Evidence for the Coexistence of Two Bond-Stretch Isomers in Solution (pages 4880–4883)
Amor Rodriguez, Ryan A. Olsen, Nima Ghaderi, David Scheschkewitz, Fook S. Tham, Leonard J. Mueller and Guy Bertrand
Article first published online: 16 SEP 2004 | DOI: 10.1002/anie.200460475
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.