Enantioselective Rhodium-Catalyzed Addition of Potassium Alkenyltrifluoroborates to Cyclic Imines

摘要

Chiral α-branched allylic amines are important building blocks for organic synthesis, and several catalytic asymmetric methods have been developed for their synthesis. For example, the enantioselective metal-catalyzed amination of allylic electrophiles and the rearrangement of allylic imidates have proven to be highly effective. An alternative approach to chiral allylic amines that can be advantageous from the viewpoint of convergency is the catalytic enantioselective union of an alkenyl nucleophile with an imine. In view of the widespread success of enantioselective Rh~I-catalyzed additions of arylboron reagents to imines as a means to access chiral α-aryl-branched amines,the development of the corresponding reactions of alkenylbofon reagents to prepare chiral α-branched allylic amines should be an attractive goal. Surprisingly, however, only very limited precedent exists for this transformation. Brak and Ellman have developed highly diastereoselective Rh~I-catalyzed additions of alkenylboron reagents to N-tert-butanesulfinyl aldi-mines (Scheme 1 A). The only existing enantioselective variant is that of Shintani, Hayashi, and co-workers, who, as part of a study involving additions of potassium aryltrifluor-oborates to N-sulfonyl ketimines, also described one example using an alkenyltrifluoroborate (Scheme IB).Also of relevance is a single example of an enantioselective Rh~I-catalyzed addition of an alkenylsilane to an N-sulfonyl aldimine. Therefore, a general enantioselective Rh~I-cata-lyzed addition of alkenylboron reagents to imines remains undeveloped.