Bridged Annulenes

Abstract

The synthesis and aromaticity of 1,6‐methano[10]annulene and its analogues containing a heteroatom bridge suggested the possibility that the homologous series of bridged [4n + 2]annulenes 1, 2, 3, etc., formally derived from the acene series, could be developed. In the pursuit of this concept a set of systematically bent bridged [14]annulenes which permitted a scrutinization of the steric prerequisite of the Hückel rule — the requirement of a planar ring skeleton — were built up. Moreover, a new synthetic strategy based on the synthon cycloheptatriene‐1,6‐dicarboxaldehyde has provided access to the two next higher homologues (syn‐series) of 1,6‐methano[10]annulene, i.e., syn‐1,6:8,13‐bismethano[14]annulene and syn, syn‐1,6:8,17:10,15‐trismethano[18]annulene, respectively.

The aromatic character of [10]annulenes and dicupra[10]annulenes from current density calculations

“…5,10,11 Peaks of aromatic protons were found in the 1 H NMR spectrum of the bridged bicyclic molecule. 10 Therefore, one would expect that a prerequisite for aromatic properties of the dicupra [10]annulenes is a planar structure of the annulene core.…”

“…Classical examples of aromatic 10 π-electron systems are naphthalene and azulene, both of which have planar geometry. The hydrocarbon [10]annulene also possesses 10 π electrons, however, it does not exhibit any strong aromatic character. 1,2 The planar all-cis structure suffers from bond-angle strains.…”

“…[3][4][5][6][7][8][9] The energy differences and the barriers between the conformations are very small because the potential energy surface of [10]annulene is flat. 1 The planarity of the [10]annulene core can be forced, for example, by introducing a methylene bridge, forming 1,6-methano [10]annulene. 5,10,11 Peaks of aromatic protons were found in the 1 H NMR spectrum of the bridged bicyclic molecule.…”

“…1 The planarity of the [10]annulene core can be forced, for example, by introducing a methylene bridge, forming 1,6-methano [10]annulene. 5,10,11 Peaks of aromatic protons were found in the 1 H NMR spectrum of the bridged bicyclic molecule. 10 Therefore, one would expect that a prerequisite for aromatic properties of the dicupra [10]annulenes is a planar structure of the annulene core.…”

“…In 2015, Wei et al synthesised copper-based metallanaphthalenes from substituted 1,4-dilithio-1,3-butadienes and Cu(I) salts. 12 The main product they isolated was dicupra [10]annulene with three trimethylsilyl (TMS) and three phenyl groups at the carbon atoms (Figure 1), which crystallised with four lithium counterions, two on each side of the core structure (molecule B 4 ).…”