Ipso attack in aromatic nitration

Abstract

Aromatic Nitration 287ate), the necessary internal return results in insignificant oxygen ~c r a m b l i n g .~~ If ion pairs formed under solvolytic conditions undergo rearrangement, attack by an added nucleophile, although rate limiting, must always suppress rearrangement20~38kv3Q since no example of skeletal rearrangement accompanying second-order substitution has ever been reported, even for the potentially favorable neopentyl case.75 These ion pairs must be "more intimate than intiand must thus behave like polarized RX molecules. This is what we believe they are, but we see no reason to call them ion pairs, for in such terms a methyl iodide molecule could be regarded as an ion pair by virtue of the C-I bond polarity.43 This would be fine in the present context since mechanistic distinction would be lost, but chemical problems cannot be solved by such semantic extensions, although we agree with the view that genuine ion pairs may possess a degree of covalent character. In fact, the postulation of ion-pair intermediates (and their formation with solvent assistance) in simple primary and secondary substrate solvolysis in reasonably nucleophilic solvents may be incorrect, and sN2 solvent attack may be the favored mode. We know of no evidence that demands intermediates in such simple solvolyses, and specifically intermediates formed without solvent or neighboring group assistance. If so, (74) A. F. Diaz, I. Lazdins, and S. Winstein, J. Am. Chern. Soc., 90, 1904 (1968). If such scrambling were detected it would still be difficult to prove that it did not occur in a mechanistic blind alley (see also ref 5). (75) Reference 1, p 742. See also G. M. Fraser and H. M. R. Hoffmann, Chem Cornrnun, 561 (1967). (76) Reference 7, p 366.there is no reason why they should intervene in nonsolvolytic substitutions with stronger nucleophiles.In summary, the "extra-intimate ion pair" is in our opinion a polarized RX molecule. The polarization arises from approach of N-, and the energy buildup needed to cross the sN2 potential barrier arises largely from solvent re~rganization~~ attendant on the merging of the original Nand RX solvation shells. We deliberately choose these words to mimic Bordwell's description of an ion-sandwich intermediate,26 noting that Bordwell himself has not been definite in postulating actual intermediacy. His results could equally well be rationalized in terms of a loose sN2 transition state (4) for his tertiary substrate and a tighter transition state (3) for the model primary system.In conclusion, the verdict which must be passed on Sneen's proposed universal unification of s N 1 and SN2 mechanisms is one which is unique to the Scottish legal system, and so I turn to the words of my forbears in stating "not proven". A more positive conclusion is not presently available, but we note that strides in this direction are being made through heavy atom isotope effect studies.45 I warmly thank Professors P. B. D. de la Mare, J. M. Harris, P. v. R. Schleyer, J. Seyden-Penne, R. A . Sneen, and J. Warkentin for ex...

Experimental Evidence for Intramolecular ipso Substitution of Alkyl Groups

“…[9] Moreover, intermolecular ipso attack [10] has mostly been studied for nitration [10a,10b] and halogenation [10c,10d] reactions. A plausible mechanism for this type of intramolecular FriedelϪCrafts alkene alkylation promoted by Brønsted acid is proposed in Scheme 5.…”

An Effective Synthesis of 5,4'-Disubstituted Flavones via a Cesium Enolate Assisted Intramolecular ipso-Substitution Reaction

“…c) The major product was 4j obtained via the plausible mechanism shown in Fig. 2, if an intramolecular ipso-substitution reaction 12) of an enolate proceeds.…”

Neues über die elektrophile Aromatensubstitution

“…Ausfuhrlich wurde unter diesem Gesichtspunkt bisher lediglich die Nitrierung von Alkylaromaten untersucht [9] ; in vie1 gerin- …”