Browsing by Author "Dechert, Sebastian"

Abstract Indolizine-carbaldehydes with the easily modifiable carbaldehyde group are important synthetic targets as versatile precursors for distinct indolizines. However, the efficient one-pot construction of trisubstituted indolizine-2-carbaldehydes represents a long-standing challenge. Herein, we report an unprecedented recyclable stereoauxiliary aminocatalytic approach via aminosugars derived from biomass, which enable the efficient one-pot synthesis of desired trisubstituted indolizine-2-carbaldehydes via [3+2] annulations of acyl pyridines and α , β -unsaturated aldehyde. Compared to the steric shielding effect from α -anomer, a stereoauxiliary effect favored by β -anomer of D-glucosamine is supported by control experiments. Furthermore, polymeric chitosan containing predominantly β -D-anhydroglucosamine units also shows excellent catalytic performance in aqueous solutions for the conversion of various substrates, large-scale synthesis and catalytic cycling experiments. Thus, our approach advances the existing methodologies by providing a rich library of indolizine-2-aldehydes. In addition, it delivers an efficient protocol for a set of late-stage diversification and targeted modifications of bioactive molecules or drugs, as showcased with 1,2,3-trisubstituted indolizine-2-carbaldehydes.

A hexanuclear cluster containing six {Cp*Dy} units shows a high energy barrier ( U eff = 561 K) for magnetic relaxation and butterfly-shaped hysteresis loops up to 4.5 K.

A comprehensive understanding of the ligand field and its influence on the degeneracy and population of d-orbitals in a specific coordination environment are crucial for the rational design and enhancement of magnetic anisotropy of single-ion magnets (SIMs). Herein, we report the synthesis and comprehensive magnetic characterization of a highly anisotropic CoII SIM, [L2Co](TBA)2 (L is an N,N'-chelating oxanilido ligand), that is stable under ambient conditions. Dynamic magnetization measurements show that this SIM exhibits a large energy barrier to spin reversal U eff > 300 K and magnetic blocking up to 3.5 K, and the property is retained in a frozen solution. Low-temperature single-crystal synchrotron X-ray diffraction used to determine the experimental electron density gave access to Co d-orbital populations and a derived U eff, 261 cm-1, when the coupling between the d x 2 - y 2 and dxy orbitals is taken into account, in very good agreement with ab initio calculations and superconducting quantum interference device results. Powder and single-crystal polarized neutron diffraction (PNPD, PND) have been used to quantify the magnetic anisotropy via the atomic susceptibility tensor, revealing that the easy axis of magnetization is pointing along the N-Co-N' bisectors of the N,N'-chelating ligands (3.4° offset), close to the molecular axis, in good agreement with complete active space self-consistent field/N-electron valence perturbation theory to second order ab initio calculations. This study provides benchmarking for two methods, PNPD and single-crystal PND, on the same 3d SIM, and key benchmarking for current theoretical methods to determine local magnetic anisotropy parameters.

Hydrogen bonds involving the oxygen atoms of intermediates that result from copper‐mediated O2 activation play a key role for controlling the reactivity of Cux/O2 active sites in metalloenzymes and synthetic model complexes. However, structural insight into H‐bonding in such transient species as well as thermodynamic information about proton transfer to or from the O2‐derived ligands is scarce. Here we present a detailed study of the reversible interconversion of a μ1,2‐peroxodicopper(II) complex ([1]+) and its μ1,1‐hydroperoxo congener ([2]+) via (de)protonation, including the isolation and structural characterization of several H‐bond donor (HBD) adducts of [1]+ and the determination of binding constants. For one of these adducts a temperature‐dependent μ1,2‐peroxo/μ1,1‐hydroperoxo equilibrium associated with reversible H+‐translocation is observed, its thermodynamics investigated experimentally and computationally, and effects of H‐bonding on spectroscopic parameters of the CuII2(μ1,2‐O2) species are revealed. DFT calculations allowed to fully map and correlate the trajectories of H+‐transfer and μ1,2‐peroxo→μ1,1‐peroxo rearrangement. These findings enhance our understanding of two key intermediates in bioinspired Cu2/O2 chemistry.

A weak-coordination-auxiliary amino-catalyzed approach enables directed [3 + 2] cyclization of α,β-unsaturated ketones and N-heteroaryl ketones for the desired 2-acylindolizines via an iminium ion/enamine tandem sequence.