Stereoelectronic interaction

Computational investigations of vinylsilanes indicate that there is a groimd-state interaction between the alkene n oibital and the carbon-silicon bond which raises the energy of the n HOMO and enhances reactivity. Furthermore, this stereoelectronic interaction favors attack of the electrophile anti to the silyl substituent. 

Stereoelectronic interaction between heteroatoms in 2-hetero-substituted 1-azaadamantanes 99PAC385. 

The asymmetric induction cannot be explained simply by steric interaction because the R group in the aldehyde is far too remote to interact with the tartrate ester. In addition, the alkyl group present in the tartrate ligand seems to have a relatively minor effect on the overall stereoselectivity. It has thus been proposed that stereoelectronic interaction may play an important role. A more likely explanation is that transition state A is favored over transition state B, in which an n n electronic repulsion involving the aldehyde oxygen atom and the /Mace ester group causes destabilization (Fig. 3-6). This description can help explain the stereo-outcome of this type of allylation reaction. 

Sproviero and Burton367 have studied the contribution of stereoelectronic interactions to 3J(F1,F[) and 4J(F1,F[) couplings using RPA calculations and NBO analyses. As model compounds they chose ethane, propane, propene and methylcyclopropane. 

A tetrahedral intermediate in which the central carbon atom is substituted by two oxygen atoms and a nitrogen atom can also be generated by the addition of water or hydroxide to an imidate salt. Least motion considerations can then affect the two possible modes of decomposition of this intermediate, to ester and amine or to amide and, alcohol. The balance between these two modes of fragmentation will depend on protonation-deprotonation processes as well as any least motion effects. However, despite attempts to rationalise results with various acyclic imidates in terms of interplay between steric and stereoelectronic interactions (Deslongchamps, 1983, pp. 118-147), it is quite clear from the incisive work of... 

Dioxetane mode becomes highly selective when allylic hydrogens cannot align appropriately to give the ene-reactions due to steric and/or stereoelectronic interactions. For olefins 119 the ratio of 1,2-addition vs. ene reaction is increased by lowering the temperature (from 25 to —78 °C) and by increasing the bulkiness of the alkoxy group (Sch. 68) [124],... 

As you will probably realize, it s not only in six-membered rings that stereoelectronic interactions between filled and unfilled orbitals stabilize some conformations more than others. Stereoelectronic effects control the conformations of many types of molecules. We shall look at three common compounds that.are stabilized by stereoelectronic effects in two cases, the stabilization is specific to one conformation, and we can use stereoele ronics to explain what would otherwise be an unexpected result. 

Figure 9.6 is a graph of this equation. Notice how J reaches its maximum values at 0° (J = 11 Hz) and 180° (13 Hz) and its minimum value at 90° (2 Hz). This is because the stereoelectronic interaction betweeifthe two vicinal molecular orbitals (bonds) is at its maximum when the orbitals are parallel (dihedral angles of 0° or 180°) and decreases to nearly zero when the orbitals are perpendicular (a dihedral angle of 90°). This is quite analogous to the requirement forp orbitals to be parallel to form a n bond. 

Ethoxy-2-oxo-l,4,2-oxazaphosphinanes (2S,5S)- and (2R,5S) (528) were synthesized. Both diastereomers were used in NMR and X-ray crystallographic studies that permitted unequivocal configurational assignment, as well as examination of the consequence of Uo O p o stereoelectronic interactions on structural properties (Figure 110). " ... 

Any conformation in which a lone pair is anti-periplanar to a low-energy antibonding orbital will be stabibzed by a stereoelectronic interaction. 

The effect of stereoelectronic interactions on three- and four-bond proton-proton couplings in hydrocarbons has been studied by Sproviero and Burton. ... 

Several different studies of the 1,3-dioxane series have been published. These include MNDO calculations of protonation equilibria <89ZOR1059> and MO calculations used in HOMO determination <85izvi433>. The structure of the radical cation derived from 1,3-dioxane has been investigated in a semi-empirical SCF-MO study <83JCS(P2)1285>. In a publication by Juaristi et ai, ab initio Hartree-Fock 6-31G and other calculations were reported of variously sustituted 1,3-dioxanes, dithianes, and oxathianes in an examination of stereoelectronic interactions in each molecule <94JA5796>. 

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