Thionyl chloride, in the presence of catalytic amounts of pyridine, apparently adds across the double bonds of ,/3-unsaturated carboxylic acids to form /3-chloro-n-chlorosulfinyl acid chloride intermediates which are converted to «,/3-dichloro-a-chlorosulfenyl acid chlorides which can be isolated. Thus, trans-crotonic acid (11) gave 2chlorosulfenyl-2,3-dichlorobutanoyl chloride (12, 55%) which upon heating decomposed to trans-2-chloro-2-butenoyl chloride (13) and sulfur dichloride, trans-Cinnamic acid (9) furnished 3-chloro-2-chlorocarbonylbenzo-[b]thiophene (4, 69%) and trans-2-chloro-3-phenylpropenoyl chloride (5, 23%) as a result of further transformation of the sulfenyl chloride 3 which could be isolated in 19% yield. Mechanism of the cyclization of 3 to 4 was studied by product analyses in the reaction with m-nitrocinnamic acid (20), m-methoxycinnamic acid (26), p-nitrostilbene (31), 3-(3-methoxyphenyl)propanoic acid (36), and 3-(3-formylphenyl)propanoic acid (45). Both electrophilic and nucleophilic cyclizations were rejected by successful isolation of the benzo[5]thiophenes 21 and 22 from 20 and of 27, 28, 29, and 30 from 26. Isolation of only benzo[6]thiophene 34 from the reaction of 31 eliminated episulfide 33 as an intermediate. Formation of 3-unchlorinated benzo[b]thiophenes 37, 38, and 39 furnished evidence for a concerted elimination-cyclization (CEC) mechanism. The reaction of 45 did not give the expected 3-unchlorinated benzo[6]thiophenes, but the benzo[6]thiophenes 50. Formation of 50 could be rationalized in terms of the stereochemical consequence of the CEC mechanism in the transition state. The CEC mechanism appeared to be the most reasonable explanation for the formation of the minor products, 22 and 51.
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.
Indian J. Chem., 11, 744 (1973); )b) H. W. Altland and G. A. Molander, manuscript In preparation. (21) Mass spectra were determined on a Hitachi Perkln-Elmer RMS-4 spectrometer. 1H NMR spectra were measured with an A-60 Varían Associates or with a Perkln-Elmer R-32 (90 MHz) NMR spectrometer. The 13C spectra were measured on a Broker Model HX-90 (22.63 MHz) NMR spectrometer.Me2SO-£fe was used as the solvent and Me4SI as the internal standard for all NMR spectra determinations. All of the compounds in the tables gave satisfactory mass and 1H NMR spectra.Oxidations by Thionyl Chloride. 8. A Convenient Synthesis of Benzo[b]thiophenes from Carboxylic Acids and Ketones1•2
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