Organoaluminums formation

Reactions of enamines with aluminum hydrogen dichloride (540,541) (UAIH4 and AICI3) or aluminum hydrogen dialkyl compounds (542) led to organoaluminum intermediates which could be hydrolyzed to tertiary amines or oxidized to aminoalcohols. The formation of olefins by elimination of the tertiary amine group has also been noted in these reactions. 

Additional evidence that reduction is not the role of R AlCla.. in catalyst formation is provided by the observation that the complexes [Bu4N] [Mo(CO)5X] and [R4N] [Mo(CO)5COR L in which the molybdenum is in a low oxidation state, require an organoaluminum reagent for catalytic activity (44, 45). In these examples, the function of the organoaluminum is most likely the removal of CO ligands to make available sites for olefin coordination. Molybdenum hexacarbonyl alone is reported to be a disproportionation catalyst in this case expulsion of the CO groups is attained thermally at 98 °C (46). 

Finally, ring expansions of cyclic ketones are an important method for the preparation of carbocyclic systems. Diazoalkanes allow this reaction to occur and involve C—C bond formation. When combined with carefully chosen organoaluminum compounds, this process can be performed in a highly stereoselective fashion (equation 148)541. 

The reactivity of organotitanium compounds is similar to that of organoaluminum compounds, and attack at the most polarized C-O bond is usually observed (Scheme4.71). The lack of stereoselectivity of the second example in Scheme4.71 suggests the intermediate formation of carbocations or radicals. 

Diene polymerization via lanthanide propanolate-based catalytic systems was first reported by U. Pedretti et al. [162], Several types of binary and ternary initiator systems have been developed since the late 1970s (see Table 7), including zz- and zsopropanolalc ligands [162], homo- and heteroleptic lanthanide species [163-165], and different types of organoaluminum cocatalysts [162,166-168]. Despite such extensive studies, structural evidence of the formation of alkylated rare-earth metal centers remained scarce. 

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