A comparative study of various widely used methods of reductive amination is reported. Specifically, such reducing agents as H2, Pd/C, hydride reagents [NaBH4, NaBH3CN, NaBH(OAc)3], and CO/Rh2(OAc)4 system were considered. For understanding the selectivity and activity of the reducing agents reviewed herein, different classes of starting materials were tested, including aliphatic and aromatic amines, as well as aliphatic and aromatic aldehydes and ketones. Most important advantages and drawbacks of the methods, such as selectivity of the target amine formation and toxicity of the reducing agents were compared. Methods were also considered from the viewpoint of green chemistry.
An
efficient protocol for synthesis of indenyl rhodium complexes
with arene ligands has been developed. The hexafluoroantimonate salts
[(η5-indenyl)Rh(arene)](SbF6)2 (arene = benzene (2a), o-xylene (2b), mesitylene (2c), durene (2d), hexamethylbenzene (2e), and [2.2]paracyclophane (2g)) were obtained by iodide abstraction from [(η5-indenyl)RhI2]
n
(1) with AgSbF6 in the presence of benzene and its
derivatives. The procedure is also suitable for the synthesis of the
dirhodium arene complex [(μ-η:η′-1,3-dimesitylpropane){Rh(η5-indenyl)}2](SbF6)4 (3) starting from 1,3-dimesitylpropane. The structures of [2e](SbF6)2, [2g](SbF6)2, and [3](SbF6)4 were determined by X-ray diffraction. The last species has a sterically
unfavorable conformation, in which the bridgehead carbon atoms of
the indenyl ligand are arranged close to the propane linker between
two mesitylene moieties. Experimental and DFT calculation data revealed
that the benzene ligand in 2a is more labile than that
in the related cyclopentadienyl complexes [(C5R5)Rh(C6H6)]2+. Complex 2c effectively catalyzes the reductive amination reaction between aldehydes
and primary (or secondary) amines in the presence of carbon monoxide,
giving the corresponding secondary and tertiary amines in very high
yields (80–99%). This protocol is the most active in water.
A total synthesis of the vasicinone
family of natural products
from bulk chemicals was developed. Reductive condensation of o-nitrobenzaldehydes with amines utilizing iron pentacarbonyl
as a reducing agent followed by subsequent oxidation leads to a great
variety of polycyclic nitrogen-containing heterocycles under mild
conditions. Enantiomerically pure vasicinone, rutaecarpine, isaindigotone,
and luotonin were synthesized from readily available starting materials
like hydroxyproline, nitrobenzaldehyde, pyrrolidine, and piperidine
in two to four operational steps without chromatography. The antifungal
activity of all products was tested.
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