Dipolar repulsions

Heteroatom substituents also introduce polar effects. In the case of a-alkoxy aldehydes the preferred TS appears to be F and G for the E- and Z-enolates, respectively. These differ from the normal Felkin TS for nucleophilic addition. The reactant conformation is believed to be determined by minimization of dipolar repulsion between the alkoxy substituent and the carbonyl group.96 This model predicts higher 3,4-anti ratios for Z-enolates, and this is observed. 

X-ray diffraction has settled the conformation of stilbene dibromide. The meso form is that when both phenyl groups and bromine atoms are anti so that steric and dipolar repulsions are at a minimum. The dl form crystallizes in conformation in which the bromines are approximately gauche, and thus the steric repulsion of the bulky phenyl groups is at a minimum, even though there is steric and dipolar repulsions of the bromine atoms. 

In this case, severe dipolar repulsion can destabilize the cyclic conformation relative to the noncyclic geometry. 

The suprafacial shift along the carbon framework is not restricted to cyclic systems but may also prevail in acyclic cases. In the example given in Scheme 11, minimization of dipolar repulsion between the two C-0 bonds mandates a preferred conformation of the initial radical, leading to a stereo-chemically defined alkene radical cation and, ultimately, to a single diastere-omer of the product [119]. 

Richardson, the extent of dipolar repulsion (and ease of displacement) was dependent upon (a) the proximity, (b) the angular orientation, and (c) the numbers of permanent dipoles, with the most significant dipolar repulsions occurring when vicinal groups and the developing transitory dipoles in the S/y2 transition state are parallel and aligned (Scheme 42). 

Dipolar repulsion when dipoles are aligned parallel... 

The second hydrogen is probably more important in reducing the dipolar repulsion between oxygen atoms than in stabilization of the complex through binding. 

The complete 7r-facial selectivity observed for these reactions, which is probably the highest so far reported with acyclic sulfinyl dienophiles, can be rationalized by assuming that conformational equilibrium around the C-S bond was completely restricted as a consequence of an important dipolar repulsion between cyano and sulfinyl groups. It determines that the latter one adopts the s-... 

The appearance of crown forms in tetroxocane and other compounds with heteroatoms in the 1,3,5 and 7 positions probably results from two stabilizing effects (a) a slightly lower eclipsing barrier for the —O—CHg— versus the —CH2—CH2 fragment in the crown there are eight partially eclipsed bonds which would benefit from relief of torsional strain (b) distorted crowns, i.e. the chair-chair and twist-chair-chair may suffer from higher dipolar repulsions than does the crown. Unfortunately, there are no conformational energy calculations available on any of the heterocyclic systems. 

Dipolar repulsion between the carboxylate moiety and the developing diol centers in intermediates (5) and (7) would account for the (trans) relative stereochemistry between these stereocenters. Following cyclization, intermolecular reduction of the Sm -chelated c< nplex (8) by the second equivalent of Snd2 and protonolysis by alcohol irreversibly drives the reaction to completion, generating the observed products. 

When the substituent in 54 or 55 is axial, and also in the case of the a-anomer of a sugar, it is clear that the destabilizing C-O dipole effects, present when oxygen of OMe is equatorial, are minimized when this oxygen becomes axial. The absence of any significant dipolar repulsions in the latter case has been proposed as the cause of the anomeric effect (see Perrin et al.1). Though its cause is still uncertain the anomeric effect is well documented and it commonly has a magnitude of ca. 8-10 kJ mol, and is solvent dependent. 

Unfortunately, the reaction of PhCHO with 2.71 (RE = H, Rz = Me) is not enantioselective [730], The results with a-substituted reagents can be inteipreted by cyclic transition states in which dipolar repulsions between the C-X (X = MeO, Cl) and B-O bonds are crucial. With E-substituted reagents, transition state 6.58 is preferred. However, for 2-substituted boronates, A(l,3) strain disfavors geometries with the C-X bond axial so that 6.59 is preferred when X = Me (Figure 6.51). 

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