Diazadioxa[8]circulenes: Planar Antiaromatic Cyclooctatetraenes

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid‐mediated oxidative dimerization of 3,6‐dihydroxycarbazoles to yield the diazadioxa[8]circulenes in high yields. The synthetic protocol appears to be general, and is a one‐pot transformation in which two CC bonds and two CO bonds are formed with the loss of two molecules of water. We also present a detailed characterization of the optical and electrochemical properties of this new class of stable planar cyclooctatetraenes. The properties of the diazadioxa[8]circulenes are compared with the properties of isoelectronic tetraoxa[8]circulenes and azatrioxa[8]circulenes. We discuss the antiaromatic nature of the planar central cyclooctatetraene moiety. The antiaromatic nature of the planar cyclooctatetraenes was studied by using computational methods (NICS calculations), and these calculations reveal that the central eight‐membered ring has antiaromatic character. Plane to see: New [8]circulenes are planar when an adequate number of heteroatoms are in the outer ring. This leads to the antiaromatic nature of the central cyclooctatetraene ring. A new class of antiaromatic [8]circulenes, the diazadioxa[8]circulenes, have been prepared by means of a new acid‐mediated oxidative dimerization of 3,6‐dihydroxycarbazoles to yield the new [8]circulenes in high yields (see scheme; R=benzyl or propyl).