Polymerization of Alkyl Diazoacetates Initiated by Pd(Naphthoquinone)/Borate Systems: Dual Role of Naphthoquinones as Oxidant and Anionic Ligand for Generating Active Pd(II) Species

For polymerization of alkyl diazoacetates, the combination of a Pd complex bearing 1,4-naphthoquinone (NQ) or its derivatives as a ligand and borate, NaBPh 4, was found to be an efficient initiating system. The polymerization of ethyl diazoacetate by a Pd(0) complex having two NQ molecules [Pd(nq) 2 ] with NaBPh 4 proceeded to give poly(ethoxycarbonylmethylene)s with a relatively high molecular weight (up to M n = 36 kDa) in good yield (∼70%). This initiating system was also effective for polymerizing other diazoacetates, benzyl and cyclohexyl diazoacetates. In addition, in the presence of NaBPh 4, a novel Pd(II) complex bearing an anionic naphthoquinonyl ligand derived from 2,3-dichloro-1,4-naphthoquinone (dichlone), [Pd(cod)(Cl-nq)Cl] (cod = 1,5-cyclooctadiene), which was newly prepared by treatment of a Pd(0) precursor, Pd 2 (dba) 3 ·CHCl 3 [dba = ( E,E )-dibenzylideneacetone], with COD and dichlone, was capable of affording poly(alkoxycarbonylmethylene)s with much higher stereoregularity compared to the previously reported Pd-based initiating systems, despite a rather low polymer yield (∼20%). For both Pd(nq)-based initiating systems, Pd–Ph species generated by transmetalation with NaBPh 4 were responsible for the initiation of diazoacetates based on MALDI-TOF-MS analyses of the resulting polymers, and naphthoquinones played unique important dual roles as both an oxidant and an anionic ligand for generating the active Pd(II) species.