Conformational dependence, steric

Wipke, WT, Gund P. Congestion. Conformation-dependent measure of steric environment. Derivation and application in stereoselective addition to unsaturated carbon./Am Chem Soc 1974 96 299-301. 

Substituents may be divided into three categories based on the degree of conformational dependence of their steric effects ... 

The foregoing examples of differential reactivities of rotamers may be summarized by saying that the reactivity is controlled by the steric factor. The difference in the reactivities of rotamers of 9-(2-bromomethyl-6-methyl-phenyl)fluorene (56) in SN2 type reactions falls in the same category (176). However, the substituent effect is not limited to a steric one there can be conformation-dependent electronic effects of substituents as well. A pertinent example is found in the reactivity of the bromomethyl compound (56) when the rotamers are heated in a trifluoroacetic acid solution (Scheme 10). The ap form gives rise to a cyclized product, whereas the sp form remains intact (176). The former must be reacting by participation of the it system of the fluorene ring. 

Conformational dependence of the sterlo effect. Some groups show little or no conformational dependence of their steric effeots( ). Non-oonformationally dependent groups are monatomic (halogen or hydrogen). HZ symmetric top (M Is hybridized sps), HHg or HZ ,... 

While the perfluorinated acetates do prefer insertion, they are still capable of forming 1,3-dipoles and have demonstrated interesting effects on the regioselectivity of intramolecular cycloaddition reactions, presumably through Lewis acid-mediated effects on the dipolarophile [83]. Other chemoselectivity effects have been noted in the intramolecular cycloaddition reactions and may or may not be partially induced by conformation and sterics [84]. It was further demonstrated thaL when possible, O-H insertion is the predominant outcome over other types of insertion for rhodium]II)-car-benes, independently of the catalyst. However, cycloaddition reactions have been demonstrated to be hgand-dependent [85]. 

If the tBu group in 4 and 5 is replaced by the skeletal group, G, we have a system which models that from which the uCh2ch2x constants have been derived. As the steric effect of a CH2CH2X group is conformationally dependent and the conformation is a function at least in part of 2 2 on o, reasonable. 

Acetals and Related Functions 5 B, and respectively. The remaining conformers , , and F possess a plane of symmetry. Consequently, the acetal function can in principle exist in the six different conformations A-F. Experimental evidence will be described which shows that the relative stabilities of these various conformers depend on stereoelectronic effects and the standard steric interactions. 

We will see in the next section that quaternization of azines and azoles has been used to test the applicability and validity of steric parameters, to show their conformational dependence, and to propose new steric parameters suitable for ortho positions. 

There has been a decisive evolution in the treatment of steric effects in heteroaromatic chemistry. The quantitative estimation of the role of steric strain in reactivity was first made mostly with the help of linear free energy relationships. This method remains easy and helpful, but the basic observation is that the description of a substituent by only one parameter, whatever its empirical or geometrical origin, will describe the total bulk of the substituent and not its conformationally dependent shape. A better knowledge of static and dynamic stereochemistry has helped greatly in understanding not only intramolecular but also intermolecular steric effects associated with rates and equilibria. Quantum and molecular mechanics calculations will certainly be used in the future to a greater extent. 

Structural features of a dendron are the rather soft geometrically defined shape, the definition of an outer rim B and a terminal (inner) functional group A (Figure 30). Whether or to what extent the dendron adopts a wedge like conformation depends only partially on the rigidity of the subunits but mostly on the sterical constraints by the surrounding. 

The C-H bonds on the chair cyclohexane ring are of two types. One set of six C-H bonds are parallel to the axis of the ring and are known as axial bonds (see 1.21). The other set of six bonds point out of the horizontal plane of the ring and are known as the equatorial bonds (see 1.22). When these C -H bonds are replaced by substituents, the substituents experience different interactions depending on their conformation. These steric relationships play an important role in the influence of the carbon skeleton of a particular compound on the reactivity of its functional groups. 

The results from the MM calculations were consistent with the authors NMR experiments and supported the conclusion that the observed conformations depend on steric interactions between ligands. Stolevik and Bakken have calculated rotational barriers for X2N—NX2, X2N—PX2, and X2P—PX2 (X = F, Cl, Br, or A trans conformation was predicted for these molecules, with some having additional low energy gauche conformations. 

The staggered conformations (1, 3, and 5) are lower in energy than the eclipsed conformations (2, 4, and 6), but how do the energies of the individual staggered and eclipsed conformations compare to each other The relative energies of the individual staggered conformations (or the individual eclipsed conformations) depend on their steric strain. 

The catalyst 6 forms an imminium species (not shown) via a nucleophilic attack of the Lewis basic amine at the carbonyl moiety of substrate 7. The imminium ion is deprotonated to give enamine 8. Coordination of the electrophile 9, which can be an aldehyde, ketone or azodicarboxylate, leads to the transition state 10. In this way, the facial orientation of the substrates is more or less fixed in a cyclic conformation, depending on the steric- and electronic properties of the constellation. The electron-donating properties of the nitrogen atom then directs the addition to the electrophile that is saturated by the Bronsted acid, in this case the carboxylic acidic proton. 

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