(E,E)-1,2,3,4-Tetracyclopropylbuta-1,3-diene: Synthesis and Some of Its Properties

(E,E)-1,2,3,4-Tetracyclopropylbuta-1,3-diene (3) and (Z,Z)-1,4-diodo-1,2,3,4-tetracyclopropylbuta-1,3-diene (4) were prepared from dicyclopropylacetylene via an intermediate 2,3,4,5-tetracyclopropyltitanacyclopentadiene (2) in 91 and 77% yield, respectively. In the crystal, 3 adopts a conformation with an almost coplanar (phi = 163 degrees) 1,3-diene core, with the inner vinylcyclopropane units in an orthogonal and the outer vinylcyclopropane moieties in an s-trans orientation. This diene, like 2,3-cyclopropylbuta-1,3-diene (5), undergoes facile concerted [4+2] cycloadditions at 130 degrees C with dimethyl acetylenedicarboxylate as well as N-phenylmaleimide and at 0 degrees C with N-phenyltriazolinedione. An X-ray crystal structure analysis of 2,3-dicyclopropylbuta-1,3-diene (5) also reveals a coplanar inner core with the vinylcyclopropane units in essentially orthogonal (phi(av)=89.3 degrees) orientation. Differential scanning calorimetry (DSC) measurements indicate that 3 undergoes significant internal reorganization on going from the liquid to the crystalline phase, and a gauche conformer of 3 may well be favored over the s-trans conformer in the liquid.