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.
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