Crowding, intramolecular

Group Branch Extension of Aromaticity Sigma/Rho Fragment Interaction Special ortho effects Crowding Intramolecular Hydrogen Bonding... 

FIGURE 16.15 Orientation effects in intramolecular reactions can be dramatic. Steric crowding by methyl groups provides a rate acceleration of 2.5 X 10 for the lower reaction compared to the upper reaction. (Adaptedfrom Milstien,. S., and Cohen, L. A., 1972. Stereopopnlation control I. Rate enhancements in the laetonization of o-hyelroxyhyeJroeinnamie acid. Journal of the American Chemical Society 94 9158-9165.)... 

Unexpectedly, tho hydrolysis of jV-(2,4-dinitrobenzoyl)imidazole at 25 °C was found to be slower by a factor of 25 in comparison to iV-(4-nitrobenzoyl)imidazole. This lower reactivity of iV-(2,4-dinitrobenzoyl)imidazole was explained by a combination of steric crowding at the reaction center and intramolecular stabilization of the reactant state.[19]... 

These relations both demonstrate the inhibition of formation of the tetrahedral state which can be clearly attributed to steric crowding. Such a correlation as (5) confirms the attack at the neighbouring carbonyl group and this intramolecular catalysis for all this series. The activation parameters for the alkaline hydrolysis of these esters were also measured and are shown in Table 1. The enthalpies of activation of the 2-formyl, 2-acetyl, 2-propionyl, 2-isobutyryl and 2-pivaloyl esters are exceptionally small. These are... 

Similar statements for IP(tt4) can be made for the two diaminonaphthalenes (15,16). Peri-substituted naphthalenes are examples of molecules with strong intramolecular crowding which can result in unique physicochemical properties. 1,8-Bis(dimethylamino)naphthalene (19) ( proton sponge 88,89) has been found to have an... 

All these deviations have one similarity they occur in compounds having relatively crowded chiral centers bearing at least one highly anisotropic substituent. The differential effect of the CSA on the intramolecular anisotropic environment of a given nucleus (e.g., on aryl rotation) cannot presently be stated. 

B group) units and fewer dendritic (no unreacted B groups) units. The polydispersity in molecular weight and isomerism, the intramolecular cyclization, and the molecular shape variations due to steric crowding contribute to the random nature of hyperbranched polymers. 

Both the experimental and calculated equilibrium constants indicate the great thermodynamic instability of hemiorthoesters with respect to the corresponding esters and show why it is normally impossible to detect the tetrahedral intermediates in acyl-transfer reactions. On going from intermolecular to intramolecular reactions the tetrahedral intermediate becomes relatively more stable, and if the structure is more rigid (cf. [120], [121] in Table 17) or more sterically crowded (cf. [119]) the tetrahedral intermediate is more stable still. However, it is only with structues as rigid as tetrodotoxin or with the trifluoroacetate of pinacol that the hemiorthoester is more stable than the ester (see Section 1). ... 

Cyclization of nitrile oxides with a four-atom intervening chain to the alkene always leads to 5,6-fused bicylic isoxazolines possessing a bridgehead C—N double bond. This is in contrast to nitrone cycliza-tions where competition to form bridged bicyclic isoxazolidines is observed. The alkenyl oximes (73) and (74) cyclize in typical fashion via nitrile oxide intermediates (Scheme 21).33a>36 The stereochemistry of cyclization here was studied both experimentally and by calculation. The higher stereoselectivity observed with the (Z)-alkene is typical. (Z)-Alkenes cycloadd much slower than ( >alkenes in intermole-cular reactions this is attributed to greater crowding in the transition state. Thus, intramolecular cycloaddition of (Z)-alkenes depends on a transition state that is heavily controlled by steric factors. 

Figure 3.5 Schematic representation of the effect of steric hindrance generated by bulky side groups on a cyclic trimeric and a high polymeric phosphazene. Depolymerization of a high polymer to a cyclic trimer relieves the intramolecular crowding.

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