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Fuchter et al.
SCHEME 3. Synthesis of the ROMPsphere
seco-Porphyrazine
illuminated with a simple 40 W desk lamp. All the polymers
performed with a similar efficiency (approximately 30% conver-
sion after 24 h). Since competitive degradation of the soluble
polymers occurred after 8 h, whereas, slow, continuous conver-
sion was observed for the insoluble, cross-linked polymers, the
insoluble polymers became the focus of the study. In an effort
to improve the proportion of endoperoxide product 11, the
reaction conditions were optimized. Out of the conditions
surveyed, the largest effect was observed by decreasing the
temperature to -20 °C. Presumably this was simply due to the
increased solubility of oxygen at this temperature. Increasing
the catalyst loading to 10 mol % increased conversion; however,
it was hoped this higher loading could be avoided. Since no
dramatic increase in conversion could be achieved, all the cross-
linked polymers were examined for their swelling properties in
a variety of solvents. Unfortunately, none of the polymers
showed any significant swelling in methanol, acetonitrile,
acetone, carbon tetrachloride, or dichloromethane. Presumably,
the poor rate of singlet oxygen formation was the result of
photosensitization by peripheral seco-porphyrazine units on the
polymer particles only and not by bulk groups. Consistent with
this, rates of conversion were improved using mechanically
broken up polymer particles (76% conversion after 8 h with 1
mol % catalyst).
As a consequence of the limitations with ROM polymer seco-
porphyrazines, we sought to examine the corresponding ROMP-
sphere reagents.16,23,24 We considered that such graft copolymers
should provide insoluble beads with a solvent-swellable ROM
surface layer coated onto a polystyrene core, a system with
potentially improved photosensitizing properties. Therefore,
using the conditions of Miokowski and co-workers,25 vinylpoly-
styrene (13) was prepared and subsequently converted to the
catalysts 15a,b (Scheme 3).16 Initially, exposure of 15a to a
solution of porphyrazine 6 and 7 (1.4:98.6) in dichloromethane
gave the ROMPsphere 16 with 41% monomer incorporation
by weight. No dye was recovered from the reaction solution,
indicating the porphyrazine had been fully incorporated into the
polymer, giving a porphyrazine loading of 0.12 mmol g-1
(Scheme 3). Interestingly, the copolymerization using diol 8
instead of 7 gave a polymer with 35% monomer incorporation
by weight, but with virtually no porphyrazine incorporation.
Pleasingly, the photooxygenation of R-terpinene using 1 mol
% catalyst 16 proceeded in >99% conversion after 4 h (Scheme
2). This result is comparable to those of other methods for the
formation of endoperoxide 11. For example, 11 has been
previously synthesized using methylene blue (86%),26 C60
(90%),27 or a soluble porphyrazine (95%)4 as the photocatalyst.
Unfortunately, the preparation of subsequent samples of ROMP-
sphere catalyst 16 showed significant batch variability. To
combat this issue, the reaction conditions for the synthesis of
16 were screened with focus on the solvent, concentration,
temperature, ruthenium catalyst (i.e., 14a or 14b), ruthenium
loading and vinyl loading on polystyrene 13. The optimum
conditions were shown to be the use of catalyst 15b at reflux
in dichloromethane (0.5 M monomer) using a low-loading
vinylpolystyrene precursor (13) (0.5 mmol g-1). These polym-
erization reaction conditions minimized release of ruthenium
carbenes into solution with subsequent solution-phase ROMP,
producing soluble polymers18 rather than the required graft
copolymers. Under these optimum conditions the ROMPsphere
catalyst 16 was isolated with 69% monomer incorporation by
weight (Scheme 3). The seco-porphyrazine loading of ROMP-
sphere 16 was determined by quantitative UV-vis spectra of
the recovered filtrate to be 0.049 mmol g-1. Alternatively,
microanalysis of ROMPsphere 16 was consistent with a loading
of 0.043 mmol g-1
.
ROMPsphere 16 was used to catalyze the conversion of 1,3-
dienes into endoperoxides (Table 1). In all cases the starting
material was smoothly converted to the product(s), with little
1
or no decomposition observable by H NMR. For entries 1-4
the yields of photooxygenated products were comparable to
those obtained with a soluble seco-porphyrazine.4 Interestingly,
however, significant amounts of hydroperoxide compounds were
obtained for cyclohexadiene (entry 1) and R-phellandrene (entry
3), resulting from a singlet oxygen ene reaction.28,29 In addition,
the elimination product p-cymene was produced during the
oxygenation of R-phellandrene (entry 3), a transformation
(23) Ahmed, M.; Barrett, A. G. M.; Braddock, D. C.; Cramp, S. M.;
Procopiou, P. A. Tetrahedron Lett. 1999, 40, 8657.
(24) Ahmed, M.; Arnauld, T.; Barrett, A. G. M.; Braddock, D. C.;
Procopiou, P. A. Synlett 2000, 1007.
(25) Sylvain, C.; Wagner, A.; Mioskowski, C. Tetrahedron Lett. 1998,
39, 9679.
(28) Dechy-Cabaret, O.; Benoit-Vical, F.; Loup, C.; Robert, A.; Gorni-
tzka, H.; Bonhoure, A.; Vial, H.; Magnaval, J.-F.; Seguela, J.-P.; Meunier,
B. Chem.sEur. J. 2004, 10, 1625.
(26) Jefford, C. W.; Jaber, A.; Boukouvalas, J. J. Chem. Soc., Chem.
Commun. 1989, 1916.
(27) Tokuyama, H.; Nakamura, E. J. Org. Chem. 1994, 59, 1135.
(29) Matusch, R.; Schmidt, G. HelV. Chim. Acta 1989, 72, 51.
(30) Kaneko, C.; Sugimoto, A.; Tanaka, S. Synthesis 1974, 876.
(31) Atkins, R.; Carless, H. A. J. Tetrahedron Lett. 1987, 28, 6093.
726 J. Org. Chem., Vol. 71, No. 2, 2006