Synergistic catalysis, a type of
plural catalysis which utilizes
at least two different catalysts to enable a reaction between two
separately activated substrates, has unlocked a plethora of previously
unattainable transformations and novel chemical reactivity. Despite
the appreciable utility of synergistic catalysis, specific examples
involving two transition metals have been limited, as ensuring a judicious
choice of reaction parameters to prevent deactivation of catalysts,
undesirable monocatalytic event(s) leading to side products, or premature
termination and other potentially troublesome outcomes present a formidable
challenge. Excluding those driven by photocatalytic mechanisms, this
review will highlight the reported examples of reactions that make
use of two simultaneous catalytic cycles driven by two transition
metal catalysts.
A multifunction
Pd/Sc(OTf)3/ionic liquid catalyst for
the tandem one-pot conversion of phenol to ε-caprolactam is
reported. Pd and Sc(OTf)3 cooperate to catalyze the hydrogenation
of phenol to cyclohexanone with excellent conversion (>99.9%) and
selectivity (>99.9%), whereas Sc(OTf)3 and an ionic
liquid,
[bmim][PF6], cooperate to catalyze the tandem transformation
of the resulting cyclohexanone to cyclohexanone oxime and the Beckmann
rearrangement affording ε-caprolactam.
The first example of a highly regio- and stereoselective catalytic method for the three-component one-pot synthesis of highly functionalized α-vinylated γ-oxo-β-amino esters is disclosed. In this catalytic triad, the Cu(I)-catalyst selectively catalyzes the cycloaddition of the 1-alkyne and sulfonyl azide first resulting in the corresponding 1-sulfonyl-1,2,3-triazole. An α-imino Rh(II)-carbene is generated from an open-chain α-imino diazo of the triazole, and this species reacts with γ-hydroxy α,β-unsaturated esters to form allylic (Z)-amino vinyl ethers. Rapid deconjugative [3,3]-sigmatropic rearrangement affords the α-vinyl γ-oxo-β-amino esters in high yields with high levels of diastereoselectivity.
A novel, one-pot route for the synthesis of nonaromatic ring-fused 1,4-oxazepines and 1,4-oxazines has been developed. The reaction features a sequential rhodium(II)-catalyzed reaction of N-sulfonyl-1,2,3-triazoles with glycidols, followed by a regioselective Lewis acid Mg(OBu)-catalyzed intramolecular ring-opening reaction. It has been found that the regioselectivity in the epoxide ring-opening was largely determined by the substituents on the glycidols. Thus, substituted glycidols (R ≠ H) afforded seven-membered oxazepine derivatives selectively, while unsubstituted glycidols (R = H) afforded six-membered oxazine derivatives. Plausible reaction pathways are elucidated and supported by experiments with several glycidols bearing different substituents around the epoxide functionality.
A rhodium(ii)-catalyzed coupling of 1-sulfonyl-1,2,3-triazoles, prepared from 1-alkynes and sulfonyl azides, with Morita-Baylis-Hillman (MBH) adducts afforded highly functionalized α-methylene-δ-oxo-γ-amino esters in excellent yields with broad functional group tolerance. This transformation can also be successfully accomplished as a multicomponent all-in-one-pot reaction of 1-alkynes, sulfonyl azides and MBH adducts in the presence of Cu(i) and Rh(ii) catalysts.
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