C-POF

Orbital structures. Show the orbital distribution of electrons in the outer energy level of the central atom in (a) SiFe ", (b) AsCls, (c) POF (phosphorus oxyfluoride), (d) C103 , (e) SeF4, (f) BrFs. 

The pronounced proclivity of phosphoric monoester monoanions to eliminate POf is not always recognizable from the characteristic pH profile of Fig. 1. The hydrolysis rate maximum at pH w 4 may be masked by a faster reaction of the neutral phosphoric ester, as in the case of a-D-glucose 1-phosphate63) or on hydrolysis of monobenzyl phosphate 64). In the latter case, the known ability of benzyl esters to undergo SN1 and SN2 reactions permits fast hydrolysis of the neutral ester with C/O bond breakage. The fact that the monoanion 107 of the monobenzyl ester is hydrolyzed some 40 times faster than the monoanion 108 of the dibenzyl ester at the same pH again evidences the special hydrolysis pathway of 107, rationalized by means of the metaphosphate hypothesis. 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

The highly electrophilic character of the POf ion would suggest a very unselective phosphorylation behavior. For example, the ratio of alkyl phosphate to inorganic phosphate obtained in hydrolyses of phosphoric esters in water/alcohol mixtures should reflect the molar ratio of water and alcohol. This is indeed found in numerous cases, e.g. in the hydrolysis of phenyl and 4-nitrophenyl phosphate monoanions 97) or of 4-nitrophenyl phosphate dianions 65) at 100 °C in methanol/ water mixtures of various compositions, as also in the solvolysis of the acetyl phosphate dianion at 37 °C 97) or of phosphoenol pyruvate monoanions 82). Calculations of the free energy of the addition reactions of water and ethanol to the POf ion support the energetic similarity of the two reactions 98) (Table 4). 

When monomeric metaphosphate anion POf (102) is generated form the phos-phonate dianion 170 in the presence of the hindered base 2,2,6,6-tetramethylpiper-idine, it undergoes reaction with added carbonyl compounds147), Thus, it phosphoryl-ates acetophenone to yield the enol phosphate, whereas in the presence of acetophenone and aniline the Schiff base is formed from both compounds, probably by way of the intermediate C6H5—C(CH3) (OPO e) ( NH2C6HS). This reactivity pattern closely resembles that of monomeric methyl metaphosphate 151 (see Sect. 4.4.2). 

The absorption and emission spectra of 10-CPT in water-methanol mixtures at room temperature (ca. 22 °C) in the pH range from neutral to basic in general exhibit a well-known naphthol-type behavior [4]. The absorption spectra in neutral water and methanol are nearly identical, the latter having a 3 nm bathochromic shift. The equilibrium between neutral (ROH, 380 nm) and Odeprotonated (RO, 420 nm) forms of 10-CPT is characterized by a pof 8.9 in a 68% mole H2O mixture (Fig. 1 right). The protonation of quinolinium nitrogen has a pKa of around 1.0, more than 3 pK units lower than the parent 6HQ. 

Figure 6. Brensted plot of the logarithm of the hydronium ion catalyzed hydrolysis rate constants vs. the pof the conjugate acid of the leaving group for a series of substituted aryloxytriethylsilanes in 48.6% water/ethanol at 25 C ( ) and for a series of substituted aryloxytributylsilanes in 40% water/ dioxane at 30°C (A ).

Ethyl anion reacts as a Br0nsted base to remove a proton from the alkyne. The proton at C-l is removed because it is the most acidic, having a pof approximately 25. 

The hydrolysis of (i-C IIyO POF was studied at different acidities. The apparent first-order rate constant, k, at a particular temperature was found to depend on pH but not on the nature or concentration of the buffer used to regulate the pH. The value of k was fairly constant from pH 4 to pH 7, but rose from this constant value with decreasing pH below 4 or with increasing pH above 7. What is the nature of the cause of this behavior ... 

Substituents are expected to alter the electron density at the multiply bonded nitrogen atom, and therefore the basicity, in a manner similar to that found in the pyridine series. The rather limited data available appear to bear out this assumption. The additional ring nitrogen atoms in triazoles, oxadiazoles, etc. are quite strongly base-weakening cf. diazines are weaker bases than pyridine. As regards C-substituents, their effects on the pof the parent compounds are as follows ... 

If// is taken to be the HF Hamiltonian, so that V= F pOf Eq.(3.33), then certain simplifications occur. In particular, all diagrams containing the loop structure cancel with corresponding diagrams having the potential symbol — in the same location except in first order. For example, diagrams B and C of Fig. 3.3 cancel since the value of diagram C is... 

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