Autoxidation of Miscellaneous Substrates

Copper complexes were used as efficient catalysts for selective autoxidations of flavonols (HFLA) to the corresponding o-benzoyl salicylic acid (o-BSH) and CO in non-aqueous solvents and at elevated temperatures (124-128). The oxidative cleavage of the pyrazone ring is also catalyzed by some cobalt complexes (129-131). 

The third ligand was assumed to be coordinated to the metal center via the deprotonated 3-hydroxy and 4-carbonyl groups. This coordination mode allows delocalization of the electronic structure and intermolecu-lar electron transfer from the ligand to Cu(II). The Cu(I)-flavonoxy radical is in equilibrium with the precursor complex and formed at relatively low concentration levels. This species is attacked by dioxygen presumably at the C2 carbon atom of the flavonoxyl ligand. In principle, such an attack may also occur at the Cu(I) center, but because of the crowded coordination sphere of the metal ion it seems to be less favourable. The reaction is completed by the formation and fast rearrangement of a trioxametallocycle. 

Essentially the same mechanism was proposed for the [CuI(FLA) (PPh3)] (127) and [Cun(FLA)(IDPA)]+ (IDPA = 3,3 -iminobis(Al,Al-di-methylpropylamine) (128) catalyzed reactions. In the former case, composite kinetic features were found in the initial phase of the reaction indicating a slow transformation of the complex presumably to the same catalytically active species which was postulated in the Cu(FLA)2 system. 

For the reaction of [CuII(FLA)(IDPA)]+ straightforward kinetics were reported, as an induction period was not observed at the beginning of the reaction, and the rate was first-order in the catalyst and 02 concentrations but independent of the substrate concentration, with k = 4.2 x 10 2 M ls- at 130°C in DMF. In the absence of flavonol, the catalyst is also oxidized in an overall second-order process and the 

A comparison of the rate constants for the [Cun(FLA)(IDPA)]+-cata-lyzed autoxidation of 4/-substituted derivatives of flavonol revealed a linear free energy relationship (Hammett) between the rate constants and the electronic effects of the para-substituents of the substrate (128). The logarithm of the rate constants linearly decreased with increasing Hammett o values, i.e. a higher electron density on the copper center yields a faster oxidation rate. 

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