2.3- Dihydro-2-substituted-thiophenes

The numbering in thiophene 1 starts at the sulfur atom and continues around the ring. The C-2 and C-5 carbon atoms are also designated as C-a the C-3 and C-4 carbons as C-/3. Substituted thiophenes arc named similarly to substituted benzenes. Some of the more common radicals (2-5) are shown in Figure 1. There are two dihydro derivatives, 2,3- 4 and 2,5-dihydrothiophene 5 and a single tetrahydrothiophene 6. The benzo derivatives benzo[/)]- 7 and benzo[r]thiophene 8 are considered analogues of naphthalene while dibenzothiophene (DBT) 9 is related to phenanthrene. 

In other work on the concentrated H2S04-Zn system [129, 130], mono- and disubstituted thiophenes gave 2,5-dihydrothiophenes and thiophanes (2,3,4,5-tetrahydrothiophenes) with a significant preference for the former. The optimized conditions (thiophene 89% sulfuric acid zinc = 1 30 110 in dichloromethane at 20°C) converted alkyl-substituted thiophenes into a mixture of 2,5-dihydrothiophenes and tetrahydrothiophenes with the former predominating. For 2-formyl- and 2-acetylthiophenes, under slightly modified conditions, 2,5-dihydro-... 

Benzo[b]thiophene, 2,3-dihydro-reactions, 4, 849 3-substituted synthesis, 4, 872... 

Phenanthro[l,2-d][l,2,3]selenadiazole, 10,11 dihydro- H NMR, 6, 348 synthesis, 6, 353 Phenanthro[b]thiophenes synthesis, 4, 914 Phenanthro[4,5-bcd]thiophenes synthesis, 4, 883, 907, 914 Phenanthro[9,10-ej[l, 2,4]triazines synthesis, 3, 434 Phenarsazin synthesis, 1, 561 Phenazine dyes, 3, 196-197 nitration, 3, 177 UV Spectra, 2, 127 Phenazine, 3-amino-2-hydroxy-in colour photography, 1, 374 Phenazine, 1-chloro-nucleophilic substitution, 3, 164-165 5-oxide... 

Benzo[c]thiophene is a source of 3-substituted derivatives of 3,4-dihydro-1//-2-benzothiopyrans via ring opening to the dianion and subsequent reaction with electrophiles <96JOC1859>. 

Substituted 3,4-diamino- (107) and 3,6-diaminothieno[3,2-6j-thiophene (108) condense with aromatic aldehydes to form dyes. Dann and Dimmling showed that 2,3-dihydro-4,6-dimethylthieno-[3,4-6]thiophen-3-one (252) was oxidized by alkaline ferricyanide to give a thioindigoid dye (253) [Eq. (90)] useful for dyeing cotton red with a Me Me Me... 

When the aromaticity of the thiophene ring in benzo[6]thiophene is destroyed, substitution is governed by the same factors which govern substitution in benzene derivatives. Thus, benzo[6]thiophene-l,l-dioxides and their 2,3-dihydro derivatives undergo substitution in the... 

Supercritical carbon dioxide with a minute co-solvent addition is an effective medium for the 1,3-dipolar cycloaddition of azomethine ylides with DMAD to produce substituted pyrroles.67 The 1,3-dipolar cycloaddition of nitrile ylides [e.g. benzonitrile (4-nitrobenzylide) and 4-nitrobenzonitrile(benzylide)] with acrylamides provided a synthesis of 3,4-dihydro-2//-pyrroles with moderate to good yields.68 The Pt(II)-or Au(III)-catalysed 3 + 2-cycloaddition of the transition metal-containing azomethine ylide (63) with electron-rich alkenes provided a carbene complex (64), which yields tricyclic indoles (65) having a substituent at 3-position (Scheme 17).69 The 1,3-dipolar cycloadditions of azomethine ylides with aryl vinyl sulfones are catalysed by Cu(MeCN)4C104-Taniaphos with nearly complete exo- selectivity and enantioselec-tivities up to 85% ee.10 The 3 + 2-cycloaddition of benzol/>]thiophene 1,1-dioxide... 

Substituted 3,4-dihydro-l //-2-benzothiopyrans 499 are formed from benzo[c]thiophenes by ring opening to the dianion and reaction with electrophiles (Scheme 184) <1996JOC1859>. 

Thiophene is not reducible by direct electroreduction, but by indirect reduction (Chapter 29) in DMF using biphenyl radical anion as electron transfer agent it is possible to reduce thiophene to 2,5-dihydrothiophene and tetrahydrothiophene in high yield [192] thiophenes substituted at C-2 with carboxyl, however, may be reduced by direct electrolysis to the 2,5-dihydro derivatives [193]. 

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