Electronic nature of the

Polarization and dipole moment studies for alkyl-, aryl-, carbonyl- hydroxy- (keto-) and amino-isoxazoles have been compiled and likewise support the low electron nature of the ring 63AHC(2)365, 62HC(l7)l,p. 177). More recent studies predict the order of electrophilic substitution to be 5>4> 3 on frontier electron density values of 0.7831, 0.3721 and 0.0659, respectively 7lPMH(4)237,pp.245,247). This contrasts with earlier reports of 4>5>3 on density values of —0.09, -t-0.14 and -t-0.18 in that order 63AHC(2 365). 

For cyclopentanone, cyclohexanone, and cycloheptanone, the K values for addition are 48, 1000, and 8 M , respectively. For aromatic aldehydes, the equilibria are affected by the electronic nature of the aryl substituent. Electron donors disfavor addition by stabilizing the aldehyde whereas electron-accepting substituents have the opposite effect. 

The addition of imidazole to the ethyl hemiacetal of tnfluoroacetaldehyde provides 1 -(1 -hydroxy-2, 2, 2 -tnfluoroethyl)imidazoles in yields depending upon the electronic nature of the substiments [5] (equation 5) (Table 1)... 

Although cTi estimates by different methods or from different data sets may disagree, it is generally held that the inductive effect of a substituent is essentially independent of the nature of the reaction. It is otherwise with the resonance effect, and Ehrenson et al. have defined four different ctr values for a substituent, depending upon the electronic nature of the reaction site. An alternative approach is to add a third term, sometimes interpreted as a polarizability factor, and to estimate the inductive and resonance contribution statistically with the added parameter the resonance effect appears to be substantially independent of reaction site. " " ... 

Figure 8-7 illustrates schematically the electronic natures of the polar ground state (an ion pair) and the less polar excited state. This is, therefore, a case of p.g. 

There are three main criteria for design of this catalytic system. First, the additive must accelerate the cyclopropanation at a rate which is significantly greater than the background. If the additive is to be used in substoichiometric quantities, then the ratio of catalyzed to uncatalyzed rates must be greater than 50 1 for practical levels of enantio-induction. Second, the additive must create well defined complexes which provide an effective asymmetric environment to distinguish the enantiotopic faces of the alkene. The ability to easily modulate the steric and electronic nature of the additive is an obvious prerequisite. Third, the additive must not bind the adduct or the product too strongly to interfere with turnover. 

The reactivity of an acid derivative toward substitution depends both on the steric environment near the carbonyl group and on the electronic nature of the substituent, Y. The reactivity order is acid halide > acid anhydride > thioester > ester > amide. 

Mataka and coworkers further studied the exo/endo selectivity of outside attack products in the reactions of 96 and 97 with A-(5-X-phenyl)maleimides [56]. They found that the endo/exo selectivity is markedly dependent on the electronic nature of the substituent X (Scheme 47). The electro-withdrawing substituents such as NOj and Cl enhance enrfo-selectivity. The relative order of the enr/o-selectivity is NOj > Cl > H > OCH3. 

The derivatives 15a-i exhibit characteristic 31P NMR spectroscopic data (see Table I), which are distinctly different from those observed for the P-organo-substituted derivatives 12 and 13 (8(3IP) = 65.8-136.0).27 However, the similar Si chemical shifts and. /(Si, P) coupling constants observed for 12,13, and 15 clearly show the identical electronic nature of the low-coordinate silicon centers in these derivatives. 

The electronic nature of the NSN fragment was studied, using both ab initio and DFT methods, for a series of 1,2,5-thiadiazoles and compared to the established zwitterionic structure of naphtha[l,8-rz/][l,2,6]thiadiazine 6 (Figure 1). 

Oxidation of cyclic and acyclic hydroxylamines with yellow mercuric oxide appears to proceed with high regioselectivity (109-115). Regioselectivity is determined by the electronic nature of the substituents (116). The oxidative regioselectivity of Mn02 is comparable to that of HgO, but due to its lower toxicity, it has been proposed to use Mn02 rather than HgO (Table 2.2) (117). 

Subsequent examination of a tethered alkyne-VCP with rhodium(i) resulted in the first metal-catalyzed [5 + 2]-reaction. Excellent yields were obtained with a variety of substrates (Scheme 3) irrespective of the steric and electronic nature of the R1 group. Notably, quaternary centers are accessed in high yield. Since this first report, in-depth studies on catalysts, substrate scope, selectivity, and applications to total synthesis have been carried out. Work in this area has been reviewed.23-26... 

The two-substituted-Quinazolinap-derived rhodium complexes proved extremely efficient catalysts for the hydro-boration-oxidation of vinylarenes (Table 6). For styrene derivatives, in most cases quantitative conversions were obtained after just 2 h at the relevant temperature (entries 1-6). Higher enantioselectivities were afforded with a 4-methoxy substituent (up to 95% ee, entry 3) compared to the 4-chloro or unsubstituted styrene analogs (entries 5 and 1), a trend also observed in hydroboration with rhodium complexes of QUINAP 60. This highlights that both the electronic nature of the substrate combined with the inherent steric properties of the catalyst are important for high asymmetric induction. It is noteworthy that in most cases, optimum enantioselectivities were afforded by the... 

Nickel oxide is a classical nonstoichiometric oxide that has been studied intensively over the last 30-40 years. Despite this, there is still uncertainty about the electronic nature of the defects present. It is well accepted that the material is an oxygen-excess phase, and the structural defects present are vacancies on cation sites. Although it is certain that the electronic conductivity is by way of holes, there is still hesitancy about the best description of the location of these charge carriers. 

These reactions were carried out using /V-mesyIoxazaborolidine 96 (0.5 eq.) and allylsilane 98 (1.5 eq.) in CH2CI2 at —50°C. Conversion of 97 as high as 63% was observed, and the remaining (2S,4R)-97 was recovered in 92% ee. Modification of the electronic nature of the aryl substituent attached to the acetal carbon at the para position of 97 did not affect the enantioselectivity of ring cleavage. 

J(P1)427>. The regioselectivity of the second radical cyclization depends on the electronic nature of the homoallylic double bond pyrrolizinones 240 which result from a final 5-o -cyclization mode are preferred in the case of electron-poor carbon-carbon double bonds, such as enones or enoates electron-rich double bonds lead to indolizinones via a final 6-f db-cyclization. The best yields of pyrrolizinones were observed with iodide precursors. The cir-isomers of 240 predominate in this 5-f rf6i-5-f3co-cyclization. 

---