Intramolecular complex

Carboxylic ionophores selectively transport cations by using intramolecular complexation in the uptake process of cations (basic region). A new ion transport system has been developed which incorporates a structural device which assists in the release process by using intramolecular complexation of an [18]crown-6 ring and a primary ammonium ion 48>. The experimental conditions are shown in Fig. 7. All these com-... 

Protein tyrosine kinases (PTKs) are enzymes (EC 2.7.1.112) that catalyze the transfer of the y-phosphate group of ATP to tyrosine residues of protein substrates. The activity of PTKs is controlled in a complex manner by posttranslational modifications and by inter- and intramolecular complex formations. 

If crossing the central barrier is not rate-controlling in TST, then trapping in the ion-dipole complex must be incorporated into the statistical model and it is more difficult to represent the effect of central barrier recrossings correcting TST with the K factor is not sufficient. The recrossings and presence of both intermolecular and intramolecular complexes are expected to affect the k, kisom, and k rate constants in equation 6. The value for k should be smaller than that of a capture model, and kisom and k 8 should disagree with the predictions of RRKM theory. 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

Reaction of germylenes with elemental chalcogens (sulfur, selenium, tellurium) also allowed the synthesis of stable >Ge = E species. This was first exemplified by the isolation of terminal chalcogenido complexes of germanium 170152 and 171-173153 (Scheme 34). Stabilization in these cases is achieved by intramolecular complexation of the metal atom by Lewis bases. 

T. M. Li and J. F. Burd, Enzymic hydrolysis of intramolecular complexes for monitoring theophylline in homogeneous competitive protein-binding reactions, Biochem. Biophys. Res. Commun. 103, 1157-1165 (1981). 

It has also been proposed that during ring closure of this I-chroma-1,3,5-hexatriene intramolecular complexation of the terminal C-C double bond may be important [330] (Figure 2.25). 

It should also be mentioned that cases are possible when polarity of the solvent allows transforming charge-transfer intramolecular complexes into molecules containing the cation- and anion-radical... 

VI. OXYGENATED ORGANOLITHIUMS INTRAMOLECULAR COMPLEXATION AND ENTHALPIES OF FORMATION... 

If the enthalpy of formation of 4-lithiobutyl methyl ether is interpolated between the values for the lithiopropyl and the lithiopentyl ethers to be —285 kJ moP, then the enthalpy of isomerization to the less stable 3-lithiobutyl methyl ether is - -10 klmoP, which is about half that of isomerization of n-butyl lithium to 5ec-butyl lithium (-1-21.3 kJmol ). However, a linear interpolation assumes the same strain energy for the 6-membered 4-lithiobutyl ether as for the above 5- and 7-membered cu-lithioalkyl methyl ethers. If it is less strained, then the isomerization enthalpy would be larger. How much of the isomerization enthalpy difference is due to other differences, such as intramolecular complexation and/or aggregation among the various species, is not known. Unfortunately, there is no enthalpy of formation measurement for the delithiated 7-methoxynorbornane. 

Interestingly, the protodelithiation enthalpy of 2-lithio-l,3-dimethoxybenzene is very nearly the same as that for the single methoxy species o-lithioanisole. If the stabilization of lithium by an ortho ether group is due mainly to intramolecular complexation or... 

Worth mentioning are some relevant studies on the prototropic exchanges of azoles in excited states involving the formation of hydrogen-bonded complexes [(7a) - (7b)] via Si (85JPC399) in complexes with acetic or via Ti in complexes with benzoic acid (83JA6790), or intramolecular complexes, as in the case of 2-(2 -hydroxy-5 -methylphenyl)benzotriazole (82JCP4978). 

If the proposed structure was true, then a solvent of higher solvating power, such as dme, should decrease the stability of the intramolecular complex. Using sodium as the counterion part of the active centres should exist in the form of externally solvated contact ion pairs or even solvent-separated ion pairs. With caesium as the counterion there should be little change, because this cation is only poorly solvated by both solvents and consequently the possibility of solvent-separated ion pairs to be found should be extremely small. 

Polyhalosilanes form neutral intermolecular hexacoordinate complexes with donor molecules such as pyridine, triethylamine, 2,2 -bipyridine and 1,10-phenanthroline. This topic has recently been reviewed6. It was demonstrated that electronegative substituents on the silicon are essential for the formation of intermolecular complexes. Thus, while SiCLt and Cl2CHSiCl3 react with 1,10-phenanthroline and with 2,2 -bipyridine to form hexacoordinate chelates, MeSiCl3 does not react224,225. For completion we discuss here a few examples, and compare some of the properties of intermolecular complexes with those of the intramolecular complexes. 

An interesting example of an intermolecular complex is the trisilicon complex 194, in which only the central silicon is coordinated to the bidentate donor molecule225. The structure is a regular octahedron, with two tetrahedral termini. The silicon nitrogen bonds are rather short (2.012 and 1.991 A), and are comparable to those of octahedral intramolecular complexes (Table 23). 194 permits a comparison of Si—Cl bonds in a tetrahedral silicon moiety (2.03 to 2.07 A) with Si—Cl bonds trans to the dative bond in a hexacoordinate silicon (2.39 and 2.21 A). As expected, the latter are substatntially longer than the regular covalent bonds. 

When ArNH2 is o-phenylenediamine (80), the reaction is poorly catalysed by the second amino group, but it is mainly catalysed by an external molecule of amine. As a consequence, internal catalysis by an intramolecular complex such as 81 is unlikely. In competition with the substitution (Scheme 34), when the nucleophile (or a base) attacks a hydrogen atom in a fi position with respect to the leaving group, an elimination reaction takes place. 

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