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The reaction is acid catalyzed and the reaction type is nucleophilic addition of the amine to the carbonyl compound followed by transfer of a proton from nitrogen to oxygen to a stable hemiaminal or carbinolamine. With primary amines, water is lost in an elimination reaction to an imine. With aryl amines, especially stable Schiff bases are formed.
The nucelophilic amine reacts at the carbon of the carbonyl group to form a hemiaminal species; reversible loss of one molecule of water from the hemiaminal species by alkylimino-de-oxo-bisubstitution to form the imine intermediate. [3] The equilibrium between aldehyde/ketone and imine is shifted toward imine formation by dehydration. [2]
In recent years, several reagents such as Tris(2,2,2-trifluoroethyl)borate [B(OCH 2 CF 3) 3], [14] pyrrolidine [15] or titanium ethoxide [Ti(OEt) 4] [16] have been shown to catalyse imine formation. Rarer than primary amines is the use of ammonia to give a primary imine. [17] In the case of hexafluoroacetone, the hemiaminal intermediate can be ...
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).
Hemiaminals are intermediates in imine formation from an amine and a carbonyl by alkylimino-de-oxo-bisubstitution. [1] Hemiaminals can be viewed as a blend of aminals and geminal diol. They are a special case of amino alcohols.
In the Crabbé reaction, the secondary amine serves as the hydride donor, which results in the formation of the corresponding imine as the byproduct. Thus, remarkably, the secondary amine serves as Brønsted base, ligand for the metal ion, iminium-forming carbonyl activator, and the aforementioned two-electron reductant in the same reaction.
One variation of the Bucherer–Bergs reaction is the treatment of carbonyl compound with carbon disulfide and ammonium cyanide in methanol solution to form 2,4-dithiohydantoins. [8] In addition, the reaction of ketones with ammonium monothiocarbamate and sodium cyanide will yield 5,5-disubstituted 4-thiohydantoins.
The first step in this reaction is the formation of an imine, followed by a hydrophosphonylation step where the phosphonate P-H bond across the C=N double bond. [5] The starting carbonyl component is usually an aldehyde and sometimes a ketone. The reaction can be accelerated with a combination of dehydrating reagent and Lewis acid.