1,2-Benzisothiazole

Fig. 1. HeterocycHc amines usedia azo dyes, (a) 2-Amino-6-nitrohenzothiazo1e [6285-57-0], (b) 3-amiao-5-nitro-2,l-benzisothiazole [14346-19-1], (c) 3-amiQo-4JT-l,2,4-triazole [65312-61 -0], (d) 5-amiQo-l,2,4-thiadiazole [7552-07-0, (e) 4,4 -diamiQo-2,2 -biphenylsulfone [6259-19-4], (f)...

Couplers which form scarlet dyes with 4-nitroani1ine and red dyes with 2-amiQO-6-nitrobenzothiazole yield blue dyes with 3-amiQo-5-nitro-2,l-benzisothiazole [14346-19-1]. 

In contrast to thiazoles, certain isothiazoles and benzisothiazoles have been directly oxidized to sulfoxides and sulfones. 4,5-Diphenyl-l,2,3-thiadiazole is converted by peracid into the trioxide (146). Although 1,2,5-thiadiazole 1,1-dioxides are known, they cannot be prepared in good yield by direct oxidation, which usually gives sulfate ion analogous to the results obtained with 1,2,4- and 1,3,4-thiadiazoles (68AHC 9)107). 

There are many related examples which are now known as the general Dimroth rearrangement. For example, 3-ethylamino-l,2-benzisothiazole (419) is in equilibrium in aqueous solution with the 2-ethyl-3-imino isomer (420) <72AHCf 14)43). Dimroth rearrangements are known in the 1,2,4-thiadiazole series (421- 422), and in the 1,3,4-thiadiazole series as products of reactions of halogeno-l,3,4-thiadiazoles see Section 4.02.3.9.1 <68AHC(9)165). For a similar example in the 1,2,3,4-thiatriazole series, see Section 4.02.3.1.9. 

X-Ray diffraction studies have been carried out on other 1,2-benzisothiazoles (72JCS(P2)2125, 74CSC535, 76G769) and complexes of substituted 1,2-benzisothiazoles with... 

Table 8 NMR Chemical Shifts of 2,1-Benzisothiazoles in CDCI3 <75CJC836)...

There is very little published information on the UV spectra of 1,2-benzisothiazoles, though more data are available on the 2,1-isomers. The spectra are complex with as many as six maxima above 200 nm. Representative wavelengths of maxima are collected in Table 12. In all cases the most intense bands (e > 15 000) are those at short wavelengths, but all the bands indicated in the table have molar absorptivities greater than 4000, except those of 3-amino-2,l-benzisothiazole. Saccharin absorbs weakly at 350 nm and 277 nm, with intense bands below 230 nm (ethanol solvent) (82UP41700>. It exists as the anion except in acid solutions. The UV spectra of cations formed from 3-amino-2,l-benzisothiazole are discussed in (69CB1961>. Further applications of UV spectroscopy in studying tautomeric... 

Table 12 UV Absorption Maxima of Benzisothiazoles (solvent ethanol or methanol)...

The exoeyelie earbonyl group of isothiazol-3-ones absorbs in the region 1610-1660 em <7lJHC59l). 2-Methylisothiazol-3-one itself has the C=0 and C=C bands at 1660 and 1629 em respeetively, in CCI4 solution <64TL1477). The low earbonyl frequeney is due in part to eontributions from the resonanee form (20b). The earbonyl frequeney inereases in sulfoxides (1660-1730 em ) and 1,1-dioxides (1690-1740 em ) where sueh forms are not favourable. Sulfoxides (1060-1190 em ) and sulfones (1330-1360 and 1150-1190 em ) absorb in the regions expeeted (e.g. saeeharin, 1353 and 1162 em ), but resonanee forms related to (13) eause a reduetion of the frequeney of the asymmetrie SO2 vibration to near 1280 em (70CB3166). A similar situation arises in 3-amino-1,2-benzisothiazole 1-oxides. 

Table 14 Melting and Boiling Points of some Isothiazoles and Benzisothiazoles ...

Data are given for substituents in the 4-position of isothiazole and the 3-position in the benzisothiazoles, except where indicated. Boiling points of liquids (at 760 mmHg) are given in brackets. 

All data for benzisothiazoles taken from (72AHC(14H3). 

Isothiazole behaves as a typical stable aromatic molecule. Thermolysis of substituted isothiazoles at 590 °C leads to the formation of thioketenes (80MI41700) and phenyl-isothiazoles undergo photoisomerism (Section 4.17.6.2) (73BSF1743, 81T3627). 1,2-Benzisothiazole boils at 220 °C without appreciable decomposition, and the 2,1-isomer... 

Annular tautomerism does not occur in isothiazoles or benzisothiazoles. Substituent tautomers can sometimes be distinguished by chemical methods, but it is important that reaction mechanisms and the relative rates of interconversion of tautomeric starting materials or isomeric reaction products are carefully investigated. Physical methods only will be considered in this section, and references to original publications can be found in a comprehensive review (76AHC(S1)1). 

Thiol-thione tautomers have not been extensively studied, but UV and IR evidence show that 5-phenylisothiazole-3-thiol exists in the SH form. Ring-chain tautomerism of 2,3-dihydro derivatives of 1,2-benzisothiazole can occur (26a 26b) and the position of equilibrium depends very much on the solvent, physical state and nature of the substituents (69JOC919, 81KGS1209). 

Benzisothiazoles suffer straightforward ring cleavage, but their 1,1-dioxides, 2,1-benzisothiazoles and derivatives of saccharin give products containing no sulfur. 

Isothiazoles and 2,1-benzisothiazoles are stable to most nucleophilic reagents, but quaternized compounds are dequaternized and/or suffer cleavage of the N—S bond. 

Benzisothiazoles also suffer N—S bond cleavage, following attack at sulfur, but 1,2-benzisothiazole 1,1-dioxides are cleaved at the C—N bond. Saccharin derivatives are attacked at the carbonyl function. In cases where N—S bond cleavage occurs, recyclization can sometimes occur, often producing thiophene compounds. 

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