248-nm Laser flash photoprotonation of mesitylene, hexamethylbenzene, and 1,3,5-trimethoxybenzene in 1,1,1,3,3,3-hexafluoroisopropyl alcohol. Formation and electrophilic reactivities of the cyclohexadienyl cations

Ring-opening hydroarylation of monosubstituted cyclopropanes enabled by hexafluoroisopropanol

Ring-opening hydroarylation of cyclopropanes is typically limited to substrates bearing a donor-acceptor motif. Here, the transformation is achieved for monosubstituted cyclopropanes by using catalytic Brønsted acid in hexafluoroisopropanol (HFIP) solvent, constituting a rare example where such cyclopropanes engage in intermolecular C-C bond formation. Branched products are obtained when electron-rich arylcyclopropanes react with a broad scope of arene nucleophiles in accord with a simple SN1-type ring-opening mechanism. In contrast, linear products are obtained when cyclopropylketones react with electron-rich arene nucleophiles. In the latter case, mechanistic experiments and DFT-calculations support a homo-conjugate addition pathway.

NXS, Morpholine, and HFIP: The Ideal Combination for Biomimetic Haliranium-Induced Polyene Cyclizations

In contrast to Nature that accomplishes polyene cyclizations seemingly with ease, such transformations are difficult to conduct in the lab. In our program dealing with the development of selective halogenations of alkenes, we now asserted that standard X⁺ reagents are perfectly suited for the biomimetic cation-π cyclization of both electron rich and poor linear polyenes in the presence of the Lewis base morpholine and the Lewis acid HFIP. The method stands out due to its broad substrate scope and practicability together with high chemical yields and excellent selectivities, even for highly challenging chloriranium-induced polyene cyclizations.

The 1,2,4-triazolyl cation: thermolytic and photolytic studies

The generation of the 1,2,4-triazolyl cation (1) has been attempted by the thermolysis and photolysis of 1-(1,2,4-triazol-4-yl)-2,4,6-trimethylpyridinium tetrafluoroborate (2) and the thermolysis of 1- and 4-diazonium-1,2,4-triazoles, using mainly mesitylene as the trapping agent. Thermolysis of 2 gave mostly 1,2,4-triazole, together with 3-(1,2,4-triazol-4-yl)-2,4,6-trimethylpyridine, 4-(1,2,4-triazol-4-ylmethyl)-2,6-dimethylpyridine, and 4-(2,4,6-trimethylbenzyl)-2,6-dimethylpyridine. Thermolysis of each of the diazonium salts in the presence of mesitylene again gave mainly triazole together with very low yields of 1-(1,2,4-triazol-1-yl)-2,4,6-trimethylbenzene and the corresponding -4-yl isomer in about the same ratio. On the other hand, photolysis of 2 in mesitylene gave mainly 1-(1,2,4-triazol-1-yl)-2,4,6-trimethylbenzene. A photoinduced electron transfer from mesitylene to 2 has been observed and preliminary laser flash photolyses of 2 and the corresponding 2,4,6-triphenylpyridinium salt have been carried out. The observed transients are explained as arising from the first excited states of the pyridinium salts rather than from 1. Ab initio MO calculations are reported and indicate that the predicted electronic ground-state of the triazolyl cation is a triplet state of B1 symmetry with five pi electrons, which corresponds to a diradical cation (1c). Possible mechanisms for the formation of the various products are proposed.

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.