Current examples of photoredox catalytic processes for CO, CN, CHal and CC bond formation are described with emphasis on the basic mechanistic scenarios. The focus is on the special aspects of photochemical redox catalysis especially on the electron transfer energetics, the use of electron transfer (ET) sensitizers and mediators as well as sacrificial reagents.magnified image

The incorporation of surface-functionalized spindle-like hematite nanoparticles as particulate cross-linkers in poly(acrylamide) hydrogel matrices delivers ferrogels with a covalent type of particle−matrix interaction. By systematic investigation of the stability and the internal architecture of the resulting gels, the regime of resilient gels is identified. The swelling properties and the rheological behavior are in accordance with a network structure based on particle nodes interlinked by long polymer segments and a significant fraction of loops.

SummaryThe homogeneous titanium- and dye-catalyzed as well as the heterogeneous semiconductor particle-catalyzed photohydroxymethylation of ketones by methanol were investigated in order to evaluate the most active photocatalyst system. Dialkoxytitanium dichlorides are the most efficient species for chemoselective hydroxymethylation of acetophenone as well as other aromatic and aliphatic ketones. Pinacol coupling is the dominant process for semiconductor catalysis and ketone reduction dominates the Ti(OiPr)4/methanol or isopropanol systems. Application of dilution effects on the TiO2 catalysis leads to an increase in hydroxymethylation at the expense of the pinacol coupling.

The photochemically induced coupling of aromatic ketones with cyclic ethers such as tetrahydrofuran, tetrahydropyran, and 1,4-dioxane was studied. Direct photolysis of the substrates with UV-A light centered at 350 nm does not lead to photoinduced hydrogen transfer whereas the addition of a mixture of the Lewis acid catalysts Ti(O(i)Pr)4 and BF3 enables the formation of the hydroxyalkylation products.

The allylic hydroperoxide 2 (myrtenyl hydroperoxide), available from singlet oxygen photooxygenation of beta-pinene (1), is converted into the azido bis-hydroperoxide 3 by an electron-transfer induced azidyl radical formation and trapping of the initial tertiary carbon radical by triplet oxygen. The azido bis-hydroperoxide 3 is reduced to the azido 1,2-diol 4 or the amino diol 5, respectively. Beside classical fluorescent PET sensitizers such as rhodamines, also nanosized semiconductor particles as well as lucigenin were applied as catalysts. The electron transfer rate of azide oxidation was determined for lucigenin by fluorescence quenching analysis.

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