Optically Active Hyperbranched Polyglycerol as Scaffold for Covalent and Noncovalent Immobilization of Platinum(II) NCN-Pincer Complexes. Catalytic Application and Recovery

Abstract

New optically active hyperbranched polymers have been used as microenvironments as well as scaffolds for noncovalent and covalent immobilization of pincer platinum(II) complexes, respectively. The catalytic activity/selectivity of the incorporated platinum(II) complexes in these polymeric chiral supports was investigated. Chiral amphiphilic hyperbranched polyglycerols with core−shell “nanocapsules” structure, namely (−)-P(G40C160.5) (1) and (+)-P(G73C160.5) (2), have been prepared in two straightforward steps by ring-opening multibranching polymerization (ROMBP) of either (−)- or (+)-glycidol, resulting in (−)-PG40 (Mn = 3000, Mw/Mn = 1.3) or (+)-PG73 (Mn = 5500, Mw/Mn = 1.6), respectively. Polymerization was followed by partial esterification of a certain fraction of OH groups with palmitoyl chloride. Multifunctional hydrophilic NCN-pincer platinum(II) complexes with sulfonate groups, [PtCl(NCN-SO3H)] (3; NCN = 2,6-bis[(dimethylamino)methyl]phenyl anion), were encapsulated in 1 and 2. The NCN-pincer platinum(II) species encapsulated in the chiral nanocapsules 1 + 3 and 2 + 3 show only slight CD activity with intensity bands of opposite sign, +0.4 mdeg and −0.5 mdeg, respectively, at 320 nm. Pertosylation of the chiral polyglycerols (−)-PG40 and (+)-PG73 afforded the fully tosylated hyperbranched polymers (−)-PG40Tos1.0 (4) and (+)-PG73Tos1.0 (5). Partial displacement of the tosyl groups of 4 with potassium carboxylate functionalized NCN-pincer platinum(II) complexes [PtI(NCN-COOK)] (7) resulted in covalent attachment of the NCN-pincer complexes to the polyglycerol (8). CD spectra of the tosylated compounds 4 and 5 were recorded, showing opposite signals at 256 nm of −1.8 and +2.0 mdeg, respectively, which confirm retention of the chirality during the pertosylation. The decrease in CD intensity of 8 compared to 4 might be attributed to either partial inversion of chirality or racemization during functionalization. The noncovalent encapsulated Pt(II) complexes 1 + 3 and 2 + 3 as well as the covalently attached polymers 8 showed catalytic activity in Michael additions between methyl vinyl ketone and ethyl α-cyanopropionate, although no enantiomeric excess was observed. Quantitative recuperation of these catalysts from catalytic reactions was achieved by dialysis. The combination of the nanosize of the hyperbranched polymer with the catalytic activity of pincer platinum complexes renders these materials promising with respect to application in membrane reactors.