Synthetic and Mechanistic Aspects of Intramolecular Aliphatic CH Bond Activation by Ti(IV) and Zr(IV) Metal Centers

Article abstract of DOI:10.1021/ja00307a024

The complexes M(OAr')2(CH2Ph)2 (OAr' = 2,6-di-tert-butylphenoxide; M = Ti, Zr) undergo the loss of equiv of toluene on mild thermolysis and formation of a monometalated complex M(OC6H3Bu^tCMe2CH2)(OAr')(CH2Ph), in which one of the CH bonds of a tert-butyl group has been activated and cleaved.Addition of 2,6-di-tert-butylphenol (HOAr') to this complex yields M(OC6H3Bu^tCMe2CH2)(OAr')2 and 1 equiv of toluene while pyridine gives the adducts M(OC6H3Bi^tCMe2CH2)(OAr')(CH2Ph)(py) as yellow (M = Ti) and white (M = Zr) powders.Although these complexes were found unsuitable for crystallographic study, the related compound Ti(OC6H3Bu^tCMe2CH2)(OAr')(CH2SiMe3)(py) has been isolated and a single-crystal X-ray diffraction study has confirmed the presence of the six-membered metallocycle.The molecule is found to adopt a pseudo-tbp geometry in the solid state with the pyridine nitrogen atom and the oxygen atom of the cyclometalated aryl oxide occupying the axial sites.The unmetalated aryl oxide oxygen atom and two carbon atoms form the equatorial plane.Crystal data for Ti(OC6H3Bu^tCMe2CH2)(OAr')(CH2SiMe3)(py) at -165 deg C follow: a = 18.837 (8) Angstroem, b = 17.823 (6) Angstroem, c = 10.923 (3) Angstroem, β = 99.62 (1) deg, Z = 4, d calcd = 1.146 g cm^-3 in space group P2₁/a.Of the 4734 unique intensities collected with Mo Kα the 3372 with F₀ > 2.33δ(F) were used in the least-squares refinement to give residuals R(F) = 0.0662 and Rw(F) = 0.024.Kinetic measurement of the cyclometalation step showed the reaction to be unimolecular with the following activation parameters for M = Ti(Zr): ΔH^<*> = 23.0 (21.6) kcal mol^-1, ΔS^<*> = -13 (-19) eu.These data are argued to be consistent with those for a multicenter transition state for the activation of the CH bonds at these d⁰ metal centers.The reaction has also been shown to be only slightly sensitive to substituents on the benzyl leaving group.The possible mechanistic implications of these data are discussed.