Organic photocatalysts are emerging as viable and more sustainable tools than metal complexes. Recently, the field of organo‐photocatalysis has experienced an explosion in terms of applications, redesign of well‐established systems, and identification of novel scaffolds. A rational approach to the structural modification of the different photocatalysts is key to accessing unprecedented reactivity, while improving their catalytic performances. We herein discuss the concepts underpinning the scaffold modification of some of the most recently used photocatalysts and analyze how specific structural changes alter their physicochemical and redox properties.
Twelve naphthochromenone photocatalysts (PCs) were synthesized on gram scale. They absorb across the UV/Vis range and feature an extremely wide redox window (up to 3.22 eV) that is accessible using simple visible light irradiation sources (CFL or LED). Their excited‐state redox potentials, PC*/PC.− (up to 1.65 V) and PC.+/PC* (up to −1.77 V vs. SCE), are such that these novel PCs can engage in both oxidative and reductive quenching mechanisms with strong thermodynamic requirements. The potential of these bimodal PCs was benchmarked in synthetically relevant photocatalytic processes with extreme thermodynamic requirements. Their ability to efficiently catalyze mechanistically opposite oxidative/reductive photoreactions is a unique feature of these organic photocatalysts, thus representing a decisive advance towards generality, sustainability, and cost efficiency in photocatalysis.
The introduction
of thianthrene as a linchpin has proven to be
a versatile strategy for the C–H functionalization of aromatic
compounds, featuring a broad scope and fast diversification. The synthesis
of aryl thianthrenium salts has displayed an unusually high
para
regioselectivity, notably superior to those observed
in halogenation or borylation reactions for various substrates. We
report an experimental and computational study on the mechanism of
aromatic C–H thianthrenation reactions, with an emphasis on
the elucidation of the reactive species and the nature of the exquisite
site selectivity. Mechanisms involving a direct attack of arene to
the isolated
O
-trifluoracetylthianthrene
S
-oxide (
TT
+
-TFA
) or to the thianthrene
dication (
TT
2+
) via electron transfer under
acidic conditions are identified. A reversible interconversion of
the different Wheland-type intermediates before a subsequent, irreversible
deprotonation is proposed to be responsible for the exceptional
para
selectivity of the reaction.
The
first light-driven
method for the α-trifluoromethoxylation
of ketones is reported. Enol carbonates react with
N
-trifluoromethoxy-4-cyano-pyridinium, using the photoredox catalyst
4-CzIPN under 456 nm irradiation, affording the α-trifluoromethoxy
ketones in ≤50% isolated yield and complete chemoselectivity.
As shown by 29 examples, the reaction is general and proceeds very
rapidly under batch (1 h) and flow conditions (2 min). Diverse product
manipulations demonstrate the synthetic potential of the disclosed
method in accessing elusive trifluoromethoxylated bioactive ingredients.
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