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Primary amines are usually not used for enamine synthesis due to the preferential formation of the more thermodynamically stable imine species. [11] Methyl ketone self-condensation is a side-reaction which can be avoided through the addition of TiCl 4 [12] into the reaction mixture (to act as a water scavenger).
When R = H, the compound is a primary imine, when R is hydrocarbyl, the compound is a secondary imine. If this group is not a hydrogen atom, then the compound can sometimes be referred to as a Schiff base. [9] When R 3 is OH, the imine is called an oxime, and when R 3 is NH 2 the imine is called a hydrazone.
The mechanism of the Knorr pyrrole synthesis begins with condensation of the amine and ketone to give an imine. The imine then tautomerizes to an enamine, followed by cyclization, elimination of water, and isomerization to the pyrrole. Mechanism of Knorr pyrrole synthesis
The Stork enamine synthesis: formation of an enamine from a ketone; addition of the enamine to an alpha, beta-unsaturated aldehyde or ketone; hydrolysis of the enamine back to a ketone; The Stork enamine reaction. The reaction also applies to acyl halides as electrophiles, which results in the formation of 1,3-diketones (Stork acylation). [2]
The enamine then attacks the carbonyl carbon of the α-haloketone (4). This is followed by the loss of H 2 O, giving an imine ( 5 ). This intermediate undergoes an intramolecular nucleophilic attack, forming a 5-membered ring ( 6 ).
The intermediate imine can be isolated or reacted in-situ with a suitable reducing agent (e.g., sodium borohydride) to produce the amine product. [2] Intramolecular reductive amination can also occur to afford a cyclic amine product if the amine and carbonyl are on the same molecule of starting material.
The reaction is also stereoselective because the enamine addition occurs with a diastereomeric preference for trans addition without formation of the cis isomer. This is in contrast to traditional Diels–Alder reactions, which are stereospecific based on the alkene geometry. Scheme 2. regio- and diastereoselective Povarov reaction
The ketene formation during the deprotonation process for substrates possessing Evans' oxazolidinone is also a main side reaction for the related alkylation reactions. Development in the field of enamine chemistry and the utilization of imine derivatives of enolates managed to provide an alternative for enolate alkylation reactions.