Under dry, anaerobic conditions, TiO2 photocatalysis
of carboxylic acid precursors resulted in carbon–carbon bond-forming
processes. High yields of dimers were obtained from TiO2 treatment of carboxylic acids alone. On inclusion of electron-deficient
alkenes, efficient alkylations were achieved with methoxymethyl and
phenoxymethyl radicals. In reactions with maleic anhydride or maleimides,
phenoxyacetic acid produced chromenedione derivatives in addition
to adducts. These photocatalytic reactions are simple and cheap to
perform, and the TiO2 is easily removed by filtration.
The anaerobic photocatalysis strategy offers a range of synthetic
possibilities.
We report an efficient and scalable synthesis of azidotrifluoromethane (CF N ) and longer perfluorocarbon-chain analogues (R N ; R =C F , C F , C F ), which enables the direct insertion of CF and perfluoroalkyl groups into triazole ring systems. The azidoperfluoroalkanes show good reactivity with terminal alkynes in copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), giving access to rare and stable N-perfluoroalkyl triazoles. Azidoperfluoroalkanes are thermally stable and the efficiency of their preparation should be attractive for discovery programs.
Photochemical
reactions employing TiO2 and carboxylic
acids under dry anaerobic conditions led to several types of C–C
bond-forming processes with electron-deficient alkenes. The efficiency
of alkylation varied appreciably with substituents in the carboxylic
acids. The reactions of aryloxyacetic acids with maleimides resulted
in a cascade process in which a pyrrolochromene derivative accompanied
the alkylated succinimide. The selectivity for one or other of these
products could be tuned to some extent by employing the photoredox
catalyst under different conditions. Aryloxyacetic acids adapted for
intramolecular ring closures by inclusion of 2-alkenyl, 2-aryl, or
2-oximinyl functionality reacted rather poorly. Profiles of reactant
consumption and product formation for these systems were obtained
by an in situ NMR monitoring technique. An array of different catalyst
forms were tested for efficiency and ease of use. The proposed mechanism,
involving hole capture at the TiO2 surface by the carboxylates
followed by CO2 loss, was supported by EPR spectroscopic
evidence of the intermediates. Deuterium labeling indicated that the
titania likely donates protons from surface hydroxyl groups as well
as supplying electrons and holes, thus acting as both a catalyst and
a reaction partner.
A titania photoredox catalysis protocol was developed for the homocoupling of C-centered radicals derived from carboxylic acids. Intermolecular reactions were generally efficient and selective, furnishing the desired dimers in good yields under mild neutral conditions. Selective cross-coupling with two acids proved unsuccessful. An intramolecular adaptation enabled macrocycles to be prepared, albeit in modest yields.
The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely, methyl carbonate (methoxycarbonyloxyl) radical, complemented by DFT computations at the CAM-B3LYP level applied to the bicarbonate radical itself. Methyl carbonate radicals were generated by UV irradiation of oxime carbonate precursors. Kinetic EPR was employed to measure rate constants and Arrhenius parameters for their dissociation to CO2 and methoxyl radicals. With oleate and cholesterol lipid components, methyl carbonate radicals preferentially added to their double bonds; with linoleate and linolenate substrates, abstraction of the bis-allylic H atoms competed with addition. This contrasts with the behavior of ROS such as hydroxyl radicals that selectively abstract allylic and/or bis-allylic H atoms. The thermodynamic and activation parameters for bicarbonate radical dissociation, obtained from DFT computations, predicted it would indeed have substantial lifetime in gas and nonpolar solvents. The acidity of bicarbonate radicals was also examined by DFT methods. A noteworthy linear relationship was discovered between the known pKa's of strong acids and the computed numbers of microsolvating water molecules needed to bring about their ionization. DFT computations with bicarbonate radicals, solvated with up to eight water molecules, predicted that only five water molecules were needed to bring about its complete ionization. On comparing with the correlation, this indicated a pKa of about -2 units. This marks the bicarbonate radical as the strongest known carboxylic acid.
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