Electronic Influences on the Dynamic Range of Photoswitchable Dithienylethene–Thiourea Organocatalysts

Abstract

Thiourea-based organocatalysts bearing a photoswitchable dithienylethene (DTE) core and a wide range of substituents were prepared and extensively tested for their ability to accelerate the Michael reaction between acetylacetone and trans-β-nitrostyrene. There is a strong correlation between the Hammett parameter of the modulating groups and catalytic activity following UV irradiation. Electron-withdrawing groups afford the largest reactivity difference between the catalysts in their ring-open form and their ring-closed isomer, with evidence for electronic coupling between the two halves in both oDTE and cDTE.

Dithienylethene-based photoswitchable phosphines for light-controlled palladium-catalyzed Stille coupling reaction

“…41,42 As expected, this result corroborates that the phosphanyl and electron-modulating groups in the non-planar structure of DTE o -COCF 3 When two equivalents of the phosphine ligands were used to react with trans-[PdCl 2 (PhCN) 2 ], monometallic palladium(II) 1:2 type complexes were formed (Figure 1a). For all these compounds, an isolated singlet was registered in their 31 P{ 1 H} NMR spectra that shifted downfield to δ ≈ 12 ppm, thereby corroborating metal complexation and the formation of exclusively one isomer (cis or trans). In addition, negligible differences were found in the chemical shift of this 31 P{ 1 H} NMR signal for the three Pd complexes prepared, which again demonstrates the lack of electronic communication between the external thiophene substituents in the open isomer of the ligands.…”

“…This behavior is characteristic of closed state DTEs, 45 as confirmed by TD-DFT calculations (Table S5, Figures S23-S25) and additional NMR spectroscopic measurements. In particular, we observed the appearance of a new set of signals for the closed isomer in the 1 H, 31 P (proton-coupled and decoupled) and, when applicable, 19 F NMR spectra of the UV-irradiated ligands in toluene-d 8 , which were upfield ( 1 H NMR) or downfield shifted ( 31 P, 31 P{ 1 H} and 19 Analysis of this NMR data also revealed that the photocyclization reaction of DTE-based ligands was not quantitative. Instead, a photostationary state (PSS) composed of an equilibrium mixture of the ligands' o and c states was obtained in all the cases, since both isomers absorb at the UV excitation wavelength used and, therefore, should simultaneously undergo ring-closing and ringopening reactions.…”

“…In this work we propose to explore one of these https://doi.org/10.26434/chemrxiv-2023-vw3zz ORCID: https://orcid.org/0000-0002-1126-4138 Content not peer-reviewed by ChemRxiv. License: CC BY-NC-ND 4.0 4 additional features: the variation in electronic communication between the external thiophene substituents upon photoconversion, which has already been utilized to control chemical reactivity with light-responsive organocatalysts 30,31 and organic substrates. 32,33 In particular, herein we devised the synthesis of asymmetric DTE derivatives as photoswitchable ligands bearing (i) a metal-binding phosphine group at one thiophene ring, and (ii) an electron-withdrawing substituent at the other (Scheme 1b).…”

Dithienylethene-Based Photoswitchable Phosphines for the Palladium-Catalyzed Stille Coupling Reaction

“…In contrast, other electronic features that accompany DTE photoisomerization have yet to be exploited in metal-based photoswitchable catalysis. In this work, we proposed to explore one of these additional features: the variation in electronic communication between the external thiophene substituents upon photoconversion, which has already been utilized to control chemical reactivity with light-responsive organocatalysts , and organic substrates. , In particular, herein, we devised the synthesis of nonsymmetric DTE derivatives as photoswitchable ligands bearing (i) a metal-binding phosphine group at one thiophene ring and (ii) an electron-withdrawing substituent at the other (Scheme b). As these two groups must be electronically insulated in the open state of the system and become selectively conjugated upon ring-closing, our molecular design should allow modulating the electron density of the phosphine ligand with light and, eventually, the catalytic activity of metal centers upon coordination.…”