1,2-Thio migration

Rearrangement of jS-thio-a-diazo carbonyl compounds (44) occurred upon decomposition of the diazo function by metals, especially Rh(II).43 1,2-Thio migration adducts (45) were obtained with moderate to high diastereoselectivities. The outcome of the decomposition of (46) by Rh(II) was shown to be highly dependent on the nature of the X substituent.44 When X = OH, (47) has been exclusively observed, whereas (48) was the only product isolated when X = NHC(0)CC13. 

A recent paper [44] shows that the treatment of silyl thioketones 68 with lithium diethylphosphite proceeds via a thiophiUc attack followed by a thio-phosphate mercaptophosphonate (69 70) carbanionic rearrangement and the migration of the silyl group from the carbon to the sulfur atom leading to the S-silylated sulfanylphosphonate carbanion 71. The last step represents the first example of the thia-Brook rearrangement (Scheme 18). 

The most popular method for generation of a-thio-carbanion (migration terminus) is direct lithiation (deprotonation) with alkyllithium or lithium amide. These deprotonation methods are widely applicable to various substrates, not only benzyl or allyl sulfides , but also dithioacetals 142 which form 143 (equation 83), and a phosphonate substituted system 144 which gives 145 (equation 84). ... 

In the cyclization of arylthioacetones under strong acid conditions a partial migration of the methyl group can occur, to produce mixtures of both the 2- and 3-methylbenzo[6 ]thio-phenes. For example, cyclization of (109 R = H) in PPA at 160-180°C gave a mixture composed of about 82-85% (110) and 15-18% of (111). Similar results were obtained when R in (109) was alkyl, but not when R was chlorine or bromine (68JCS(C)2733). 

Cyclodehydrations of ortho- or para-substituted (arylthio)acetones afford 7- and 5-substituted 3-methylbenzo[6]thiophenes,respectively mete-substituted (arylthio)acetones afford mixtures of 4- and 6-substituted 3-methylbenzo[6]thiophenes (see, e.g., Matsuki and Ito105). However, in these cases a partial migration of the 3-methyl group to the 2-position may be an added complication.297,306 Such isomerizations occur during the preparation of 3-methylbenzo[6]thio-phene and its 5-alkyl,297 306 but not its 5-halo 297 derivatives. A mixture of 3-methylbenzo[6]thiophene and its 5- (the expected product) and 6-JerJ-butyl derivatives is obtained on cyclization of (p-terf-butyl-... 

An example of the photodecomposition of a dithiin to give a stable dithiet, with elimination of ethylene, has been reported.206 A different reaction is preferred in the dithiins (261), which on irradiation, are converted to benzene derivatives (262) via the 1,4-dithiocins (263).207 The cyclic trimer of thio-acetophenone on irradiation in cyclohexane yields thioacetophenone.208 An initial carbon-sulfur bond homolysis is also responsible for the conversion, on irradiation in methanol, of a 3-cephem to two thiazoles,209 whereas a 1,2-alkyl migration followed by a further photochemically induced carbon-sulfur bond homolysis has been proposed to account for the photorearrangement of the sulfonium ylides (264) to the pyrimido-l,4-benzo[h]thiazepines (265).210 The novel photorearrangemenl of a 1,3-thiazine to a cyclopro-pathiazolidine has also been rationalized in terms of an initial carbon-sulfur... 

The Willgerodt Reaction allows the synthesis of amides from aryl ketones under the influence of a secondary amine and a thiating agent. The mechanism involves the formation of an enamine which undergoes thiation, and the carbonyl group migrates to the end of the chain via a cascade of thio-substituted iminium-aziridinium rearrangements. 

In the presence of a Lewis acid, 1-seleno- and l-thio-2-silylethenes react with 2-phosphonoacrylates to give cyclopropane products (Scheme 2).31,31a The [2 + l]-cycloaddition would involve 1,2-silyl migration of an cr-seleno-or a-thio-/3-silylcarbenium ion intermediate and subsequent ring closure. 

Lastly, the 2- versus 3-positional reactivity order for benzo[6]thio-phene appears to be affected by steric hindrance from the sulfur d orbitals [82JCS(P2)1489], and this is discussed in detail in Section 2.A.C. For indoles it has been proposed that 2-substitution may take place by initial attachment of the electrophile to the 3-position, followed by migration (69T227). 

Several thiopropargylimines 32 underwent ring-closure to the pyrroles 33 upon treatment with Cul in hot A V-dimethylacetamide (DMA). This process involves a propargyl-allenyl isomerization followed by a 1,2-migration of the thio-group in the intermediate allenes <03AG(E)98>. 

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