Aryl
halides are a fundamental motif in synthetic chemistry, playing
a critical role in metal-mediated cross-coupling reactions and serving
as important scaffolds in drug discovery. Although thermal decarboxylative
functionalization of aryl carboxylic acids has been extensively explored,
the scope of existing halodecarboxylation methods remains limited,
and there currently exists no unified strategy that provides access
to any type of aryl halide from an aryl carboxylic acid precursor.
Herein, we report a general catalytic method for direct decarboxylative
halogenation of (hetero)aryl carboxylic acids via ligand-to-metal
charge transfer. This strategy accommodates an exceptionally broad
scope of substrates. We leverage an aryl radical intermediate toward
divergent functionalization pathways: (1) atom transfer to access
bromo- or iodo(hetero)arenes or (2) radical capture by copper and
subsequent reductive elimination to generate chloro- or fluoro(hetero)arenes.
The proposed ligand-to-metal charge transfer mechanism is supported
through an array of spectroscopic studies.
Developments
in the field of photoredox catalysis that leveraged
the long-lived excited states of Ir(III) and Ru(II) photosensitizers
to enable radical coupling processes paved the way for explorations
of synthetic transformations that would otherwise remain unrealized.
While first row transition metal photocatalysts have not been as extensively
investigated, valuable synthetic transformations covering broad scopes
of olefin functionalization have been recently reported featuring
photoactivated chlorobis(phenanthroline) Cu(II) complexes. In this
study, the photochemical processes underpinning the catalytic activity
of [Cu(dmp)2Cl]Cl (dmp = 2,9-dimethyl-1,10-phenanthroline)
were studied. The combined results from static spectroscopic measurements
and conventional photochemistry, ultrafast transient absorption, and
electron paramagnetic resonance spin trapping experiments strongly
support blue light (λex = 427 or 470 nm)-induced
Cu–Cl homolytic bond cleavage in [Cu(dmp)2Cl]+ occurring in <100 fs. On the basis of electronic structure
calculations, this bond-breaking photochemistry corresponds to the
Cl → Cu(II) ligand-to-metal charge transfer transition, unmasking
a Cu(I) species [Cu(dmp)2]+ and a Cl atom, thereby
serving as a departure point for both Cu(I)- or Cu(II)-based photoredox
transformations. No net photochemistry was observed through direct
excitation of the ligand-field transitions in the red (λex = 785 or 800 nm), and all combined experiments indicated
no evidence of Cu–Cl bond cleavage under these conditions.
The underlying visible light-induced homolysis of a metal–ligand
bond yielding a one-electron-reduced photosensitizer and a radical
species may form the basis for novel transformations initiated by
photoinduced homolysis featuring in situ-formed metal–substrate
adducts utilizing first row transition metal complexes.
Adding Fe3+ or Al3+ to the electrolyte resulted in fast promotion or poisoning, respectively, of catalysis for oxygen evolution at nickel–borate, and both effects were accompanied with anodic shifts in the redox peaks with potential scanning.
Showcasing the concept of light-induced homo¬lysis for the generation of radicals, the CuII-photocatalyzed decarboxylative oxygenation of carboxylic acids with molecular oxygen as the terminal oxidant is described. Two CuII-carboxylate complexes...
We report a new water soluble and stable thiolate/disulfide redox couple (T(-)/DS) and its use with a new zwitterionic and thiocyanate-free dye (T169) in a 100% aqueous electrolyte system. A DSSC incorporating T169 and the T(-)/DS showed the highest photocurrents (Jsc = 13.30 mA cm(-2)) and IPCE% (84%) values reported to date. In addition, a 2000 h long-term stability measurement was performed, where Jsc and Voc of the above mentioned DSSC stayed somehow the same except for the fill factor (FF) which decreased from 0.62 to 0.48 and consequently lowered the total efficiency (from η = 4.5% on day 1 to η = 3.3% after 2000 h).
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