Consider what happens when we treat cyclooctatetraene with a powerful reducing agent

•	If 1,3,5,7-tetramethylcyclooctatetraene (refcode TMCOTT) is treated with alkali metals a dianion is formed (refcode TMOCKE).

•	Look closely at the structures of 1,3,5,7-tetramethylcyclooctatetraene (refcode TMCOTT) and the resultant dianion (refcode TMOCKE). How do these two compounds differ structurally?

•	The dianion is planar and all bonds lengths are equivalent (within experimental error). Whereas the neutral compound is non-planar (“tub” shaped) with alternate double and single bonds lengths of 1.48Å and 1.33Å.

 

left: “tub” shape of 1,3,5,7-tetramethylcyclooctatetraene (refcode TMCOTT); right: the resulting planar dianion (refcode TMOCKE)

 

•	By reducing 1,3,5,7-tetramethylcyclooctatetraene we are adding electrons. The difference between the anion and the neutral compound is therefore the number of electrons in the π system. The following table summarises what we have discovered so far:

 

Compound and CSD Refcode	Diagram	π Electrons	Geometry
cyclooctatetraene   ZZZSAE01		8	non-pla­nar
1,3,5,7-tetramethyl cyclooctatetraene   TMCOTT		8	non-pla­nar
1,3,5,7-tetramethyl cyclooctatetraene dianion   TMOCKE		10	planar