Stabilizing electronic factors

Figure 19 (a) Electronic factors stabilizing side-bound over bottom-bound metallacycle. 

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

The reaction rates and product yields of [2+2] cycloadditions are expectedly enhanced by electronic factors that favor radical formation. Olefins with geminal capto-dative substituents are especially efficient partners (equations 33 and 34) because of the synergistic effect of the electron acceptor (capto) with the electron donor (dative) substituents on radical stability [95]... 

The Hammond postulate says that any factor stabilizing the intermediate carbocation should increase the rate of an S l reaction. Solvation of the carbocation—the interaction of the ion with solvent molecules—has just such an effect. Solvent molecules orient around the carbocation so that the electron-rich ends of the solvent dipoles face the positive charge (Figure 11.14), thereby lowering the energy of the ion and favoring its formation. 

Electronic factors also influenced the outcomes of these cyclization reactions cyclization of pyrrole 84 to bicyclic amine 85 is catalyzed by the sterically open complex 79a. In this reaction, initial insertion into the Y - H bond occurred in a Markovnikov fashion at the more hindered olefin (Scheme 19) [48]. The authors proposed that the Lewis basic aromatic ring stabilizes the electrophilic catalyst during the hydrometallation step, overriding steric factors. In the case of pyrroles and indenes, the less Lewis basic nitrogen contained in the aromatic systems allowed for the cyclization of 1,1-disubstituted alkenes. 

For PR3/P(OR)3-stabilized nickel complexes, there are two borderline cases known from the experimental investigation of Heimbach et al. 1 which, unlike the usual behavior, redirect the cyclo-oligomerization reaction into the Ci2-cyclo-oligomer production channel. Catalysts bearing either strong a-donor ligands that must also introduce severe steric pressure (e.g., PBu Pr2) or sterically compact n-acceptors (like P(OMe)3) are known to yield CDT as the predominant product. From a statistical analysis it was concluded,8a,8c that the C8 Ci2-cyclo-oligomer product ratio is mainly determined by steric factors (75%) with electronic factors are less important. 

Electronic factors in oligomerization, 175-176, 197-205, 212-214 Electronic properties, polysilane, 143-145 Electron lone pairs, 230, 231, 233 on base-stabilized monomers, 280,... 

The stability of o-sulfonylbenzonitrile oxides and their thiophene analogs probably depends on electronic factors. The same factors do not prevent dimerization, as can be seen from data concerning several differently substituted nitrile oxides of the thiophene series (103). Sterically stabilized 3-thiophenecarbonitrile oxides 18 (R = R1 = R2 = Me R = R2 = Me, R1 = i -Pr), when boiled in benzene or toluene, isomerized to isocyanates (isolated as ureas on reaction with aniline) while nitrile oxides 18 with electron-withdrawing substituents (R1 and/or R2 = SOiMe, Br) dimerized to form furoxans 19. 

Equilibrium constants have been reported for the stepwise conversion of MeRe(NAr)2(PR3)2 to the mixed-phosphine and then the (PR3)2 derivative. Both steric and electronic factors come into play, typical of phosphines. The stability ordering is PMe3 > dmpe>PMe2Ph > P(OMe)2Ph > PEt3 > P(OEt)3 > PMePh2 > P(OEt)Ph2 > PPh3 (58). 

Section III.C A Hydrido(methyl)carbene Complex of Platinum(IV) (223) and Methyl(hydrido)platinum(IV) Complexes with Flexible Tridentate Nitrogen-Donor Ligands (224) are structurally related to the system shown in Scheme 13 and give additional information on how steric and electronic factors influence the stability of platinum(IV) methyl hydrides. 

Common Anomeric Groups, Common Activators Both conformational and electronic factors have been exploited to obtain selectivity in one-pot glycosylations using common anomeric groups and activators. Selectivity results from differences in the stability of oxonium ion intermediate between the two prospective glycosyl donors. 

Experimentally, the cis isomer of N2F2 is found to be about 3.0 kcal/mol more stable than the trans isomer87,88 Another electronic factor which contributes to the substantial stabilization of the cis isomer will be discussed in a subsequent section. 

Infrared and Raman spectroscopic studies of XNSO, where X=F, Cl, Br, I, indicate that these compounds have a cis configuration in the gas phase89). This result has also been confirmed by electron diffraction study of CINSO90. Once again, an additional important electronic factor is responsible for the greater stability of the cis isomer and this will be discussed in a later section. 

We now focus on conformational preference within the cis and trans isomers, respectively. If steric effects play the major role in dictating the preferred conformation in the trans isomer, one would expect that the relative order of stability will be Tss > Tse > Tee- However, the results of ab initio calculations shown above predict that the order of stability of the trans isomers is Tee > Tse > Tss in the calculations utilizing the STO—3G basis set and Tee > Tss > Tse in the 4—31G computations. An additional electronic factor which can account for the greater stability of the more sterically crowded Tee isomer will be discussed in a subsequent section. 

In summary, we expect that 2,3-substitution will not greatly alter the conformational preference of the diene system, L e. conformational isomerism of 2,3-difluoro-butadiene will be subject to the same electronic factors as that of the unsubstituted diene and these factors operate in the same direction in both cases. It is expected that, by analogy to 1,3-butadiene, the order of conformer stability of 2,3-difluoro-butadiene will be dictated by steric effects, L e. it will be tram > gauche > cis. 

It is clear that the variation in the carbene stabilization modes dramatically modify the carbene properties in terms of steric and electronic factors, which is very essential in the design of new ligands for transition metals. In comparison to classical NHCs, non-NHC carbenes offer a large range of possible structural variations, which should rapidly enhance the interest in non-NHC-based complexes. We can anticipate that the fine tuning of the properties of stable non-NHC carbenes makes these ligands very promising for the development of novel and efficient catalysts. 

The weakest bonds in an explosive will often determine its sensitivity to impact and such bonds are usually present in the explosophoric groups. Steric and electronic factors also play an important role. Unsurprisingly, factors which increase explosive performance usually have a detrimental effect on stability and sensitivity, and so a compromise must be made. As the database of energetic materials and their properties is ever increasing this task becomes... 

High selectivity of 4,4 -DIBP was observed in the catalysis of HM. The selectivity of 4,4 -DIBP was constant during the reaction with the accumulation of 2- and 3-IPBP and decrease of the selectivity of 4-IPBP. These results show that the alkylation proceeds by a consecutive reaction mechanism. The alkylation of 4-IPBP occurred regloselectlvely to give 4,4 -DIBP. Other Isomers, 2- and 3-IPBP, do not participate in the reaction because these Isomers are too sterically bulky to enter the pore of HM. On the other hand, catalyses of HY and HL were nonselective for the formation of 4,4 -DIBP. Three isomers of IPBP take part in the alkylation, which is controlled by the electronic factor of reactant molecules at low temperatures and by the stability of product molecules at higher temperatures. 

Electron centre, C033 (C3 symmetry) A pyramidal AB3-type molecular ion with 25 electrons is stabilized by a trivalent impurity such as Y3+ at room temperature and forms a complex of Y3+ - C033-. The shift of -factors from the free electron value ge = 2.0023 and the hyperfine (hfs) constant, A can be estimated by molecular orbital calculation of AB325 -type molecule ... 

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