Imine compounds stabilized nitrile imines

In the late 1980s, accustomed thinking about the namre of nitrile imines was upturned by the discovery that these intermediates could be stabilized by the use of appropriate C and N substituents to such a degree that they could be obtained as stable isolable compounds (15,16). Some examples are shown below 16-24. Most of them are quite thermally stable and are insensitive to air and water (e.g., 16). The first such nitrile imine (16) to be prepared has a mp of 100 °C. Even example 23 with carbon-based substituents has a mp of 60 °C. It appears that the major factor in the kinetic stabihty of these compounds is the steric bulk of the C and N substiments. 

Thus in the N-silyl substituted series, 17 and 18, which rearrange thermally to the corresponding diazo compounds, the stability increases through the series R=Me, Ph, i-Pr. As discussed below, these compound undergo the usual cycloaddition and electrocyclization reactions of nitrile imines and are not simply overstabilized curiosities. The usefulness in synthesis of those with P—C bonds is probably limited since these bonds are not easily broken, but products derived from those with C—Si and C—B bonds (e.g., 21 and 22) should be capable of further... 

Two general methods are available for the assembly of the sterically stabilized species 16-24, both starting from metallic derivatives of diazo compounds (147) (16). The latter have two nucleophilic centers and can, in principle, react with electrophiles at C giving the functionalized diazo compounds (148), or at N, which leads to the nitrile imines (149). 

Reactions with Active Methylene Compounds. Enolates of ketones," esters," enediolates," 1,3-dicarbonyl compounds," amides and lactams," as well as nitrile-stabilized carbanions," can be alkylated with benzyl bromide. Cyclohexanone may be benzylated in 92% ee using a chiral amide base." Amide bases as well as alkoxides have been employed in the case of nitrile alkylations." Benzylation of metalloenamines may be achieved and enantioselective reactions are possible using a chiral imine (eq 3). However, reactions between benzyl bromide and enamines proceed in low yield. The benzylation of a ketone via its enol silyl ether, promoted by fluoride, has been observed. ... 

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