Electronic structure stereoelectronic effect

Molecular orbital calculations have also provided theoretical justification for these stereoelectronic effects in tetracovalent and pentacovalent phosphorus species (2-7). As has been shown in molecular orbital calculations on the X -P-X2 (X = 0,N) structural fragments, the X.-P bond is strengthened (as indicated by an increase in the Mulliken overlap population) while the P-X3 bond is weakened when the X atom lone pair is app to the P-X3 bond. Thus, in the g,t conformation of dimethyl phosphate (Structure ll the overlap population for the trans P-0 bond is. 017 electron lower than the overlap population for the gauche P-0 bond. As shown for g,t dimethyl phosphate one lone pair (shaded in 1) on the gauche bond oxygen is app to the trans bond, while no lone pairs on the trans bond oxygen are app to the gauche bond. Thus, the weakest X.-P bond has one app lone pair and no lone pairs on X. app to the P=X2 bond. 1... 

Some tri- and tetracyclic alkanes contain two cyclopropane moieties locked into a fixed orientation. These arrangements might give rise to interesting stereoelectronic effects in the interaction between the cyclopropane rings. For example, CIDNP effects during electron-transfer reactions of syn- and a/jn -tricyclo[5.1.0.0 ]octane syn-, anfi-89) indicate significantly different structures for their radical cations. 

Stereoelectronic effects of various remote substituents on the basicity of the carbonyl is yet another intriguing question. Electronic effects of remote substituents have been postulated to be responsible for stereoselective additions to carbonyls in rigid adamantanone systems. If present, such stereoelectronic effects would similarly be expected to bias the energetics and structural features of Lewis acid complexation. 

Stereoelectronic effect An effect on structure and reactivity due to the orientation and alignment of bonded or nonbonded electron pairs [108]. 

The electronic structure of 11 sparteines has been studied by Galasso et a/. by the use of experimental and DFT calculated NMR parameters, special attention being paid to spin-spin couplings. In particular, the influence of stereoelectronic hyperconjugative effects on A JMCeq/Vncax has been correctly accounted for by the DFT results. 

Molecular stability results from the tug-of-war of multiple interactions including a variety of electronic, steric, electrostatic factors, hi a given molecule, a significant stereoelectronic interaction can be overridden by the combination of several effects that, taken together, can change the more stable structure into the one where the stereoelectronic effect of interest is absent (or considerably weakened). However, even a disappearing stereoelectronic interaction can still be consequential for reactivity and make an impact both on the observed reaction rate and on the nature of reaction products. 

Houk has suggested that stereoelectronic effects have influence over the stereochemical course of hydroboration reactions of allylic alcohols [19]. Because borane is an electrophilic reagent, it exhibits a preference for electron-rich partners in hydroboration reactions. The more reactive conformer of an allylic alcohol is that in which the olefin avoids additional hyperconjugative interactions that would render it electron-deficient, such as jic=c ( c-x (allylic). Therefore, allylic hydroxy or alkoxy substituents tend to avoid the anti position with respect to the partially formed bonds (cf. transition structure 36). Altogether, both steric and electronic effects work in concert to support the predominance of transition structures 28 and 36,... 

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