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96
Kazemi, Kiasat, and Ebrahimi
ing thiiranesby an oxygen– su lfur exchange reaction. Sulfur introducing
[3,4]
[
2,3]
reagent su sch asinorganic thiocyanat e, s
sulfide,
thiourea,
dimethylthioformamide,
phosphine
[7]
[5]
[6]
3-methylbenzothiazol-2-thione,
silica-gel supported potassium thiocyanate, and polymer supported
[
8]
[9]
thiocyanate have already been proposed in the literature for this goal.
Recently, conversion of epoxides to thiiranes in the presence of efficient
[10–12]
catalysts
wasal so reported.
Application of oxiranesasconvenient st arting materialsand thiourea
as sulfured agent for this conversion was reported under wet conditions
[
solvent free) or in aqueous ethanol, but the reactions su ffer from long
3]
(
reaction times, low yield and occurrence of desulfuration of obtained
episulfide to olefin in some cases. Recently, application of poly
[
11]
[12]
(
4-vinylpyridine)–Ce(OTf )4
as catalysts for this conversion was reported.
and tin(IV) mesotetraphenylporphyrin
The successful applications of LiBF , asa l so w BF
4
releasing
3
[13–15]
source,
LiBF in organic synthesis
and in continuation of our studies on application of
16,17]
[
prompted usto explore the potential of
4
this lithium salt as a catalyst for conversion of epoxides to thiiranes with
thiourea. Now we wish to report an improved, efficient and easy synthesis
of thiiranesfrom oxiranesunder mild non-aqueousconditionsin excellent
yieldsin the pre se nce of catalytic amountsof LiBF
4
.
[
18]
The catalyst, LiBF , can be easily prepared from tetrafluoroboric
4
acid and LiOH and in comparison with BF ÁOEt is solid and stable.
3
2
We examined the catalytic ability of LiBF for conversion of epoxides
4
to thiiraneswith thiourea in acetonitrile under reflux condition .s This
catalyst acted very efficiently and it was observed that 0.2 molar equi-
valent of the catalyst is enough to convert epoxides to their corresponding
thiiranesin excellent yieldswithin 10–140 min.
The effects of other solvents such as acetone, ether, carbon tetrachloride
and t-butanol were also studied, but in comparison with acetonitrile the reac-
tionstimeswere longer and the yieldswere found to be con si derably lower.
Different typesof oxiran e, s except cyclooctene oxide, carrying
activated and deactivated groupsare converted to the corre ps onding
thiiranesasexclui sv e and virtually pure productsaccording to TLC
1
and H NMR, in considerably short times and in excellent yields.
The results are shown in Table 1.
The promoting effect of LiBF wasdefinitely confirmed by reaction
4
of styrene oxide with thiourea under similar conditions, without adding
LiBF , GC, and TLC analysis of the reaction mixture did not show
4
formation of any product after 5 h.
In reports where an aqueous solvent has been used, control of pH is
[
1]
important to obtain a high yieldsof epi su lfideswithout polymerization.