Production of mixed halogenated congeners of the natural product heptachloro-1'-methyl-1,2'-bipyrrole (Q1) by photolysis in the presence of bromine.

Halogenated 1'-methyl-1,2'-bipyrroles (MBPs) are a class of marine halogenated natural products that have been detected in biota from all over the world. However, structures and standards of many mixed chlorinated/brominated MBPs are not available. For this reason, the known 2,3,3',4,4',5,5'-heptachloro-1'-methyl-1,2'-bipyrrole (Q1 or MBP-79) was UV-irradiated in the presence of bromine with the goal of inducing a chlorine-->bromine exchange. A few drops of bromine were added to a solution of Q1 and 10 mL of either CH2Br2, CH2Cl2, or CHCl3. The experiments were performed both at room temperature and elevated temperature for 30 min. At least four out of five possible bromohexachloro-1'methyl-1,2'-bipyrroles (BrCl6-MBPs), at least seven out of 13 possible Br(2)Cl(5)-MBPs, as well as traces of Br(3)Cl(4)-MBPs and Br(4)Cl(3)-MBPs were obtained in this way. Selective fragment ions in the GC/ECNI-MS spectra as well as electrophilic bromination of hexachloro-MBP solutions were used to verify the structures of the BrCl6-MBP isomers. The BrCl6-MBPs eluted from DB-5-like columns in the order of 4'-bromo-2,3,3',4,5,5'-hexachloro-1'-methyl-1,2'-bipyrrole (Br-MBP-76), which co-eluted with 3'-bromo-2,3,4,4',5,5'-hexachloro-1'-methyl-1,2'-bipyrrole (Br-MBP-78), followed by 2-bromo-3,3',4,4',5,5'-hexachloro-1'-methyl-1,2'-bipyrrole (Br-MBP-75), 3-bromo-2,3',4,4',5,5'-hexachloro-1'-methyl-1,2'-bipyrrole (Br-MBP-77), and 5'-bromo-2,3,3',4,4',5-hexachloro-1'-methyl-1,2'-bipyrrole (Br-MBP-74). These BrCl6-MBPs were also detected in a sample of cetacean blubber from Australia, but the abundance pattern was different. While Br-MBP-76/Br-MBP-78 dominated in the cetacean, irradiation of Q1 (MBP-79) in the presence of bromine led to high proportions of Br-MBP-75. The suitability of the UV-induced Cl-->Br exchange was confirmed by the Br-assisted UV-irradiation of pentachloroanisole (PCA). This experiment produced at least two bromotetrachloro- and three dibromotrichloroanisoles, the last eluting in each case being the most relevant. Thus, this method is most likely generally suited for the production of mixed-halogenated aromatic organohalogen compounds which are not readily obtainable by synthesis.