Hydrocarbon Activation with metal Halides: Zirconium Tetrachloride Catalyzing the Jacobsen Reaction and Assisting the Trimerization of Alkynes via the Formation of eta6-Arene-Zirconium(IV) Complexes

Solubilization of the highly insoluble polymeric ZrCl4 is achieved using 1,2,4,5-Me4C6H2 (durene) as a carrier in a halogenated solvent, such as CH2Cl2, CHCl3, 1,2-Cl2C2H4, or o-Cl2C6H4. Solubilization can reach the level of 40 g of ZrCl4 in 100 mL of CH2Cl2 using an equimolar amount of durene. This isa very successful approach to making available soluble ZrCl4 in a noncoordinating solvent. The solubilization occurs via the formation of a eta(6)-arene complex, which has been structurally characterized in the form of the hexamethylbenzene derivative [(eta(6)-Me6C6)Zr-2(mu-Cl)(3)Cl-5] (3) This complex is in equilibrium, as revealed by the H-1 NMR spectrum in CH2Cl2, with free C6Me6 and a complex (4) which contains a higher Zr/C6Me6 ratio. A careful analysis of the CH2Cl2 solution, containing originally ZrCl4 and durene, showed that durene underwent transformation to C6Me5H and C6Me6. These species have been trapped, bonded to zirconium in complex [(eta(6)-C6Me5R)Zr-2(mu-Cl)(3)Cl-5] (2, R = H, 50%; R = Me, 50%), which has been structurally characterized. The methyl redistribution reaction in CH2Cl2 can result in the inconvenient formation of byproducts derived from the Zr-assisted Friedel-Craft reaction of CH2Cl2 on the arene. The intermediate of such a reaction has been isolated as [C6Me6CHCl2](+)[Zr2Cl9](-) (5), whose X-ray structure is available. In order to avoid the intervention of the solvent and to speed up the reaction, the ZrCl4 rearrangement of methylbenzenes was carried out in neat hydrocarbon at 90 degrees C for 6 h. This rearrangement has a number of peculiarities: (i) unlike the original Jacobsen reaction, it involves intermolecular methyl transfer; (ii) it reaches an equilibrium of methyl distribution; and (iii) it is catalytic in zirconium. The very high stability of 3 accounts for the isolation of the same compound derived from the ZrCl4-assisted trimerization of 2-butyne in n-hexane.