Preparation of silanethiols by reaction of silanes with triphenylphosphine sulfide and with alkanethiols

Article abstract of DOI:10.1016/S0040-4039(01)00801-2

Silanes react with triphenylphosphine sulfide by a radical-chain mechanism to give the corresponding silanethiols in good yield. Silanethiols are similarly formed when silanes react with tert-dodecanethiol. Enantiomerically pure (S)-Bu^tMePhSiH gave racemic silanethiol with Ph₃P=S, but with tert-dodecanethiol silanethiol with an ee up to 60% has been obtained, although in low chemical yield.

Full text of DOI:10.1016/S0040-4039(01)00801-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4581–4584
Preparation of silanethiols by reaction of silanes with
triphenylphosphine sulfide and with alkanethiols
Yudong Cai and Brian P. Roberts*
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
Received 9 April 2001; accepted 11 May 2001
Abstract—Silanes react with triphenylphosphine sulfide by a radical-chain mechanism to give the corresponding silanethiols in
t
good yield. Silanethiols are similarly formed when silanes react with tert-dodecanethiol. Enantiomerically pure (S)-Bu MePhSiH
gave racemic silanethiol with Ph PꢀS, but with tert-dodecanethiol silanethiol with an ee up to 60% has been obtained, although
3
in low chemical yield. © 2001 Elsevier Science Ltd. All rights reserved.
Organosilanethiols (R SiSH) now have a number of
in good yield when the other groups attached to silicon
are alkyl or aryl. However, poor yields are obtained
when alkoxy groups are attached to silicon.
The many similarities between phosphines and carbon
monoxide, together with the high affinity of silicon for
bonding to the electronegative sulfur atom, suggested
that phosphine sulfides might serve as sulfur-transfer
reagents in place of the carbonyl sulfide (Scheme 1;
3
1
applications in organic chemistry. Triphenylsilanethiol
2
and triisopropylsilanethiol function as H S equivalents
2
for the synthesis of alkanethiols and thioethers, when
CꢁS bond formation can be accomplished by radical or
3
heterolytic pathways. The silanethiols (Me Si) SiSH
3
3
4
and (Me Si) MeSiSH have been used as homolytic
3
2
reducing agents for alkyl halides and silanethiols [in
t
particular Ph SiSH and (Bu O) SiSH] have been used as
3
3
M=PR ). The reaction of COS proceeds via an inter-
3
protic polarity-reversal catalysts to promote the overall
abstraction of electron-rich hydrogen by one nucle-
ophilic radical to produce a second nucleophilic
mediate thioacyl radical 1, while addition of the silyl
radical to R PꢀS would give an intermediate thiophos-
3
7
5
phoranyl radical 2, a-scission of which could yield the
radical.
8,9
phosphine and a silanethiyl radical.
6
We have reported recently that silanes react by a
radical-chain mechanism with carbonyl sulfide to give
silanethiols and carbon monoxide (Scheme 1; M=CO)
In the present paper we report that silanethiols can
indeed be prepared by the radical-chain reaction of
silanes with phosphine sulfides and that this reaction is
more tolerant of the presence of alkoxy groups attached
to silicon than is the corresponding reaction with COS.
While this work was in progress, a report by Chat-
11
gilialoglu and co-workers appeared in which these
authors described the reduction of phosphine sulfides
and selenides with tris(trimethylsilyl)silane to give the
parent phosphine. Although, the silicon-containing
products of these reactions were not examined, it was
Scheme 1.
’
thought that the intermediate radical (Me Si) SiX (X=
3
3
S or Se) rearranged by a 1,2-shift of a trimethylsilyl
group from Si to X more rapidly than it abstracted
hydrogen from the silane.
Keywords: silicon and compounds; phosphorus and compounds; thi-
ols; radicals and radical reactions.
*
Corresponding author.
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00801-2

4
582
Y. Cai, B. P. Roberts / Tetrahedron Letters 42 (2001) 4581–4584
When a dioxane solution containing triphenylsilane and
triphenylphosphine sulfide (1.2 equiv.) was heated for
silanethiol from the triphenylphosphine produced was
facilitated by addition of elemental sulfur to the reac-
2
.5 h at 60°C under argon, in the presence of di-tert-
tion mixture, to affect its conversion back to the polar
1
2
16,17
butyl hyponitrite (TBHN; 5 mol% based on silane) as
a thermal source of initiating tert-butoxyl radicals, 97%
of the silane was converted into triphenylsilanethiol (as
and relatively insoluble Ph PꢀS.
3
We have reported previously that the glucosidic thiol 3
undergoes efficient desulfurisation at C-1 to give 4
when heated at 60°C in dioxane in the presence of
triphenylsilane (1.2 equiv.) and TBHN (5 mol% based
1
judged by H NMR spectroscopy). Essentially no
silanethiol was formed in the absence of the initiator,
confirming the radical-chain nature of the reaction. The
same result was achieved using azobis(isobutyronitrile)
18
on silane). It was pointed out that triphenylsilanethiol
was also a product of the reaction, although this com-
pound was not isolated. We now report that this reac-
tion, which shows promise as a method for the
desulfurisation of aliphatic thiols, can also provide an
efficient route to silanethiols. The propagation cycle of
the proposed radical-chain mechanism is shown in
(
AIBN) at 85°C or in refluxing octane (bath tempera-
ture 140°C, internal temperature ca. 130°C) as solvent
13
with 2,2-di-tert-butylperoxybutane (DBPB) as initia-
tor. The results are summarised in Table 1.
Tributylphosphine sulfide and methyldiphenylphos-
phine sulfide were less effective sulfur-transfer reagents
than triphenylphosphine sulfide (entries 4 and 5), per-
haps because the presence of alkyl groups attached to
phosphorus increases the basicity of the corresponding
phosphine and consequently reduces the rate of PꢁS
bond cleavage in the intermediate thiophosphoranyl
Scheme 2, although the S
with the thiol to displace an alkyl radical might take
2 reaction of the silyl radical
H
1
9
place via a discrete sulfuranyl radical intermediate.
AcO
AcO
AcO
AcO
AcO
AcO
O
O
SH
radical. Triphenylphosphine sulfide also reacts with
OAc
OAc
t
MePh SiH and with racemic Bu MePhSiH to give high
2
3
4
yields of the corresponding silanethiols (entries 6–8),
1
4
although the (S)-enantiomer of the latter silane gave
extensively (at 60°C) or completely racemic (at 130°C)
product, showing that the configuration of the pyrami-
R'SH
R Si
3
R'SH
t
’
dal intermediate silyl radical Bu MePhSi is scrambled
1
5
during the reaction (entries 9 and 10). The (S)-silane
also gave completely racemic silanethiol when treated
with COS in dioxane at 60°C, under the conditions
R SiH
3
R SiSH
3
R'S
R'
6
described previously.
R'H
R'SH
In contrast to the low yield obtained from its reaction
6
t
with COS, Bu OPh SiH was efficiently converted to
2
the silanethiol by reaction with Ph PꢀS (entries 11–13),
3
especially at higher temperatures. The separation of the
Scheme 2.
Table 1. Thiols prepared by the reaction of phosphine sulfides with silanes
Entry
Silanea
Phosphine sufide
Conditionsb
Initiator (bath temp./°C)
Solventc
Productd (conversion %)
1
2
3
4
5
6
7
8
9
Ph SiH
Ph PS
A
A
A
A
A
A
A
B
B
B
A
A
B
TBHN (60)
AIBN (85)
DBPB (140)
TBHN (60)
TBHN (60)
TBHN (60)
AIBN (85)
DBPB (140)
TBHN (60)
DBPB (140)
TBHN (60)
AIBN (85)
DBPB (140)
D
D
O
D
D
D
D
O
D
O
D
D
O
Ph SiSH (97)
3
3
3
Ph SiH
Ph PS
Ph SiSH (97)
3
3
3
Ph SiH
Ph PS
Ph SiSH (98)
3
3
3
Ph SiH
MePh PS
Ph SiSH (57)
3
2
3
Ph SiH
Bu PS
Ph SiSH (11)
3
3
3
MePh SiH
Ph PS
MePh SiSH (93)
2
3
2
MePh SiH
Ph PS
MePh SiSH (95)
2
3
2
t
t
e
f
Bu MePhSiH
Ph PS
Bu MePhSiSH (97)
Bu MePhSiSH (86)
Bu MePhSiSH (97)
Bu OPh SiSH (53)
Bu OPh SiSH (54)
Bu OPh SiSH (95)
3
t
t
(S)-Bu MePhSiH Ph PS
3
t
t
g
10
11
12
13
(S)-Bu MePhSiH Ph PS
3
t
t
Bu OPh SiH
Bu OPh SiH
Bu OPh SiH
Ph PS
2
3
2
t
t
Ph PS
2
3
2
t
t
h
Ph PS
2
3
2
a
b
c
Scale 1–10 mmol; concentration ca. 0.5 M; reaction time 2.5 h.
A=1.2 equiv. phosphine sulfide and 5 mol% initiator based on silane; B=1.5 equiv. phosphine sulfide and 7.5 mol% initiator based on silane.
D=dioxane; O=octane.
d
e
Conversion estimated by 1H NMR spectroscopy.
Isolated yield 73%; bp 98–100°C/2 mmHg.
ee=7%, retention of configuration at Si assumed.
ee=0%.
f
g
h
Isolated yield 72%; bp 112–114°C/0.02 mmHg.

Y. Cai, B. P. Roberts / Tetrahedron Letters 42 (2001) 4581–4584
Table 2. Thiols prepared by the reaction of alkanethiols with silanes
4583
Silanea
Alkanethiolb
Initiator (bath temp./°C)
c
Solventd
Product (conversion %)
e
Entry
f
1
2
3
4
5
6
7
8
9
Ph SiH
TDT
TDT
PhCH SH
TBHN (60)
AIBN (85)
TBHN (60)
TBHN (60)
TBHN (60)
DBPB (140)
DBPB (140)
TBHN (60)
AIBN (85)
DBPB (140)
D
D
D
D
D
O
O
D
D
O
Ph SiSH (94)
3
3
Ph SiH
Ph SiSH (95)
3
3
Ph SiH
Ph SiSH (51)
3
2
3
t
t
g
i
(S)-Bu MePhSiH
(S)-Bu MePhSiH
(S)-Bu MePhSiH
TDT
TDT
Bu MePhSiSH (41)
Bu MePhSiSH (16)
Bu MePhSiSH (49)
Bu MePhSiSH (27)
Bu OPh SiSH (12)
Bu OPh SiSH (5)
Bu OPh SiSH (15)
t
h
t
t
t
j
TDT
TDT
t
h
t
k
(S)-Bu MePhSiH
t
t
Bu OPh SiH
TDT
TDT
TDT
2
2
t
t
Bu OPh SiH
2
2
t
t
1
0
Bu OPh SiH
2
2
a
b
c
d
e
f
Scale generally 1.0 mmol; concentration ca. 0.5 M.
1
5
.05 equiv. of alkanethiol based on silane, unless stated otherwise.
mol% initiator based on silane.
D=dioxane; O=octane.
Estimated by 1H NMR spectroscopy.
Isolated yield 75%.
g
h
i
ee=29%, retention of configuration at silicon assumed.
2
.0 equiv. of alkanethiol.
ee=60%.
ee=3%.
ee=23%.
j
k
When a dioxane solution containing triphenylsilane and
We conclude that the reaction of silanes with
triphenylphosphine sulfide constitutes a useful method
for the preparation of silanethiols, especially when the
latter are liquids and thus readily isolated from solid
by-products. So far, the reaction of aliphatic thiols with
silanes is more limited in scope, but can be useful for
the preparation of solid silanethiols (especially if these
are water sensitive) and might perhaps be developed
into a method for the synthesis of homochiral
silanethiols.
2
0
tert-dodecanethiol
heated at 60°C under argon, in the presence of TBHN
5 mol% based on silane), the silane was smoothly
(1.05 equiv.) was stirred and
(
converted to triphenylsilanethiol (94% by NMR); the
results are summarised in Table 2. No reaction
occurred in the absence of initiator and, as for sulfur
transfer from Ph PꢀS, AIBN was equally effective as
3
initiator (entry 2). Although benzyl mercaptan has a
relatively weak CꢁS bond, this reagent gave lower
yields of silanethiol than TDT (entry 3), perhaps
because of the presence of benzylic CH groups that lead
to side reactions and to inhibition of the radical-chain
References
2
1
process. Although the yields of silanethiol from (S)-
t
Bu MePhSiH were lower than those obtained from the
1. Hach e´ , B.; Gareau, Y. Tetrahedron Lett. 1994, 35, 1837.
reaction of Ph PꢀS under similar conditions (entries
3
2. Miranda, E. I.; Diaz, M. J.; Rosado, I.; Soderquist, J. A.
t
4–7), the Bu MePhSiSH produced was not racemic and
Tetrahedron Lett. 1994, 35, 3221.
showed an ee up to 60% (entry 5). These observations
imply that the silyl radical reacts with TDT more
rapidly than with the phosphine sulfide under compara-
ble conditions. Although increasing the amount of
TDT used to 2 equiv. led to the expected increase in ee
of the silanethiol produced (entries 5 and 7), the chem-
ical yield was reduced considerably. When attempting
to understand these results, it must be borne in mind
that the overall rate of a radical-chain reaction does not
depend simply on the rate of any single propagation
step. Furthermore, the mechanisms of both types of
sulfur-transfer reaction are likely to be more complex in
detail than shown in Schemes 1 and 2. For example, the
3
. Ballestri, M.; Chatgilialoglu, C.; Seconi, G. J. Organomet.
Chem. 1991, 408, C1.
4. Darszewski, J.; Lusztyk, J.; Degueil, M.; Navarro, C.;
Maillard, B. J. Chem. Soc., Chem. Commun. 1991, 586.
5. Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25.
6. Cai, Y.; Roberts, B. P. Tetrahedron Lett. 2001, 42, 763.
7. Giles, J. R. M.; Roberts, B. P. J. Chem. Soc., Perkin
Trans. 2 1981, 1211.
8. Roberts, B. P. Adv. Free-Radical Chem. 1981, 6, 225.
9. The phosphoranyl radical [Bu OPBu ] undergoes a-scis-
t
’
3
sion to give butyl radicals so rapidly that it is not
8,10
detectable by EPR spectroscopy even at −100°C.
In
contrast, the EPR spectrum of the thiophosphoranyl
t
’
radical [(Bu O) SiSPBu ] , generated by photolysis of the
3 3
addition of the silyl radical to R PꢀS (Scheme 1) may
3
be reversible under the reaction conditions and abstrac-
tion of hydrogen from TDT by the silyl radical is also
likely to be reversible (Scheme 2).
bis(tri-tert-butoxysilyl) disulfide in the presence of tri-
butylphosphine, is readily detectable [a(P) 614.5 G, a(6 H)
3.2 G, g 2.0079 in cyclopropane at −73°C]. There was no
spectroscopic evidence for cleavage of this thiophospho-
ranyl radical to give butyl radicals at even higher temper-
t
The radical-chain reaction between Bu OPh SiH and
2
t
’
atures and the spectrum of [(Bu O) SiSPBu ] was still
3 3
TDT gives low yields of the corresponding silanethiol
and is not synthetically useful (entries 8–10).
10
readily detectable at +23°C. It seems likely that the

4
584
Y. Cai, B. P. Roberts / Tetrahedron Letters 42 (2001) 4581–4584
unpaired electron in the thiophosphoranyl radical is
2 h, the benzene was removed by rotary evaporation and
light petroleum was added to the residue to precipitate
triphenylphosphine sulfide. After cooling to −10°C to
7
confined to the PS s* orbital and such a structure would
have a tendency to fragment by PꢁS bond cleavage,
8
rather than by loss of a butyl radical.
complete precipitation, the Ph PS was removed by filtra-
3
1
1
0. Fielding, A. J.; Roberts, B. P., unpublished results.
1. Romeo, R.; Wozniak, L. A.; Chatgilialoglu, C. Tetra-
hedron Lett. 2000, 41, 9899.
tion. The solvent was evaporated from the filtrate under
reduced pressure and the residue was distilled to give
tert-butoxydiphenylsilanethiol (1.04 g, 72%) as a colour-
less oil, bp 112–114°C/0.02 mmHg. NMR (500 MHz for
1
2. (a) Kiefer, H.; Traylor, T. G. Tetrahedron Lett. 1966,
1
13
6
163; (b) Mendenhall, G. D. Tetrahedron Lett. 1983, 24,
H, 125.7 MHz for C, CDCl solvent); lH 0.51 (1 H, s,
3
t
4
51. TBHN was prepared from sodium hyponitrite
SH), 1.38 (9 H, s, Bu ), 7.36–7.71 (10 H, m, Ph); lC 31.9,
75.6, 127.8, 130.1, 134.3, 136.9. MS (EI, 70 eV) 288 (M ,
12b
+
(Aldrich), using the method of Mendenhall.
1
1
3. Available from Aldrich as a 50% w/w solution in mineral
oil.
1.3), 273 (2.4), 255 (4.2), 45 (100%). IR (KBr disc) 2558
−
1
cm (SH str.). Found: C, 66.4; H, 6.9. C H OSSi
16
20
4. Jankowski, P.; Schaumann, E.; Wicha, J.; Zarecki, A.;
Adiwidjaja, G. Tetrahedron: Asymmetry 1999, 10, 519.
The enantiomeric purity of the silane used was 99%, as
determined by chiral-stationary-phase GLC using a beta-
DEX 120 column (Supelco Inc.).
requires C, 66.6; H, 7.0%.
tert-Butylmethylphenylsilanethiol was prepared in a simi-
lar way from racemic tert-butylmethylphenylsilane as a
colourless oil, bp 98–100°C/2 mmHg. lH −0.11 (1 H, s,
t
SH), 0.62 (3 H, s, Me), 0.97 (9 H, s, Bu ), 7.37–7.65 (5 H,
1
5. The enantiomeric purity of tert-butylmethylphenylsi-
lanethiol could not be determined directly and it was
m, Ph); l −2.6, 19.1, 26.1, 127.7, 129.7, 134.5, 135.4. MS
C
+
+
t
(EI, 70 eV) 210 (M , 5), 153 (M −Bu , 100%). IR (KBr
t
−1
converted to Bu MePhSiSCH CO Et by stirring with
disc) 2562 cm (SH str.). Found: C, 62.9; H, 8.7.
2
2
ethyl bromoacetate and potassium carbonate in benzene
at room temperature. The ee of the resulting ester was
determined by HPLC using a Chiralcel-OD column
C H SSi requires C, 62.8; H, 8.6%.
11
18
17. Attempts to use triphenylphosphine to catalyse the reac-
tion between triphenylsilane and elemental sulfur to give
the silanethiol were unsuccessful. It seems likely that
triphenylsilyl radicals react with sulfur in preference to
(Daicel Chemical Industries) and eluting with hexane–iso-
propyl alcohol (99.5:0.5).
1
6. Representative procedure: tert-Butoxydiphenylsilane (1.28
g, 5.0 mmol), triphenylphosphine sulfide (2.21 g, 7.5
mmol), DBPB (50% w/w in mineral oil, 0.18 g, 0.38
Ph PS under these conditions, leading to chain inhibition.
18. Haque, M. B.; Roberts, B. P.; Tocher, D. A. J. Chem.
Soc., Perkin Trans. 1 1998, 2881.
19. Gara, W. B.; Roberts, B. P. J. Organomet. Chem. 1977,
135, C20.
20. This is the isomeric mixture of tertiary thiols as obtained
from Aldrich.
3
3
mmol) and dry octane (10 cm ) were placed in a dry,
argon-filled flask, containing a magnetic stirrer bar and
equipped with a condenser. The flask was then immersed
in an oil bath, pre-heated to 140°C, and the reaction
mixture was stirred for 2.5 h and then allowed to cool to
room temperature. The excess sulfide was removed by
filtration and the filter cake was washed thoroughly with
light petroleum. The solvents were removed from the
21. Methyl thioglycolate (MeO CCH SH) gave results simi-
2
2
lar to those obtained using TDT. Thus, with triphenylsi-
lane, otherwise under the conditions of Table 2, entry 1,
conversion to Ph SiSH was 98% and the isolated yield
3
3
filtrate under reduced pressure and benzene (8 cm ) and
was 84%. An advantage of using methyl thioglycolate is
that the methyl acetate produced as a by-product is very
easily removed by evaporation.
powdered sulfur (0.16 g, 5.0 mmol) were added to the
residue. The mixture was stirred at room temperature for
.