Synthesis and Application of Phosphorus Dendrimer Immobilized Azabis(oxazolines)

Abstract

Phosphorus dendrimer immobilized azabis(oxazoline) ligands can be efficiently synthesized up to the third generation with 48 ligand molecules being attached to the periphery using click chemistry. The so-assembled macromolecules were evaluated in copper(II)-catalyzed asymmetric benzoylations, showing good yields and enantioselectivities. Moreover, the copper(II)-catalysts could be readily recovered and reused in several cycles. The globular structure of the dendritic ligands seems to prevent interference of the triazole moieties in the catalysis, contrasting MeOPEG or polystyrene bound ligands of the same type.

Polytopic Oxazoline‐Based Chiral Ligands for Cyclopropanation Reactions: A New Strategy to Prepare Highly Recyclable Catalysts

“…[7] Ligands 3 were obtained in high purity by metalation of azabis(oxazoline) 1 [8] with butyllithium and subsequent trapping with α,α'-dibromo-p-xylene, followed by recrystallization in acetone (> 80% yield).…”

“…[8] The same strategy has also been described for the immobilization of other chiral ligands. [9] Copper-catalyzed [3+2]-cycloaddition [10] between 2 and two equivalents of 1,4-bis(azidomethyl)benzene were reacted to give rise to 5 in approximately 90% yield.…”

“…The starting azabis(oxazolines) and 1,4-bis(azidomethyl)benzene were prepared according to literature procedures. [8] Synthesis of (4S,4'S)-N,N'-(1,4-phenylenebis(methylene))bis(4-tert-butyl-N-(4-tert-butyl-N-(4-tert-butyl-4,5-dihydrooxazol-2-yl)-4,5-dihydrooxazol-2-amine) (tBuDAX) (3a). (S)-4-tert-butyl-4,5-dihydro-oxazol-2-yl)-(4,5-dihydro-oxazol-2-yl)-amine (1.00 mmol, 267 mg) was dissolved in tetrahydrofuran (10 mL) and a 15% solution of n-butyllithium in hexane (1.50 M, 688 µL) was added at -78 ºC.…”

Combining Fluorous and Triazole Moieties for the Tagging of Chiral Azabis(oxazoline) Ligands

“…[6] Recent studies on catalyst immobilization by us [6] and other groups [7] revealed that the copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a facile method for grafting catalysts onto polymers or nanoparticles. Using an azido-functionalized support and an alkyne-substituted ligand or metal-ligand complex, high loading of the immobilized derivative close to the maximum capacity of the support can be achieved.…”

Efficient Heterogeneous Asymmetric Catalysis of the Mukaiyama Aldol Reaction by Silica‐ and Ionic Liquid‐Supported Lewis Acid Copper(II) Complexes of Bis(oxazolines)

“…In this regard, phosphorus dendrimer-immobilized azabis(oxazolines) form highly efficient catalysts for the asymmetric benzoylation of rac-diols with excellent recyclability. [13] In addition to the problem of leaching, the activity of an immobilized catalyst can be strongly influenced by the environment around the metal on the surface and as a result, many reactions are mass transfer limited and suffer from significant catalyst instability as well as a reduction in enantioselectivity. [14] Ionic liquids have been attracting increasing interest as a medium for catalysis as their low volatility and immiscibility with non-polar organic solvents makes them ideal candidates for catalyst immobilisa-tion and recycling.…”