Nitrogen scrambling

We had not expected to find ring expansion in this system. Nevertheless, the N-labeling showed that the products 131 and 132 both had undergone nitrogen scrambling, at least to the extent of 70% and probably more [Eq. (43)] 97,ioi)... 

A few data exist on the oxidation by ferricyanide. This is simple second-order (in oxidant and neutral hydrazine), and leads to quantitative production of nitrogen in accordance with scheme (71)-(74) with A 4 k and A i. No scrambling occurs during oxidation of N-labelled N2H4 indicating that all N2 is formed via di-imine" Di-imine so prepared is capable of hydrogenating added unsaturated compounds, for example, phenylpropiolic acid gives m-cinnamic acid" " . 

Thus, upon photolyzing a mixture of two nitrites, one of which contains an isotopically labeled nitrogen, the products should show the original isotopic distribution if mechanism (a) is operative, the original isotopic distribution in unreacted nitrite but scrambling in the nitroso product if (c) is operative, and complete isotopic scrambling if (b) is operative. 

In order to clarify the possible existence of these intermediates, 6-chloro-5-cyano-4-phenyl[l(3)- N]pyrimidine (20) (the label is scrambled over both nitrogens) and the radioactive 6-chloro-5-[ " C-cyano]-4-phenylpyrimidine (23) were synthesized as substrates. Because of the presence of the cyano function at C-5, one can expect that 20 (and 23) would undergo amination involving an Sn(ANRORC) mechanism. This has indeed been found. When 20 was reacted with potassium amide in liquid ammonia, two products were obtained as main product, 6-amino-5-cyano-4-phenylpyrimidine (21, 75%), and as minor product, a-amino-jS,jS -dicyanostyrene (22, about 20%) (Scheme 11.15). 

Experiments with mono- N-labeled amidines (the label is equally scrambled over both nitrogen atoms) have shown that in the 2-R-5-nitropyrimidines the excess of the enrichment is lower than that in the amidine (see Table III.2) (86JOC71). These measurements clearly indicate that during the reaction some labeling is lost. This result leads to the conclusion that besides formation of a mono- N-labeled 2-phenyl-5-nitropyrimidine (108), in which the N-label is scrambled over both nitrogens, a mono- N-labeled 2-phenyl-5-nitropyrimidine (109) is also obtained in which only one nitrogen is N-labeled (the benzamidine acts in this reaction as N-C donor) (Scheme III.55). 

Scrambling of the nitrogen atoms during irradiation does not take place. This was substantiated by irradiating 2-15N-5-phenylthiatriazole to give partial transformation and analyzing the remaining material by means of 15N NMR spectroscopy. 

A variation of this method that removes the water produced in the pyrrole/aldehyde or hydrated aldehyde condensation has been used to produce meso-tetrakis(perfluoroalkyl)porphyrins. Lindsey s laboratory has also reported a rational synthesis of weyo-substituted porphyrins bearing one nitrogen heterocyclic group that can readily be used for preparing other mono-functionalized porphyrins, methods for minimi zing scrambling in the synthesisof tran -porphyrins from dipyrromethanes and aldehydes, and a rational synthesis of porphyrins bearing up to four different meso substituents. ... 

Studies on the photolysis of simple aliphatic and alicyclic nitrites have confirmed the need for a six-membered transition state, which decides the point of attack on a carbon chain [ 18]. The photolysis of a nitrite (9) gives an active alkoxy radical and a molecule of nitric oxide, which is itself a stable free radical. These radicals must at first be confined within a solvent cage (lo), for no significant scrambling of isotopic nitrogen occurred when a mixture of the nitrite (14) and... 

---