Steric stabilization-anomalous stability

Steric Stabilization and Anomalous Stability. An explanation of the anomalous stability of Iler s silica sols in terms of steric stabilization effects requires that oligomeric or polymeric silicate species are present at the silica-water interface and that steric repulsion results during overlap of such layers. This mechanism is appealing in that soluble silicates, usually sodium silicates, are universal dispersants of many... 

Steric Stabilization and Anomalous Stability. 250 Hydration Effects. 251... 

Experimental work by Sundararaj and Macosko, nicely contrasted the competing effects of droplet breakup and droplet coalescence in both Newtonian and non-Newtonian mixtures. They concluded that the extent of interfacial tension reduction due to the presence of block copolymer was insufficient to be the primary reason for the reduction of the droplet size, and the primary effect of the copolymer was to prevent droplet coalescence through steric stabilization of the droplets. Sundararqj and Macosko also noted that the droplet size as a function of shear rate for a pure blend decreased to a minimum value, then increased at higher shear rates. No data was given for the compatibilized blend and they referred to this shear rate dependence of the dispersed phase size for the pure blend as "anomalous". Sundararaj and Macosko noted that this anomalous behavior has been observed previously by... 

On the basis of distribution of products obtained in these reactions with the change of solvents, temperature and molar ratio of reactants, a mechanism has been suggested for the anomalous annulation which does not involve an initial attack of the tetrasub-stituted isomer of the enamine (equation 16)51. Alkylation of the more stable cis isomer of the enamine (80) with methyl vinyl ketone (MVK) would afford zwitterion 81 (attack by the other side of the enamine leads to strong steric interactions in the transition state). Reaction of the thermodynamically less favorable trans isomer 83 gives rise to zwitterions 84 and 89 (both without axial-axial interactions), and ion 84 is sterically able to undergo intramolecular proton shift to afford enamine 85. Zwitterionic intermediates 81 and 89 can be stabilized by conversion to dihydropyrans 82 and 90, or protonated to immonium ions. The pair 81-82 will lead to enamine 85, while the pair 89-90 will afford enamine 91. Then, cyclization of 85 or 91 will afford the enone expected from the normal enamine version of the Robinson annulation. 

The problem we are facing is that of defining just what factor or factors are responsible for the difference in energies of the two transition states. Is the exo transition state stabilized by a-bridging Or is it the endo transition state that is anomalous — destabilized by some factor, such as steric hindrance to ionization" ... 

The role of steric interactions on anomalous stereoselectivity in the Wittig reaction has been examined. " DFT calculations combined with a distortion/interaction energy analysis show that the anomalous Z selectivity observed in Wittig reactions of o-substituted benzaldehydes is not caused by phosphorus-heteroatom interactions in the addition transition state but is predominantly steric in nature. The calculations reproduced correctly the stereoselectivity preferences for a wide range of reactant pairs as well as relative reactivities for different substituent types. An efficient synthesis of olefins by the coupling of stabilized, semi-stabilized, and non-stabilized phosphorus ylides with various carbonyl compounds in the presence of silver carbonate has been reported. Wittig olefination of aromatic, heteroaromatic, and aliphatic aldehydes (yields up to 97%) and a ketone (yield 42%) was demonstrated. 

In nucleotides however, position 5 is not free for acetal formation and the additional hydroxyl functions of the side chain can not form a macrocyclic ring for steric reasons (24). The anomalous thiohemiacetal stability towards hydrolysis cannot be simulated at the lumiflavin or riboflavin level, while experiments with 8a-oxo-FMN or -FAD are lacking. 

---