2//-Isoindole

The name isoindole is admissible for benzo[c]pyrrole. Although isoindole has an ortho-qai-nonoid structure, it can be isolated it is characterized spectroscopically and chemically [56]  

A tautomeric equilibrium exists between isoindoles unsubstituted in the 2-position and the corresponding 1//-isoindoles  

Isomerizations in which an H-atom changes its position in a heterocyclic system are known as annular tautomerisms. This is a special case of prototropy (prototropic rearrangement) [57]. For isoindole, the position of the equilibrium depends to a large extent on the type of the substituent in the 1- and/or 3-position [58]. The 1-phenyl compound exists in the 2//-form and couples with benzenediazonium chloride in the 3-position. 

The numbering of carbazole (benzo[Z ] indole) predates the introduction of systematic nomenclature and is retained for historical reasons. 

Carbazoles behave like 0,0-disubstituted diphenylamines. However, the basicity of carbazole, Ka = -4.94, is much lower than that of diphenylamine (p = 0.78), and also lower than that of indole and pyrrole. As a consequence, carbazole is insoluble in dilute acids but only soluble in coned H2SO4 with protonation of the N-atom. On pouring the solution into water, carbazole precipitates without polymerization. 

2-(Methoxycarbonyloxy)-l,3-dihydroisoindole (1, obtained from 2-hydroxy-1,3-dihydro-isoindole and methyl (4-nitrophenyl)carbonate) was pyrolyzed in vacuo at 500 °C, and isoindole condensed on a cold-finger condenser filled with Hquid nitrogen. Isoindole crystaUizes in colorless needles, darkens at room temperature, and polymerizes. Solutions in dichloromethane kept under nitrogen are more stable. They give a red color with 

Ehrlich s reagent and form the corresponding Diels-Alder adducts (endoxxo = 2 3) with N-phenyhnaleinimide. 

Isoindoles substituted in the pyrrole ring are thermally more stable. They can be prepared on several routes [209], 

The insertion products 9 can be isolated, when 7 is reacted with ArNH2 in the presence of Rh2(OAc)4. 

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Hydroxy-THISs react with electron-deficient alkynes to give nonisol-able adducts that extrude carbonyl sulfide, affording pyrroles (23). Compound 16 (X = 0) seems particularly reactive (Scheme 16) (25). The cycloaddition to benzyne yields isoindoles in low- yield. Further cyclo-addition between isoindole and benzyne leads to an iminoanthracene as the main product (Scheme 17). The cycloadducts derived from electron-deficient alkenes are stable (23, 25) unless highly strained. Thus the two adducts, 18a (R = H, R = COOMe) and 18b (R = COOMe, R = H), formed from 7, both extrude furan and COS under the reaction conditions producing the pyrroles (19. R = H or COOMe) (Scheme 18). Similarly, the cycloadduct formed between 16 (X = 0) and dimethylfumarate... 

Unlike quinoline and isoquinoline which are of comparable stability the compounds indole and isoindole are quite different from each other Which one is more stable Explain the reason for your choice... 

Indole is a heteroaromatic compound consisting of a fused benzene and pyrrole ring, specifically ben2o[ ]pyrrole. The systematic name, IJT-indole (1) distinguishes it from the less stable tautomer 3JT-indole [271-26-1] (2). Iff-Indole [120-72-9] is also more stable than the isomeric ben2o[ ] pyrrole, which is called isoindole, (2H, (3) and IH (4)). A third isomer ben2o[i ]pyrrole is a stable compound called indoli2idine [274-40-8] (5). 

A general method for the preparation of 2JT-isoindol-4,7-diones, eg (97) [72726-02-4] involves 1,3-dipolar addition of oxa2ohum-5-oxides (sydnones) (98) to 2-meth5l-l,4-ben2oquinone (96). Yield of (97) is 37%. 

A new cyanide dye for derivatizing thiols has been reported (65). This thiol label can be used with a visible diode laser and provide a detection limit of 8 X 10 M of the tested thiol. A highly sensitive laser-induced fluorescence detector for analysis of biogenic amines has been developed that employs a He—Cd laser (66). The amines are derivatized by naphthalenedicarboxaldehyde in the presence of cyanide ion to produce a cyanobenz[ isoindole which absorbs radiation at the output of He—Cd laser (441.6 nm). Optimization of the detection system yielded a detection limit of 2 x 10 M. 

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

The five-membered ring heterocycles possess Diels-Alder reactivity of varying degree. This is most pronounced in the case of furan and benzo[c] fused heterocycles such as isoindole. In this capacity they are functioning as heterocyclic analogues of cyclopentadiene, and high Diels-Alder reactivity can be correlated with low aromaticity. 

Although it has not been possible to study the protonation of isoindole itself, it is clear that isoindoles are more basic than indoles or pyrroles. For example, 2,5-dimethyl-1,3-diphenylisoindole (40) has a p/sTa of 4-2.05 protonation of isoindoles occurs at positions 1 or 3. The pK for protonation of indolizine (10) at position 3 is 4-3.94 and that for carbazole (41) for protonation on nitrogen is estimated at -6.0. 

It is of interest to compare the stability of the protonated forms of benzo[u], benzo[Z>] and benzo[c] fused pyrroles, i.e. the cations derived from indolizines, indoles and isoindoles. Indolizine gives a stable pyridinium ion and does not polymerize in the presence of acid. 

Reduction of isoindoles with dissolving metals or catalytically occurs in the pyrrole ring. Reduction of indolizine with hydrogen and a platinum catalyst gives an octahydro derivative. With a palladium catalyst in neutral solution, reduction occurs in the pyridine ring but in the presence of acid, reduction occurs in the five-membered ring (Scheme 38). Reductive metallation of 1,3-diphenylisobenzofuran results in stereoselective formation of the cw-1,3-dihydro derivative (Scheme 39) (80JOC3982). 

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

Benz-7-azabicyclo[2.2.1]heptenes in isoindole synthesis, 4, 351 Benz[e][l, 2]azaborine, 2-chloro-1,2-dihydro-synthesis, 1, 647... 

Benzene, o-dibenzoyl-isoindole synthesis from, 4, 330 Benzene, diethynyl-polymerization, 1, 295 polymers... 

Benzimidazolo[l,2-c]thiatri azoles synthesis, 6, 612 Benz[cd]indazole antiaromaticity, 5, 208 Benz[cd]indazole, dihydroaromaticity, 5, 208 Benz[e]indolizine synthesis, 4, 466 Benzipiperylon biological activity, 5, 296 Benz[/]isoindoles synthesis, 4, 266... 

Benzylamine, o-chloro-N-(cyanomethyl)-N-methyl-isoindole synthesis from, 4, 323 Benzylisoquinoline alkaloids synthesis, 1, 446 Benzynes... 

Isoindole, l-benzyl-3-formyl-, 4, 222 Isoindole, 1-cyano-synthesis, 4, 148 Isoindole, 2-cyano-synthesis, 4, 149 Isoindole, 1,3-dimethyl-... 

Isoindole, 2,5-dimethyl-1,3-diphenyl-protonation, 4, 47 Isoindole, 1,3-diphenyl-mass spectrometry, 4, 187... 

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