Transition metal catalysis that utilizes N-heterocyclic
carbenes as noninnocent ligands in promoting transformations has not
been well studied. We report here a cyclic (alkyl)(amino)carbene (CAAC)
ligand-promoted nitro deoxygenative hydroboration with cost-effective
chromium catalysis. Using 1 mol % of CAAC–Cr precatalyst, the
addition of HBpin to nitro scaffolds leads to deoxygenation, allowing
for the retention of various reducible functionalities and the compatibility
of sensitive groups toward hydroboration, thereby providing a mild,
chemoselective, and facile strategy to form anilines, as well as heteroaryl
and aliphatic amine derivatives, with broad scope and particularly
high turnover numbers (up to 1.8 × 106). Mechanistic
studies, based on theoretical calculations, indicate that the CAAC
ligand plays an important role in promoting polarity reversal of hydride
of HBpin; it serves as an H-shuttle to facilitate deoxygenative hydroboration.
The preparation of several commercially available pharmaceuticals
by means of this strategy highlights its potential application in
medicinal chemistry.
An unprecedented and general hydroboration of alkenes with BX3 (X=Br, Cl) as the boration reagent in the presence of iPr2NEt is reported. The addition of iPr2NEt not only suppresses alkene polymerization and haloboration side reactions but also provides an “H” source for hydroboration. More importantly, the site‐fixed installation of a boryl group at the original position of the internal double bond is readily achieved in contrast to conventional transition‐metal‐catalyzed hydroboration processes. Further application to the synthesis of 1,n‐diborylalkanes (n=3–10) is also demonstrated. Preliminary mechanistic studies reveal a major reaction pathway that involves radical species and operates through a frustrated Lewis pair type single‐electron‐transfer mechanism.
The mono-base-coordinated 1,2-diboranylidenehydrazine derivatives exhibiting the BNN-1,3-dipolar reactivity toward unactivated arenes and CO2 are reported.
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