C. Gilmartin et Jrl. Smith, ALKENE EPOXIDATION BY IODOSYLBENZENE CATALYZED BY IRON(III) 5,10,15,20-TETRA(2,6-DICHLOROPHENYL)PORPHYRIN COORDINATED TO PYRIDINE-MODIFIED SILICA, Perkin transactions. 2, (2), 1995, pp. 243-251
Six silicas, with different surface areas, pore sizes and pore volumes
, have been surface modified with pyridine groups and used to support
iron(III) 5,10,15,20-tetra (2,6-dichlorophenyl) porphyrin ((FeTDCPP)-T
-III) by ligation to the metal ion. Comparison of these materials (SiP
y-(FeTDCPP)-T-III) as catalysts for the epoxidation of (Z)-cyclooctene
by iodosylbenzene in dichloromethane reveals that all have essentiall
y the same catalytic activity and that the heterogeneous reactions are
significantly slower (ca. ten-fold) than the homogeneous analogue. In
small scale repeat-use experiments the catalysts achieve 2200 catalys
t turnovers without loss of activity. however, larger scale reactions
(7900 turnovers) lead to catalytic oxidative bleaching and > 50% reduc
tion in activity. A Hammett study of the epoxidation of styrene and 4-
substituted styrenes gives rho values (vs. sigma(+)) of -0.95 and -0.9
8 for oxidations catalysed by (FeTDCPP)-T-III and a supported catalyst
SiPy-(FeTDCPP)-T-III, respectively. The similarity of these values to
each other and to values from other iron(III) porphyrin-catalysed epo
xidations is discussed. A minor pathway in the oxidation of the styren
es in air that leads to benzaldehydes is attributed to dioxygen interc
epting an intermediate in the reaction. Oxidations of (Z)- and (E)-4-m
ethylpent-2-ene catalysed by (FeTDCPP)-T-III and by SiPy-(FeTDCPP)-T-I
II show a marked difference. With the former the relative reactivity o
f (Z)- to (E)-alkene is 13:1, whereas the latter fails to catalyse the
epoxidation of the (E)-alkene and instead is converted into a green m
aterial; this Is possibly an iron N-alkylporphyrin.