Chiral guanidinium salts

In the context of the phospha-Mannich reaction, secondary phosphine oxides were added on the C = N bond of imines in the presence of chiral guanidinium salt catalysts to give the a-aminophosphine oxides in an enantioselective way (Scheme 29). " ... 

Figure 16.1 Chiral guanidinium salt 1, an example of an organocatalyst that works by hydrogen bond activation.

Scheme 40.37 Enantioselective Claisen rearrangement catalyzed by chiral guanidinium salts.

Figure 12.12 Chiral guanidinium salt phase-transfer catalysts.

Generally, the chiral quaternary ammonium salts, derived from cinchona alkaloids [26, 28, 29, 46, 56-60] and C2-symmetric chiral binaphthyl or biaryl structures [61-68, 71-73, 77], were demonstrated to be very enantioselective catalysts for the asymmetric alkylation of 65a, and provided the corresponding chiral a-amino acids in over 95% ee in most cases. The tartaric acid-derived catalysts 48a [81] and 52 [84] and chiral guanidinium salts 53 [85] and 54 [86] were also found to be suitable for this alkylation reaction. However, in terms of catalytic activity, only a... 

Chiral guanidinium-based ligands have also been used for recognition of diastereomeric salts of saccharides [45]. Some promising ligands with guani-dinium structure have not been studied yet [46], and some of them have been used as catalysts for the nitroaldol reaction [47] and Michael addition to a,P-unsaturated ketones [48]. 

In 2007, Ooi and coworkers introduced chiral tetraaminophosphonium salts as a new class of Bronsted acids [166]. Similar to the guanidine/guanidinium case, these tetraaminophosphonium salts act as Bronsted bases in their neutral/ deprotonated (triaminoiminophosphorane) form, while they can also be used as mono-functional Bronsted acids in their protonated, phosphonium form. Phos-phonium salt 67, when neutralized in situ with KO Bu, was shown to be a highly effective catalyst in the enantioselective Henry reaction of nitroalkanes with various aromatic and aliphatic aldehydes (Scheme 10.65). The same strategy was further applied to the catalytic asymmetric Henry reaction of ynals [167] and hydrophosphonylation of ynones (Scheme 10.66) [168]. Brfunctional catalysis using this scaffold were also obtained using the carboxylate salts of tetraaminophosphoniums in the direct Mannich reaction of sulfonyl imines with azlactones (Scheme 10.67) [169]. 

Chiral Cuanidinium Salts Guanidinium salts can be considered to be effective phase-transfer catalysts because of their ability to form stabihzed complexes... 

Murphy and coworkers prepared similar guanidinium salts 54 from commercially available ethyl (J )-3-hydroxybutyrate and (S)-(—)-mahc acid [86]. The application of the catalysts 54 in the phase-transfer-catalyzed epoxidation of the chalcones delivered high enantioselectivities. Recently, Tan and coworkers developed a series of chiral pentanidium salts 55, which could be successfully applied to the asymmetric phase-transfer-catalyzed conjugate addition [87] as well as a-hydroxylation reactions [88]. 

Recently, also a guanidinium camphor salt has been prepared by Afonso et al. (together with other chiral salts), simply combining the tetra-n-hexyl-dimethylguanidmium cation with readily available chiral crystalline anions. Noticeably, the IL with lactate as anion has low viscosity and a lower Tg value (Scheme 20). ... 

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