Phosphorane dianion

In the second model system 3.28 [56], a proton inventory study indicates that the guanidinium group is involved in hydrolysis as a general acid, rather than simply affording electrostatic stabilization of the phosphorane dianion. The nucleophilic part of the reaction, the cyclization of the 2-hydroxypropyl phosphate, is much less efficient than for 3.25, and the leaving group is a phenol so the rate determining step will be the formation of the phosphorane, as shown (3.28, Scheme 2.28). 

In the aprotic solvent acetonitrile, the trianion of cyclic trimetaphosphoric acid 115 is formed via the phosphorane 112 and the pyrophosphate dianion 113 protic solvents differ by reacting via the phosphorane 114 to give the phosphates 104 or H2PO . 

The common nucleophile in ribonuclease enzymes, and thus in relevant models, is the 2 -OH group of the central nucleotide. The work of the Williams group [31] confirmed the mechanism of hydrolysis of uridyl esters (Scheme 2.14, base = U) with good, substituted-phenol leaving groups as a relatively simple process, described by the simple general base catalysis mechanism (Bronsted p = 0.67), with 2.10 2.11 as the rate determining step (Scheme 2.14), followed by rapid breakdown of the presumed phosphorane (pentacovalent addition) intermediate dianion... 

On the other hand, phosphorane intermediates are not expected to be involved in the hydrolysis of phosphate monoesters, so the effective observed catalysis by the carboxyl group of salicyl phosphate 3.21 [51] (Scheme 2.26) is presumed to be concerted vith nucleophilic attack. (The hydrolysis reaction involves the less abundant tautomer 3.22 of the dianion 3.21, and the acceleration is >10 -fold relative to the expected rate for the pH-independent hydrolysis of the phosphate monoester dianion of a phenol of pK 8.52.) However, this system differs from the methoxy-methyl acetals discussed above, in that there is a clear distinction between neutral nucleophiles, which react through an extended transition structure similar to 3.16 in Scheme 2.23, and anions, which do not react at a significant rate, presumably because of electrostatic repulsion. This distinction is well-established for the dianions of phosphate monoesters with good leaving groups (p-nitrophenyl [52] and... 

Elsasser, B., Valiev, M., 8c Weare, J. H. (2009). A dianionic phosphorane intermediate and transition states in an associative A(N)+D-N mechanism for the ribonucleaseA hydrolysis reaction. Journal of the American Chemical Society, 131, 3869. 

The dianionic species (39) can be generated directly from 2,4-dimethyl-3-furoic acid (LDA-THF, -78 °C) and behaves as a good nucleophile in some trial reactions. Various routes to pyridylpropiolic acids (40) have been investigated pyrolysis of the appropriate phosphorane was found to be the most expeditious method. 

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