Y. Cai, B. P. Roberts / Tetrahedron Letters 42 (2001) 763–766
765
References
residue was shown by NMR spectroscopy to consist
mainly of TPST (96%) together with a trace of unre-
acted silane. In the absence of TBHN but under other-
wise identical conditions, no conversion of TPS to
TPST was detectable, confirming the radical-chain
nature of the reaction. Several other organosilanes and
triphenylgermane were converted to the corresponding
metalloidal thiols by the same method and the results
are collected in Table 1. In some cases yields were
improved if the reaction was carried out in a thick-
walled glass bottle under a pressure of COS ca. 2.5 bar
above atmospheric. Azobis(isobutyronitrile) (AIBN)
and dilauroyl peroxide (DLP) were investigated as al-
ternative initiators (at 80–85°C) and sometimes gave
better results than TBHN. In this context, it must be
borne in mind that use of TBHN leads to the produc-
tion of tert-butyl alcohol which may react with some of
the metalloidal thiols to replace the SH group by a
ButO group and liberate H2S.
1. Cole, S. J.; Kirwan, J. N.; Roberts, B. P.; Willis, C. R.
J. Chem. Soc., Perkin Trans. 1 1991, 103.
2. Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25.
3. Lesage, M.; Simo˜es, J. A. M.; Griller, D. J. Org. Chem.
1990 55 5413. These authors showed that triphenylsi-
lane-mediated reduction of 1-bromohexadecane to hex-
adecane, initiated by dibenzoyl peroxide in heptane at
90°C, was 80% complete after 4 h.
4. Wu, Y.-D.; Wong, C.-L. J. Org. Chem. 1995, 60,
821.
5. (a) Kiefer, H.; Traylor, T. G. Tetrahedron Lett. 1966,
6163; (b) Mendenhall, G. D. Tetrahedron Lett. 1983 24
451. TBHN was prepared from sodium hyponitrite,
which is available from Aldrich.
6. Barton, D. H. R.; Jang, D. O.; Jaszberenyi, J. Cs. Tet-
rahedron 1993, 49, 7193.
7. Barton, D. H. R.; Jacob, M. Tetrahedron Lett. 1998, 39,
1331.
The yield of silanethiol drops rapidly as the phenyl
groups in TPS are replaced sequentially with tert-
butoxy groups (entries 10–12). Attachment of an elec-
tronegative oxygen atom to silicon probably
strengthens the SiꢁH bond and decreases the electron
density at the hydrogen atom, making the latter less
readily abstracted by the electrophilic silanethiyl radical
(cf. Scheme 1). It is also likely to render the silicon
atom in the corresponding silyl radical ‘harder’ and less
thiophilic, reducing the rate of its addition to the CꢀS
group in carbonyl sulfide.
8. (a) Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin
Trans. 2 1984, 1717; (b) Baban, J. A.; Roberts, B. P. J.
Chem. Soc., Perkin Trans. 2 1988, 1195.
9. Cholestanyl xanthate is also reduced by TPS to give
cholestane, in 83% yield, using 2,2-di-tert-butylperox-
ybutane (5 mol%; Aldrich) as initiator in refluxing
octane and without any added thiol catalyst.
10. Haque, M. B.; Roberts, B. P.; Tocher, D. A. J. Chem.
Soc., Perkin Trans. 1 1998, 2881.
11. Crich, D.; Quintero, L. Chem. Rev. 1989, 89, 1413.
12. Although all reactions were carried out under nominally
anhydrous conditions, as always in such situations it is
difficult to exclude the possibility that traces of moisture
could be present. Hydrolysis or alcoholysis of
Ph3SiSC(ꢀO)SMe could produce traces of a thiol cata-
lyst (TPST or MeSH).
Triphenylgermane reacts with COS in a similar way to
the silanes and gives the germanethiol in essentially
quantitative yield (entries 14 and 15). This thiol also
functioned as an effective protic polarity-reversal cata-
lyst for the radical-chain addition of triphenylsilane to
the methylene lactone 5 and complete (]98%) conver-
sion to the adduct 6 was obtained in its presence (5
mol%) under the conditions described above for cataly-
sis by TBST.
13. Birkofer, L.; Ritter, A.; Goller, H. Chem. Ber. 1963, 94,
3289.
14. Representative procedure: a-Naphthyldiphenylsilane15
(1.60 g, 5.15 mmol), TBHN (45 mg, 5 mol%) and dry
dioxane (10 cm3) were placed into a dry, argon-filled
two-necked 50 cm3 round-bottomed flask, containing a
magnetic stirrer bar and equipped with a condenser and
a silicone rubber septum in the side arm. Carbonyl
sulfide (ca. 250 cm3, ca. 10 mmol; Aldrich) was intro-
duced as a slow stream of small bubbles into the stirred
We conclude that (i) simple organosilanes, in particular
triphenylsilane, are excellent, relatively cheap and envi-
ronmentally-sound replacements for tributyltin hydride
in Barton–McCombie-type deoxygenations via xan-
thate esters, (ii) the possible role of thiols generated in
situ from thiocarbonyl compounds should always be
considered in mechanistic discussions of their reactions
with metallic or metalloidal hydrides and (iii) COS
reacts with organosilanes under mild conditions to
provide a convenient and clean method for the synthe-
sis of silanethiols. As demonstrated by the reaction of
triphenylgermane with COS, it seems likely that the
method can be extended to the preparation of other
metallic and metalloidal thiols.
solution, via
a thin PTFE tube which terminated
beneath the surface of the liquid, and the flask was then
immersed in an oil bath pre-heated to 60°C. After 2.5 h,
the reaction mixture was allowed to cool to room tem-
perature, the solvent was removed under reduced pres-
sure and the residue was purified by recrystallisation
from diethyl ether–light petroleum to give a-naph-
thyldiphenylsilanethiol (1.46 g, 83%) as a white solid,
mp 99–101°C. NMR (500 MHz for 1H, 125.7 MHz for
13C, CDCl3 solvent); lH 0.68 (1 H, s, SH), 7.37–7.97 (17
H, m, aromatic H); lC 125.0, 125.7, 126.1, 128.1,
129.0(0), 129.0(2), 130.2, 131.5, 131.7, 133.6, 134.8,
135.4, 136.4, 137.1. MS (EI) 342 (M+, 21%), 309 (100),
263 (25), 231 (18). IR (KBr disc) 2564 cm−1 (SH).
Found: C, 76.9; H, 5.5. C22H18SiS requires C, 77.1; H,
Acknowledgements
We thank the EPSRC for financial support.
.