Studies on reactions of thioketones with trimethyl(trifluoromethyl)silane catalyzed by fluoride ions

Article abstract of DOI:10.1002/1522-2675(200206)85:6<1644::AID-HLCA1644>3.0.CO;2-8

Treatment of 2,2,4,4-tetramethylcyclobutane-1,3-dione (6) in THF with CF₃SiMe₃ in the presence of tetrabutylammonium fluoride (TBAF) yielded the corresponding 3-(trifluoromethyl)-3-[(trimethylsilyl)oxy]cyclobutanone 7 (Scheme 1) via nucleophilic addition of a CF₃^- anion at the C=O group and subsequent silylation of the alcoholate. Under similar conditions, the 'monothione' 1 reacted to give thietane derivative 8 (Scheme 2), whereas in the case of 'dithione' 2 only the dispirodithietane 9, the dimer of 2, was formed (Scheme 3). A conceivable mechanism for the formation of 8 is the ring opening of the primarily formed CF₃ adduct A followed by ring closure via the S-atom (Scheme 2). In the case of thiobenzophenones 4, complex mixtures of products were obtained including diarylmethyl trifluoromethyl sulfide 10 and 1,1-diaryl-2,2-difluoroethene 11 (Scheme 4). Obviously, competing thiophilic and carbophilic addition of the CF₃^- anion took place. The reaction with 9H-fluorene-9-thione (5) yielded only 9,9′-bifluorenylidene (14; Scheme 6); this product was also formed when 5 was treated with TBAF alone. Treatment of 4a with TBAF in THF gave dibenzhydryl disulfide (15; Scheme 7), whereas, under similar conditions, 1 yielded the 3-oxopentanedithioate 17 (Scheme 9). The reaction of dithione 2 with TBAF led to the isomeric dithiolactone 16 (Scheme 8), and 3 was transformed into 1,2,4-trithiolane 18 (Scheme 10).

Full text of DOI:10.1002/1522-2675(200206)85:6<1644::AID-HLCA1644>3.0.CO;2-8

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Helvetica Chimica Acta Vol. 85 (2002)
Studies on Reactions of Thioketones with Trimethyl(trifluoromethyl)silane
Catalyzed by Fluoride Ions
by Grzegorz Mloston¬*1)2)
¬
¬
¬
¬
Section of Heteroorganic Compounds, University of Ëodz, Narutowicza 68, PL-90-136 Ëodz
and G. K. Surya Prakash and George A. Olah
Loker Hydrocarbon Institute, University of Southern California, Los Angeles, CA 90089
and Heinz Heimgartner*
Organisch-chemisches Institut der Universit‰t Z¸rich, Winterthurerstrasse 190, CH-8057 Z¸rich
Treatment of 2,2,4,4-tetramethylcyclobutane-1,3-dione (6) in THF with CF3SiMe3 in the presence of
tetrabutylammonium fluoride (TBAF) yielded the corresponding 3-(trifluoromethyl)-3-[(trimethylsilyl)oxy]-
cyclobutanone 7 (Scheme 1) via nucleophilic addition of a CFÀ3 anion at the CˆO group and subsequent
silylation of the alcoholate. Under similar conditions, the −monothione× 1 reacted to give thietane derivative 8
(Scheme 2), whereas in the case of −dithione× 2 only the dispirodithietane 9, the dimer of 2, was formed
(Scheme 3). A conceivable mechanism for the formation of 8 is the ring opening of the primarily formed CF3
adduct A followed by ring closure via the S-atom (Scheme 2). In the case of thiobenzophenones 4, complex
mixtures of products were obtained including diarylmethyl trifluoromethyl sulfide 10 and 1,1-diaryl-2,2-
difluoroethene 11 (Scheme 4). Obviously, competing thiophilic and carbophilic addition of the CFÀ3 anion took
place. The reaction with 9H-fluorene-9-thione (5) yielded only 9,9'-bifluorenylidene (14; Scheme 6); this
product was also formed when 5 was treated with TBAF alone. Treatment of 4a with TBAF in THF gave
dibenzhydryl disulfide (15; Scheme 7), whereas, under similar conditions, 1 yielded the 3-oxopentanedithioate
17 (Scheme 9). The reaction of dithione 2 with TBAF led to the isomeric dithiolactone 16 (Scheme 8), and 3 was
transformed into 1,2,4-trithiolane 18 (Scheme 10).
Introduction. In recent years, there has been a growing interest in fluorinated
organic compounds [1 3]. The presence of F-atoms, especially in CF3 groups, modifies
the physicochemical and biological properties of these materials [2 4]. One of the
most important features is their increased lipophilicity. Among several methods
applied for the preparation of fluorinated systems, the nucleophilic trifluoromethyla-
tion with trimethyl(trifluoromethyl)silane (CF3SiMe3, −Rupert×s compound× [5]) is the
method of choice for the introduction of a CF3 group [6]. In the presence of FÀ,
CF3SiMe3 reacts efficiently with ketones and aldehydes as well as with carboxylic acid
derivatives. Whereas reactions with carbonyl compounds give trifluoromethylated
alcohols [7][8], carboxylic esters are converted under controlled conditions to
trifluoromethyl ketones [9][10]3). In contrast to carbonyl compounds, imines are less
reactive towards CF3SiMe3, and special conditions are necessary to perform the
1
)
)
Sabbatical at the Loker Hydrocarbon Institute, USC, Los Angeles, CA 90089, 09.96 03.97.
Presented in preliminary form by G. M. at the 18th International Symposium on the Organic Chemistry of
Sulfur, Florence, 1998.
For reactions with other CˆO compounds see [6].
2
3
)

Helvetica Chimica Acta Vol. 85 (2002)
1645
reaction. In the presence of N-(trimethylsilyl)imidazole and CsF, the expected
secondary amines with the CF3 group at C(a) are obtained [11a]. A similar reaction
with 2H-azirines was also reported [11b]. Prakash et al. have reported on trifluoro-
methylation of sulfonaldimines as well as sulfinimines [11c f].
Fluorinated sulfurorganic compounds, especially trifluoromethyl sulfides, attract
attention as agrochemicals [12] and pharmaceuticals [13]. The main reason for the
biological activity of these compounds is their high lipophilicity; the CF3S group is
recognized as the most lipophilic substituent known [4b][14]. In a recent review, the
methods applied for the preparation of trifluoromethyl sulfides are summarized [15].
On the one hand, the CF3S group can be introduced by the reaction of CF3SCl or
CF3SSCF3 with organometallic reagents and, on the other hand, organic sulfides
undergo substitution reaction with CF3X (X ˆ Cl, Br, I) under different conditions
(e.g., phase-transfer catalysis). Another approach reported recently is based on the
reaction of CF3SiMe3 with disulfides or thiocyanates in the presence of FÀ ions
[16][17a]. Similarly, N-substituted trifluorothioacetamides were obtained in reactions
with isothiocyanates [17b]. Under these conditions, the nucleophilic CFÀ3 anion is the
reactive intermediate.
Thiocarbonyl compounds, especially thioketones, can also be considered useful
starting materials for the preparation of trifluoromethyl sulfides via thiophilic addition
of a CFÀ3 anion generated from CF3SiMe3. So far, very little is known about reactions of
thiocarbonyl compounds with silanes. However, two recent publications show that
aromatic thioketones and some sulfines undergo thiophilic addition with allyl(tri-
methyl)silane in the presence of tetrabutylammonium fluoride (TBAF) to afford allyl
sulfides and sulfoxides, respectively, in good yields [18]. These observations prompted
us to investigate the behavior of aromatic and cycloaliphatic thioketones towards
CF3SiMe3 with TBAFas a catalyst. As model compounds, we selected the easily available
and relatively stable 2,2,4,4-tetramethyl-3-thioxocyclobutanone (1), 2,2,4,4-tetrame-
thylcyclobutane-1,3-dithione (2), adamantanethione (3), thiobenzophenones 4a d,
and 9H-fluorene-9-thione (5). To compare the behavior of the thiones with that of the
parent ketone, 2,2,4,4-tetramethylcyclobutane-1,3-dione (6) was also included [18b].
R
S
S
S
X
Y
R
3
5
1 X = O, Y = S
2 X = Y = S
6 X = Y = O
4a R = H
b R = Me
c R = F
d R = MeO
Results and Discussion.
The original protocol applied for −nucleophilic
trifluoromethylation× of benzophenone, adamantanone, etc. with CF3SiMe3 and TBAF
[7] was modified by using commercially available TBAF solution (1m in THF) carefully
dried over activated molecular sieves (4 ä). This modification proved to be crucial for

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Helvetica Chimica Acta Vol. 85 (2002)
successful conversion of carboxylic esters to trifluoromethyl ketones [9] (cf. also [10]).
Under these conditions, the reaction of dione 6 with ca. 1.2 equiv. of CF3SiMe3 at ca. 08
was completed after 2 min, and no evolution of gaseous CHF3 was observed4). The
analysis of the crude mixture by 19F-NMR spectroscopy revealed the presence of two
signals at À 62.24 (minor) and À 67.70 ppm (major)5). A spectrum taken after 1 h
showed no significant changes of the mixture. The minor signal was indicative of
CF3SiMe3 and the major one corresponded to the 1:1 adduct 7 (Scheme 1), which was
isolated after aqueous workup in 74% yield. It is worth mentioning that the
cyclobutanone ring was conserved6). This result can be explained by the fact that the
initially formed alcoholate is silylated immediately [18b].
Scheme 1
CF3SiMe3
CF3
O
O
O
OSiMe3
TBAF, THF
0°
6
7
Under similar conditions, the reaction with monothione 1 was finished after 15 min
when the red color of the solution disappeared. The 19F-NMR spectrum of the crude
mixture indicated the formation of a new product containing a CF3 group absorbing at
1
À 67.50 ppm7). Unexpectedly, the H-NMR spectrum showed four signals for Me
groups at 1.70, 1.46, 1.41, and 1.40 ppm indicating that the strained cyclobutanone ring
was transformed into a new nonsymmetric system. Characteristic signals in the
13C-NMR spectrum are two singlets for olefinic C-atoms at 127.5 and 119.2 ppm. Based
on these data, the structure of thietane derivative 8 was proposed (Scheme 2). The
colorless product, isolated after chromatography in 70% yield, was found to be stable at
room temperature.
Analogous to the reaction with 6, the CFÀ3 anion generated in the system CF3SiMe3/
FÀ undergoes nucleophilic addition onto the more electrophilic CˆO group to give
intermediate A, which opens the ring to form the stabilized thiolate B. Ring closure via
the more nucleophilic S-atom leads to the formation of thietanolate C, which is trapped
by CF3SiMe3 to yield 8. As a consequence, the CFÀ3 anion is set free to start the next
reaction cycle. Analogous ring-opening/ring-closure processes leading to the four-
membered thiolactone isomer of 1 were reported to occur on treatment with NaH in
DMSO [20].
On treatment of dithione 2 with CF3SiMe3/TBAF at 08, the orange-red color of the
solution changed to pale red within 3 min, and a red solid precipitated. No new CF3-
containing product could be detected by 19F-NMR spectroscopy. The red solid isolated
by means of chromatography showed only one Me signal at 1.39 ppm. The 13C-NMR
spectrum indicated high symmetry in the molecule, which, according to its MS (m/z
4
)
It is known that TBAF is a very hygroscopic substance, and its commercial THF-solution contains a
significant amount of H2O, which immediately decomposes CF3SiMe3.
As internal standard, CFCl3 was used.
Under several basic conditions, ring-opening reactions of 6 were observed [19].
For the purified product in CDCl3, this signal appeared at À 76.04 ppm (see Exper. Part).
5
6
7
)
)
)

Helvetica Chimica Acta Vol. 85 (2002)
1647
Scheme 2
CF3SiMe3
O
O
S
S
O
S
F3C
TBAF, THF
0°
–Me3SiF
F3C
A
B
1
S
CF3SiMe3
O
Me3SiO
F3C
O
S
S
–
F3C
–CF3
F3C
8
C
B'
345), is a dimer of 2. All data confirm structure 9 for the new product8) (Scheme 3).
The formation of 9 is remarkable! In contrast to other thioketones, which
spontaneously form dimers and trimers [21][22], 2 is known not to undergo such
oligomerizations.
Scheme 3
CF3SiMe3
S
S
S
S
S
S
TBAF, THF
0°
2
9
In the case of adamantanethione (3), no reaction was observed even after 8 h
when it was treated with CF3SiMe3/TBAF under the above-mentioned conditions.
Aromatic thioketones 4 reacted very sluggishly with CF3SiMe3/TBAF, and a
conversion was achieved only under rigorous exclusion of moisture. Reactions were
carried out at room temperature, and, after 3 4 h, the initially blue solutions turned
yellow. The crude mixtures were analyzed quantitatively by 19F-NMR spectroscopy
with weighed amounts of trifluoromethylbenzene as the internal standard. In all cases,
two fluorinated products were present revealing 19F-resonances at ca. À 40 and À 90 ppm,
respectively9). The isolation of both remarkably nonpolar products was achieved by
means of preparative TLC. Yields of fluorinated products as established by NMR, were
not satisfactory (ca. 30 40% in total), and, therefore, it was presumed that some
8
)
)
The structure of the isomeric 1,2-dithietane (head-to-head dimer) can be excluded on the basis of the NMR
data.
In addition, several minor products containing F-atoms were detected but not identified.
9

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Helvetica Chimica Acta Vol. 85 (2002)
products were nonfluorinated. For this reason, the mixture obtained from the reaction
of 4a was thoroughly separated.
As the least polar fractions, the fluorinated products 10a and 11a were obtained,
followed by tetraphenylthiirane (12) and a small portion of tetraphenylethene (13)
(Scheme 4). The main fluorinated product 10a showed a signal in the 19F-NMR
spectrum at À 41.12 ppm, and this chemical shift is typical for the CF3S group [23]. This
substance is the expected product of the thiophilic addition of CFÀ3 onto 4a. The second
fluorinated product was identified as 11a (cf. [24]). Its formation can be explained by a
carbophilic addition of CFÀ3 , followed by elimination of FÀ by cyclization to give 2,2-
difluoro-3,3-diphenylthiirane, which spontaneously extrudes the S-atom. An analogous
conversion of this thiirane was observed earlier in the reaction of 4a with
difluorocarbene [24].
Scheme 4
CF3SiMe3
Ar
Ar
Ar
Ar
Ar
Ar
F
SCF3
H
S
TBAF, THF
r.t.
F
4a–d
10a–d
11a–d
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
S
12 Ar = Ph
13 Ar = Ph
a Ar = Ph b Ar = 4-MeC6H4 c Ar = 4-FC6H4 d Ar = 4-MeOC6H4
The formation of 12 in the reaction of 4a with CF3SiMe3/TBAF is rather
unexpected. Plausible mechanistic explanations are shown in Scheme 5. The inter-
mediate carbanion D, which is the precursor of 10a, can react with 4a to give G, which,
via an intramolecular SN2 reaction, forms the thiirane 12. On the other hand, a
dissociation of D leads to diphenylcarbene E, which intercepts 4a to give thiocarbonyl
ylide F that cyclizes to yield 12. Desulfurization of 12 under the reaction conditions
leads to 13.
When a THF solution of 5 and CF3SiMe3 was treated with TBAF, a vigorous reaction
ensued, the olive-green color of the solution changed immediately to orange, and 9,9'-
bifluorenylidene (14; Scheme 6) precipitated as an orange solid. No other product was
detected in the mixture, and the known 14 [25] was isolated in almost quantitative yield.
9H-Fluorene-9-thione (5) shows a strong tendency to undergo thiophilic reactions
with nucleophiles [26]. For this reason, a reaction of 5 with CFÀ3 can easily afford an
anion of type D (Scheme 5), which, subsequently, decomposes to give fluorenyli-
dene10). Its dimerization is probably the source of 1411).
10
)
)
A reaction mechanism without participation of CF3SiMe3 may also be taken into account (see below).
Many routes leading to 14 starting from 5 or its dimer (the hetero-Diels-Alder adduct [27]) are described
[28]. In these procedures, the corresponding thiirane was never observed. For preparation of the thiirane,
see [29].
11

Helvetica Chimica Acta Vol. 85 (2002)
1649
Scheme 5
Ph
Ph
Ph
S
CF3
–
CF3
F3CS
+
C
4a
C
Ph
D
E
4a
4a
Ph
Ph
Ph
Ph
F3CS
Ph
12
12
–F3CS
S
Ph
Ph
S
Ph
F
G
Scheme 6
S
CF3SiMe3
TBAF, THF
r.t.
5
14
In extension of the above described studies, we decided to investigate reactions of
thioketones mediated by FÀ. Despite the fact that the reactivity of thioketones has been
extensively studied [30], no reactions induced by FÀ have been reported so far. On the
other hand, FÀ gained great importance as a powerful catalyst in organic synthesis [31]
as shows properties of a strong base with poor nucleophilicity. In Pearson−s
classification, FÀ belongs to the hard bases [32].
The reactions described below were carried out using commercially available TBAF
dissolved in THF after rigorous drying over activated molecular sieves (4 ä).
Thiobenzophenone (4a) was treated with equimolar amounts of TBAF at room
temperature, and, after 12 h, the blue color of 4a disappeared. Chromatographic
workup gave a colorless solid as the main product12). During determination of the
melting point (153 1558), the substance turned blue indicating that 4a was formed by
thermal decomposition. The 1H-NMR spectrum shows a multiplet (7.40 7.30 ppm) for
aromatic H-atoms and a singlet at 4.80 ppm (ratio 10 :1). In the 13C-NMR spectrum, in
addition to only one set of signals for Ph, a signal for CH (DEPT analysis) appears at
.
‡
59.7 ppm. Whereas the EI-MS shows a peak with m/z 198 (82%) corresponding to M
of 4a, the CI-MS (NH3) indicated the presence of a dimeric compound ‡ 2 H-atoms
‡
(m/z 418, i.e., [M of 15 ‡ NH4] ). Finally, the structure of the product has been
unequivocally established by X-ray crystallography as dibenzhydryl disulfide (15,
Scheme 7, Figure).
12
)
Several minor products were also formed but not characterized.

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Scheme 7
TBAF
THF
Ph2CH
S
S CHPh2
4a
15
Ph2C
S
Ph2CH
S
H
I
Figure. ORTEP Plot [33] of the molecular structure of 15 (50% probability ellipsoids, arbitrary numbering of
atoms)
The formation of 15 from 4a has been described previously, e.g., by Staudinger and
Freudenberger on treatment with NaSH in EtOH [34]. Ohno et al. obtained this
compound in quantitative yield by treatment of 4a with EtONa in EtOH at room
temperature in the dark [35a]. A mechanism via a single-electron transfer (SET) to
give the radical anion H as an intermediate has been proposed. Dimerization followed
by protonation then yields 15. Alternatively, H can be protonated to give the radical I,
which dimerizes. The reaction of 4a with EtONa in THF also yielded 15 along with
benzhydryl sulfide as the main product. Alper and Paik reported the formation of 15
from 4a in the reaction with cyclopentadienylmetal carbonyl anions in THF [35b].
Similar to the previous paper, a SET-mechanism was postulated and the solvent was
indicated as the source of H-radicals. It is worth mentioning that a dithietane structure
(−head-to-tail× dimer) was ascribed to the product isolated after the treatment of 4a
with sodium acetylide in liquid NH3 [36a], but this conclusion is questionable, as the
reported melting point (152 1538) is almost identical to that of 15 (151 1528)

Helvetica Chimica Acta Vol. 85 (2002)
1651
reported by Ohno et al. [35a]13). The experiment was repeated by Schˆnberget al ., and
the product was described as dibenzhydryl (15) [36c]. The reproduced IR spectrum
(KBr) of the compound is in perfect agreement with that for product 15 isolated in our
experiment. Whereas no convincing data on the formation of 1,3-dithietanes by base-
catalyzed dimerization of aromatic thioketones are available, dimerization of some
aliphatic representatives to yield 1,3-dithietanes in the presence of pyridine was
reported [36c].
Under similar conditions to those described for 4a, 5 reacted immediately to give 14
(Scheme 6) as the sole product. Apparently, the reactive thioketone 5 behaves
identically in the presence of CF3SiMe3/TBAF or only TBAF.
Treatment of the cycloaliphatic dithione 2 with TBAF solution led to the formation
of a single product that was identified as the isomeric thietanethione 16 [37]
(Scheme 8). The reaction was completed after 5 min at 08 and afforded 16 in high yield.
The mechanistic pathway leading to 16 corresponds to earlier presented ring-opening/
ring-closure processes. In the present case, FÀ attacks the CˆS group in a carbophilic
fashion, in agreement with the HSAB concept, to give intermediate K, which
undergoes ring opening to L.
Scheme 8
F
F
F
S
S
2
S
S
S
-F
S
K
16
L
To our surprise, the reaction of TBAF with monothione 1 afforded a completely
1
different product. By means of H-NMR analysis of the crude mixture, dione 6 and a
new product 17 with a set of 3 singlets for Me groups and the typical signals for an i-Pr
group were present. After separation of the two products by chromatography, 17 was
obtained as a yellow oil, which solidified at room temperature. Based on EI-MS (m/z
314) and high-resolution (HR) MS (314.1370; C16H26O2S2), we concluded that the
molecule has been formed from two molecules of 1. The most surprising fact is the
presence of two additional H-atoms, which is unequivocally confirmed both by 1H- and
13C-NMR spectra. Taking into account all these data, the structure of the product can
tentatively be ascribed to 3-oxopentanedithioate 17 (Scheme 9)14). In analogy to the
reaction of 4a, a likely explanation is an initial SETwith participation of FÀ and THFas
the H donor.
The reaction of adamantanethione (3) with TBAF solution also led to an
astonishing result. After 4 h at room temperature, the reaction mixture was separated
chromatographically and colorless crystals of 1,2,4-trithiolane 18 were isolated as the
sole product (Scheme 10). Its structure was confirmed by comparison with an authentic
sample [22][38].
13
)
)
The information given in [21] concerning the dimer of thiobenzophenone is wrong; the indicated paper
relates to dimers of aliphatic thioketones exclusively (see [36b]).
At the moment, a convincing explanation for the formation of 17, as well as of 15, is missing. Further studies
are needed to gain more insight on the reactions of thioketones and the naked FÀ.
14

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Helvetica Chimica Acta Vol. 85 (2002)
Scheme 9
S
O
TBAF
THF
(H2O?)
S
O
S
O
O
O
H
H
6
17
1
Scheme 10
S
TBAF
THF
S
S
S
3
18
F
S
S
S
F
S
S
M
N
O
P
In the light of the known mechanism leading to 1,2,4-trithiolanes [38][39], the
formation of 18 results from the 1,3-dipolar cycloaddition of an intermediate M
(adamantanethione S-sulfide) with 3. The generation of M from 3 under the described
conditions is not clear15). However, it is likely that the thiophilic addition of FÀ to 3
gives an intermediate anion N, which, subsequently, decomposes to adamantylidene
(O) by transfer of FSÀ to another molecule of 3 to give P. By elimination of FÀ, the
latter converts to M.
Conclusions. The presented results show that in general thioketones are less
reactive towards CF3SiMe3 than ketones. The course of the reaction strongly depends
on the type of thioketone. Thiobenzophenones 4 react to give products that are formed
via addition of CF3À in the thiophilic and carbophilic mode. The anion generated by the
thiophilic addition is probably also the key intermediate for the formation of thiiranes.
Furthermore, thioketones were shown to undergo reactions with FÀ. The type of
products formed in TBAF solutions differs depending on the structure of the
thioketone. Explanations of reaction pathways leading to the isolated products require
the assumption of thiophilic as well as carbophilic addition of FÀ onto the CˆS group.
We thank the Polish State Committee for Scientific Research (Grant No. 3 TO9A 007 16) and F. Hoffmann-
La Roche AG, Basel, for financial support, Dr. A. Linden, University of Z¸rich, for performing the X-ray
crystal-structure determination, and the analytical laboratories of our institutes for spectra and analyses.
15
)
The formation of 18 from 3 has been observed under different conditions, e.g., on treatment with nitrile
sulfides or S8 [40], with organic azides [38][41], sodium thiophenolate [42], P4S10 [43], or with SiO2 [22].

Helvetica Chimica Acta Vol. 85 (2002)
1653
Experimental Part
1. General. M.p.: Melt-Temp.-II apparatus (Aldrich); in capillary; uncorrected. IR Spectra: Specord-75 IR
1
spectrometer; in KBr or neat for oily materials; absorptions in cmÀ1. H-, 19F-, and 13C-NMR Spectra: Varian
1
Gemini 200 or Varian VXR 200 instruments; in CDCl3; d in ppm rel. to TMS (ˆ0 ppm) for H and 13C or to
CFCl3 (ˆ0 ppm) for 19F. MS: Hewlett-Packard GC/MS 5890/5971A or Finnigan MAT INCOS 50. Elemental
analyses were performed at the Galbraith Laboratory (Knoxville, Tennessee).
2. StartingMaterials . Trimethyl(trifluoromethyl)silane (CF3SiMe3) was synthesized from bromo(trifluoro)-
methane and chloro(trimethyl)silane according to [44]. 2,2,4,4-Tetramethyl-3-thioxocyclobutanone (1) and
2,2,4,4-tetramethylcyclobutane-1,3-dithione (2) were obtained as a mixture after heating 2,2,4,4-tetramethylcy-
clobutane-1,3-dione (6) [45] with P4S10 in pyridine and were separated by chromatography (see [46]).
Adamantanethione (3) was prepared from adamantanone and P4S10 [47]. 9H-Fluorene-9-thione (5) was
synthesized from 9H-fluoren-9-one by treatment with HCl and H2S [48] (for a modified procedure, see [28]).
Thiobenzophenone (4a) and its derivatives 4b, 4c, and 4d were prepared from the corresponding ketones by
heating in toluene soln. with Lawesson×s reagent (LR) according to [49] and were purified by chromatography
on SiO2. Activated molecular sieves (4 ä) were prepared by heating small portions (2 3 g) of commercial
sieves in a round-bottomed flask with a flame at ca. 10À1 Torr for ca. 10 min and were immediately used for the
reaction. The THF soln. of tetrabutylammonium fluoride (TBAF, 1m) was purchased from Sigma-Aldrich.
3. Fluoride-Ion-Mediated Reactions with CF3SiMe3. 3.1. Reaction with 6. To a soln. of 6 (280 mg, 2 mmol)
and CF3SiMe3 (355 mg, 2.5 mmol) in abs. THF (2 ml) in a 25-ml round bottomed flask (N2 atmosphere)
equipped with a magnetic stirrer were added ca. 300 mg of freshly activated molecular sieves (4 ä)16). The flask
was closed with a rubber septum and stirred at r.t. for 2 h. In another flask, similarly equipped with a magnetic
stirrer, a diluted soln. of TBAF (0.5 ml of commercial 1m soln. in THF was diluted with 2 ml of abs. THF) was
treated under N2 with ca. 300 mg of freshly activated molecular sieves (4 ä). The flask was closed with a rubber
septum, and the soln. was stirred magnetically for 2 h. Then, by means of a syringe 0.25 ml of the predried soln.
containing ca. 0.05 mmol TBAF was added dropwise to the soln. of 6, cooled with an ice bath. No evolution of
gaseous products was observed and, after 2 min, a sample was taken via syringe to check the progress of reaction
(19F-NMR). A small excess of CF3SiMe3 was evidenced by the presence of an s at À 62.24 ppm; another s at
À 67.70 ppm was assigned to the product 7. After 1 h, the mixture showed virtually the same ratio of CF3SiMe3
and 7. The mixture was diluted with 20 ml of CH2Cl2 and washed with 20 ml of H2O (3Â). The org. phase was
dried (MgSO4), and the solvent was evaporated to give 450 mg of a colorless oil. After microdistillation, an anal.
pure product was isolated as a colorless liquid.
2,2,4,4-Tetramethyl-3-(trifluoromethyl)-3-[(trimethylsilyl)oxy]cyclobutanone (7): 420 mg (74%). Colorless
liquid, distilled at 95 978/20 Torr. IR (neat): 2971s, 1783vs (CˆO), 1472s, 1256vs, 1026vs, 874s, 839vs. 1H-NMR:
1.26( s, 2 Me); 1.23 (q, J(H,F) ˆ 1.8, 2 Me); 0.18 (q, J(H,F) ˆ 0.6, Me3Si). 13C-NMR: 218.9 (CˆO); 125.6( q,
1J(C,F) ˆ 284.4, CF3); 79.9 (q, 2J(C,F) ˆ 28.1, C(3)); 63.0 (C(2), C(4)); 22.1 (2 Me); 18.8 (q, J(C,F) ˆ 3.2, 2 Me);
‡
1.4 (q, J(C,F) ˆ 1.8, Me3Si). 19F-NMR: À 67.7 (CF3). EI-MS: 267 (4, [M À Me] ), 213 (7), 212 (53, [M À
.
.
‡
‡
Me2CˆCˆO] ), 120 (39), 77 (41), 70 (100, [Me2CˆCˆO] ). Anal. calc. for C12H21F3O2Si (282.38): C 51.04,
H 7.50; found: C 50.98, H 7.32.
3.2. Reaction with 1. According to the procedure in 3.1, 1 (312 mg, 2 mmol) was reacted with CF3SiMe3
(355 mg, 2.5 mmol). After 15 min at 0 58, the reaction was finished and, after aq. workup and evaporation of
CH2Cl2, 415 mg of a colorless oil were obtained. Anal. pure product, 8, was obtained by distillation at ca. 20 Torr.
2-Isopropylidene-3,3-dimethyl-4-(trifluoromethyl)-4-[(trimethylsilyl)oxy]thietane (8): 415 mg (70%). Col-
orless liquid, distilled at 108 1108/20 Torr. IR (neat): 2950s, 1280s, 1240s, 1180vs, 1130s, 1080s, 850vs. 1H-NMR:
1.70, 1.46, 1.41 (3s, 3 Me); 1.40 (q, J(H,F) ˆ 1.6, MeÀC(3) cis to CF3); 0.22 (s, Me3Si). 13C-NMR: 124.6( q,
1J(C,F) ˆ 282.7, CF3); 127.5, 119.2 (CˆC); 84.2 (q, 2J(C,F) ˆ 30.3, C(4)); 58.5 (C(3)); 24.9, 22.4 (br.); 21.4, 19.4
.
‡
‡
(4 Me); 1.20 (Me3Si). 19F-NMR: À 76.04 (s, CF3). EI-MS: 298 (56, M ), 283 (23, [M À Me] ), 209 (22), 187
(21), 127 (17), 96(100), 81 (55). Anal. calc. for C 12H21F3OSSi (298.44): C 48.30, H 7.09, F 19.10, S 10.74; found:
C 48.70, H 7.28, F 18.84, S 10.95.
3.3. Reaction with 2. According to the general procedure in 3.1, 2 (328 mg, 2 mmol) was reacted with
CF3SiMe3 (355 mg, 2.5 mmol). After 3 min at 0 58, the ruby-red color of the mixture changed to pale red, and a
colored solid precipitated from the THF soln., which dissolved completely after warming the soln. to r.t. After
16
)
After activation, molecular sieves were immediately transferred to the reaction flask and used for drying of
the TBAF soln. All operations have been carried out under a stream of N2 to prevent contact with air
moisture.

1654
Helvetica Chimica Acta Vol. 85 (2002)
typical aq. workup and evaporation of the org. solvents, the red residue (310 mg) was chromatographed on prep.
TLC plates (SiO2; petroleum ether/CH2Cl2 9 :1) to afford 240 mg of a red crystalline material. Anal. pure
product, 9, was obtained after crystallization.
1,1,3,3,7,7,9,9-Octamethyl-5,10-dithiadispiro[3.1.3.1]decane-2,8-dithione (9): 240 mg (70%). Red needles.
M.p. 194 1958 (hexane). IR (KBr): 2968s, 2920m, 1448s, 1377m, 1360m, 1302s, 1146m (CˆS), 924s. 1H-NMR:
1.39 (s, 8 Me). 13C-NMR: 280.3 (CˆS); 68.0 (C(1), C(3), C(7), C(9)); 55.1 (C(4), C(6)); 26.0 (8 Me). EI-MS:
.
‡
345 (100, M ), 173 (11), 172 (7), 96(11). HR-MS: calc. for C 16H24S4: 345.083913; found: 345.085700. Anal. calc.
for C16H24S4 (344.63): C 55.77, H 7.02, S 37.21; found: C 55.92, H 7.03, S 37.70.
3.4. Attempted Reaction with 3. According to the general procedure in 3.1, after 8 h at r.t. the orange color of
the soln. was unchanged. After dilution with CH2Cl2, washing with H2O, drying (MgSO4), and evaporation, the
orange residue obtained was identified as unchanged 3 (13C-NMR).
3.5. Reaction with 4a d. In all cases, the reaction was carried out according to the general procedure. After
3
4 h, the initially blue-colored soln. turned yellow. A sample of decolorized mixture was diluted with CDCl3
and examined by 19F-NMR spectroscopy. Two major fluorinated products revealing absorptions at ca. À 40 and
À 90 ppm, resp., were found in all cases and left unchanged after the aq. workup. The yields of fluorinated
products 10 and 11 were established directly in the crude mixture by standard NMR analysis with a weighed
portion of a,a,a-trifluorotoluene. The mixtures were separated on prep. TLC plates (SiO2; pentane or pentane
with increasing amount of CH2Cl2). In the cases of 4b d, separation was limited to fluorinated products. As a
rule, geminal-difluoroethenes 11 were isolated as the less polar fractions.
Diphenylmethyl Trifluoromethyl Sulfide (10a): Yield: crude 10a 30%; isolated 78 mg (22%). Colorless
liquid, b.p. 105 1088/0.75 Torr. 1H-NMR: 7.40 7.36, 7.24 7.21 (m, 10 arom. H); 5.67 (br. s, CH). 13C-NMR:
139.2 (2 arom. C); 130.1 (q, 1J(C,F) ˆ 306.4, CF3); 128.8, 128.2, 127.9 (10 arom. CH); 53.5 (CH). 19F-NMR:
.
‡
‡
À 41.12 (CF3S). EI-MS: 268 (0.3, M ), 167 (100, [M À CF3] ), 165 (34), 152 (15). Anal. calc. for C14H11F3S
(268.30): C 62.67, H 4.13, S 11.95; found: C 62.73, H 4.22, S 12.07.
1,1-Difluoro-2,2-diphenylethene (11a). Yield: crude 11a 8%; isolated 15 mg (6%). Colorless liquid, b.p. 92
948/0.75 Torr ([50]: 91 978/1.5 Torr). 13C-NMR: 153.8 (t, 1J(C,F) ˆ 291.2, CF2); 134.3 (2 arom. C); 129.6, 128.4,
127.5 (10 arom. CH); 97.3 (t, 2J(C,F) ˆ 17.5, CˆCF2). 19F-NMR: À 88.32 (CF2).
2,2,3,3-Tetraphenylthiirane (12): 109 mg (45%), after chromatography with pentane/CH2Cl2 8 :2. Colorless
crystals, m.p. 172 1748 (MeOH) ([50]: 177 1798). 13C-NMR: 140.1 (1 arom. C); 130.9, 127.1, 126.6 (10 arom.
CH); 66.6 (C(2), C(3)).
1,1,2,2-Tetraphenylethene (13): 12 mg (3%), after chromatography with pentane/CH2Cl2 8 :2. Colorless
crystals. M.p. 201 2068 ([50]: 203 2058).
Bis(4-methylphenyl)methyl Trifluoromethyl Sulfide (10b): Yield: crude 10b 26%; isolated 84 mg (21%).
Colorless oil, purified by distillation at 105 1088/0.75 Torr. 13C-NMR: 137.6, 136.4 (4 arom. C); 131.6 (q,
1J(C,F) ˆ 306.4, CF3); 129.4, 128.0 (8 arom. CH); 53.1 (br., CH); 21.0 (2 Me). 19F-NMR: À 40.72 (CF3S). Anal.
calc. for C16H15F3S (296.35): C 64.85, H 5.10, S 10.82; found: C 64.79, H 5.13, S 10.28.
1,1-Difluoro-2,2-bis(4-methylphenyl)ethene (11b). Yield: crude 11b 15%; isolated 29 mg (9%). Colorless
needles. M.p. 54 568 (MeOH). 13C-NMR: 153.6( t, 1J(C,F) ˆ 290.3, CF2); 137.2, 131.5 (4 arom. C); 129.4, 128.9
(8 arom CH); 95.9 (t, 2J(C,F) ˆ 18.0, CˆCF2); 21.1 (2 Me). 19F-NMR: À 89.17 (CF2). Anal. calc. for C16H14F2
(244.29): C 78.67, H 5.78, F 15.55; found: C 79.09, H 6.11, F 15.14.
Bis(4-fluorophenyl)methyl Trifluoromethyl Sulfide (10c): Yield: crude 10c 28%; isolated 110 mg (20%),
after chromatography and distillation at 90 938/0.75 Torr. 13C-NMR: 162.3 (d, 1J(C,F) ˆ 246.3, 2 arom. CF);
134.7 (2 arom. C); 129.7 (q, 1J(C,F) ˆ 306.7, CF3); 129.8, 116.0 (8 arom. CH); 52.0 (CH). Anal. calc. for
C14H9F5S (304.28): C 55.26, H 2.98, S 10.54; found: C 54.97, H 2.91, S 10.45.
1,1-Difluoro-2,2-bis(4-fluorophenyl)ethene (11c): Yield: crude 11c 5%; not isolated in pure form. Identified
in the mixture with 10c after attempted chromatographic separation on prep. TLC plates. 19F-NMR: À 88.46
(CF2).
Bis(4-methoxyphenyl)methyl Trifluoromethyl Sulfide (10d). Yield: crude 10d 29%; isolated 92 mg (21%).
1
Colorless crystals, m.p. 36 38 8 (MeOH). 13C-NMR: 159.1, 131.3 (4 arom. C); 129.9 (q, J(C,F) ˆ 306.0, CF3);
129.2, 113.7 (8 arom. CH); 55.1 (MeO); 52.6(br., CH). Anal. calc. for C 16H15F3O2S (328.35): C 58.53, H 4.60, S
9.77; found: C 58.81, H 4.68, S 9.89.
1,1-Difluoro-2,2-bis(4-methoxyphenyl)ethene (11d): Yield: crude 11d 7%; isolated 26mg (5%). Colorless
crystals. M.p. 50 528 (pentane) ([51]: 53 548). 19F-NMR: À 90.38 (CF2).
4. Fluoride-Ion-Induced Conversions of Thioketones. General Procedure. A mixture of 2 mmol of the
corresponding thioketone in 2 ml of abs. THF and ca. 300 mg of freshly activated molecular sieves (4 ä) was
stirred in a Schlenk flask. In a second Schlenk flask, 5 ml of commercial TBAF soln. in THF was diluted with

Helvetica Chimica Acta Vol. 85 (2002)
1655
3 ml of abs. THF and dried with activated molecular sieves. After 2 h at r.t., 3 ml of predried TBAF soln. (ca.
1.5 mmol. TBAF) were added portionwise by means of a syringe to the soln. of a thioketone cooled to 0 58. The
cooled soln. was stirred for 30 min, then the cooling bath was removed, and stirring was continued at r.t., until
the initial color of thioketone disappeared or completely changed. The mixture was diluted with 20 ml of
CH2Cl2, washed with H2O (29 ml, 3 Â ), and the org. phase was dried and evaporated. Separation of products
was achieved by means of chromatographic techniques, followed by crystallization. Reported yields refer to
amounts obtained after chromatography.
4.1. Reaction with 4a. Reaction time 12 h; chromatography: SiO2, petroleum ether. Bis(diphenylmethyl)
Disulfide (15): 195 mg (49%). Colorless crystals, m.p. 153 1558 (MeOH/CH2Cl2) ([34]: 151 1528). IR: 3026w,
1
1495m, 1450m, 1082w, 1030w, 750m, 721m, 700s, 6 27m. H-NMR: 7.40 7.30 (m, 20 arom. H); 4.80 (s, 2 CH).
.
13C-NMR: 140.1 (2 arom. C); 128.9, 128.3, 127.4 (10 arom. CH); 59.7 (2 CH). EI-MS: 198 (82, Ph2CS ), 165
‡
.
‡
‡
(100), 121 (93), 77 (43). CI-MS (NH3): 417 (7), 416(21, [ M ‡ NH4] ), 352 (10), 199 (14, Ph2CHS ), 184 (5),
182 (6), 168 (14), 167 (100). Anal. calc. for C26H22S2 (398.59): C 78.35, H 5.56, S 16.09; found: C 77.95, H 5.65, S
16.73.
4.2. Reaction with 5. The first drop of TBAF soln. initiated a vigorous reaction to give an orange-colored
product. Chromatographic workup (SiO2 column, hexane/CH2Cl2 8 :2) gave 198 mg (60%) of 9-(9H-fluoren-9-
ylidene)-9H-fluorene (14): Orange crystals. M.p. 182 1858 (Et2O) ([25]: 182 1878).
4.3. Reaction with 2. Five min after the addition of the TBAF soln., the red color of the mixture changed to
orange, and TLC revealed complete conversion of 2. The sole product of the reaction was isolated
chromatographically (prep. TLC plates, SiO2; petroleum ether/CH2Cl2 9 :1). Physical and spectral properties
matched those reported for 4-isopropylidene-3,3-dimethylthietane-2-thione (16) [20]. Yield: 293 mg (85%).
Orange oil, distilled at 108 1108/12 Torr ([20]: 112 116/20 Torr). 1H-NMR: 1.80 (s, Me); 1.6 7 (s, Me); 1.40
(s, 2 Me). 13C-NMR: 243.0 (CˆS); 130.7, 122.3 (CˆC); 73.9 (C(3)); 25.0 (2 Me); 21.7 (Me); 19.9 (Me).
4.4. Reaction with 1. After 3 h, the red color of the soln. changed to yellow-green, and the conversion of 1
was complete (TLC). After typical workup, the crude mixture was examined by means of 1H-NMR
spectroscopy. The spectrum revealed an s at 1.32 ppm (assigned to 6) along with three other s at 1.64, 1.43, and
1.17. The ratio of 6 and the new product, established on the basis of integrals of Me groups, was ca. 1:1. The
mixture was separated (SiO2 column; petroleum ether with increasing amount of CH2Cl2), and two fractions
were isolated.
Compound 6: Chromatography with petroleum ether/CH2Cl2 1:1:37 mg (12%). Colorless crystals,
identical with an authentic sample [45].
2,2,4,4-Tetramethyl-3-oxocyclobutyl 2,2,4-Trimethyl-3-oxopentanedithioate (17): 147 mg (47%), isolated as
a slightly more polar fraction than 6, with petroleum ether/CH2Cl2 1:1 and purified by crystallization. Yellow
crystals. M.p. 62 638 (hexane at À 208). 1H-NMR: 4.20 (s, HÀC(3)); 2.84 (sept., J ˆ 6.6, Me2CH); 1.64
(s, 2 Me); 1.43 (s, 2 Me); 1.17 (s, 2 Me); 1.09 (d, J ˆ 6.6, Me2CH). 13C-NMR: 241.0 (CˆS); 218.6(C ˆO,
cyclobutanone); 211.8 (C(3)ˆO); 69.3, 61.0 (C(1'), C(2)); 57.5 (C(2'), C(4')); 36.9 (C(4)); 26.7 (2 Me); 24.9
.
.
‡
‡
‡
(2 Me); 21.7 (2 Me); 20.3 (2 Me). EI-MS: 314 (3, M ), 299 (2, [M À Me] ), 244 (15, [M À Me2CˆCˆO] ),
174 (18), 173 (19), 158 (21), 131 (5), 98 (15), 97 (100), 87 (8). HR-MS: Calc. for C16H26O2S2: 314.13724; found:
314.13700. Anal. calc. for C16H26O2S (314.50): C 61.11, H 8.33, S 20.39; found: C 61.45, H 8.57, S 20.89.
4.5. Reaction with 3: The reaction was complete after 1 h at 0 58 and 4 h at r.t., and the mixture was
separated chromatographically.
Dispiro[adamantane-2,2'-[1,2,4]trithiolane-5',2''-adamantane] (18): 233 mg (64%). Colorless prisms. M.p.
203 2068 (EtOH/CH2Cl2) ([52]: 193 1958). Identified by comparison of the 13C-NMR spectra with that of an
authentic sample.
5. X-Ray Crystal-Structure Determination of 1517). All measurements were made on a Nonius KappaCCD
diffractometer [53] with graphite-monochromated MoKa radiation (l 0.71073 ä) and an Oxford Cryosystems
Cryostream 700 cooler. The data collection and refinement parameters are given in the Table, and a view of the
molecule is shown in the Figure. Data reduction was performed with HKL Denzo and Scalepack [54]. The
intensities were corrected for Lorentz and polarization effects, and an absorption correction based on the multi-
scan method [55] was applied. The structure was solved by direct methods using SIR92 [56], which revealed the
17
)
Crystallographic data (excluding structure factors) for the structure of 15 reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC-
179554. Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: ‡44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).

1656
Helvetica Chimica Acta Vol. 85 (2002)
positions of all non-H-atoms. The molecule has C2 symmetry. The non-H-atoms were refined anisotropically. All
of the H-atoms were fixed in geometrically calculated positions (d(CÀH) ˆ 0.95 ä), and each was assigned a
fixed isotropic displacement parameter with a value equal to 1.2 Ueq of its parent C-atom. Refinement of the
structure was carried out on F using full-matrix least-squares procedures, which minimized the function Sw(j Fo j
Àj Fc j )2. A correction for secondary extinction was not applied. The absolute structure was determined by
refinement of the absolute structure parameters [57], which yielded a value of 0.01(4). Neutral-atom scattering
factors for non-H-atoms were taken from [58a] and the scattering factors for H-atoms from [59]. Anomalous
dispersion effects were included in Fc [60]; the values for f ' and f '' were those of [58b], and the values of the
mass attenuation coefficients were those of [58c]. All calculations were performed using the teXsan
crystallographic software package [61].
Table. Crystallographic Data of 15
Crystallized from
Empirical formula
AcOH
C26H22S2
398.58
Mr [g molÀ1
]
Crystal color, habit
Crystal dimensions [mm]
Temp. [K]
colorless, needle
0.08 Â 0.20 Â 0.35
160(1)
Crystal system
Space group
monoclinic
C2
Z
2
Reflections for cell determination
16093
2q range for cell determination [8]
Unit-cell parameters a [ä]
4 À 60
17.7978(3)
5.7598(1)
12.3326(2)
124.5945(9)
1040.71(3)
1.272
b [ä]
c [ä]
b [8]
V [ä3]
Dx [g cmÀ3
]
m(MoKa) [mmÀ1
Scan type
]
0.264
f and w
6
2q(max) [8]
0
Transmission factors (min; max)
Total reflections measured
Symmetry independent reflections
Reflections used [I > 2s(I)]
Parameters refined
0.889; 0.985
15892
3017
2847
127
Final R
0.0330
0.0364
2.270
wR (w ˆ [s2(Fo) ‡ (0.005Fo)2]À1
)
Goodness-of-fit
Final Dmax/s
D1 (max; min) [e ¥ äÀ3
0.0006
0.20; À 0.21
]
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Received March 12, 2002