Alkaline Earth-Centered CO Homologation, Reduction, and Amine Carbonylation

Reactions of β-diketiminato
magnesium and calcium hydrides
with 1 atm of CO result in a reductive coupling process to produce
the corresponding derivatives of the cis-ethenediolate
dianion. Computational (DFT) analysis of this process mediated by
Ca, Sr, and Ba highlights a common mechanism and a facility for the
reaction that is enhanced by increasing alkaline earth atomic weight.
Reaction of CO with PhSiH3 in the presence of the magnesium
or calcium hydrides results in catalytic reduction to methylsilane
and methylene silyl ether products, respectively. These reactions
are proposed to ensue via the interception of initially formed group
2 formyl intermediates, an inference which is confirmed by a DFT analysis
of the magnesium-centered reaction. The computational results identify
the rate-determining process, requiring traversal of a 33.9 kcal mol–1 barrier, as a Mg–H/C–O σ-bond
metathesis reaction, associated with the ultimate cleavage of the
C–O bond. The carbonylation reactivity is extended to a variety
of magnesium and calcium amides. With primary amido complexes, which
for calcium include a derivative of the parent [NH2]− anion, CO insertion is facile and ensues with subsequent
nitrogen-to-carbon migration of hydrogen to yield a variety of dinuclear
and, in one case, trinuclear formamidate species. The generation of
initial carbenic carbamoyl intermediates is strongly implicated through
the isolation of the CO insertion product of a magnesium N-methylanilide derivative. These observations are reinforced by a
DFT analysis of the calcium-centered reaction with aniline, which
confirms the exothermicity of the formamidate formation (ΔH = −67.7 kcal mol–1). Stoichiometric
reduction of the resultant magnesium and calcium formamidates with
pinacolborane results in the synthesis of the corresponding N-borylated methylamines. This takes place via a sequence
of reactions initiated through the generation of amidato­hydridoborate
intermediates and a cascade of reactivity that is analogous to that
previously reported for the deoxygenative hydroboration of organic
isocyanates catalyzed by the same magnesium hydride precatalyst. Although
a sequence of amine formylation and deoxygenation may be readily envisaged
for the catalytic utilization of CO as a C1 source in the
production of methylamines, our observations demonstrate that competitive
amine–borane dehydrocoupling is too facile under the conditions
of 1 atm of CO employed.