Author:
Coller BAW,Heffernan ML,Jones AJ
Abstract
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
"pseudo-alternant" 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 H�ckel 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 290-700� to give variable quantities of 3,4-dimethylenecyclobutene, fulvene,
and benzene is described. 3,4-
Dimethylenecyclobutene is thermally rearranged to benzene at 680�.
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