Polymeric indolizines

It is of interest to compare the stability of the protonated forms of benzo[u], benzo[Z>] and benzo[c] fused pyrroles, i.e. the cations derived from indolizines, indoles and isoindoles. Indolizine gives a stable pyridinium ion and does not polymerize in the presence of acid. 

Indolizine is much more basic than indole (p Ta = 3.9 vs. —3.5), and the stability of the cation makes it less reactive and resistant to acid-catalyzed polymerization. Protonation occurs at C-3, although 3-methylindolizine protonates also at C-l. Introduction of methyl groups raises the basicity of indolizines. Electrophilic substitutions such as acylation, Vilsmeyer formylation, and diazo-coupling all take place at C-3. Nitration of 2-methylindolizine under mild conditions results in substitution at C-3, but under strongly acidic conditions it takes place at C-l, presumably via attack on the indolizinium cation. However, the nitration of indolizines often can provoke oxidation processes. 

Substituted vinylindolizines sometimes polymerize spontaneously and also copolymerize with styrene. Polymers of similar structure have been prepared by formation of indolizines on the polymer chain using methods outlined in Sections 3.08.3 and 3.08.6 (see Scheme 36 for an example). Moreover, indolizine dyes such as (214) have been bound to an ethyl acrylate/acrylic acid copolymer by heating to give dyes that do not migrate in photographic colour film emulsions. 

Various indolizines containing vinyl groups in the pyridine ring have been synthesized by standard procedures, e.g., Ref. 212. Turchinovich and co-workers found that whereas 2-methyl-6-vinyl- and 1,2-dimethyl-6-vinylindolizine would not polymerize, even in the presence of an initiator, 3-acetyl-l,2-dimethyl-6-vinylindolizine polymerized spon-... 

Furthermore, cationic dyes 179 of the diazahemicyanine class are derived from indolizines. This invention related to water-soluble azo dyestuffs is useful for the colorations of synthetic polymeric materials particularly polymers and copolymers of acrylnitrile and dicyanoethylene as well as modified polyesters and polyamides (81GBP2075540, Scheme 64). 

Indolizine is a base (plQ, = +3.9) much stronger than indole (pfQ, = -3.5). Indolizine is protonated at C-3 to give the indolizinium ion 2 its relative stability makes 2 less reactive and thus indolizine (in contrast to pyrrole) resistant to acid-induced polymerization ... 

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