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Supporting Information

Silicon Carbide Passive Heating Elements in

Microwave-Assisted Organic Synthesis

Jennifer M. Kremsner and C. Oliver Kappe*

Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstrasse 28, A-8010 Graz, Austria.

oliver.kappe@uni-graz.at

Table of Contents

General Experimental Details and Microwave Equipment S2

Heating Profiles for Solvents, Solvent Mixtures, and Reactions S3-S17

Images of Heating Elements S12-S13

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General Experimental Details: 1H NMR and 13C NMR spectra were recorded on a 360 MHz

instrument at 360 and at 90 MHz respectively. Chemical shifts (δ) are expressed in ppm downfield from

TMS as internal standard. The letters s, d, t, q and m are used to indicate singlet, doublet, triplet,

quadruplet and multiplet. FTIR spectra were recorded using KBr pellets. Low resolution mass spectra

were obtained on a LC/MS instrument using atmospheric pressure chemical ionization (APCI) in

positive or negative mode. Analytical HPLC analysis was carried out on a C18 reversed-phase (RP)

analytical column (119 × 3 mm, particle size 5 mm) or a reversed-phase column (150 × 4.6 mm, particle

size 5 mm) at 25 °C using a mobile phase A (water/acetonitrile 90:10 (v/v) + 0.1 % TFA) and B (MeCN

+ 0.1 % TFA) at a flow rate of 0.5-1.0 mL/min. The following gradient was applied: linear increase

from solution 30% B to 100 % B in 7 min, hold at 100% solution B for 2 min. Dry-flash

chromatography was performed on silica gel 60 H (< 45 nm particle size). Melting points were obtained

on a standard melting point apparatus in open capillary tubes. TLC analyses were performed on pre-

coated (silica gel 60 HF254) plates. All anhydrous solvents (stored over molecular sieves), and chemicals

were obtained from standard commercial vendors and were used without any further purification.

Microwave Irradiation Experiments: Heating curves of solvents were recorded using a single-mode

Discover System from CEM Corporation using either custom-made high purity quartz or standard Pyrex

vessels (capacity 10 mL) as appropriate, sealed with a Teflon septum cap. The temperature profiles of

the solvents (power control) were monitored either using a calibrated infrared temperature control

mounted underneath the reaction vessel, or a fiber-optic probe inserted into the reaction vessel protected

by a sapphire immersion well. Chemical transformations in the presence of heating elements (SiC, 10 x

18 mm cylinder) described in this article were either run in a CEM Discover, or in Biotage Emrys

Synthesizer or Initiator Eight EXP instruments in the standard configuration (temperature control,

remote IR temperature sensor, sealed Pyrex vessels). Microwave reactions in multimode instruments

were performed in a SYNTHOS 3000 instrument from Anton Paar GmbH using 100 mL Teflon sealed

reaction vessels and a 16 position rotor. Graphical representations of SiC heating elements (mp ca. 2700

°C, density 3.10 g/cm3, specific heat capacity 650 J/kgK, thermal conductivity (100 °C) 120 W/mK,

thermal expansion (20-1000 °C) 4.1 x 10-8 K-1) and reaction vessels are shown in Figure S10.

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PyrexQuartz

Figure S1. Microwave heating profiles for CCl4 in Pyrex and quartz reaction vessels at constant 150 W

magnetron output power for 5 min (CEM Discover). Single mode irradiation, 4 mL sample volume,

fiber-optic temperature measurement, sealed 10 mL reaction vessel, magnetic stirring. The heating of

the microwave transparent solvent to 109 °C in the case of the Pyrex vessel is the result of indirect

heating by conduction and convection phenomena via the hot surface of the self-absorbing Pyrex glass.

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fiber opticIR

Figure S2. Microwave heating profiles for a standard 10 mL empty Pyrex vessel at constant 150 W

magnetron output power for 10 min (CEM Discover) using fiber-optic or IR temperature measurement.

The self-heating of the conventional Pyrex reaction vessels is clearly demonstrated by comparison of

heating profiles using IR and fiber-optic temperature measurements. Since the IR sensor directly

monitors the surface temperature of the glass (rather than of its contents), the observed effects are more

pronounced using this type of monitoring method. In the experiment described herein, microwave

transparent air was used as medium.

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PyrexQuartz

Figure S3. Microwave heating profiles for ethanol in quartz and Pyrex reaction vessels at constant 150

W magnetron output power (CEM Discover). Single mode irradiation, 4 mL sample volume, IR

temperature measurement, sealed 10 mL reaction vessel, magnetic stirring. There is virtually no

difference between the heating profiles using the microwave transparent quartz and the to some extend

absorbing Pyrex vessel, since ethanol is a strongly microwave absorbing solvent.

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0% EtOH0.9% EtOH2.1% EtOH4.5% EtOH9.0% EtOH16% EtOHpure EtOH

Figure S4. Microwave heating profiles for samples of 2 mL (3.20 g) of CCl4 doped with varying

amounts of ethanol in a 10 mL quartz reaction vessel at constant 150 W magnetron output power. Single

mode microwave irradiation, IR temperature sensor, magnetic stirring (CEM Discover). Profiles were

recorded for: pure CCl4, 30 mg (0.9 %, w/w) ethanol, 70 mg (2.1 %, w/w) ethanol, 150 mg (4.5 %, w/w)

ethanol, 310 mg (9 %, w/w) ethanol, 610 mg (16 %, w/w) ethanol, and pure ethanol . In the case of the

16 % ethanol/CCl4 mixture, the experiment was terminated after ca 150 s at ca 140 °C when the

pressure limit of the instrument (20 bar) was reached (bp CCl4 = 76 °C). Even small amounts of

strongly microwave absorbing ethanol are capable of changing the overall absorption characteristics of

the solvent mixture to a large extent allowing significant heating by microwave dielectric effects.

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0.0 M0.001875 M0.00375 M0.0075 M0.015 M0.03 M0.06 M0.09 M

NaCl Concentration:

Figure S5. Microwave heating profiles at 150 W constant magnetron output power for aqueous NaCl

solutions of varying salt concentrations. Single mode irradiation, 5 mL sample volume, sealed 10 mL

quartz reaction vessel, IR temperature measurement, magnetic stirring (CEM Discover). Pure water is

rather difficult to heat by microwave irradiation, in particular at higher temperatures as the loss tangent

(tan δ = 0.123 at 25 °C) decreases significantly with increasing temperature.

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0.0 M0.03 M0.015 M0.0075 M

TBAB Concentration:

Figure S6. Microwave heating profiles for aqueous tetrabutylammonium bromide (TBAB) solutions at

constant 150 W magnetron output power. Single mode irradiation, 5 mL sample volume, sealed 10 mL

quartz reaction vessel, IR temperature measurement, magnetic stirring (CEM Discover).

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2 W 5 W 25 W 50 W 75 W 100 W

Figure S7. Microwave heating profiles for the ionic liquid N-butyl-N’-methylimidazolium

hexafluorophosphate (bmimPF6). Single mode microwave irradiation, 5 ml sample volume, sealed 10

ml quartz reaction vessel, fiber-optic temperature probe, magnetic stirring (CEM Discover). Profiles

were recorded at 2 W, 5 W, 25 W, 50 W, 75 W and 100 W magnetron output power. The very strong

coupling characteristics are demonstrated by the fact that only 5 W of microwave energy suffices to heat

the ionic liquid to 160 °C within less than 5 minutes. For experiments reaching temperatures above 200

°C the cooling profiles using compressed air cooling are also shown.

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Ionic Liquidhexane

Figure S8. Differential heating of a biphasic mixture of N-butyl-N’-methylimidazolium

hexafluorophosphate (bmimPF6, 2.5 mL, bottom layer IL) and hexane (2.5 mL, top layer, hex) as

monitored by fiber-optic probe temperature measurements at 2 W constant power. Single mode

microwave irradiation, 4 mL total sample volume, sealed 10 mL quartz reaction vessel, fiber-optic

temperature probe measurements at different positions in the vessel (see insert), no stirring (CEM

Discover). The heating curves are strongly dependent on the position of the temperature probe,

reflecting the different temperatures in the two immiscible phases (differential heating). There is a more

than 40 °C temperature gradient after 100 s between the strongly absorbing ionic liquid phase and the

poorly absorbing hexane layer. Temperature measurements using the standard remote IR sensor

technique, either from the bottom (CEM Discover platform) or from the side (Biotage Emrys and

Initiator platforms) will lead to erratic results and will make it difficult to reproduce experiments using

different equipment.

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without PHECarboflonWeflon

Figure S9. Microwave heating profiles (150 W constant magnetron output power) for CCl4 in the

presence of the fluoropolymer-derived heating elements WeflonTM (5 x 20 mm cylinder, ca. 0.8 g,

source: Milestone) and Carboflon® (6 x 13 mm cylinder, ca. 0.9 g, source: CEM). Single mode

microwave irradiation, 4 mL sample volume, sealed 10 mL quartz reaction vessel, IR temperature

sensor, magnetic stirring (CEM Discover).

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A

B C D

E F

Figure S10 (see next page for caption)

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Figure S10. Compatibility of SiC passive heating elements (PHEs) with common reaction vessels

used in microwave synthesis. (a) PHE cylinders: 10 x 18 mm (4.35 g) and 10 x 8 mm (1.94 g). (b) PHE

cylinder (10 x 18 mm) inside a standard 10 mL microwave vial used in CEM Discover or Biotage

Emrys/Initiator platforms, solvent volume 2 mL. Note that the design allows for magnetic stirring of the

reaction mixture during the reaction with a standard stir bar placed below the PHE cylinder. (c) PHE

cylinder (10 x 8 mm) inside a standard 5 mL conical microwave vial used in Biotage Emrys/Initiator

platforms, solvent volume 2 mL. (d) PHE cylinder (10 x 18 mm) inside a standard 20 mL microwave

vial used in Biotage Initiator EXP platforms, solvent volume 15 mL. (e) PHE inside a 100 mL Teflon

microwave vessel for use in an Anton Paar SYNTHOS 3000 multimode reactor. In the presence of a 2

cm magnetic stir bar, efficient stirring is possible. For clarity the vessel wall has been cut. (f) IR thermal

image of a 10 x 8 mm SiC heating element exposed to microwave irradiation inside a multimode

microwave cavity. The recorded temperature after 2 min of irradiation at 400 W was 247 °C (shown) or

262 °C after 1 min at 600 W power.

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CCl4hexanetoluenedioxaneTHF

Figure S11. Microwave heating profiles for non-polar solvents (2 mL) in the presence of a SiC heating

element. Single mode microwave irradiation, 150 W constant magnetron output power, sealed 10 mL

Pyrex reaction vessel, IR temperature sensor, magnetic stirring (CEM Discover). The following solvents

are displayed: CCl4, toluene, THF, dioxane and hexane. The maximum reached temperatures reflect a

pressure of ca 20 bar at which the experiments were aborted and cooling by compressed air commenced.

In the absence of the heating elements there is very little heating for most solvents (see Table 1 in the

manuscript).

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[°C] dioxane

chloroformtoluenexyleneMeCNwaterCCl4

Figure S12. Microwave heating profiles for 100 mL samples of non-polar solvents (10 mL solvent,

SiC heating element, and 2 cm stir bar per vessel; 10 vessels in a 16-vessel rotor). Multimode

microwave irradiation, 1000 W constant power for 10 min, sealed 100 mL Teflon reaction vessel, gas

balloon temperature sensor, magnetic stirring (Anton Paar SYNTHOS 3000). The following solvents

are displayed: H2O, toluene, CCl4, CHCl3, dioxane, xylene and acetonitrile.

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Figure S13. Heating profiles for the Diels-Alder cycloaddition of 2,3-dimethylbutadiene and

acrylonitrile in toluene, carried out on a 100 mmol scale (total solvent volume ca 120 mL). The

experiment was carried out using a multimode microwave instrument (Anton Paar SYNTHOS 3000) in

Teflon reaction vessels housed in a 16 vessel rotor. Heating ramp to 240 °C (7 min), temperature control

using the feedback from the reference vessel temperature measurement (constant 240 °C, 7-27 min), and

forced air cooling (27-50 min). The reaction was performed in 4 Teflon vessels each containing ca 30

mL of reaction volume (see Experimental Section for details). Shown is the temperature measurement in

one reference vessel via internal gas balloon thermometer (T), the surface temperature monitoring of the

4 individual vessel by IR thermography (IR 1-4), and the magnetron power (P, 0-1400 W). Note that the

power of the magnetron suffices to follow a heating ramp from 20 to 240 °C in 7 min. After the

maximum temperature has been reached ca 320 W power is used to keep the reaction temperature at 240

°C. Also note that the individual IR vessel surface temperatures deviate by less than 10 °C. For clarity,

the pressure graph is not shown.

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T QuartzT Pyrex + SiCT PyrexP QuartzP Pyrex + SiCP Pyrex

Figure S14. Microwave heating profiles for acetonitrile (2 mL) in temperature control mode. Single

mode microwave irradiation, 120 °C set maximum temperature, sealed 10 ml Pyrex or quartz reaction

vessels, IR temperature sensor, magnetic stirring (CEM Discover). Displayed are temperature and

power profiles for: a) heating in a quartz vessel without SiC element; b) heating in a Pyrex vessel

without SiC element; c) heating in a Pyrex vessel with SiC element. It is clearly demonstrated that the

heating of acetonitrile at 120 °C in a quartz reaction vessel requires the highest power (200 W). Due to

the self-absorbance of the Pyrex vessel the required energy is significantly reduced (140 W) when the

experiment is conducted in this vessel type. In the presence of the SiC heating element less than 50 W

of energy are required to maintain a solvent temperature of 120 °C.