Basicity ortho effects

A detailed analysis of substituent effects on the pKa values of imidazoles and tetrazoles as well as benzimidazoles and naphthimidazoles has been made.308 The ortho effect is shown to parallel that of 2-substituted pyridines and quinolines47 190 and application of the Hammett equation to the tautomerism of these systems is also considered. The equation also satisfies the effect of substituents on the basicity of the nitrogen in the 1-position for 1-pyrazolines.309... 

Methoxy-substituted aromatic compound 4 is lithiated metalation with Buli in THF, a step in which it proves useful to include lithium chloride. Because of the greater basicity of /t-butyllithium relative to 4. direct metallation is in fact possible thermodynamically, but /i-butyllithium is generally present in solution as a tetra-mer, and this reduces its reactivity. Addition of lithium chloride destroys these aggregates, and that eliminates the kinetic inhibition. Lithiated aromatic species 18 is further stabilized through chelate formation between lithium and the orr/icr-methoxy groups (ortho effect).8... 

Once again we see the operation of the ortho effect (Sec. 18.14). Even electron-releasing substituents weaken basicity when they are ortho to the amino group, and electron-withdrawing substituents do so to a much greater extent from the ortho position than from the meta or para position. 

The present review intends neither to report on the analytical importance of electron impact induced ortho effects nor to summarize structurally different compounds, where the hydrogen transfer according to (1) has been observed. Instead, it is the purpose of this article to show which mechanisms are the basic principles for the interaction of vicinal substituents in compounds of the general structure, 1. On the other hand, more general aspects will be discussed, which might lead to a better understanding of the gas phase chemistry of ionic species. 

Pb NMR has been used to investigate 20 lead(IV) tetracarbox-ylates both in solution and in the solid state. From an analysis of the range of, and trends in, the NMR chemical shifts, circumstantial evidence has been presented for augmentary coordination by Lewis basic ortho-aromatic substituents of the carboxylate groups. The Pb CSA parameters have been extracted from analysis of the spinning sideband manifolds of the CPMAS spectra. Thirteen of the 20 CSA analyses performed show effectively axial CSA tensors. A simple shielding model which rationalizes this typical CSA pattern has been presented... 

The basicity-decreasing effect of nitro substitution in the 3-position is almost entirely the result of its inductive effect, whereas that of nitro substitution in the 4-position is attributable to both inductive and resonance effects. In the case of para substitution (as well as ortho substitution), delocalization of the lone pair on the amino nitrogen involves not only the carbons of the aromatic ring but also oxygen atoms of the nitro group. 

The same situation is observed in the series of alkyl-substituted derivatives. Electron-donating alkyl substituents induce an activating effect on the basicity and the nucleophilicity of the nitrogen lone pair that can be counterbalanced by a deactivating and decelerating effect resulting from the steric interaction of ortho substituents. This aspect of the reactivity of thiazole derivatives has been well investigated (198, 215, 446, 452-456) and is discussed in Chapter HI. 

Bunnett (1991) expresses doubts that the aryldiazene can be formed at all under these strongly basic conditions, after Huang and Kosower (1968) showed that phenyldiazene is destroyed in water at pH 13.8 (25 °C) with a half-life of <10s, doubtless via C6H5-N2. In addition, Broxton s proposal (Scheme 8-53) does not provide a satisfying explanation of why the ortho halogen has such a strong effect in favor of the carbanion mechanism. 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

The literature on basic- and acid-catalyzed alkylation of phenol and of its derivatives is wide [1,2], since this class of reactions finds industrial application for the synthesis of several intermediates 2-methylphenol as a monomer for the synthesis of epoxy cresol novolac resin 2,5-dimethylphenol as an intermediate for the synthesis of antiseptics, dyes and antioxidants 2,6-dimethylphenol used for the manufacture of polyphenylenoxide resins, and 2,3,6-trimethylphenol as a starting material for the synthesis of vitamin E. The nature of the products obtained in phenol methylation is affected by the surface characteristics of the catalyst, since catalysts having acid features address the electrophilic substitution in the ortho and para positions with respect to the hydroxy group (steric effects in confined environments may however affect the ortho/para-C-alkylation ratio), while with basic catalysts the ortho positions become the... 

The effect of temperature on the catalytic performance of Mg/Fe/O is reported in Figure 3. The behavior was quite different from that of the Mg/Al/O catalyst. The conversion of m-cresol with Mg/Fe/O was always lower than that with Mg/Al/O. The selectivity to 3-MA was almost negligible in the whole range of temperature. The selectivity to polyalkylates and to 3,4-DMP was also much lower than that observed with Mg/Al/O. Therefore, the catalyst was very selective to the products of ortho-C-methylation, 2,3-DMP and in particular 2,5-DMP. This behavior has to be attributed to specific surface features of Mg/Fe/O catalyst, that favor the ortho-C-methylation with respect to O-methylation. A different behavior of Mg/Al/O and Mg/Fe/O catalysts, having Mg/Me atomic ratio equal to 4, has also been recently reported by other authors for the reaction of phenol and o-cresol methylation [5], The effect was attributed to the different basic strength of catalysts. This explanation does not hold in our case, since a similar distribution of basic strength was obtained for Mg/Al/O and Mg/Fe/O catalysts [4],... 

Contreras and Pardey reported on the activity of ds-[Rh(CO)2(amine)2]PF6 complexes.141 They found that the turnover frequencies of the Rh/amine catalysts were strongly dependent on the nature of the amine ligand, as pointed out earlier by Laine and Wilson.77 They suggested that there is either a promoting effect related to the basicity of the ligand, or a steric hindrance effect of two ortho methyls. Results are displayed in Table 32. Conditions T = 100 °C Pco =1.9 atm acetonitrile/H20 = 8 2 v/v 10 x 10-3 mol Rh complex. 

The propenyl group can then supply electrons to the cobalt atom by a resonance effect. This basic mechanism may also be applied to explain the marked accelerating effect of the aroyl group. Except in cases where ortho substitution sterically hinders formation of a cyclic transition state substitution on the aromatic ring has relatively little effect on the rate of decomposition. 

It is the position ortho to the maximum number of substituents that is most electron rich in an anion-radical of a starting aromatic compound. Being a less basic species, anion-radicals exhibit a more selective primary isotope effect than their more basic (and therefore... 

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