/z: 216 [M]+. 1H NMR
(DMSOꢀd6), δ: 3.77 (s, 3 H, OMe); 4.17 (s, 3 H, OMe); 5.46 (q,
1 H, CH, J = 9 Hz).
Received October 22, 2002;
in revised form January 28, 2003