Photocatalytic reactions alkane

Reaction of Ruthenium Carbonyls with Alkyl Radicals Boese and Goldman reported that in the presence of aryl ketones, d8 metal carbonyls such as Ru(CO)3(dmpe) mediate photocatalytic carbonylation of alkanes via a free radical mechanism.161 The activity was proposed to be initiated by the addition of an alkyl radical to the metal carbonyl and the formation of a metal-acyl radical intermediate. The transition states and the products of the reaction between alkyl radicals and ruthenium carbonyls were studied utilizing the B3LYP level of theory.162 The methyl addition to a carbonyl of Ru(CO)5 or Ru(CO)3(dmpe) was computed to be about 6 kcal/mol more exothermic than addition to free CO. 

The reaction between alkanes (especially methane) and water is endothermic and produces syngas (see books, reviews [51], recent papers [52] and patents [53]). The photocatalytic oxidation of methane with water to produce methanol and molecular hydrogen has been patented [54]. 

The chief advantages to using dense CO2 as the reaction medium were claimed to be its inertness, its general ability to dissolve the reactants, and its easy separation from the reaction mixture. No mechanistic work was reported, but the same mechanism as previously proposed in alkane solvents was suggested, that is, the key step was oxidative addition to the excited state of the metal complex. The C02/Rh complex system was also reported to be an effective photocatalytic system for the dehydrogenation of cyclooctane. 

A number of reactions involving alkane photooxidation by polyoxometalates have been run under aerobic conditions. Those by PWi2O40 and Wio032 are summarized at the end of Table I (see section 5). Unfortunately these reactions, collectively, are rather uninformative with respect to mechanism. Radical chain autoxidation processes are undoubtedly present and few if any of the reported papers have addressed the requisite experiments to differentiate autoxidation from other oxidation pathways. A similar general limitation exists in the literature on photocatalytic aerobic oxidation of organic compounds by semiconductor materials [66]. 

Applicability of the photocatalytic hydrogenation of alkenes to alkanes was investigated using various aromatic or aliphatic alkenes, and Table 9.1 shows results of the photocatalytic hydrogenation in methanolic suspensions of Pd-Ti02 particles under deaerated conditions. Hydrogenation reaction proceeded in all cases, and high yield was obtained. 

---