Photochemical, nitrogen fixation

In the following section we discuss the problem of contamination by adventitious ammonia and discuss how the use of isotope labelling could lead to a clearer picture of the reported photochemical nitrogen fixation process. 

Surface emissions include technological sources (industry, transportation and other fossil fuel combustion processes), biomass burning, nitrogen fixation in soils and oceanic activity and totals 39 Tg yr"F The photochemical production (31 Tg yr ) from HCHO photolysis accounts for about 45% of the total source of H2. 

In addition to photocleavage of water with photocatalysts, other photosynthetic processes such as photochemical C02 reduction, resulting in the formation of CO or methane, and the photochemical fixation of nitrogen are of great interest ... 

The Fixation of Carbon Dioxide and Nitrogen.—As reduced carbon compounds are convenient fuels, the possibility of achieving the reductive fixation of COa in vitro is appealing, if remote. There are very few data on photochemical reactions involving carbon dioxide, for it has no low-lying excited states and has not historically been of much interest to the photochemist. However, what appears to be the first example of photofixation of COa in a non-biological system has been briefly reported.18 Photoirradiation (with a high-pressure mercury lamp) of phenanthrene in the presence of an amine and C02 in a polar solvent (MeaSO or HCONMea) yielded 9,10-dihydrophenanthrene-9-carboxylic acid, in unspecified quantum yield. The mechanism appears to involve formation of COaT by electron transfer from the photoexcited amine, followed by attack of COaT on position 9 of phenanthrene. Similar reductive carboxylation of anthracene, pyrene, naphthalene, and biphenyl was observed. 

Pronounced optical induction was observed in the addition of mercaptan to N-acyldehydroalanyl-L-proline amides (73), acyl-D-cysteinyl-L-proline amides (75) being formed almost exclusively 287, 348). This stereoselective course is observed only when a strongly basic catalyst is used, and only when the proline component of the unsaturated dipeptide is present as amide. The stereoselectivity is considerably less when piperidine is the catalyst, and vanishes completely on photochemical addition. Alkylation of both nitrogen atoms is without effect on the course of reaction, while addition to acyldehydroalanylproline ester proceeds with virtually no stereoselectivity. These findings suggest a carbanion intermediate (74) fixed spatially in such a way that it can be approached by the proton from one direction only. One possible explanation seems to be the fixation of the carbanion within a carbanion-immonium ion pair (74). 

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