Benzene ring, complexes, electronic substituent effects

The competing effects of the electron-releasing ability of the non-complexing benzene ring against the electron-withdrawing character of the substituents (acetyl, nitrile) are illustrated in Fig. 9. 

If the electrophile attacks the benzene ring at a position ortho or para to a + / substituent (i.e., to one electron-donating by inductive effect), the activated complex will be similar to 81 or 82, respectively. Resonance structures 81c and 82c are of particularly low energy because in these the positive charge is localized on the carbon that bears the + / group. Attack at the meta position does not allow such a resonance structure to be drawn. 

Asymmetric MPV type reduction can be achieved by using a chiral Sm(III) complex 38 (Scheme 29) [93]. Several aryl methyl ketones are reduced in an excellent optical yield, up to 97%. Electronic properties of substrates profoundly affect the reactivity. For example, a p-chloro substituent in the benzene ring accelerates the reaction, whereas a p-methoxy substituent decelerates the reduction. The presence of an o-chloro or o-methoxy group effectively enhances the... 

A suitable mechanism for the homogeneous platinum catalyzed exchange must rationalize two important experimental observations (1) low reactivity at the ortho positions of substituted benzenes and (2) absence of substantial electronic effects of substituents in the benzene ring (57). For these reasons, a mechanism involving the direct attack of external D+ on the ti complexed benzene has been rejected. 

In the reactions of aryl halides with various complexes, the effect of para-substituents in the benzene ring on the reaction rate has been used as a probe of the reaction pathway. The reaction of [Pd(PPh3)j] with p-RC H I as shown in Scheme 16 is influenced by the substituent electron-withdrawing groups enhance the rate in the order NOj > CN > PhCO > H. This... 

A series on substituent effects on the solvolysis of l,l-diphenyl-2,2,2-trifluoroethyl systems has continued in a study of the effects of electron-donating substituents in the fixed aryl moiety. " The fixed substituent in one benzene ring was p-MeO, p-PhO, or p-Me, and the substituent in the other ring was varied. The substrates were chlorides and bromides, and the reactions were carried out in 80% aqueous ethanol at 25.0 °C. The substituent effects were complex and required detailed analysis through Hammett-Brown and Yukawa-Tsuno treatments. The complexities arise largely from the twisting of the aryl groups out of coplanarity. 

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