Abstract
An efficient cobalt-catalyzed reductive coupling reaction of organic triflates and tosylates with electron-withdrawing alkenes (CH2=CR1EWG, EWG = electron-withdrawing group) in the presence of water and zinc powder in acetonitrile for 10 to 24 hours to give the corresponding Michael-type addtiton product (RCH2CR1EWG) was described. The methodology is versatile such that unactivated primary, secondary alkyl triflates and tosylates and various conjugated alkenes including acrylates, acrylonitrile, methyl vinyl ketone and phenyl vinyl sulfone all successfully participate in this coupling reaction. It is a unique method employing CoI2(dppe) or CoI¬2(PPh3)2, zinc and alkyl triflates and tosylates, affording conjugate addition products in high yields. Mechanistically, the catalytic reaction is likely to be initiated by the reduction of Co(II) to Co(I) by zinc power and followed by oxidative addition of organic triflates and tosylates to give Co(III) intermediate. The coordination followed by insertion of double bond to the Co center and then transmetalation of the latter with the zinc generates zinc intermediate. Protonation of this zinc intermediate with water forms the final reductive coupling product.