Developing potentially useful organometallic Lewis acid catalysts: (η-cyclopentadienyl)zirconium trichloride derivatives

Add time:08/06/2019    Source:sciencedirect.com

The organometallic Lewis acid (η-cyclopentadienyl)zirconium trichloride (2) was obtained from bis(cyclopentadienyl)zirconium dichloride (1) and chlorine by means of a radical induced metal-Cp bond cleavage. Reaction of 2 with tetrahydrofuran gave the CpZrCl3-bis(tetrahydrofuran) adduct 3; treatment of either 2 or 3 with excess dimethylformamide furnished CpZrCl3(dmf)2 (4). (η-Methylcyclopentadienyl)bis(tetrahydrofuran)zirconium trichloride (7) was prepared by treatment of ZrCl4(thf)2 (6) with one molar equivalent of the (methylcyclopentadienyl)thallium reagent 5. Complexes 3, 4 and 7 were characterized by X-ray diffraction studies and their dynamic NMR spectra were recorded. Complex 3 crystallizes in space group P21/n with cell parameters a 8.155(1), b 13.342(2), c 15.775(1) Å, β 104.81(1)°, Z = 4, R = 0.041, Rw = 0.056. CpZrCl3(dmf)2 (4) crystallizes in space group P21/c with cell parameters a 7.790(1), b 15.445(2), c 13.893(1) Å, β 93.94(1)°, Z = 4, R = 0.032, Rw = 0.052. Complex 7 crystallizes in space group P21/n with cell parameters a 10.242(2), b 12.836(2). c 13.683(1) Å, β106.37 (1)°, Z = 4, R = 0.037, Rw = 0.039. The (cyclopentadienyl)bis(tetrahydrofuran)zirconium trichloride adduct 3 selectively catalyzed the Diels-Alder addition of 2,3-dimethylbutadiene and methacrolein at ambient temperature to give 1,3,4-trimethyl-3-cyclohexenyl-1-carbaldehyde (13). However, the CpZrCl3-Lewis acid catalyzed rearrangement of 13 to 1,3,4-trimethylbicyclo[2.2.1]heptan-2-one (14) does not take place at a significant rate until ca. 150°C.

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