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ever, they suffer from drawbacks such as expensive
reagents, use of basic conditions and incompatibility of
reactive functional groups (e.g. NO2, CN) under Grig-
nard conditions. In this paper, we report the prepara-
tion of thiocarboxylic S-esters from olefins and
thiobenzoic acids in the presence and absence of clay
catalysts giving rise to Markovnikov and anti-
Markovnikov addition products, respectively (Scheme
1).
Surprisingly, the addition of both thiophenol and
thiobenzoic acid onto alkenes takes place in an anti-
Markovnikov fashion leading to linear structures 1
either at 25 or 80°C with high conversions (>90%). To
obtain more information on the nature of the interme-
diate, an experiment was carried out in the presence of
galvinoxyl radical as a spin trap at 25°C under nitro-
gen. However, the addition reaction still proceeded to
give a linear structure in high yields, thus eliminating
the possibility of involvement of radical species. The
formation of anti-Markovnikov products may be
explained by presuming that the addition of SꢀH onto
CꢁC bonds takes place in a concerted manner with
steric factors controlling the regioselectivity.
Table 1 presents the results of the regioselective addi-
tion of thiophenol and thiobenzoic acid onto styrene
both in the presence and absence of catalysts. For
example, in the absence of any catalyst, the SꢀH addi-
tion takes place in an anti-Markovnikov fashion lead-
ing to the linear structure 1 in high yields. However, in
the presence of Lewis acid-type catalysts, such as clays
(e.g. AlCl3, etc.), the SꢀH addition onto CꢁC proceeded
in a Markovnikov manner giving branched structure 2
in high yields. A search of the literature revealed the
lack of a systematic study carried out to establish the
nature of the products resulting from the addition of
thiophenol to olefins in the absence of a catalyst.16
Table 3 presents the results of the addition of various
thiols and thiobenzoic acids onto a variety of olefins in
the presence of Mont K 10 clay as the catalyst in
refluxing benzene. Toluene was also used with equiva-
lent results. Evidently, the addition of thiols or
thioacids onto olefins in the presence of a clay catalyst
proceeded largely in a Markovnikov fashion. However,
in the case of aliphatic olefins (entries 12 and 13),
considerable amounts of linear structures (30%) are
formed. Also, poor selectivity to branched structures
was observed in the case of 4-bromostyrene. Surpris-
ingly, the addition of thiobenzoic acid onto methyl-
methacrylate in the absence or presence of acidic, basic
or neutral clays gave, almost exclusively, anti-
Table 2 shows the results of the regioselective addition
of thiophenol and thiobenzoic acid onto various olefins
in the absence of any catalyst. All reactions were car-
ried out in nitrogen using freshly distilled reagents to
eliminate oxygen and peroxide impurities, respectively.
Table 1. Regioselective addition of thiols/thioacids onto styrenea
Entry
Thiols/thioacids
Catalyst
Conv. (%)b
Product distribution (%)b
1
2
Othersc
1
2
3
4
5
6
Thiophenol
Thiophenol
Thiobenzoic acid
Thiobenzoic acid
Thiobenzoic acid
Thiobenzoic acid
None
Mont K 10
None
Mont K 10
Mont KSF
AlCl3
95
93
100
97
89
96
97
6
98
3
2
5
2
80d
–
89
92
95
1
14
2
8
6
–
a Styrene (10 mmol), thiol/thioacid (10 mmol), catalyst (10% wt), benzene (25 ml), 80°C, 24 h.
b Based on GC–MS analysis.
c Mixtures of product, uncharacterized.
d Identical results were obtained with recovered catalyst in a subsequent experiment.
Table 2. Addition of thiols/thioacids onto olefins in the absence of catalyst: formation of anti-Markovnikov productsa
Entry
Olefin
Thiols/thioacids
Temp. (°C)
Conv. (%)b
Product distribution (%)b
1
2
Othersc
1
2
3
4
5
6
7
Styrene
Styrene
Styrene
Styrene
1-Hexene
4-Acetoxystyrene
Methylmethacrylate
Thiophenol
Thiophenol
25
97
99
94
92
93
91
96
94
98
97
95
80
85
93
2
2
–
3
5
5
2
4
–
3
80d
25
Thiobenzoic acid
Thiobenzoic acid
Thiobenzoic acid
Thiobenzoic acid
Thiobenzoic acid
80d
25
25
2
15
10
5
25
a Olefin (10 mmol), thiol/thioacid (10 mmol), benzene (25 ml), 24 h, N2.
b Based on GC–MS analysis, major products were thoroughly characterized by IR, 1H and 13C NMR, and MS analyses.
c Mixtures of product, uncharacterized.
d 10 h.