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

Synthesis and Mechanochemical Activity of Peptide-Based Cu(I) Bis(N-heterocyclic carbene) Complexes

“…Subsequently, the catalytic activity of the synthesized catalysts were investigated using a model CuAAC “click” reaction of benzyl azide ( 14 ) and phenylacetylene ( 15 ). Cu(I) bis(NHC) complexes are known for their mechanoresponsive behavior since they can be switched from their latent, inactive state to their active state after exposure to mechanical stress, for example in the form of ultrasound (in solution) or by compression (in bulk) [9,60,63]. In its latent state the Cu(I) center is surrounded by the two shielding NHC ligands which prevent the alkyne from coordination to the copper(I) center.…”

Mechanochemical Activation of Fluorogenic CuAAC “Click” Reactions for Stress‐Sensing Applications

“…Transforming mechanical force into a chemical response has become a major issue in material science as a new approach of triggering and tuning chemistry . A plethora of chemical bonds have been designed to enable not only the defined breaking of a specific bond (such as cyclopropanes, (benzo)‐cocyclobutenes, spiropyrans, anthracenes, diarylbenzofuranes, or 1,2‐dioxetanes) but also the induction of catalytic reactions based on decoupling metal–ligand complexes . Thus the mechanical force can be directly used to trigger chemical crosslinking reactions such as the copper(I)‐catalyzed alkyne/azide “click” cycloaddition (CuAAC) or ROMP for self‐healing applications.…”

Biomimetic Elastin-Like Polypeptides as Materials for the Activation of Mechanophoric Catalysts

“…The switching of latent Cu(I) bis (NHC) complexes to their active state after exposure to mechanical stress, for example in the form of ultrasound (in solution) or compression (in bulk), has been previously described. 28,29,31,33,42 While in its latent state, the Cu(I) center is shielded by the two NHC ligands, preventing the alkyne from coordination to the Cu(I). Once an external force is applied, one of the NHC ligands is cleaved off from the Cu(I) center due to the rupture of the copper-carbene bond.…”

“…12,19,20 Mechanophores contain one or more mechanically labile bonds, functional groups that are inactive in their initial state but can respond to an external applied force. Once activated, mechanophores can undergo a variety of mechanochemical transformations such as color changes, [21][22][23] chemiluminescence, 24,25 activation of latent metal catalysts, [26][27][28][29][30][31][32][33][34] biased reactivity, 35 release of small molecules, 36,37 generation of protons, 38 and also remodeling of polymers. 39 Besides the correct positioning of the mechanophoric bond near the center of the molecule, the attached polymer chains play a crucial role since they take up and transmit the force along the polymer backbone.…”

“…40,41 We previously have designed latent Cu(I) bis(N-heterocyclic carbene) (NHC) catalysts as mechanophores for the time-and spatial-controlled copper(I)catalyzed alkyne-azide "click" reaction (CuAAC). 28,29,31,33 The ability of the attached polymer chains to transmit an applied force was controlled via the T g of the chosen polymer [poly(isobutylene), poly(styrene), and poly(tetrahydrofuran)]. Exchanging the synthetic polymer handles with peptides thus is a promising way to study force transmission, since the resulting β-spiral structures could be exploited as "molecular springs.…”