(possibly short-lived) CuI-phenoxide radical species in the key
step, spectroscopic data support a CuII-phenolate ground state
description for the red intermediate 2. While mechanistically
distinct from the type 3 dicopper sites (which shuttle between
the CuIICuII and CuICuI states without involvement of 1eꢁ
radical chemistry), some new cooperative two-centre reactivity
has resulted from the present bioinspired approach. Cu-
mediated formation of MDP from TMP in the presence of
methanol has previously been suggested to proceed via 1,6-
nucleophilic addition of methanol to a quinone methide
intermediate,8 which would require an initial 2eꢁ transfer that
is not observed here. Further studies are now directed at
elucidating details of the electronic structure of 2, the activa-
tion of dioxygen by the mixed-valent species 3 with the
possible generation and involvement of superoxide, and finally
a full mechanistic understanding of the various reaction steps.
Fig. 4 ORTEP plot (30% probability thermal ellipsoids) of the
structure of 20. For the sake of clarity all hydrogen atoms except H2
have been omitted. Cu1ꢀ ꢀ ꢀCu2 4.309(1) A; see ESIw for other atom
distances and angles.
Notes and references
z Crystal data for 20 are given in the ESIw.
CCDC 659295. For crystallographic data in CIF format see DOI:
10.1039/b718162k
spectroscopy (see ESIw): 1 is EPR silent at 15 K but at 50 K
shows a broad absorption around 3000 G with a weak half-
field signal, characteristic of an exchange-coupled antiferro-
magnetic dicopper(II) system (J = ꢁ35.3 cmꢁ1 was derived
from SQUID data4). In contrast, the spectrum of the frozen
yellow-green solution containing 3 recorded at 15 K shows a
typical axial EPR spectrum for a single CuII, suggesting that 3
is a valence-localized CuICuII species on the EPR time scale.
Furthermore, spectral parameters for 3 are very similar to
those of a CuICuII compound prepared independently from
the pyrazolate/imidazole-based ligand and each one equivalent
of a CuI and CuII salt (gII = 2.25, g> = 2.06, |AII| = 157 G,
|A>| = 20 G).
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Obviously, molecular oxygen is not involved in the initial
C–C coupling, but only in subsequent re-oxidation of the semi-
reduced dicopper complex and transformation of TMBB to
the final product TMSQ. Control experiments show that
TMBB itself is not oxidized to TMSQ in air, but it is rapidly
oxidized upon exposure to air in the presence of catalyst 1.
In conclusion, a highly preorganized dicopper complex has
allowed the development of a new catalytic procedure for the
para C–H activation of TMP with aerial dioxygen as the
oxidant. Selective formation of either TMSQ or MDP is
achieved depending on the solvent used. The reaction proceeds
through initial coordination of TMP to the dicopper site,
followed by benzylic C–C coupling that leads to TMBB as
the first product. The resulting semi-reduced CuICuII species is
then re-oxidized by dioxygen to revive catalyst 1. A model
complex for the first key intermediate, featuring a coordinated
and strongly hydrogen-bonded phenolate within the bimetallic
pocket, has been isolated and structurally characterized.
Whereas the benzylic C–C coupling of TMP to give TMBB
may suggest a radical-based process and involvement of a
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unpublished results.
´
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ꢂc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 1005–1007 | 1007