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.

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes.

In this account we give an overview of the synthesis and properties of heterocyclic [8]circulenes. Much of the interest in studying heterocyclic [8]circulenes stems from the planar cyclooctatetraene core often contained in these compounds, which in principle is antiaromatic. We start with a short introduction to the hydrocarbon [n]circulenes and proceed to describe the synthetic chemistry involved in creating tetraoxa[8]circulenes, with particular focus on the acid-mediated oligomerization of benzo-or naphthoquinones, resulting in some simple rules for predicting the outcome of the oligomerization reactions. These rules have guided the synthetic strategies for the preparation of azatrioxa[8]circulenes and diazadioxa[8]circulenes, which will be described in separate sections of this account. More traditional synthetic strategies have been applied in the preparation of octathia[8]circulene, tetrathiatetraselena[8]circulene, and a number of other heterocyclic [8]circulenes, and these synthetic efforts will be highlighted. Finally, a section describing structures that are closely related to the heterocyclic [8]circulenes will be presented, and at the end we will comment on the extensive theoretical work regarding the question of aromaticity/antiaromaticity of the central cyclooctatetraene of heterocyclic[8]circulenes. 1 Introduction 2 Synthesis of [n]circulenes 2.1 [4]Circulene 2.2 [5]Circulene 2.3 [6]Circulene 2.4 [7]Circulene 2.5 [8]Circulene 3 Synthesis of Tetraoxa[8]circulenes: A Historical Perspective 4 Synthesis of Azatrioxa[8]circulenes 5 Synthesis of Diazadioxa[8]circulenes 6 Synthesis of Other Heterocyclic [8]Circulenes 7 Synthesis of Structurally Related Compounds 8 Hetero[8]circulenes and Related Compounds as Tools to Study Aromaticity and Antiaromaticity 9 Conclusion and Outlook

Insights to the subtle reactivity patterns of hydroxy-substituted carbazoles allows the precise synthesis of unsymmetrical azatrioxa[8]circulenes by the reaction of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with two different 1,4-benzoquinones in the presence of an oxidant (chloranil) and a Lewis acid (BF3OEt2). The unique synthetic control obtained originates from the selectivity obtained upon reacting N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with an electron-rich benzoquinone to give first the C-C bond formation and then subsequently the dibenzofuran formation with high regioselectivity. Herein the first synthesis of unsymmetrical antiaromatic azatrioxa[8]circulenes and the full characterization using NMR spectroscopy, optical spectroscopy, electrochemistry, computational techniques and single crystal X-ray crystallography is reported. The controlled stepwise condensation of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with two different 1,4-benzoquinones gives selectively the unsymmetrical azatrioxa[8]circulenes.

Synthesis of Heterocyclic [8]Circulenes and Related Structures -[68 refs. + subrefs.]. -(HENSEL, T.; ANDERSEN, N. N.; PLESNER, M.; PITTELKOW*, M.; Synlett 27 (2016) 4, 498-525, http://dx.doi.org/10.1055/s-0035-1560524 ; Dep. Chem., Univ. Copenhagen, DK-2100 Copenhagen, Den.; Eng.) -Y. Steudel