304
M. Benaglia
from diethyl ether, but it was recovered as a very thick oil. However, by
using the mesylate 3 of MeOPEG 2000 the expected PEG-supported porphyrin 5 was obtained as a solid readily precipitated with diethyl ether
in 87% yield as a pure compound. The irradiation of a 0.01 M methylene chloride solution of bisdialine 6 with a 100-W halogen lamp, in the
presence of 3 mol% of PEG supported tetrahydroxyphenyl-porphyrin
(PEG-TPP, 5) as sensitizer gave a 82/18 mixture of supra and antara
diastereoisomeric endoperoxides 7 in quantitative yield after 1 h (equation a, Scheme 1). The polymer-bound catalyst not only showed the
same activity of the non-supported species, but it greatly simplified isolation of the product (Fabris et al. 2002). At the end of the reaction the
solvent was concentrated in a vacuum and diethyl ether was added to the
PEG-supported porphyrin which was then recovered quantitatively by
filtration. From this concentrated filtrate the endoperoxides were easily
isolated by crystallization from ethanol. The one-pot oxidation of olefin
to α,β-unsaturated ketones was applied to convert dicyclopentadiene
8 into the corresponding dicyclopentadienone 9, a useful starting material for the preparation of enantiopure diols (equation b, Scheme 1).
Once again, the soluble polymer-bound porphyrin 5 favourably compares with the non-supported catalyst. It is worth mentioning that it is
possible to run the reaction on gram scale, using an even smaller amount
of sensitizer; the oxidation of 13 mL. of dicyclopentadiene, promoted by
220 mg of PEG–TPP 5 (0.025% mol equivalent), gave 11 g of ketone
9, isolated, after filtration of the supported catalyst, by simple evaporation of the organic solvent as analytically pure compound requiring
nofurther purification.
Recycling of the PEG-bound porphyrin 5 was also studied. The catalyst was reused a second time in the photooxidation of the bisdialine to
give the product 7 in 97% yield and 80/20 diastereoisomeric ratio after
a 1-h reaction. The recovered PEG–TPP was recycled a third time without showing any decrease in the catalytic activity; the PEG-supported
sensitizer was recycled six times with no appreciable loss of chemical
or stereochemical efficiency.
Immobilization of an organic catalyst can be used also to facilitate
the process of catalyst’s optimization. Surprisingly, there is just a single
example of the application of this methodology reported to date and so
it deserves a special mention (Jacobsen and Sigman 1998).
M. Benaglia
from diethyl ether, but it was recovered as a very thick oil. However, by
using the mesylate 3 of MeOPEG 2000 the expected PEG-supported porphyrin 5 was obtained as a solid readily precipitated with diethyl ether
in 87% yield as a pure compound. The irradiation of a 0.01 M methylene chloride solution of bisdialine 6 with a 100-W halogen lamp, in the
presence of 3 mol% of PEG supported tetrahydroxyphenyl-porphyrin
(PEG-TPP, 5) as sensitizer gave a 82/18 mixture of supra and antara
diastereoisomeric endoperoxides 7 in quantitative yield after 1 h (equation a, Scheme 1). The polymer-bound catalyst not only showed the
same activity of the non-supported species, but it greatly simplified isolation of the product (Fabris et al. 2002). At the end of the reaction the
solvent was concentrated in a vacuum and diethyl ether was added to the
PEG-supported porphyrin which was then recovered quantitatively by
filtration. From this concentrated filtrate the endoperoxides were easily
isolated by crystallization from ethanol. The one-pot oxidation of olefin
to α,β-unsaturated ketones was applied to convert dicyclopentadiene
8 into the corresponding dicyclopentadienone 9, a useful starting material for the preparation of enantiopure diols (equation b, Scheme 1).
Once again, the soluble polymer-bound porphyrin 5 favourably compares with the non-supported catalyst. It is worth mentioning that it is
possible to run the reaction on gram scale, using an even smaller amount
of sensitizer; the oxidation of 13 mL. of dicyclopentadiene, promoted by
220 mg of PEG–TPP 5 (0.025% mol equivalent), gave 11 g of ketone
9, isolated, after filtration of the supported catalyst, by simple evaporation of the organic solvent as analytically pure compound requiring
nofurther purification.
Recycling of the PEG-bound porphyrin 5 was also studied. The catalyst was reused a second time in the photooxidation of the bisdialine to
give the product 7 in 97% yield and 80/20 diastereoisomeric ratio after
a 1-h reaction. The recovered PEG–TPP was recycled a third time without showing any decrease in the catalytic activity; the PEG-supported
sensitizer was recycled six times with no appreciable loss of chemical
or stereochemical efficiency.
Immobilization of an organic catalyst can be used also to facilitate
the process of catalyst’s optimization. Surprisingly, there is just a single
example of the application of this methodology reported to date and so
it deserves a special mention (Jacobsen and Sigman 1998).