Alkali amides dissociation

The main reason for the discrepancy remaining in the nature of the catalytic species is the lack of knowledge of the dissociation of the alkali amides. The main contribution of this work is a careful analysis of this dissociation, made with the help of the existing data and of a few new experimental results. 

All data presented in this chapter have been drawn from results obtained at —33.5°C. It can be shown (6) that between —33.5°C. and —70.0°C. the dissociation of alkali amides does not vary significantly with temperature. 

Both types of bond are relatively stable. Only the bonds of the most electropositive metals of Groups IA and IIA of the periodic table with the electronegative oxygen or nitrogen atoms exhibit sufficient ionic character for the dissociation of the respective salts to ions and ion pairs at low temperatures and in relatively non-polar media. In this way, initiating anions are formed which are mostly more stable (and therefore also less reactive) than carbanions. Alkali metal hydroxides and amides have often been used in the past. 

The hydrogen exchange between liquid NH3 and dissolved H2 is catalyzed by amide ions [8]. The dissociated ion was identified to be the active particle on account of the fact that the reaction rate depends on the concentration of the free anion which varies with the counterion and other alkali salts added [9]. Intermediates of the exchange reaction were deduced by the interacting-bond method [10]. The amide ion in liquid NH3 also catalyzes the conversion of para-H2 to ortho-H2 [8]. 

Barret and Hill have extensively studied the reactivity and catalytic activity of NHC-supported Group 2 amido complexes. Similar to several reports with the alkali metals, tricoordinate NHC-alkaline earth metal complexes 58 could be formed directly from the corresponding conjugate acid by addition of a metal amide salt (Scheme 5.8). Whereas carbene-Li complexes were found to be excellent carbene transfer reagents, 58 could function as a stable carbene equivalent. Indeed, in the presence of Lewis base donors such as triphenylphosphine oxide or protic substrates such as 2-methoxyethylamine, liberation of the free carbene was observed by and NMR. While the authors did not attempt to isolate this free carbene or investigate any additional reactivity, they claimed that the carbene was dissociated under catalyt-ically relevant conditions. 

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