L-Bromo-3-chloro-5-iodobenzene

Polyhalogenation is a potential problem at several stages of the synthesis of l-bromo-3-chloro-5-iodobenzene. Describe what tactics are used to avoid polyhalogenation at each of these stages. 

Senn and Ziegler find the dissociative pathway to be the preferred one in their calculations in THF solution of the oxidative addition of chloro-, bromo- and iodobenzene to Pd(P-P) complexes, where P-P is a chelating diphosphine ligand of the type l,2-bis(dimethylphosphino)ethane or (P)-2,2 -bis(dimethylphosphino)-1,1 -biphenyl. The barriers from the prereaction complex are in fact quite low 31, 12 and 5 kJ/mol for the chloro, bromo and iodo species, respectively. For methyl-(lS,5R)-5-chlorocyclohex-3-ene-l-carboxylate in presence of neutral palladium, Espinet and Echavarren observed retention of configuration in apolar solvent (benzene) and inversion in polar solvent (acetonitrile) [13]. 

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