Dimeric complexation

Bicyclo[2.2.1]hepta-2,5-diene rhodium (I) chloride dimer (norbornadiene rhodium chloride complex dimer) [12257-42-0] M 462, m 240°(dec). Recrystd from hot CHCl3-pet ether as fine crystals soluble in CHCI3 and C H but almost insoluble in Et20 or pet ether. [7 Chem Soc 3178 1959.]... 

Also, reactive silylene complexes of iron and chromium can be generated at low temperatures and subsequently derivatized by trapping reagents. In THF as solvent, first labile THF adducts are formed, which are converted to the more stable HMPA adducts. The THF complexes dimerize above —40 °C with loss of THF. The silylene complexes can be utilized for reactions if they are generated in the presence of reagents like dimethylcarbonate. The resulting reaction products... 

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

In be complexes bci complexes of mitochondria and bacteria and b f complexes of chloroplasts), the catalytic domain of the Rieske protein corresponding to the isolated water-soluble fragments that have been crystallized is anchored to the rest of the complex (in particular, cytochrome b) by a long (37 residues in bovine heart bci complex) transmembrane helix acting as a membrane anchor (41, 42). The great length of the transmembrane helix is due to the fact that the helix stretches across the bci complex dimer and that the catalytic domain of the Rieske protein is swapped between the monomers, that is, the transmembrane helix interacts with one monomer and the catalytic domain with the other monomer. The connection between the membrane anchor and the catalytic domain is formed by a 12-residue flexible linker that allows for movement of the catalytic domain during the turnover of the enzyme (Fig. 8a see Section VII). Three different positional states of the catalytic domain of the Rieske protein have been observed in different crystal forms (Fig. 8b) (41, 42) ... 

Scheme 8 Zn2 + ( OEt) promoted cleavage of 32 where two of the initially formed 32 Zn2 + ( OEt) complexes dimerize and subsequently undergo and intramolecular cyclization.

The ruthenium complex dimer (3.06 mg, 0.25 mol%) and the chiral ligand (5.08 mg, 2 mol%) were then weighed into the round-bottomed flask and any moisture was azcotropically removed via evaporation of benzene (5x5 mL) at reduced pressure. 

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

Rates of deprotonation of a simple ketone (89) by lithium diisopropylamide (LDA) in THF at -78 °C show a first-order dependence on ketone, and an order of 0.58 ( 0.06) in base. Alternative pathways involving the LDA monomer and its solvent-complexed dimer (90) are considered. 

In the solid state NMR study, uncomplexed phenyllithium, assumed to be a tetramer, as well as the TMEDA complexed dimer and the PMDTA complexed monomer were investigated. Both Li and Li isotopes were used in the preparations. The C spectra of the complexes are presented in Figure 12. It is evident that the substitution of Li with Li has profound effects on the Unewidths, especially of the ipso-carbon at ca 180 ppm in the aggregated uncomplexed system (Figure 12a and 12b, respectively). This is in accordance with the previously mentioned study of methyllithium. However, even the other positions are affected by the dipolar couplings to the four quadrupolar lithium cations, but to a lesser extent due to the larger C-Li distances. 

The observation of a bonding arrangement of the type shown in Fig. 20a in (AlO)2-centred dimers is a recurrent feature of aluminium hydroxides and organooxides of simple, electron rich ligands [44, 99, 110, 125, 134, 204, 205, 214-231]. Those more complex dimers which have been noted take two main forms both polydentate terminal and bridging ligands allow multiple interactions with the Group 13 metal centres. 

First isolated from human urine, biopterin (Fig. 15-17) is present in liver and other tissues where it functions in a reduced form as a hydroxylation coenzyme (see Chapter 18).338 It is also present in nitric oxide synthase (Chapter 18).341/342 Other functions in oxidative reactions, in regulation of electron transport, and in photosynthesis have been proposed.343 Neopterin, found in honeybee larvae, resembles biopterin but has a D-erythro configuration in the side chain. The red eye pigments of Drosophila, called drosopterins, are complex dimeric pterins containing fused 7-membered rings (Fig. 15-17).344 345... 

Figure 17.4 Intracellular steroid receptor activation by hormone ligands, (a) Steroid hormones diffuse across the cell membrane into the cell. (b) Steroid hormone receptors in the basal state bound to accessory proteins, (c) Steroid hormones bind to receptors and accessory proteins are dissociated from the receptors. (d) Hormone receptor complexes dimerize. (e) Dimer complexes enter the nucleus and initiate transcription of responsive genes.

The observation that fractions of the rosey periwinkle, Catharanthus rosea, produced severe leukopenia, resulted in the isolation and development of two major anticancer drugs, vincristine and vinblastine. These two complex, dimeric indole-indoline alkaloids are important in the treatment of acute childhood leukemia (vincristine), Hodgkin s disease (vinblastine), and metastatic testicular tumors (vinblastine), and continue to be manufactured today by mass cultivation and processing of their natural source. 

To mimic c T-[(NH3)2Pt(H20)2]2+ with the more rapidly exchanging Pd11 and to prevent isomerization, it is necessary to employ the complex of ethylenediamine (en), [(en)Pd(H20)2]2+, that through chelation is necessarily cis. Upon titration with standard base an endpoint is reached after the addition of only one equivalent of base at pH 7.5, but the reversible titration curve is flattened on the pH axis and cannot be fitted with the equilibrium expression for a simple deprotonation. It was proposed that the mono-hydroxo complex dimerizes to abinuclear dihydroxo-bridged dimer [9], The two reactions and their equilibrium constant expressions follow. 

The corresponding dimeric analogues of lipids are also important, owing to their occurrence in biological membranes. For example cardiolipins, which constitute a class of complex dimeric phospholipids that occur mainly in the... 

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

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