The photosolvolysis reactions, in methanol, of six substituted benzyl acetates (7a-9 and benzyl pivalates @a-9 were studied. Five major benzylic products were formed from two critical intermediates. The ethers (9) were formed from the ion pair, 15, and all of the other products (10-f4) were formed from the radical pair, 16. Quenching studies showed that only excited singlet state reactivity was important. The product yields were found to be highly substituent dependent. For instance, for the acetate esters, the yield of ether (9) varied from 2% for X = 4-OCH3 to 32% for X = 3-OCH3. Most of the differences in the yields could be attributed to ground state processes that occur after bond cleavage. The important competition is between electron transfer, converting the radical pair to the ion pair, and decarboxylation of RCOz'. The rates of electron transfer are shown to fit Marcus theory in both the normal and inverted regions. Direct heterolytic cleavage to form the ion pair is of minimal importance.

Aminopyrine is associated with a high incidence of agranulocytosis. It is known to be oxidized by peroxidases and hypochlorous acid to a blue cation radical. It has been proposed that the mechanism by which hypochlorous acid oxidizes aminopyrine to a cation radical involves N-chlorination followed by loss of a chlorine radical. Another possible mechanism is loss of HCl to form an iminium ion and subsequent reaction with another molecule of aminopyrine and a hydrogen ion to form two radical cations. This mechanism would lead to incorporation of a hydrogen from water; however, using a deuterated analog, we found no hydrogen incorporation, thus providing strong evidence against this mechanism. Using a stopped-flow diode array spectrophotometer to study the reaction between aminopyrine and hypochlorous acid, an intermediate with a lambda max at approximately 420 nm was observed in the formation of the cation radical. We propose that this represents a dication formed by the loss of chloride ion from N-chloroaminopyrine. This intermediate is very reactive, with a half-life of approximately 15 ms, and in addition to being the precursor of the cation radical, it also appears to react with two molecules of water to form several other products that were observed and are consistent with the proposed dication intermediate. Similar stable products were formed when amino-pyrine was oxidized by the combination of myeloperoxidase, hydrogen peroxide, and chloride or activated neutrophils. The reactive dication formed by neutrophil-derived hypochlorous acid could be responsible for aminopyrine-induced agranulocytosis.

This paper reports the use of laser flash photolysis (LFP) techniques to show that cyclohexadienyl cations (cr 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-l,3-dimethoxybenzene (4) in 1,1,1,3,3,3-hexafluoroisopropyl 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 Ph,CHOCH(CF,), (7), the species expected if the cyclohexadienyl cation formed in the C2 protonation cleaved Ph2CH+. These products are, however, accompanied by 4-diphenylmethyl-l,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 (S), is formed within 106200 ps, and reacts with k = 9 x 10' s-I, 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 x lo5 s-'. The precursor for this has been identified as the 1-diphenylmethyl-2,4-dimethoxybenzenium ion (lo), the thermodynamically more stable isomer of 5. A mechanistic model is proposed in which excited 4 is C2 protonated in HFIP with k 2 1 x 10" s-' to form 5, which loses Ph2CH+ with k = 3 x 10's-I in competition with rearrangement to 10 with k = 6 x 10' s-'. The cation 10 serves as the second source of Ph,CH+, losing Ph,CH+ with k = 4 x lo5 s-'; in competition 10 is deprotonated by HFIP to give 8 with k = 8 x lo4 s-'. 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.RCsumC : Dans ce travail, on prCsente un rapport sur l'utilisation des techniques de photolyse Cclair au laser (<< LFP *) pour dCmontrer que les cations cyclohexadiCnyles (complexes a), impliquCs dans la rkaction de Friedel-Crafts du 1,3-dimCthoxybenzbne avec le cation diphCnylmCthyle, se transposent ii 1'Cchelle de temps de la ns, sans qu'il se produise de skparation entre le composC aromatique et llClectrophile. Ce rCsultat a Ct C dCmontrC par le biais d'une Ctude du comportement photochimique du 2-diphknylmCthy1-l,3-dimCthoxybenzbne (4) dans l'alcool 1,1,1,3,3,3-hexafluoroisopropylique (HFIP). I1 a Ct C dCmontrC antCrieurement que, par excitation dans le HFIP, les dCrivCs du 1,3-dimCthoxybenzbne se protonent ~Clectivement en C2; en fait, les pro...

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