New Reactivity Modes of Chromium(0) Fischer Carbene Complexes: Unprecedented Insertion of a Carbene Ligand into an Active B−H Bond

The reaction of α,β-unsaturated chromium(0) complexes with hydrides leads to different
products, depending on the electronic properties of the reducing agent and the carbene carbon.
Electrophilic alkynylcarbene complexes react with NaCNBH3 to produce compounds derived
from the insertion of the carbene ligand into the B−H bond in a process that has no precedent
in the literature. This reaction does not take place if the electrophilicity of the carbene carbon
is diminished by the presence of a strong electron-donating group. However, the behavior of
alkenylcarbene complexes toward NaCNBH3 can be interpreted by initial addition of the
hydride to the C−Cr bond followed by evolution of the intermediate thus formed through
different reaction pathways. These include chromium 1,3-migration and alkoxide extrusion
followed by dimerization or oxidation. This route also justifies the reactivity of alkenyl and
alkynyl alkoxy Fischer carbene complexes toward other boron or aluminum hydrides. Finally,
when a bulky reagent such as K-Selectride is employed, the 1,2-hydride addition is disfavored
and only products derived from the reduction of the C−C multiple bonds are obtained.