A. 1,1-Dibromo-2,2-bis(chloromethyl)cyclopropane (1). Into a 1-L, three-necked, round-bottomed flask, equipped with an efficient mechanical stirrer, a thermometer, and a condenser equipped with a potassium hydroxide drying tube, are placed 54.1 g (0.403 mol) of 3-chloro-2-(chloromethyl)propene (Note 1), 212 g (0.805 mol) of bromoform (Note 2), 1.70-2.00 g (14.4-16.9 mmol) of pinacol (Note 3), and 1.45 g (3.94 mmol) of dibenzo-18-crown-6 (Note 4). With very vigorous stirring (Note 5), 312 g of an aqueous 50% sodium hydroxide solution that has been cooled to 15°C is added in one portion. The reaction mixture turns orange, then brown, then black within 5 min, and the temperature of the reaction mixture begins to rise. Within 20 min, the internal reaction temperature is 49-50°C at which point the reaction flask is cooled with a room-temperature water bath, and the reaction temperature decreases to ca. 20°C. After 1 hr, the bath is removed and the reaction mixture is heated to 40°C (internal temperature) with an oil bath. The vigorously stirred mixture is maintained at this temperature for 4 days (Note 6). The reaction mixture is cooled to room temperature, diluted with 500 mL of water, and filtered through a pad of Celite on a glass-fritted funnel (pore size C), using a water aspirator (Note 7). Up to an additional 1-L of water is used to rinse the thick black reaction mixture from the flask. The resulting golden-yellow filtrate is discarded (Note 8). The black, solid residue in the frit, and any material remaining in the reaction flask are transferred to a 1-L beaker with pentane, acetone, and a glass rod, and the solution is vigorously stirred with an additional 500 mL of a 1:1 (v/v) solution of acetone and pentane for 30 min. This mixture is filtered through a glass-fritted funnel using a minimal layer of Celite. The Celite pad is washed thoroughly with 1:1 pentane/acetone solution, and the resulting brown filtrate is dried over magnesium sulfate . Concentration using a rotary evaporator, followed by distillation under reduced pressure (bp 75-85°C/0.4 mm), and low temperature (ca. −20°C) recrystallization from pentane (ca. 1 mL/g of product), provide 70-95 g (60-80% yield) of the product as small, white crystals (mp 47.5-50°C) (Note 9) and (Note 10).

B. [1.1.1]Propellane (2). A 500-mL, three-necked, round-bottomed flask that has been flame dried under reduced pressure and purged with argon, is equipped with a vacuum adapter equipped with an argon balloon, an efficient mechanical stirrer, and a pressure-equalizing, 150-mL addition funnel equipped with a rubber septum. The flask is charged with a 25-g (0.085 mol) portion of material obtained from part A, and 25 mL of pentane (Note 11). A diethyl ether solution of methyllithium, 132 mL of a 1.4 M (0.185 mol) solution (Note 12), is transferred to the addition funnel via cannula under a flow of argon. The reaction flask is cooled to −78°C and the methyllithium is added over 15 min with vigorous stirring. The reaction mixture is maintained at −78°C for 10 - 15 min; then the −78°C cooling bath is replaced with an ice-water bath (0°C), and the addition funnel is replaced with a rubber septum equipped with an argon balloon. Stirring is continued for an additional hour; then the volatile materials are transferred under reduced pressure to a flame-dried flask that is cooled to −196°C (Note 13). Based on the recovery of the thiophenol adduct, the yield of [1.1.1]propellane is between 75 and 88%.

Determination of the yield of propellane. Wiberg and Waddell have shown that propellane reacts spontaneously in normal room light with thiophenol to yield bicyclo[1.1.1]pentyl phenyl sulfide in 98% yield.² A slight modification of their method provides a good estimate of the yield of propellane. Thus, a 3.0-mL portion of the propellane solution is transferred via a gas-tight syringe to a tared, flame-dried, argon-purged, 10-mL, round-bottomed flask equipped with a stirring bar and maintained under a static atmosphere of argon. Thiophenol is added in slight excess via syringe and the mixture is stirred under room light for 15 min. Concentration of the solution provides a mixture of thiophenol and bicyclo[1.1.1]pentyl phenyl sulfide. The ratio of compounds may be determined by ¹H NMR or GLC, and the yield of propellane calculated (Note 14). Alternatively, the mixture may be diluted with pentane, washed with 1 M sodium hydroxide, dried with magnesium sulfate, and concentrated to provide bicyclo[1.1.1]pentyl phenyl sulfide from which the yield of propellane is calculated. The reaction of thiophenol with propellane is assumed to be quantitative; thus yields for bicyclo[1.1.1]pentyl sulfide are 75 to 88% based on 1.

Wiberg and Waddell also noted that propellane reacts with iodine in 88% yield.² Alber and Szeimies report a somewhat lower yield (61%) when an ethereal solution of propellane is treated with a solution of iodine in ether/pentane while being irradiated.³ Thus, titration of a portion of the resulting propellane solution with iodine provides an estimate of the minimum yield. A 3.0-mL portion of the propellane solution is transferred as noted above. Small pieces of iodine are added with stirring under room light over 10 min until just before the dark color persists. Concentration of the solution provides 1,3-diiodobicyclo[1.1.1]pentane from which the minimal concentration of the propellane solution and minimal yield of propellane may be estimated if iodine is not added in excess. Alternatively, a solution of iodine in diethyl ether/pentane may be added instead of neat iodine.