Photosensitized dehydrogenation

Photosensitized dehydrogenation.1 Irradiation of flavanones with this sensitizer provides flavones in moderate (—60%) yield. This dehydrogenation has been carried out with CAN or DDQ. 

Irradiation of tryptophan with visible light in acetic or formic acid solution in the presence of methylene blue gives good conversion to a P-carboline (215). The reaction probably proceeds via OH abstraction from the solvent molecule by photoexcited methylene blue, with formation of an acyl RC = 0 radical, which attacks the 2-position of the indole substrate. The 2-acyl derivative cyclizes to the dihydro-p-carboline (82) which is aromatized by methylene blue photosensitized dehydrogenation to (83). p-Carbolines variously substituted in the 1- or 3-position have been obtained by an appropriate choice of the solvent and indole side chain. 

The catalytic activity of various semiconducting oxides and mixtures of oxides for the dehydrogenation of aromatic hydrocarbons is increased by ultraviolet irradiation 65>. The carbon monoxide oxidation photosensitized by ZnO has been studied by several authors (see below) 66 70>. 

Anodic dehydrogenations, e.g., oxidations of alcohols to ketones, have been treated in Sect. 8.1 and formation of olefins by anodic elimination of C02 and H+ from carboxylic acids was covered in Sect. 9.1. Therefore this section is only concerned with anodic bisdecarboxylations of v/odicarboxylic acids to olefins. This method gives usually good results when its chemical equivalent, the lead tetraacetate decarboxylation, fails. Combination of bisdecarboxylation with the Diels-Alder reaction or [2.2] -photosensitized cycloadditions provides useful synthetic sequences, since in this way the equivalent of acetylene can be introduced in cycloadditions. 

The photochemical dehydrogenation of alcohols can be achieved using decatungstate as photosensitizer. Near-u.v. light is required to drive the reactions, which involve multi-electron reduction of the tungstate, and colloidal Pt is mandatory for Hj liberation. Similar processes occur with vanadates, ... 

HRhLi, (L = PPh3).82 The reduction of acetophenone and aromatic aldehydes by [Rh(nbd)L2] /NaOPrl In PrlOH decreases with decreasing electron donor ability of L or electron richness of the aldehyde.83 The photolytic dehydrogenation of isopropanol to give acetone and H2 Is catalysed by c1s-fRhCl(C0)(dppm) U In the presence of acetone as photosensitizer.84... 

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