Does phosphoric acid form amide?
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Phosphoric Acid and Amide Bond Formation
Chiral Phosphoric Acid-Catalyzed Amide Bond Formation
Chiral phosphoric acids have been shown to effectively catalyze the formation of amide bonds. In one study, a chiral phosphoric acid bearing a 2,4,6-trimethyl-3,5-dinitrophenyl group at the 3,3'-position was used to catalyze the kinetic resolution of a 2-pyridyl ester via amide bond formation with high selectivity. The addition of a Lewis acid further enhanced both the reactivity and selectivity of the substrate containing a methoxy group.
Synthesis of Phosphoric Acid Amides
The synthesis of phosphoric acid amides can be challenging due to the instability of the final compounds, which are prone to hydrolysis. However, a method involving the silylation of phosphates and phosphonates followed by the removal of silyl groups under mild conditions has been successfully employed to obtain these labile derivatives. This approach uses trialkylhalosilanes as silylating agents and intermediate bis(silyl) ethers to synthesize difficult-to-obtain amides of phosphoric and thiophosphoric acids.
Catalytic Enantioconvergent Rearrangement
Chiral phosphoric acids also play a role in the catalytic enantioconvergent 2-aza-Cope rearrangement of α-stereogenic-β-formyl amides. This process results in the formation of β-imino amides with high diastereo- and enantiocontrol. These β-imino amides can be further deprotected to primary β-amino amides or reduced to corresponding diamines.
Condensation Reactions Involving Phosphoric Acid Amides
Condensation reactions involving N-alkyl-N-methylol diethyl phosphoric acid amides with carboxylic acids have been explored. These reactions produce N-alkyl-N-acyloxymethyl diethyl phosphoric acid amides, demonstrating the versatility of phosphoric acid amides in forming various derivatives through condensation with different carboxylic acids.
Asymmetric Transfer Hydrogenation
Chiral phosphoric acids have been identified as highly efficient organocatalysts for the asymmetric transfer hydrogenation of α-imino esters and amides. Using Hantzsch esters as hydrogen donors, this method yields highly enantioenriched α-amino esters and their derivatives with enantiomeric excesses of up to 98%.
Palladium-Catalyzed Arylation
In the context of C-H activation reactions, chiral phosphoric amides and acids have been used to control the stereoselectivity during the enantioselective arylation of secondary β-C(sp3)-H bonds of 8-aminoquinoline amides. This method highlights the utility of chiral phosphoric compounds in achieving high enantioselectivity in complex organic transformations.
Phosphorylation of Amides
Phosphoric anhydride can phosphorylate both acyclic and cyclic amides. The phosphorylated acyclic amides exhibit a tendency for N-O reversible or irreversible phosphorotropy, whereas this behavior is less specific in cyclic amides. This phosphorylation process underscores the reactivity of phosphoric anhydride with amides.
Organophosphorus-Catalyzed Amidation
An efficient method for amide bond formation involves organophosphorus-catalyzed amidation reactions between unactivated carboxylic acids and amines. Poly(methylhydrosiloxane) serves as a green reducing agent for the in situ reduction of phosphine oxide to phosphine, enabling the synthesis of a wide range of secondary and tertiary amides in high yields.
Brønsted Acid-Catalyzed Imine Amidation
A novel method for Brønsted acid-catalyzed addition of amide nucleophiles to imines has been developed, producing protected aminal products. Simple Brønsted acids like phenyl phosphinic acid and trifluoromethanesulfonimide act as excellent catalysts, achieving high yields. Additionally, a catalytic asymmetric imine amidation using sulfonamides as nucleophiles was successful with a hindered biaryl phosphoric acid catalyst, achieving excellent yields and enantioselectivities.
Conclusion
Phosphoric acid and its derivatives play a significant role in the formation of amide bonds through various catalytic and synthetic methods. These include chiral phosphoric acid-catalyzed kinetic resolutions, silylation techniques for synthesizing labile amides, and organophosphorus-catalyzed amidation reactions. The versatility and efficiency of phosphoric acid in these processes underscore its importance in organic synthesis.
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