title: On the bromination of the dihydroazulene/vinylheptafulvene photo-/thermoswitch.
abstract: BACKGROUND
  The dihydroazulene (DHA)/vinylheptafulvene (VHF) system (with two cyano groups at C1) functions as a photo-/thermoswitch. Direct ionic bromination of DHA has previously furnished a regioselective route to a 7,8-dibromide, which by elimination was converted to a 7-bromo-substituted DHA. This compound has served as a central building block for functionalization of the DHA by palladium-catalyzed cross-coupling reactions. The current work explores another bromination protocol for achieving the isomeric 3-bromo-DHA and also explores the outcome of additional bromination of this compound as well as of the known 7-bromo-DHA.
  
  
  RESULTS
  Radical bromination on two different VHFs by using N-bromosuccinimide/benzoyl peroxide and light, followed by a ring-closure reaction generated the corresponding 3-bromo-DHAs, as confirmed in one case by X-ray crystallography. According to a (1)H NMR spectroscopic study, the ring closure of the brominated VHF seemed to occur readily under the reaction conditions. A subsequent bromination-elimination protocol provided a 3,7-dibromo-DHA. In contrast, treating the known 7-bromo-DHA with bromine generated a very labile species that was converted to a new 3,7-dibromoazulene, i.e., the fully unsaturated species. Azulenes were also found to form from brominated compounds when left standing for a long time in the solid state. Kinetics measurements reveal that the 3-bromo substituent enhances the rate of the thermal conversion of the VHF to DHA, which is opposite to the effect exerted by a bromo substituent in the seven-membered ring.
  
  
  CONCLUSIONS
  Two general procedures for functionalizing the DHA core with a bromo substituent (at positions 3 and 7, respectively) are now available with the DHA as starting material.
authors.affiliation: Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark ; Sino-Danish Center for Education and Research (SDC), Niels Jensens Vej 2, DK-8000 Aarhus C, Denmark.
authors.forename: Virginia
authors.initials: V
authors.lastname: Mazzanti
authors.forename: Martina
authors.initials: M
authors.lastname: Cacciarini
authors.forename: Søren L
authors.initials: SL
authors.lastname: Broman
authors.forename: Christian R
authors.initials: CR
authors.lastname: Parker
authors.forename: Magnus
authors.initials: M
authors.lastname: Schau-Magnussen
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Bond
authors.forename: Mogens B
authors.initials: MB
authors.lastname: Nielsen