Lowering the temperature breaks the σ bond in the interconversion of 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl and its bicyclo[1.1.0]butane bond-stretch isomer (see picture). Variable-temperature NMR and UV/Vis experiments demonstrate for the first time the existence of two bond-stretch isomers that feature a transannular bonding π overlap, which allows for the thermal ring closure and opening processes.
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Rodríguez et al. (2004) studied this question.
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