Lanthanum trifluoromethanesulfonate is an effective single-component catalyst for synthesizing a variety of amides directly from esters and amines under mild conditions. Highly selective amidation of esters and amines, as well as catalyst-controlled amidation of esters, demonstrated the effectiveness of the catalyst system.

Mechanistic studies and expansion of the substrate scope of direct enantioselective alkynylation of α-ketiminoesters catalyzed by adaptable (phebox)rhodium(III) complexes are described. The mechanistic studies revealed that less acidic alkyne rather than more acidic acetic acid acted as a proton source in the catalytic cycle, and the generation of more active (acetato-κ(2)O,O')(alkynyl)(phebox)rhodium(III) complexes from the starting (diacetato)rhodium(III) complexes limited the overall reactivity of the reaction. These findings, as well as facile exchange of the alkynyl ligand on the (alkynyl)rhodium(III) complexes led us to use (acetato-κ(2)O,O')(trimethylsilylethynyl)(phebox)rhodium(III) complexes as a general precatalyst for various (alkynyl)rhodium(III) complexes. Use of the (trimethylsilylethynyl)rhodium(III) complexes as precatalysts enhanced the catalytic performance of the reactions with an α-ketiminoester derived from ethyl trifluoropyruvate at a catalyst loading as low as 0.5 mol % and expanded the substrate scope to unprecedented α-ketiminophosphonate and cyclic N-sulfonyl α-ketiminoesters.

A green way to amino acids: α-Tetrasubstituted α-amino acid derivatives are formed in high yield and enantioselectivity by using a Rh-catalyzed enantioselective alkynylation of α-ketiminoesters. This reaction, which involves a proton transfer and can be conducted at room temperature, has high substrate scope (see scheme; Cbz = benzyloxycarbonyl, Fmoc = 9-fluorenylmethyloxycarbonyl).

Catalytic addition reactions to N-unsubstituted (N-unprotected) imines can provide a more straightforward synthesis of amines. This direct process eliminates the unnecessary protecting-group manipulations that are required with N-substituted (N-protected) imines and can contribute to the development of green chemistry. Although their use has been limited due to difficulties associated with the nature of N-unsubstituted imines, recently developed catalytic methods enable the use of N-unsubstituted imines as electrophiles in various catalytic addition reactions. To facilitate an understanding of the state of the art development of synthetic methodologies, herein we review recent progress on catalytic addition reactions to N-unsubstituted imines. We begin with an overview of the chemistry of N-unsubstituted imines, followed by a summary of recent progress categorized according to the reaction type. We hope this review will help to stimulate further development of greener syntheses of nitrogen-containing compounds.