Preparation of Organoaluminum Compounds by Hydrozirconation-Transmetalation

Organozirconium(IV) complexes produced by hydrozirconation of olefins or acetylenes with Cp₂Zr(H)Cl (Cp = η⁵-C₅H₅) are precursors of organoaluminum compounds. By stoichiometric transmetalation, the η¹ organic ligand can be transferred stereospecifically from zirconium to aluminum by treatment with aluminum halides. Organoaluminum dichlorides can act as mild alkylating agents; they are precursors of ketones through acylation with acid chlorides (this acylation has been found to proceed well for primary, saturated alkyl- and alkenylaluminum dichlorides). Transmetalation from Zr to A1 occurs faster for vinylic substituents than for alkyl ones and proceeds predominantly with retention of configuration at carbon. A mechanism for transmetalation is postulated in which the transition state (or intermediate) involves a transition metal-main group metal complex containing a bridging alkyl (or vinylic) group. This mechanism is analogous to the one reported for electrophilic cleavage of alkylzirconium complexes. Acylaluminum dichlorides have been formed by transmetalation between Cp₂Zr(Cl)(acyl) and aluminum chloride. They have been shown to be reactive toward electrophiles at both the carbon and the oxygen of the acyl group. Treatment of an acylaluminum dichloride with D₂0 results in an aldehyde which is at least 83% -C(O)-D. However, treatment with acetyl chloride gives an a-chloroalkyl acetate as the major product (attack at O) and an a-diketone as a minor product (attack at C).