4-amino-5-chloro-1,3-benzodioxole 858 mg (5.00 mmol) was dissolved in toluene 20mL and 4M anh. HCl solution in dioxane 1.5mL (6 mmol) was added dropwise. The formed slurry was stirred vigorously for 10 min. Phosgene 20% solution in toluene 15 mL (Fluka, approx 1.9M solution; 28.5 mmol) was added to the mix, the flask was equipped with a Drierite-filled gas outlet tube and the mixture was stirred on oil bath at 70C for 45 min and then at 90C for additional 45 min. (The hydrochloride slurry gradually dissolved with gas evolution). The reaction mixture was cooled to RT, filtered (the fritted funnel was washed with toluene) and the filtrates were evaporated to dryness. The obtained solid residue was dissolved in cyclohexane 10mL with gentle heating, the solution was diluted with hexane 10 mL and allowed to crystallize at RT overnight. The first crop (large needles, 388mg) was obtained by decanting the supernatants and rinsing the precipitate briefly with pentane, then drying on highvac. Cooling the supernatants to -15C in a freezer overnight, a second pure fraction (465mg) crystallized. Combined yield was 853mg (86%) of coarse light tan needles.
1H(CDCl3, 400MHz, ref. TMS): 6.867(d, 8.4Hz, 1H), 6.605(d, 8.4Hz, 1H), 6.047(s, 2H); 13C(CDCl3, 100MHz, ref TMS): 147.27, 142.41, 128.48(br s), 122.61, 121.92, 115.28(br s), 106.06, 102.63
4-amino-5-chloro-1,3-benzodioxole preparation was posted in Org Prep Daily on Sept 24 (2006)
Using the same procedure, 5-chloro-2,1,3-benzothiadiazol-4-yl isocyanate was prepared from 4-amino-5-chloro-2,1,3-benzothiadiazole (TCI-US) in 59% yield as pale yellow cotton-like fibrous needles (recryst from cyclohexane, to RT).
Note: There was a significant phosgene gas evolution from the heated reaction mixture and phosgene also condensed in the rotavap receiving flask. Using an efficient fume hood and cleaning the rotavap after the evaporation is a necessity: Unlike zombies, the sweet-smelling phosgene gas eats lungs without causing an immediate discomfort.