Photochemical and thermal rearrangements of protonated 2,3-homotropones

Abstract

The 2-hydroxyhomotropylium cation 10 and 8-endo-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 FS03H. 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 I-hydrgxyhomotropylium 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 I-phenylpropanal ( k = 2.5 X s-I at 37 "C). The 8,8-dimethyl cation 23 isomerized back to 13 ( k = 3.1 X I 0-4 S-I at -23 "C), which underwent a further series of rearrangements to give eventually protonated 8,8-dimethylbicyclo[ 3.1 .O]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 15 it was concluded that the 8,8-dimethyl-3-hydroxyhomotropylium cation must also be a n intermediate in the conversions of 13 to 29 and 13 to 32.

Acid catalysed rearrangements of 8,8-dimethyl-2,3-homotropone

“…Instead we have uncovered a variety of isomerization pathways and a remarkable acid dependence on the relative energies of these paths. In FSO,H, where the various ketones are fully protonated, 2H isomerized essentially exclusively to 3H and then to either 4H or 5H (2). None of the products found under the acid catalysed conditions used here was detected on rearrangement of 2H in FS0,H.…”

“…Hydroxyhomotropylium cations undergo a variety of photochemical and thermal isomerizations (1)(2)(3). Thermally induced rearrangements include exo-endo interconversion of substituents on the bridging C(8) carbon by a ring flipping process (1,4), isomerizations to protonated aromatic ketones (2)(3)(4), and circumambulatory rearrangements of the bridging carbon (2,5).…”

“…Thermally induced rearrangements include exo-endo interconversion of substituents on the bridging C(8) carbon by a ring flipping process (1,4), isomerizations to protonated aromatic ketones (2)(3)(4), and circumambulatory rearrangements of the bridging carbon (2,5). This latter process becomes facile when the bridging carbon bears charge stabilizing groups.…”

“…This latter process becomes facile when the bridging carbon bears charge stabilizing groups. An example of these circumambulatory rearrangements is shown in Scheme 1 for the 8,8-dimethyl substituted systems (2). 0 One feature of the reactions shown in Scheme 1 2 is that the ultimate product is the protonated bicyclo[3.2.…”

“…Thus lH, 2H, and 4H all isomerize to 5H.' At first glance the thermodynamic stability of 5H is surprising inasmuch as this cation can formally be regarded as "antibishomoaromatic" (6), while lH, 2H, 3H, and 4H are homoaromatic ions (2,7). However, there is a considerable difference in strain energy expected between the two types of cation and this could override any difference in stability due to delocalization.…”