An Investigation of Wall Effects in Microwave-Assisted Ring-Closing Metathesis and Cyclotrimerization Reactions

Abstract

Challenging Ru-catalyzed ring-closing metathesis transformations leading to eight-membered-ring systems and Ni- or Co-catalyzed [2+2+2] cyclotrimerizations were evaluated at elevated temperatures applying microwave dielectric heating or conventional thermal heating in order to investigate the role of wall effects. All reactions were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes for both types of heating modes. For ring-closing metathesis best results were achieved using an open vessel-gas sparging protocol in 1,2-dichloroethane at reflux temperature (83 degrees C), while cyclotrimerizations were performed under sealed vessel conditions in toluene between 80 and 160 degrees C. For all studied transformations the results achieved in a single-mode microwave reactor could be reproduced by conventional heating in an oil bath by carefully matching the temperature profiles as close as possible during the entire heating and cooling cycle. In contrast to previous literature reports, no evidence that direct in-core microwave heating can increase catalyst lifetime by minimization or elimination of wall effects was obtained. At the same time, no indication for the involvement of nonthermal microwave effects in these homogeneous transition metal-catalyzed transformations was seen.

Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

“…[37] Not unlike ionic liquids, the distinctly polar properties of TREAT-HF make it very suitable as a reagent (and/or solvent) in microwave-heated transformations. In fluorinechlorine exchange reactions involving (trichloromethylthio)-benzene at 250 8C (Scheme 6) [36] we did, however, notice a significant corrosion of standard Pyrex microwave reaction vials in the vapor space above the liquids surface. Although generally considered noncorrosive, [37] TREAT-HF can release hydrogen fluoride at elevated temperatures and will therefore attack borosilicate glass to some extent.…”

“…Performing an oil bath experiment with a similar temperature profile as in the microwave run also gave full conversion and a similar isolated product yield (75 %). [36] However, using the SiC vial under identical conditions consistently provided conversion in the range of only 45-60 %. As in the case of the Leuckart reductive amination described above, we ascribe this phenomenon to an enhanced thermal decomposition of the Co catalyst on the hot SiC vessel surface of higher porosity.…”

“…[35] A more recent investigation using accurate internal reaction temperature control and matching heating and cooling profiles for both types of heating modes as close as possible has demonstrated that no microwave effects exist for this [2 + 2 + 2] cyclotrimerization reactions. [36] The Co-catalyzed [2 + 2 + 2] cyclotrimerization required 160 8C to lead to complete conversion within 20 min under microwave conditions and furnished a 78 % isolated product yield. Performing an oil bath experiment with a similar temperature profile as in the microwave run also gave full conversion and a similar isolated product yield (75 %).…”

“…[24] Based on these facts we have employed SiC vials for conducting microwave-assisted, aliphatic fluorine-chlorine exchange reactions using triethylamine trihydrofluoride (TREAT-HF, Et 3 N·3 HF) as a reagent. [36] TREAT-HF is a commercially available, colorless, hygroscopic liquid with a shelf life of at least one year and has been reported not to corrode borosilicate glassware at ambient conditions. [37] Not unlike ionic liquids, the distinctly polar properties of TREAT-HF make it very suitable as a reagent (and/or solvent) in microwave-heated transformations.…”

Microwave-specific Effect on Enantioselective Reactions

“…RCM reactions have been extensively used for the syntheses of medium-sized rings and macro cycles, and the synthetic routes for total syntheses of natural products have completely changed after the introduction of RCM. Microwave irradiation has been widely applied to RCM since the first report in 2000 17) . The advantages of applying microwave irradiation to RCM are mainly shortening of the reaction time and suppression of the wall effect.…”