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Results and Discussion
Flavin 2 was synthesized from 3,4-diaminobenzoic acid (3)
and alloxane monohydrate (4) in good yield according to
standard procedures (Scheme 1).9 The first step afforded flavin
5 in quantitative yield,in 1:1 ratio of the 7- and 8-hydroxycarbonyl
regioisomers,and was subsequently methylated by methyl
iodide to form flavin 6 in 86% yield,in a 3:2 ratio of the 7-
and 8-regioisomers.In this step also the acid moiety was
methylated to give the ester.The ester was hydrolyzed by
reaction with concentrated hydrochloric acid,which afforded
flavin 7 in 85% yield in a 3:2 ratio of the 7- and 8-regioisomers.
The precatalyst 2 was obtained in 67% yield by reductive
alkylation of 7.
Our previous studies on the substituent effects have shown
that electron-withdrawing substituents on the flavin increase its
oxidation potential and hence the rate of the corresponding
flavin-catalyzed oxidation.9 Thus,flavin 2 is expected to give
a faster sulfoxidation with H2O2 compared to 1.Furthermore,
and more importantly,the carboxylate group makes an immobilization
of 2 in an ionic liquid very efficient.Flavin 2 is
most likely present in its zwitterionic form or as its carboxylate
ion.The 1H NMR spectrum of 2 is consitent with a zwitterionic
form with the nitrogen in the 5-position protonated since in both
isomers the two hydrogens of the CH2 group in the ethyl group
on N-5 are coupled with an additional proton.Furthermore,the
1H NMR shows that the two protons in the CH2 group are
nonequivalent and appear at different shifts (¢ä ) 0.48 ppm;
see the Experimental Section).In previous studies we have used
flavin 1 in ionic liquid [BMIm]PF6 for the recycling of NMM
to NMO in the osmium-catalyzed dihydroxylation.18 One
problem with flavin 1 is that it leaches on recycling and attempts
to immoblize 1 in [BMIm]PF6 for sulfoxidation by H2O2 were
unsuccessful due to severe leaching.17 p-Tolyl methyl sulfide
(8a) was used as a model substrate in the tuning of reaction
conditions.We found that sulfide 8a was efficiently oxidized
to sulfoxide 8b using 2 mol % of flavin 2 and 1.5 equiv of
H2O2 in the ionic liquid [BMIm]PF6/methanol mixture.The
immobilized catalytic system showed high activity and no
overoxidation was detected.The new system was applied to
several other sulfides and the results are presented in Table 1.
The sulfides were oxidized to sulfoxides by H2O2 in high
yields at room temperature with short reaction times (Scheme
2).In all cases a high selectivity for sulfoxide was obtained
with no overoxidation to sulfone.The system tolerates both
electron-rich and electron-deficient substrates,although electrondeficient
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Results and Discussion
Flavin 2 was synthesized from 3,4-diaminobenzoic acid (3)
and alloxane monohydrate (4) in good yield according to
standard procedures (Scheme 1).9 The first step afforded flavin
5 in quantitative yield,in 1:1 ratio of the 7- and 8-hydroxycarbonyl
regioisomers,and was subsequently methylated by methyl
iodide to form flavin 6 in 86% yield,in a 3:2 ratio of the 7-
and 8-regioisomers.In this step also the acid moiety was
methylated to give the ester.The ester was hydrolyzed by
reaction with concentrated hydrochloric acid,which afforded
flavin 7 in 85% yield in a 3:2 ratio of the 7- and 8-regioisomers.
The precatalyst 2 was obtained in 67% yield by reductive
alkylation of 7.
Our previous studies on the substituent effects have shown
that electron-withdrawing substituents on the flavin increase its
oxidation potential and hence the rate of the corresponding
flavin-catalyzed oxidation.9 Thus,flavin 2 is expected to give
a faster sulfoxidation with H2O2 compared to 1.Furthermore,
and more importantly,the carboxylate group makes an immobilization
of 2 in an ionic liquid very efficient.Flavin 2 is
most likely present in its zwitterionic form or as its carboxylate
ion.The 1H NMR spectrum of 2 is consitent with a zwitterionic
form with the nitrogen in the 5-position protonated since in both
isomers the two hydrogens of the CH2 group in the ethyl group
on N-5 are coupled with an additional proton.Furthermore,the
1H NMR shows that the two protons in the CH2 group are
nonequivalent and appear at different shifts (¢ä ) 0.48 ppm;
see the Experimental Section).In previous studies we have used
flavin 1 in ionic liquid [BMIm]PF6 for the recycling of NMM
to NMO in the osmium-catalyzed dihydroxylation.18 One
problem with flavin 1 is that it leaches on recycling and attempts
to immoblize 1 in [BMIm]PF6 for sulfoxidation by H2O2 were
unsuccessful due to severe leaching.17 p-Tolyl methyl sulfide
(8a) was used as a model substrate in the tuning of reaction
conditions.We found that sulfide 8a was efficiently oxidized
to sulfoxide 8b using 2 mol % of flavin 2 and 1.5 equiv of
H2O2 in the ionic liquid [BMIm]PF6/methanol mixture.The
immobilized catalytic system showed high activity and no
overoxidation was detected.The new system was applied to
several other sulfides and the results are presented in Table 1.
The sulfides were oxidized to sulfoxides by H2O2 in high
yields at room temperature with short reaction times (Scheme
2).In all cases a high selectivity for sulfoxide was obtained
with no overoxidation to sulfone.The system tolerates both
electron-rich and electron-deficient substrates,although electrondeficient
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