Ipso substitution steric effects

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

Ion pairs (contd.) intimate, 91, 249, 291 solvation, 45, 57, 81, 390 solvent separated, 91 Ipso substitution, 161, 169 steric effects in, 162 Isocyanate intermediates, 49, 122 Isomer distribution, 158 Isomerisation, cis- trans, 315 Isoprene, 323... 

From this it is possible to calculate the overall theoretical rate ratio for acetylation of m-xylene relative to benzene, since this is one-sixth the sum of the partial rate factors (in this case 1130), and the isomer distribution if the reaction is kinetically controlled. The overall rate ratio actually is 34769 and the calculated and observed isomer distributions are listed in Table 11.2.69 In this case, and in many others, agreement is fairly good, but many cases are known where the effects are not additive.70 For example, the treatment predicts that for 1,2,3-trimethylbenzene there should be 35% 5 substitution and 65% 4 substitution, but acetylation gave 79% 5 substitution and 21% of the 4 isomer. The treatment is thrown off by steric effects, such as those mentioned earlier (p. 511), by products arising from ipso attack (p. 512) and by resonance interaction between groups (for example, 24), which must make the results deviate from simple additivity of the effects of the groups. 

The effects of different substituents (R) (both steric and electronic) on the 1,5-ipso substitution reactions of radicals of the type (73) to give (74) and finally biaryls (75) after loss of SO2 have been examined (Scheme 31).154,155 The results indicated that the introduction of either an electron-donating or -releasing group in the ortho position... 

The factors that control the site of attack of nucleophiles on arene complexes are complex, and the position of attack is controlled by the electronic properties of the substituents. These properties dictate whether attack will occur meta or para to the substituent. Steric effects discourage attack ortho to the substituent. In many cases, the attack of nucleophiles on coordinated arenes is reversible. Thus, the final productive reaction may result from addition at a site of the arene that is not the kinetic site for attack. For example, nucleophilic attack onto the ortho, meta, or para positions of the phenyl chloride coordinated to Cr(CO)j in Equation 11.53 is reversible, and it is only attack at the ipso position that leads to productive substitution chemistry. - ... 

The main purpose of this chapter is to discuss the directing effects of functional groups in these reactions. As one would expect, steric effects direct internal nucleophile addition to the less hindered of the electrophilic centers as in the formation of 49. With a C-1 OMe substituent, a addition has been proposed [235]. Internal nucleophile addition to the less hindered of the electrophilic centers has also been reported for 50 [236]. Other examples give mixtures [237]. On the other hand, 1,4-dimethyl substitution on the tj cydoheptadienyl ligand shows regiocon-trol dominated by the C-1 Me group (y selectivity ipso to the C-4Me group) [229]. 

Silicon group introduction by DoM also provides opportunity for design of E+-induced ipsodesilylation and sterically determined reactions. In the former, the operation of a /3-cation effect induces rate enhancements in ipso-substitu-tion in systems which are unreactive (EWG) or para-reactive (EDG) toward typical electrophilic reagents (24). The presence of strong EDG overrides the /3-cation effect but the bulk of the silane increases the ortho/para ratio which enables development of substitution patterns and types which are not easily derived by classical electrophilic reaction means (25) [23],... 

Even with an excess of tert-butylmethylphosphide, the desired compound 55 was not obtained and only 20 could be isolated, probably due to both steric and electronic reasons. Accordingly, the X-ray structure of 20 shows effective shielding of the ipso carbon of the remaining fluorine atom by the phosphine borane group. Deboronated 20 (Scheme 5.10) did not react with the phosphide either. Unexpectedly, when the reaction mixture containing 19 and the phosphide treated with acid at low temperature the main product turned out to be the p ra-substituted diphosphine borane 54. Compound 54 arises from nucleophilic attack at the meta position to the fluoro group therefore the... 

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