Aluminium alkoxides determination

More recently Teyssie determined the rate constants in the polymerization of e-caprolactone (eCL) initiated with aluminium alkoxides, believing that the covalent species are the only ones responsible for propagation [4]. For the same monomer Yamashita estimated tentatively rate coefficients of propagation using an anionic initiator [ ]. Lenz in his studies of substituted g-propiolactones (gPL) observed peculiar influence of structure on reactivity that can have its origin in the multiplicity of ionic structures involved [fi]. 

The available experimental data supports a mechanism for the Oppenauer oxidation, involving an initial complexation of a carbonyl group with the aluminium from an aluminium alkoxide, followed by a rate-determining hydride transfer via a six-membered transition state.22... 

An outstanding input into the chemistry of metal alkoxides was made by the dissertation of V. E. Tishchenko, which was devoted to aluminium alkoxides. It was published in 1899 (unfortunately only in Russian) [1585] and—in contrast to his works on organic chemistry — is practically unknown by foreign chemists. A number of general synthetic approaches to metal alkoxides have been developed. The main regularities for the variation of the properties in the homologous sales and ramification of the radical were found to determine the main directions of the thermal decomposition. He made also the proposal to use the Al(OR)3 in ester condensation of aldehydes (Tishchenko reaction) (see also Chapter 12, Section 12.5). 

Due to the lack of X-ray data the structure of Ln-Al alkoxide has been proposed on the basis of indirect information. The molecular weight determination, the mass-spectra, the NMR and IR spectroscopy data suggest that lanthanoid alumoalkoxides have a structure similar to those of tetrameric aluminium alkoxides [53, 109, 112, 115]. 

Polymers with isopropoxy carbonyl and hydroxyl groups were isolated after qumiching the reactions with some proton donor. The kinetic order in initiator was less straightforward to determine, and also "the numbo of active alkoxide groups per aluminium" was a point of controversy 2<... 

The Tishchenko reaction is more than 100 years old and, with its different variants, has grown over the decades into a niche method for the selective production of dimeric esters from two aldehyde molecules. The reaction also known as the Claisen-Tishchenko reaction, occurs via a Lewis acid-catalysed disproportionation reaction involving a hydride shift (the rate determining step) as depicted in Scheme 18.23. Catalysts with higher Lewis acidity (like pure aluminium(m) alkoxides) afford in most cases the esters. 

---