Strain energy cyclic systems

Since 1-azirines are well known, it does not seem reasonable to attribute the nonexistence of 2-azirines exclusively to strain energy. An unfavorable electronic situation is most likely responsible for their alleged instability. As mentioned previously, the 2-azirine ring system is a cyclic conjugated necessarily planar n system containing four 7r electrons and Hiickel s rule would not predict it to be stabilized by delocalization. In fact, HMO theory predicts that delocalization results in a less stable 7r system than the open chain analog.5 Such systems are predicted to be relatively unstable and have recently been designated as antiaromatic.8,9... 

A unique treatment of cyclopropane has been advanced by Dewar, who introduced the concept of a aromaticity, which explains some of the anomalous chemical and physical properties of cyclopropane. The notion of cr-conjugation implies that three cr-bonds form a cyclic system of six electrons thus cyclopropane is aromatic by the (4n -1- 2) rale. This explanation well accounts for the strain energy of cyclopropane. The actual value (27.5 kcal mol ) is much lower than the predicted value of 104 kcal mol (1 cal = 4.2 J), calculated from the C—C—C bending force constants obtained spectroscopically. - A similar comparison for cyclobutane (antiaromatic by the above notion and the 4n rule) underestimated the strain energy. o -Aromaticity also accounts for such observations as H NMR chemical shifts and the reactivity of cyclopropane toward electrojAiles. 

The intra- and intermolecular interaction between cationic center and tt-electron system have attracted many chemists and much knowledge on the interaction has been accumulated (1). In 1975, we intended to apply such a kind of interaction to cyclopolymerization. Thus, it is expected that the transition state leading to a strained cyclic unit in the polymerization could be stabilized by the intramolecular attractive interaction between cationic growing-end and ir-system There had been few examples of cyclopolymerizations giving strained units. A paracyclophane unit was employed as a strained cyclic unit in the polymer main chain. In Table I are summarized some cyclophanes with their strain energies. 

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