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yields were observed with other heterocyclic thiols such as pyri- highest Z-selectivity with only a trace amount of the E-isomer
dine-2-thiol 1c, pyrimidine-2-thiol 1e, 1,3,4-thiadiazole-2-thiol 1g (Table 3, entry 3).
and 5-mercapto-1-methyltetrazole 1h.
Unfortunately, aliphatic thiols did not participate in the
We next turned our attention to check the applicability of reaction under these conditions. Efforts to activate the aliphatic
our method with thiols other than heterocyclic thiols. When thiols by elevating the temperature or altering the microwave
4-methyl thiophenol 4d was reacted with ethyl propiolate 2a at power also failed and starting compounds were recovered.
500 W in a closed vessel inside a microwave reactor, mixture of
E- and Z-vinyl suldes was obtained with the Z-isomer as the
predominant product (Table 3, entry 4).
Conclusions
Although the reaction exhibited reduced selectivity, the In conclusion, we have developed a simple, efficient and ster-
transformation was rapid and yield was found to be excellent. eoselective method for the formation of vinyl suldes via
Subsequently, other aromatic thiols having electron with- hydrothiolation reaction between heterocyclic thiols and acti-
drawing and donating substituent were employed and similar vated terminal alkynes under microwave irradiation. The key
results were obtained (Table 3, entries 1–3 and 5–6). It is features of the methodology are short reaction time, good to
noteworthy that under the current microwave reaction condi- excellent yields, absence of any side products and the achieve-
tions, ortho substituted aromatic thiols like 2-methyl and ment of 100% stereoselectivity in the case of heterocyclic thiols
2-methoxy thiophenol also underwent the vinylation smoothly with no additional requirement of any catalyst and/or additives.
to obtain a mixture of E and Z-vinyl suldes in short time This makes it an attractive protocol to obtain Z-vinyl suldes.
(Table 3, entry 1 and 2). 4-Nitro thiophenol exhibited the
Acknowledgements
NR thanks UGC, New Delhi for the award of a research fellow-
ship. We also thank the Director, NEIST, Jorhat, for his keen
Table 3 Vinylation of aromatic thiolsa
interest and constant encouragement.
Notes and references
Time Yieldb
(min) (%)
1 (a) A. H. F. Lee, J. Chen, D. Liu, T. Y. C. Leung, A. S. C. Chan and
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Entry Thiol, 4
Product
1
4
4
5
3
84 (1 : 4)
2
3
4
80 (1 : 6)
75 (trace : 99)
95 (1 : 3.1)
3 R. D. Miller and R. Hassig, Tetrahedron Lett., 1985, 26, 2395–
2398.
4 B. M. Trost and A. C. Lavoie, J. Am. Chem. Soc., 1983, 105,
5075–5090.
5 For reviews on C–S bond formation reactions, see: (a)
T. Kondo and T.-A. Mitsudo, Chem. Rev., 2000, 100, 3205–
3220; (b) F. Alonso, I. P. Beletskaya and M. Yus, Chem. Rev.,
2004, 104, 3079–3159.
6 (a) H.-L. Kao and C.-F. Lee, Org. Lett., 2011, 13, 5204–5207; (b)
M. S. Kabir, M. Lorenz, M. L. Van Linn, O. A. Namjoshi,
S. Ara and J. M. Cook, J. Org. Chem., 2010, 75, 3626–3643; (c)
M. S. Kabir, O. A. Namjoshi, R. Verma, R. Polanowski,
S. M. Krueger, D. Sherman, M. A. Rott, W. R. Schwan,
A. Monte and J. M. Cook, Bioorg. Med. Chem., 2010, 18, 4178–
5
3
3
92 (1 : 4.7)
93 (1 : 7)
6
a
Reaction conditions: thiol (1 mmol), ethyl propiolate (1.1 mmol) were
b
irradiated under microwave heating till reaction is completed. Yields
of isolated products: values in parentheses indicate E/Z ratios which
was determined by H NMR spectrum of the crude product.
1
7836 | RSC Adv., 2014, 4, 7834–7837
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