C2-Symmetric 1,2-di(fluorenylidene)cyclobutanes: syntheses, structures, rearrangements and reactivity

Abstract

Attempted deprotonation of 1,2-di(fluorenylidene)cyclobutane, 8, using n-butyl or tert-butyl-lithium led instead to 1,4-addition across the 1,3-butadiene unit to yield the 1-(9H-fluorenyl-2-(9-butyl-9H-fluorenyl)cyclobutene 10 or 11, respectively; analogously, n-butyl-lithium and trans-3,4-bis(trimethylsilyl)-1,2-di(fluorenylidene)cyclobutane, 12, furnished the corresponding 1,4-adduct 13. Bromination of 8 with N-bromosuccinimide led to mono-, di- and tri-bromo derivatives whereby, in the trans-di-bromo systems, the helical character of the trans bromines could either match (in 16) or oppose (in 17) the helicity of the fluorenylidenes. In accordance with DFT calculations, the diastereomer 17 is the favoured product, and a pure sample of 16 in solution underwent equilibration via cleavage of the C(3)–C(4) linkage such that 17 became the major isomer. Attempted addition of bromine to 3,4-di(fluorenylidene)-1,2-diphenylcyclobutene, 24, unexpectedly yielded the diketone 2,3-di(9H-fluoren-9-ylidene)-1,4-diphenylbutane-1,4-dione, 25. The structures of 10, 11, 13, 16, 17 and 25 were determined by X-ray crystallography.