Studies On The Electrophilic Addition Reactions Of Allenes

Allenes are a class of compounds with a 1,2-diene functionality possessing two mutally perpendicularÏ€-orbitals.The unique reactivity of allenes was due to the presence of the two perpendicularÏ€-orbitals and the substituent-loading capability.In this dissertation,much attention has been focused on the study of the electrophilic addition reactions and electrophilic cyclization reactions in the presence of electrophiles.The whole dissertation can divided into three parts:Partâ… :Electrophilic addition of 1,2-allenylic sulfones with Br₂ and I₂.1.We have established the highly regio- and stereoselective E-bromohydroxylation E-bromination-elimination,and E-thirranation of 1,2-allenyl sulfones.The reaction pathway depends largely on the substitution pattern of the allene functionality.The intermediacy of these reactions has been fully established by isolation,X-ray diffraction characteritation,and the reactivity study of the five-membered intermediates.2.Based on the study of electrophilic addition reactions of 1,2-allenyl sulfones with Br₂,we have developed the highly regioselective E-iodohydroxylation E-iodonation-elimination reactions of 1,2-allenyl sulfones.Partâ…¡:Oxidative hydroacetoxylationof 1,2-allenylic sulfoxides.We have demonstrated that the reaction of 1,2-allenyl sulfoxides with HOAc afforded sulfonylallyl acetate esters highly regio- and stereoselectively.The axial chirality can be transferred into the center chirality in the final products highly efficiently.Partâ…¢:Electrophilic fluorination reactions of 2,3-allenoic acids and 3-aryl-1,2-allenes1.We have developed a convenient method for the efficient monofluorination via the electrophilic fluorocyclization reactions of 2,3-allenoic acids with Selectfluor in MeCN in the presence of 10 equiv.H₂O or even in pure water to affordβ-fluorobutenolides in moderate to high yields.2.We have developed a convenient method for the efficient synthesis of 2-fluoroalken-3-ols alcohols by the highly regioselective fluorohydroxylation reaction of 3-aryl substituted 1,2-allenes with Selectfluor.The observed results showed that fluorohydroxylation reaction demonstrates an aryl effect,which may be explained by its stabilization effect in the cationic allylic intermediate.