Hydrocarbons dinuclear activation

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

The M 114.02 clusters were more active than the Mn30 clusters and the Mn clusters also catalyzed t-butyl hydroperoxide decomposition ( 1%) to acetone and methanol, which prevented reliable analysis of methane activation results. We could not compare the dinuclear manganese complexes to their tri and tetra analogues because of relative solubility differences however, we were able to do this with several iron di and tetra clusters and found that the Fe402 clusters were more active with the hydrocarbons studied. Therefore, higher nuclearity or a variety of ligands may provide the shape selectivity we seek fa ultimate methane activation with Mn and Fe clusters. 

To begin to develop functionalization catalysts based on these more electron-rich systems we have been investigating the development of catalytic cycles based on the reaction of O-donor metal-alkoxo complexes with CH bonds as shown in Fig. 7.41 B). This reaction is intriguing because as shown, the reaction leads to the simultaneous CH activation of the hydrocarbon as well as the formation cf a desired oxy-junetionalized product, ROH, in one step. We recently reported [24] the first intermolecular example of such a transformation with trans-(acac-0,0)2lr(0CH3)(CH30H), MeOH-fr-OMe and the corresponding pyridine complex, Py-Ir-OMe. There is no precedent for this type of CH activation reaction with alkoxo complexes [31] and such complexes typically decomposition by facile 8-hydride elimination reactions or formation of inert dinuclear complexes. 

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