Freeze-thaw degassing

Carry out the freeze-thaw-degassing procedure in the same time vacuum and purge the vessel with dry argon or nitrogen to ensure all oxygen is removed from the... 

Quantitative nitroxide-trapping experiments should be carried out under thoroughly de-oxygenated conditions since oxygen will act as a competitive radical trap for carbon-centred radicals. Consequently, several freeze-thaw degassing cycles using pressures <10 3 mm Hg are usually required. 

Materials. (Bicyclo[2.2.1]hepta-2,5-diene) rhodium(I) chloride dimer and silver hexafluorophosphate were purchased from Strem Chemical the rhodium complex was recrystallized from acetone prior to use. All silanes (Aldrich) were distilled under a nitrogen atmosphere, freeze-thaw degassed and stored under a nitrogen atmosphere. 4-cyclopentene-l,3-dione (CPDK) from Aldrich was recrystallized from diethyl ether and stored in the dark under a nitrogen atmosphere at -40°C. All solvents were distilled under nitrogen from appropriate drying agents immediately prior to use. 

Dried, freeze-thaw degassed solvent (e.g. acetonitrile, 5 mL) is then vacuum transferred into the evacuated, cooled (-196°C) reaction vessel, which is then closed and allowed to warm to 25°. Our experience has been that gas evolution is visually evident at this temperature. While gas evolution is often complete within one hour, we have experienced cases where up to 24h is required. The most objective procedure would be to let the oxidation proceed with the reaction vessel open to a manometer, and use the cessation of pressure changes to signal completion of the reaction. 

A Schlenk tube equipped with a magnetic stirring bar. 100.1 mg (1.0 mmol) of 2.4-pentanedione and 196.2 mg (1.0 mmol) of the vinylcyelopropane is capped with a rubber septum and cooled to ca. — 190 C. The septum is removed. 0.023 g (2 mol%) of [Pd(PPh3)4] is added quickly against a moderate flow of Ar, and the septum is immediately replaced. Any catalyst that is adhered to the inner walls of the Schlenk tube is washed and flushed into the bottom of the vessel when 2 mL of THF are added. The mixture is freeze/thaw degassed three times and then refluxed for 7 h. The solvent is removed in vacuo, and the residue is purified by flash chromatography (25% EtOAc in hexane, Rf 0.2) to give a yellow oil yield 263.7mg (89%). 

Preparation of polymer 3-CI2. A 100-mL Schlenk flask containing a magnetic stir bar was charged with I-CI2 (3.0 g, 0.0118 mol) followed by 2 drops of Pt-DVTMDSO and 1,1,3,3-tetramethyldisiloxane (1.59 g, 0.0118 mol). The mixture was freeze-thaw-degassed two times. The reaction mixture was then vigorously stirred while heated in an oil bath at 45 C. The reaction progress was measured by... 

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