Catalyst-Controlled Stereoselective Barton-Kellogg Olefination

Overcrowded alkenes are expeditiously prepared by the versatile Barton-Kellogg olefination and have remarkable applications as functional molecules endowed by their unique stereochemical features. The induced stereodynamics thereby enable the controlled motion of molecular switches and motors while the high configurational stability prevents undesired isomeric scrambling that would impact their essential molecular topology. Bistricyclic aromatic enes are prototypical overcrowded alkenes with outstanding stereochemical properties, but their stereocontrolled preparation is challenging and was thus far only feasible in stereospecific reactions and by the use of chiral auxiliaries. Here, we now report that direct catalyst control is achieved by means of a stereoselective Barton-Kellogg olefination with enantio- and diastereocontrol in the preparation of various bistricyclic aromatic enes. Using Rh2(S-PTAD)4 as catalyst, several diazo compounds were selectively coupled with a thioketone to give one of the four anti -folded overcrowded alkene stereoisomers upon reduction. Moreover, complete stereodivergence was reached by catalyst control in combination with distinct thiirane reductions, providing access to all four stereoisomers with an e.r. of up to 99:1. We envision that the catalyst controlled synthesis of overcrowded alkenes and the possibility for stereodivergence in the Barton-Kellogg olefination will provide a direct and effective route for a broad range of topologically unique overcrowded alkenes for functional molecules, catalysis, energy- and electron transfer, or bioactive compounds.