Direct Observation of Triplet States in the Isomerization of Alkenylboronates by Energy Transfer Catalysis

Abstract

Alkenylboronates are versatile building blocks for stereocontrolled synthesis owing to the traceless nature of the boron group that can be leveraged to achieve highly selective geometric isomerization. Using thioxanthone as an inexpensive photocatalyst, the photoisomerization of these species continues to provide an expansive platform for stereodivergent synthesis, particularly in the construction of bioactive polyenes. Although mechanistic investigations are consistent with light-driven energy transfer, direct experimental evidence remains conspicuously absent. Herein, we report a rigorous mechanistic investigation using two widely used alkenylboronates alongside relevant reference compounds. Through the combination of irradiation experiments, transient absorption spectroscopic studies, kinetic modeling, and DFT calculations with all isomers of the model compounds, it has been possible to unequivocally detect and characterize the perpendicular triplet generated by energy transfer. Our results serve not only as a blueprint for mechanistic studies that are challenging with organic sensitizers, but these guidelines delineated have also enabled the development of more sustainable reaction conditions: for the first time, efficient organocatalytic isomerization under sunlight irradiation has become feasible.

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

“…Nanosecond laser flash photolysis (LFP) studies upon 355 nm excitation of 4a 2+ in the presence of benzene concentrations allowing up to 90% * 4a 2+ quenching were carried out to detect both quenching products. However, we only observed baseline-like spectra without any evidence for the benzene radical cation and 4a • + , despite the high sensitivity of our LFP setup . Unproductive and ultrafast back-electron transfer in the so-called solvent cage is spin-allowed for this reaction, which can be regarded as a general problem for photoinduced electron transfer reactions with photoexcited singlet states .…”

Isonitriles as Alkyl Radical Precursors in Visible Light Mediated Hydro‐ and Deuterodeamination Reactions**

“…Moreover, the calculated triplet state energy for an isonitrile ( � 110 kcal mol À 1 ) is higher than that for 4CzIPN (58.4 kcal mol À 1 ). [40] Although a non-vertical ET process cannot be ruled out, [41,42] we do not have any experimental evidence to support this idea. Moreover, the rapid reverse ISC (T 1 to S 1 ) and the resultant thermally activated delayed fluorescence (TADF) inevitably present significant competition to a hypothetical intermolecular triplet-triplet energy transfer (TTET) between the photosensitizer and 1 a or 1 o, particularly under the diffusion-limited conditions.…”

Isonitriles as Alkyl Radical Precursors in Visible Light Mediated Hydro‐ and Deuterodeamination Reactions**

“…Moreover, the calculated triplet state energy for an isonitrile ( � 110 kcal mol À 1 ) is higher than that for 4CzIPN (58.4 kcal mol À 1 ). [40] Although a non-vertical ET process cannot be ruled out, [41,42] we do not have any experimental evidence to support this idea. Moreover, the rapid reverse ISC (T 1 to S 1 ) and the resultant thermally activated delayed fluorescence (TADF) inevitably present significant competition to a hypothetical intermolecular triplet-triplet energy transfer (TTET) between the photosensitizer and 1 a or 1 o,…”