Monoalkyl H-phosphonate anion

In the NMR spectrum of the methylammonium salt of the monomethyl ester of H-phosphonic add, the phosphorus signal appears at 4.55 ppm as a doublet of quartets with V(P,H) = 600 Hz indicating phosphonate structure. Kers et al. [23] confirm that in the reaction mixture of diethyl H-phosphonate and pyridine or pyridine with 2 equiv. of tertiary base (TEA), there were no signals that could be assigned to Iriethyl phosphite. The results obtained show that the basic activation in these reactions involves at the first step formation of the monoalkyl H-phosphonate anion [(R0)P(0)(H)0] , which further plays the key role in interactions where the phosphorus atom acts as a nucleophile. 

This monoalkyl H-phosphonate anion is the actual base, which then deprotonates the dialkyl H-phosphonate. It has been established that alkylanunonium or metal salts, containing monoalkyl H-phosphonate anions, promote the Atherton-Todd reaction [87], Taking into account the observation of Kong and Engel that the Atherton-Todd reaction does not take place in the absence of a base, these results indicate that the monoalkyl H-phosphonate anion is playing a key role as an intermediate in the amine-promoted Atherton-Todd reaction. A recent theoretical ab-initio study [88] of the mechanism of the Atherton-Todd reaction favors the following reaction scheme, which is based on energetic evaluation of alternative reaction pathways ... 

In this new scheme, the initial step is the reaction of the amine with dialkyl H-phosphonate to form the corresponding monoalkyl phosphonium salt. The subsequent three steps form the catalytic cycle that leads to product formation. The anion of the monoalkyl H-phosphonate, acting as a base, first deprotonates the dialkyl H-phosphonate to generate the reactive diaUcyl phosphite anion. In the subsequent step, which is the same as in the originally proposed mechanism of that reaction, dialkyl chlorophosphate is formed along... 

The increased reactivity of the monoalkyl H-phosphonate salt of the type I toward nucleophilic substitution at the second a-carbon has been explained in terms of complex formation between the phosphonate anion and the metal Ihrongh the oxygen donors [306]. 

The possible role of the monoalkyl phosphonate anion [(R0)PH(0)0] as a proton acceptor from dialkyl H-phosphonate has also been discussed in the mechanism proposed by M. Kabachnik and E. Tsvetkov [175] for addition of dialkyl H-phosphonates to carbonyl compounds. 

The nitrogen atom of amines attacks the a-carbon atom of the alkoxy group of dialkyl H-phosphonates to give the corresponding alkylanunonium salts of the monoalkyl ester of phosphonic acid. These results are in contrast to the oflen-expressed opinion—especially in the cases of amine-promoted reactions of dialkyl phosphonates—that amines deproto-nate dialkyl H-phosphonates to form dialkyl phosphite anions II (Scheme 3.4.). 

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