Green Synthesis of Cinnamic Acid and Derivatives Using Palladium N-Heterocyclic Carbene Catalyst

Abstract

Cinnamic acids and esters are widely used intermediates for the fine chemical and pharmaceutical Industries. In the current studies, cinnamic acids and cinnamates were synthesized via Mizoroki-Heck cross-coupling reaction mediated through palladium N-heterocyclic carbene as catalyst. The catalyst was synthesized through double N-benzylation of benzimidazole to yield 1,3-di benzylbenzimidazolium bromide (the pre-carbene) (2). The pre-carbene was then reacted with palladium bromide (PdBr₂) in 3-methylpyridine, to yield the pre-catalyst (Dibromido[(1,3-dibenzylbenzimidazole-2-ylidene) (3-methylpyridine)] palladium (II) (3)). The intermediates and the pre-catalyst were characterized using ¹H and FT-IR analyses which supported the proposed structures. The pre-catalyst was used to develop a method for the synthesis of cinnamic acids and cinnamates via Mizoroki-Heck cross-coupling reactions.  In the substrate scope studies, the pre-catalyst (3) demonstrated excellent air and moisture stability and compatibility with both activated and un-activated and deactivated aryl bromides.