Nonsymmetrical 3,4-dithienylmaleimides by cross-coupling reactions with indium organometallics: Synthesis and photochemical studies

Article abstract of DOI:10.1002/chem.201403736

The synthesis and photochemical study of novel nonsymmetrical 1,2-dithienylethenes (DTEs) with a maleimide bridge have been carried out. The synthetic approach to the DTEs was based on successive selective palladium-catalyzed cross-coupling reactions of 5-susbtituted-2-methyl-3-thiophenyl indium reagents with 3,4-dichloromaleimides. The required organoindium reagents were prepared from 2-methyl-3,5-dibromothiophene by a selective (C-5) coupling reaction with triorganoindium compounds (R3In) and subsequent metal-halogen exchange. The coupling reactions usually gave good yields and have a high atom economy with substoichiometric amounts of R₃In. The results of photochemical studies show that these novel dithienylmaleimides undergo a photocyclization reaction upon irradiation in the UV region and a photocycloreversion after excitation in the visible region, thus they can be used as photochemical switches. ON-OFF operations can be repeated in successive cycles without appreciable loss of effectiveness in the process.

Full text of DOI:10.1002/chem.201403736

DOI: 10.1002/chem.201403736
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&
Photochromic Compounds
Nonsymmetrical 3,4-Dithienylmaleimides by Cross-Coupling
Reactions with Indium Organometallics: Synthesis and
Photochemical Studies
ꢀngeles Mosquera,[a] M. Isabel Fꢁrnandez,[b] Moisꢁs Canle Lopez,[b] Josꢁ Pꢁrez Sestelo,*[a] and
Luis A. Sarandeses*[a]
Dedicated to the memory of Jean-Louis Luche
Abstract: The synthesis and photochemical study of novel
nonsymmetrical 1,2-dithienylethenes (DTEs) with a maleimide
bridge have been carried out. The synthetic approach to the
DTEs was based on successive selective palladium-catalyzed
cross-coupling reactions of 5-susbtituted-2-methyl-3-thio-
phenyl indium reagents with 3,4-dichloromaleimides. The re-
quired organoindium reagents were prepared from 2-
methyl-3,5-dibromothiophene by a selective (C-5) coupling
reaction with triorganoindium compounds (R3In) and subse-
quent metal–halogen exchange. The coupling reactions usu-
ally gave good yields and have a high atom economy with
substoichiometric amounts of R3In. The results of photo-
chemical studies show that these novel dithienylmaleimides
undergo a photocyclization reaction upon irradiation in the
UV region and a photocycloreversion after excitation in the
visible region, thus they can be used as photochemical
switches. ON–OFF operations can be repeated in successive
cycles without appreciable loss of effectiveness in the pro-
cess.
Introduction
Organic photochromic compounds have received increased in-
terest due to their potential applications in optoelectronic mo-
lecular devices, such as molecular switches, logic gates, and in-
formation storage devices.[1] Among the different families of
photochromic materials, 1,2-diarylethenes, in particular 1,2-di-
thienylethenes (DTEs), which contain a cyclic ethylene bridge
bound to two functionalized 2-substituted-3-thiophenyl moiet-
ies, are one of the most interesting due to their thermal stabili-
ty and fatigue resistance.[2] The molecular switching of DTEs is
based on an efficient and reversible ON–OFF control photocyc-
lization reaction between colorless ring-opened and colored
ring-closed isomers by selective irradiation (Scheme 1).
Scheme 1. Photochromism of DTEs.
Both isomeric forms of the DTEs show marked differences,
not only in their absorption spectra, but also in other physical
properties, such as the fluorescence intensity,[3] redox poten-
tials,[4] refractive indices,[5] dielectric constants, and geometric
structure.[6] The photochemical behavior of these systems has
been exploited in numerous fields, for example, the develop-
ment of optical memory, optoelectronic devices, and organic
semiconductors,[7] the control of biological activity in living or-
ganisms,[8] the direct conversion of light into mechanical
work,[9] or as molecular regulators in DNA interactions.[10] All of
these appealing applications have stimulated the development
of new synthetic approaches to prepare novel DTEs with selec-
tive functions.
[a] Dr. ꢀ. Mosquera, Prof. Dr. J. Pꢁrez Sestelo, Prof. Dr. L. A. Sarandeses
Departamento de Quꢂmica Fundamental and
Centro de Investigaciones Cientꢂficas Avanzadas (CICA)
Universidade da CoruÇa, 15071A CoruÇa (Spain)
Fax: (+34)981167065
E-mail: luis.sarandeses@udc.es
sestelo@udc.es
[b] Dr. M. I. Fꢁrnandez, Prof. Dr. M. Canle Lopez
Chemical modifications of DTEs have focused on new ethyl-
ene bridges and functionalized thiophene units.[11] As a result,
there are a large number of DTEs furnished with a perfluoro-
ethylene bridge,[12] although DTEs with cyclopentene,[13]
maleimide,[14] maleic anhydride,[15] azulene,[16] and benzothia-
diazole[17] bridges have been designed.[18] The ethylene unit is
Departamento de Quꢂmica Fꢂsica e EnxeÇerꢂa Quꢂmica I
and Centro de Investigaciones Cientꢂficas Avanzadas (CICA)
Universidade da CoruÇa, 15071A CoruÇa (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403736. This includes all experimental
details, compound characterization, copies of spectra of new compounds,
and photochemical studies.
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usually functionalized with two identical thiophene units, thus
symmetrical compounds are afforded by double anionic substi-
tution of fluoro substituents of the perfluoroethylene bridge,
double cross-coupling reaction, or construction of the cyclic
bridge in the final steps. On the other hand, nonsymmetrical
DTEs are also compounds of interest; their synthesis by cou-
pling reactions are less common due to low selectivity and are
generally prepared by cyclization sequences or selective func-
tionalization of simple dithienylcyclopentenyl precursors.[12b,19]
In recent years, our group and others have shown that
indium organometallic reagents are useful for metal-catalyzed
cross-coupling reactions.[20,21] The main features of these re-
agents are their high efficiency, versatility, and selectivity.
Indeed, triorganoindium reagents (R3In) have been successfully
applied in selective coupling reactions with 3,4-dihalo-
maleimides, 2,5-dibromothiophene, and dihalopyrimidines.[22]
Herein, we report the synthesis and photochemical studies of
novel nonsymmetrical DTEs with a maleimide bridge. The com-
pounds were prepared by selective, sequential, iterative cross-
coupling reactions with R3In.
Table 1. Selective palladium-catalyzed cross-coupling reactions of triorga-
noindium reagents with 3,5-dibromo-2-methylthiophene (4).
Entry
R
T [8C]
Product
Yield [%]
1
2
3
4
5
phenyl
rt
5a
5b
5c
5d
5e
86
77
74
82
91
naphthalen-1-yl
thiophen-2-yl
benzo[b]thiophen-2-yl
phenylethynyl
80
80
80
80
other promising groups for optoelectronic applications.[2]
These organoindium reagents could be prepared from 3,5-di-
bromo-2-methylthiophene (4; Table 1) by another selective (C-
5) coupling reaction with an indium organometallic reagent.
Further metal–halogen exchange should allow the successive
use (in an iterative process) of thiophene 4 as an electrophile
and a nucleophile.[25] Overall, the synthetic process can be con-
sidered in three main steps: 1) preparation of substituted 2-
methyl-3-bromothiophenes 5 as precursors to the organoindi-
um reagents, 2) synthesis of monosubstituted 3-halo-
maleimides 6 by selective cross-coupling between a tri(3-meth-
ylthiophen-3-yl)indium 3 and a 3,4-dihalomaleimide 2, and
3) synthesis of DTEs 7 by a second coupling reaction.
The 3,4-disubstituted maleimide ring is an important moiety
in photochromic compounds[23] and light-emitting diodes[24]
with promising applications in material science. However, the
use of this unit as an ethylene bridge in DTEs has been less-
widely studied. Furthermore, the synthesis of nonsymmetrical
DTEs will enhance the chemical diversity of these compounds
and provide access to further selective functionalization and
specific reactivity. Finally, these new compounds should lead
to a new class of DTEs with novel photochemical behavior.
The synthesis started with the preparation of 5-substituted-
3-bromo-2-methylthiophenes 5 by selective coupling of 4.[26]
Initial screening with different palladium complexes showed
that the best results were achieved with [PdCl2(dppf)] (dppf=
1,1’-bis(diphenylphosphino)ferrocene) as the catalytic system,
a palladium complex previously used in our selective coupling
with halothiophenes.[22c] Accordingly, the reaction of Ph3In[27]
(40 mol%) with 4 in THF catalyzed by [PdCl2(dppf)] (5 mol%)
gave the monocoupling product 5a in 86% yield after 15 h at
room temperature (Table 1, entry 1). Interestingly, the reaction
took place selectively at the C-5 position and all three organic
groups attached to indium were efficiently transferred to the
electrophile.[28] Under these conditions, the reaction of
tri(naphthalen-1-yl)indium (40 mol%) with 4 gave the coupling
product 5b in low yield (<30%), but at 808C the yield in-
creased to 77% with a reaction time of only 2 h (Table 1,
entry 2).
Results and Discussion
Synthesis
Given the high selectivity exhibited by triorganoindium re-
agents in coupling reactions to electrophiles with various reac-
tive positions,[22] it was envisaged that nonsymmetrical DTEs
with a maleimide bridge (1; Scheme 2) could be synthesized
The methodology was extended to other triorganoindium
reagents. As mentioned before, DTEs with heteroaryl moieties,
such thiophenyl and benzo[b]thiophenyl, are promising candi-
dates for optoelectronic applications.[2,7] In this case, the reac-
tion of tri(thiophen-2-yl)indium with 4, in the presence of
[PdCl2(dppf)] (5 mol%), in THF at 808C gave the bisthiophene
5c in 74% yield (Table 1, entry 3). Analogously, the reaction of
4 with tri(benzo[b]thiophen-2-yl)indium selectively afforded
the monocoupling product 5d in 82% yield (Table 1, entry 4).
The great interest in alkynylated (hetero)arenes in the fields
of organic materials, natural products, and pharmaceuticals[29]
led us to assay the coupling reaction with a representative
Scheme 2. Synthetic plan for the synthesis of nonsymmetrical dithienylma-
leimides 2.
by sequential cross-coupling reactions of a 3,4-dihalomalei-
mides (2) with substituted 2-methyl-3-thiophenylindium re-
agents (3).
As nucleophilic partners, to take advantage of the versatility
of the methodology, we chose 5-substituted-2-methyl-3-thio-
phenyl indium reagents furnished with aryl, heteroaryl, and
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conjugated
alkyne,
namely
tri(phenylethynyl)indium
Table 2. Palladium-catalyzed mono-cross-coupling reactions of triorga-
noindium reagents with 2a.
(40 mol%). In this case the coupling product 5e was obtained
in excellent yield (91%, Table 1, entry 5). In general, these reac-
tions showed high selectivity, without detection of the double
coupling products, and gave good yields after short reaction
times.
With the 5-substituted-3-bromothiophenes 5a–e in hand,
we studied their conversion into the corresponding triorgano-
indium reagents and the coupling reaction with 3,4-dihaloma-
leimides 2. By using our general protocol,[22b] we found that
the reaction between the triorganoindium reagent 3a
(120 mol%) and 3,4-dichloromaleimide 2a, catalyzed by
[PdCl2(PhCN)2] (5 mol%), selectively gave the monocoupling
product 6a in 70% yield after 18 h at room temperature
(Table 2, entry 1). Interestingly, the reaction with the 3,4-
dibromomaleimide analogue gave lower selectivity and yield
(51%).
Entry
1
R1 In
Product 6
Yield [%]
3
3a
3b
3c
70
2
3
76
54
Under the same experimental conditions, the reaction of
3,4-dichloromaleimide 2a with the tri(thiophen-3-yl)indium re-
agents 3b–e bearing aryl, heteroaryl, or alkynyl groups at C-5
(prepared from 5b–e) afforded the monocoupling products
6b–e selectively in good yields (52–76%, Table 2, entries 2–5).
To broaden the chemical diversity of the maleimides, we also
performed the selective monocoupling reaction with function-
alized indium derivatives, such as the 2-methyl-5-hydroxyme-
thylthiophen-3-yl indium derivative 3 f or tri(2-methylbenzo[b]-
thiophen-3-yl)indium (3g). The latent hydroxyl group in 3 f
could be used as linker or for subsequent functionalization of
the resulting DTE, whereas the benzo[b]thiophenyl moiety
could be useful to compare thermal stabilities and fatigue re-
sistance. In both cases the coupling product was obtained in
good yield (Table 2, entries 6 and 7). Overall, the selectivity of
the reactions at room temperature is remarkable given that
only 40 mol% of organoindium reagent was used and the
double coupling products were not detected.
4
3d
3e
59
5
6
7
52
62
83
The last step for the synthesis of nonsymmetrical DTEs was
the incorporation of a second thienyl unit into 3-chloro-4-thi-
enylmaleimides 6. Under previously developed conditions, the
reaction of 6a with 3b (60 mol%) afforded the coupling prod-
uct in low yield (<50%), probably due to high steric hin-
drance. Fortunately, after studying different palladium sources
([Pd(OAc)2, [Pd2dba3] (dba=dibenzylideneacetone), Pd(PtBu3)2])
and ligands (SPhos=2-dicyclohexylphosphino-2’,6’-dimethoxy-
biphenyl; XPhos=2-dicyclohexylphosphino-2’,4’,6’-triisopropyl-
biphenyl; and CataCXium A=di(1-adamantyl)-n-butylphos-
phine), we found that [Pd2dba3] (2.5 mol%) and SPhos
(10 mol%) in THF heated at reflux gave the best results: dithie-
nylmaleimide 7 was obtained in excellent yield (96%) after
1
20 h at 808C (Table 3, entry 1). Interestingly, H NMR spectro-
maleimide 6g with various organoindium reagents (3a–3 f)
provided the dithienylmaleimides 9–13 in good yields (79–
96%, Table 3, entries 3–7). In all cases, except for 8, the novel
DTEs were isolated as a mixture of open and closed isomers in
variable ratios (from 89:11 for 10 to 54:46 for 12; see Table 5
below). The isomers were isolated by HPLC (except for 13) and
crystallization to enable the photochemical and kinetic proper-
ties to be studied.
scopic analysis of 7 showed a 32:68 mixture of the open/
closed isomers (see Table 5 below) from the photochemical
cyclization under ambient light. This observation further em-
phasizes the behavior of these dithienylmaleimides as photo-
chemical switches.
Under the same reaction conditions, the coupling of the di-
thienylchloromaleimide 6c with 3b afforded DTE 8 in 66%
yield (Table 3, entry 2) and the coupling reactions of chloro-
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Photochemical and kinetic studies
Table 3. Palladium-catalyzed cross-coupling reactions of triorganoindium
reagents with 3-chloro-4-thienylmaleimides 6.
The open isomers resulted in bright-yellow solutions in CHCl3
and their UV spectra showed an intense absorption in the UV
range l=290–340 nm, probably due to the presence of aro-
matic moieties, and a broad absorption band (l=400–420 nm)
in the visible range (Table 4). The latter is usually attributed to
charge transfer from the electron-rich thiophene to the elec-
tron-deficient maleimide ring.[14e,f] The general short range of
the absorption band indicates that the substitution at the thio-
phene has a small effect on the absorption characteristics of
the DTEs.
Entry
6
R1 In
Product
Yield [%]
3
1
6a
3b
3b
3a
3b
3d
3e
3 f
96
The irradiation of bright-yellow solutions of the compounds
in CHCl3 with UV light (l=313 nm) resulted in a rapid color
change to dark red (except for 8, for which photocyclization
was not observed), with an increase in the band at lꢀ410 nm
in the UV spectra and the appearance of a new band in the
visible region at l=509–574 nm (extinction coefficient (e)=
2.2–19.7ꢃ103 mꢁ1 cmꢁ1, Table 4). The l and e values are associ-
ated with a p!p* transition allowed both by spin and symme-
try, which corresponds to the closed isomers of the irradiated
compounds.
2
3
4
5
6
7
6c
6g
6g
6g
6g
6g
66
92
96
81
90
79
Interestingly, the solutions of cyclic isomers became colorless
again upon irradiation with visible light (l>550 nm). The pho-
tocycloreversion to the opened isomers can be monitored by
changes in the UV spectra. The progressive disappearance of
the band in the visible range (l=509–574 nm) and decrease in
intensity of the band at lꢀ410 nm until, finally, the initial
spectra are obtained indicates the reversibility of the process.
The presence of the same isosbestic points in the spectra con-
firm the reversibility of the process without secondary reac-
tions. As an example, absorption spectra for the photoconver-
sion of 7 are shown in Figure 1a.
To test the fatigue resistance and the thermal stability of the
materials, the closing–opening cycle (coloration–bleaching)
was performed at least twice (Figure 1b). The sequence was
also followed by HPLC (Table 5). Irradiation of pure 7 at l=
313 nm showed the photocyclization of the 13:87 ratio of
open/closed isomers in the photostationary state (Table 5,
entry 1). For 9 and 10, the photoconversion ratio was reduced
to 36:64 and 46:54, respectively, which indicated a lower effi-
ciency for the photocyclization process (Table 5, entries 2 and
3). For 11 and 12 the conversion ratio was close to that ob-
served for 7 (Table 5, entries 4 and 5). Interestingly, the irradia-
tion of these mixtures with visible light (l>550 nm) resulted
in the regeneration of the open isomers with higher ratios
(open/closed up to 96:4, Table 5). Notably, these values were
essentially maintained in a second coloration–bleaching cycle
(Table 5).
Dithienylethene 8 is special case. In solution it has a charac-
teristic orange color with a sharp absorption band at l=
323 nm and an intense broad band at l=480 nm (e=15.8ꢃ
103 mꢁ1 cmꢁ1; Table 4, entry 2). In this case, photochromism was
not detected under irradiation with UV or visible light. Relative
to other DTEs prepared, the absorption in the visible range
was bathochromically shifted by 65–78 nm and had a higher
intensity. This photochemical behavior could be attributed to
Overall, this synthetic sequence enabled us to prepare
a series of nonsymmetrical dithienylmaleimides functionalized
with valuable groups such as naphthyl, benzo[b]thiophenyl, or
alkynyl (10–12) or a group with the possibility of further func-
tionalization, as in 13. The high selectivity observed for the
coupling reactions is remarkable and allows easy access to
novel DTEs by using indium organometallic reagents.
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Table 4. Absorption characteristics of photochromic dithienylmaleimides 7–13.[a]
Entry
Compound
lO [nm] (e=[103 Mꢁ1 cmꢁ1])
lC [nm] (e=[103 Mꢁ1 cmꢁ1])
FO-C
FC-O
1
7
8
9
10
11
12
13
293 (29.2), 321 (25.2) 410 (5.0)
323 (27.8), 480 (15.8)
290 (22.7), 415 (5.1)
322 (31.3), 367 sh[b] (37.5), 387 (43.3), 574 (14.1)
–
0.39
–
0.03
–
2[c]
3
317 (22.6), 413 (11.0), 542 (7.8)
0.31
0.34
0.15
0.27
–
0.11
0.26
0.01
0.08
–
4
5
6
7[d]
298 sh (14.4), 413 (4.3)
322 (11.5), 335 (11.5), 405 (9.5), 531 (5.8)
318 sh (24.8), 347 (29.6), 404 (14.6), 555 (12.9)
318 sh (44.4), 343 (52.1), 416 (23.2), 550 (19.7)
246 (18.6), 271 sh (12.1), 331 (6.7), 395 (6.9), 509 (2.2)
318 sh (24.9), 339 (28.5), 417 (4.9)
299 sh (24.2), 329 (27.7), 413 (5.0)
247 (18.5), 288 sh (8.3), 401 (4.6)
[a] lO and lC are the wavelengths of the absorption band maxima for open (after HPLC separation) and cyclic forms (stationary state after irradiation at l=
313 nm), respectively. The contribution of the open form to lC was not subtracted. Quantum yields for the photocyclization (FO-C) and photocycloreversion
reactions (FC-O) were calculated by the literature method.[31] [b] sh=shoulder. [c] Photoreaction was not detected for compound 8. [d] The two isomers of
13 were not separated by HPLC. The extinction coefficients were estimated according the extinction coefficient at the isosbestic point at l=313 nm.
Table 5. Photoconversion ratios of dithienylethenes 7 and 9–12 in the
photostationary state after two irradiation sequences at l=313 nm and
l> 550 nm and with ambient light.[a]
First irradiation cycle[c]
Second irradiation
cycle[c]
Cmpd Ambient
light[b]
l1 =313 nm l1 >550 nm l2 =313 nm l2 >550 nm
1
2
3
4
5
7
9
10
11
12
32:68
58:42
89:11
63:37
54:46
13:87
36:64
46:54
21:79
16:84
94:6
93:7
96:4
79:21
94:6
14:86
33:67
47:53
14:86
16:84
92:8
97:3
97:3
82:18
91:9
[a] Photoreaction was not detected for compound 8 and compound 13
was not separated by HPLC. [b] Values correspond to the open/closed
ratio of isomers measured by HPLC in the photostationary state after irra-
diation. [c] l1 and l2 correspond to the wavelengths of the irradiation
light in the first and second cycle, respectively.
Kinetic studies showed that all the photocyclizations follow
first-order kinetics (Table 6) and are almost complete after 150–
200 min, except for 13, for which the photostationary state is
achieved after 40 min of irradiation. Interestingly, the photocy-
cloreversion requires a longer reaction time (about 400 min)
and 13 only required 40 min. This fact can be explained by the
conformation of the compound (see below). The reaction rates
for these compounds are lower relative to those of other DTEs
(i.e., with a perfluorocyclopentene bridge), but similar to those
reported previously for other 3,4-dithienylmaleimides. This be-
havior can be explained by charge transfer between the thio-
phene and the maleimide bridges, which can affect the effi-
ciency of the photochemical cyclization and diminish the rate
and yield of these reactions and, in some cases, can even
afford compounds that do not cyclize under irradiation.[14e,f] Fi-
nally, the rate constants for the second cycle are slightly faster
than the first, probably due to a conformational effect if the re-
quired antiparallel conformation was favored after the first
photocycloreversion.
Figure 1. a) Time-resolved spectra for photocyclization of 7 (l=313 nm,
[7]~10ꢁ4 m, T=258C) and b) cycling between the open and closed forms of
7 (g), 9 (c), 11 (c), and 12 (a) by repetitive irradiation at
l=313 nm (ring closure) and l>550 nm (ring opening) in CHCl3 at 258C
(values at l=410 nm).
the occurrence of a high charge transfer from the thiophene
rings to the electron-deficient maleimide bridge to result in
complete inhibition of the photoreaction. The reduction or in-
hibition of the photoreaction efficiency can be explained by
the formation of a twisted intramolecular charge transfer (TICT)
excited state, which is characterized by a twist of the single
bonds between the thiophenes and the maleimide
bridge.[14f,30]
To determine the efficiency of the process, the photocycliza-
tion and photocycloreversion quantum yields (F) were deter-
mined by a photokinetic method based on time-resolved ab-
sorbance measurements for both the photocoloration and
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a novel entry to the synthesis of
nonsymmetrical DTEs, and the
high reactivity, selectivity, and
atom economy shown by trior-
ganoindium reagents in cou-
pling reactions makes them
useful and versatile synthetic
tools in this context. In general,
photochemical and kinetic stud-
ies showed that these com-
pounds undergo a photocycliza-
tion reaction upon irradiation
with UV light and a photocyclore-
version when treated with visible
Table 6. Kinetic rate constants for the photochemical reactions of 7 and 9–13 after two irra-
diation sequences at l=313 and 550 nm.[a]
First cycle
k550 [sꢁ1
Second cycle
Entry Compound k313 [sꢁ1
]
]
k313 [sꢁ1
]
k550 [sꢁ1
]
1
2
3
4
5
6
7
9
10
11
12
13
(5.8ꢂ0.1)ꢃ10ꢁ2
(6.7ꢂ0.2)ꢃ10ꢁ2
(2.9ꢂ0.1)ꢃ10ꢁ3
(8.89ꢂ0.07)ꢃ10ꢁ2 (5.0ꢂ0.2)ꢃ10ꢁ3
(2.02ꢂ0.09)ꢃ10ꢁ2 (9.6ꢂ0.2)ꢃ10ꢁ2
(3.1ꢂ0.1)ꢃ10ꢁ2
(7.0ꢂ0.4)ꢃ10ꢁ2
(5.31ꢂ0.07)ꢃ10ꢁ2 (3.2ꢂ0.1)ꢃ10ꢁ2
(2.40ꢂ0.03)ꢃ10ꢁ2 (2.4ꢂ0.1)ꢃ10ꢁ3
(5.46ꢂ0.06)ꢃ10ꢁ2 (8.1ꢂ0.4)ꢃ10ꢁ3
(7.8ꢂ0.1)ꢃ10ꢁ2
(3.40ꢂ0.04)ꢃ10ꢁ2 (4.3ꢂ0.2)ꢃ10ꢁ3
(7.65ꢂ0.08)ꢃ10ꢁ2 (1.21ꢂ0.05)ꢃ10ꢁ2
0.106ꢂ0.008
0.110ꢂ0.002
0.106ꢂ0.004
0.10ꢂ0.01
[a] k313 and k550 correspond to the rate constants for photocyclization (l=313 nm) and photo-
cycloreversion (l=550 nm), respectively, in the first and second irradiation cycles.
photobleaching processes (Table 4).[31] The photocyclization
(open–closed form) quantum yields fall in the range FO-C
light. This behavior could allow the use of these molecules as
photochemical switches. ON–OFF operations can be repeated
in successive cycles without an appreciable loss in effective-
ness of the process. Further studies on the synthesis and pho-
tochemical studies of novel DTEs are currently ongoing in our
laboratories.
=
0.15–0.39, whereas the photocycloreversion (closed–open
form) quantum yields (FC-O) are lower, especially for 7, 11, and
12. These results are consistent with those observed from ki-
netic data for analogous dithienylethene systems.[1b] Interest-
ingly, X-ray analysis of 10 and 13 showed a parallel conforma-
tion for 10, with the thiophene moieties aligned perpendicular-
ly to the maleimide bridge, and an antiparallel conformation
for 13 (Figure 2).[32] It is known that the open-ring isomers of
Acknowledgements
We are grateful to the Spanish Ministerio de Ciencia e Innova-
ciꢄn (MICINN) and Ministerio de Economꢅa y Competitividad
(MINECO) (grant numbers CTQ2009–07180, CTQ2012–31200,
and ACI2010–1093), the Xunta de Galicia (grant number INCI-
TE08PXIB103167PR), and the EDRF funds for financial support.
A.M. also thanks the Spanish Ministerio de Educaciꢄn y Ciencia
(MEC) for a predoctoral fellowship (Formaciꢄn de Personal In-
vestigador). We also acknowledge the assistance of Prof. Alber-
to Fernꢆndez for processing the X-ray data.
Keywords: CꢁC coupling · indium · palladium catalysis ·
photochemical reactions · photochromism
Figure 2. ORTEP drawings (showing 40% probability displacement ellipsoids)
of the crystal structures of 10 and 13.[32] The hydrogen atoms are omitted
for clarity.
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Received: May 29, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 8
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7
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FULL PAPER
&
Photochromic Compounds
ꢀ. Mosquera, M. I. Fꢁrnandez,
M. Canle Lopez, J. Pꢁrez Sestelo,*
L. A. Sarandeses*
&& – &&
Photochromic compounds: Triorga-
noindium reagents are useful reagents
for the synthesis of nonsymmetrical 3,4-
dithienylmaleimides by palladium-cata-
lyzed cross-coupling reactions (see
scheme). Photochemical studies show
that these novel compounds undergo
photocyclization–photocycloreversion
upon irradiation, which allows their use
as photochemical switches.
Nonsymmetrical 3,4-
Dithienylmaleimides by Cross-
Coupling Reactions with Indium
Organometallics: Synthesis and
Photochemical Studies
&
&
Chem. Eur. J. 2014, 20, 1 – 8
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8
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!