Formation of BCBH/BCBCl Four-Membered Rings by Complexation of Boron- and Nitrogen-Substituted Acetylene with Hydro-/Chloroboranes

Reactions of boron- and nitrogen-substituted acetylene toward hydro-/chloroboranes are described. The X-ray crystallography revealed the formation of BCBH/BCBCl four-membered rings in the solid state. By NMR and IR spectroscopic analysis, the BCBH four-membered ring was retained even in the solution. In contrast, the BCBCl four-membered ring exists as an association/dissociation equilibrium mixture in solution. The B–H–B moiety forms a three-center-two-electron bond through a spherical 1s orbital of a hydrogen atom; therefore, the four-membered ring does not have significant strain. On the other hand, due to the weak coordination bond from Cl to B, the BCBCl four-membered ring was labile. In addition, the rearrangement of the BCBH ring under heating conditions is described.