Torsional effect

From an FMEA of the system design, a Severity Rating S) = 1 was allocated, relating to a safety critical failure in service. It is required to find the optimum unequal angle section size from the standard sizes available. It is assumed that the load is carried at the section s centre of gravity, G, and only stresses due to bending of the section are considered, that is, the torsional effects are minimal. The combined weight of the beam and tie rod are not to be taken into account. 

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

Houk and co-workers examined the role of torsional effects in the stereoselectivity of enolate alkylation in five-membered rings, and their interpretation can explain the preference for C(5) alkylation syn to the 2-methyl group in trans-2,3-dimethylcyclopentanone.59... 

Torsional effects are important in cyclic systems. A PM3 study of the high stereoselectivity of compounds 4a-d found torsional effects to be the major difference between the diastereomeric TSs.81 The computed TSs for 4a are shown in Figure 12.10. The structures all show similar stereoselectivity, regardless of the presence and nature of a 3-substituent. 

The torsional effect as propsed by Schleyer57 and steric hindrance of the departing group according to Brown57 both have been discussed as interpretations of these reactivity series. 

In an investigation of the stereoselectivity of nucleophilic addition to larger ring systems, ethyl-, vinyl-, and ethynyl-lithium and -Grignard reagents have been added to 2-(3 -phenylpropyl)cycloheptanone (69). In all cases, the predominant product is the cw-alcohol, and calculations have been used to identify the steric and torsional effects in the transition state that favour this stereochemistry. 

Nucleophilic additions of ethyl-, vinyl-, and ethynyl-lithium and Grignard reagents to a 2-alkyl-substituted cycloheptanone have been found to yield the corresponding cw-cycloheptanol preferentially. The selectivity, which increases with size of the nucleophile, has been attributed to a combination of steric repulsions and torsional effects in the transition state. Application of the MM2 force field to analyse nucleophilic attack of hydride and ethynyl lithium on 2-methylcycloheptanone gave results comparable to those obtained experimentally and from ab initio calculations. 

The product ratio of exo-halide 28 and endo-isomer 29 was largest for large halogen transfer agents XY. XY apparently approaches 2 7 preferentially from the less shielded exo-side. The torsional effect discussed before is probably also of importance. Similar results were obtained more recently for the transfer of hydroxy groups from peracids to 27 ... 

Torsional effects and non-perpendicular attacks are second in importance. Computational estimates indicate that rotational barriers involving torsional interations with partially formed bonds may be as large as - 3 kcal mol-1.93 To the best of our knowledge, all recent models incorporate non-perpendicular attacks and torsional effects. 

The Evans electrostatic model for 1,2-asymmetric induction incorporates Dunitz-Biirgi attack and torsional effects into the Cornforth model ... 

The influence of the 4,6-O-benzylidene acetal in p-mannosylations was believed to be due to its torsional disarming effect. In an earlier work, Bert Fraser-Reid and coworkers found that a 4,6-O-benzylidene acetal locks the pyranose ring in a stable chair conformation [41], Therefore, it disfavors the formation of an intermediate oxocarbenium ion, which requires rehybridization and flattening of the sugar ring, usually leading to ahalf-chair conformation. This torsional effect, combined with the strong endo-mom nc effect in mannose, favors the a-triflate intermediate. 

Torsional effects (linked to the preference for staggered arrangements at the transition state) are more likely to be at the origin of this selectivity (Scheme 3). 

These results have been interpreted in terms of the stereodivergent nature of the two fast-equilibrating and differently reactive envelope conformations of such cyclopentenes. Specifically, torsional effects may dictate the ul topicity of the former substrates, whereas normal steric factors may account for the opposite diastereoselection of the latter cases. A similar syn selectivity has been observed in the osmylation of other cyclopentene derivatives103 and of f/.v-disubstituted cyclobutenes102. 

Stereoselectivity in the epoxidation of monocyclic or fused cyclohexene derivatives in a halfchair conformation is governed by steric repulsions torsional effects are mostly negligible. One exception is the epoxidation of sterpurene (1), which forms a single epoxide 2 structural assignment of this epoxide was not attempted171. 

The force-field calculation confirms complete exo diastereoselection in both cases especially in the [3.2.1] system 7, enhanced conformational rigidity and torsional effects ( norbor-nene effect ) are responsible for the calculated extremely high d.r. (99.9998 0.0002 at 0°C). 

Fraser-Reid B, Wu Z, Andrews CW, Skowronski E. Torsional effects in glycoside reactivity saccharide couplings mediated by acetal protecting groups. J. Am. Chem. Soc. 1991 113 1435-1437. 

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