Dynamic Stereochemistry of 10-Substituted 9-Mesityl-9,10-dihydro-9-boraanthracene Compounds

The free energies of activation for rotation around the B-C (mesitylenic) bond have been determined by NMR spectroscopy in a series of several substituted 9-mesityl-9,10-dihydro-9-boraanthracenes. These molecules, both in solution and in the solid state, adopt a ground-state conformation in which the mesityl ring is nearly perpendicular to the boraanthracene ring. The free energies of activation of the topomerization process observed have been found to depend on the solvent employed, which has a major influence on the coalescence temperature. A comparison between the energy barriers for rotation in our molecules and related systems, i.e., triarylboranes, 9-arylfluorenes, and 9-arylxanthenes, is also made. © 1980, American Chemical Society. All rights reserved.