Synthesis and Characterization of Polymer Linked Copper(I) Bis(N-heterocyclic carbene) Mechanocatalysts

In this paper, the synthesis and characterization of a series of latent polymeric bis(N-heterocyclic carbene) (NHC) copper(I) complexes is reported, which can be activated for the copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) via ultrasound. To prove the influence of chain length and nature of the polymer towards the activation, poly(isobutylene) (PIB), poly(styrene) (PS) and poly(tetrahydrofuran) (PTHF) are synthesized via living polymerization techniques (LCCP, ATRP, CROP) obtaining different chain lengths (from 2500 to 9000 g/mol), followed by quaternization with N-methylimidazole, generating the corresponding N-methylimidazolium-telechelic polymers. The deprotonation of these macroligands via strong bases like sodium tert-butoxide (NaOtBu) or potassium hexamethyldisilazide (KHMDS) yields the free N-heterocyclic carbenes (NHCs), which are used to coordinate to tetrakis(acetonitrile) copper(I) hexafluorophosphate, forming the final polymer-based mono-and bis(N-methylimidazole-2-ylidene) copper(I) X complexes. The structural proof of these complexes is accomplished via H-1-NMR spectroscopy, MALDI-TOF-MS and GPC-techniques. The activation of the copper(I) biscarbene catalysts by ultrasound is studied by GPC, revealing the cleavage of one shielding NHC-ligand. The initial catalytic latency and the via ultrasound introduced catalytic activation is successfully demonstrated monitoring a CuAAC "click" reaction of benzyl azide and phenylacetylene by in situ H-1-NMR spectroscopy introducing thus "click" conversions up to 97%. (C) 2017 Wiley Periodicals, Inc.