Conspectus
Transition
metal catalyzed asymmetric
hydrofunctionalization of
readily available unsaturated hydrocarbons presents one of the most
straightforward and atom-economic protocols to access valuable optically
active products. For decades, noble transition metal catalysts have
laid the cornerstone in this field, on account of their superior reactivity
and selectivity. In recent years, from an economical and sustainable
standpoint, first-row, earth-abundant transition metals have received
considerable attention, due to their high natural reserves, affordable
costs, and low toxicity. Meanwhile, the earth-abundant metal catalyzed
hydrofunctionalization reactions have also gained much interest and
been investigated gradually. However, since chiral ligand libraries
for earth-abundant transition-metal catalysis are limited to date,
the development of highly enantioselective versions remains a significant
challenge.
This Account summarizes our recent efforts in developing
suitable
chiral ligands for iron and cobalt catalysts and their applications
in the highly enantioselective hydrofunctionalization reactions (hydroboration
and hydrosilylation) of alkenes and alkynes. In ligand design, we
envisioned that chiral unsymmetric NNN-tridentate
(UNT) ligand scaffolds could promote these enantioselective transformations
with earth-abundant metals. Therefore, several types of chiral UNT
ligands were designed and prepared in our laboratory, utilizing readily
available natural amino acids as chiral sources. In the very beginning,
chiral oxazoline iminopyridine (OIP) ligands were proposed and investigated
through the rational combination of nitrogen-containing ligand scaffolds.
After a systematic survey of the ligand effects, the imine moiety
in the rigid OIP ligands was replaced by a conformationally more flexible
amine unit, leading to the construction of reactive oxazoline aminoisopropylpyridine
(OAP) ligands. Subsequently, imidazoline iminopyridine (IIP) and thiazoline
iminopyridine (TIP) ligands were prepared by altering the oxygen atom
of oxazoline with nitrogen and sulfur linkers, respectively. To further
expand the chiral ligand library, other tridentate ligands containing
a twisted pincer, anionic, and nonrigid backbone were also designed
and synthesized, including iminophenyl oxazolinyl phenylamine (IPOPA)
and imidazoline phenyl picolinamide (ImPPA). The efficacy of these
chiral UNT ligands for asymmetric induction in iron and cobalt catalysis
has been demonstrated through asymmetric hydrofunctionalization of
alkenes and asymmetric sequential hydrofunctionalization of alkynes,
which exhibit excellent reactivity as well as high chemo-, regio-,
and stereoselectivity with broad functional group tolerance. Notably,
highly regio- and enantioselective hydrofunctionalization of challenging
substrates, such as 1,1-disubstituted aryl alkenes and terminal aliphatic
alkenes, was also achieved. Furthermore, the development of asymmetric
sequential isomerization/hydroboration of internal alkenes and sequential
hydrofunctionaliz...
