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Publisher Summary This chapter discusses the medium-large and large π-excessive heteroannulenes. The chapter discusses the structural demands imposed by π-electron mobility on medium-large and large monocyclic π ribbons. The ability of a heteroannulene to exist in planar form depends on molecular size and, within a specified size, on the relative number of cis and trans double bonds and the sequence in which these groups occur. The nine-membered system, the heteronins, is the largest member of the family capable of attaining planarity while in possession of an all- cis perimeter. There is ample indication that the increased rigidity of the heavily “restricted” heterocyclic perimeters significantly relaxes the requirement of high lone-pair mobility for the development of π delocalization. The replacement of two or more double bonds by heteroatomic π -excessive units, as in the diheterocins, the triheteronins, and the dithia[10]annulene, significantly reduces lone-pair mobility chiefly as a result of enhanced electron repulsion. |
