Abstract:Selective photoexcitation of phenanthrene in a radical transfer from phenanthrene cation radicaI to mixture of phenanthrene, a-methylstyrene, and nza-methylstyrene. dicyanobenzene in MeCN and in MeCN-MeOH ( 1 : 1) gave 1,4-dimethyl-1-phenyl-1,2,3,4-tetrahydronaphtha-RECENTLY the photodimerization of olefins via cation lene and 1,4-dimethyl-1-methoxy-1 ,kdiphenylbutane, radicals has been extensively studied and a cation radical respectively, products which are formed from the atransfer mechanism has been propos… Show more
“…For this purpose we have chosen the acid-catalyzed dimerization of α-methylstyrene ( 1 ), which is known to give rise to the olefinic dimers 2 together with the indanic dimer 3 (Scheme ). − Other reaction mechanisms afford different dimers such as 4 or 5 and 6 (Scheme ). − Thus, by merely determining the product distribution, it is possible not only to estimate changes in acid strength but also to assess if other reaction mechanisms besides that involving a carbenium ion are intervening. 2 Dimer Ratios at Low Conversions: 2a (9), 2b (1), 3 (4) 3 Characteristic Dimers Formed Under Radical ( 4 ) or Radical Cation ( 5 and 6 ) Reaction Mechanisms …”
The 9-(4-methoxyphenyl)xanthylium ion (AnX + ), a bulky cation that according to molecular modeling simulations cannot enter through the 7.4 Å pore opening of tridirectional large-pore zeolites Y and β, has been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure involves two steps: (i) generation of xanthylium cation (X + ) adsorbed within the zeolite by treatment of xanthydrol onto the H + form of these solids; (ii) electrophilic attack of the cation to anisole within the framework of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with some unidentified adventitious material. Textural and acidic properties of these composites were tested using R-methylstyrene dimerization and were found to be very similar to the original HY or Hβ zeolites, thus indicating an internal location for AnX + . Entrapment within the zeolite framework restricts AnX + conformational mobility, thwarting the deactivation pathway occurring in solution. This has allowed for the first time characterization of its triplet excited state as a long-lived transient species (tens of microseconds), as well as the observation of room-temperature fluorescence in the nanosecond range.
“…For this purpose we have chosen the acid-catalyzed dimerization of α-methylstyrene ( 1 ), which is known to give rise to the olefinic dimers 2 together with the indanic dimer 3 (Scheme ). − Other reaction mechanisms afford different dimers such as 4 or 5 and 6 (Scheme ). − Thus, by merely determining the product distribution, it is possible not only to estimate changes in acid strength but also to assess if other reaction mechanisms besides that involving a carbenium ion are intervening. 2 Dimer Ratios at Low Conversions: 2a (9), 2b (1), 3 (4) 3 Characteristic Dimers Formed Under Radical ( 4 ) or Radical Cation ( 5 and 6 ) Reaction Mechanisms …”
The 9-(4-methoxyphenyl)xanthylium ion (AnX + ), a bulky cation that according to molecular modeling simulations cannot enter through the 7.4 Å pore opening of tridirectional large-pore zeolites Y and β, has been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure involves two steps: (i) generation of xanthylium cation (X + ) adsorbed within the zeolite by treatment of xanthydrol onto the H + form of these solids; (ii) electrophilic attack of the cation to anisole within the framework of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with some unidentified adventitious material. Textural and acidic properties of these composites were tested using R-methylstyrene dimerization and were found to be very similar to the original HY or Hβ zeolites, thus indicating an internal location for AnX + . Entrapment within the zeolite framework restricts AnX + conformational mobility, thwarting the deactivation pathway occurring in solution. This has allowed for the first time characterization of its triplet excited state as a long-lived transient species (tens of microseconds), as well as the observation of room-temperature fluorescence in the nanosecond range.
“…The radical cations of both pyrene (22) and 11 (70) have been detected by means of laser flash photolysis in acetonitrile in the presence of 1. Furthermore, Pac and co-workers (46) have calculated the theoretical free energy change for the transfer process, using the Weller equation, and came to the conclusion that transfer from the excited singlet state of several aromatic hydrocarbons to 1 should be a highly favoured process.…”
Section: Irradiations In Ncetonitrile Solutionmentioning
Can. J. Chem. 62, (785 (1984). ' The photosubstitution (electron transfer) reaction between 1,4-dicyanobenzene (1) and 2,3-dimethyl-2-butene (2), which gives I-(4-cyanophenyl)-2,3-dimethyl-2-butene (3) and 3-(4-cyanopheny1)-2,3-dimcthyl-I-butene (4). has been extended to other dicyanobenzene-olefin mixtures. Substitution of a cyano group occurs when both 1 or 1,2-dicyanobenzene (5) are irradiated in acetonitrile solution, in the presence of 2 or cyclohexcne (16). Under comparable conditions, 1.3-dicyanobenzene (6) failed to react. Little or no substitut~on was observed in any case when the olefin was methylpropene (19). ' The results for 1 and 5 are in agreement with empirical free energy calculations (Weller equation) for the electron transfer process which, however, fail to explain the general lack of reactivity of 1,3-dicyanobenzene. Phenanthrene (11) has been shown to photosensitize the photosubstitution reaction between dicyanobenzenes and 2. Under these conditions the olefin reacts with 6 predominantly at the 4-position, resulting in overall substitution of a hydrogen atom. This reaction occurs regiospecifically, resulting in the formation of only one of the two possible isomeric side chains. The mechanistic details of these reactions have been substantiated by means of deuterium labelling studies. The aromatic nitriles also undergo photosubstitution by 2, in acetonitrile-methanol solution, resulting in methanol-incorporated products. Whereas reaction with 1 or 5 results in substitution of a cyano group, 6 was observed to give isomeric dicyanocyclohexenes, resulting from initial reaction at the 4-position, followed by reduction. A detailed mechanism for this secondary photoreduction has been substantiated by deuterium labelling studies. The anomalous behaviour of 1,3-dicyanobenzene has been attributed to a difference in the react~vity of the radical anion. marquages par le deutCrium permettent de confirmer les dttails mkcanistiques dc ces rCactions. Les nitriles aromatiques, en solution dans des milanges d'acktonitrile et de methanol, subissent aussi des photosubstitutions par 2; il y a formation de produits ayant incorporC du mCthanol. Alors que la rCaction des composCs 1 ou 5 conduit a la substitution d'un groupe cyano, on a observC que le compost 6 donne des dicyanocyclohextnes isomkres qui resultent d'une rkaction initiale en position 4, suivie d'une riduction. Des Ctudes impliquant des marquages par du deuterium ont permis de confirmer le mCcanisme dCtaillC de cette photorCduction secondaire. On attribue Ie comportement anormal du dicyano-I ,3 benzkne a la diffkrence de rCactivitC de I'anion radical.[Traduit par le journal]
“…What is remarkable is the retention of the stilbenoxide stereochemistry. This is a result of an allowed photochemical disrotatory opening of the stilbene oxide, leading to a planar carbonyl ylide 130 whose geometry is preserved through resonance delocalization.…”
Section: Reactions Of 9-cyanophenanthrenementioning
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