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Synthesis and characterization of polymer linked copper(I) bis(N‐heterocyclic carbene) mechanocatalysts

ABSTRACT

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 1H‐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 1H‐NMR spectroscopy introducing thus “click” conversions up to 97%. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3893–3907

The synthesis and characterization of an initially latent mechanocatalyst based on PIB‐, PS‐, and PTHF‐based copper(I) bis(N‐heterocyclic carbene) (NHC) complexes is reported demonstrating their force‐dependent activation via ultrasound. Systematic GPC investigations show the chain length dependent cleavage of one shielding NHC ligands forming the monocarbene complex. The mechanocatalysts are activated for the copper(I)‐catalyzed azide/alkyne cycloaddition (CuAAC) of benzyl azide and phenylacetylene, revealing “click” conversions of up to 97%.

Authors:   Philipp Michael, Shima Khazraee Sheidaee Mehr, Wolfgang H. Binder
Journal:   Journal of Polymer Science Part A: Polymer Chemistry
Volume:   55
edition:   23
Year:   2017
Pages:   3893
DOI:   10.1002/pola.28775
Publication date:   29-Sep-2017
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