Cooperative
catalytic systems are making significant advances in
modern organic synthesis due to the potential to combine multiple
catalytic cycles or enable enzyme-like proximity effects. We report
the rational design of a bifunctional helical peptide catalyst that
displays an imidazolidinone catalyst in close proximity to a thiourea
binding site and enables proximity-enhanced reactivity and selectivity.
The helical structure of the peptide and the binding of both reactants
are shown to be essential for enhanced reactivity in Diels–Alder
and indole alkylation reactions, and up to 28 000 catalyst
turnovers are achieved. A variety of Lewis basic functional groups
facilitate binding and proximity-enhanced reactivity, and product
selectivity is observed that cannot be achieved in the absence of
the peptide template.
We
report the synthesis of bimetallic Pd(I) and Pd(II) complexes
with bidentate 2-phosphinoimidazole ligands and their catalytic activity
to generate substituted naphthalenes. This process involves the coupling
of an aryl iodide and 2 equiv of a ketone via sequential ketone α-arylation
and then annulation to generate disubstituted and tetrasubstituted
naphthalenes in a regioselective manner. Excellent substrate scope
for both aryl iodide and ketone partners is demonstrated, including
that for heteroaryl iodides. Bimetallic Pd complexes are much more
reactive than monometallic Pd catalysts in this transformation. Density
functional theory calculations, isotope effect experiments, and substrate
competition experiments were used to examine bimetallic mechanisms,
reactivity, and selectivity.
The palladium-catalyzed [3 + 2] cycloaddition
using in
situ generated sulfone-TMM species to construct various chiral
cyclopentyl sulfones in a highly regio-, diastereo- (dr >15:1),
and
enantioselective (up to 99% ee) manner is reported. The present strategy
can tolerate different types of sulfone-TMM donors and acceptors,
and enables the construction of three chiral centers in a single step,
specifically with a chiral center
bearing the sulfone moiety. The robust chiral diamidophosphite ligand
is the key to the reactivity and selectivities of this transformation.
We report the synthesis of bimetallic Pd(I) and Pd(II) complexes scaffolded on bidentate 2-phosphinoimidazole
ligands. These complexes display unique catalytic activity and enable the expeditious formation of 1,3-disubstituted naphthalenes via an unprecedented coupling of
aryl iodides and methyl ketones in the presence of silver triflate. Excellent substrate
scope for naphthalene formation is also demonstrated. Mechanistic studies suggest
that the transformation proceeds via Pd-catalyzed arylation of a methyl ketone, followed by cyclization with a second equivalent of ketone. Importantly, this ketone
arylation processes occurs under oxidizing conditions, suggesting involvement of
higher oxidation state dimeric Pd catalysts. Based on experiments and DFT calculations, we propose a mechanism involving high oxidation state Pd(III) bimetallic catalysis. These new bimetallic complexes
possess reactivity that is not seen with monometallic Pd catalysts and we confirm the importance of the palladium catalyst
for both arylation and cyclization for naphthalene formation.
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