Methylenecyclobutene and related substances

Methylenecyclobutene was synthesized by a modification of the Willstatter cyclobutene synthesis, with the use of the Hofmann reaction to introduce the intracyclic double bond. A novel feature of the synthesis was the use of anthracene as a blocking group for the exocyclic double bond during the malonic-ester ring closure. 4,4-Dimethylmethylenecyclobutene was synthesized from pinene, and 3-phenylmethylenecyclobutene was obtained by condensation of allene and phenylacetylene. The structures of the methylenecyclobutene dibromides and of the polymers formed by three different mechanisms were investigated. The results were shown to be consistent with the predictions of simple molecular orbital theory. 3-Methylenetrimethylene oxide was synthesized and its structure investigated by electron diffraction. The structures of the dibromides of methylenecyclobutane were determined and found to be consistent with a carbonium-ion mechanism for bromine addition