Bistriazoles Connected Through a B-B Bridge, Synthesized by Highly Selective Dipolar Cycloaddition Reactions of a Diazido-diborane(4).

In this work we report the first cycloaddition reactions between a diazido diborane(4) and terminal alkynes, providing unique access to bis-1,2,3-triazoles connected by a B-B bridge. The catalyst-free reactions are highly selective, yielding exclusively the thermodynamically disfavored bis-1,4-triazoles. The reactions are enabled by the high thermal stability of the diazido-diborane [B(hpp)(N 3 )] 2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate). Due to the tetra-coordinate boron atoms in this reagent, the reactions are tolerant with respect to the introduction of Lewis-basic groups at the alkyne. The scope and limitations of the new reactions are discussed.