The addition of a
B–H bond to an unsaturated bond (polarized
or unpolarized) is a powerful and atom-economic tool for the synthesis
of organoboranes. In recent years, s-block organometallics have appeared
as alternative catalysts to transition-metal complexes, which traditionally
catalyze the hydroboration of unsaturated bonds. Because of the recent
and rapid development in the field of hydroboration of unsaturated
bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca,
Sr, Ba) metals, we provide a detailed and updated comprehensive review
that covers the synthesis, reactivity, and application of s-block
metal catalysts in the hydroboration of polarized as well as unsaturated
carbon–carbon bonds. Moreover, we describe the main reaction
mechanisms, providing valuable insight into the reactivity of the
s-block metal catalysts. Finally, we compare these s-block metal complexes
with other redox-neutral catalytic systems based on p-block metals
including aluminum complexes and f-block metal complexes of lanthanides
and early actinides. In this review, we aim to provide a comprehensive,
authoritative, and critical assessment of the state of the art within
this highly interesting research area.
A magnesium‐catalyzed hydroboration of alkynes providing good yields and selectivities for a wide range of terminal and symmetrical and unsymmetrical internal alkynes has been developed. The compatibility with many functional groups makes this magnesium catalyzed procedure attractive for late stage functionalization. Experimental mechanistic investigations and DFT calculations reveal insights into the reaction mechanism of the magnesium catalyzed protocol.
A magnesium-catalyzed
regiodivergent C–O bond cleavage protocol
is presented. Readily available magnesium catalysts achieve the selective
hydroboration of a wide range of epoxides and oxetanes yielding secondary
and tertiary alcohols in excellent yields and regioselectivities.
Experimental mechanistic investigations and DFT calculations provide
insight into the unexpected regiodivergence and explain the different
mechanisms of the C–O bond activation and product formation.
A Bi-catalyzed synthesis
of sulfonyl fluorides from the corresponding
(hetero)aryl boronic acids is presented. We demonstrate that the organobismuth(III)
catalysts bearing a bis-aryl sulfone ligand backbone revolve through
different canonical organometallic steps within the catalytic cycle
without modifying the oxidation state. All steps have been validated,
including the catalytic insertion of SO
2
into Bi–C
bonds, leading to a structurally unique O-bound bismuth sulfinate
complex. The catalytic protocol affords excellent yields for a wide
range of aryl and heteroaryl boronic acids, displaying a wide functional
group tolerance.
Efficient reduction of cyclic and linear organic carbonates catalyzed by a readily available earth alkaline catalyst has been achieved. The described homogenous reaction based on a ligand-free magnesium catalyst provides an indirect route for the conversion of CO 2 into valuable alcohols. The reaction proceeds with high yields under mild reaction conditions, with low catalyst loading and short reaction times, and shows a broad applicability toward various linear and cyclic carbonates. Additionally, it can be applied for the depolymerization of polycarbonates.
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