Heteroaromatics metallated five-membered systems

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

One such problem is the possibility of the existence and aromatic character of analogues of five-membered heteroaromatics with one heteroatom and those of azoles in which one or several ring C atoms are substituted by a metal atom, e.g. structures (289)-(305) (M = Ge, Sn, Pb X = O, S, NR). The other specific problems involve antiaromaticity of 871-electron carbenoids (306) and homoaromaticity of 67t-electron systems with a tetravalent metal (307). 

Isomeric thienothiophenes and their derivatives are of considerable interest as electron donors or acceptors for the design of new types of charge-transfer complexes and as ligands coordinating metal ions. As mentioned above (see Section IV.D), charge-transfer complexes of some five-membered fused heteroaromatic systems with TCNE were studied by UV and IR spectroscopy and their relative stabilities were discussed using the association constants and enthalpies of formation (82CS214). 

MetaUation.— In the course of a wider research programme on the metal-lation of five-membered heteroaromatic systems, Micetich has extended the work of Caton et al. on the lithiation of isothiazoles. Unlike 3,5-dimethylisoxazole, which undergoes chiefly lateral lithiation to the 5-lithiomethyl derivative capable of furnishing heteroarylacetic acids (HetCHa HetCHaLi -> HetCHaCOaH), 3,5-dimethylisothiazole (72) is... 

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