DIVERSIFIED REACTIONS OF ENOL DIAZOACETATE IN CHEMICAL CATALYSIS
Doyle, Michael P
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Silylated enol diazoacetates, prepared by the selective O-silylation of diazoacetoacetates in the presence of a mild base, are extremely valuable in synthesizing noval heterocyclic compounds. The incorporation of a silyl enol unit in the α-diazoacetate moiety creates a unique molecule that displays multifaceted reactivities under the catalysis of conceptually different catalytic systems. Upon catalytic dinitrogen extrusion of silylated enol diazoacetate by dirhodium catalysis, the corresponding rhodium bound enol carbene intermediate resembles a metallo-1,3-dipolar species and undergoes a formal dearomatizing [3+3]-cycloaddition with isoquinolinium/pyridinium methylides that furnishes substituted quinolizidines. The development of the catalytic asymmetric variant of this cycloaddition reaction by catalysis with the previously unreported chiral dirhodium carboxylate catalyst−Rh2(S-PTIL)4 has allowed convenient access to highly enantioenriched quinolizidines. Coordination of the Lewis basic methylides to dirhodium(II) prompts the rearrangement of the enol carbene that is bound to dirhodium to produce a donor−acceptor cyclopropene which undergoes a diastereoselective [3+2]-cycloaddition with isoquinolinium/pyridinium methylides. With the overall reaction outcome controlled by the amount of catalyst used, an increase in the mol % of catalyst loading suppresses the [3+2]-cycloaddition pathway. In a reaction that proceeds under mild conditions with remarkable functional group tolerance, catalytically generated rhodium enol carbene intermediate reacts with nucleophilic silylated ketene imines and produced structurally diverse 3-amino-2-cyclopentenones bearing a quaternary carbon at the 4-position in high efficiency. The key step for the overall transformation emanates from the [3+2]-cycloaddition of silylated ketene imine and rhodium enol carbene with the nucleophilic silylated ketene imine attacking the vinylogous position of rhodium enol carbene. Under Lewis acid catalysis, silylated enol diazoacetates participates in diastereoselective [3+2]-cycloaddition reactions with azomethine imines to produce highly functionalized β-methylene-β-silyloxy-β-amido-α-diazoacetates. Catalyst-directed selectivity of competitive 1,2-C→C, -O→C and -N→C migrations from the β-methylene-β-silyloxy-β-amido-α-diazoacetates demonstrates that the differential selectivities rely on the control of the stereoelectronic property of the catalytically generated metal carbenes.