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45.12 Product Class 12: Radialenes

DOI: 10.1055/sos-SD-045-00477

Iyoda, M.Science of Synthesis, (201045507.

General Introduction

Radialenes are cross-conjugated cyclic hydrocarbons with a general structure 1 (Scheme 1) of formula C2nH2n, with n ring atoms and n exocyclic double bonds. They are named similarly to annulenes, according to their ring size, i.e. [n]radialene.[‌1‌‌3‌] However, specific examples in this section will be named according to standard nomenclature conventions as oligoalkylidenecycloalkanes. Radialenes (e.g., 25) and their derivatives have been receiving growing attention from synthetic and physical chemists, materials scientists, and especially theoretical chemists.[‌4‌‌8‌] The introduction of electron-donating or -withdrawing substituents can convert radialenes into π-donors or π-acceptors, respectively, for preparing novel charge-transfer complexes and ionic salts.[‌9‌‌11‌] Furthermore, radialenes have been employed as molecular scaffolding[‌12‌] and molecular machinery.[‌13‌]

Scheme 1 Radialenes

Although the first synthetic approach toward trimethylenecyclopropane (2; [3]radialene) was reported in 1959,[‌14‌] Hopf and Wick reported the first member of the radialene family in 1961.[‌15‌] This hexamethyl[6]radialene 7 was synthesized as a mixture of diastereomers from hexabromide 6 (X=Br) or the corresponding chloride 6 (X=Cl) using magnesium metal (Scheme 2). Since this pioneering work, a variety of synthetic methodologies have been applied to the construction of radialenes and their π-expanded analogues.

Scheme 2 Synthesis of a Hexamethyl[6]radialene[‌15‌]