Phosphoric acid derivatives carbonyl compounds

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

Until 2006, a severe limitation in the field of chiral Brpnsted acid catalysis was the restriction to reactive substrates. The acidity of BINOL-derived chiral phosphoric acids is appropriate to activate various imine compounds through protonation and a broad range of efficient and highly enantioselective, phosphoric acid-catalyzed transformations involving imines have been developed. However, the activation of simple carbonyl compounds by means of Brpnsted acid catalysis proved to be rather challenging since the acid ity of the known BINOL-derived phosphoric acids is mostly insufficient. Carbonyl compounds and other less reactive substrates often require a stronger Brpnsted acid catalyst. 

The chemistry of the metal carbonyl hydrides and metal carbonylates remained the principal research topic for Hieber until the 1960s. He mentioned in his account [25], that it was a particular pleasure for him that in his laboratory the first hydrido carbonyl complexes of the manganese group, HMn(CO)5 and HRe(CO)5, were prepared by careful addition of concentrated phosphoric acid to solid samples of the sodium salts of the [M(CO)5] anions, giving the highly volatile hydrido derivatives in nearly quantitative yield [45, 46]. In contrast to HCo(CO)4 and its rhodium and iridium analogues, the pentacarbonyl hydrido compounds of manganese and rhenium are thermally remarkably stable, and in... 

Disubstituted ferrocenyl amino alcohols 20a—u were derived from 3, 21 — 23, and 25 by treatment with n-butyllithium followed by reaction with a carbonyl compound. When aldehydes were used as carbonyl component, two chromatogra-phically separable diastereomers (20a—b, 20c—d, 20i—j, 201—m, 20q—r) were obtained. In the cases of 20h, 20k, and 20u, the diastereomeric mixtures were isomerized to single diastereomers without separation by treatment with aqueous phosphoric acid. The yields and the properties of 20 are summarized in Table 3-4. The absolute configuration of the two diastereomers was tentatively assigned on the basis of their NMR spectra, their stability to aqueous phosphoric acid, and spectral comparison with 20n, absolute configuration of which was confirmed by single-crystal X-ray analysis [35]. 

In general, binol-derived phosphoric acids are not good catalysts for the activation of simple carbonyl compounds such as aromatic, aliphatic, or a,p-unsaturated aldehydes or ketones [159]. In order to overcome these limitations, Nakashima and Yamamoto in 2006 [160] synthesized binol-derived A-trifyl phosphoramides (Figure 2.37). These compounds are 10 times more acidic than the corresponding phosphoric acids [157], a fact that can account for their higher catalytic efficiencies in reactions involving unsaturated ketones[160] or (3-keto esters [161]. 

Until 2006, the activation of simple carbonyl compounds by means of Br0nsted acid catalysis proved to be challenging since the acidity of the known BINOL-derived phosphoric acids were mostly insufficient. Indeed, carbonyl compounds and other less reactive substrates often require... 

As mentioned above, a range of enantioselective transformations have been established using chiral phosphoric acids via the activation of imines and related nitrogen-substituted substrates. In contrast, the activation of carbonyl compounds, including a,P-unsaturated carbonyl compounds, by chiral phosphoric acids has been limited. In 2008, Zhou, He, and coworkers achieved the asymmetric Friedel-Crafts alkylation reaction of indole derivatives with 1,3-diaryl a,p-unsaturated... 

The Mukaiyama aldol reaction is beyond doubt a brilliant triumph of modem synthetic organic chemistry however, the reaction products are contaminated with pre-activated silyl enol ethers derived from the carbonyl compounds with stoichiometric amounts of silylation agent and base. In addition, silylated wastes are inherently formed. Circumventing the pre-activation process improves atom efH-ciency in this case, the carbonyl nucleophiles react directly with the carbonyl electrophiles in the presence of catalyst. The first Bronsted acid-catalyzed direct aldol reactions have been achieved using chiral Hg-BINOL-derived phosphoric acid 96 (Scheme 28.12) [66], The aldol products (127) have syn-configurations and, thus, this reaction is complementary to (S)-proHne catalysis in Brpnsted acids, which in general yields the anti configuration [11]. 

The enantioselective organocatalytic functionaUzation of carbonyl compounds at the 7 position represents a highly challenging and persistent problem for asymmetric synthesis [33]. Recently, Melchiorre and coworkers [34] reported a chiral Bronsted acid-assisted dienamine catalysis for the direct asymmetric y-alkylation of a-substituted linear a,(l-unsaturated aldehydes (Scheme 43.22). In this reaction, the chiral phosphoric acid 106 can induce the formation of a chiral contact ion-pair from alcohol 103 [35], which may synergistically engage in a matched combination with the chiral covalent dienamine intermediate derived from chiral primary... 

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