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A. R. Jennings et al. / Tetrahedron Letters 55 (2014) 6773–6775
Table 1
formation of a resonance stabilized benzenethiyl radical, which
Results from Series 1, reactions with ethanethiol
would be less reactive than its alkyl counterparts. Despite this,
excellent yields were still obtained by simply increasing the
reaction times. Performing the reactions shown in Table 3 in ether
significantly increased the reaction times. For example, when the
synthesis of 3a was attempted in ether, the reaction was incom-
plete even after 8 h of stirring under the UV light source (see
Fig. S48, Supporting information).
hν, 4h, N
1 mol% PI
2
SiMe Cl
3-n n
SiMe Cl
SH
S
3-n
n
n
Compound
Yield (%)
1
2
3
1a
1b
1c
98 (99)a
95 (94)a
100 (99)a
In the fourth and final study, this novel selectivity was applied
to a multifunctional thiol that was previously synthesized in our
laboratory (Table 4).22 The reactions with the multifunctional thiol
were performed in ether. When attempted under bulk conditions,
the reactions were incomplete after 4 h (see Fig. S49, Supporting
information). The multifunctional thiol that was employed is more
viscous than the thiols used in Series 1–3. Thus, it was necessary to
use a solvent to improve the reagents’ miscibility in order to obtain
completely reacted products after 4 h.
The compounds shown in Tables 1–4 were characterized using
multi-nuclear NMR spectroscopy (1H, 13C, 29Si) and all analyses
demonstrated no need for further purification (see Figs. S3–38,
Supporting information). In some cases, trace amounts of benzo-
phenone can be seen in the 1H and 13C NMR spectra, but the signals
are easily distinguishable from the product peaks (see Figs. S1 and
S2, Supporting information). Additionally, when performing the
thiol-ene reaction there is a small percentage of thiol that adds
to the interior carbon atom of the –ene, yielding what is known
as the alpha product. The formation of the alpha product can easily
be distinguished by a characteristic upfield doublet in the 1H NMR
spectrum, which corresponds to the methyl protons coupled to the
methine proton. As previously mentioned, chlorosilanes are very
reactive toward moisture so the products should be stored in a
glove box. Attempts to obtain combustion analysis data on the
products from Series 1–4 were unsuccessful, and were attributed
to the reactive nature of chlorosilanes under normal atmospheric
conditions.
a
Yields obtained in ether.
Table 2
Results from Series 2, reactions with 1,2-ethanedithiol
HS
SiMe Cl
SH
2
3-n
n
hν, 4h, N
2
1 mol% PI
S
Cl Me Si
n
3-n
S
SiMe Cl
3-n
n
n
Compound
Yield (%)
1
2
3
2a
2b
2c
97 (91)a
99 (87)a
97 (91)a
a
Yields obtained in ether.
Table 3
Results from Series 3, reactions with benzenethiol
SH
S
hν, N
1 mol% PI
2
SiMe Cl
3-n n
SiMe Cl
3-n
n
n
Compound
Reaction time (h)
Yield (%)
1
2
3
3a
3b
3c
8
14
10
97
98
97
To demonstrate that the compounds synthesized in Tables 1–4
could be used for further chemical transformations, compounds
2a–2c were reacted with the appropriate amount of 2-mercap-
toethanol (2, 4, or 6 equiv), producing Series 5, branched com-
pounds 5a–5c, through an orthogonal reaction we reported
earlier.22 Table 5 summarizes these results.
Table 4
Results from Series 4, reactions with a multifunctional thiol
Compounds 5a–5c were obtained in good yield and character-
ized by multi-nuclear NMR spectroscopy (1H, 13C, 29Si) and
combustion (C, H, N) analysis (see Figs. S39–S47, Supporting
information). Compounds 5b and 5c could serve as cores for hydro-
lytically sensitive dendrimers.
O
SiMe Cl
n
4
Si
SH
3-n
4
hν, 4h, N
2
Et O, 1 mol% PI
2
O
SiMe Cl
3-n n
Si
S
4
In all cases, the orthogonality of the reactions was evident. The
chlorovinylsilanes reacted with the thiols exclusively via thiol-ene
n
Compound
Yield (%)
1
2
3
4a
4b
4c
100
100
100
Table 5
Results from the synthesis of Series 5
Cl Me Si
S
n
3-n
S
SiMe Cl
3-n
n
(Table 2). This set of reactions could also be performed in ether;
however, the yields were slightly improved when bulk conditions
were employed.
OH
2n
HS
2n
Et N
3
To further demonstrate the scope of this newly discovered
selectivity, an aromatic thiol was employed in Series 3 (Table 3).
During the studies conducted with benzenethiol, it was
determined that significantly longer reaction times were required;
however, this was not an unexpected result. The thiol-ene reaction
proceeds by a free-radical mechanism, the rate of which is
dependent on a number of factors including the chemical nature
of the –ene and thiol as well as the stability of the resulting radi-
cal.9,10 The decreased rate for this set of reactions is attributed to
Me
Si
3-n
O
HS
S
S
Si
SH
n
O
n
Me
3-n
n
Compound
Reaction conditions
Yield (%)
1
2
3
5a
5b
5c
THF, RT, 30 min, 55 °C, overnt.
THF, RT, 30 min, 55 °C, overnt.
THF/Et2O (1:1), RT, 30 min, 55 °C, 60 h
99
92
83