Quaternizing Diboranes(4): Highly Divergent Outcomes and an Inorganic Wagner–Meerwein Rearrangement

Apart
from a few compounds under heavy use in organic chemistry,
diboranes are relatively exotic and poorly understood. Recently, interest
in these molecules has intensified with the advent of so-called “sp2–sp3” diboranes which exhibit useful
reactivity toward organic substrates. In our hands, addition of Lewis
bases to dihalodiorganyl diboranes(4) has previously shown some very
surprising reactivity, including a substituent exchange between the
boron atoms, and diboranes in which halide atoms bridge the B–B
bond. Herein we have expanded the range of diborane(4) and Lewis base
reaction partners, in the process uncovering three new reactivity
patterns as well as some cases where mixtures are obtained. Trends
are established for the preferential formation of certain products
which rationalize the results based on electronic and steric effects.
The substituent exchange, clearly based on an inorganic version of
the well-known Wagner–Meerwein rearrangement, was also found
to be an equilibrium reaction with the halide-bridged Lewis adducts
on the other side.