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

Alkyl and dialkylammonium tetrafluoroborate promoted cis-trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane (1) in DMSO-d(6) were studied. The isomerization equilibrium constant K are within the range of 3.74-3.30 from 22 to 47 degreesC. Thermodynamic parameters of DeltaHdegrees and DeltaSdegrees for the isomerization were -0.95 kcal/mol and -0.59 cal/mol-K respectively. The isomerization rate is first order in [cis-1] 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)(6)NH3+ > n-C8H17NH3+ > n-C16H33NH3+ > Me3CNH3+ > PhCH2NH3+ > Et2NH2+ much greater than Ph2CHNH3+, Et3NH+. Inversion region was observed in the plot of ln(k(f)/T) versus (1/T) with the ceiling located at around 38 degreesC. The positive activation enthalpy of 9 kcal/mol was estimated at 22-32 degreesC. The activation enthalpy turns to be slightly negative at T > 38 degreesC. (C) 2004 Elsevier Ltd. All rights reserved.