Parliament and Councils of Mediaeval Ireland. H. G. Richardson , G. O. Sayles

Oxidations by thionyl chloride. VI. Mechanism of the reaction with cinnamic acids

“…In the presence of a catalytic amount of a tertiary amine, thionyl chloride generally oxidizes carboxylic acids and ketones at a carbon atoms to form a-chloro-a-chlorosulfenyl derivatives and their subsequent reaction products. Thus, 3-phenylpropanoic acid (1), for example, when treated with an excess of thionyl chloride and a small amount of pyridine, can be converted to sulfenyl chloride 2, which then undergoes further reaction to form benzo [6]thiophene 4 and -chlorocinnamoyl chloride (5) via intermediate 3 (eq l).3 Another example is the conversion of methyl ketones to 3-thietanones 7 presumably through intermediates 6, that offered a one-step synthesis of the four-membered hetereocycles (eq 2).4…”

“…Under milder conditions (bath temperature 88°, 24 hr) sulfenyl chloride 3 could be isolated in 19% yield. Sulfenyl chloride 3, a yellow, viscous oil, showed in the infrared spectrum characteristic3•7 multiple bands (5.55, 5.65, and 5.70 urn) in the carbonyl region and NMR absorption at 2.54 (aromatic) and 4.21 (methine proton).…”

“…Acid 20 was treated with 3.5 equiv of thionyl chloride and 0.12 equiv of pyridine at 135°for 1.5 hr. The product mixture was treated with methanol and separated on an alumina column to furnish four benzo [5]thiophenes, 21a, 21b (23.4%), 22a (1.2%), and 22b (1.8%). Although 21a was a, X = N02; Y = H b, X = ; Y = N02 a major product, only a small amount of it was isolated owing to its difficult separation from 21b.…”

“…Since a m-nitro group deactivates an aromatic ring to electrophilic substitution, the ease of benzo [5]thiophene formation from 20 indicated that the ring closure involved neither path a from 23a nor path b from 24a, but it rather suggested a nucleophilic process of episulfide 25a, path c (see Scheme I).…”

“…a, X = NO, b, X = OCH3 Episulfide Mechanism. In view of a number of reported instances18 for rearrangement (and related transformations) of -halo episulfides to benzo [5]thiophenes the episulfide mechanism (path c) was particularly attractive. The behavior of 20 appeared to be consistent with this mechanism.…”