Degenerate 1,2-shift of methylethynyl group in long-lived 9,10-dimethyl-9-methylethynyl-phenanthrenium ion has been detected by NMR. This is the first example of ethynyl migration in a long-lived carbocation.

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The behavior of 10,10-dimethyl-9-trimethylsiloxy-9,10-dihydrophenanthrene-9-carbonitrile in trifluoromethanesulfonic acid and acid systems CF 3 SO 3 H-CD 2 Cl 2 , HSO 3 F-SO 2 ClF-CD 2 Cl 2 , and CF 3 COOH-CD 2 Cl 2 were studied by NMR spectroscopy. Principal reaction schemes were determined; the first step in these schemes is protonation of the initial compound at the oxygen or nitrogen atom.We found [1] that 10,10-dimethyl-9-trimethylsilyloxy-9,10-dihydrophenanthrene-9-carbonitrile (I) in superacid media undergoes ionization with formation of long-lived 9-cyano-9,10-dimethylphenanthrenyl cation (II) (Scheme 1). Apart from signals belonging to cation II, the NMR spectra contained a number of additional signals indicating other schemes of reactions of nitrile I with superacids. The goal of the present work was to elucidate these schemes. located in the region δ 7.6-8.9 ppm. In the 13 C NMR spectrum we observed only signals from conformer IIIa, δ C , ppm: 27.1 (9-CH 3 ), 46.1 (C 9 ), 215.7 (C 10 ); aromatic carbon atoms resonated in the range δ C 120-160 ppm. According to the NMR data, the ratio of cations II and III was about 1 : 1.