THERMOLYSIS OF TRIFLUOROMETHYLSILANES - NOVEL FLUOROMETHYLSILANES BY INSERTION OF CF2 AND CHF INTO SI-H BONDS

The thermal decomposition in the gas phase of CF3SiH3 and (CF3)2SiH2 begins at ∼200 and ∼100°C, respectively. It is catalyzed by KF, and involves as initial step a clean CF2 elimination with an α-fluorine shift. Reactive species such as HBr trap CF2 quantitatively (to give in this case CHF2Br), while addition to the less reactive cyclohexene (to give 7,7-difluorobicycloheptane) is accompanied by secondary reactions. These dominate in the absence of an efficient CF2 trapping agent, and spectroscopic product analyses reveal that they mainly arise from insertion of the carbenes CHnF2-n into SiH bonds followed by CHn+1F1-n elimination (n = 0, 1). This sequence corresponds to H/F exchange at the Si atom. Insertion of CF2 (generated by thermolysis of CF3SiF3 below 100°C) into an SiH bond of H3SiF to give CHF2SiH2F in good yield is the first example of such a reaction and demonstrates its usefulness for the selective synthesis of CHF2Si compounds. In addition, some dismutation of SiHnF3-n moieties accompanies the carbene elimination/insertion reactions and the resulting novel fluoromethylsilanes were characterized by 1H and 19F NMR and (in part) IR spectroscopy. The synthesis of (CF3)2SiHBr by cleavage of (CF3)2Si(H)N(i-Pr)2 with BBr3 and its conversion with LiAlH4 to (CF3)2SiH2 are reported. The mechanism of the CF2/SiH insertion reaction is discussed. © 1990.