Steric effects intramolecular

The variation in the add-catalyzed hydrolysis of aliphatic carboxylic esters RCOOEt is mostly subject to the steric effect intramolecularly exerted by the substituent R on the formation of the tetrahedral reaction intermediate. Taft4) defined the steric parameter E, as Eq. 3,... 

Deviations from this generalization may have several sources, including charge repulsion, steric effects, statistical factors, intramolecular hydrogen bonding, and other structural effects that alter electron density at the reaction site. Hague - ° P has discussed these effects. 

Another comparison is between o-hydroxy and o-methoxy compounds, as in 15 and 16 the electronic and steric effects will be, very roughly, similar, but only the hydroxy group is capable of intramolecular catalysis. 

In addition to the geometric, steric effects which flexibility directly modifles, all intramolecular electronic properties of a flexible molecule are also coupled to molecular structure. In a real material, such properties as molecular dipole. 

Synclinal and antiperiplanar conformations of the TS are possible. The two TSs are believed to be close in energy and either may be involved in individual systems. An electronic tt interaction between the stannane HOMO and the carbonyl LUMO, as well as polar effects appear to favor the synclinal TS and can overcome the unfavorable steric effects.161bi 162 Generally the synclinal TS seems to be preferred for intramolecular reactions. The steric effects that favor the antiperiplanar TS are not present in intramolecular reactions, since the aldehyde and the stannane substituents are then part of the intramolecular linkage. 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

The significance of B—C ir-bonding in determining the course and the ease of chemical reaction with organoboranes is obviously a more difficult question for both the experimentalist and the theoretician. Again, both inductive and resonance effects can, in principle, play a role but in addition, steric effects now become important, either for intramolecular reactions, as in Eq. (5), or for intermolecular reactions, as in Eqs. (7, 8). 

Certain specific steric effects are operative on intramolecular nitrile oxide— olefin cycloadditions. These effects are governed by both ring size and character of substituents. Thus, cycloadditions to the exomethylene group are successful with substituted methylenecyclohexanones 334 (m = 1, 2 n = 2) and gave tricyclic 335 (m = 1, 2), but do not occur with methylenecyclopentanones 334 (m = 1, 2, 3 n = 1). Activation energies calculated by molecular mechanics are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives cA-decalone 336 (403). 

In conclusion to this section, the two step reaction requires the presence of a metal that can be easily oxidized (d8 over d6). However the 1,3 migration step does not appear to be easy in an intramolecular way. An intermolecular 1,3 shift seems feasible but is probably highly sensitive to steric effects. The pathways for the isomerization reactions with d6 and d8 metal complexes are significantly different. They have in common the preference for the hydrogen to move as a proton, whether the shift starts from Ca or from the metal center. 

Experimental Rate Constant Data Illustrating the Role of Steric Effects in Strengthening the Intramolecular Hydrogen Bond in the Monoanion of 2,2-Disubstituted Malonic Acids in Aqueous Solution (25 C, 0.1 M NaClO )... 

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

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