Benzene, halo-, halogen displacement

The chemical reactivity of the thiocyano groups in these chelate rings has not been investigated, but the halogen atoms in the 3-halo metal acetylacetonates have been found to be quite inert and their behavior is different from that of aryl halides, since treatment of the tris(3-bromo-2,4-pentanediono)chromium(III) chelate with magnesium or lithium in benzene or tetrahydrofuran resulted in no reaction. Attempted nucleophilic displacement of the bromine atoms in this chelate by azide, acetate, nitrate, and iodide ions in hot dimethylformamide also failed. In most of these... 

Such substitutions follow the same mechanistic route as the displacement of halide from 2- and 4-halo-nitro-benzenes, i.e. the nucleophile first adds and then the halide departs. By analogy with the benzenoid situation, the addition is facilitated by (i) the electron-deficiency at a- and 7-carbons, further increased by the halogen substituent, and (ii) the ability of the heteroatom to accommodate negative charge in the intermediate thus produced. Once again, a comparison of the three possible intermediates makes it immediately plain that this latter is not available for attack at a /3-position, and thus (3 nucleophilic displacements are very much slower - for practical purposes they do not occur. 

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