Cultivating hybrids: The synthesis of new hybrid aerobic oxidation catalysts that include an oxygen‐activating motif and a hydroquinone is reported. These hybrid catalysts proved to be efficient for the aerobic reoxidation of palladium and ruthenium catalysts in a set of test oxidations (see graphic).

In this work, we report the preparation and crystal structures of three new oligonuclear complexes, Ru2(bbpmp)(μ‐OAc)3 (4), [Co2(bbpmp)(μ‐OAc)(μ‐OMe)](PF6) (5), [Cu4(Hbbpmp)2(μ‐OAc)(H2O)2](OAc)(PF6)2 (6) {H3bbpmp = 2,6‐bis[(2‐hydroxybenzyl)‐(2‐pyridylmethyl)aminomethyl]‐4‐methyl‐phenol (3)}. The structures of the complexes were determined by single‐crystal X‐ray diffraction. The oxidation states of ruthenium, cobalt and copper in the complexes are +3, +3 and +2, respectively. In 4 and 5, RuIII and CoIII are coordinated to four oxygen and two nitrogen atoms in an octahedral geometry, while in 6, CuII adopts both octahedral (CuN2O4) and square‐pyramidal (CuN2O3) geometry. The potential of the three complexes as oxidation catalysts has been investigated.

Biomimetic aerobic oxidation of secondary alcohols has been performed using hybrid catalyst 1 and Shvo's catalyst 2. This combination allows mild reaction conditions and low catalytic loading, due to the efficiency of intramolecular electron transfer. By this method a wide range of different alcohols have been converted into their corresponding ketones. Oxidation of benzylic as well as aliphatic, electron‐rich, electron‐deficient and sterically hindered alcohols could be oxidized in excellent yield and selectivity. Oxidation of (S)‐1‐phenylethanol showed that no racemization occurred during the course of the reaction, indicating that the hydride 2b adds to the quinone much faster than it re‐adds to the ketone product. The kinetic deuterium isotope effect of the oxidation was determined by the use of 1‐phenylethanol (3a) and 1‐deuterio‐1‐phenylethanol (3a‐d1) in parallel and competitive manner, which gave the same isotope effect within experimental error (kH/kD ≈ 2.8). This indicates that there is no strong coordination of the substrate to the catalyst.

Hybrid catalysts A and B have recently been found to efficiently transfer electrons from a metal catalyst to molecular oxygen in biomimetic oxidations. In the present work hybrid catalysts A and B were synthesized in high yield from inexpensive starting materials. The key step is an efficient Suzuki cross‐coupling, which allows the use of unprotected aldehyde 5. The new synthesis of the title hybrid catalysts is easy to carry out and can be scaled up. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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