- Photocycloadditions of chloranil to homobenzvalene, norbornadiene, and quadricyclane
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Solutions of chloranil (CA) in toluene or benzene have been irradiated in the presence of the C7H8 valence isomers homobenzvalene (HB), norbornadiene (N), and quadricyclane (Q). In the case of HB, the adducts 3-6 were obtained, all of them having a rearranged structure. By a separate irradiation it was shown that 4 is a consecutive product of 3. The structure of 4 was established by X-ray diffraction analysis. These reactions are most probably initiated by an electron transfer (ET) from HB to 3CA, continue by bond formation between the radical ions to give the zwitterion 14, which undergoes Wagner-Meerwein rearrangements, and are completed by ring-closure between the charged centres of the resulting zwitterions. The oxidation potential of N is not larger than that of HB and the products 8-10 may well be formed through an ET to 3CA. However, the pathway leading to product 3 most probably involves diradical intermediates. This should also be the case for the adducts 11 and 12, which were produced upon irradiation of methyl phenylglyoxylate (PG) in the presence of N. In the case of Q, an ET to 3CA is again highly likely, followed by a combination of the radical ions to generate the zwitterion 29, which eventually collapses to produce the oxetane 8 as the sole identifiable product. The photoreactions of CA with N and Q proceeded much less efficiently in acetonitrile solution.
- Braun, Max,Christl, Manfred,Deeg, Oliver,Rudolph, Marcus,Peters, Eva-Maria,Peters, Karl
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p. 2093 - 2101
(2007/10/03)
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- Chloranil-Sensitized Photolysis of Benzyltrimethylsilanes. Solvent Effect on the Competition between Carbon-Hydrogen and Carbon-Silicon Bond Cleavage
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A steady state and laser flash photolysis study of the chloranil (CA)-sensitized oxidation of benzyltrimethylsilane (BTS) and (4-methoxybenzyl)trimethylsilane (MBTS) in different solvents (benzene, CH2Cl2, and MeCN) has been carried out.In benzene, BTS reacts with 3CA* to give exclusively α-substituted benzyltrimethylsilane, whereas with MBTS the α-substituted silane is formed together with benzylic desilylation products.The latter situation also holds in CH2Cl2 for BTS.Only desilylation products are obtained from MBTS in CH2Cl2 and from both BTS and MBTS in MeCN.Higher quantum yields in the reactions with BTS than in those with MBTS have been observed in benzene and CH2Cl2.In MeCN, no significant change in quantum yield has been observed on going from BTS to MBTS.In MeCN with both BTS and MBTS and in CH2Cl2 with MBTS, the laser photolysis experiments have shown evidence for the formation of transients which can be attributed to MBTS(.+) and CA(.-) (MBTS in MeCN and CH2Cl2) and to CA(.-) (BTS in MeCN).This indicates that quenching of 3CA* has taken place via an electron transfer process.Once formed, both BTS(.+) and MBTS(.+) undergo exclusive C-Si bond cleavage.In CH2Cl2, this reaction is promoted by CA(.-), and accordingly, MBTS(.+) decays by a second-order kinetics.In MeCN, MBTS(.+) decays by a first-order kinetics and desilylation is promoted by the solvent itself.The same holds for BTS(.+) as the decay reaction of this cation radical in MeCN appears much faster than that of CA(.-).In fact, only the transient assigned to the latter species has been observed in the laser photolysis experiments.A different situation has been found for both MBTS and BTS in benzene and BTS in CH2Cl2, where quenching of 3CA* occurs via a partial charge transfer (CT) triplet complex.The reversible formation of this complex with MBTS in benzene is clearly indicated by the dependence of the observed rate constant for 3CA* quenching on the substrate concentration, which has allowed the association constant for the complex (ca. 400 M-1) to be determined.With BTS in benzene and CH2Cl2, the formation of the CT triplet complex is irreversible and rate determining, and the main evidence in this respect comes from the absence of a sizable deuterium kinetic isotope effect for the 3CA* quenching rate.With BTS in benzene, the CT complex undergoes C-H bond cleavage as the exclusive chemical reaction.With MBTS in benzene and BTS in CH2Cl2, both C-H and C-Si bond cleavages take place, and it is suggested that the cleavage of the latter bond requires more transfer of charge in the complex than the cleavage of the C-H bond.Alternatively, in CH2Cl2, the CT complex might evolve in part to a solvent-separated radical ion pair, exclusively leading to the C-Si bond cleavage products.The above results have also allowed an assessment of the scope of the benzyltrimethylsilane probe to detect electron tr...
- Baciocchi, Enrico,Giacco, Tiziana Del,Elisei, Fausto,Ioele, Marcella
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p. 7974 - 7983
(2007/10/03)
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- Reactions of Aryldiazomethanes with Chloranil
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The reactions of several aryldiazomethanes (1a: p-tolyl; 2b: phenyl; 1c: p-chlorophenyl) with chloranil gave stilbenes (3) and spiro-oxetanes (4) at 20 deg C in tetrahydrofuran or 1,2-dichloroethane.The isomer ratios of 3 were ca. 2 to 3:1 in favor of cis, depending on the solvents and on the substituents 1.In the case of 4, however, stable trans-isomers were selectively formed.On the other hand, the presence of added CH3OH supressed the formation of 3 and 4 and, instead induced redox reactions giving α,α-dimethoxyarylmethanes and tetrachlorohydroquinone (6).In these redox reactions, the acid decompositions of 1 with 6 were also found.However, the presence of CF3CH2OH allowed the formation of considerable amount of 3 and 4, along with the redox products.The mechanism of these reactions will be discussed.
- Oshima, Takumi,Nagai, Toshikazu
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p. 726 - 730
(2007/10/02)
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