Received: October 20, 2014 | Accepted: December 2, 2014 | Web Released: December 11, 2014
CL-140960
Synthesis of Oxidized Thioxanthene-type Base Amplifiers
and Their Application to Photoreactive Materials
Koji Arimitsu,* Hiroshi Kitamura, Ryuta Mizuochi, and Masahiro Furutani
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba 278-8510
(E-mail: arimitsu@rs.noda.tus.ac.jp)
We propose base amplifiers (BAs) that are autocatalytically
decomposed into base molecules by a small amount of trigger
H2NR
base molecules from photobase generators. We report here novel
BAs having oxidized thioxanthene skeletons. It is confirmed that
they are decomposed autocatalytically in solution or polymer
matrix. They are applied to UV curing systems to improve their
photosensitivity.
X
X
+
H2NR
O
NHR
Δ
-CO2
O
1 (X = SO2)
2 (X = SO)
D-1 or D-2
Photoreactive materials are indispensable for modern
chemical industries. They are utilized in many fields such as
electronics, cars, residential equipment, and other areas. To
improve their photosensitivity, the concept of a proliferation
reaction has been proposed by our group, where molecular
catalysts themselves are generated autocatalytically by photo-
generated catalysts, which is clearly distinguished from the
chemical amplifications.1 Secondary increase of the amount
of catalyst would make the appearance quantum yields of
photolatent catalysts greater than one. Acid amplifiers were
initially developed for the concept2 and have been applied to
types of acid-reactive materials with photoacid generators.3
However, acids generated in the proliferation reactions are
problematic in that the erosion of metallic substrates could
proceed.
To solve this problem, base amplifiers (BAs) have also been
developed by our group.4 Because photobase generators (PBGs)
for base-reactive materials generally have low photosensitivity,
BAs can be of great assistance when using photoreactive
materials based on base-catalytic reactions.4-6 9-Fluorenylcar-
bamates are one of the representative BAs inspired by a
protecting group of aliphatic amino groups in artificial peptide
syntheses.4a,4b,5,7 This type of BA has been integrated into
various photopolymers.8-10 In this study, we considered oxidized
thioxanthenylcarbamates for designing BA molecules that were
also reported as a protecting group of amino groups by Carpino
et al. (Scheme 1),11 because it would be easy for such protecting
groups to change their decomposition rate by modifying the
oxidation state of the sulfur atom. The sulfone-type 1 and
sulfoxide-type 2 were both synthesized, in the expectation that
the decomposition rate of the former would be faster than that of
the latter because of the stabilization effect of the sulfone moiety
on the anionic intermediate.
H2NR (amine):
H2N
NH2
H2N
for 1a and 2a
for 1b
Scheme 1. Autocatalytic fragmentation of BAs 1 and 2 having
oxidized thioxanthene moieties.
100
80
60
40
20
0
100
80
60
40
20
0
0
20
40
60
Heating time / min
Figure 1. The consumption of 1a (conversion, closed symbols) and
the formation of D-1 (yield, open symbols) as a function of heating
time in the presence (red) and the absence (blue) of 13 mM of
piperidine in 1,4-dioxane-d8 at 100 °C.
form. The solubility of sulfone-type 1a was slightly greater than
that of sulfoxide-type 2a.
A BA should fulfill the following three requirements.4b First,
a BA molecule should undergo a base-catalyzed decomposition
reaction to liberate a base, leading to autocatalytic decomposi-
tion. Second, the BA should be thermally stable in the absence
of other base molecules under reaction conditions where both
the autocatalytic decomposition and subsequent base-catalyzed
reaction proceed. Third, the liberated base should be strong
enough to catalyze the subsequent chemical reactions, resulting
in a nonlinear chemical transformation. The thermal decompo-
sition behavior of 1a in 1,4-dioxane-d8, at 100 °C, was monitored
by 1H NMR spectroscopy. Figure 1 shows both the consumption
BAs 1 and 2 were both synthesized with 9H-thioxanthene as
a starting compound.12 9H-Thioxanthene was oxidized again by
m-chloroperbenzoic acid, and 9H-thioxanthene-10,10-dioxide
for 1 was obtained with 3 equiv of the peroxide, while 9H-
thioxanthene-10-oxide for 2 was obtained with 1.3 equiv of the
peroxide. The overall yields were 29% (1a), 27% (1b), and 16%
(2a) after four synthetic steps. These BAs were soluble in some
organic solvents such as methanol, acetone, THF, and chloro-
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