Theoretical study of the mechanism of oxidative addition of allyl-ammoniumand -iminium salts to low-valent metal complexes. Rationalization of selective C-N and N-H bond activation

A theoretical study has been carried out for the mechanism of the reactions of allylammonium and -iminium substrates with Ni(0) complexes using the B3 LYP density functional method. The main findings are as follows: (1) The ac tual active catalyst in the oxidative addition of ammonium and iminium salt s is the bisphosphine-nickel complex. (2) For allylammonium salts the react ion is found to proceed via an associative mechanism (rather than a dissoci ative path) which involves (i) coordination of the allylammonium cation to the active metal catalyst, (ii) oxidative addition of the C-N bond to the N i(0) complex, (iii) coordination of NH3 to yield a pentacoordinated interme diate, and (iv) loss of a phosphine ligand. The analogous reaction involvin g allyliminium salts does not follow the same mechanistic pattern; once the allyliminium cation is coordinated to the metal, the reaction prefers to p roceed in a dissociative fashion. This preference is likely to be enhanced in solution. (3) The main difference between allylammonium and -iminium rea ction mechanisms lies in the relative barriers corresponding to the inserti on of the catalyst into the C-N bond leading to the pentacoordinated interm ediate. For allylammonium cations, such a step is calculated to be less ene rgy demanding than phosphine loss, whereas for allyliminium, the situation is reversed. (4) Finally the observed preference of allyliminium substrates to undergo C-N cleavage rather than N-H cleavage has been also investigate d. The reason that allyl salt does not yield hydrido complexes is the kinet ic impairment found for the N-H bond cleavage as compared to C-N.;Intramole cular M-H activation is not allowed because of the high barriers correspond ing to the three-centered transition states involved in the reaction. The s tructural rearrangements required for the coordinated iminium to form an et a (2)-NH complex make the process prohibitive.