A photochemical retro-Friedel–Crafts alkylation. Rapid rearrangement of cyclohexadienyl cations

This paper reports the use of laser flash photolysis (LFP) techniques to show that cyclohexadienyl cations (σ complexes) of the Friedel–Crafts reaction of 1,3-dimethoxybenzene and the diphenylmethyl cation rearrange on the ns time scale without separating the aromatic compound and the electrophile. This is demonstrated through a study of the photochemical behaviour of 2-diphenylmethyl-1,3-dimethoxybenzene (4) in 1,1,1,3,3,3-hexafIuoroisopropyl alcohol (HFIP). Derivatives of 1,3-dimethoxybenzene have previously been found to selectively protonate at C2 upon excitation in HFIP, and indeed the principal products with 4 are 1,3-dimethoxybenzene (6) and Ph2CHOCH(CF3)2 (7), the species expected if the cyclohexadienyl cation formed in the C2 protonation cleaved Ph2CH+. These products are, however, accompanied by 4-diphenylmethyl- 1,3-dimethoxybenzene (8), a rearranged isomer of 4. A portion of this product is explained by the combination of Ph2CH+ and 1,3-dimethoxybenzene as the latter accumulates during the irradiation. However, 11.5% of 8 is also seen upon extrapolation to zero time. LFP experiments on the ps time scale reveal that the C2 protonated cation, the 1-diphenylmethyl-2,6-dimethoxybenzenium ion (5), is formed within 100–200 ps, and reacts with k = 9 × 108 s−1, with absorbance for Ph2CH+ growing in as 5 decays. LFP studies on the ns time scale reveal that there is a second quantity of Ph2CH+ that grows in, with k = 5.0 × 105 s−1. The precursor for this has been identified as the 1-diphenylmethyl-2,4-dimethoxybenzenium ion (10), the thermodynamically more stable isomer of 5. A mechanistic model is proposed in which excited 4 is C2 protonated in HFIP with k ≥ 1 × 1010 s−1 to form 5, which loses Ph2CH+ with k = 3 × 108 s−1 in competition with rearrangement to 10 with k = 6 x 108 s−1. The cation 10 serves as the second source of Ph2CH+, losing Ph2CH+ with k = 4 × 105 s−1; in competition 10 is deprotonated by HFIP to give 8 with k = 8 × 104 s−1. The 11.5 % of the rearranged 8 that is observed at zero conversion is thus shown to come from an intramolecular pathway in which the key step is the migration of a diphenylmethyl group without separation: 4 → 4* → 5 → 10 → 8. Key words: cyclohexadienyl, photoprotonation, Friedel–Crafts, diphenylmethyl, 1,3-dimethoxybenzene.