Photochemical and Thermal Rearrangements of Protonated 2,3-Homotropones

The 2-hydroxyhomotropylium cation 10 and 8-e/nfo-methyl-, 8-exo-methyl-, 8,8-dimethyl-, 1,8,8-trimethyl, and 3,8,8-trimethyl-2-hydroxyhomotropylium cations, 12,11,13,14, and 15, respectively, were prepared by protonation of the corresponding 2,3-homotropones in FSO3H. On the basis of a comparison of the ‘H NMR spectra of the 2-hydroxyhomotropylium ions with nonaromatic systems it is concluded that they can properly be regarded as homoaromatic cations. Ions 10, 11, 12, and 13 isomerized when irradiated in FSO3H to give the corresponding l-hydr9xyhomotropylium cations 19, 21, 22, and 23, respectively. The thermal isomerization of these ions has been investigated. Cation 22 was shown to isomerize to 21 at -39.5 °C (k = 4.3 X 10-4 s-i). An equilibrium was set up between these two ions consisting of 6% 22 and 94% 21 at 0 °C. At higher temperatures 21 rearranged to protonated 1-phenylpropanal (k = 2.5 X 10-4 s-1 at 37 °C). The 8,8-dimethyl cation 23 isomerized back to 13 (A: = 3.1 X 10~4 s_l at -23 °C), which underwent a further series of rearrangements to give eventually protonated 8,8-dimethylbicyclo[3.1.0]octa-3,6-dien-2-one (29). The symmetrical 8,8-dimethyl-4-hydroxyhomotropylium cation 32 was observed as an intermediate in the isomerization of 13 to 29. On the basis of the thermal isomerizations of 14 and IS it was concluded that the 8,8-dimethyl-3-hydroxyhomotropylium cation must also be an intermediate in the conversions of 13 to 29 and 13 to 32.