Oligomeric species, equilibrium

As previously pointed out in Chapter 2, monomeric stannylene can be in equilibrium with oligomeric species which are formed by tin-tin or tin-substituent inter-molecular interactions. The tendency for the formation of the oligomers increases the more the molecules approach one another. Thus, when passing from the vapor to the liquid phase and finally to the solid state, the molecules usually exhibit quite different structures. In Table 13 examples of the corresponding structural changes are given. 

The alkylation of substrates 16 or 34 by catalyst 33 was found to exhibit a pronounced positive non-linear effect [34]. In addition, there was an optimal concentration of catalyst 33 in these reactions, with higher or lower catalyst concentrations being detrimental to the asymmetric induction [41]. These results are consistent with a dimeric form of catalyst 33 being catalytically active and in equilibrium with catalytically inactive monomeric and oligomeric species. On the basis ofthese results,... 

In acid, the predominent species are V3+, V02+, and cis-V02 (Fig. 1). For simplicity, coordinated water molecules have been omitted from these formulas. As the pH is raised, hydrolysis takes place and a number of monomeric and oligomeric species are formed, only some of which are shown in Figure 1. Because of the multiple equilibria involved and the tendency for equilibrium to be attained slowly, the elucidation of all of the species present and their respective formation constants is a difficult task. 

The molecular weight of the polymer at equilibrium during cyclosiloxane polymerizations is controlled by including a hexaalkyldisiloxane such as MM or an oligomeric species such as MDM or MD2M. The equilibrium is represented as follows ... 

Although the presence of mixed bridging ligands may favour the formation of high nuclearity assemblies, it remains difficult to pinpoint the factors that give rise to these unusual cyclic products. In accordance with the previous discussion, it seems likely that these multi-component species may exist in solution in equilibrium with related oligomeric species their isolation will thus be influenced by both the position of the equilibrium as well as by the relative solubilities of the respective species in the chosen reaction solvent. 

On the other hand, the total equilibrium concentration of the combined oligomers relative to polymer is inversely dependent upon total RSi03/2 levels e.g., with 87% solvent in the system, 66% of the ViSi03,2 is present in discrete oligomeric species. The... 

This sequence of condensation requires both depolymerization (ring opening) and the availability of monomers, which are in solution equilibrium with the oligomeric species and/or are generated by depolymerization (reverse of Eqs. 9 and 10). However in alcohol-water solutions normally employed in sol-gel processing, the depolymerization rate is lower than in aqueous media, especially at low pH [1]. Under these conditions. Her [1] proposed ... 

To avoid the intrinsic instability of cyanohydrins and their silyl ether, Saa and coworkers reported catalytic asymmetric cyanophosphonylation reaction of aldehydes with commercially available diethyl cyanophosphonate [58]. In these works, Lewis acid-Lewis base bifunctional catalyst (65) prepared by mixing BI-NOLAM ligand with amino arms as Lewis base and Et2AlCl was found to work nicely (Scheme 6.46). Since a strong positive nonlinear effect was observed in this reaction, actual catalyst is in equilibrium with some oligomeric species of the aluminum complexes. Bifunctional catalyst (65) could also catalyze cyanosilylation of... 

Consider the formation of the oligomeric mixed-charge weak acid species, (AH A) . In addition to the required equilibrium Eqs. (2a) and (10a), the addi-honal reaction is needed to describe the model ... 

In favorable cases for some single olefmic species, thermodynamic equilibria with respect to double-bond position can be attained at low temperatures without significant oligomerization. For example, ris-2-bu-tene has been isomerized with the P(CH3)3-modified catalyst at -20°C to give the thermodynamic equilibrium mixture having the composition 78.8% trarts-2-butene, 19.5% m-2-butene, and 1.7% 1-butene. The isomerization of 4-methyl-1-pentene (or of 2-methyl-1-pentene) in the presence of the same catalyst at 0°C also leads to the thermodynamic equilibrium mixture with the composition shown in Table II. If the reaction is carried out at low temperature, the individual isomerization steps can be followed At -58°C, isomerization to ci i-4-methyl-2-pentene occurs and is followed by isomerization to the trans isomer this is then con-... 

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