Head-to-tail coupling reactions

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

To explain this unusual result, two basically different mechanisms were proposed (24). The first, the uncoupled electron mechanism assumed that oligomeric phenoxy radicals combined by a head-to-tail coupling reaction. This implied then that a resonance structure could be written... 

Similar results were obtained by Cooper and by Mijs et ah (24) from a number of other substituted dimers. With lightly substituted dimers, however, Mijs observed that at low temperatures the initial products consist almost entirely of tetramers the tetramer, moreover, is that corresponding to the quinone ketal rearrangement, rather than to head-to-tail coupling (Reaction 20). 

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