1.1- Enediamines protonation

Although this reaction was discussed earlier, it is mentioned here because it is catalyzed by solid acids such as NH4SCN, which provide H+ ions that bond to the pairs of electrons after breaking them loose from the metal. Over a rather wide range of catalyst concentrations, the rate is linearly dependent on the amount of solid acid. Once the NH4+ ion donates a proton, NH3 is lost and the protonated ethyl-enediamine molecule is the acid that remains and continues to catalyze the reaction. While base catalyzed reactions of complexes may be better known, there are many acid catalyzed reactions as well. 

Results obtained so far have shown that protonation occurs, without exception, on the / -carbon atom of 1,1-enediamines29,95. This agrees well with the structural characteristics of 1,1-enediamines, i.e. with the increased electron density on the / -carbon atom due to the delocalization effect. 

Rajappa and coworkers20 used isothiocyanates as a probe to examine the enaminic reactivity of nitro-substituted enamines and enediamines. The results were usually consistent with predictions based on the chemical shift of the vinyl proton and on extended Huckel calculations. However, cyclic enediamine 7 was found to be unreactive toward aryl and alkyl isothiocyanates (see Section II.A). Very recently, the same reaction has been re-examined21 and it has been found that cyclic enediamine 7 indeed reacts easily with aryl isothiocyanate to give the addition products 176 in 54-65% yield (equation 68). 

Ring transformation from benzoyl-substituted 1,1-enediamines 93 to isoxazoles 244 has been achieved recently184. In acidic media, 93 reacts with hydroxylamine to give a moderate yield of substituted 5-(2-aminoethylamino)isoxazoles 244, A reasonable mechanism which involves protonation, oximation, cyclization and deamination steps has been proposed (Scheme 15). 

T. Inabe, I. Luneau, T. Mitani, Y. Mamyama, and S. Takeda, Proton transfer in A-(2-hydroxy-l-naphthylmethylene)-l -pyreneamine and AW4>is(2-hydroxy-1 -naphthylmethylene)-/ -phenyl-enediamine crystals, Bull Chem. Soc. Jpn. 67, 612-621 (1994). 

Bis-//-oxodicopper(in)-phenolate intermediate (6) can be observed at -120 C in the rapid oxidation of 2,4-di-t-butylphenolate by [Cu202(A,A/ -di-t-butyldiethyl-enediamine)2] to a mixture of catechol and quinone. A hybrid DFT study, based on the calculated free-energy profile, suggests that the first step is the 0-0 bond cleavage in the peroxo complex which subsequently coordinates to one of the copper ions in the bis-//-oxodi-Cu(in) complex to yield the phenolate intermediate (6). The rate-limiting decay of (6) involves C-O bond formation, followed by coupled internal proton and electron transfer, and electron transfer coupled to proton transfer from an external donor. 

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