The development of
a switchable strategy to control the catalytic
action of dual active species is of great significance toward the
precise manipulation of a cascade reaction. Herein, by combining water-soluble
thermoresponsive polymer and hollow-shell-structured mesoporous silica
as an integrated support, we develop a form of switchable-type supported
molecule catalysts by tethering the achiral organic functionality
in the outer polymer coating layers and anchoring the chiral ruthenium/diamine
functionality in the inner mesoporous silica’s nanochannels.
As presented in this study, the created on and off modes of water-soluble
thermoresponsive polymer coating layers on the external surface of
silica shell can open and close the entrances of the nanochannels,
thereby selectively initiating or terminating the catalytic action
of the chiral ruthenium/diamine species within the nanochannels. As
we envisaged, the switchable bifunctional catalysts are able to manipulate
catalytic cascade sequences for the Suzuki cross-coupling/asymmetric
transfer hydrogenation of iodoacetophenones and aryl boronic acid,
and the aza–Michael addition/asymmetric transfer hydrogenation
of enones and arylamines.
The combination of biocatalysis and transitionmetal catalysis can complement synthetic gaps only in a chemical or biological process. However, the intrinsic mutual deactivation between enzymatic and chemical species is a significant challenge in a single operation. To address the above issue, we developed an encapsulated Au/carbene combined with a free amine dehydrogenase as a co-catalyst system that enables an efficient hydration/amination enantioselective cascade process to be accomplished. The mechanistic investigation discloses dual catalysis comprised of alkyne hydration, followed by a reductive amination process.
Development of an efficient cocatalyst
system to eliminate the
intrinsic conflict of the cross-interactions in a pair of cocatalysts
and to overcome the extrinsic conflict of the reaction conditions
in an unmatched reaction environment has great significance in asymmetric
dual catalysis. Herein, a compartmentalization method involving the
integration of a cocatalyst system has been developed, which enables
an efficient Michael addition/reduction enantioselective dual-catalysis
process to be accomplished from a noncompatible system. In this process,
the chiral squaramide species is encapsulated within the cavity of
one hollow-shell-mesostructured silica support, whereas the chiral
organoruthenium/diamine species is entrapped within the cavity of
another water-soluble thermoresponsive polymer-coating hollow-shell-mesostructured
silica support. This shielding feature together with the reversible
on–off mode of the water-soluble thermoresponsive polymer not
only controls the cross-interactions of dual species but also harmonizes
the reaction conditions. As we envisioned, the one-pot sequential
Michael addition of α-nitrosulfone and enones followed by asymmetric
transfer hydrogenation provides various 1,4-distereocentered chiral
δ-hydroxysulfones with enhanced yields and enantio/diastereoselectivities.
Cycloisomerization of 1,6-enynes to access azabicyclo[4.1.0]heptenes was achieved in the presence of commercially available [RuCl 2 (CO) 3 ] 2 after a comprehensive study on the electronic properties of ruthenium complexes. A series of 1,6-enynes bearing an internal/terminal alkyne motif proved to be good candidates for substrates, giving their corresponding bicyclo[4.1.0]heptenes in good to excellent yields. Substitution patterns on either alkyne or alkene have no significant influence on the yields of bicyclic adducts.[a] Dr.
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