Phosphoramides BINOL-derived

In 2006, Yamamoto and Nakashima picked np on this and designed a chiral A -triflyl phosphoramide as a stronger Brpnsted acid catalyst than the phosphoric acids based on this concept. In their seminal report, they disclosed the preparation of new chiral BINOL-derived A -triflyl phosphoramides and their application to the asymmetric Diels-Alder (DA) reaction of a,p-unsaturated ketones with sily-loxydienes [83], As depicted in Scheme 59, chiral A-triflyl phosphoramides of the general type 4 are readily synthesized from the corresponding optically active 3,3 -substituted BINOL derivatives 142 through a phosphorylation/amidation route. 

Scheme 59 Preparation of chiral BINOL-derived iV-tiiflyl phosphoramides...

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]. 

FIGURE 2.37. Chiral binol-derived A -trifyl phosphoramides. 

The first example of 1,3-dipolar cycloaddition of nitrones catalysed by a chiral organocatalyst was that developed by Yamamoto et al, concerning the cycloaddition with ethyl vinyl ether promoted by a chiral BINOL-derived phosphoramide. These reactions yielded the endo products as the major diastereomers with high diastereoselectivities of up to 94% de, excellent yields and high enantioselectivities of up to 93% ee, as shown in Scheme 6.14. 

As with other P(III) compounds, the binaphthol (BINOL) derivatives have been by far the most successful for a wide range of synthetic methods. Several reviews and reports have been published on the incorporation of BINOL-derived phosphoramidites in rhodium-catalysed asymmetric hydrogenations Novel chiral phosphine-phosphoramidite ligands, HY-Phos (61) and THNAPhos (62), have also been successful in rhodium-catalysed asymmetric hydrogenations. Asymmetric synthesis of the Roche ester and its derivatives (63) by rhodium-INDOLPHOS-catalysed hydrogenation was reported and a number of other BINOL-derivered phosphoramidates have been employed in iridium-catalysed hydrogenations and allylic alkyla-... 

A-Triflyl phosphoramides derived from BINOL are highly acidic chiral Brpnsted acids, and they are powerful catalysts for enantioselective C-C and C-X bond-forming reactions. Their design, structural features, and applications since their development in 2006 have been reviewed. 

Although phosphoric acid has been employed for the activation of carbon-nitrogen double bond, Yamamoto et at. designed a stronger chiral Bronsted acid in order to expand the scope of the chiral Bronsted acid catalyzed reactions. A N-triflyl phosphoramide (50), bearing BINOL backbone, catalyzed the Diels-Alder reaction of a,p-unsaturated ketone with electron-rich diene ((2, )-siloxydiene is major) to give cyclohexene derivatives in high ees (Scheme 2.106) [184]. 

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