Pyrrole, reactions with—continued

Other factors which are known to lower the yield of 2,2 -bipyridine include dilution of the pyridine with a solvent (such as xylene) and the presence of pyrroles. The formation of pyrroles in the reaction, and the accumulation of 2,2 -bipyridine, are no doubt responsible for the fact that the production of 2,2 -bipyridine ceases after about 50 hr. The catalyst can be used for longer periods only if the reaction is carried out under conditions of continuous flow, or if the products of the reaction are removed as they are formed. 

Reviews dealing with a specific reaction or property from the heterocyclic point of view have been rarer—tautomerism (continued from Volume 1), free radical substitution, metal catalysts and pyri-dines, acid-catalyzed polymerization of pyrroles, and diazomethane reactions have been covered in this volume. 

The high yields of pyrroles and their A-vinyl derivatives are achieved not only when the reaction is carried out in an autoclave under an acetylene pressure of 10-20 atm (75MIP1), but also, as stated previously, in the simplest apparatus equipped with a stirrer (with a continuous acetylene feed) under only slightly elevated (l.S atm) or atmospheric pressure (79MIP1). The synthesis of pyrroles in an autoclave was performed at 120-140°C. With a large excess of acetylene and KOH in amounts of 1-30% of the ketoxime mass and a DMSO/ketoxime volume ratio within 1 8-1 16, the reaction is over in 1-3 hr. The yield of pyrroles is 72-95%. If the alkali content is increased to 1-2 mol per 1 mol of ketoxime, the reaction proceeds much faster even under atmospheric pressure and lower temperature (93-97°C), it takes only about 3 hr for the reaction to be over, the yield remaining high (80%). 

The amination of aryl halides and triflates catalyzed by palladium complexes is suitable for use in complex synthetic problems. Many substrates will produce high yields of mixed arylamines with one of the existing catalyst systems. Nevertheless, there are many combinations of substrates for which the amination chemistry may be substantially improved. For the most part, these reactions involve nitrogen centers, such as those in pyrroles, indoles, amides, imidazoles and other heterocyclic groups that are less basic than those in standard alkylamines. Although mild reaction conditions have been developed for many substrates, the harsh conditions used in many of the applications indicate that continued studies on developing mild condi-... 

A mechanism involving the coupling of cation radicals has also been considered for the electropolymerization of benzene compounds [306,313]. This mechanism occurs by a sequence of events similar to those proposed for the electropolymerization of pyrroles. The first step is the oxidation of benzene to a cation radical (471). Two of these cation radicals combine to form a dication dimer (478). The neutral aromatic dimer (479) is formed upon loss of two protons. This dimer is then reoxidized to a cation radical (480). Chain growth is accomplished by the coupling reaction of this cation radical with other cation radicals followed by deprotonation to form aromatic structures. Polymer growth continues by this sequence of steps until precipitation from solution occurs (Fig. 72). 

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