Enantioselective rhodium-catalyzed addition of potassium alkenyltrifluoroborates to cyclic imines.

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, enantioselective metal-catalyzed amination of allylic electrophiles 1 , 2 , 3 ] and 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, development of the corresponding reactions of alkenylboron 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-tertbutanesulfinyl aldimines (Scheme 1A). The only existing enantioselective variant is that of Shintani, Hayashi, and coworkers who, as part of a study involving additions of potassium aryltrifluoroborates to N-sulfonyl ketimines, also described one example using an alkenyltrifluoroborate (Scheme 1B). Also of relevance is a single example of an enantioselective Rh(I)-catalyzed addition of an alkenylsilane to an N-sulfonyl aldimine. 18 ] Therefore, a general enantioselective Rh(I)-catalyzed addition of alkenylboron reagents to imines remains undeveloped. Herein, we demonstrate that cyclic imines are highly effective substrates for enantioselective Rh(I)-catalyzed additions of potassium alkenyltrifluoroborates, providing products in excellent enantioselectivities and generally good yields. The cyclic structure of these imines, where the C=N bond is constrained in the Z-geometry, appears to be N