NUCLEOPHILIC SUBSTITUTION REACTION OF ARYL TRIFLATES VIA ARYL CATIONS

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The solvolytic generation of aryl cations from aryl triflates with trimethylsilyl group(s) and/or tert-butyl group(s) on the both ortho positions in the aromatic rings was demonstrated as a new type of nucleophilic aromatic substitution reactions.The trimethylsilyl groups on the both ortho positions of the aryl cations play important roles not only in stabilizing the cations, whose hyperconjugative stabilization energy was calculated to be more than 30 kcal <mol^(-1)> , but also in controlling sterically the attack of the nucleophilee on the cationic center.The selectivity values of the aryl cations toward several nucleophiles are obtained from the ration of the products from the reactions of the cations with the nucleophiles and the products from that with the solvent TFE in the solvolysis of the triflate in TFE in the presence of the nucleophiles; <k_(Nu)> / <k_(TFE)> = 7.7( <I^-> ), 6.5(PhCN), 5.8(MeCN), 4.3( <Br^-> ), 3.5 (EtSH), and 1.7(MeOH).The solvolysis of the triflate in HFIP, on the other hand, gave the products from the reactions with the external nucleophiles exclusively and no hexafluoroisopropyl ether from that with HFIP was formed due to the steric hindrance among the both trimethylsilyl groups of the distorted aryl cation and the bulky HFIP.A first example of the generation of the aryl cations was also shown in the photochemical solvolysis of the aryl triflates and halides, where the initially formed radical pairs by homolytic cleavage of the C-X bond undego subsequent electron transfer to afford an ion pair and cationic products. The carbene-cation character of the aryl cations was indicated in the formation of indane by the intramolecular C-H insertion reaction of 2-propylphenyl cation generated in the photolysis of 2-propylphenyl triflate.