Dirhodium complex immobilization on modified cellulose for highly selective heterogeneous cyclopropanation reactions

Abstract

A novel, efficient approach for the functionalization of microcrystalline cellulose (MCC) is presented. The as-obtained material allows the immobilization of chiral dirhodium catalysts preserving their enantioselectivity in asymmetric cyclopropanation reactions. As model, microcrystalline cellulose is modified with a polyethylene glycol derived linker, and Rh2(S-DOSP)4 is grafted on the material to produce a heterogeneous catalyst. SEM images at different stages of the immobilization show an unchanging uniform morphology, providing constantly good separation characteristics. The modification of the cellulose material with the polyethylene derived linker and the immobilization process are monitored using DNP enhanced 1H → 13C CP MAS NMR, quantitative 19F MAS NMR, TGA and ICP-OES analysis, confirming the success of the immobilization as well as the stability of bonds between the used linker molecule and the cellulose material. Finally, the evaluation of the produced catalyst is demonstrated in the asymmetric cyclopropanation reaction between styrene and methyl(E)-2-diazo-4-phenylbut-3-enoate showing excellent enantioselectivity with an ee of nearly 90% over a wide temperature range as well as good recyclability characteristics in four consecutive catalysis cycles.

Peptides as Model Systems for Biofunctionalizations of Cellulose─Synthesis and Structural Characterization by Advanced Solid-State Nuclear Magnetic Resonance Techniques

“…This strong correlation is attributed to the proximity of the C�O groups to CH 2 of the glycine in the peptide sequence. This hypothesis is supported by the 13 ms (Figure S11), where a correlation is visible at (171 ppm, 42 ppm), which is attributed to the proximity of the 13 C�O group to CH 2 of the glycine.…”

“…The small chemical shift difference of about 1 ppm in the f2 dimension compared to the abovediscussed one (172 vs 171 ppm) may be related to the highly disordered structure of our investigated model system. Surprisingly, a correlation of the signal of 13 C�O with the C 1 signal is not visible in the spectra, probably due to the long distance between C 1 and 13 C�O in the system.…”

“…5 ) to 45.8 ± 3.2 μmol/g when using 2 equiv of the peptide to 1 equiv of glucose units in MCC in the synthesis. This low value let us assume that for deeper structure analysis of the peptide on MCC, i.e., by 1D or 2D 13 C solid-state MMR techniques, a sensitivity boost is required. This sensitivity enhancement can in principle be reached by combination of solid-state NMR with DNP and/or isotope labeling as discussed in the next section.…”

“…Employing DNP-enhanced 1 H → 13 C CP MAS NMR, the intactness of the peptide after the reaction with MCC has been confirmed. For analysis of the binding of the peptide, L-glycine (Gly) has been replaced by 13 C-labeled L-glycine (Gly( 13 C)) in the sequence. DNP-enhanced 13 C− 13 C DARR spectra have proven the proximity between the terminal carbon of the 13 C�O peptide and the MCC carrier.…”

“…The mixing time (τ mix ) was set to 500 or 50 ms. The π/2 pulse length on the 13 C channel was set to 4 μs. Spectra were recorded with 832 slices, where each was accumulated with 64 scans.…”