Figure 1. Photostationary states of 2-benzal-5-methylcyclopentanone (1) as a function of sensitizer triplet energy: •, unselective sensitizers; O, selective sensitizers.predicts the selectivities of sensitizers with £T > 59 kcal/ mol but fails to predict that all lower energy sensitizers are unselective.1 234 This result is predictable if there is an isomer of 1 having a ~59-kcal/mol T2 state with , * character. Lower energy potentially selective sensitizers would then have insufficient energy to populate the T2 state, and exothermic energy transfer would ensue causing population of the -, * hydrogen abstraction intermediate. The observed occurrence of isomerization even with the lower energy sensitizers could be due to competing nonvertical energy transfer to give a distorted 3( , *) state or to isomerization of the 3( , *) state.The changes in the photostationary ratios on reducing the triplet energies of the sensitizers also suggest a 2-, * state of 1. The decrease in the trans-cis ratio with lower energy selective sensitizers suggests that the cis-7r,7r* triplet is more energetic than the trans-, * triplet and therefore is populated less efficiently on lowering the sensitizer energies. The increase in the trans-cis ratio with lower energy unselective sensitizers implies that, conversely, with these sensitizers the efficiency of excitation to the trans-, * triplet falls off faster than to the cis-, * triplet. Analogy with sensitization of oxygen to its 4 8+ state suggests that excitation to T2 states may fall off rapidly with sensitizers having energies up to 10 kcal/mol above the T2 energy.5 Thus if the , * triplet of trans-1 is a T2 state, excitation to this state with unselective sensitizers could become inefficient earlier than excitation of cz's-1 to its higher energy , * state.(4) This is strictly true only if the nearly identical triplet energies given for the two phenylnaphthalene sensitizers are reversed.(5) D. R.
To distinguish between a mechanism in which thionyl chloride oxidizes a methylene group from one in which thionyl chlorides oxidizes a methyl group while converting monosubstituted acetones into 3-thietanones, a series of 4-aryl-2-butanones was subjected to oxidation conditions (thionyl chloride, 25-85O, 5 min-3 hr). Butanones. with the following ring substitution gave only 3-thietanones as products: 3-NO2, 3-F, 4-N02,4-C1,4-CH30. However, 4-(3-hydroxyphenyl)-2-butanone gave only 2-acetyl-5-hydroxybenzo [ b] thiophene, and 4-( 3-methoxypheny1)-2-butanone gave both 2-(3-methoxybenzylidene)-3-thietanone and 2-acetyl-5-methoxybenzo[b]thiophene as products. The results are consistent only with thionyl chloride oxidation proceeding exclusively at the methylene position.A few years ago we reported the oxidative conversion of @-aryl carboxylic acids and certain ketones to benzo[ blthiophenes2 and of methyl ketones to 3-thietan0nes.~ The unusual nature of these products and the generality of the reactions prompted us to examine the mechanism by which these products were being formed. The results of our investigation of the benzothiophene case have been reported.2The key to the mechanism was the intermediacy of sulfenyl chlorides by oxidation of a methylene group adjacent to the carbonyl function.On the reasonable assumption that the 3-thietanones were being formed through the intermediacy of a structurally similar sulfenyl chloride, we proposed3 a mechanism for 3-thietanone formation (eq 1). However, this mechanism did not seem to account for the lack of benzothiophene formation in those cases where Ar (in eq 1) is an aromatic ring. If for some reason oxidation of the methyl ke-0 CI ArCH,
This study investigated the effect of high-irradiance light-curing on exposure time and pulpal temperature of adequately-cured composite. Composite placed in a molar preparation was cured using high-irradiance light-curing units (Flashmax P3, Valo, S.P.E.C. 3 LED, Cybird XD) and tested for hardness occlusal-gingivally. The first group had exposure times set according to manufacturer settings (recommended), second group to yield 80% of maximum hardness at the 2 mm depth (experimental), and third group was set at 20 s (extended). Exposure time necessary to adequately polymerize the composite at 2 mm depth was 9 s for the Cybird XD and Valo and 12 s for S.P.E.C. 3 LED and Flashmax P3. None of the high-irradiance light-curing units adequately polymerized the composite at the manufacturer-recommended minimum-exposure times of 1-3 s. Exposure times necessary to adequately polymerize composite at 2 mm resulted in a maximum pulpal-temperature increase well below the temperature associated with possible pulpal necrosis.
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