Comparison of tris(dimethylphenylsilyl)methyllithium and tris(trimethylsilyl)methyllithium reactions with carbon disulfide: A new route for the synthesis of some 1,1-bis(benzylthio) and bis(allylthio)-2,2- bis(dimethylphenylsilyl)ethenes

Article abstract of DOI:10.1080/17415993.2013.879307

A series of bis(benzylthio) and bis(allylthio)-2,2-bis(dimethylphenylsilyl) ethenes has been obtained by the reaction of (PhMe₂Si) ₃CLi and CS₂ with benzyl and allyl bromides. A plausible mechanism for the formation of these compounds has been proposed. The reactivity of (PhMe₂Si)₃CLi toward CS₂ has been compared with that of (Me₃Si)₃CLi. The reaction of (PhMe ₂Si)₃CLi with CS₂ and 2-methyloxirane gave a cyclic thiocarbonate(equation presented). 2014

Full text of DOI:10.1080/17415993.2013.879307

Journal of Sulfur Chemistry, 2014
Vol. 35, No. 3, 304–311, http://dx.doi.org/10.1080/17415993.2013.879307
Comparison of tris(dimethylphenylsilyl)methyllithium and
tris(trimethylsilyl)methyllithium reactions with carbon disulfide:
a new route for the synthesis of some 1,1-bis(benzylthio)- and
bis(allylthio)-2,2-bis(dimethylphenylsilyl)ethenes
Kazem D. Safa∗ and Khatereh Ghorbanpour
Organosilicon Research Laboratory, Faculty of Chemistry, University of Tabriz, 51664 Tabriz, Iran
(Received 1 October 2013; final version received 24 December 2013)
A series of bis(benzylthio)- and bis(allylthio)-2,2-bis(dimethylphenylsilyl) ethenes has been obtained by
the reaction of (PhMe2Si)3CLi and CS2 with benzyl and allyl bromides. A plausible mechanism for the
formation of these compounds has been proposed. The reactivity of (PhMe2Si)3CLi toward CS2 has been
compared with that of (Me3Si)3CLi. The reaction of (PhMe2Si)3CLi with CS2 and 2-methyloxirane gave
a cyclic thiocarbonate.
Keywords: organosilicon-sulfur;mercaptobis-(trimethylsilyl)-thione;tris(dimethylphenylsilyl)methyllithium;
tris(trimethylsilyl)methyllithium; carbon disulfide
1. Introduction
Vinyl silanes and vinyl sulfides have received significant attention as potential synthons in organic
synthesis, because the C(sp2)-Si and C(sp2)-S bonds undergo numerous transformations.[1–4]
We have recently reported a convenient route for the synthesis of 1,1-bis(trimethylsilyl)alkenes
=
by the reaction of (RMe2Si)3CLi (R H or Me) with non-enolisable aldehydes and investigated the
nucleophilic and electrophilic reactions of them.[3,5–8] In addition, we have compared the reac-
tivity of various (RMe2Si)3CLi such as (Me3Si)3CLi and (PhMe2Si)3CLi with electrophiles.[9]
Organosilicon compounds containing bulky groups – tris(trimethylsilyl)methyl (Me3Si)3C- and
tris(dimethylphenylsilyl)methyl (PhMe2Si)3C – exhibit unusual reactivity and novel structures
because of the severe steric hindrance at the centers to which they are attached. (PhMe2Si)3CLi
∗Corresponding author. Email: dsafa@tabrizu.ac.ir
© 2014 Taylor & Francis

Journal of Sulfur Chemistry 305
is produced in good yield, but has abnormally low reactivity. For example, it does not react with
aldehydes, whereas (Me3Si)3CLi gives 1,1-bis(trimethylsilyl)alkenes.[3,6]
We have also reported the reaction of (Me3Si)3CLi with carbon disulfide in the presence of
various electrophiles such as allyl, benzyl and alkyl halides and synthesized new mercaptobis-
(trimethylsilyl)-thiones by unusual rearrangements.[10,11] To explore the versatility of these
reactions and compare the reactivity of (PhMe2Si)3CLi and (Me3Si)3CLi toward carbon disulfide,
we have investigated the reaction of (PhMe2Si)3CLi and CS2 with halides such as allyl and benzyl
bromides and electrophiles such as epoxides, aldehydes and acyl chlorides.
2. Results and discussion
It was shown previously that reactions of (Me3Si)3CLi and CS2 with alkyl iodides, allyl and benzyl
bromides gave mercaptobis-(trimethylsilyl)-thiones (1) by an unusual rearrangement (Scheme 1).
These compounds were obtained in high yields and short reaction times.[10,11]
Scheme 1. Reaction of (Me3Si)3CLi and CS2 with alkyl iodides, allyl and benzyl bromides.
In a logical extension of our initial study, it was envisaged that the reaction of (PhMe2Si)3CLi
and CS2 with alkyl, allyl and benzyl halides would give mercaptobis-(triorganosilyl)-thiones.
However, contrary to our expectation, the desired compounds (1) were not obtained, and the major
products were the 1,1-bis(benzylthio)- or bis(allylthio)-2,2-bis(dimethylphenylsilyl)ethenes (2)
(Scheme 2).
Scheme 2. Reaction of (PhMe2Si)3CLi and CS2 with some halides.
Tris(dimethylphenylsilyl)methane, (PhMe2Si)3CH, obtained via the reaction of PhMe2SiCl
and CHBr3 with n-BuLi at −78◦C, was metalated with MeLi under the conditions used for
(Me3Si)3CH in the previous study. The much lower reactivity of (PhMe2Si)3CLi, compared with
that of (Me3Si)3CLi, was attributed to steric hindrance from the phenyl groups, which shield the
−
Li C bonds from attack.[9]
The reaction of (PhMe2Si)3CLi and CS2 with various halides was studied (Scheme 2). The
reaction of 4-bromobenzyl bromide with (PhMe2Si)3CLi and CS2 was chosen as a model for

306 K.D. Safa and K. Ghorbanpour
Table 1. Investigation of temperature and time effects on the reaction of (PhMe2Si)3CLi, CS2 and 4-bromobenzyl
bromide.
Entry
Temperature (◦C)
Timea
Yieldb (%)
1
2
3
4
5
−46
25
25
0
2 h
15 min
48 h
15 min
2 h
–
50
30
60
60
0
aThe reactions were followed by TLC.
bYields obtained by PTLC.
optimization (Table 1). This reaction was carried out at different temperatures and for various
times. The best conditions were found to be 0◦C and 15 min. As can be seen from Table 1, at
−46◦C (cyclohexanone/N2), no reaction was observed, but at 0◦C (Entries 4 and 5), the reaction
was complete after 15 min. At room temperature, the reaction was complete in 15 min (Entry 2)
and after that unidentified compounds were formed (Entry 3).
After optimizing the reaction conditions, we examined the generality of these conditions in reac-
tions with various benzyl bromides, alkyl halides, acetyl chloride and allyl bromide. The results
are summarized in Table 2. In the case of allyl and benzyl bromides, the products were obtained
in good yields. In contrast, no reaction was observed with benzyl chloride, alkyl iodides, alkyl
bromides and acetyl chloride. When carbon disulfide was added to a solution of (PhMe2Si)3CLi
at 0◦C and the mixture was stirred at the same temperature for 5 min, the yellowish mixture
became deep red indicating the formation of new anion. When the electrophile was added,
the color change indicated the formation of compound 2. In the case of benzyl chloride, alkyl
iodides, alkyl bromides and acetyl chloride, the deep red color persisted. In an attempt to sep-
arate the intermediates, the anion resulting from initial attack of (PhMe2Si)3CLi on CS2 was
immediately protonated by hydrolysis at 0◦C. Unfortunately, we were unable to trap the pre-
−
sumed intermediates and separate a specific compound under these conditions. The GC MS
spectrum of+this reaction mixture showed peaks with m/z (EI): 286 ([PhMe2SiOSiMe2Ph]+), 271
−
([286 Me] ), that the intermediates were not stable and decomposed to 1,1,3,3-tetramethyl-1,3-
diphenyldisiloxane.
The mechanism in Scheme 3 is proposed on the basis of spectral data and previous works
(Scheme 3).[10–18] It involves the initial nucleophilic attack of (PhMe2Si)3CLi on the carbon
of CS2(A) and migration of an SiMe2Ph group to sulfur (B), then the formation of 1,1-
bis(silyl)enethiolates (C) and (E). Intermediates (C) and (E) react with allyl and benzyl bromides
to produce compounds 2.
(PhMe2Si)3CLi was also treated with CS2 and 2-methyloxirane (Scheme 4). When a solution of
(PhMe2Si)3CLi in THF was generated from the reaction of (PhMe2Si)3CH with MeLi. CS2 was
added at 0◦C and the mixture stirred at the same temperature for 5 min. The yellowish mixture
became deep red indicating the formation of new anion. Then, the epoxide was added and a color
change was observed. At 0◦C, the mixture was stirred for 15 min and compound 3 was formed
in 80% yield. At the end of the reaction, as monitored by TLC in n-hexane: ethyl acetate 9:1,
compound 3 and (PhMe2Si)3CH were separated but no other product was observed. The structure
of the previously synthesized 3 was determined from its 1H and 13C NMR spectra.

Journal of Sulfur Chemistry 307
Table 2. Reaction of (PhMe2Si)3CLi and CS2 with various halides.
RX
Temperature (◦C) Time (min)
Product
Yielda (%)
60
0
15
Br
Br
0
15
60
55
Br
0
0
15
15
Br
Br
35
Br
Br
0
0
60
10
–
–
Cl
66
H
C
Br
H2C
CH3I
CH3I
CH3CH2Br
CH3COCl
0
25
0
60
60
60
60
–
–
–
–
–
–
–
–
0
aYields obtained by PTLC.
One of the most efficient methods for synthesizing cyclic thiocarbonates is from an epox-
ide and carbon disulfide with an appropriate catalyst.[19–21] It is likely that in our reaction,
(PhMe2Si)3CLi activates the CS2 to react with the epoxide to give the cyclic thiocarbonate.
=
In previous work, the reaction of (RMe2Si)3CLi (R Me or Ph) with aldehydes was investi-
gated. It was found that (Me3Si)3CLi gave 1,1-bis(trimethylsilyl)alkenes under reflux conditions,
but with (PhMe2Si)3CLi no reaction was observed.[3,6] Therefore, we studied the reactions
=
of (RMe2Si)3CLi (R Me or Ph) with aldehydes in the presence of CS2. In the reaction of
(PhMe2Si)3CLi and CS2 with aldehydes no product was formed. However, the reaction of
(Me3Si)3CLi with CS2 and aldehydes at −78◦C gave 1,1-bis(trimethylsilyl)vinyl derivatives 4
in high yield and very short periods of time (Path 2). Without CS2 these reactions required reflux
conditions for long periods (Path 1). The results are compared in Scheme 5.

308 K.D. Safa and K. Ghorbanpour
Scheme 3. The proposed mechanism for the reaction of (PhMe2Si)3CLi and CS2 with allyl and
benzyl bromides.
Scheme 4. Reaction of (PhMe2Si)3CLi and CS2 with 2-methyloxirane.
Scheme 5. Comparison of reactions of (PhMe2Si)3CLi and (Me3Si)3CLi with aldehydes.
3. Conclusion
Our results show that (PhMe2Si)3CLi is much less reactive than (Me3Si)3CLi because of
severe steric hindrance. Although the reaction of (Me3Si)3CLi and CS2 with halides gives

Journal of Sulfur Chemistry 309
mercaptobis-(triorganosilyl)-thiones, 1,1-bis(organothio)-2,2-bis (dimethylphenylsilyl)ethenes
have been produced by the reaction of (PhMe2Si)3CLi and CS2 with allyl and benzyl bromides.
These compounds are new and can be important intermediates in organic synthesis. Furthermore,
treatment of (PhMe2Si)3CLi with CS2 and 2-methyloxirane gave the known cyclic thiocarbonate.
No reaction was observed with aldehydes or acyl chlorides. (Me3Si)3CLi reacted with CS2 and
aldehydes at −78◦C to give 1,1-bis(trimethylsilyl)vinyl derivatives.
4. Experimental
4.1. Solvents and reagents
The reactions were carried out under dry argon. Solvents and CS2 were dried by standard methods.
Substrates for the preparation of (Me3Si)3CLi and (PhMe2Si)3CLi, namely Me3SiCl (Merck),
PhMe2SiCl (Merck), PhBr (Merck), n-BuLi (Merck), Li (Merck), MeI (Merck), CHCl3 (Merck),
CHBr3 (Merck), and substrates for the preparation of 1,1-bis(benzylthio)- and bis(allylthio)-2,2-
bis(dimethylphenylsilyl)ethenes, viz. alkyl halides, benzyl and allyl bromide (Merck), n-hexane
and ethyl acetate (Merk) for TLC, were used as received. (Me3Si)3CLi and (PhMe2Si)3CLi were
prepared according to the literature methods.[22,23]
4.2. Instrumentation
The 1H and 13C NMR were recorded with a Bruker FT-400 MHz spectrometer at room tempera-
ture and CDCl3 as solvent. The FTIR spectra were recorded on a Bruker-Tensor 270 spectrometer.
Elemental analyses were obtained with aVario EL III instrument. The mass spectra were obtained
−
with a GC MS Agilent, quadrupole mode 5973N instrument, operating at 70 eV. All the com-
−
pounds (2a–2e) decomposed during GC MS analys+is to give the following fragments with m/z
+
+
=
−
(EI): 296 ([(PhMe2Si)2C CH2] ), 281 ([296 Me] ), 135 ([SiMe2Ph] ).
4.3. Typical procedure for the preparation of 1,1-bis(benzylthio)- and
bis(allylthio)-2,2-bis(dimethylphenylsilyl)ethenes (2a–2e)
To a stirred solution of (PhMe2Si)3CLi (1 mmol) in THF, carbon disulfide (1.2 mmol) was added at
0◦C under an argon atmosphere. The mixture was stirred for 5 min and then electrophile (2 mmol)
was added at this temperature and the stirring was maintained as the mixture was allowed to end
the reaction. The progress of the reaction was followed by TLC with n-hexane: ethyl acetate 9:1
as solvent. The mixture was poured into water and extracted with CH2Cl2. The organic layer was
washed with water, dried with Na2SO4 and filtered. The solvent was evaporated from the filtrate
and the residue was purified by preparative thin layer chromatography (pTLC) on silica gel with
n-hexane: ethyl acetate 9:1 as eluent.
4.3.1. 1,1-Bis(4-bromobenzylthio)-2,2-bis(dimethylphenylsilyl)ethene (2a)
Yellow liquid (Rf = 0.70). FTIR (KBr, cm−1): 3065 (CHvinyl), 3018 (CHAr), 2959 (CH), 1589–
1
=
−
−
1486 (C C), 1250 and 873 (C Si), 1105 and 1071 (C S); H NMR (400 MHz, CDCl3): δ 0.38
(s, 12H, SiMe2), 3.86 (s, 4H, CH2), 7.16 (d, J 8.3, 4H, Ar), 7.37–7.53 (m, 14H, Ar); 13C NMR
=
(CDCl3): δ − 2.01 (SiMe2), 38.32 (CH2), 120.75–134.46 (Cvinyl, Ar), 135.57 (C(S)2vinyl); Anal.
calcd for C32H34Br2S2Si2: C, 55.0; H, 4.9; S, 9.2. Found: C, 54.8; H, 4.9; S, 9.3%.

310 K.D. Safa and K. Ghorbanpour
4.3.2. 1,1-Bis(benzylthio)-2,2-bis(dimethylphenylsilyl)ethene (2b)
Yellow liquid (Rf 0.57). FTIR (KBr, cm−1): 3064 (CHvinyl), 3029 (CHAr), 2959 (CH), 1600–1425
=
1
=
−
−
(C C), 1254 and 875 (C Si), 1105 and 1064 (C S); H NMR (400 MHz, CDCl3): δ0.22 (s, 12H,
SiMe2), 4.63 (s, 4H, CH2), 7.23–7.44 (m, 20H, Ar); 13C NMR (CDCl3): δ − 1.95 (SiMe2), 40.52
(CH2), 125.55–134.26 (Cvinyl, Ar), 139.80 (C(S)2vinyl); Anal. calcd for C32H36S2Si2: C, 71.1; H,
6.7; S, 11.9. Found: C, 71.4; H, 6.9; S, 11.7%.
4.3.3. 1,1-Bis(2-bromobenzyl)-2,2-bis(dimethylphenylsilyl)ethene (2c)
Yellow liquid (Rf 0.73). FTIR (KBr, cm−1): 3061 (CHvinyl), 3018 (CHAr), 2957 (CH), 1566–1435
=
1
=
−
−
(C C), 1251 and 844 (C Si), 1105 and 1071 (C S); H NMR (400 MHz, CDCl3): δ0.37 (s,
12H, SiMe2), 3.90 (s, 4H, CH2), 7.11–7.55 (m, 18H, Ar); 13C NMR (CDCl3): δ − 2.00 (SiMe2),
38.40 (CH2), 123.73–134.92 (Cvinyl, Ar), 136.5 (C(S)2vinyl); Anal. calcd for C32H34Br2S2Si2: C,
55.0; H, 4.9; S, 9.2. Found: C, 54.8; H, 5.0; S, 9.2%.
4.3.4. 1,1-Bis(3-bromobenzyl)-2,2-bis(dimethylphenylsilyl)ethene (2d)
Yellow liquid (Rf 0.71). FTIR (KBr, cm−1): 3065 (CHvinyl), 3011 (CHAr), 2957 (CH), 1580–1446
=
1
=
−
−
(C C), 1257 and 843 (C Si), 1105 and 1073 (C S); H NMR (400 MHz, CDCl3): δ0.32 (s,
12H, SiMe2), 3.82 (s, 4H, CH2), 7.14–7.45 (m, 18H, Ar); 13C NMR (CDCl3): δ − 2.11 (SiMe2),
38.37 (CH2), 121.9–131.01 (Cvinyl, Ar), 137.8 (C(S)2vinyl); Anal. calcd for C32H34Br2S2Si2: C,
55.0; H, 4.9; S, 9.2. Found: C, 54.7; H, 5.1; S, 9.3%.
4.3.5. 1,1-Bis(allylthio)-2,2-bis(dimethylphenylsilyl)ethene (2e)
Yellow liquid (Rf 0.54). FTIR (KBr, cm−1): 3068 (CHvinyl), 3011 (CHAr), 2957 (CH), 1638–
=
1
=
−
−
1422 (C C), 1254 and 829 (C Si), 1116 and 1062 (C S); H NMR (400 MHz, CDCl3): δ0.33
=
=
=
(s, 12H, SiMe2), 4.04 (d, J 6.7, 4H, CH2allyl), 5.20 (d, J 10.0, 2H, CH2vinyl), 5.32 (d, J 16.9,
2H, CH2vinyl), 5.82–5.92 (m, 2H, CHvinyl), 7.27–7.55 (m, 10H, Ar); 13C NMR (CDCl3): δ −
1.15 (SiMe2), 37.68 (CH2allyl), 117.65 (CH2vinyl), 125.51–133.14 (CHvinyl, Cvinyl, Ar), 137.80
(C(S)2vinyl); Anal. calcd for C24H32S2Si2: C, 65.4; H, 7.3; S, 14.5. Found: C, 65.7; H, 7.4; S,
14.4%.
4.4. Reaction of (PhMe2Si)3CLi with CS2 and 2-methyloxirane
To a stirred solution of (PhMe2Si)3CLi (1 mmol) in THF, carbon disulfide (1.2 mmol) was added
at 0◦C under an argon atmosphere. The mixture was stirred for 5 min and then 2-methyloxirane
(1 mmol) was added at this temperature and the stirring was maintained until the reaction was
complete (15 min) as shown by TLC with n-hexane and ethyl acetate 9:1 as solvent. Then, the
mixture was poured into water and extracted with CH2Cl2. The organic layer was washed with
water, dried with Na2SO4 and filtered. The solvent was evaporated from the filtrate and the residue
was purified by PTLC on silica gel with n-hexane: ethyl acetate 9:1 as eluent.
1
=
Colorless liquid, 80% (Rf 0.28), H NMR (400 MHz, CDCl3): δ1.59 (s, 3H, CH3), 3.31–3.39
=
=
(dd, J 9.9 Hz, 1H, CH2), 3.59–3.64 (dd, J 6.5, 11.0 Hz, 1H, CH2), 5.16–5.25 (m, 1H, CH);
C NMR (CDCl3): δ18.04 (CH3), 39.61 (CH2), 86.96 (CH), 213.62 (C S).
13
=
Acknowledgement
We thank Dr J. D. Smith for helpful comments.

Journal of Sulfur Chemistry 311
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