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.
Extensive effort has been expended to utilize π-allyl palladium-complexes as electrophilic allyl donor intermediates in cooperative dual catalysis, but their counter anions such as carboxylates or alkoxides are almost always discarded as waste. We have developed a cooperative Pd(0)/Rh(II) dual catalysis system that utilizes both the electrophilic allyl and nucleophilic counter anion functionalities inherent in the starting allylic substrates. In this cooperative catalysis, redox compatible Pd(0) and Rh(II) catalysts selectively activate allylic substrates and N-sulfonyl-1,2,3-triazoles to generate π-allyl Pd(II)-complexes and 1,3-ambivalent equivalent aimino Rh(II)-carbenoid intermediates, respectively. The counter anion of the π-allyl Pd(II)-complex acts as a nucleophile transferring to the electrophilic carbenic carbon to form Pd/Rh-associated zwitterionic intermediates, in which the cationic palladium species may coordinate with both counter anion and imine nitrogen in the same plane establishing the (Z)-geometry of the products.
Described herein is an organoboranecatalysed consecutive borylative reduction of quinolines and isoquinolines to furnish tetrahydro(iso) quinolines bearing a C(sp 3 )À B bond β to the nitrogen atom. The installed CÀ B bond is oxidatively transformed to the hydroxy group in one pot. The present double hydroboration is proposed to proceed via a stepwise ionic mechanism involving a boronium ion. The stereo-outcome was found to be dependent on the position (C2 vs C4) of the substituents in quinolines.
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