SYNTHETIC AND MECHANISTIC INVESTIGATION OF THE REARRANGEMENT OF ETA-2-IMINOACYL LIGANDS OF GROUP-4 METALS TO VINYLAMIDO GROUPS VIA A FACILE 1,2-HYDROGEN SHIFT

Reaction of the metallocene dichloride compounds Cp2MCl2(M = Zr, Hf) with (2- (6-methylpyridyl)-methyl)lithium (LiCH2-py-6Me) leads to the bis(alkyl) compounds Cp2M(CH2-py-6Me)2(M = Zr (1a), Hf (1b)). A single-crystal X-ray diffraction analysis of 1a shows the compound to contain one chelated and one terminal pyridylmethyl ligand in the solid state. Solution studies with1and13C NMR spectroscopy indicate equivalent pyridylmethyl ligands for 1, presumably due to rapid dissociation and recoordination of the pyridine nitrogen atoms. Treatment of compounds 1 with 2,6-dimethylphenyl isocyanide (xyNC) leads initially to the formation of the corresponding η2-iminoacyl derivatives Cp2M(η2-xyNCCH2-py-6Me)(CH2-py-6Me) (M = Zr (2a), Hf (2b)). The η2binding of the iminoacyl group is strongly implied by the downfield shift of the iminoacyl carbon atom in the13C NMR spectrum: δ 246.7 ppm for 2a and δ 254.4 ppm for 2b. The iminoacyl compounds 1 exhibit thermal instability in solution. Over a period of hours at 25-30 °C they rearrange via a 1,2-hydrogen shift to the corresponding vinylamido compounds Cp2M[xyNCH=CH-py-6Me](CH2-py-6Me)(M = Zr (3a), Hf (3b)) in almost quantitative yield. Further reaction of 3 with xyNC finally leads to the bis(vinylamido) compounds Cp2M[xyNCH=CH-py-6Me]2(5) via the corresponding intermediate η2-iminoacyl 4. On the basis of1H NMR data and a single-crystal X-ray diffraction analysis of 5b, a trans arrangement can be assigned for the hydrogen atoms attached to the vinyl group. Treatment of Cp2HfCl2with 1 equiv of LiCH2-py-6Me or Cp2Hf(CH2-py-6Me)21b leads to the formation of Cp2Hf(CH2-py-6Me)Cl (6). Again treatment of 6 with xyNC generates the intermediate η2-iminoacyl compound Cp2Hf(η2-xyNCCH2-py-6Me)Cl (7), which isomerizes to the corresponding vinylamido compound 8. A kinetic study of the conversion of 2a,b to 3a,b and 7 to 8 shows the reactions to be first order in η2-iminoacyl at 28 °C and the 1,2-hydrogen shifts to proceed at essentially tbe same rate, demonstrating little dependence on metal and ancillary ligation. The use of the deuterated reagent LiCD2-py-6CD3allows the formation of Cp2Hf(η2-xyNCCD2-py-6CD3)Cl 7. The isomerization of 7 to 8 proceeds at a much slower rate than for the protio compound, yielding kH/kD= 11 (2) at 28 °C. The introduction of a phenyl substituent onto the α-carbon of the alkyl ligand totally inhibits the 1,2-hydrogen shift. Hence, the compound Cp2Hf[CH(Ph)-py]Cl (9) reacts with xyNC to yield the stable, structurally characterized η2-iminoacyl Cp2Hf[xyNCCH(Pb)-py]Cl (10). These results combined with the thermal stability of the corresponding benzyl-derived η2-iminoacyl compounds are discussed in terms of possible mechanisms for the overall 1,2-hydrogen shift. Crystal data at 22 °C for Cp2Zr(CH2-py-6Me)2(1a): a = 7.679 (3) Å, b = 11.239 (3) Å, c = 12.672 (2) Å, α = 95.67 (2)°, β = 100.57 (2)° γ = 68.37 (3)°, Z = 2, dcalcd= 1.432 g cm-3in space group PĪ Crystal data for Cp2Hf(xyNCH=CH-py-6Me)2(5b) at -155 °C: a = 14.490 (7) Å, 6 = 8.925 (4) Å, c = 15.943 (9) Å, β = 91.03 (3)°, Z = 2, dcalcd= 1.263 g cm-3in space group I2. Crystal data for Cp2Hf[η2-xyNCCH(Ph)-py]Cl (10) at 22 °C: a = 11.012 (1) Å, b = 14.821 (1) Å, c = 19.067 (2) Å, β = 103.263 (8)°, Z = 4, dcalcd= 1.613 g cm-3in space group P21/n. In the solid-state structure of 10 he phenyl and pyridyl groups were disordered. © 1990, American Chemical Society. All rights reserved.