Stereochemical strain

Compounds originally thought to have the stereochemically strained structure (20) are now understood to be mesoionic compounds <57Qr15) (see Section 4.12.2.5). 

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 )... 

The stereochemical outcome of these reactions can be rationalized as arising from attack on a ground-state conformation in which the sulfoxide lone pair and the alkene double bond are syn coplanar2. Such a conformation would minimize 1,3-allylic strain. 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

The relative stabilities of the species involved appear to be responsible for the stereochemical outcomes. Relief of ring strain must play a role in determining the course of the reaction. An explanation for the different reaction paths on using different Grignard reagents must wait further experimentation. 

The stereochemical predictions for the intramolecular cycloadditions described in Scheme 10 and Eq. (9) in terms of MMX calculations are compared to experimental results in Table 9. The most striking trend from inspection of the data is the predominance of trans cycloadduct. The computed energy difference between the transition states 88 and 89 (Fig. 1) when R = H corresponds to 1.42 kcal/mol. The experimental preference for the trans-cis isomers 72 a and 86 a is correctly predicted by the MMX calculations. It seems reasonable to invoke A strain which is present in the transition state 88 leading to the cis product 71 a to explain the trans over cis preference. Since the substituent R is... 

These results can be interpreted in terms of competition between recombination of the diradical intermediate and conformational equilibration, which would destroy the stereochemical relationships present in the azo compound. The main synthetic application of azo compound decomposition is in the synthesis of cyclopropanes and other strained-ring systems. Some of the required azo compounds can be made by 1,3-dipolar cycloadditions of diazo compounds (see Section 6.2). 

Intramolecular 1,3-cycloadditions of nitrile oxides (INOC) provide a useful tool for the construction of fused cyclic ring systems. The stereochemical outcome of this reaction is presumed to be a consequence of reaction through the transition state that minimizes allylic 1,3 strain (Scheme 8.19).103... 

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