Alkyl and dialkylammonium tetrafluoroborate catalyzed cis–trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane

Alkyl and dialkylammonium tetrafluoroborate promoted cis–trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane () in DMSO-d6 were studied. The isomerization equilibrium constant K are within the range of 3.74–3.30 from 22 to 47 °C. Thermodynamic parameters of ΔH° and ΔS° for the isomerization were −0.95 kcal/mol and −0.59 cal/mol-K respectively. The isomerization rate is first order in [] and second order in [RnNH4−nBF4]. Both components of RnNH4−n+ and BF4− are essential for the catalytic cis–trans isomerization. The catalytic strength follows the decreasing order of +H3N(CH2)6NH3+>n-C8H17NH3+>n-C16H33NH3+>Me3CNH3+>PhCH2NH3+>Et2NH2+≫Ph2CHNH3+, Et3NH+. Inversion region was observed in the plot of ln(kf/T) versus (1/T) with the ceiling located at around 38 °C. The positive activation enthalpy of 9 kcal/mol was estimated at 22–32 °C. The activation enthalpy turns to be slightly negative at T>38 °C.