Bicyclo butane, from 1,3-butadiene

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

We also know the enthalpies of formation of the triene, l,6-(butane-l,4-diyl)-tropilidene (bicyclo[4.4.1]undeca-l,3,5-triene, 88) as well as of the related tetraene [ 1,6-(2-butene-1,4-diyl)-tropilidene, 89] and pentaene [l,6-( 1,3-butadiene-1,4-diyl)-tropilidene, 90], respectively. Choosing Roth s suggested value for the enthalpy of formation of the parent tropilidene so that all four species are taken from the same primary source, we find that attachment of these varying 4-carbon chains increase the enthalpy of formation by —40, 74 and 136 kJ mol respectively. Upon affixing these same 4-carbon chains to benzene to form tetralin, 1,4-dihydronaphthalene and naphthalene (91, 92 and 7, respectively) the corresponding enthalpy of formation changes by —57, 51 and 68 kJmoU. ... 

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