Hydrocarbon Activation with Metal Halides: Zirconium Tetrachloride Catalyzing the Jacobsen Reaction and Assisting the Trimerization of Alkynes via the Formation of η6-Arene−Zirconium(IV) Complexes

Solubilization of the highly insoluble polymeric ZrCl 4 is achieved using 1,2,4,5-Me 4 C 6 H 2 (durene) as a carrier in a halogenated solvent, such as CH 2 Cl 2, CHCl 3, 1,2-Cl 2 C 2 H 4, or o -Cl 2 C 6 H 4 . Solubilization can reach the level of 40 g of ZrCl 4 in 100 mL of CH 2 Cl 2 using an equimolar amount of durene. This is a very successful approach to making available soluble ZrCl 4 in a noncoordinating solvent. The solubilization occurs via the formation of a η 6 -arene complex, which has been structurally characterized in the form of the hexamethylbenzene derivative [(η 6 -Me 6 C 6 )Zr 2 (μ-Cl) 3 Cl 5 ] ( 3 ). This complex is in equilibrium, as revealed by the 1 H NMR spectrum in CH 2 Cl 2, with free C 6 Me 6 and a complex ( 4 ) which contains a higher Zr/C 6 Me 6 ratio. A careful analysis of the CH 2 Cl 2 solution, containing originally ZrCl 4 and durene, showed that durene underwent transformation to C 6 Me 5 H and C 6 Me 6 . These species have been trapped, bonded to zirconium in complex [(η 6 -C 6 Me 5 R)Zr 2 (μ-Cl) 3 Cl 5 ] ( 2, R = H, 50%; R = Me, 50%), which has been structurally characterized. The methyl redistribution reaction in CH 2 Cl 2 can result in the inconvenient formation of byproducts derived from the Zr-assisted Friedel−Craft reaction of CH 2 Cl 2 on the arene. The intermediate of such a reaction has been isolated as [C 6 Me 6 CHCl 2 ] + [Zr 2 Cl 9 ] - ( 5 ), whose X-ray structure is available. In order to avoid the intervention of the solvent and to speed up the reaction, the ZrCl 4 rearrangement of methylbenzenes was carried out in neat hydrocarbon at 90 °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 ZrCl 4 -assisted trimerization of 2-butyne in n -hexane.