Strain-Release Promoted Thermal [2 + 2] Cycloaddition and Photochemical [2 + 2] Cycloreversion of a Tetraorganylaminoborane

Tetraorganyl aminoboranes are typically monomeric species, while their hydrides or halides often are dimers or higher oligomers. Aminoboranes composed of 9-borafluorene and epiminoacenes (9,10-dihydro-9,10-anthracene, 1, and 1,4-dihydro-1,4-naphthalene, 2) are reported to spontaneously dimerize at room temperature according to NMR spectroscopy and X-ray crystallography. The reason is elucidated to be associated with the strain of the CNC angle that favors trigonal pyramidal coordination over trigonal planar coordination at nitrogen. As the consequence, the distortion energy of the transition state is low, resulting in a reduced barrier for dimerization of 1 and 2. The dimer of 1 undergoes photoinduced cleavage into monomers.