Structural Characterization Of Hydro-, Chloro- And Fluoroorganylsilanes With Substituents Of Varying Electron Withdrawing Character

We report on the molecular structure of trimethylsilanes Si(CH3)(3)X in which the substituents X, namely (Z)-pentafluoropropen-1-yl, trifluoropropyn-1-yl, pentafluoroethyl, trifluorovinyl, vinyl, propyn-1-yl, di- and trichloromethyl, display electron withdrawing effects of varying strength. The lengths of the bonds between the silicon and the carbon atoms of X correlate with the hybridization of the respective orbitals and the steric demand of X rather than with the electron withdrawing capability. In case of chlorinated substituents dispersion effects seem to shorten the Si-C bond. Furthermore, a route for generating trifluoropropyn-1-yllithium from the cryogen 2,3,3,3-tetrafluoropropene (HFO-1234yf) and n-butyllithium is described. Tetrafluoropropen-1-yllithium is slowly formed at -80 degrees C but even at this temperature spontaneous elimination of LiF occurs. Deprotonation of the formed 3,3,3-trifluoropropyne requires temperatures of above -60 degrees C leading to trifluoropropyn-1-yllithium which appears as relatively stable at room temperature.