
%0 Journal Article
%A Coller, BAW 
%A Heffernan, ML 
%A Jones, AJ 
%T Benzene isomers. II. 3,4-Dimethylenecyclobutene
%J Australian Journal of Chemistry
%D 1968/07/01
%V 21
%N 7
%P 1807-1826
%K 
%R https://doi.org/10.1071/CH9681807
%U https://www.publish.csiro.au/paper/CH9681807 
%X Theoretical considerations indicate that the electron distribution in 3,4,-dimethylenecyclobutene is non-uniform and a dipole moment of 0.71 D has been predicted by inclusion of non-neighbour core resonance integrals. Preliminary studies of the microwave spectrum of this compound provide a dipole moment of 0.618 ± 0.011 D. The term &quot;pseudo-alternant&quot; is proposed to describe this phenomenon. In the present paper additional physical and chemical properties of 3,4,-dimethylene-   cyclobutene are described.   The observed low-field proton chemical shifts for 3,4-dimethylenecyclobutene are not accounted for using a ring current model and a small calculated paramagnetic current indicates properties usually associated with Hückel 4n-hydrocarbons. A local anisotropy model provides a better account of the observed shift. The carbon-13-   proton satellite spectrum of 3,4-dimethylenecyclobutene indicates similarities in the geometry of the four-membered ring to that in cyclobutene. In addition, infrared and ultraviolet spectra support the conclusion that this hydrocarbon be regarded as a cross-conjugated diene.   A detailed discussion of the products arising from the bromination of 3,4-dimethylenecyclobutene under a variety of conditions indicates consecutive 1,4-addition of bromine across the four-membered ring, a property which has been associated with cross-conjugation. Hydrogenolysis, chlorination, iodination, and attempted Diels-Alder addition reactions are also described.   3,4-Dimethylenecyclobutene was prepared by the thermal rearrangement of   hexa-1,5-diyne. The mechanisms of thermal and photolytic rearrangement of hexa-1,5-diyne are discussed using Woodward-Hoffmann postulates. 1,2-Dideutero-,1-ethyl-, and 1-n-propyl-3,4-dimethylenecyclobutene were also prepared by the thermal rearrangement of the corresponding diyne. In addition, the thermal rearrangement of hexa-1,5-diyne over the temperature range 290-700º to give variable quantities of 3,4-dimethylenecyclobutene, fulvene, and benzene is described. 3,4-   Dimethylenecyclobutene is thermally rearranged to benzene at 680º. 
