2,6-Diisopropylphenylphosphane:: A new, bulky primary phosphane and its mono- and disilylated Si(CH3)3 and Si(CH3)2-t-Bu derivatives -: A synthetic, crystallographic, and dynamic NMR investigation

The bulky primary phosphane 2,6-diisopropylphenylphosphane, DipPH(2), has been prepared from 1-bromo-2,6-diisopropylbenzene via the reaction of the Grignard reagent with PCl3. The resulting mixed phosphonous dihalides DipP(Cl,Br)(2) are reduced with LiAlH4 to yield the title compound in reasonable yield and on a synthetically useful scale. DipPH(2) is also used to prepare the monosilylated derivatives DipPHSi(CH3)(3) and DipPH{Si(CH3)(2)-t-Bu} as well as the disilylated compounds DipP{Si(CH3)(3)}(2) and DipP{Si(CH3)(2)-t-Bu}(2). All products have been fully characterized by IR, mass, and NMR spectroscopy. The crystal structure of DipPH{Si(CH3)(2)-t-Bu} was determined from single-crystal diffraction data: C18H33PSi, P2(1)/c, Z = 4, a = 8.5768(10), b = 28.104(3), c = 8.1102(4) Angstrom, beta = 93.341(3)degrees (R = 0.0518). Changes in the NMR spectrum of DipPH{Si(CH3)(2)-t-Bu} were observed over the temperature range 178-380 K. Barrier heights were determined from the peak separation at low temperature and the coalescence points: DeltaG(double dagger) approximately equal to 40 kJ mol(-1) for C(aryl)-P bond rotation and approximately equal to 72 kJ mol(-1) for pyramidal inversion at phosphorus.