Tri-r-butylbenzenes

Another ion for which the large number of degrees of freedom results in unmanageable kinetic shifts is the tri-r-butylbenzene ion, ° Equation (8). This is... 

With very few exceptions, the final step in the nitration mechanism, the deprotonation of the CT complex, is fast and has no effect on the observed kinetics. The fast deprotonation can be confirmed by the absence of an isotope effect when deuterium or tritium is introduced at the substitution site. Several compounds such as benzene, toluene, bromobenzene, and fiuorobenzene were subjected to this test and did not exhibit isotope effects during nitrationThe only case where a primary isotope effect has been seen is with 1,3,5-tri-r-butylbenzene, where steric hindrance evidently makes deprotonation the slow step. ... 

Cleavage of r-butyl groups has been observed in halogenation reactions. Minor amounts of dealkylated products are formed during chlorination and bromination of r-butyl-benzene. The amount of dealkylation increases greatly in the case of 1,3,5-tri-r-butylbenzene, and the principal product of bromination is 3,5-dibromo-r-butylbenzene. ... 

The photoisomerization of aromatic rings has also been studied using 1,3,5-tri-r-butylbenzene. The composition of the photostationary state is shown below. ... 

It has been shown by studies of kinetic isotope effects that the deprotonation step is fast and does not influence the rate of nitration. The cases in which this has been demonstrated include, among others, nitration of benzene, toluene, nitrobenzene, and bromobenzene in nitric acid-sulfuric acid nitration of benzene, toluene, and fluorobenzene with nitronium tetrafluoroborate and nitration of toluene in nitric acid-nitromethane. The only exception, where primary isotope effects are observed, is in the case of substituted 1,3,5-tri-r-butylbenzenes. Here, steric hindrance to deprotonation is apparently sufficiently large to make deprotonation slow relative to the first two steps in the nitration mechanism. 

Fig. 3-1. Separation of racemic 3,5-dinitrobenzamido leucine Al.A -diallylamide on silica and polymer-based chiral stationary phases. Conditions column size 150 x 4.6 mm i.d. mobile phase 20 % hexane in dichloromethane flowrate 1 mL min injection 7 pg. Peaks shown are l,3,5-tri-rert.-butylbenzene (1), R-enantiomer (2) 5-enantiomer (2 ). (Reprinted with permission from ref. [8]. Copyright 1997 American Chemical Society.)...

Subsequently, rate coefficients were determined for the zinc chloride-catalysed bromination of benzene, toluene, i-propyl-benzene, r-butylbenzene, xylenes, p-di-f-butylbenzene, mesitylene, 1,2,4-trimethyl-, sym-triethyl-, sym-tri-f-butyl-, 1,2,3,5-and 1,2,4,5-tetramethyl- and pentamethylbenzenes, all at 25.4 °C and in acetic acid, and it was shown that the reaction was inhibited by HBr.ZnCl2 which accumulates during the bromination and was considered to cause the first step of the reaction (formation of ArHBr2) to reverse320. The second-order coefficients for bromination of o-xylene at 25.0 °C were shown to be inversely dependent upon the hydrogen bromide concentration and the reversal of equilibrium (155)... 

Liquid-phase MPVO reactions were performed in 25 ml isopropanol (reductions) or 25 ml 2-butanone (oxidations) at 85 °C using 2.5 mmol of the appropriate substrate 4-r-butylcyclohexanone (4-Bu-ONE), 4-methylcyclohexanone (4-Me-ONE) or 4-t-butylcyclohexanol (4-Bu-OL, cis/trans mixture) 0.5 g zeolite or 0.25 mmol aluminium isopropoxide as the catalyst and 1,3,5-tri-f-butylbenzene as the internal standard. Samples were taken at regular intervals and analyzed by GC on a Carbowax CP-52 column and GC/MS. 

Co-deposition of a monoatomic lanthanide vapor (Sm, Eu, Tm, Yb) and tri-/-butylbenzene, 1 TS-Bu jCt.Hj, onto a cold (77 K) surface afforded matrices that contained zero-valent bis(r/ -arene (lanthanide complexes of the form Ln( 6-C6H3But3-l,3,5)2 as formed in macroscale co-condensation reactions using metal vapor synthetic (MVS)... 

---