Complexes formed from silver triflate and (R) or (S)-Binap 9, were isolated at different temperatures and further characterized by X-ray diffraction analysis by Yamamoto’s group.20 These studies revealed that mixture of structures 13-15 are in equilibrium and at room temperature, being the 1:1 complex 14 the most abundant system (Figure 2).
Figure 2. [(R)-Binap]AgOTf complexes.
In spite of equimolar [(S)-Binap]-AgClO4 and [(S)-Binap]-AgOAc complexes gave identical chemical yields of product endo-12aa and very high enantioselection (>99 and 99% ee, respectively, Table 1, 4 and 7) the presumed major complex 16 was much more insoluble in toluene than the analogous formed by AgOAc. This property allowed the separation of the complex 16 from the reaction mixture by simple filtration (see experimental part). Surprisingly, complexes (R)- and (S)-16 exhibited a high stability and any apparent decomposition occurred upon the light exposure. Both complexes 16 and 17 were prepared and isolated by reaction with 1 and 2 equiv of (R)- or (S)-Binap together to 1 equiv. of AgClO4, respectively. The mixture was stirred for 1h at room temperature and the complexes were obtained in quantitative yield. Complex (S)-16 was further characterized by ESI-MS experiments showing an M++1 signal at 731 and a tiny one at 1353 (Graphic 1). In the case of complex (S)-17 (Graphic 2),the same experiment revealed a peak at 1353 and a very small one at 731. However, these two in situformed Binap complexes 16 and 17 could not be differentiated by 31P NMR spectroscopy. Unfortunately, we could not obtain appropriate crystals for their comprehensive and definitive characterization by X-ray diffraction analysis.
Figure 3. Structures of complexes (R)-16, (S)-16, and (S)‑17.