Attack, adjacent

M s-, outer-sphere reduction by Co(CN) -. Remote attack. Adjacent attack. J. H. ... 

A system which is very instructive in considering the factors that may affect the rate of reaction by adjacent attack is Craq+2 reacting with [(NHa) -... 

The chemical reactivities of groups in the apical and equatorial positions of pentacovalent intermediates are different.664 In particular, elimination of a nucleophilic group to form a tetrahedral phosphate is easier from an apical position than from an equatorial position. For the in-line displacement of Eq. 12-27 elimination of RO should be easy. However an adjacent attack would leave - OR in an equatorial position. Before it could be eliminated, the intermediate would probably have to undergo a permutational rearrangement by which -OR would be transferred from an equatorial to an apical position. 

Although inversion was not observed with the E. colt alkaline phosphatase, it has been observed for ribonucleases and many other hydrolytic enzymes and for most kinases transferring phospho groups from ATP. The difference lies in the existence of a phospho-enzyme intermediate in the action of alkaline phosphatase (see Eq. 12-38). Each of the two phosphotransferase steps in the phosphatase action apparently occurs with inversion. The simplest interpretation of all the experimental results is that phosphotransferases usually act by in-line -like mechanisms which may involve metaphosphate-ion-like transition states that are constrained to react with an incoming nucleophile to give inversion. An adjacent attack with pseudorotation would probably retain the original configuration and is therefore excluded. 

The methyl ester was crystallized and its absolute stereochemistry was determined by x-ray diffraction to be as in equation 8.37. This product corresponds to an in-line attack. When incubated with ribonuclease in aqueous solution, the methyl ester re-forms the original cyclic phosphorothioate (structure 8.36). This result is expected from the principle of microscopic reversibility, since the forward and reverse reactions must go through the same transition state. But it does show directly that the cyclization step involves an in-line attack an adjacent attack of the ribose hydroxyl in the cyclization of the methyl ester as in the right-hand structure 8.38 would give the enantiomer of structure 8.36. 

The X-ray data do not yet, and cannot conclusively, provide an answer to this question. The current interpretation does distinctly allow for the possibility that transphosphorylation occurs by an opposite attack (see Section VI,G,3) and that hydrolysis occurs by an adjacent attack. The dinucleotide phosphonate as depicted in Fig. 23 seems poised for an attack of 02 opposite C5"-CH2 while W, is in good position to make an... 

The question of opposite vs. adjacent attack and the need for pseudorotation in the case of the latter but not the former has been brought to the forefront by Usher (521) and Dennis et al. (528) who both worked with Westheimer on the organic chemistry of phosphorus. The various possibilities relevant to the current discussion are illustrated in Fig. 31. A set of statements and preference rules summarizing the findings are as follows ... 

N=C=S—OlOHj) ] are produced. (1 has been postulated ihat both remote and adjacent attacks are involved in the formation of these products. Draw bridging intermediates consisicni with this view for the formation of both producls. ... 

A review of one-electron oxidants certainly would not be complete without a discussion of the effect of changing the organic ligand L in [CoCNHj ljL] . Oxidations by [Co(NH3)jOC(0)R] are possible by adjacent attack, presumably at the carbonyl oxygen , or attack at a site remote from the Co(III) center, as when L is fumaric acid . The organic ligand itself also may accept an electron for a time long compared to nuclear vibrations, and the chemical mechanism often operates . 

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