The strained thoracyclobutane Cp′2ThCH2C(CH3)2CH2 (1, Cp′ = η5-(CH3)5C5) undergoes facile ring-opening C-H activation reactions with saturated hydrocarbons and related molecules, RH, to yield complexes of the type Cp′2Th(R)-[CH2C(CH3)3]. All new complexes have been characterized by standard spectroscopic/analytical methodology. Approximate relative rates of the R-H functionalization are Sn(CH3)4 ≳ Si(CH3)4 > cyclopropane ≈ P(CH3)3 > benzene > CH4 ≳ C2H6 ≫ cyclohexane. For Si(CH3)4, the reaction obeys the rate law v = k[Si(CH3)4] with k = 7.0 (5) × 10-5 M-1 s-1 at -10 °C. In the case of Si(CH3)4, Sn(CH3)4, and P(CH3)3, further reaction (cyclometalation) after ring opening affords the heteroatom-substituted metallacycles Cp′2ThCH2Si(CH3)2CH 2, Cp′2ThCH2Sn(CH3)2CH 2, and Cp′2ThCH2P(CH3)CH2. NMR data indicate that the metallacyclic ring of the latter complex is probably not planar and that the phosphorus lone pair does not interact with the thorium ion. In the case of cyclopropane and benzene, a follow-up C-H activation reaction leads to the corresponding Cp′2ThR2 complexes and neopentane. The CH4/CD4 activation process by 1 exhibits a substantial kinetic isotope effect, kH/kD = 6 (2) at 60°C, and the deuterium distribution in the products gives no evidence of significant Cp′ methyl group involvement in the methane functionalization. The ethane reaction with 1 does not lead to a stable ethyl complex, but rather thorium hydride products are detected (suggesting follow-up β-hydride elimination). There is no evidence of a reaction between 1 and cyclohexane. The reaction of 1 with propylene and ethylene does not involve C-H activation, but rather insertion of the C=C double bond into the Th-C σ bond occurs to yield the metallacyclohexanes Cp′2ThCH2C(CH3)2CH 2CH(CH3)CH2 and Cp′2ThCH2C(CH3)2CH 2CH2CH2, respectively. The courses of most of the transformations reported herein can be readily understood on the basis of Th-ligand and R-H bond disruption enthalpy data. Mechanistically, a heterolytic "four-center" pathway appears to be the most viable description of the Th(IV)-centered C-H activation process.
|Number of pages||13|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1986|
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