C-H hydroxylation - oxaziridines - hydrogen peroxide - hydrophobic aggregation
Significance
<P>Based on the authors’ previous work in this field (
J. Am. Chem. Soc. 2005,
127, 15391) a second-generation oxaziridine
2-mediated catalytic process for C-H
hydroxylation is reported. The revised reaction protocol features
the use of aqueous H
2O
2 in AcOH-H
2O
(1:1), conditions which efficiently generate the reactive oxaziridine
intermediate
2 in situ from the benzoxathiazine
catalyst
1. Among all catalysts
1 tested, catalyst
1e (R = C
6F
5)
turned out to be the most effective suggesting hydrophobic catalyst-substrate
aggregation as proposed by the authors. Mechanistically, oxaziridine-mediated
electrophilic O-atom insertion into C-H bonds likely proceeds in
a concerted, asynchronous fashion.</P>
Comment
<P>Most catalytic C-H bond hydroxylation processes
rely on transition metal complexes to support reactive metal-oxo
or metal-peroxo species (for an example, see: M. S. Chen, M. C. White
Science 2007,
318, 783). Curci and co-workers have
used dioxiranes - strained, electrophilic heterocycles - for
the oxygenation of saturated hydrocarbons (
Acc.
Chem. Res. 2006,
39, 1).
In contrast the Du Bois group employs in situ generated oxaziridines
2 for the same purpose. Compared to their
first-generation process the advanced method not only simplifies
the experimental protocol by avoiding Ar
2Se
2 as
co-catalyst but also has a significantly broader substrate scope.
</P>
