Salts tetraaminophosphonium

Ooi has recently reported application of chiral P-spiro tetraaminophosphonium salt 37 as a catalyst for the highly enantio- and diasterioselective direct Henry reaction of a variety of aliphatic and aromatic aldehydes with nitroalkanes (Scheme 5.51) [92]. Addihon of the strong base KO Bu generates in situ the corresponding catalyhcally active triaminoiminophosphorane base A. Ensuing formation of a doubly hydrogen-bonded ion pair B positions the nitronate for stereoselective addition to the aldehyde. This catalyst system bears many similarities to guanidine base catalysis. 

Tetraaminophosphonium salts such as 77 for catalyzing the Henry reaction have been developed. 1,2-Diamines bearing cbiral information to form metal complexes often can serve as catalyst for tbe reaction, and sucb is tbe case of A -(2-pyridylmetbyl)isobomylamine (78) with Cu(OAc)2, and a Cu(n) complex of the salen 79. The bimetaUic-salen complex 80A shows catalytic activity for bringing about anti-selective Henry reaction. ... 

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]. 

The latter mode of activation, in which a charged tetraaminophosphonium salt acts as a Bronsted acid, was realized by the same research group in 2009. Enanti-oselective aza-Michael reaction of 2,4-dimethoxyaniline with nitroalkenes afforded the conjugate addition products in excellent yields and high enantiomeric excesses (Scheme 10.70) [173, 174]. Similarly, chiral diaminooxaphosphonium salts have been used as Br0nsted acids in the asymmetric protonation reactions of ketene disilyl acetals [175]. 

Newly designed chiral tetraaminophosphonium salt 184 possessing an organic anion cooperatively catalyzes asymmetric Mannich-type reaction of azlactones (182) with N-sulfonyl imines (183) (Scheme 28.20). The basic carboxylate anion deprotonates the active methine proton of the azlactone (182) to form the corresponding chiral phosphonium enolate with a defined hydrogen bonding network (ion pair), and then highly stereoselective bond formation proceeds [92]. 

Scheme 28.20 Tetraaminophosphonium salt

The use of chiral P-spiro salts 5-8 in a nitroaldol reaction was recently reported by the Ooi group (Table 29.1) [10]. The reaction proceeded in the presence of 5 mol% of the chiral tetraaminophosphonium salt and KO Bu (5 mol%). It was postulated that the nitroalkane would be deprotonated by triaminoiminophospho-rane I, which would be the actual catalyst generated in situ from 5-8 and a strong base, forming a chiral phosphonium nitronate complex. An acyclic transition state model II predicting the formation of the anti adduct was proposed in which the Si-face of the nitronate would be set up to approach the Re-face of the aldehyde without significant sterical hindrance (Scheme 29.5). 

As shown in the Table 29.1, the screening of several chiral tetraaminophosphonium salts, in the test Henry reaction of nitromethane and benzaldehyde, showed that the aromatic substituent (Ar) on the diazaphosphacycle has a significant effect on catalyst efficiency. Thus, while catalyst 5 (Ar = Ph, 86% yield and 89% ee), and especially 6 (Ar = m-Xylyl, 36% yield and 45% ee), shown suboptimal performance, catalyst 7 (Ar = P-CF3-C6H4) provides the corresponding final nitroaldol with... 

Scheme 28.20 Tetraaminophosphonium salt-catalyzed Mannich reactions.

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