Tertiary enamines reactions with electrophiles

A further difference to electrophilic alkenes shown by DAD and other diimides is that tertiary amines, such as A-cyclohexylpyrrolidine (167), are dehydrogenated to the enamine. Further reaction then occurs to give the product of 2,6-disubstitution (168)352. Diacyl diimides, such as dibenzoyldiimide (DBD), are even more remarkable. The action of DBD on 167 at room temperature results in dehydrogenation of the pyrrolidine ring and reaction with a further two equivalents of DBD to give 169353 (Scheme 178). 

The reaction of electrophiles with tertiary enamines can be formulated as having several stages and is illustrated by equation 1. The fact that C-substitution can be forced, even when initial reaction occurs at the enamine nitrogen, is one of the synthetic virtues of the Stork enamine synthesis7. 

Nucleophilicity parameters N and % for electrophile-nucleophile combination based on the Mayr equation log k=+E) were reported for fluorides in protic solvents, enamines derived from imidazolidinones, trimethylsilyl enol ethers with perfluori-nated substituents at the a-carbon, O-methylated Breslow intermediates, anions of nucleobases and their subunits, enamides, symmetrical and unsymmetrical hydrazines, and heteroarylboron compounds. Of note, replacement of CH3 with CF3 and C6H5 with CgF5 in the trimethylsilyl enol ethers reduces the nucleophilicity by 8 and 4.5 orders of magnitude, respectively. Hydrazines have very similar nucle-ophilicities to alkyl amines in other words, there is no evidence of an a-effect. With NH2NMC2, there is a fast reversible reaction corresponding to addition of the tertiary amine, followed by a slow irreversible reaction for addition at the primary amine, with a 3000-fold difference in nucleophilicity between the two sites. ... 

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