Pyridine—continued halogenation

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The preparation of fluorinated pyridine derivatives continues to be of considerable importance due to the effect that the fluorine atom can have on the physical, chemical, and biological properties of the heterocycle. Despite this, there are few reports on the direct electrophilic fluorination of pyridines. The treatment of various quinoline derivatives with elemental fluorine in acidic reaction media afforded mono- and difluorinated products where the halogenation occurred on the benzene ring of the heterocycles <2004JFC661>. 

The electrophilic aromatic substitution of the pyridine ring continues to be explored as a way to prepare substituted pyridines. Bw-( 5y w-collidine)-iodine(I) and bromine (I) are effective iodinating or brominating agents, respectively, of pyridinols, although the di- or tri-halogenated products are obtained <97TL2467>. lodination of 3-pyridinol produces 5-hydroxy-... 

There are a variety of well-studied approaches to halogenated pyridines however, new approaches that incorporate regiocontrol and/or provide access to the fluoropyridnes continue to be of value. In a recent example, pyridine iV-oxides can be transformed into chlorinated pyridines in... 

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