Abstract
The boracarboxylation of vinyl arenes catalyzed by copper(I) is an alkene difunctionalization reaction that provides synthetically useful beta-boryl-alpha-aryl propanoic acid derivatives. Drawbacks of the original reaction methodology are high catalyst loading and vinyl arene scope limited largely to electron-rich systems. Herein, we demonstrate that catalytic additives, specifically triphenylphosphine or electron-rich styrene, leads to improved catalyst efficiency and broadening of substrate scope. With the addition of PPh3, comparable yields of previously reported boracarboxylated substrates as well as two novel beta-boronated non-steroidal anti-inflammatory drugs (bora-fenoprofen and bora-flurbiprofen) were achieved at lower catalyst loading. Boracarboxylation of electron-deficient substrates could be achieved through superstoichiometric addition of vinyl arene or addition of secondary phosphine ligand. Reactivity optimization and competition experiments have provided preliminary insights into the ability of PPh3 to aid in the catalytic cycle, specifically carboxylation of electron-rich benzyl-copper(I) intermediate. Additional competition studies revealed that a catalytic amount of an electron-rich vinyl arene could be used in addition to the copper(I) catalyst to promote boracarboxylation of an electron-deficient styrene derivative.