Hemiaminal
A hemiaminal is a functional group or type of chemical compound that has a hydroxyl group and an amine attached to the same carbon atom: -C(OH)(NR2)-. R can be hydrogen or an alkyl group. Hemiaminals are intermediates in imine formation from an amine and a carbonyl by alkylimino-de-oxo-bisubstitution.
An example is of an hemiaminal is that obtained from reaction of secondary amine carbazole and formaldehyde [1] [2]
reaction of carbazole with formaldehyde to Carbazol-9-yl-methanol
Those generated from primary amines are unstable to the extent that they have never been isolated and very rarely been observed directly. In a 2007 study a hemiaminal substructure trapped in the cavity of a host-guest complex was studied with a chemical half-life of 30 minutes. Because both amine and carbonyl group are isolated in a cavity, hemiaminal formation is favored due to a high forward reaction rate comparable to a intramolecular reaction and also due to restricted access of external base (another amine) to the same cavity which would favor elimination of water to the imine [3].
Hemiaminal formation is a key step in a asymmetric total synthesis of saxitoxin [4]:
In this reaction step the alkene group is first oxidized to an intermediate acyloin by action of osmium(III) chloride, oxone (sacrificial catalyst) and sodium carbonate (base).
Aminal
An aminal or aminoacetal is a functional group or type of chemical compound that has two amine groups attached to the same carbon atom: -C(NR2)(NR2)-. Again R can be hydrogen or an alkyl group.
The aminal and the hemiaminal groups are the cousins of hemiacetals and acetals with nitrogen replaced by oxygen. Aminals are encountered in for instance the Fischer indole synthesis.
Reflist
- ↑ Carbazol-9-yl-methanol Milata Viktora, Kada Rudolfa, Lokaj J¨¢nb Molbank 2004, M354 open access publication [1]
- ↑ Reaction in methanol in reflux with potassium carbonate. Acid catalysis turns the hemiaminal to the aminal N,N´-biscarbazol-9-yl-methane.
- ↑ Stabilization of Labile Carbonyl Addition Intermediates by a Synthetic Receptor Tetsuo Iwasawa, Richard J. Hooley, Julius Rebek Jr. Science 317, 493 (2007) doi:10.1126/science.1143272
- ↑ (+)-Saxitoxin: A First and Second Generation Stereoselective Synthesis James J. Fleming, Matthew D. McReynolds, and J. Du Bois J. Am. Chem. Soc., 129 (32), 9964 -9975, 2007. doi:10.1021/ja071501o