Halogenation coordinated nitrogen atoms

Halogen or nitrogen atoms of cisplatin are in the cis position. But, organotin compounds are not formed in planar four coordination as the cisplatin. However, the octahedral structure of six-coordination is able to form partly cis structure on halogen or nitrogen atoms similar to cisplatin. 

Each of these intermediates can be hthiated in the 2-position in good yield. The reactivity toward hthiation is due to the inductive effect of the nitrogen atom and coordination by oxygen from the N-substituent. A wide variety of electrophiles can then carry out substitution at the 2-position. Lithiation at other positions on the ring can be achieved by halogen—metal exchange 3-hthio and 5-hthioindoles have also been used as reactive intermediates. 

In these reaetions, the most reliable mechanism is considered to involve the initial metal-coordination at the nitrogen atom of the pyridine ring and the subsequent attack of an alkyl or aryl anion at the most probable cationic sites on the ring, namely, the 2- and/or 4-position of the ring. If a 2-halogen substituted pyridine is used, the nucleophilic anion attacks the 6-position. Thus, the addition is a more prefered reaction than the ipso-substitution as shown in reaction (29). The substitution of amide or phenyl-... 

As a first approximation, the reactions of pyridines with electrophiles can be compared with those of trimethylamine and benzene. Thus, pyridine reacts easily at the nitrogen atom with reagents such as proton acids, Lewis acids, metal ions, and reactive halides to form salts, coordination compounds, complexes, and quaternary salts, respectively. Under much more vigorous conditions it reacts at ring carbons to form C-substitution products in nitration, sulfonation, and halogenation reactions. 

The fundamental chemistry and compounds reported prior to 1984 are summarized in CCC (1987). The development and expansion of coordination chemistry provided many possibilities and opportunities for the selection of ligands. Many combinations of coordinated atoms have been reported including halogen, chalcogen, nitrogen, phosphorus, carbon, and hydrogen. Initially, classical halides and their derivatives are described, followed successively by less electronegative atoms. 

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