A Striking Case of Enantioinversion in Gold Catalysis and Its Probable Origins.

The cyclization of the hydroxy-allene 2 to the tetrahydrofuran 3 catalyzed by the gold-phosphoramidite complex 1, after ionization with an appropriate silver salt AgX, is one of the most striking cases of enantioinversion known to date. The major reason why the sense of induction can be switched from (S) to (R) solely by changing either the solvent or the temperature or the nature of the counterion X is likely found in the bias of the organogold intermediates to undergo assisted proto-deauration. Such assistance can be provided by a protic solvent, a reasonably coordinating counterion or even by a second substrate molecule itself; in this case, the reaction free energy profile gains a strong entropic component that can ultimately dictate the stereochemical course.