Pd-promoted homocoupling reactions of unsaturated silanes in aqueous micelles

Article abstract of DOI:10.1002/ejoc.201000021

Palladium-promoted homocoupling reaction of vinyl- and polyenylsilanes in aqueous conditions has been investigated. The reaction is catalyzed by PdCl ₂ in the presence of the reoxidizing system. CuCl₂/LiCl and occurs at room temperature in aqueous solutions containing nonionic amphiphiles. Symmetrically α,ω-disubstituted stereodefined all-trans polyenes have been obtained in mild conditions and in good yields (65-87 %), higher than those previously reported for the same reactions carried, out in methanol or HMPA. A comparison between two commercially available surfactants, Triton X-100 and PTS, has been performed.

Full text of DOI:10.1002/ejoc.201000021

FULL PAPER
DOI: 10.1002/ejoc.201000021
Pd-Promoted Homocoupling Reactions of Unsaturated Silanes in Aqueous
Micelles
[a]
[b]
[b]
[b]
Stefania R. Cicco, Gianluca M. Farinola,* Carmela Martinelli, Francesco Naso, and
Matteo Tiecco[
c]
Keywords: Silanes / Palladium / Green chemistry / Surfactants / Micelles
Palladium-promoted homocoupling reaction of vinyl- and
polyenylsilanes in aqueous conditions has been investigated.
have been obtained in mild conditions and in good yields
(65–87%), higher than those previously reported for the same
The reaction is catalyzed by PdCl
2
in the presence of the re- reactions carried out in methanol or HMPA. A comparison
oxidizing system CuCl /LiCl and occurs at room temperature
in aqueous solutions containing nonionic amphiphiles. Sym-
2
between two commercially available surfactants, Triton X-
100 and PTS, has been performed.
metrically α,ω-disubstituted stereodefined all-trans polyenes
Introduction
pounds, to the best of our knowledge micellar versions of
these processes have never been investigated so far. In recent
years, in connection with our continuous interest in devel-
opment of synthetic methodologies for organic materials
In recent years the use of water as solvent for organic
reactions has been actively investigated[ since aqueous
conditions combined with efficient catalyst recycle could
strongly reduce the environmental impact of several syn-
thetic sequences. In many cases the use of surfactants in
water effectively improves the reaction outcome compared
with the same processes carried out in pure water, an effect
which is called micellar catalysis.[ A micellar system is
composed of spherical amphiphilic aggregates of surfactant
molecules having nanometric dimension range suspended in
water or in media similar to water, and it can be considered
a nano-heterogeneous two-phase system. In fact, such sup-
ramolecular structures create an environment that can ac-
celerate the reactions by hydrophobic and concentration ef-
fects, due to the segregation of the reagents in the nano-
cores of the micellar structures. In particular, micellar catal-
ysis represents a possible approach to convert common
transition-metal catalyzed cross-coupling reactions into
green chemistry processes carried out in water and appropri-
ate experimental protocols have been set up for efficient
1]
[7]
for photonics and electronics, we have disclosed several
efficient and selective protocols based on coupling reactions
of unsaturated silanes to obtain stereodefined conjugated
[8]
oligomers and polymers. Therefore, we considered inter-
esting to investigate whether aqueous micellar conditions
can positively affect the outcome of some of these processes.
Accordingly, as a first exploration in this direction, we re-
port herein our study on the micellar version of a Pd-pro-
moted homocoupling process of vinyl- and polyenylsilanes
that we had previously shown to be an efficient methodol-
ogy to synthesize extended stereodefined all-trans polyenes
up to eight conjugated double bonds long, by doubling in
one step the conjugated system of the starting unsaturated
2]
[9]
silanes.
The significance of polyenes in studies dealing with or-
ganic electronics is manifold: conjugated polyenes have
[10]
been investigated as molecular conductors
and
and
[11]
[12]
[13]
switches,
optical conductors,
optical diodes,
[
3]
[4]
[5]
Heck, Sonogashira, and Suzuki–Miyaura reactions.
In spite of the well-recognized synthetic utility of metal
[14]
nonlinear optical (NLO) materials.
Moreover, due to
their well-defined chemical structure and possibility of se-
lective functionalization, they represent valuable model sys-
tems for validation of predictions based on theoretical cal-
culations and for developing new concepts in organic elec-
[
6]
catalyzed coupling reactions of unsaturated silanes, due
to the easy handling and low toxicity of organosilicon com-
[
[
a] Istituto di Chimica dei Composti Organometallici, ICCOM-
CNR di Bari,
[
15]
tronics.
via Orabona 4, 70125 Bari, Italy
b] Dipartimento di Chimica, Università degli Studi di Bari,
via Orabona 4, 70125 Bari, Italy
Results and Discussion
We report here the application of micellar aqueous con-
ditions to homocoupling reactions of vinyl- and polyenyl-
trimethylsilanes promoted by PdCl2 in the presence of
Fax: +39-080-5442076
E-mail: farinola@chimica.uniba.it
[
c] CEMIN – Centro di Eccellenza Materiali Innovativi Nanostrut-
turati,
Dipartimento di Chimica, Università di Perugia,
Via Elce di Sotto 8-I, 06123 Perugia, Italy
CuCl and LiCl.
2
Eur. J. Org. Chem. 2010, 2275–2279
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2275

S. R. Cicco, G. M. Farinola, C. Martinelli, F. Naso, M. Tiecco
FULL PAPER
As previously reported,[9,16] silanes 1 afford dimerization lipophilic core of the micelles. Moreover, the CuCl pro-
2
products 2 (Scheme 1) with yields ranging from 30 to 61% moted halogenation of olefins is favoured by solvents that
by treatment with catalytic[
16]
to substoichiometric
[9]
can coordinate copper, and this does not occur in the case
amounts of PdCl in methanol or HMPA, in the presence of the long polyether chain of the Triton X-100 surfac-
2
[
17]
of a large excess of CuCl and LiCl, which are necessary to tant.
reoxidize the Pd generated in the reductive elimination step
2
0
leading to the new carbon–carbon bond formation. The re- Table 1. Homocoupling reaction of 3 in the presence of Triton X-
00.[
a]
1
action is highly stereoselective, affording only the all-E
Entry Catalyst[b] (% PdCl2) Time [h] GC conversion [%] Yield[c] [%]
stereoisomer of the product polyenes.
1
2
3
3
10
25
25
100
24
24
24
24
6
45
60
100
45
n.d.[e]
36
87
[
d]
[e]
4
n.d.
[f]
5
100
87
[
a] Reactions carried out at room temperature in 15 wt.-% Triton
X-100/H 0. [b] Molar ratio PdCl /CuCl /LiCl, 1:8:3. [c] Based on
2
2
2
isolated materials. [d] Reaction carried out “on water”: without the
presence of surfactant. [e] n.d.: not determined. [f] In this case the
reoxidant system CuCl /LiCl was obviously not required.
2
Scheme 1. Homocoupling reactions of unsaturated silanes.
Competing proton-[17] and chlorodesilylation[18] pro-
cesses are side reactions which decrease the yield of dimer-
ized products. The former process is promoted by the hy-
drogen chloride derived from the methanolysis of the tri-
methylsilyl chloride generated in the reaction medium,
while chlorodesilylation is due to CuCl2.[
When the reactants were stirred in aqueous suspension
entry 4) in the absence of surfactant, a protocol denoted
(
[
19]
as “on water” conditions,
only 45% conversion was
achieved after 24 hours and a black colloidal palladium
precipitate was formed in the reaction mixture.
9]
Equally good yield (87%) was obtained with a stoichio-
We started our investigation by reacting (E)-1-p-tolyl-2-
metric amount of PdCl , in the absence of the reoxidizing
2
(trimethylsilyl)ethene (3) (Scheme 2) in water with different
system (CuCl /LiCl) (entry 5, Table 1). The same reaction
2
amounts of the catalytic system composed of PdCl , CuCl ,
2
2
carried out in methanol occurred with a slightly lower yield
79%), but the addition of one equivalent of dicyclohexyl-
LiCl, in the presence of the commercially available nonionic
amphiphile Triton X-100 (Figure 1).
(
ethylamine was necessary to prevent protodesilylation of
the starting silane by HCl generated in the reaction environ-
[
16]
ment.
Complete conversion of the reactants in the stoichio-
metric conditions was fast (6 h), likely due to the higher
availability of the palladium(II) in the nano-confined micel-
Scheme 2. Homocoupling reaction of (E)-1-p-tolyl-2-(trimethyl- lar environment.
silyl)ethene 3.
The investigation was then extended to the homocou-
pling reaction of keto-polyenylsilanes (Scheme 3). The con-
jugated polyenic carbonyl compounds are useful structural
[
20]
intermediates in organic synthesis, especially in the field
[
21]
of natural products. The use of these starting compounds
enables a straightforward comparison with the results ob-
tained in our previous studies dealing with the same reac-
Figure 1. Structure of Triton X-100.
A minimum 25% palladium catalyst with respect to the tion carried out in methanol or HMPA.[9]
starting silane 3 was necessary to reach the complete con-
version (Table 1, entry 3). In these conditions, the reaction
produces stereoselectively the dimerization product
(1E,3E)-1,4-bis(p-tolyl)butadiene 4 with unexpectedly high
yield (87%) compared with the yields obtained when the
reactions were carried out in MeOH or HMPA.[
9,16]
The
increased yield is due to the suppression of proto- and chlo-
rodesilylation side reactions which negatively affect the pro-
Scheme 3. Homocoupling reactions of keto-polyenylsilanes 5 and
cess in non-micellar conditions.[
9,16]
Inhibition of protode- 7 in the presence of Triton X-100.
silylation can be attributed to the lipophilic nano-environ-
ment of the micellar core which protects the organic sub-
All the reactions were carried out in surfactant/water me-
strate from the reaction with HCl and likely reduces or dia, at room temperature and in conditions ranging from
eliminates its formation. Similarly, the CuCl salt responsi- catalytic to substoichiometric in PdCl and the results are
2
2
ble for the chlorodesilylation reaction would not enter the summarized in the Table 2.
2276
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 2275–2279

Homocoupling of Unsaturated Silanes in Aqueous Micelles
Table 2. Homocoupling reactions of keto-polyenylsilanes in the presence of Triton X-100.[a]
Entry
Silyl ketone
Catalyst[b] (% PdCl
2
)
Time [h]
GC Conversion [%]
Product
Yield[c] [%]
1
2
3
4
5
5
5
7
3
12
12
12
24
30
45
100
100
6
6
6
8
8
10
25
25
20
85
65
[a] Reactions carried out at room temperature in 15 wt.-% Triton X-100/H
2
2 2
O. [b] Molar ratio PdCl /CuCl /LiCl, 1:8:3. [c] Based on
isolated materials.
Three experiments were carried out using the silylated
ketone 5 as the substrate to give (2E,4E)-1,6-diphenyl-2,4-
butadiene-1,6-dione (6). The results for these reactions are
similar to those recorded with 3: the highest yield in the
polyene 6 was obtained performing the homocoupling reac-
tion with 25% PdCl (Table 2, entry 3), while complete con-
2
version was not reached with catalytic percentages (3%, Scheme 4. Homocoupling reaction of unsaturated silyl ketones in
0%, Table 2, entries 1, 2) of the PdII salt. Again, no side
PTS.
1
reactions (protodesilylation and chlorodesilylation) of the
starting silyl ketone 5 were detected. Finally, in the opti-
mized conditions, the conjugated triene 7 afforded the di-
keto-polyene 8 in 65% yield (Table 2, entry 4).
Reducing the concentration of the surfactant to 2 wt.-%
PTS/H O resulted in a lower rate of the dimerization reac-
2
tion (Table 3, entries 1, 2). As with Triton X-100, the con-
version critically depends on the amount of catalyst em-
ployed (Table 3, entries 1,3). Increasing the amount of the
Among the wide number of nonionic micelle-forming
surfactants employed in micellar catalysis, the Lipshutz’s
[
22]
group
reported studies on the vitamin-E-based amphi-
surfactant to 15 wt.-% PTS/H O (Table 3, entry 5), no sub-
2
phile PTS (polyoxyethanyl α-tocopheryl sebacate) (Fig-
ure 2) which can be employed as micelle-forming system in
transition-metal catalyzed cross-coupling reactions carried
out in neat water. Recently, aqueous solutions containing
small amounts of PTS were also used as reaction media for
stantial improvement of the reaction yield was observed
compared with the use of 7 wt.-% PTS/H O (Table 3, entry
2
4) and a very viscous reaction mixture was generated that
was difficult to stir without heating.
Finally, conjugated diketo-polyenes with four and six
double bonds have been obtained in good yield by dimer-
ization of the keto-polyenylsilanes 5 and 7 in the optimized
conditions (Table 3, entries 7, 8), while reducing the amount
of surfactant in the case of the substrate 5 resulted in longer
reaction time and lower yield (Table 3, entry 6).
[23]
efficient olefin cross-metathesis
and ring closing meta-
[
24]
thesis reactions.
Figure 2. Structure of PTS (n ≈ 13; MW ≈ 1200).
Conclusions
Therefore, we extended our investigation to homocou-
In summary, we have reported here a study on the use
pling of 3 in the presence of this alternative surfactant of micellar conditions applied to the palladium-catalyzed
Scheme 4). When using 7 wt.-% PTS in water and 25% homocoupling reactions of trimethylsilyl polyenes, as a first
(
PdCl , no substantial difference in the reaction outcome be- example of studies directed towards the investigation of re-
2
tween Triton X-100 and PTS was observed, except a slight activity of unsaturated silanes in coupling processes in
reduction of the yield (80%, Table 3 entry 4).
aqueous environment and in the presence of surfactants.
Table 3. Homocoupling reactions of unsaturated silyl ketones in the presence of PTS as surfactant.
Entry Silyl ketone
Catalyst[b] (% PdCl
2
)
Surfactant (PTS/H
2
O % weight)
Time [h]
GC Conversion [%]
45[e]
Yield[c] [%]
[a]
[a]
[a]
[a]
d]
1
2
3
4
3
3
3
3
3
5
5
7
10
25
10
25
25
25
25
25
2
2
7
7
15
2
48
48
24
24
24
48
24
24
36
65
55
80
80
60
65
65
[e]
100
[e]
86
[e]
100
100
100
100
[
[e]
5
6
7
8
[
[
[
a]
a]
a]
[e]
[e]
7
7
[e]
100
2 2
[a] Reactions carried out at room temperature. [b] Molar ratio PdCl /CuCl /LiCl, 1:8:3. [c] Based on isolated materials. [d] Good stirring
with a slight heating (40 °C) is needed for solubilizing the reactants due to the high viscosity of the mixture. [e] Determined by GLC
analysis.
Eur. J. Org. Chem. 2010, 2275–2279
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2277

S. R. Cicco, G. M. Farinola, C. Martinelli, F. Naso, M. Tiecco
FULL PAPER
The results of our experiments demonstrate that under mi- ture. The reaction was monitored by TLC or GC–MS analysis until
the disappearance of 3. The reaction mixture was quenched with a
saturated aqueous sodium chloride solution and then extracted
cellar conditions the reactions afford dimerization products
in higher yields compared with those obtained using meth-
twice with ethyl acetate. The organic layers were combined and
anol or HMPA as the solvent, maintaining the high stereo-
2 4
dried with anhydrous Na SO . After evaporation of the solvent at
selectivity. The observed improvement is likely due to sup-
pression of proto- and chlorodesilylation side reactions in
the nano-segregated lipophilic micellar environment. Stud-
ies are in progress in our laboratory to extend the micellar
conditions to other transition metal-catalyzed reactions of
unsaturated silanes.
reduced pressure, the crude product was subjected to flash
chromatography, using petroleum ether as eluent, and a white solid
was isolated (0.054 g, 87% yield); m.p. 199–200 °C (ethanol) that
was identified as 4 on the basis of the following spectroscopic data:
1
H NMR (500 MHz, CDCl ): δ = 2.35 (s, 6 H), 6.58–6.68 (m, 2
3
H), 6.86–6.97 (m, 2 H), 7.14 (d, J = 7.9 Hz, 4 H), 7.34 (d, J =
1
3
8
.0 Hz, 4 H) ppm. C NMR (125.7 MHz, CDCl
3
): δ = 21.32,
1
26.13, 128.38, 129.25, 132.13, 134.57, 137.22 ppm. MS (EI,
+
Experimental Section
70 eV): m/z (%) = 234 (49) [M ], 219 (100), 204 (37), 115 (10), 105
15), 101 (4).
(
General: All chemicals were purchased from Aldrich, Alfa Aesar
and Acros and used without further purification. TRITON X-100
and PTS are commercially available from Sigma–Aldrich. Com-
mercially available copper(II) chloride and anhydrous lithium chlo-
ride were used without further purification. (2E,4E)-1-phenyl-5-
The stereochemistry of the product 4 can not be directly derived
from the H NMR spectrum on the basis of the coupling constants
1
of the protons of the diene moiety, because they appear as a second
order AAЈBBЈ spin system. However, the analysis of the spin sys-
tem by means of MestReC NMR software, as we previously re-
ported for similar dienes,[ enabled us to define the coupling con-
stant values J(AB) = J(AЈBЈ) = 17 Hz, J(BBЈ) = 9 Hz, J(ABЈ) ≈
(trimethylsilyl)penta-2,4-dien-1-one (5), (2E,4E,6E)-1-phenyl-7-(tri-
27]
methylsilyl)hepta-2,4,6-trien-1-one (7) were prepared as reported in
[
25]
the literature.
Column chromatography was performed using silica gel 60 (0.04–
.063 mm) from Merck. Petroleum ether was the 40–70 °C boiling
0.5 Hz, J(AAЈ) = 0 Hz which are characteristic for a (1E,3E)-diene
structure.
0
fraction. Thin-layer-chromatography analysis was conducted using
Merck Silica gel 60 F254 aluminium sheets. GLC analysis was per-
formed with Varian 3900 (dimethylpolysiloxane, 30 mϫ25 mmϫ
Procedure for the Homocoupling Reaction of the (2E,4E)-1-Phenyl-
5-(trimethylsilyl)-2,4-pentadien-1-one 5 Using 25% of PdCl2 in
Aqueous Micelles with Triton X-100 (15 wt.-% Triton X-100/H O):
2
0
5
.25 µm). MS spectra were acquired on a Shimadzu GC–MS-QP
A 25 mL round-bottomed flask was charged with (2E,4E)-1-
000 spectrometer. 1H and C NMR spectra were recorded in
13
phenyl-5-(trimethylsilyl)-2,4-pentadien-1-one 5 (0.1 g, 0.43 mmol),
PdCl2 (0.019 g, 0.109 mmol), CuCl2 (0.116 g, 0.86 mmol) LiCl
(0.014 g, 0.327 mmol) in 5 mL of surfactant water solution and the
mixture was stirred at room temperature. The reaction was moni-
tored by TLC. The reaction mixture was quenched with saturated
aqueous sodium chloride and the organic compound was extracted
twice with ethyl acetate. The organic layers were combined and
CDCl
25.7 MHz, respectively. The residual CHCl
and the CDCl signal at δ = 77.0 ppm were used as standards for
3
on a Bruker AM 500 spectrometer at 500 MHz and
1
3
signal at δ = 7.24 ppm
3
13
1
H NMR and C NMR spectra, respectively.
(E)-1-p-Tolyl-2-(trimethylsilyl)ethene (3): A three-neck 250 mL
round-bottomed flask equipped with a magnetic stir bar and
00 mL dropping funnel was charged with (E)-(2-bromovinyl)tri-
methylsilane (4.0 g, 22.3 mmol), NiCl (dppe) (0.36 g, 0.69 mmol)
2 4
dried with anhydrous Na SO . After evaporation of the solvent at
reduced pressure, the product 6 was isolated as a yellow solid after
1
2
flash chromatography, using petroleum ether/diethyl ether (6:4) as
and 50 mL of freshly distilled THF, under a nitrogen atmosphere.
The solution was cooled to 0 °C and p-tolylmagnesium bromide
1
eluent (0.057 g, 85% yield): H NMR (500 MHz, CDCl
3
): δ = 6.59–
6
7
.75 (m, 2 H), 6.69–6.85 (m, 2 H), 7.15 (d, J = 14.8 Hz, 2 H), 7.38–
(0.37  solution in THF, 80 mL, 29.2 mmol) was added dropwise.
1
3
.59 (m, 8 H), 7.89–7.95 (m, 4 H) ppm. C NMR (125.7 MHz,
): δ = 126.91, 128.48, 128.57, 132.9, 134.82, 137.96, 140.51,
43.52, 190.15 ppm. Mp: 213-214 °C (chloroform/diethyl ether).
The solution was maintained at 0 °C during the addition and then
warmed to room temperature overnight. The reaction mixture was
then quenched with water and extracted twice with hexane. The
CDCl
1
3
Spectroscopic characterizations of the product 6 are in agreement
2 4
organic layers were combined, dried with anhydrous Na SO , and
with the data reported in literature.[
9]
the solvent was distilled under reduced pressure to yield a light
yellowish liquid. Purification by column chromatography using pe-
troleum ether as solvent afforded 4.54 g (yield 87%) of a colourless
liquid. Spectroscopic characterizations of the product 3 are in
Procedure for the Homocoupling Reaction of the (2E,4E,6E)-1-
Phenyl-7-(trimethylsilyl)-2,4,6-heptatrien-1-one (7) Using 25% of
PdCl
00/H
2E,4E,6E)-1-phenyl-7-(trimethylsilyl)-2,4,6-heptatrien-1-one
0.1 g, 0.391 mmol), PdCl (0.017 g, 0.098 mmol), CuCl (0.105 g,
.78 mmol) LiCl (0.012 g, 0.292 mmol) in 5 mL of surfactant water
2
in Aqueous Micelles with Triton X-100 (15 wt.-% Triton X-
O): A 25 mL round-bottomed flask was charged with
(7)
agreement with the data reported in the literature.[
500 MHz, CDCl
J = 19.1 Hz, 1 H), δ = 7.00 (d, J = 19.1 Hz, 1 H), δ = 7.24 (d, J =
Hz, 2 H), δ = 7.46 (d, J = 8 Hz, 2 H) ppm. 13C NMR
125.7 MHz, CDCl ): δ = 145.35, 137.65, 135.62, 129.10, 128.03,
26]
1
H NMR
3
): δ = 0.3 (s, 9 H), δ = 2.4 (s, 3 H), δ = 6.56 (d,
1
(
(
2
(
2
2
8
0
(
1
3
solution and the mixture was stirred at room temperature. The re-
action was monitored by TLC and quenched with a saturated aque-
ous sodium chloride solution. The mixture was then extracted twice
+
26.17, 21.35, –0.70 ppm. MS (EI, 70 eV): m/z (%) = 190 (36) [M ],
175 (100), 159 (63), 149 (53).
Procedure for the Homocoupling Reaction of the (E)-1(-p-Tolyl)-2- with dichloromethane. The organic layers were combined and dried
(
trimethylsilyl)ethene (3) Using 25% of PdCl
2
in Aqueous Micelles
2 4
with anhydrous Na SO . Concentration of the solvent at reduced
with Triton X-100 (15 wt.-% Triton X-100/H
2
O): A 25 mL round- pressure gave a crude product that was recrystallized from chloro-
bottomed flask was charged with (E)-1-(p-tolyl)-2-(trimethylsilyl)-
ethene (3) (0.1 g, 0.53 mmol), PdCl (0.023 g, 0.13 mmol), CuCl
0.143 g, 1.06 mmol), LiCl (0.017 g, 0.39 mmol) in 5 mL of surfac-
tant water solution and the mixture was stirred at room tempera-
form/diethyl ether isolating 8 as an orange solid (0.046 g, 65%
yield); m.p. 221–222 °C (chloroform/diethyl ether). H NMR
1
2
2
(
(500 MHz, CDCl
3
): δ = 6.40–6.55 (m, 6 H), 6.71 (dd, J = 14.8,
10.2 Hz, 2 H), 7.01 (d, J = 14.9 Hz, 2 H), 7.38–7.65 (m, 8 H), 7.88–
2278
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 2275–2279

Homocoupling of Unsaturated Silanes in Aqueous Micelles
7
1
1
8
.98 (m, 4 H) ppm. 13C NMR (125.7 MHz, CDCl
): δ = 125.63, [9] F. Babudri, A. R. Cicciomessere, G. M. Farinola, V. Fiand-
3
28.26, 128.51, 132.33, 132.54, 134.61, 135.88, 138.50, 141.57,
anese, G. Marchese, R. Musio, F. Naso, O. Sciacovelli, J. Org.
Chem. 1997, 62, 3291–3298.
44.21, 190.29 ppm. Spectroscopic characterizations of the product
are in agreement with the data reported in the literature.[9]
[10] C. George, H. Yoshida, W. A. Goddard, S. S. Jang, Y.-H. Kim,
J. Phys. Chem. B 2008, 112, 14888–14897; H. Braatza, S.
Hechtb, H. Seiferta, S. Helmb, J. Bendigb, W. Rettig, J. Pho-
tochem. Photobiol. A: Chem. 1999, 123, 99–108.
Acknowledgments
[11] N. M. Dickson, J. E. Donehue, T. L. Gustafson, Abstracts,
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[13] O. Kwon, S.-J. Kwon, M. Jazbinsek, F. D. Bruner, J.-I. Seo, C.
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Funct. Mater. 2008, 18, 3242–3250; D. Laage, W. H. Thomp-
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This work was financially supported by Ministero dell’Istruzione,
dell’Università
e
della
Ricerca
(MIUR)
(project
PRIN 2007 PBWN44) and by Università degli Studi di Bari.
1
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