P—O Bonds

In each case the P—O bonds have some multiple character. Phosphinic acid is a moderately strong monobasic acid. On heating the acid and its salts they disproportionate evolving phosphine ... 

The next example shows how different search queries can be combined to shed more light onto a series of related reactions. A reaction substructure search for reactions that break a P-O bond provided 304 reactions as hits. Figure 10.3-27 shows one of the reactions in this hit list. 

Figure 10.3-27 Example of a reaction which breaks and makes a P-O bond.

ATP IS the mam energy storing molecule for practically every form of life on earth We often speak of ATP as a high energy compound and its P—O bonds as high energy bonds This topic is discussed m more detail m Sections 28 4 and 28 5... 

One of the most widely used methods for the formation of phosphate esters involves the conversion of a P-N bond of a phosphorus(III) compound to a P-O bond by ROH, catalyzed by l//-tetrazole, followed by oxidation to the phosphorus(V) derivative ... 

It is interesting to determine which bond (the P—O bond marked a or the O—P bond marked b) is cleaved by hydrolysis (reaction with water). 

Phosphonates are organic phosphates characterized by a C-P-O bond, which is much more resistant to hydrolysis than the polyphosphate bond (O-P-O) or the phosphate ester bond (C-O-P), making them suitable for many types of BW treatment formulation. Phosphonates were originally developed for the industrial and institutional (I I) cleaning market in the 1960s, but are commonly employed in a myriad of applications. 

Various phospono- and phosphinopolycarboxylic acids (PCAs) are available in the market. These polymers are similar to phosphonates and some actually are phosphonates. They tend to exhibit varying degrees of both deposit control and corrosion control properties. For BW applications, the acrylic acid/organic phosphate polymer (PCA type 16) is the only important phosphinopolycarboxylic and has a C-P-C bond (phosphonates have a C-P-O bond). 

The relative strengths of bonds are important for understanding the way that energy is used in bodies to power our brains and muscles. For instance, adenosine triphosphate, ATP (35), is found in ever)- living cell. The triphosphate part of this molecule is a chain of three phosphate groups. One of the phosphate groups is removed in a reaction with water. The P O bond in ATP requires only 276 kjmol-1 to break and the new P—O bond formed in H2P04 releases 350 kj-mol-1 when it forms. As a result, the conversion of ATP to adenosine diphosphate, ADP, in the reaction... 

The carbonyl n face of the adamantan-2-one with an electron-withdrawing group at the 5-position is nnsynunetrized by interaction of the P o bonds antiperiplanar to the C-H bonds and to the C-R bond. The orbital phase environment of the carbonyl n orbital (7) is nnsynunetrized by the more electron-donating orbitals at the P-position, which is consistent with the observed syn preference. 

Although, as stated above, we wiU mostly focus on hydrolytic systems it is worth discussing oxidation catalysts briefly [8]. Probably the best known of these systems is exemphfied by the antitumor antibiotics belonging to the family of bleomycins (Fig. 6.1) [9]. These molecules may be included in the hst of peptide-based catalysts because of the presence of a small peptide which is involved both in the coordination to the metal ion (essential co-factor for the catalyst) and as a tether for a bisthiazole moiety that ensures interaction with DNA. It has recently been reported that bleomycins will also cleave RNA [10]. With these antibiotics DNA cleavage is known to be selective, preferentially occurring at 5 -GpC-3 and 5 -GpT-3 sequences, and results from metal-dependent oxidation [11]. Thus it is not a cleavage that occurs at the level of a P-O bond as expected for a non-hydrolytic mechanism. 

A. Pentoses.—t-Ascorbic acid 2- and 3-phosphates, together with their phosphate esters, give a characteristic colour with ferric chloride and this colour reaction has been used in a study of the hydrolysis of L-ascorbic acid 3-phosphate (58). The acid-catalysed, pseudo-firsi-order hydrolysis proceeds with P—O bond fission, as does the bromine oxidation of its phenyl ester. Both of these observations can be rationalized if (58) is... 

Metal ions have a profound influence on the hydrolysis of acetyl phosphate, Thus, in the magnesium(ii)-catalysed system P—O bond flssion... 

The force constants of the Ni—P bond in P " nickel carbonyl complexes increase in the order MeaP < PHg < P(OMe)a < PFs. This order is different from that of the donor-acceptor character, as estimated from uco-The lengthening of the P—O bond of triphenylphosphine oxide upon complexation with uranium oxide has been estimated by i.r. spectroscopy. However, A -ray diffraction shows little difference in the P-O bond lengths (see Section 7). Some SCF-MO calculations on the donor-acceptor properties of McaPO and H3PO have been reported. 

The crystal structures of a number of diphosphine disulphides (121) and (122) show a remarkable constancy in the bond lengths. Two types of molecule are observed in the crystal of the tetramethyl compound (121, X = Y = Me). The crystal structure of triphenylphosphine oxide (P—C 176 pm, P—O 164 pm) varies little from that observed in the uranium oxide complexes, and does not confirm P—O bond lengthening in complexes, as indicated by vp=.o (see Section 3C). 

The reaction with phosphite esters is known as the Michaelis-Arbuzov reaction and proceeds through an unstable trialkoxyphopsphonium intermediate. The second stage is another example of the great tendency of alkoxyphosphonium ions to react with nucleophiles to break the O—C bond, resulting in formation of a phosphoryl P—O bond. 

A structurally characterized example of a dinuclear zinc complex with a bridging phosphate monoester was provided by Kitajima and co-workers using the tris(pyrazolyl)borate ligand system. The P—O bond in a tris- or bis-phosphate ester is cleaved by a hydroxo zinc complex resulting in the monoester compound.443... 

A kinetic isotope effect 160/180 of 2% in the spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion, whose ester oxygen is labeled, suggests a P/O bond cleavage in the transition state of the reaction, and thus also constitutes compelling evidence for formation of the metaphosphate 66,67). The hydrolysis behavior of some phosphoro-thioates (110) is entirely analogous 68). 

It should be mentioned, however, that the phosphoramidothioate 202 can undergo hydrolysis by another mechanism which becomes operative above all in polar solvents (e.g. aqueous KOH, and less so in methanol or acetone). P—O bond cleavage occurs, presumably via an addition/elimination mechanism, while the metaphosphorimidate pathway is characterized by P—S bond cleavage. 

---