Electron-rich Phosphines Catalyzed Head-to-Tail Dimerization of Renewable Alkyl Crotonates

Herein, a facile head-to-tail dimerization of alkyl crotonates using electron-rich N,N'-bis(imidazolyl) guanidinylphosphines (BIG-Ps) as organocatalysts was firstly developed. This reaction proceeded smoothly under mild reaction conditions, effectively generating a series of functionalized 2-ethylidene-3-methylpentanedioates in excellent yields and stereoselectivity. Furthermore, this method was successfully applied for the synthesis of unsaturated polyesters by employing alkyl dicotonates as monomers. More importantly, thermal performance and crystallinity of the resulting polyesters could be effectively regulated through post-polymerization modification using a thiol-ene "click" reaction. The results of mass spectrometry experiments revealed that the head-to-tail dimerization of alkyl cotonates tends to proceed via a BIG-Ps-mediated nucleophilic addition mechanism, analogous to that proposed for the classical Rauhut-Currier reaction.