Annulyne

Annulynes or 1,2-didehydroannulenes are conjugated monocyclic hydrocarbons with alternating alkene bonds in addition to one alkyne bond. They are related to annulenes who have just alternating double bonds. The smallest member of this class is [4]annulyne and never observed because the molecule carries too much angle strain. The next member is [6]annulyne or benzyne which is a reactive intermediate well known in organic chemistry. [8]annulyne is known to exists but quickly dimerizes or trimerizes. The compound has been trapped as its radical anion and observed by EPR spectroscopy. [10]annulyne, like [4]annulyne only exists in theory. [12]annulyne on the other hand is a stable compound and can be observed in solution by NMR spectroscopy at room temperature. Reaction of 1,5-hexadiyne and potassium tert-butoxide yields two isomers 5,9-di-trans-[12]-annulyne and 3,11-di-trans-[12]annulyne in a 1:1 ratio. The reaction sequence involves a as yet unspecified electron transfer reaction. A third single isomer 3,9-Di-trans-[12]annulyne can be obtained in three steps from hexabromocyclododecane. Unlike other annulynes, the [12]annulyne isomers are very stable and do not self-condense. These annulynes do react with potassium metal to form radical anions and dianions. The NMR chemical shifts of two internal protons of the dianion are negative due to an aromatic diamagnetic ring current. The external proton next to the alkyne group has a chemical shift of nearly 14 because it experiences the positive charge of the potassium cation coordinated to it.