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.

[8]Annulyne trimerization in presence of base

[12]annulyne has been observed in 2005 by Stevenson et al. in solution by NMR spectroscopy at room temperature ^[1]. Reaction of 1,5-hexadiyne and potassium tert-butoxide was reported to yield two isomers 5,9-di-trans-[12]-annulyne and 3,11-di-trans-[12]annulyne in a 1:1 ratio. The proposed reaction sequence involved an unspecified electron transfer reaction. A third single isomer 3,9-Di-trans-[12]annulyne could be obtained in three steps from hexabromocyclododecane.

Unlike other annulynes, the [12]annulyne isomers were found to be very stable and did not self-condense. These annulynes reacted with potassium metal to form radical anions and dianions. The NMR chemical shifts of two internal protons of the dianion were negative and attributed to an aromatic diamagnetic ring current. The external proton next to the alkyne group had a chemical shift of nearly 14 and attributed to the positive charge of the potassium cation coordinated to it.

Proposed [12]Annulyne synthesis from 1,5-hexadiyne, Stevenson 2005

Two new [12]annulyne isomers were reported in 2008 by the same group ^[2]

The Stevenson findings were challenged by Christl and Hopf in 2009 ^[3]. They were unable to conceive a plausable mechanism and argued that based on the available spectroscopic data the products formed were in fact two linear 1,3-hexadien-5-yne isomers and not macrocycles.

References

^ [12]Annulynes Matthew N. Gard, Matthew K. Kiesewetter, Richard C. Reiter, and Cheryl D. Stevenson J. Am. Chem. Soc.; 2005; 127(46) pp 16143 - 16150; (Article) doi:10.1021/ja053886l
^ The Isomers of [12]Annulyne and their Reactive Relationships to Heptalene and Biphenyl Brad D. Rose, Richard C. Reiter, and Cheryl D. Stevenson Angew. Chem. 2008, 120, 8842–8846 doi:10.1002/ange.200803863
^ [12]Annulynes from 1,5-Hexadiyne and Potassium tert-Butoxide? Franz Sondheimers Hexadienynes! Manfred Christl, Henning Hopf Angew. Chem. Int. Ed. 2009, 48 doi:10.1002/anie.200901741

[1] [12]Annulynes Matthew N. Gard, Matthew K. Kiesewetter, Richard C. Reiter, and Cheryl D. Stevenson J. Am. Chem. Soc.; 2005; 127(46) pp 16143 - 16150; (Article) doi:10.1021/ja053886l

[2] The Isomers of [12]Annulyne and their Reactive Relationships to Heptalene and Biphenyl Brad D. Rose, Richard C. Reiter, and Cheryl D. Stevenson Angew. Chem. 2008, 120, 8842–8846 doi:10.1002/ange.200803863

[3] [12]Annulynes from 1,5-Hexadiyne and Potassium tert-Butoxide? Franz Sondheimers Hexadienynes! Manfred Christl, Henning Hopf Angew. Chem. Int. Ed. 2009, 48 doi:10.1002/anie.200901741

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