Strain steric

One possible explanation for the diastereoface selection (AAGt -78°C 3 kcal/mol) observed for these chiral enolates is illustrated in Scheme 23. In the respective aldol transition states derived from conformers A and B leading to erythro diastereomers A and B, it may be assumed that developing imide resonance (118) will lock the chiral auxiliary in one of the in-plane conformations illustrated in products A or B. Based on an examination of models, it is projected that developing CHj R, allyhc strain steric interaction (37) disfavors that transition state leading to A. These steric considerations are largely attenuated in the transition state leading to the observed erythro adduct B. ... 

Carbon-hydrogen spin-spin coupling constants are included in a review by Hansen and discussed in terms of ring strain, steric effects, electronegativity, lone pair effects and electric field effects. Additivity of these effects in a few systems is commented on. 

Strain, steric strain, 154, 163, 166, 866 Strained endoperoxides, peroxyacetalization, 277-85... 

Non-bonded atoms (or groups) that just touch each other—that is, that are about as far apart as the sum of their van der Waals radii—attract each other. If brought any closer together, they repel each other such crowding together is accompanied by van der Waals strain (steric strain) (Secs. 1.19, 3.5). 

F strain substituent front strain -> steric descriptors... 

Substituent front strain, steric energy difference, - Joshi steric descriptor and - Joshi electronic descriptors are examples of molecular descriptors calculated from the standard enthalpy of formation and the steric energy. 

In addition to angle strain and torsional strain, steric strain is yet a third factor that contributes to the overall strain energy of cycloalkanes. As in gauche butane (Section 4.3), two nonbonded atoms in a molecule repel each other if they approach too closely and attempt to occupy the same... 

The classical front strain steric effect plays a significant role in infinencing the hydrogen-bond acidity of oriLo-substituted phenols. The introdnction of one ortho-a ky gronp into phenol lowers log LT (Table 3). A 2,6-dialkyl snbstitution prodnces a more severe steric inhibition to hydrogen-bond formation (Table 3) and 2,6-di-/-propylphenol and 2,6-di-f-butylphenol become so weak that they were exclnded from the analysis (Fignre 4). 

The gauche (or synclinal) conformation is higher in energy (by 4 Id mol 1) as the two methyl groups are near one another (60° separation), resulting in steric strain. Steric strain is the repulsive interaction between two groups, which are closer to one another than their atomic radii allow. 

The shape of cycloalkanes is determined by torsional strain, steric strain and angle strain. 

However, on top of the inclusion phenomena, cyclophanes have more interesting properties to offer. In his first publication on cyclophanes, Cram [13] had already expressed his opinion about some peculiarities to be expected in cyclophanes, which he outlined as follows a) electronic interaction between aromatic rings placed face to face , b) the resulting influence on substitution reactions in the aromatic rings by transannular electronic effects, c) intramolecular charge transfer complexes and d) ring strain, steric strain and transannular strain. These effects have been studied on the parent compounds in detail [3]. 

Anti and gauche conformers do not have the same energy because of steric strain. Steric strain is the strain (i.e., the extra energy) put on a molecule when atoms or groups are too close to one another, which results in repulsion between the electron... 

Stilbenes synthesis by Heck coupling, 43 Stille coupling aryl triflate + organylstannane, 42 Stobbe condensation, 58-59 Stork reagent, 328-329 Strain, steric. See Steric strain Strecker s synthesis of a-amino acids, 50, 301 L-Streptose synthesis, 267 Styrene. See Benzene, ethenyl-Succinic acid. See Butanedioic acid Succinimide. See 2,5-Pyrrolidinedione Sugars. See Monosaccharides Oligosaccharides inexpensive derivs. pr. (table), 263-264 Sulfafurazole, 307 Sulfamethoxazole, 308 Sulfenic acids, esters ... 

Torsional Strain Steric Hindrance and Energy Barriers... 

While the cw-product predominates, the origin of this preference for 59 and 60 are thought to be due to a combination of A 1,2 strain, torsional strain, steric approach of the aromatic ring in the transition state, and secondary stabilization effects of the intermediate. 

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