Commercially available magnesium perchlorate is reported as an extremely efficient catalyst for the synthesis of alpha-aminophosphonates. A three-component reaction (3-CR) of an amine, an aldehyde or a ketone, and a di-/trialkyl phosphite (Kabachnik-Fields reaction) took place in one pot under solvent-free conditions to afford the corresponding alpha-aminophosphonates in high yields and short times. The use of solvent retards the rate of the reaction and requires a much longer reaction time than that for neat conditions. The reactions involving an aldehyde, an aromatic amine without any electron-withdrawing substituent, and a phosphite are carried out at rt. The reactions involving cyclic ketones, aromatic amines with an electron-withdrawing substituent, and aryl alkyl ketone (e.g., acetophenone) require longer reaction times at rt or heating. Magnesium perchlorate was found to be superior to other metal perchlorates and metal triflates during the reaction of 4-methoxybenzaldehyde, 2,4-dinitroaniline, and dimethyl phosphite. The catalytic activity of various magnesium compounds was influenced by the counteranion, and magnesium perchlorate was found to be the most effective. The reaction was found to be general with di-/trialkyl phosphites and diaryl phosphite. The Mg(ClO4)2-catalyzed alpha-aminophosphonate synthesis in the present study perhaps represents a true three-component reaction as no intermediate formation of either an imine or alpha-hydroxy phosphonate was observed that indicated the simultaneous involvement of the carbonyl compound, the amine, and the phosphite in the transition state.
Zirconium(IV) compounds are reported as excellent catalysts for a three-component one-pot reaction of an amine, an aldehyde or a ketone, and a di/trialkyl/aryl phosphite to form alpha-aminophosphonates under solvent-free conditions at rt. Among the various zirconium compounds, ZrOCl2 x 8 H2O and ZrO(ClO4)2 x 6 H2O were most effective. The reactions were faster with dialkyl/diaryl phosphites than with trialkyl/triaryl phosphites. No O-Me cleavage occurs with aryl methyl ether and methyl ester groups. alpha,beta-Unsaturated carbonyl moiety does not undergo conjugate addition with the phorphorous moiety.
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