A Retro-[1,4]-aza-Brook Rearrangement

It's well documented that silyl group has a tendency to migrate to the neighboring location as long as the geometry favors the move. The retro-[1,4]-aza-Brook rearrangement is a chemical transformation involving the intramolecular migration of a silyl group from a nitrogen atom to a carbon atom. It is the reverse process of the [1, 4]-aza Brook rearrangement, which typically migrates silicon from carbon to nitrogen. During my graduate study, an 1, 4-N to C anionic migration was observed, in which the Me3Si group on the allylic amine nitrogen underwent an intramolecular migration to the vinylic carbon anion generated by transmetallation of Bu3Sn group that has a Z-configuration ( Corriu and Geng et al: J. Org. Chem., 1993, 58 , 1443). It could be envisioned that the silyl group migrates via a pentacoordinated silicon intermediate. In contrast, the trans configured N, N-bis(Me3Si)-E-Bu3Sn substituted allylic amine gives no such migration upon lithiation, which confirmed the steric requirement. 

 

A similar 1, 4-N to C anionic migration was recognized by an Italian group a bit earlier (Ricci et al: Synlett., 1991, 712). In this case the allylic anion was produced by deprotonating the allylic C-H in N, N-bis(Me3Si)-E-Me3Si substituted allylic amine. Interestingly, over three decades later, these remain as the only two examples of 1, 4-N to C-silyl migration. During the last 30 years, a number of other 1, 4-silyl migrations have been reported, such as, from O to C ( Mitchell et al: Tet. Lett., 1999, 55, 1285; Lautens et al: J. Org. Chem., 1992, 57, 3270), from C to O (Takeda et al: Chem. Commun., 2002, 2218) and from C to C (Kawachi et al: Organometallics, 2006, 25, 2390). All these 1,4-silyl migrations will eventually find interesting synthetic applications.