Crowding, steric strain

The steric crowding introduced in the latter by the four ethyl substituents inhibits nucleophilic attack at platinum, so that complexes of this type tend to undergo substitution by a dissociative mechanism [89]. The complex of the more rigid ligand, 2,2, 2"-terpyridyl, Pt(terpy)Cl+, is found to be about 103 to 104 times more reactive to substitution than the dien analogue this is ascribed to steric strain [90], which is reflected in the short Pt—N bond to the central nitrogen (Pt-N some 0.03 A shorter than the other two Pt-N bonds) and N—Pt—N bond angles of 80-82°). 

The structure of the cyclic ketone is of utmost importance. Reduction of cyclic ketone by complex hydrides is started by a nucleophilic attack at the carbonyl function by a complex hydride anion. The approach of the nucleophile takes place from the less crowded side of the molecule (steric approach or steric strain control) leading usually to the less stable alcohol. In ketones with no steric hindrance (no substituents flanking the carbonyl group or bound in position 3 of the ring) usually the more stable (equatorial) hydroxyl is generated (product development or product stability control) [850, 851, 852, 555]. The contribution of the latter effect to the stereochemical outcome of... 

When in compound 60 R = t-Bu, the s-trans form should be sterically crowded, and an increase of the s-cis form would be expected. The IR spectra (74SA(A)1471) seem to point out that relief of steric strain is better reached by distortion toward an s-gauche conformation than by changes in the relative amount of conformers. 

Steric strain is maximal when the addition of monomer causes the substituents of the preceding unit to become crowded by bulky groups on both sides. For example, in the conversion of living a-methylstyrene dimer into trimer... 

As expected, the two trans compounds are more stable than their cis counterparts. The cis-trans difference is much more pronounced for the tetramethyl compound, however. Build a model of c/s-2,2,5,5-tetramethyl-3-hexene and notice the extreme crowding of the methyl groups. Steric strain makes the cis isomer much less stable than the trans isomer and causes cis A//°hydrog to have a much larger negative value than trans A//°hydrog for the hexene isomers. 

In the boat form. Fig. 9-4(6), the following pairs of C—C bonds are eclipsed C —with C —C, and —C with —C. Furthermore, the H s on C —C and C —C are also eclipsed. Additional strain arises from the crowding of the flagpole H s on C and C, which point toward each other. This is called steric strain because the H s tend to occupy the same space. In the chair form, Fig. 9-4(a), all C—C bonds are skew and one pair of H s on adjacent C s is anti and the other pair is gauche (see the Newman projection). 

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). 

The thermodynamic driving force for fructan biosynthesis is the sterically strained nature of the crowded glycosidic linkage in sucrose. 

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