Reactions which modify the Pyrrole Nucleus

For general remarks regarding these reactions see p. VII. It is interesting to notice that among them are the small number of instances of nucleophilic attack upon the pyrrole nucleus the reductions to pyrrolines and the reactions with sodium bisulphite and with hydroxylamine. 

Like other aromatic systems, pyrroles reveal their unsaturated condition in certain addition reactions. The most obvious of these are partial or complete reduction to pyrrolines or pyrrolidines. Sodium and alcohol are without effect on pyrrole s but zinc and acid produce A -pyrro-lines 39, 233, 303 This reaction may involve attack of hydride ion upon the a-protonated pyrrole cation. Many pyrroles have been reduced by a variety of catalytic procedures to the fully saturated pyrrolidines and electrolytic reduction also converts pyrrole into pyrrolidine i. 

Pyrrole forms with sodium bisulphite an addition compound, C4H7N0eS2Na2.2H20, which has been formulated as the dihydrate of the disulphonate (49). The reaction has been represented as a nucleophilic attack upon the polarized pyrrole molecule. Mechanistically it could be very satisfactorily represented as a nucleophilic attack by bisulphite anion upon the highly reactive -protonated cation of pyrrole in a manner analogous to the formation of pyrrole trimer (p. 86). However, under the conditions used 4, the concentration of cation must be extremely small. 

The formation from pyrroles and dienophiles of substances which are the results of Michael addition has been mentioned (p. 70). In some cases Diels-Alder addition does occur. Thus, with methyl acetylenedicarboxylate, methyl pyrrole-1-carboxylate gives (50), formed by elimination of acetylene from the initial Diels-Alder product . From 1-methylpyrrole, (51) is obtained , formed from the initial Diels-Alder product by reaction with a second molecule of methyl acetylenedicarboxylate . 1-Benzylpyrrole 

In contrast to pyrrole itself (p. 71), 1-methyl- and 1-benzyl-pyrrole undergo Diels-Alder addition with benzyne giving (53) and substances derived from these, 308. pyrryl magnesium iodide gives a-naphthylamine formed by aromatization of the Diels-Alder intermediate os. 

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