Solvent complexes

Simplest examples are prepared by the cyclic oligomerization of ethylene oxide. They act as complexing agents which solubilize alkali metal ions in non-polar solvents, complex alkaline earth cations, transition metal cations and ammonium cations, e.g. 12—crown —4 is specific for the lithium cation. Used in phase-transfer chemistry. ... 

Dardi P S and Dahler J S 1993 A model for nonadiabatic coupling in the photodissociation of l2-solvent complexes J. Chem. Phys. 98 363-72... 

The stereoselectivity of an addition reaction is considerably lower when the reactions are conducted in polar solvents, complexing additives such as /V./V,A. A, -tetramethylethylenedi-arnine arc used, or when the stereogenic center carries a methoxy group instead of a hydroxy group. This behavior is explained as competition between the cyclic model and a dipolar model, proposed for carbonyl compounds bearing a polar substituent such as chlorine with a highly... 

Under the same conditions, but in moist solvents, complexes of type 100 with terminal alkynes 90 gave 2-acyl-3-amino- 107 and 2-acyl-3-ethoxycy-... 

In any solvent system, the essential factors required for dissolution of cellulose include adequate stabihty of the electrolyte/solvent complex cooperative action of the solvated ion-pair on hydrogen bonding of cellu-... 

Polymeric forms have also been reported. One example, which also includes germanium heteroatoms terminating the chain, is the oligomer (RO)Ge(RO)2Co(RO)2Co(RO)2Ge(OR), (97) where each Co center is surrounded by four bridging tert-butoxide ions.416 These form via a photochemically induced labile solvent complex, or else through thermally induced substitution... 

The possible mechanisms for solvolysis of phosphoric monoesters show that the pathway followed depends upon a variety of factors, such as substituents, solvent, pH value, presence of nucleophiles, etc. The possible occurrence of monomeric metaphosphate ion cannot therefore be generalized and frequently cannot be predicted. It must be established in each individual case by a sum of kinetic and thermodynamic arguments since the product pattern frequently fails to provide unequivocal evidence for its intermediacy. The question of how free the PO ion actually exists in solution generally remains unanswered. There are no hard boundaries between solvation by solvent, complex formation with very weak nucleophiles such as dioxane or possibly acetonitrile, existence in a transition state of a reaction, such as in 129, or SN2(P) or oxyphosphorane mechanisms with suitable nucleophiles. 

T Higuchi, S Dayal, I Pitman. Effects of solute-solvent complexation reactions on dissolution kinetics Testing of a model by using a concentration jump technique. J Pharm Sci 61 695, 1972. 

At room temperature in methanol as solvent, complex OsH(t 2-H2BH2)(CO) (P Pr3)2 decomposes to give the dihydride-dihydrogen compound OsH2(t 2-H2) (CO)(P Pr3)2. If the decomposition, however, is carried out under reflux the cis-dihydride-ds-dicarbonyl derivative OsH2(CO)2(P Pr3)2 is formed.13... 

The catalyst system employed was the cationic rhodium solvent complex [Rh(P-P)S2]+ (P-P = BINAP, CHIRAPHOS, S = solvent). The BINAP ligand enhances the activity of the complex (Table 10), although additional studies have revealed that both the solvent and the substituents on Si influence the levels of enantioselectivity (Scheme 29).131,132... 

Favretto and Tunis [198] extracted polyoxyethylene alkyl phenyl ethers as picrates into an organic solvent, complexed the polymer with sodium ion, and measured the absorption complex. This is one of the few specific methods available. 

Survival of the ions is assisted by the solvent. Complexes with bridging tin atoms, and mixed complexes, are particularly interesting. Oxidation of the latter is followed by cleavage of either of the tin-metal bonds as in the following example ... 

As discussed above, Li+ can be isolated from a solution containing other cations by complexation with specific macrocyclic ligands and subsequent extraction of the Li+-ligand complex into organic solvents. Complexation of the ligand alters the absorption profile in the UV-vis region and can therefore be followed spectrophotometrically. 

The catalytic asymmetric hydrogenation with cationic Rh(I)-complexes is one of the best-understood selection processes, the reaction sequence having been elucidated by Halpern, Landis and colleagues [21a, b], as well as by Brown et al. [55]. Diastereomeric substrate complexes are formed in pre-equilibria from the solvent complex, as the active species, and the prochiral olefin. They react in a series of elementary steps - oxidative addition of hydrogen, insertion, and reductive elimination - to yield the enantiomeric products (cf. Scheme 10.2) [56]. 

The common further treatment of the approach - assumption of steady-state conditions for the intermediate substrate complexes, consideration of the catalyst balance ([catalyst]0=[solvent complex] + [IRe] + [Isi] + [IIRe] + [Hsi]) and of the stoichiometry of the hydrogenation - provides the rate of hydrogen consumption under isobaric conditions (Eq. (13)) [57f]. A more general derivation can be found in [59]. 

The value l/KM corresponds to the ratio of concentrations of the sum of all catalyst-substrate complexes to the product [solvent complex]-[substrate], and thus is a measure of how much catalyst-substrate complex is present [60]. 

As explained earlier, the pre-equilibria are characterized by the limiting values of Michaelis-Menten kinetics. In the case of first-order reactions with respect to the substrate, we have Kfvl [S]0. Since the pre-equilibria are shifted to the side of educts during hydrogenation, only the solvent complex is detectable. In contrast, in the case of zero-order reactions only catalyst-substrate complexes are expected under stationary hydrogenation conditions in solution. These consequences resulting from Michaelis-Menten kinetics can easily be proven by var-... 

For both cases in these examples only the solvent complex is detectable, besides traces of non-hydrogenated COD-precatalyst (cf. Fig. 10.20). 

In these cases, the 31P-NMR spectrum exhibits only signals of substrate complexes there is almost no solvent complex visible. This is illustrated for (Z)-2-ben-zoylamino-3-(3,4-dimethoxyphenyl)-methyl acrylate with [Rh((S,S)-DIPAMP) (MeOH)2]BF4 in Figure 10.22. 

Curve c as the solvent complex ([Rh(Et-DuPHOS)(MeOH)2]BF4). Reaction conditions for each case 0.01 mmol catalyst, 1.0 mmol substrate, 15.0 mL MeOH, 1.0 bar total pressure, 25.0°C. 

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