Abstract:We have previously shown that the use of ligand-free palladium employing Pd(OAc) 2 as catalyst precursor in the Heck reaction of aryl bromides is possible if low catalyst loadings, typically between 0.01 -0.1 mol % are used. We have now tested this phenomenon, which we have dubbed "homeopathic" palladium, in biaryl formation using the Suzuki, the Negishi and the Kumada cross-coupling reactions. The Suzuki reaction of aryl bromides, both activated and deactivated, is possible using 0.02 -0.05 mol % of Pd(OAc) 2 . In this reaction turnover frequencies up to 30,000 have been reached with activated substrates. Even aryl chlorides could be reacted if strongly electron-withdrawing substituents were present. The Negishi coupling with a variety of arylzinc halides was possible on aryl bromides containing electronwithdrawing substituents. The Kumada reaction only gave low yields of products under "homeopathic conditions.

Dedicated to Prof. Roger Sheldon on the occasion of his 60th birthday.

The reactivity of palladium(0) complexes, [Pd(0) (2)(dba-n,n'-Z)(3)] (n,n'-Z=4,4'-F; 4,4'-CF(3); 4,4'-H; 4,4'-MeO) and [Pd(0)(dba-n,n'-Z)(2)] (n,n'-Z=4,4'-CF(3); 4,4'-H; 3,3',5,5'-OMe), used as precursor catalysts with suitable donor ligands (e.g. phosphines, N-heterocyclic carbenes), has been correlated in several palladium(0)-mediated cross-coupling processes. Increasing the electron density on the aryl moiety of the dba-n,n'-Z ligand increases the overall catalytic activity in the majority of these processes. This effect primarily derives from destabilization of the L(n)Pd(0)-eta(2)-dba interaction (in dpi-pi* synergic bonding, n=1 or 2), which ultimately increases the global concentration of catalytically active L(n)Pd(0) available for reaction with aryl halide in the first committed step in the general catalytic cycle(s) (oxidative addition). Decreasing electron density on the aryl moiety of the dba-n,n'-Z ligand stabilizes the Pd(0)-eta(2)-dba interaction, reducing catalytic activity. The specific type of dba-n,n'-Z ligand appears to also play a stabilizing role in the catalytic cycle, preventing Pd agglomeration, and increasing catalyst longevity. A subtle balance therefore exists between the L(n)Pd(0) concentration (and the associated catalytic activity) and catalyst longevity. Changing the type of dba-n,n'-Z ligand controls the concentration of L(n)Pd(0) and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation (or carbopalladation) and reductive elimination. The role of dba-n,n'-Z ligands in Heck arylation is more convoluted and dependent on the alkene substrate employed, although trends have emerged. Changes in the structure of dba-n,n'-Z had a minimal affect on Buchwald-Hartwig aryl amination processes. A secondary Michael reaction of dba-n,n'-Z with amine and/or base effectively lessens its interference in the catalytic cycle.

A new liquid-phase catalytic oxidation system for the low temperature, high conversion benzylic mono-oxyfunctionalization of 10,11-dihydrocarbamazepine (1) into oxcarbazepine (4) with dioxygen has been developed. The method is based on a co-oxidation of 1 with benzaldehyde in the presence of a four-component catalyst system consisting of Co(-OAc) 2 , Ni(OAc) 2 , Cr(NO 3 ) 3 , and N-hydroxyphthalimide (NHPI). The influence of the catalyst system on the formation and decomposition of the crucial hydroperoxide intermediate

CuCl ligated with b-diketones were found to be good catalysts for the amination of aryl bromides. Crucial is the concentration of the substrates: at 5 M the rate and selectivity improves substantially. In addition, K 2 CO 3 can be used as base instead of expensive Cs 2 CO 3 . Primary and secondary amines, heterocycles and anilines could be arylated in good yields.

Primary and secondary (homo-)allylic and (homo-)propargylic alcohols can be oxidized under slightly acidic conditions at or below room-temperature with sodium periodate in the presence of sodium dichromate as the catalyst to the corresponding carboxylic acids and ketones, respectively.

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