Fluorous BINAP

A fluorous version of the chiral bis-phosphine BINAP has been developed for asymmetric Heck reactions [5], Several fluorous-derivatized binaphthols and BINAP derivatives have been reported, (Scheme 10.4) [6], The silane spacer group present in one of the ligands was used to maximize the percentage fluorine on the molecule. Even so, the partition coefficient1 between FC-72 (see Chapter 3) and benzene was only 2.85, and not surprisingly, the reuse of the catalyst was poor. 

Fluorous BINAP ligands such as 1, prepared from the bromo-BINOL precursor(s), coordinate with RuCl2 to form reusable catalysts that have shown activities in hydrogenation of... 

Stuart and co-workers reported the first synthesis of a light fluorous BINAP, (R)-6,6 -bis(lH,lH,2H,2H-perfluorooctyl)-2,2 -bis(diphenylphosphino)-l,l -bi-naphthyl (F content = 38%), and its application to a Ru complex catalyzed asymmetric hydrogenation of dimethyl itaconate [Eq. (2)] [10). The reaction was carried out at ambient temperature under the same reaction conditions as those reported by Noyori [11). The chemical yield (83%) and enantioselectivity (95.7% ee) were similar to those reported (88% and 95.4% ee, respectively). However, there was no description of the recovery of the catalyst or ligand. 

Huorous compounds are also potentially useful as additives to promote organic reactions in carbon dioxide. For example, a fluorous alcohol RfCH20H assists asymmetric hydrogenations with non-fluorous ruthenium BINAP catalysts, and a fluorous aryl alkyl ether (C8F17C6H4-P-OC12H25) does so in scandium-triflate-catalyzed aldol and Friedel-Crafts reactions. These additives are presumed to act as solubilizers or emulsifiers to promote contact among the various reaction components. Since they are fluorous, they can be readily recovered from the otherwise organic reaction mixtures for reuse. 

Two new fluorous chiral 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP Table 15.1, entries 11 and 12) ligands were synthesized and their efficiency was demonstrated in an asymmetric Mizoroki-Heck reaction (Scheme 15.8) [74-76]. Aryl triflates 38 were coupled with 2,3-dihydrofuran (39) to (i )-40 in up to 93% ee. Either pure fluorous solvents or mixtures of fluorous and nonfluorous solvents were applied. Ligand 34 was compared with conventional BINAP, with the fluorous ligand giving lower reaction rates and a similar level of enantioselectivity in 24-77 h reaction time [74, 75]. The catalyst could not be recovered by reverse-phase silica gel chromatography due to its oxidation. 

Nakamura, Y., Takeuchi, S., Zhang, S. et al. (2002) Preparation of a fluorous chiral BINAP and application to an asymmetric Heck reaction. Tetrahedron Lett., 43, 3053-6. 

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