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.

A catalyst of a new type, cyclobutadiene complex [(C 4 Et 4 )Rh(p-xylene)]PF 6 , was found to promote selective reductive amination in the presence of carbon monoxide under mild conditions (1−3 bar, 90 °C). The reaction demonstrated perfect compatibility with a wide range of functional groups prone to reduction by conventional reducing agents. The developed system represents the first systematic investigation of cyclobutadiene metal complexes as catalysts.

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We demonstrate that well-known transition metal catalysts can be transformed into high-performance versions by the simple use of a fluoride anion source. In situ fluoride-activated catalysts are highly active catalytic species. The isolation may lead to degradation of the species or a decrease in catalytic activity. Fluoride activation of known, relatively simple catalysts resulted in the development of one of the most efficient catalytic systems for the asymmetric cyanation of aldehydes, asymmetric Michael addition, and synthesis of cyclic carbonates. Furthermore, the fluoride-assisted reductive opening of tetrahydrofuran (THF) with amines was developed. We believe that the proposed approach can find broad applications in the enhancement of known catalytic processes and in the design of new ones.