Strain, bond opposition

BASE STACKING BONDING ORBITAL BOND NUMBER Bond opposition strain,... 

In cyclopentane, as opposed to cyclobutane and cyclopropane, the bond angles have values close to the optimum. Therefore, the strain in the molecule arises essentially from bond opposition and is partly relieved by puckered conformations. Two flexible forms of cyclopentane exist, namely the so-called envelope (LXXVlIIa) and half-chair (LXXVIIIb) forms. The former has four carbons in the same plane, and... 

The tetrahydrodiazepine (112) was found107 to be a highly unstable substance. On the other hand, the 4-keto derivative (110, R = H) was quite stable, although refluxing a dilute aqueous hydrochloric acid solution led to the formation of what was believed to be an oxindole polymer. This ready acid hydrolysis was attributed to the essentially acetal-like character of the grouping N—CH2—N. The low basicity of 110 (R = H) (pK < 2) is of interest since A-methyl-o-toluidine has a pK of 4.87. The inductive effect plus the mesomeric influence of the amide carbonyl and internal strain effects resulting in a certain amount of bond opposition appear to explain this basicity.107... 

The products of these reactions are easily transformed into allylamines by sequential treatment with (Me3Si)2NH and KOH. Quatemization of the nitrogen of 1.25, followed by reaction with PhMgBr at -78°C, gives sulfoxides 8.4 with an excellent enantiomeric excess. In turn, these sulfoxides undergo [2,3] sigmatropic transposition to allyl alcohols 8.5. In the ene reactions of 1.24, the enophile approaches the face of the N=S double bond opposite to the phenyl substituent of the auxiliary so that the A (1,3) strain is minimized (Figure 8.2). 

Substitution in dioxolanes decreases their polymerizability. Although the strain in the monomer may increase with substitution, the conformational strain of the macromolecule apparently prevails. The increase of the strain that destabilizes the monomer comes from the replacement of the cis-geminal C4—H and C5— H bond oppositions with greater C4—H and C5-substitutent interactions (for mono-substitu-... 

The Jacobson-Stockmayer cyclization theory is based on the assumption that all rings are strainless and that the conformational probability of ring closure is given by Eq. 3-2, i.e. conformational restrictions or preferences are absent. This assumption apparently cannot be fulfilled in real systems for small rings (as shown in Sect. 3.2.2). In some systems this leads to a lowering of the concentration of cydics when compared with their equilibrium concentration. This is because the probability of small ring closure is reduced due to the strain caused not only by bond angle deformation but also by bond opposition and transannular interactions. In such a case, as in the discussed earlier l,3-dioxolane-BF3 system, the concentration of small cycles (up to 25-30 bonds) is lower than the calculated one 141... 

Bond torsion (bond opposition, Pitzer strain), related to rotational motion around the bond axis and the interaction between substituents on neighboring ring atoms... 

Torsional strain (Section 3 1) Decreased stability of a mole cule associated with eclipsed bonds trans (Section 3 11) Stereochemical prefix indicating that two substituents are on opposite sides of a ring or a double bond (Contrast with the prefix cis )... 

Conversely, processes which convert carbons to sfp- carbons are more favorable for five-membered than for six-membered rings. This can be illustrated by the data for acetolysis of cyclopentyl versus cyclohexyl tosylate. The former proceeds with an enthalpy of activation about 3kcal/mol less than the latter." A molecular mechanics analysis found that the difference was largely accounted for by the relief of torsional strain in the cyclopentyl case." Notice that there is an angle-strain effect which is operating in the opposite direction, since there will be some resistance to the expansion of the bond angle at the reaction center to 120° in the cyclopentyl ring. 

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