The Boat Conformation in Pyrazaboles. A Theoretical and Experimental Study
Experimental data are combined with theoretical calculations to study the stability of the boat conformation of the pyrazabole ring. For the experimental studies, new BH2- and BF2-pyrazaboles disubstituted with iodo or ethynyl groups at the 2- and 6-positions have been prepared and structurally characterized. We have found different molecular structures and crystal packings depending on the substituents. The iodo derivatives have a bent molecular shape due to the boat conformation of the pyrazabole ring, and the molecules are arranged in stacks with the same conformation (all-up or all-down boat conformation) along the crystallographic b axis. Stacks of molecules with the same conformation interact in a plane by means of iodo−iodo short contacts. However, the ethynyl derivatives are formed by bent-shaped molecules for BH2 and planar structures for BF2. Density functional theory and ab initio calculations have been performed on these compounds and on unsubstituted analogues to understand the effect of substituents on the conformation, inversion barrier, and NMR chemical shifts. The results of these calculations were compared with the experimental results. Theoretically, the boat conformation is an energy minimum, with the planar and chair conformations as transition states in an energy diagram. In solution, the compounds are in a boat conformation (with a boat-to-boat dynamic equilibrium) irrespective of their crystal structures.