Phosphorous substitution mechanisms

Aluminum. Previous Al NMR studies have demonstrated four possible local environments for Al in SAPO materials (3,4). These environments are illustrated in Figure 3, and may be classified as either phosphorous rich (i.e., ALPO -like) with a chemical shift ranging from 30 to 40 ppm, or silicon rich (i.e., zeolite-like) with a chemical shift greater than 48 ppm. Both types of environments are characteristic of a substitution mechanism involving silicon substitution for phosphorus. A fifth possibility for an Al environment involves two Si and two P second nearest neighbors. However, no such environment has yet been identified by NMR, either because the Al chemical shift is similar to that for the silicon- or phosporous-rich environments, or because materials with an appropriate level of Si to give rise to... 

It has generally been assumed that phosphorous oxychloride-pyridine dehydrations, the elimination of sulfonates, and other base catalyzed eliminations (see below) proceed by an E2 mechanism (see e.g. ref. 214, 215, 216). Concerted base catalyzed eliminations in acyclic systems follow the Saytzelf orientation rule i.e., proceed toward the most substituted carbon), as do eliminations (see ref 214). However, the best geometrical arrangement of the four centers involved in 2 eliminations is anti-coplanar and in the cyclohexane system only the tran -diaxial situation provides this. 

Harger has studied the rearrangement of A-substituted N-phosphinoylhydroxylamines in the presence of base . He proposed a concerted mechanism based on the observed retention of the configuration at the phosphorous center during the transposition , and on studies with 0-labelled compounds . Similar cyclic transition states 572 were proposed in the base-induced rearrangement of A,0-bis(diphenylphosphinoyl)hydroxylamines (571) (equation 254). However, in the rearrangement of O-benzoyl-A-(diphenylphosphino-thiol)hydroxylamine where a transposition of O and S atoms occurs, the proposed cyclic transition state has sulfur participation . 

Both acid and base catalysis have been used extensively to catalyze exchange in aromatic, and to a lesser extent, heterocyclic molecules. In acid exchange, the most widely used catalysts are sulfuric acid,122,129, 131 phosphoric acid,132 trifluoroacetic acid5133 perchloric acid,134 aluminum chloride,135 and the phosphoric acid-boron trifluoride complex.132 These reactions constitute the simplest electrophilic substitution. The mechanism for such substitution in benzenoid compounds is now comparatively well understood 122 however, the problem of heteroaromatic electrophilic substitution is still being clarified and has led to renewed interest in acid-catalyzed exchange in heterocyclic compounds.122... 

V. Gasmi, G. Robinet, M. Barthelat, and J. Devillers, Struct. Chem., 2, 621 (1991). Molecular Mechanics Use of an Extended Version of the MM2 Program for Pentacoordinated Phosphorous Compounds. 2. Conformational Analysis of Substituted Bicyclic Phosphoranes with a Ph2 Group. 

All glycerylphospholipids are considered as derivatives of sn-glyceryl-3-phosphoric acid that is acylated by two moieties of acjd-CoA producing 1,2-diacyl->yn-glyceryl-3-phosphoric acid. The mechanism of substitution of glyecrylphosphate of naturally occurring phospholipids produces phosphatidic acids with saturated or monounsaturated fatty acid residue in sn-l position and polyunsaturated acyl residue in sn-2 position Fig. (6). 

The thiirane part of the Dewar thiophene is not desulfurized by triphenylphosphine but isomerization to the thiophene occurs. This reaction with trivalent phosphorus compounds which are not substituted with electronegative groups is typical of Dewar thiophene. Therefore, a mechanism involving electron transfer from the phosphorous compound to the sulfur atom of the Dewar thiophene has been proposed. The reaction of Dewar thiophene with diphenylchlorophosphine proceeds via an intermediate which was isolated (124)129). 

Let us now turn to an example of nucleophilic substitution involving a group of pollutants other than alkyl halides. We consider the hydrolysis of thiometon and disulfoton, two insecticides that were among the major contaminants that entered the Rhine River after the famous accident at Schweizerhalle in Switzerland in 1986 (Capel et al., 1988). This example is representative for the hydrolysis of a variety of phosphoric and thiophosphoric acid derivatives (e.g., esters, thioesters, see Fig. 1), and it illustrates that hydrolysis of a more complex molecule may be somewhat more complicated. The kinetic data, as well as the proposed mechanisms of hydrolysis of thiometon and disulfoton, are presented in Table 4 and Figure 2, respectively. In these cases, the base catalyzed reaction... 

It is of pedagogical value to discuss the first two supercritical fluid-enzyme catalysis papers that appeared within two months of each other (Randolph et al., 1985 Hammond et al., 1985). Randolph studied the hydrolysis of the disodium salt of p-nitrophenyl phosphoric acid reacting to p-nitrophenol. Hammond studied the catalytic oxidation of p-substituted phenols to first, orthocatecholic compounds and in series to o-quinone compounds, which very quickly polymerize by chemical, not enzymatic, mechanisms to form o-quinoid polymers. The product recovered is a poly(o-quinone). 

Stereochemical information is important in the analysis of most reaction mechanisms. This is true for all substitution reactions at atoms with substituents in tetrahedral array, such as saturated carbon and tetravalent phosphorus. Enzymic substitution at phosphorus in phosphoric esters and phosphoanhydrides is not an exception to the rule. There are experimental complications, however, in that all naturally occurring biological phosphates have two or more chemically equivalent oxygens, so that none has chirally substituted phosphorus. Inasmuch as an asymmetric arrangement of substituents is required for stereochemical analysis, P-chiral substrates for stereochemical studies of phosphotransferases and nucleotidyltransferases must be synthesized with sulfur or heavy isotopes of oxygen as substituents in an asymmetric array. 

In nonenzymic substitution reactions of phosphoric esters and anhydrides, it is generally recognized that a diversity of mechanisms is allowed and that the... 

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