Ethyl toluene isomer distribution

The isomer with isopropyl groups in the narrower center of the assembly appears energetically inaccessible. But with p-ethyl-toluene the most favored isomer places the ethyl groups in the center and, unaccountably, the unsymmetrical isomer represents only a few percent of the mixture as shown in Fig. 12, although it is statistically favored. In contrast, p-methylstyrene is well behaved and shows exactly what is expected from a statistical distribution of social isomers. Simple... 

Isopropylation of toluene by isopropyl halo- and alkyl-sulfonates (equation 54) has been performed by Olah et al. A variety of cattdysts, such as AlCb, AlCb-MeNOa and Nafion-H were employed, and the isomer distribution (o, m, p) in the product was determined. Sartori et al have recently reported an unusual Friedel-Crafts alkylation of lithium phenolates with ethyl pyruvate in the presence of AlCb to afford a-(2-hydroxyphenyl)ethyl lactates (22), which are the precursors of 3-methyl-2,3-dihydrobenzo-furan-3-ols (23 Scheme 6). 

The para-selectivity of the ethyl-toluenes was very close to the para-selectivity of the xylenes and the diethyl-benzenes (DEB) also seemed to give similar isomer distribution. However, the DEB-content in the product was not sufficient to allow a quantitative analysis. 

Demethoxycarbonylation of methyl 3,6-di-/-butyl-3//-azepine-l-carboxylate by DBU in xylene gives a mixture of not only the corresponding 3,6-di-t-butyl-3//-azepine, but also the 2/f-azepine and 4/f-azepine isomers <94JCS(P1)1753>. When heated in toluene at I25°C the 2H- and 3//-azepines are each converted quantitatively into azepine mixtures consisting of 2H-, 2>H-, and 4//-azepines in the ratios 12 51 1 (from the 2/f-isomer) and 12 56 1 (from the 3//-isomer). The distribution of the azepine isomers is proportional to their relative thermal stabilities as they interconvert via an allowed 1,5-hydrogen shift. Under similar conditions, methyl 2,5-di-/-butyl-3//-azepine-l-car-boxylate gives only the 3//-tautomer. Similar thermal tautomerizations of 3-phenyl-3//-azepine to 7-phenyl-3//-azepine <89H(29)253> and ethyl 4-oxo-4//,10//-pyrimido[l,2-a]azepine-3-carboxylate <84H(22)2285> to the 4//,6//-tautomer have been observed. 

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