The first simple and stable cross‐conjugated π‐extended alkylidenecycloproparenes 5, 6 and 15−22 have been prepared as coloured crystalline derivatives from use of (E,E)‐1,5‐diphenylpenta‐2,4‐dienone, (E,E)‐1,5‐diarylpenta‐1,4‐dien‐3‐ones and (E,E)‐2,5‐(diarylmethylidene)cyclopentanones with the relevant 1,1‐bis(trimethylsilyl)cyclopropa[b]naphthalene. The permanent dipole moments of the compounds have been measured, and the conformational preference of the rotamers assessed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The Peterson olefination for alkylidenecycloproparene synthesis from a 1,1-disilylcycloproparene has been refined into five distinct protocols that have provided 43 new aryl- (5 and 6) and diaryl- (7 and 8), and aryl(phenyl)- (9 and 10) methylidene derivatives. The permanent dipole moments of these and other previously reported compounds have been measured and the direction of the dipole, to or from the cycloproparenyl moiety, established for each compound. The 13C NMR spectra are fully assigned and linear correlations of carbon chemical shift with the Hammett sigmap+ constants for each atom within the cycloproparene moiety are provided for the 8-11 compounds that comprise each substitution pattern present in 5-10.

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Reaction of 1,1-dichloro-2,5-diphenylcyclopropabenzene 6 with Meldrum's acid 8 in the presence of pyridine leads to coupling of the cycloproparenyl cation 7 with the stabilized diketo anion 9. Subsequent, spontaneous, base-induced dehydrochlorination gives the alkylidenecyclopropabenzene 11 in a one-pot reaction. Flash vacuum thermolysis of 11 at 650 degrees C ejects acetone and carbon dioxide, giving cyclopropabenzenylidenethenone 12 that is isolated in an Ar matrix at 20 K and characterized by a strong ketene band at 2107 cm(-1) in the IR spectrum. [reaction: see text]

[reaction: see text] The first derivative of the novel and highly strained rocketene (cyclobuta[a]cyclopropa[d]benzene) 5, namely, the C4 diphenyl-substituted exocyclic alkene 8, has been prepared in 16% overall yield as a yellow oil with properties fully consistent with its highly strained nature.