Effect of ortho substituents

The importance of field effects of ortho substituents has been discussed by Miller and Williams. The effect of various factors on reaction rates is depicted throughout mainly by means of transition states rather than intermediate complexes whose properties are usually not rate-controlling. 

Table 1 The effect of ortho substituents on the flattening of the phosphorus atom (OOP angle) in substituted arylphospholes (10)...

From the understanding, provided by the calculations, of the mechanism by which lb cyclizes, what can be predicted about how the rate of this reaction might be affected by substituents on the benzene ring The substiment effects would, in fact, be expected to be small, except for possible steric effects due to substituents in the ortho positions. If both ortho positions are substimted, one would expect to see a decrease in rate, relative to unsubstituted lb. On the other hand, if only one ortho position is substituted, cyclization should be about as fast as in unsubstituted lb but cyclization should preferentially occur at the unsubstituted ortho carbon. Additional (8/8)CASPT2/6-31G calculations by Bill Kamey in our group and subsequent experiments by the Platz group confirmed these qualitative predictions about the effects of ortho substituents. 

In Eq. 14, the reference of is shifted to that of H. The proximity effects of ortho substituents are well separated by Es and F terms according to Eq. 5. The coefficient values of Es and F terms, 0.17 and 0.77, are close enough to those for the alkaline hydrolysis of -substituted phenyl acetates 15). Thus, in support of the above discussion, the proximity effects of o-substituents are considered to be those on formation of the tetrahedral intermediate. 

Effects of ortho substituents CfiH5COOCH3 o-CH3C6H4COOCH3 o C2 H5C6H4COOCH3 o-(C H3)2C hc6h4cooc h3... 

The most fruitful treatment of the electronic effects of ortho-substituents involves the use of the same ay and ay -type constants as may be employed in correlation analysis for meta- and para-substituents by means of the dual substituent-parameter equation 30 or the extended Hammett equation 34 (Section n.B). Obviously it is a considerable assumption that these are valid for ortho-substituents and the implication is that in the correlation analysis any peculiarities may be adequately expressed through the coefficients of the inductive and resonance terms. Really satisfactory correlation analysis for any given reaction system requires a large amount of data and can only rarely be accomplished. 

Inductive effects of ortho substituents are important in governing the acidity of a C-H bond in substituted benzenes [106], and a variety of data indicate that the acidifying influence of fluorine falls off in the order ortho meta > para this is illustrated by Table 4.13 [107]. 

Furthermore, the introduction of two alkyl groups increases the rate of migration to the para position by a factor of 20- to 40-fold as compared to one orrAo-alkyl group. These results were rationalized in terms of steric effects of ortho substituents on the ground state energies. ... 

Sotomatsu, T. and Fujita, T. (1989). The Steric Effect of Ortho Substituents on the Acidic Hydrolysis of Benzamides. J. Org. Chem., 54,4443-4448. 

In view of the conflicting reports on the effect of ortho substituents on reduction of the pyridine ring, a series of hydrogenations modeled after the one in Section IV was carried out in this laboratory. It had been shown that piperidine caused immediate poisoning in the reaction. A comparison of the effect of 2-methylpiperidine should have a bearing on the validity of physical shielding. 2,6-Dimethylpyridine and 2,6-dimethylpiperidine were also included. 

The effects of ortho substituents are often very complex... 

Apparently, there will not be exhaustive results either with the chemical structure of the polymer on the preferential adsorption," or the influence of the tacticity on the preferential adsorption. In the last years, investigations regarding the effect of ortho-substituents in polymers with aromatic bulky side groups on the preferential adsorption and viscometric behavior have been reported for poly(phenyl methacrylate) and its dimethyl and diisopropyl ortho derivatives in tetrahydrofiiran/water. " Figure 5.4.5 from ref " shows the Xvalues for three polymers in THF/water. 

Effect of ortho substituents on patterns of benzyne substitution. 

Turning to disubstituted benzenesulfonylureas we found they retained activity (Figure 9). The 2,6-dichloro derivative is almost equivalent to the 2-chloro analog. The remaining compounds show the activating effect of ortho substituents and deactivating effect of para substituents. 

The effects of ortho substituents upon the basic strength of aniline are anomalous as compared with those of the corresponding meta and para substituents. Thus although an alkyl group in the meta or para positions raises the an alkyl group in the ortho position always decreases it. 

The introduction of substituents in the positions meta and para to the amino group of aniline leads to marked changes in the piT value which reveal clearly the two effects (inductive and mesomeric) which are operative in the molecules concerned. The effects of ortho substituents are more complicated and therefore they will be discussed separately. The pJfa values for a range of monosubstituted anilines are shown in Table 9. 

Myers and coworkers reported an unprecedented example for catalytic decar-boxylative alkenylation of ortho-substituted benzoic acids [25]. Thus, treatment with styrene in the presence of 20 mol% of Pd(02CCF3)2 and 1.2 equiv. of Ag2COg in dimethyl formamide (DMF)/dimethyl sulfoxide (DMSO) at 80-120 C gives stUbene derivatives (Scheme 4.21). density functional theory (DFT) calculations of the reaction were performed to interpret the promotion effects of ortho-substituents on benzoic acids [26]. 

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