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.