Stereo electronic effect

The first example of a stereo electronic effect in this text concerned anti elimination in E2 reactions of alkyl halides (Section 5 16)... 

Entries 6 to 8 demonstrate addition of allyl trimethylsilane to protected carbohydrate acetals. This reaction can be a valuable method for incorporating the chirality of carbohydrates into longer carbon chains. In cases involving cyclic acetals, reactions occur through oxonium ions and the stereochemistry is governed by steric and stereo-electronic effects of the ring. Note that Entry 8 involves the use of trimethylsilyl... 

Six-Membered Ring (endo-Cyclic). All the previous discussion of stereo-attack is based on steric hindrance, but in the case of a six-membered ring (endo-cyclic) enolate, the direction is affected simultaneously by stereo-electronic effects (Scheme 2-4).10 In the transition state, the attacking electrophiles must obey the principle of maximum overlap of participating orbitals by perpendicularly approaching the plane of atoms that constitute the enolate functional group. Electrophile attacks take place on the two diastereotopic... 

P. Deslongchamps, Reactions on sp2 type unsaturated systems, in Stereo-Electronic Effects in Organic Chemistry, Pergamon Press, Oxford, 1983, p. 252. 

Some unsaturated alcohols resist reaction with Mn02 due to steric reasons. Sometimes, epimeric unsaturated alcohols possess very different reactivities versus active Mn02, which points to the possible involvement of little-investigated stereo-electronic effects.35... 

Piers and co-workers (44) have reported that the cuprous chloride catalyzed 1,4-addition of isopropenylmagnesium bromide to the bicyclic enones 127 (R=H or CH3) gave exclusively the bicyclic ketones 128 (R=H or CH3) with the isopropenyl group axially oriented. Interestingly, with their trans ft/B ring junction, these enones are essentially conformationally rigid and their reactivity is in accord with the predictions made on the basis of stereo-electronic effects. 

Although the size of the leaving proton is not significant, steric hindrance for deprotonation can be appreciable. Here s an additional example The cumene cation radical prefers to lose a proton from (3-carbon rather than a-carbon. This is ascribed to the stereo electronic effect (Zhao, Ch.-X., et al. 1999). 

Cuevas, G. Juaristi, E. Manifestation of stereo-electronic effects on the calculated C—H bond lengths and one bond 1JC-h NMR coupling constants in cyclohexane, six-membered heterocydes, and cydohexanone derivatives. 

In order to experimentally assess the magnitude of the stereo electronic effect at phosphorus, we have prepared and studied the reactions of the six-membered ring cyclic esters such as... 

Draw a mechanism for the following multistep reaction. Do the cyclization steps follow Baldwin s rules What other stereo-electronic effects are involved ... 

Another very important aspect of enolisation is the "stereo electronic effect , first invoked by Corey [ii2,xx3]. This is revealed in the preferential loss of the axial proton from a conformatiohally rigid cyclohexanone. The stereoelectronic... 

A. J. Kirby, The Atiomeric Effect and Related Stereo-electronic Effects at Oxygen, Springer, Berlin, 1983, p, 75. 

The irreversible decay of the dormant alkoxamine chains stops the monomer conversion rather abruptly at the time t = H( nk. For methyl methacrylate polymerizations this stop has been observed, and it has been demonstrated that it is caused by a considerable fraction of cross-disproportionation between the nitroxide and the propagating radicals.51,97112 Unfortunately, the factors governing disproportion-ation-to-combination ratios in radical—radical reactions are not well understood up to now, but stereo-electronic effects are certainly very important.112 Hence, one cannot yet predict a nitroxide structure that will allow living methacrylate polymerizations up to large conversions. 

In-vitro rate studies on the antioxidant activity of a-tocopherol has shown that it is one of the most efficient alkylperoxyl radical traps, far better than the commercial hindered phenols, e.g., BHT (2,6-di-t r -butyl-4-methylphenol, AO 1). Its efficiency was attributed to stereo electronic effects the electronic synergy between a fully methylated aromatic ring and the chro-man moiety results in a highly stabilized tocopheroxyl radical, formed during the rate limiting step. Reaction 8, because of the interaction between the p-orbitals on the two oxygen atoms. ... 

A few other fluorinated kanamycins, including 5-deoxy-5-epi-fluoroamikacin, 5-deoxy-5-ep(-fluoroarbekacin, and their related analogs, have been prepared to study the fluorination-toxicity relationship. In contrast to the low toxicities of the 5-fluoro derivatives [121], these epi-fluoro compounds showed acute toxicity values identical to those of arbekacin and amikacin [122]. This indicates the importance of stereo-electronic effects of the fluoro group at position 5 of the 2-deoxystreptomine moiety in toxicity of aminoglycoside antibiotics. 

To successfully correlate thermodynamic or kinetic parameters with stereo-electronic effects, there must be a clean separation of the electronic from the steric effects (Eq. 1). Since there is no necessary reason for an experimentally based steric parameter to be free of electronic effects, workers have turned to applications of computational chemistry to achieve a quantitative measure of pure steric influence of a ligand. 

The preceding discussion is not meant to imply that stereo electronic effects alone are responsible for determining diastereoselection in the Diels-Alder reaction. Indeed, examples of reactions that do not conform to the endo rule abound, and these cases are not easily explained without invoking alternative hypotheses. For instance, it has been demonstrated that 1,1-disubstituted dien-ophiles can favor formation of the exo product with cyclopentadiene, sometimes to the complete exclusion of the electronically favored endo isomer [19]. There appears to be subtle interplay between steric and electronic factors, as simply switching the diene to cyclohexadiene or an acyclic diene results in a turnover in selectivity to favor the endo isomer. While the exact source of stereocontrol for a given cycloaddition is still a source of debate, this review will emphasize the practical ramifications of diastereoselection, namely, prototypical dienophiles such as a-methacrolein and a-bromoacrolein can be relied on to deliver exo cycloadducts preferentially with cyclopentadiene endo otherwise), while acrylate, crotonate, and cinnamate-derived dienophiles will generally favor the endo tran-... 

Model compound studies reveal factors which influence free—radical additions of ethers, polyethers, and crown—polyethers, to fluorinated alkenes. Stereo-electronic effects significantly affect relative reactivities of cyclic amines. Novel dienes are obtained by reactions of some oligomers of fluorinated alkenes with sodium amalgam and some reactions of these systems with nucleophiles are described. 

Abstract This chapter emphasises on the important aspects of steric and stereo-electronic effects and their control on the conformational and reactivity profiles. The conformational effects in ethane, butane, cyclohexane, variously substituted cyclohexanes, and cis- and tra/ ,v-decalin systems allow a thorough understanding. Application of these effects to E2 and ElcB reactions followed by anomeric effect and mutarotation is discussed. The conformational effects in acetal-forming processes and their reactivity profile, carbonyl oxygen exchange in esters, and hydrolysis of orthoesters have been discussed. The application of anomeric effect in 1,4-elimination reactions, including the preservation of the geometry of the newly created double bond, is elaborated. Finally, a brief discussion on the conformational profile of thioacetals and azaacetals is presented. 

An interesting stereo-electronic effect was found for the two isomeric ketones endo-6 and exo-6 (Chart 7). With the exo-isomer, the carbonyl and alkene moieties are isolated from each ofher, fhe compound undergoes ot-cleavage accompanied by CIDNP effects, and is a good initiator for radical polymerizations. In contrast, the... 

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