Electron-rich three-center systems

When the electron-rich three center bond of trithiapentalenes was discussed the carbon skeleton of these species was disregarded. Therefore it seems interesting to look for systems where this skeleton is not present. 

In attempting to approximate (CNDO/2) the nature of the 1,2,5-thiadiazole ring (2a vs. 2b), a nontraditional view has been proposed <83IJC(B)802> with two C=N bonds and one three-center two-electron 7t-bond delocalized over the N—S—N system. This would account for the electron-rich N—S—N nature of the molecule. Classical structures (2a, 2b) are viewed as unlikely versus a hybrid structure (2c). 

Germacrene-D, a ten-membered-ring system with three double bonds acting as electron-rich centers, and pinenes and menthadiene oiXylopia aethiopica EO showed a significant ability to scavenge superoxide anion radical [176]. EOs with /f-caryophyllene as the major compound showed radical-scavenging activity [177]. 

If n is odd, a three fused ring group such as anthracene or carba-zole attached at the center atom, the 9 position in the two cited, can twist to provide a tt overlap of about 30%. When n is even, the aromatic system must be elongated to overlap an example could be a 4 substituted biphenyl group. The polymerizing system is a basic heterocycle which polymerizes cationically, the 2 substituted, A2, 1,3-oxazoline. This is stable enough both as monomer and as active center that monomers with very electron rich side chain substituents can be readily polymerized. 

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