Aluminum centers

The aldehyde or ketone, when treated with aluminum triisopropoxide in isopropanol as solvent, reacts via a six-membered cyclic transition state 4. The aluminum center of the Lewis-acidic reagent coordinates to the carbonyl oxygen, enhancing the polar character of the carbonyl group, and thus facilitating the hydride transfer from the isopropyl group to the carbonyl carbon center. The intermediate mixed aluminum alkoxide 5 presumably reacts with the solvent isopropanol to yield the product alcohol 3 and regenerated aluminum triisopropoxide 2 the latter thus acts as a catalyst in the overall process ... 

A proposed mechanism (Scheme 5-44) begins with deprotonation of dimethyl phosphite to give an Al-phosphito complex (32) which can react with the aldehyde via either a chelate or open transition state, the latter possibly involving cooperative action of two aluminum centers, consistent with the observation of co-catalysis. Following P-C bond formation, several possible rearrangements could regenerate the achve catalyst and form the product... 

The thiolate species (182) and (183) do not require photo excitation in order to initiate polymerization (182) consumes 200 equivalents MMA in 18 h at 35 °C (Mn = 22,000, Mn(calc) = 20,000, Mw/ Mn= 1.12) 446 The propagating species is again believed to be an enolate.447 Propagation is accelerated upon addition of (185), with 100 equivalents of MMA requiring just 90 seconds for full conversion. The steric bulk of the Lewis acid prevents scrambling of the propagating enolate between the two aluminum centers.448 Hence, for aluminum diphenolates, ortho substitution is essential, whilst smaller Lewis acids such as Me3Al may only be used successfully at low temperatures, e.g., —40 °C. 

Further studies have shown that the degree of aggregation, and hence the number of active sites, is a function of the alkoxide substituent.813 For example, in (270) one of the i.vo-propoxide groups bridges two aluminum centers the other three are terminal ligands and all three initiate the ROP of CL. Less aggregated species such as (270) generally exhibit simple first order kinetics. 

The crystal structure of ECS-2 can be described by the stacking of alumino-silicate layers held together by phenylene groups (Fig.l). These layers are composed by aluminum centered tetrahedra bonded to [Si03C] tetrahedra. 

The striking increase in reducing power produced by the addition of electron donors to the system indicates the existence of a strong interaction between the two types of site and means, in effect, that closely adjacent aluminum centers must be involved, at least in the cases where... 

Lewis acidity, arising from electron deficient centers, does not exist in silica. In alumina this acidity is associated with surface aluminum ions which are coordinated in a tetrahedral manner. Such sites may be generated either by removal of water coordinated to an aluminum center or by dehydration of two adjacent hydroxyl groups above 500°C. 

Lewis acid centers, which were thought to be the primary catalytic sites. Boreskova et al. (51) studied the poisoning effect of quinoline on the cracking of cumene over Na, H—Y zeolite and observed a linear decrease in activity with the amount of quinoline added until a constant level of activity was reached. The catalytic activity was attributed to trivalent aluminum centers (Lewis acids), which were poisoned by coordinately bound quinoline. In a similar study of cumene cracking, Turkevich et al. (50) also concluded on the basis of magnetic resonance experiments that Lewis centers were the active sites. 

It has been suggested that alkene or alkyne complexes are mechanistically important intermediates in hydroalumination and carboalumination reactions (124-126). Clear spectroscopic evidence for n interactions stems from investigations of alkenylaluminum compounds having a suitable intramolecular separation between the aluminum center and the double bond (127). IR and NMR data of these compounds show comparably lower alkene stretching frequencies and deshielded vinylic protons. Furthermore, these molecules are monomeric in solution this indicates that the tendency toward 7i-complex formation is stronger than that toward the dimeric bonding usual in aluminum alkyls. 

Table 28 Structural parameters of heterobimetallic Ln/Al complexes featuring different alkylated aluminum centers ( mixed-alkylated complexes )... 

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