Electrophilic aromatic substitution. Part 22. The effect of methyl substituents on detritiation of the 9-position of phenanthrene; tritium migration during exchange

Abstract

Rates of protiodetritiation in trifluoroacetic acid a t 70 "C have been measured for 9-tritiated phenanthrenes containing a methyl substituent in each of the positions 1-8.The activating effects of these substituents are as follows (positions in parentheses) : 4.24(1) ; 2.77(2) ; 17.4(3) ; 2.96(4) ; 2.37(5) ; 3.1 6(6) ; 3.05(7) ; 2.67(8). The methyl substituent effects (with the exception of 6-methyl) closely parallel those obtained for the corresponding interactions in naphthalene, such differences as do exist being predicted by empirical calculations which simply consider the change in bond orders on going from ground state to transition state ; calculations of the effect of a C H z -substituent on localization energies are also satisfactory models. The reactivity of the 6-methyl compound is anomalously high and is believed to be due to migration of tritium across the 9,lO-bond accompanying the exchange, thereby giving a small amount of the more reactive 3-methyl[9-3H]phenanthrene. Likewise, detritiation of 3methyl [9-3H] phenanthrene gave a curved kinetic plot indicating the presence of an isomer, Jess reactive towards exchange, i.e. from tritium migration to give 6-methy1[9-3H]phenanthrene. This, the first example of a 1.2migration of hydrogen in non-super acid media is calculated to occur to the extent of 6-1 0% during the time of the kinetic runs. o+-Constants for all but the 3-methyl compound are closely similar and in the range -(0.415-0.44) ; the high reactivity of the 3-methyl compound for which o+ is -0.51 derives from the strong conjugative interaction leading to a p-quinonoid-like transition state.

Application of Electron Delocalization Indicators in the Study of Electrophilic Aromatic Substitution Reactions

“…These results are concordant with the experimentally observed orientation in EAS reactions in halonaphthalenes [154]. Since the effect of a substituent is generally stronger on the ring to which it is attached [2] (although its presence affects all the rings [155]), the halogens in thehalonaphtalenes orient to C2 and C4, while the most reactive positions in the -naphthalenes are C1 and C3 (see Figure 29). Moreover, the most negative values of Table 7 are for the fluoro, then chloro and finally bromonaphthalenes, with the same decreasing order of reactivity towards EAS observed experimentally [154].…”

Oxidation of Heteroleptic Diarylpalladium Compounds withtert-Butyl Hydroperoxide. Substituent Effects in Aromatic Oxidation Reactions

“…Oxidation of Diarylpalladium Compounds. It is well-known that naphthalene derivatives are generally more susceptible to electrophilic attack than benzene derivatives because the aromatic stabilization of naphthalene per ring is lower than for benzene. , For naphthalenes, the α-positions 1, 4, 5, and 8 are more susceptible than the β-positions 2, 3, 6, and 7 as a result of a more effective delocalization of the positive charge in the transition state of electrophilic attack (Wheland complex) . Also, in (aryl)palladium compounds the Pd δ+ −C i p s o δ - bond is polarized and oxygen insertion into the Pd−C bond will be influenced by the charge density on the C i p s o atom, which itself is influenced by mesomeric and inductive effects.…”