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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).
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]
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 Mannich reaction starts with the formation of an iminium ion from the amine and formaldehyde. [4]: 140 The compound with the carbonyl functional group (in this case a ketone) will tautomerize to the enol form, after which it attacks the iminium ion.
General structure of an imine. Schiff bases are imines in which R 3 is an alkyl or aryl group (not a hydrogen). R 1 and R 2 may be hydrogens General structure of an azomethine compound. In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure R 1 R 2 C=NR 3 (R 3 = alkyl or aryl, but not hydrogen).
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 1- and 3-aza-Cope have a bias towards imine formation as opposed to enamine formation, as carbon-nitrogen π-bonding is stronger than carbon-nitrogen σ-bonding, meaning the 3-aza-Cope rearrangement is thermodynamically favored, while the 1-aza-Cope rearrangement is not: the imine is nearly 10kcal/mol less in energy.
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