Trimethylaluminum, group exchange

Further evidence of possible formation of five-coordinate aluminum atoms as a pathway for exchange has been reported by Mole and co-workers (50) in a study of alkyl group exchange between trimethylaluminum and dimethylethylaluminum etherates. The exchange rate has an overall second-order dependence on total aluminum concentration and a zero-order dependence on ether concentration, indicating alkyl exchange without prior dissociation of the ether adducts. 

A1NMR spectroscopy (234). The exchange reactions are also fast in ether, but about 104 times slower in pyridine. In the latter solvent, a mixture of trimethylaluminum and triethylaluminum requires about 4 hours at 40° C for a 50% equilibration (207). From proton NMR measurements in pyridine, the following equilibrium constants were found for the exchange of phenyl with methyl groups (209),... 

This intermolecular exchange may be readily accounted for by the process represented in Eqs. (16) and (17). Bor example, in the studies by Hoffmann (60, 61, 63), Poole (115), and others (93, 98, 131, 156, 157), the exchange of larger organic groups between aluminum atoms may be represented as going through a transition state similar in nature to the trimethylaluminum dimer. 

Self-exchange has been reported, however, in trivinylgallium, which is dimeric (134). The proposed structure for this compound is similar to that of trimethylaluminum with bridging vinyl groups ... 

Information for other organometallic compounds is scarce. The corresponding methyl and ethyl derivatives of mercury, tin, and silicon were shown to undergo redistribution to yield random equilibrium mixtures of all possible metal alkyls. Rapid intermolecular exchange of the methyl groups in mixtures of trimethylaluminum-dimethylcadmium and dimethylzinc-dimethylcadmium was also demonstrated by NMR. The redistribution of mercury derivatives was found to be much slower. Further data are also available for thallium. - The four-center intermediate 154 was suggested to explain the observations [Eq. (6.149)] ... 

Capturing of the Fe + ions in the zeolite a-cation-exchange positions was first considered theoretically in [81], The Fe + ion grafted to the zeolite framework, or captured by a zeolite lattice defect =Si-0-Fe-0-Si= was proposed in [82] as an active center with low coordination of Fe +. Such structure could be emerged as the result of iron immobilization on vicinal hydroxyl zeolite groups. Also, it was found that addition of trimethylaluminum to Fe-silicalite drastically improves the catalyst activity in the process (20.10). Based on these data a conclusion about the formation of FeAlO active species was made [83],... 

---