Organic Letters
Letter
Somorjai, G. A.; Alivisatos, A. P.; Toste, F. D. J. Am. Chem. Soc. 2015,
137, 7083.
(5) (a) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638.
(b) Thomas, J. M.; Raja, R. Acc. Chem. Res. 2008, 41, 708. (c) Narayanan,
R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663.
(6) For examples of supported Pd nanoparticles in cross-coupling
catalysts are lined up from the left to the right (A−F, Figure 1)
according to the assumed declining polarity of the cross-linkers.
The visible overall trend of the substrate response to the catalyst
thus implies the polarity of the cross-linker as the likely principal
factor causing changes in the catalyst activity. Considering the
amount of the cross-linker in the polymer matrix and the overall
amount of the matrix material in the reaction mixture, the extent
of the cross-linker impact over the catalyst activity is fairly
remarkable. The close proximity of the reaction center
(palladium) and the cross-linker-based functional group may
be likely culprit behind the extent of the observed selectivity.15
Overall, the concept of a tunable heterogeneous catalyst based
on Pd embedded in a polysiloxane matrix is presented and
demonstrated in a classical Suzuki−Miyaura protocol. The
present type of gelous catalyst, which can be conveniently
prepared in a common synthetic laboratory setting, is fairly
robust and suffers minimally from metal leakage. Importantly, it
permits an effective separation of the catalyst at the end of the
process and ultimately allows for catalyst reuse. An additional
strength of the introduced PdGel system is the concept of its
catalytic activity modulation. Changes in the catalytic activity of
the metal are accomplished via alterations in the immediate
environment of the metal. These alterations are achieved through
the functionalized cross-linkers of the gel matrix. Changes in
activity of the novel Pd gel catalyst are illustrated in a series of
experiments following the Suzuki−Miyaura protocol with the
substrates matching (mismatching) chemophysical properties of
the cross-linker. While the overall alteration of the reaction
mixture may arguably be (due to a minuscule amount of the
modified cross-linker) negligible, a more profound change in the
microenvironment of the catalytic metal may explain the
observed activity variation.
catalysis, see: (a) Pagliaro, M.; Pandarus, V.; Bel
́
and, F.; Ciriminna, R.;
Palmisano, G.; Cara, P. D. Catal. Sci. Technol. 2011, 1, 736.
̀
(b) Firouzabadi, H.; Iranpoor, N.; Ghaderi, A.; Ghavami, M.; Hoseini,
S. J. Bull. Chem. Soc. Jpn. 2011, 84, 100. (c) Han, W.; Liu, Ch.; Jin, Z. Adv.
Synth. Catal. 2008, 350, 501. (d) Zhang, G. Synthesis 2005, 4, 537.
(7) For a review of microencapsulated metals as catalysts, see:
Akiyama, R.; Kobayashi, S. Chem. Rev. 2009, 109, 594.
(8) For PHMS gels containing Pt and their reactivity in hydrogenation
reactions, see: Motoyama, Y.; Kamo, K.; Nagashima, H. Org. Lett. 2009,
11, 1345.
(9) Chauhan, B. P. S.; Rathore, J. S.; Glloxhani, N. Appl. Organomet.
Chem. 2005, 19, 542.
(10) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(11) Rigorous RFA metal analysis did not show any appreciable
amount of Pt in the PdGels. (See the experimental details, SI.)
(12) Sensitivity of ETV/ICP/OES is in the range of ppb.
(13) The reaction mixture was shaken rather than vigorously stirred.
The PdGel was isolated and carefully washed after every run.
(14) Without the termination in 60 min, all of the reactions reached
quantitative conversion in a few hours.
(15) An important component of the concept is the metal topology: In
order to effectively tune the catalytic properties of the catalyst, the metal
should be embedded inside the polymer matrix. Our findings (the
analytical RFA and reactivity studies of the supernatant reaction solvent
indicating no loss of the metal to the environment) support this
scenario. See the SI for details.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
Technical details, supplementary experiments, and char-
acterization of all new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This investigation was financially supported by the Ministry of
Education of the Czech Republic (LH12012).
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REFERENCES
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D
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