Phosphorus atom

Commercial compounds are oil-soluble organic molecules containing chlorine, sulfur or phosphorus atoms (Figure 9.12). 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

In addition to the above acids and anions which contain only one phosphorus atom there are many other condensed phosphates(V)... 

Phosphorus Compounds. Acyclic phosphorus compounds containing only one phosphorus atom, as well as compounds in which only a single phosphorus atom is in each of several functional groups, are named as derivatives of the parent structures listed in Table 1.12. Often these... 

MOs can be constructed from 2s and 2p AOs on the oxygen atom and 3s and 3p AOs on the sulphur or phosphorus atom and appear similar to those shown in Figure 7.13. These linear combinations, such as 2p on oxygen and 3p on sulphur, obey all the conditions given on page 226 - notably Condition 1, which states that the energies of the AOs should be comparable. 

There is, in principle, no reason why linear combinations should not be made between AOs which have the correct symmetry but very different energies, such as the lx orbital on the oxygen atom and the lx orbital on the phosphorus atom. The result would be that the resonance integral /i (see Figure 7.12) would be extremely small so that the MOs would be virtually unchanged from the AOs and the linear combination would be ineffective. 

The reaction between esterase and phosphorus inhibitor (109) is bimolecular, of the weU-known S 2 type, and represents the attack of a nucleophilic serine hydroxyl with a neighboring imida2ole ring of a histidine residue at the active site, on the electrophilic phosphorus atom, and mimics the normal three-step reaction that takes place between enzyme and substrate (reaction ). 

Table 4. Properties and Electronegativity Differences of Phosphorus—Atom Bonds ...

Quite a number of cyclic phosphonium salts have been prepared over the years. We have not generally included compounds here which contain fewer than three potential binding sites in a ring unless they are mentioned as by-products in another reaction. Horner, Kunz and Walach have utilized the well-known alkylation approach to prepare cyclic phosphonium salts containing four phosphorus atoms. The formation of the cyclic tetraphosphonium salts is shown below in Eq. (6.14). 

Kyba and his coworkerswere also able to prepare a number of smaller, phosphorus-containing rings and several maeroeyeles containing both sulfur and phosphorus atoms. 

Photolysis of 2,3,5,6-tetrakis(trifluoromethyl)-l,4-di hoshabenzene gives 1,3,4,6 tetrakis(tnfluoromethyl)-2,5-diphosphatncyclo[3 1 0 0 ]hex-3-ene, an ana logue of benzvalene containing phosphorus atoms in the ring system [267, 16S] (equation 40)... 

The EPR spectra of the related 1,2,4,6,3,5-thiatriazadiphosphinyl radicals (3.20) reveal a distinctly different electronic structure.The observed spectrum consists of a quintet of triplets consistent with coupling of the unpaired electron with two equivalent nitrogen atoms and two equivalent phosphorus atoms [Fig. 3.4(a)]. This interpretation was confirmed by the observation that the quintet collapses to a 1 2 1 triplet when the nitrogen atoms in the ring are 99% N-enriched [Fig 3.4(b)]. Thus the spin delocalization does not extend to the unique nitrogen atom in the phosphorus-containing system 3.20. 

The heterocycles 13.1 are isolated as purple crystals (R = Mc3SiNH, Ph) or as a thermally unstable, purple oil (R = Me), which decomposes at room temperature to give 13.3 (R = Me, Ph). The structure of 13.1 (R = Ph) consists of a six-membered ring in which the phosphorus atom lies 0.26 A out of the S2N3 plane. The S-N bond lengths fall within a narrow... 

The diphosphadithiatetrazocines 13.2 (E = S) and 13.3 (E = S) contain one more R2PN unit in the ring than 13.1. The structure of the orange-red 1,3-isomer (13.2, E = S, R = Ph) consists of an essentially planar N3S2 unit with the two phosphorus atoms located on opposite sides of the plane." Like 13.1 this isomer forms a 1 1 adduct with norbornadiene via 1,3-addition across the sulfur atoms. 

The situation with phosphoric anhydrides is similar. The phosphorus atoms of the pyrophosphate anion are electron-withdrawing and destabilize PPj with respect to its hydrolysis products. Furthermore, the reverse reaction, reformation of the anhydride bond from the two anionic products, requires that the electrostatic repulsion between these anions be overcome (see following). 

DNA is not susceptible to alkaline hydrolysis. On the other hand, RNA is alkali labile and is readily hydrolyzed by dilute sodium hydroxide. Cleavage is random in RNA, and the ultimate products are a mixture of nucleoside 2 - and 3 -monophosphates. These products provide a clue to the reaction mechanism (Figure 11.29). Abstraction of the 2 -OH hydrogen by hydroxyl anion leaves a 2 -0 that carries out a nucleophilic attack on the phosphorus atom of the phosphate moiety, resulting in cleavage of the 5 -phosphodiester bond and formation of a cyclic 2, 3 -phosphate. This cyclic 2, 3 -phosphodiester is unstable and decomposes randomly to either a 2 - or 3 -phosphate ester. DNA has no 2 -OH therefore DNA is alkali stable. 

Diphenyl-2-thienylphosphine is expected to serve as a bidentate ligand by coordination of either the phosphorus and sulfur atoms or the phosphorus atom and TT-electrons of the heteroring. Reaction of this phosphine with [Rc2(CO)lo] yields the species where both the sulfur and phosphorus atoms serve as the donor sites (960M786). 

Reaction of diphenyl-2-thienylphosphine with Ru3(CO)l2 gives the tiVp) ti Ti C) coordinated species, 144, along with cluster 145 where two ligand molecules participate in coordination, one via the phosphorus atom and the C=C bond of the heteroring, and the other via the phosphorus atom only. P-Coordination in the products of such an interaction is known [95JOM(488)85]. Complex 144, the main product, interacts with carbon monoxide to yield the P-coordinated cluster, 146,... 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

JOM(400)149 96BSCF33]. The complex 164 is the first known t] -phospholyl species. The tungsten atoms have a coordination number of 9, and the carbon atoms of the phospholyl ring are coplanar. The phosphorus atom deviates from the plane of carbon atoms by 0.015 nm. The basic difference between the Ti -cyclopentadienyl and ri -phospholyl complexes is the existence of a low-lying LUMO localized mainly at the phosphorus atom. 

Acylation of the carbon atom in l-phenyl-3,4-dimethylphosphole becomes possible only after blocking the phosphorus atom by complex formation with Mo(CO)6 as in 146 (M = Mo, = Ph, = H, R = r" = Me). A series of complexes... 

Fe—Fe bond can be assigned structures 201 or 202 based on spectral data. The other product of this reaction is 193 (R = r-Bu), however, it is produced in minor amounts. Complexes 199 (R = R = r-Bu, R = Ph, R = r-Bu) were obtained. Reaction of 146 (M = Mo, R = Ph, R = R = Ft, R = r" = Me) with (benzyli-deneacetone)iron carbonyl gives rise to the bimetallic complex 200 (M = Mo), which reacts further with the free phosphole to form the bimetallic heteronuclear sandwich 203. The preferable coordination of the molybdenum atom to the dienic system of the second phosphole nucleus is rather unusual. The molybdenum atom is believed to have a greater tendency to coordinate via the trivalent phosphorus atom than via the dienic system. 

The nucleophilic properties of phosphorus in phosphaferrocene were demonstrated by reaction with n-butyllithium occurring at the phosphorus atom (81IC3252 820M312). 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

Bis(3,5-dimethylpyrazol-l-yl)cyclotriphosphazene containing phenyl substituents at the phosphorus atoms (R) reacts with molybdenum and tungsten hexacar-bonylstogive34(R = Ph M = Mo, W) [94JCS(D)63]. IfRj = N(Me)(CH2)20, compounds 34 (R2 = N(Me)(CH2)20 M = Mo, W) having better solubility in organic solvents are produced. Their structures show the novel tridentate NNN feature of the ligands. 

Benzonitrile with [(i -Cp )P W(CO)5 2] gives 82, the result of migration of the phosphorus atom, insertion of the nitrile moiety into the P-C bond and further C-H bond activation (01AGE3413). 

A transition structure model was proposed which accounts for the high selectiv-ities obtained for some of the substrates [80]. In the structure shown in Scheme 6.37 the two phosphorus atoms of the Tol-BINAP ligand and the two car-... 

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