Bicyclobutanes strain energy

Bicyclobutane is a highly strained molecule. Quantitative estimation of its strain energy depends on the group increments used. With a value of 51.9 kcalmol" for its the... 

Bicyclobutane is unique in that it does not obey the strain additivity rule of small-ring bicyclo[n.m.O]alkanes. For example, the strain energy of bicyclo[2.1.0]pentane, using the Franklin group increment is 53.6 kcal mol" This should be compared to the sum of the strain energies in cyclobutane (26.4) and cyclopropane (27.6 kcal mol" ). Yet, the sum of two cyclopropanes (55.2 kcal mol" is ca. 9 kcal mol" lower than that of bicyclobutane. 

In this respect the nearly identical strain energies of cyclobutane and cyclopropane are somewhat surprising. One of the possible origins of this phenomenon is probably the two 1-3 interactions in cyclobutane. Such an interaction was estimated by Bauld and CO workers to induce a destabilizing effect of 10-16 kcal mol" The gap between the expected and observed strain energies of bicyclobutane is suggested to be due to a single repulsive interaction of this type as shown in (5) between the two methylene carbons . 

Bicyclo[1.1.0]butanes, the smallest bicycloalkanes, have remarkably large strain energies (63-68 kcalmol ) and have attracted the attention of many chemists for more than half a century. The n-character of the central bond in bicyclobutane has been confirmed experimentally. In fact, treatment of bicyclobutane with Zeise s dimer and pyridine led to the formation of a platinabicyclo[l.l.l]pentane complex via cleavage of the central C1-C3 bond [19]. However, in most cases, transition-metal-catalyzed rearrangement of bicyclobutanes did not involve the initial cleavage of the central C-C bond. 

Seeking the limits of strain in hydrocarbon bonds is a fascinating area of research that has resulted in the synthesis of many exotic molecules. What is surprising is how much bond-angle distortion a carbon atom is able to tolerate. A case in point in the bicyclic series is bicyclobutane, whose strain energy is 66.5 kcal moF (278 kJ moF ), making it remarkable that the molecule exists at all. Yet it can be isolated and stored. 

Bicyclo[1.1.0]butane has been studied in some detail. " Again, all of the states appear to involve Rydberg transitions. The transition energies are at lower energies than those for cyclopropane, which may be a result of the strain in the bicyclobutane ring that further increases the ground-state energy. The spectrum of [l.l.ljpropellane has not as yet been subjected to a detailed analysis. 

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