Rhodium-catalyzed Cycloaddition Of Allenamides And Selective Addition Of Dicarbonyl Compounds To Internal Alkynes

Cyclobutanes are privileged structural motifs which constitute the core structure of many biologically active molecules including natural products,how to synthesize cyclobutanes through an efficient and atom-economy way is a hot topic in the organic synthesis field.In chapter one of this dissertation discloses a rhodium-catalyzed regio-and stereoselective head-to-head[2 + 2]cycloaddition of allenamides to constructed cyclobutane derivatives.After screening of the rhodium catalysts,phosphine ligands,solvents and temperature,various cyclobutane derivatives with different functional groups can be obtained in 63-89%yield.In chapter two,we had developed an efficient catalytic enantioselective[2+2]cycloaddition of allenamindes for synthesis of enantio-enriched cyclobutane derivatives(up to 99%enantioselectivity).The mechanistic studies demonstrated that rhodium catalyst coordinated with allenamide,which in turn occurring intermolecular nucleophilic capture by a zwitterion which generated from allenamide.Subsequent ring-closing process generates the cycloaddition products.In chapter three,we describes an efficient protocol for the regioselective addition of 1,3-dicarbonyl compounds to internal alkynes catalyzed by the combination of rhodium and Lewis acid catalysts.Throuth screening of the rhodium catalysts,Lewis acids,phosphine ligands,solvents and temperature,the corresponding branched and linear products can be selectively obtained in 47-99%yields and 63-90%yields,respectively.Moreover,retro-allylic alkylation process was observed in this transformation which can explain the selective formation of the branched/linear products.A possible mechanism was proposed that 1.3-dicarbonyl compounds activated by Lewis acids would undergo oxidative addition of C-H to the Rh(I)center to resulted in Rh-H species.Then,Markovnikov-selective hydrometalation to internal alkyne delivered a ?-vinylrhodium intermediate,which may undergo ?-hydride elimination to generate an allene.A subsequent Markovnikov-selective hydrometalation of this allene would afford a?-allylrhodium intermediate which may be attacked by 1,3-dicarbonyl compounds to furnish the desired branched product.