Copper-Catalyzed Formal Dehydration Polymerization of Propargylic Alcohols via Cumulene Intermediates

Abstract

Here we report a copper-catalyzed formal dehydration polymerization of propargylic alcohols. Copper catalysis allows for efficient in situ generation of [n]cumulenes (n = 3, 5) by a soft deprotonation/β-elimination pathway and subsequent polymerization via organocopper species. Alkyne polymers (M n up to 36.2 kg/mol) were produced with high efficiency (up to 95% yield) and excellent functional group tolerance. One-pot synthesis of semiconducting head-to-head poly(phenylacetylene) was demonstrated through a polymerization−isomerization sequence.

Synthesis of Polydiynes via an Unexpected Dimerization/Polymerization Sequence of C3 Propargylic Electrophiles

“…Notably, despite that a variety of nucleophiles have been explored in Cu-catalyzed propargylic substitutions, there have been very few precedents either (1) using acetylenic nucleophiles or (2) involving α-selective coupling. , The identification of such a pathway is thus interesting per se and might be useful for the preparation of 1,3-diynes and related structures in the context of small molecules. Meanwhile, this observation establishes a mechanistic connection to our recent report on the Cu-catalyzed polymerization of cumulenes generated in situ by remote elimination, where the polymerization of 9′ was shown to give the same diyne-based backbone . Therefore, we hypothesized that the elimination of C1 would furnish a transient [5]­cumulene intermediate D1 , which was the true monomer for propagation …”

“…The quest of the origin of this unexpected regiochemistry led to the identification of a highly regioselective α–α dimerization pathway of A , which is previously elusive. Based on these results and our recent finding with copper-catalyzed polymerization of C4 and C6 monomers, a tandem dimerization/chain-growth polymerization mechanism was proposed.…”

“…The energy levels of its frontier orbitals were probed electrochemically, and a band gap of 2.43 eV was estimated (Figure S48). The backbone of P9′ can be viewed as head-to-head poly­(phenylacetylene) (HH-PPA) with periodical CC bond insertions. , Such a simple modification significantly lowers both the HOMO and LUMO energies (from −5.05 to −5.85 and −2.78 to −3.42 eV, respectively) and blue-shifts the absorption band (Figure S49). A similar trend has been reported in polyenyne systems by Choi and co-workers .…”

“…Meanwhile, this observation establishes a mechanistic connection to our recent report on the Cu-catalyzed polymerization of cumulenes generated in situ by remote elimination, where the polymerization of 9′ was shown to give the same diyne-based backbone. 7 Therefore, we hypothesized that the elimination of C1 would furnish a transient [5]cumulene intermediate D1, which was the true monomer for propagation. 23 Based on the polymer structure, kinetic studies, smallmolecule reactions, and relevant literature, we have proposed a tentative mechanistic picture for C3-CPPE (Figure 5f).…”

Alkyne Polymers from Stable Butatriene Homologues: Controlled Radical Polymerization of Vinylidenecyclopropanes

“…Prominent examples include ring-opening alkyne metathesis polymerization, cascade metathesis/metallotropy polymerization, and catalyst transfer Sonogashira polycondensation . We recently disclosed the copper-catalyzed condensation polymerization of propargylic electrophiles (CPPEs) . CPPE is distinct in using in situ formed [ n ]­cumulenes ( n = 3, 5) for a chain-growth process.…”