1f was converted into the corresponding sulfide 2f with ethyl thiol
using 1 mol% of In(OTf)3. The reaction could also be run without
solvent and was completely regiospecific, affording the thiol
addition exclusively on the internal position of the diene.
Interestingly, only a single thiol addition to the diene was observed
even when using 2 equivalents of thiol. When diene 1f was subject
to thiol addition with AIBN (10 mol%), the reverse regioselectivity
was obtained, with a ratio 2f : 3f of 2 : 98.
non-activated olefins. The reaction is highly regioselective and
occurs in a Markovnikov fashion. It can be carried out without
solvent in the presence of 1 mol% of catalyst. Interestingly, the
isomerisation of the C–C double bond was observed in some cases.
Moreover, indium trifluoromethanesulfonate is stable in water and
can be recycled and reused efficiently after evaporation of the
aqueous phase.
These results report a novel example of the concept of a Lewis
acid-assisted Brønsted acid-catalysed reaction.9
For less activated terminal and 1,2-disubstituted olefins (entries
9–10), the reactions were run in refluxing nitromethane, as with 1a.
When vinylcyclohexane 1g was reacted with octanethiol, the
expected 2-(2-cyclohexylethyl)octyl sulfide was not formed. Indeed,
isomerisation of the double bond occurred (as with substrate 1b)
with the clean and selective formation of sulfide 2g in 80% isolated
yield as the single product.
Notes and references
1 K. Griesbaum, Angew. Chem., Int. Ed. Engl., 1970, 4, 273.
2 (a) T. Posner, Ber., 1905, 38, 646; (b) C. G. Screttas and M. Micha-
Screttas, J. Org. Chem., 1979, 44, 713; (c) F. Wolf and H. Finke,
Z. Chem., 1972, 12, 180; (d) S. Kanagasabapathy, A. Sudalai and
B. C. Benicewicz, Tetrahedron Lett., 2001, 42, 3791.
3 (a) T. Mukaiyama, T. Izawa, K. Saigo and H. Takai, Chem. Lett., 1973,
355; (b) M. Belley and R. Zamboni, J. Org. Chem., 1989, 54, 1230.
4 D. Prajapati, D. D. Laskar and J. S. Sandhu, Tetrahedron Lett., 2000, 41,
8639.
5 T. Ali, K. K. Chauhan and C. G. Frost, Tetrahedron Lett., 1999, 40,
5621.
6 C. G. Frost, J. P. Hartley and D. Griffin, Tetrahedron Lett., 2002, 43,
4789.
cis-Cyclooctene 1h was functionalised with octyl mercaptan
(entry 10) to afford 2h in 81% isolated yield.
From the point of view of the mechanism, no reaction occurred
without catalyst and the classical radical-type reaction occurred at
a very slow rate with light, without much selectivity and affording
the opposite regioisomer. The regiochemical outcome of the In(III)-
catalysed thiol addition to olefins corresponds to an acid-catalysed
process. However, the In(OTf)3 catalysis allows for better yields
and selectivities than the corresponding TfOH catalysis, in which
the olefinic substrates are subject to polymerisation. The In(III)-
catalysed (5 mol%) thiol addition to 1e (entry 5) was carried out in
the presence of 2,6-di-tert-butylpyridine (5 mol%) as a hindered
base and the thiol addition was inhibited. These data account for
the important role of the protons in the catalytic process. We
propose the formation of an In(III) thiolate liberating in a strong
protic acid, such as TfOH or [In(RS)(OTf)3]2H+, as a key feature
of the protic catalysis. The intermediate In(III) thiolate should also
induce a higher nucleophilicity to the thiol moiety.
7 (a) S. Muthusamy, S. Arulananda Babu and C. Gunanathan,
Tetrahedron, 2002, 58, 7897; (b) K. Kazahaya, N. Hamada, S. Ito and
T. Sato, Synlett, 2002, 9, 1535.
8 In a typical procedure, a mixture of olefin (5 mmol), thiol (5 mmol)
and In(OTf)3 (0.25 mmol) was stirred in refluxing dichloromethane
or nitromethane (5 mL) depending on the substitution of the double
bond of the olefin. The progress of the reaction was monitored by
GC analysis. On completion of the reaction, the mixture was quenched
with distilled water and extracted with Et2O. The organic layer was
washed with saturated aqueous NaHCO3 and dried with MgSO4. The
1
solvent was evaporated and the products were analysed by H and 13C
NMR and mass spectrometry. In order to recycle the catalyst, the
aqueous phase was washed with distilled Et2O and evaporated under
vacuum.
In conclusion, In(OTf)3 constitutes a good and efficient catalyst
both for the intra- and intermolecular addition of thiols to
9 H. Yamamoto and K. Futatsugi, Angew. Chem., Int. Ed., 2005, 44, 1924.
334 | Chem. Commun., 2006, 332–334
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