L_12_09 - 318N Krische Lecture 12 Th Enamines then Benzene...

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318N – Krische, Lecture 12: Th - 02/26/09, Enamines then Benzene and Aromaticity We conclude our discussion of carbonyl chemistry with the reactions of enamines. Enamines are prepared from the condensation of a secondary amine and an aldehyde or ketone. See on-line handout on the formation and reactions of enamines. Enamines are isoelectronic with enols. Enamines, however, are more nucleophilic than enols and will readily serve as nucleophiles in SN2 reactions with alkyl halides and addition-elimination reactions with acid chlorides. SEE ON-LINE HANDOUT ON ENAMINES. Through enamine formation, the α -position of a carbonyl compound is “nucleophilically activated” (like an enol). The use of enamines occurs in stages. (1) Enamine Formation: the ketone is exposed to the secondary amine in the presence of an acid catalyst to provide the enamine. (2) Enamine Alkylation/Acylation: the enamine is reacted with an electrophile (i.e. alkyl halide or acid chloride). (3) Hydrolysis-upon alkylation/acylation an iminium species results which is hydrolyzed to “unmask” the alkylated/acylated ketone.
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