[41851-49-4] · C5H5Br · 5-Bromo-1,3-cyclopentadiene · (MW 145.00)
Physical Data: reactive intermediate typically stored in CCl4 solution under nitrogen at -78 °C.
Form Supplied in: not commercially available.
Preparative Method: the compound may be prepared1a by addition of N-Bromosuccinimide to a solution of Thallium(I) Cyclopentadienide in dry CCl4 at 0 °C.
Bromocyclopentadiene reacts with 4-Phenyl-1,2,4-triazoline-3,5-dione (eq 1)1a and Dimethyl Acetylenedicarboxylate1b to form the Diels-Alder adduct stereospecifically. The 5-iodo analog reacts similarly, while 5-chlorocyclopentadiene produces both chlorine epimers.1
5-Halocyclopentadienes are much more prone to dimerization than Cyclopentadiene itself (dimerizing about 103 times faster).1a However, the cycloaddition of halocyclopentadienes with electrophilic dienes is slower than with cyclopentadiene.1a
The presence of halogen on the cyclopentadiene ring slows the rate of 1,5-hydride shift and, although iodine rapidly migrates around the ring, bromine does not.2
Both potassium2 and thallium3 bromocyclopentadienyl derivatives have been reported.
Dallas K. Bates
Michigan Technological University, Houghton, MI, USA