Amphiphilic photo-isomerisable phosphanes for aqueous organometallic catalysis

Article abstract of DOI:10.1039/c0cc02458a

Water-soluble phosphanes were tagged with a light-responding diazo group. Upon UV exposure, the diazo-isomerisation led to phosphane morphology change, resulting in an increase in the reaction rate of an aqueous palladium-catalysed cleavage reaction. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0cc02458a

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Amphiphilic photo-isomerisable phosphanes for aqueous organometallic
catalysisw
a
b
b
a
a
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Herve Bricout, Estelle Banaszak, Christophe Len, Frederic Hapiot and Eric Monflier*
Received 9th July 2010, Accepted 2nd September 2010
DOI: 10.1039/c0cc02458a
Water-soluble phosphanes were tagged with a light-responding
diazo group. Upon UV exposure, the diazo-isomerisation led to
phosphane morphology change, resulting in an increase in the
reaction rate of an aqueous palladium-catalysed cleavage
reaction.
Nature has always been a source of inspiration for chemists.
As an example, imitating the many photoresponsive systems
such as vision or photosynthesis, light-sensitive molecules have
been synthesized and incorporated in switchable materials for
biotechnology or advanced material applications. Photo-
responsive functional devices have been used as smart
polymers,1,2 liquid crystals,3 molecular switches4 and
machine.5–7 Among the most suitable light-sensitive molecules,
azobenzene derivatives proved to be very successful. They
respond to UV exposure by a transition between the more stable
(E)-isomer and the less stable (Z)-isomer, the energetic barrier
between them being 50 kJ molÀ1. Though papers have been
published on the synthesis of azo-containing phosphanes,8–13 few
has been described on their application in organometallic
catalysis under photo-illumination14,15 and nothing has been
Scheme 2 Synthesis of diazo-containing water-soluble phosphanes 3
and 7.
published on their catalytic application in water.
In this context, we report hereby that the in situ
photoinduced changes in the geometry of azo-containing
water-soluble phosphanes can greatly impact the catalytic
performances. The effect of UV exposure was investigated in
a Pd-catalyzed cleavage reaction of allylic carbonates in a two
phase system (Scheme 1).
Two new azo-containing phosphanes (3 and 7) have been
synthesized from 1 and 4 by a three-step procedure as
described in Scheme 2. Sulfonation of iodide 1 in sulfuric
oleum followed by neutralisation with NaOH gives the
sulfonated product 2 in a very good 95% yield.
Subsequent reaction with diphenylphosphane in anhydrous
DMF using palladium acetate as catalyst according to a
modified procedure of the literature16 leads to the expected
phosphane 3 in 65% yield after chromatography on a silica
column under nitrogen. The same procedure was used for
iodide 4. A mixture of sulfonated products 5 and 6 (70/30)
was then obtained, the sulfonation on the non-substituted
aromatic cycle being favoured. 6 results from combined
electronic effects of the iodide and azo function, both of them
being weakly deactivating and ortho/para-directing. After
separation, 5 reacts with diphenylphosphane to give 7 in
77% yield after chromatography. Note that substitution by
a diphenylphosphino group in ortho-position regarding the
diazo function has also been attempted but very low yields
(o5%) were obtained certainly due to steric hindrance. The
basicity of 3 and 7 has been determined using the 1JPQSe values
of their selenide derivatives.17 The higher the coupling
constant the lower the basicity. The phosphane derivatives
have been prepared by refluxing 3 or 7 in the presence of excess
selenium in ethanol under nitrogen. Their respective 31P NMR
spectra exhibit a singlet with two satellites due to only 7.6%
Scheme 1 Pd-catalysed cleavage of allyl-undecyl-carbonate.
a
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Universite de Lille Nord de France, CNRS UMR 8181,
UArtois UCCS, Faculte des Sciences Jean Perrin, rue Jean Souvraz,
´
SP 18, F-62307 Lens Cedex, France.
E-mail: eric.monflier@univ-artois.fr; Fax: +33321791755;
Tel: +33321791772
b Universite´ de Technologie de Compie`gne,
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´
Transformation Integrees de la Matiere Renouvelable,
EA 4297 UTC/ESCOM, 1 allee du reseau Jean-Marie Buckmaster,
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F-60200 Compiegne, France
w Electronic supplementary information (ESI) available: Experimental
details. Phosphanes synthesis, procedure for catalytic experiments.
See DOI: 10.1039/c0cc02458a
c
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Chem. Commun., 2010, 46, 7813–7815 7813

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Scheme 3 Water-soluble phosphane 8.
Fig. 2 UV/vis spectra of 3 for increasing duration of irradiation
(l = 365 nm) of 0, 2, 4, 6, 8, 10, 15, 20, 30, 60 and 90 min.
Fig. 1 Surface tension curves of 3 (E), 7 (m) and 8 (J) in water at 20 1C.
active selenium isotope (77Se). As the 1JPQSe value is larger for
7 than for 3 (745 vs. 742 Hz), 7 is less basic than 3. For
comparison, phosphane 8 (Scheme 3),18 used as a reference in
this study, exhibits a stronger basicity (1JPQSe = 739 Hz).
Thus, introduction of a diazo group between both phenyls
logically has an electrowithdrawing effect on the phosphorus.
Surface tension measurements revealed that phosphanes 3
and 7 are surface active and form aggregates in water (Fig. 1).
Indeed, addition of 3 or 7 in water reduces the surface tension
of water and ruptures in the slope of the g = f(c) curves are
indicative of an aggregation process. The critical aggregation
concentration (cac) for 3 and 7 were estimated to be about
0.1 mM and 0.2 mM, respectively. The difference of cac likely
originates from the more linear structure of 3. In fact, the
ability of 3 to form aggregates is greater than 7 as packing
linear structures is easier than packing bent ones. Note that the
presence of the diazo group strongly impacts the ability of the
amphiphilic molecules 3 and 7 to self-assemble into well-
organized structures such as micelles. Indeed, no critical
micellar concentration (cmc) can be detected for 3 or 7, in
contrast with 8 for which the surface tension gradually
decreases and remains constant at concentrations over the
cmc (0.6 mM).18
Fig. 3 UV/vis spectra of 7 for increasing duration of irradiation
(l = 365 nm) of 0, 2, 4, 6, 8, 10, 20, 30, 60, 150 and 210 min.
445 nm are ascribed to p–p* and n–p* of the trans form and
the cis form of azo respectively,19 this spectral variation clearly
shows the isomerization of the azo groups from the trans
photoisomer to the cis photoisomer. The sample containing 7
shows a similar variation in the UV/vis spectra (Fig. 3). HPLC
analyses have been performed to determine the percentage of
isomerised phosphanes. The maximum isomerization yield is
obtained after 90 min irradiation at 365 nm for 3 (from 91% to
54% of the trans-isomer) and after 150 min irradiation at
365 nm for 7 (from 95% to 48% of the trans-isomer) (Fig. 4).
In fact, though 3 and 7 are structurally different, they behave
similarly upon UV irradiation. The percentages of cis-isomer
are close, suggesting that steric factors do not affect the
isomerisation process. The cis-isomers 3 and 7 were found to
be stable when protected from light for 180 min. Conversely,
the trans/cis initial proportions are recovered after one night
even in the dark or in less than 10 min under daylight. Finally,
as already shown on numerous azo-derivatives, it must be
pointed out that the cis-isomer could not be isolated in pure
form.20
In order to confirm the ability of the diazo function to
isomerise under UV exposure, 3 and 7 have been analyzed by
UV/vis absorption spectroscopy. As shown in Fig. 2, UV
irradiation at 365 nm of the sample containing 3 in water at
room temperature causes a substantial change in the UV-Vis
spectra. The absorption band at around 320 nm decreases
gradually as the irradiation goes on.
The catalytic behaviour of 3 and 7 has then been evaluated
in the Pd-catalyzed cleavage reaction of allyl-undecyl-carbonate
(Scheme 1). The substrate and the allyl scavenger (diethylamine)
were dissolved in heptane when Pd(OAc)2 and excess phosphane
(9 equiv. with respect to the palladium) were dissolved in
water. In fact, the phosphane immobilizes the palladium
precursor in the aqueous phase.21,22 Both solutions were
At the same time, the bands at around 250 and 445 nm
slightly increase. Because the absorption bands at 320, 250 and
c
7814 Chem. Commun., 2010, 46, 7813–7815
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7 upon UV irradiation is more than 2.5-fold that obtained
without UV irradiation (80 vs. 30 hÀ1 respectively). Actually,
as shown above, 3 or 7 photoisomerise upon UV exposure to
give approximately 1 : 1 mixtures of trans and cis isomers. As
a consequence, under UV light, 3 and 7 form new aggregates
having a better ability to solubilise the substrate in water. The
catalytic performances are then significantly improved. In the
case of 7, these aggregates appeared even more efficient than
those stemmed from 8, indicating that photostimulation can
generate much more active catalytic species.
To sum up, we showed that the catalytic activity of a
Pd-catalyzed cleavage reaction of allyl-undecyl-carbonate
could be regulated by photoisomerization of azo-containing
water-soluble phosphane. This constitutes the first example of
a light-driven organometallic catalytic process in aqueous
media. Experiments are currently underway to determine the
nature of aggregates under UV irradiation.
Fig. 4 HPLC quantification of the trans-photoisomer ratios of 3 (J)
and 7 (E) before irradiation, after 90 min irradiation for 3 and 150 min
for 7, and for increasing duration of light protection.
We thank CNRS and the Ministere de l’Enseignement
Superieur et de la Recherche for their support.
Notes and references
1 T. Shimoboji, E. Larenas, T. Fowler, S. Kulkarni, A. S. Hoffman
and P. S. Stayton, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 16592.
2 D. Taura, S. Li, A. Hashidzume and A. Harada, Macromolecules,
2010, 43, 1706.
3 Y. Yu, M. Nakano and T. Ikeda, Nature, 2003, 425, 145.
4 S. Yasuda, T. Nakamura, M. Matsumoto and H. Shigekawa,
J. Am. Chem. Soc., 2003, 125, 16430.
5 K. Ichimura, S. K. Oh and M. Nakagawa, Science, 2000, 288,
1624.
Fig. 5 Reaction rate (average TOF (hÀ1) at 40% conv.) measured for
catalytic systems derived from phosphanes 3, 7 and 8 without UV
irradiation or upon UV exposure.
6 J. F. Morin, Y. Shirai and J. M. Tour, Org. Lett., 2006, 8, 1713.
7 T. Sasaki and J. M. Tour, Org. Lett., 2008, 10, 897.
8 M. J. Alder, K. R. Flower and R. G. Pritchard, Tetrahedron Lett.,
1998, 39, 3571.
9 E. Besson, A. Mehdi, V. Matsura, Y. Guari, C. Reye and
R. J. P. Corriu, Chem. Commun., 2005, 1775.
10 M. Yamamura, N. Kano and T. Kawashima, Inorg. Chem., 2006,
45, 6497.
11 M. D. Segarra-Maset, P. W. N. M. van Leeuwen and Z. Freixa,
Eur. J. Inorg. Chem., 2010, 2075.
12 M. J. Alder, K. R. Flower and R. G. Pritchard, J. Organomet.
Chem., 2001, 629, 153.
13 S. Sasaki, K. Kato and M. Yoshifuji, Bull. Chem. Soc. Jpn., 2007,
80, 1791.
14 M. Kawamura, R. Kiyotake and K. Kudo, Chirality, 2002, 14, 724.
15 M. J. Alder, W. I. Cross, K. R. Flower and R. G. Pritchard,
J. Organomet. Chem., 1999, 590, 123.
16 O. Herd, A. Heßler, M. Hingst, M. Tepper and O. Stelzer,
Catal. Today, 1998, 42, 413.
17 D. W. Allen, I. W. Nowell and B. F. Taylor, J. Chem. Soc., Dalton
Trans., 1985, 2505.
18 M. Ferreira, H. Bricout, N. Azaroual, C. Gaillard, D. Landy,
S. Tilloy and E. Monflier, Adv. Synth. Catal., 2010, 352, 1193.
19 F. Hamon, F. Djedaini-Pilard, F. Barbot and C. Len, Tetrahedron,
2009, 65, 10105.
20 M. C. Basheer, Y. Oka, M. Mathews and N. Tamaoki, Chem.–Eur. J.,
2010, 16, 3489.
21 D. Kirschner, T. Green, F. Hapiot, S. Tilloy, L. Leclercq,
H. Bricout and E. Monflier, Adv. Synth. Catal., 2006, 348, 379.
22 N. Six, S. Menuel, H. Bricout, F. Hapiot and E. Monflier, Adv.
Synth. Catal., 2010, 352, 1467.
poured in a quartz Schlenk tube which was covered by a black
box to shield the room light. The mixture was then irradiated
by UV light at 365 nm under stirring. In Fig. 5 are summarized
the average turnover frequencies (TOF) measured at 40%
conv. for catalytic system derived from phosphanes 3, 7 and
8. Without UV irradiation, the presence of diazo groups has a
detrimental effect on the reaction rate. Indeed, the catalytic
activities observed in the presence of phosphanes 3 and 7 are
lower than those observed in the presence of phosphane 8.
These results are rather surprising as 3 and 7 are more surface
active than 8 (Fig. 1). These observations may result from the
presence of different kinds of aggregates in the aqueous
solution. As highlighted by tensiometric measurements, 3
and 7 do not form micelles in water contrary to 8. This change
in aggregates morphology negatively affects the ability of 3
and 7 to solubilise hydrophobic substrate in water. The
situation is very different upon UV exposure. When no
difference can be detected for 8, an increase in initial activity
was noticed with the azo-containing water-soluble phosphanes
when the catalytic system is irradiated at 365 nm. The effect is
rather small for the para-substituted phosphane 3 (from 11 hÀ1
without UV to 18 hÀ1 upon UV exposure). Conversely, the
initial activity measured with the meta-substituted phosphane
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7813–7815 7815