C.V. Stevens

Structural and photophysical properties of various polypyridyl ligands: A combined experimental and computational study

L. De Bruecker, J. Everaert, P. Van der Voort, C.V. Stevens, M. Waroquier, V. Van Speybroeck
ChemPhysChem
2020
A1

Abstract 

Covalent triazine frameworks (CTFs) with polypyridyl ligands are very promising supports to anchor photocatalytic complexes. Herein, we investigate the photophysical properties of a series of ligands which vary by the extent of the aromatic system, the nitrogen content and their topologies to aid in selecting interesting building blocks for CTFs. Interestingly, some linkers have a rotational degree of freedom, allowing both a trans and cis structure, where only the latter allows anchoring. Therefore, the influence of the dihedral angle on the UV‐Vis spectrum is studied . The photophysical properties are investigated by a combined computational and experimental study. Theoretically, both static and molecular dynamics simulations are performed to deduce ground‐ and excited state properties based on density functional theory (DFT) and time‐dependent DFT. The position of the main absorption peak shifts towards higher wavelengths for an increased size of the π‐system and a higher π‐electron deficiency. We found that the position of the main absorption peak among the different ligands studied in this work can amount to 271 nm; which has a significant impact on the photophysical properties of the ligands. This broad range of shifts allows modulation of the electronic structure by varying the ligands and may help in a rational design of efficient photocatalysts.

Gold Open Access

N‐rich porous polymer with isolated Tb3+‐ions displays unique temperature dependent behavior through the absence of thermal quenching

F. Vanden Bussche, A.M. Kaczmarek, S. K. P. Veerapandian, J. Everaert, M. Debruyne, S. Abednatanzi, R. Morent, N. De Geyter, V. Van Speybroeck, P. Van der Voort, C.V. Stevens
Chemistry - A European Journal
2020
A1

Abstract 

The challenge of measuring fast moving or small scale samples is based on the absence of contact betw een sample and sensor. Grafting lanthanides onto hybrid materials arises as one of the most promising accurate techniques to obtain noninvasive thermometers. In this w ork, a novel bipyridine based Porous Organic Polymer (bpyDATPOP) wasinvestigatedastemperaturesensorafter grafting w ith Eu(acac) 3 and Tb(acac) 3 complexes. The bpyDAT POP successfully showed temperature dependent behavior in the 10 ‐ 310 K range, proving the potential of amorphous, porous organic framew orks. More intriguingly, w e observed unique temperature dependent behavior; instead of the standard observed change in emission as a result of a change in temperature for both Eu 3+ and Tb 3+ , the emission spectrumof Tb 3+ remained constant. This w ork provides framework‐ and energy‐based explanations for the observed phenomenon. The conjugation in the bpyDAT POP framew ork is interrupted, creating energetically isolated Tb 3+ environments. Energy transferfromTb 3+ toEu 3+ isthereforeabsent,norenergybacktransfer from Tb 3+ to bpyDAT POP ligand (i.e. no thermal quenching) is detected.

Open Access version available at UGent repository
Gold Open Access

Optical Properties of Isolated and Covalent Organic Framework-Embedded Ruthenium Complexes

F. Muniz-Miranda, L. De Bruecker, A. De Vos, F. Vanden Bussche, C.V. Stevens, P. Van der Voort, K. Lejaeghere, V. Van Speybroeck
Journal of Physical Chemistry A
123 (32), 6854-6867
2019
A1

Abstract 

Heterogenization of RuL3 complexes on a support with proper anchor points provides a route toward design of green catalysts. In this paper, Ru(II) polypyridyl complexes are investigated with the aim to unravel the influence on the photocatalytic properties of varying nitrogen content in the ligands and of embedding the complex in a triazine-based covalent organic framework. To provide fundamental insight into the electronic mechanisms underlying this behavior, a computational study is performed. Both the ground and excited state properties of isolated and anchored ruthenium complexes are theoretically investigated by means of density functional theory and time-dependent density functional theory. Varying the ligands among 2,2′-bipyridine, 2,2′-bipyrimidine, and 2,2′-bipyrazine allows us to tune to a certain extent the optical gaps and the metal to ligand charge transfer excitations. Heterogenization of the complex within a CTF support has a significant effect on the nature and energy of the electronic transitions. The allowed transitions are significantly red-shifted toward the near IR region and involve transitions from states localized on the CTF toward ligands attached to the ruthenium. The study shows how variations in ligands and anchoring on proper supports allows us to increase the range of wavelengths that may be exploited for photocatalysis.

Gold Open Access

Tandem addition of phosphite nucleophiles across unsaturated nitrogen-containing systems: mechanistic insights on regioselectivity

W. Debrouwer, D. Hertsen, T.S.A Heugebaert, E. Birsen Boydas, V. Van Speybroeck, S. Catak, C.V. Stevens
Journal of Organic Chemistry
82 (1), 188–201
2017
A1

Abstract 

The addition of phosphite nucleophiles across linear unsaturated imines is a powerful and atom-economical methodology for the synthesis of aminophosphonates. These products are of interest from both a biological and a synthetic point of view: they act as amino acid transition state analogs and Horner–Wadsworth–Emmons reagents, respectively. In this work the reaction between dialkyl trimethylsilyl phosphites and α,β,γ,δ-diunsaturated imines was evaluated as a continuation of our previous efforts in the field. As such, the first conjugate 1,6-addition of a phosphite nucleophile across a linear unsaturated N-containing system is reported herein. Theoretical calculations were performed to rationalize the observed regioselectivites and to shed light on the proposed mechanism.

Beyond the diketopiperazine family with alternatively bridged brevianamide F analogues

I. Wauters, H. Goossens, E. Delbeke, K. Muylaert, B.I. Roman, K. Van Hecke, V. Van Speybroeck, C.V. Stevens
Chemistry - A European Journal
80 (16), 8046-8054
2015
A1

Abstract 

A method for the preparation of 3,5-bridged piperazin-2-ones from a tryptophan–proline-based diketopiperazine is described using diphosgene to induce the ring closure. Density functional theory calculations were conducted to study the mechanism of this C–C bond formation. Several derivatives of the thus obtained α-chloroamine were synthesized by substitution of the chlorine atom using a range of O-, N-, S-, and C-nucleophiles. This novel class of brevianamide F analogues possess interesting breast cancer resistance protein inhibitory activity.

Elucidating the Structural Isomerism of Fluorescent Strigolactone Analogue CISA-1

H. Goossens, T.S.A Heugebaert, B. Dereli, M. Van Overtveldt, O. Karahan, I. Doğan, M. Waroquier, V. Van Speybroeck, V. Aviyente, S. Catak, C.V. Stevens
European Journal of Organic Chemistry
2015 (6), 1211–1217
2015
A1

Abstract 

The synthesis of a new potent strigolactone analogue (CISA-1), resulting in the formation of two interconverting structural isomers, which could not be identified, was recently reported by Rasmussen et al [Molecular Plant, 2013, 6, 100]. In the present study, a combined computational and experimental approach is used to identify the exact nature of these structural isomers. While standard experimental techniques were not able to determine the identity of the isomers, chromatographic methods excluded E/Z isomerisation. Computational 1H NMR chemical shift values and DFT calculations on interconversion barriers strongly suggest that the CISA-1 isomers were interconverting (Z)-configured atropisomers.

Synthesis of Tricyclic Phosphonopyrrolidines via IMDAF: Experimental and Theoretical Investigation of the Observed Stereoselectivity

D.D. Claeys, K. Moonen, B.I. Roman, V.N. Nemykin, V.V. Zhdankin, M. Waroquier, V. Van Speybroeck, C.V. Stevens
Journal of Organic Chemistry
73 (20), 7921-7927
2008
A1

Abstract 

During the synthesis of tricyclic phosphonopyrrolidines via intramolecular Diels−Alder reactions of 1-acylamino(furan-2-yl)methyl phosphonates, two isomers are formed in most cases. The presence of a short three-atom tether together with spectroscopic data, including difference NOE, revealed that the cycloaddition occurred exo, but the phosphonate substituent on the tether had an exo or endo orientation. This was confirmed via X-ray analysis. A thermodynamic preference for the product with the phosphonate function in the endo position was observed experimentally and was confirmed theoretically. Density functional theory methods and several high-level post Hartree−Fock procedures were used to rationalize the observed isomer ratio of the IMDAF-reactions. This was done for two different types of reagents: with the activating carbonyl group in the tether or as a substituent on the tether. For the first type of molecules there is a large steric hindrance of the bulky tether substituents that disfavors the exo-isomer. In the latter case, there was a very small energy difference between the transition states causing a mixture of epimers being formed.

The Formation of trans-Fused Macrocycles from N3,N3′-Polymethylenebis(hydantoins) by Ring-Closing Metathesis

D.D. Claeys, C.V. Stevens, N. Dieltiens
European Journal of Organic Chemistry
(1), 171–179
2008
A1

Abstract 

A straightforward ring transformation giving polymethylenebis(hydantoins) was extended, as these are HMBA analogues. Firstly, ethyl or allyl pyroglutamate was carbamoylated with a diisocyanate. Upon treatment with KOtBu in allyl alcohol the bis(carbamoyllactam) rearranged to give the hydantoin, which was followed by the ring-opening of the pyrrolidinone with formation of the allyl ester. These compounds were subsequently ring-closed in the presence of second-generation Grubbs' catalyst to form macrocycles containing the ester functionality in the ring. It was established by HSQC experiments with inverse detection that only the E isomers were formed in the cases of the 24- and 26-membered heterocycles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Exploiting the regioselectivity of pyroglutamate alkylations for the synthesis of 6-azabicyclo[3.2.1]octanes and 4-azabicyclo[3.3.0]octanes

K.G.R. Masschelein, C.V. Stevens, N. Dieltiens, D.D. Claeys
Tetrahedron
63 (22), 4712–4724
2007
A1

Abstract 

Depending on the N-protecting group of pyroglutamates, the reactivity can be directed to the formation of 6-azabicyclo[3.2.1]octanes or 4-azabicyclo[3.3.0]octanes, which are conformationally restricted glutamate analogues.

Unexpected Four-Membered over Six-Membered Ring Formation during the Synthesis of Azaheterocyclic Phosphonates: Experimental and Theoretical Evaluation

V. Van Speybroeck, K. Moonen, K. Hemelsoet, C.V. Stevens, M. Waroquier
JACS (Journal of the American Chemical Society)
128 (26), 8468-8478
2006
A1

Abstract 

The cyclization of functionalized aminophosphonates is studied on both experimental and theoretical grounds. In a recently described route to phosphono-β-lactams [Stevens C. V.; Vekemans, W.; Moonen, K.; Rammeloo, T. Tetrahedron Lett. 2003, 44, 1619], it was found that starting from an ambident allylic anion only four-membered rings were formed without any trace of six-membered lactams. New anion trapping experiments revealed that the γ-anion is highly reactive in intermolecular reactions. Ab initio calculations predict higher reaction barriers for the γ-anion due to restricted rotation about the C−N bond and due to highly strained transition states during ring closure. The sodium or lithium counterion, explicit dimethyl ether solvent molecules, and bulk solvent effects were properly taken into account at various levels of theory.

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