Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 53

More detailed studies on reactions between and amines
have revealed, at least by NMR studies (see below), the presence of
several Ru-intermediates en route to the final and co-
products (imine or nitrile, and water, eqs. 38, 39) 197 . UV-Vis studies at ~
M Ru(VI) for the reaction with (R = H, Me, Ph.) in benzene at
20°C show rapid spectral changes with clean isosbestic points
for implying the absence of substantial
amounts of any intermediates; attempted kinetic studies were thwarted by
irreproducibility, although the substituted benzylamines reacted more
slowly 185,197 . The kinetic inconsistencies possibly result from photo-
sensitivity of the or other species (see below) in the light
beam 197 . At the higher concentrations used for NMR, and in the darkness of
the probe, kinetic data became reproducible within stoichiometric conditions
(20°C under Ar). The stoichiometric reactions (cf. eq. 38) followed the
simple rate-law k [Ru] [amine] for loss of Ru(VI), over an amine
concentration range of ~ 0.01 - 0.3 M, with k values of 9.74, 1.72 and
at 20°C for the R = H, Ph, and Me systems, respectively 197 , and
there was no sign of saturation kinetics as found for the related alcohol
systems (see Section 3.6). A kinetic isotope effect measured for
the systems is perhaps consistent with formation of
an activated complex of the type Ru=O....H---N, via an H-bonding
interaction with H-atoms on the N-atom; the suggestion corresponds to that
shown in Figure 25 for the alcohol systems where a C-H bond is initially
stretched. Cleavage of the N-H bond of the amine versus the C-H bond of
the alcohol would account for the generally faster stoichiometric reactions of
the

species with amines versus alcohols under analogous

conditions.
In contrast to studies on the alcohol systems, two Ru-intermediates were
detected in the NMR studies en route to the bis(amine) product for the
slower R = Ph and Me systems (eq.40) 197 . Intermediate ( 26 ) has
symmetry and thus two different axial ligands (this being readily diagnosed

by the presence of two signals for the o -Me groups of the mesityl
substituents 49,60,196,198,199 ), while intermediate ( 27 ) has

symmetry and two

identical axial ligands. Potential intermediates such as
and were ruled out, at least for the
system, where the is readily available, as the bis(imine) and
(amine)(imine) species with readily identifiable NMR spectra were
synthesized independently from

The findings are