Chelate addition compounds

Thiadiazoles are weak bases. They form salts with mineral acids and addition compounds with heavy-metal salts. Ethenebis(triphenylphosphane)platinum(0) reacts with 1,2,4-thiadiazole-3,5-dicarbonitrile (15) to give a chelate complex. Insertion of platinum takes place at the S—N bond to give the six-membered metalloheterocycle (16) (Equation (2)) <84AG(E)971>. 

On the basis of this evidence it was postulated that a 1 to 1 complex is formed between the metal ion and the amino acid ester, in which the metal ion chelates with the amino group and the carbonyl oxygen of the ester, and that this chelate is attacked by hydroxide ion to give the products of reaction through the intermediate formation of a tetrahedral addition compound. 

In addition to the ligands above, considerable attention is given to more complex ligand systems [4,5] aromatic and heteroaromatic compounds (heteroarenes) (i.e., five- or six-member cyclic structures with delocalized 7i-bonds in the ring containing, besides carbon atoms, either N, P, As, O, S, Se, or Te compounds [6-8]), various chelate-forming compounds, such as macrocyclic crown-ethers, cryptands, porphyrins, and phthalocyanines. 

The preparation of organopalladium compounds by exchange reactions of palladium salts and organo-lead, -tin, or -mercury compounds is apparently not the only way that they can be obtained but it does seem to be the most useful way. Convincing evidence is now available to show that direct metalation of aromatic compounds with palladium salts (palladation) can occur. Since the initial report of Cope and Siekman 32> that palladium chloride reacted readily with azobenzene to form an isolable chelated, sigma-bonded arylpalladium compound, several additional chelated arylpalladium compounds have been prepared. 

With aldehydes (140a) and (140b) in hand, further 1,2-additions of Me2CuLi have been examined and found to afford results (Scheme 25) opposite to those obtained with, for example, MeMgBr, in good yields (>85%). The preference is explained on the basis of attack onto the chelated bicyclic compound... 

Aerobic respiration, on the other hand, releases CO2 owing to the oxidation of organic matter, and thus, increases the concentration of CO2 in the system. The equilibria of the carbonate buffer system are shifted in the opposite direction and a dissolution of CaCOj results (Fig. 2.1.1, fine arrows). In addition to oxidative respiration, partial degradation (fermen-taton) of organic matter may result in formation of organic acids which also contribute to the dissolution of carbonates. Furthermore, carbonate may be dissolved by the activity of chelating organic compounds, or by the enzyme carbonic anhydrase. 

The catalysts found active in the norbornadiene dimerization are various nickel tt complexes, among them Ni(CO)4, Ni(CH2=CHCN)2, Ni(0) complexes of 1,1-dicyanoethylenes, and the corresponding phosphine substitution or addition compounds (3, 32, 33, 47). Other active catalysts are Co2(CO)8, Fe2(CO)9, Co2(CO)g 2P(CgH5)3, Co(CO)3NO, and Fe(CO)2(NO)2 (3, 4, 32, 33, 44, 48). Efficient catalysts were also obtained by the in situ reduction of Ni, Co, or Fe chelates with organo-aluminum compounds (46, 49). Finally, dimerization of norbornadiene could also be effected with Rh on carbon (45). The nickel catalysts... 

Table II shows that the chelation shift Achei is quite different in benzene solution. Here again the behavior of iso-HMTT LiBr is analogous to TMED LiBu, with a large upfield shift of the N-CH2- protons. This large upfield methylene shift (large positive Achei) in benzene is general for chelated lithium compounds and is seen as a manifestation of a stereospecific collision complex between benzene and the positive end of the molecular dipole moment of Chel LiX. To study this unusual interaction we have carried out extensive experiments in mixed CH2C12— benzene solvents, some results of which are discussed below. In addition to studying the origin of this upfield methylene chelation shift in benzene, we have taken advantage of its existence in a number of ways described below.

Unfortunately, both excess sodium compound and chelated sodium compound catalyze the telomerization, even when the former is less effective, and it is difficult to separate these two reactions. With excess chelating agent, however, the equilibrium shift is clear. The data in Table III for excess chelating agent support the concept of a chelated sodium compound in equilibrium with free chelating agent and sodium compound. It is also possible, particularly below 25 °C, that catalytic species of stoichiometry chel2NaX or chel(NaX)2 are involved in addition to chel-NaX in the telomerization reaction. 

The addition of heparin to the isolated rat diaphragm previously loaded with rubidium 86 results in change of the inflow of rubidium . Heparin has a permeabilizing action which appears to be due to mobilization of bound potassium and this can be related to a reduction of calcium in the perfusion solution. Karasek and Mourek.i conclude that heparin depresses oxidative processes through an effect on cell permeability it is possible that it adsorbs certain substances such as potassium chloride or acetylcholine. In extracorporeal dialysis , heparin causes an apparent decrease of 60 per cent in plasma calcium concentration. Heparin also forms a chelate-like compound with calcium ions " . This process is usually unimportant as heparin absorbs a maximum of only 5 per cent of its weight of calcium, but with intradermal, intramuscular or subcutaneous administration, heparin may deplete the capillary walls of calcium and cause them to become fragile. Heparin alsc decreases wound strength 6-14 days after operation in some experiments and may delay the union of fractures in bone repair . Heparin, but not chondroitin sulphate or hyaluronic acid, in tissue culture increases the amount of bone resorption in the presence of suboptimal concentrations of parathyroid extracts, and thus may be a cofactor in bone metabolism. ... 

A similar chelated TPP compound 60 was used by Walker to study the effect of axial ligand plane orientation on the H-NMR shifts of the pyrrole protons in iron(III) (TPP)bis(imidazole) cotnplexes In this case the tail was made shorter to study the effect of axial ligand bond strain. In addition. Walker and Benson have used mono-(o-aminophenyl)triphenylporphyrin (57) to prepare a series of derivatives 6la-e containing a pyridine ligand bound to a zinc TPP . H-NMR and visible spectroscopy were used to study the displacement of the 3-pyridyi ligand by free 3-picoline (Scheme 23),... 

In animals and humans, at intracellular pH, nearly all natural boron exists as boric acid, which behaves as a Lewis acid, and forms molecular additive compounds with amino- and hydroxy acids, carbohydrates, nucleotides, and vitamins through electron donor-acceptor interactions. In most of the commercial dietary boron supplements now available, boron is chelated with amino acids or with hydroxy acids (i.e., glycine, aspartic acid, or citric acid) in combination with vitamins. However, little is known about the molecular structure of these boron chelates. ... 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

The Lo-Cat process, Hcensed by US Filter Company, and Dow/Shell s SulFerox process are additional Hquid redox processes. These processes have replaced the vanadium oxidizing agents used in the Stretford process with iron. Organic chelating compounds are used to provide water-soluble organometaHic complexes in the solution. As in the case of Stretford units, the solution is regenerated by contact with air. 

The apparent acid strength of boric acid is increased both by strong electrolytes that modify the stmcture and activity of the solvent water and by reagents that form complexes with B(OH) 4 and/or polyborate anions. More than one mechanism may be operative when salts of metal ions are involved. In the presence of excess calcium chloride the strength of boric acid becomes comparable to that of carboxyUc acids, and such solutions maybe titrated using strong base to a sharp phenolphthalein end point. Normally titrations of boric acid are carried out following addition of mannitol or sorbitol, which form stable chelate complexes with B(OH) 4 in a manner typical of polyhydroxy compounds. EquiUbria of the type ... 

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