In heteroaromatic systems

The calculation of localization energies in heteroaromatic systems derived from alternant hydrocarbons has been simplified by Dewar and Maitlis (57JCS2521). This approach has had considerable success the results provide a somewhat empirical index of reactivity. [Pg.7]

Qualitatively, similar relationships are ascertained in heteroaromatic systems where the same conclusion is derived by a numerical calculation. In more elaborate calculations than the Hiickel method, such as the Pariser-Parr-Pople approximation 21>23>, similar distinct parallelisms are recognized 59> (Table 4.1). Essentially the same circumstances exist also... [Pg.28]

In accordance with the general stability of alkylthio groups in heteroaromatic systems, 3-alkylthio groups in 1,2,4-thiadiazoles are not easily replaced.Thus, 3-alkylthio-1,2,4-thiadiazoles resist theaction of aniline at 100°, ammonia at 120°, or molten urea or ammonium acetate.99 On the other hand, hydrazine attacks 3-methylthio-l,2,4-thiadiazole under restrained conditions, with formation of 3-amino-... [Pg.190]

A comparison of the suitability of solvents for use in Srn 1 reactions was made in benzenoid systems46 and in heteroaromatic systems.47 The marked dependence of solvent effect on the nature of the aromatic substrate, the nucleophile, its counterion and the temperature at which the reaction is carried out, however, often make comparisons difficult. Bunnett and coworkers46 chose to study the reaction of iodoben-zene with potassium diethyl phosphite, sodium benzenethiolate, the potassium enolate of acetone, and lithium r-butylamide. From extensive data based on the reactions with K+ (EtO)2PO (an extremely reactive nucleophile in Srn 1 reactions and a relatively weak base) the solvents of choice (based on yields of diethyl phenylphosphonate, given in parentheses) were found to be liquid ammonia (96%), acetonitrile (94%), r-butyl alcohol (74%), DMSO (68%), DMF (63%), DME (56%) and DMA (53%). The powerful dipolar aprotic solvents HMPA (4%), sulfolane (20%) and NMP (10%) were found not to be suitable. A similar but more discriminating trend was found in reactions of iodobenzene with the other nucleophilic salts listed above.46 Nearly comparable suitability of liquid ammonia and DMSO have been found with other substrate/nucleophile combinations. For example, the reaction of p-iodotoluene with Ph2P (equation (14) gives 89% and 78% isolated yields (of the corresponding phosphine oxide) in liquid ammonia and DMSO respectively.4 ... [Pg.456]

In heteroaromatic systems, transitions between Hn, 7t ) w.-> 3(t , 7 ) states and1(7i, 7t ) 3(n, 7t ) states occur more readily if energy relationships... [Pg.262]

Moody, C. J., Azodicarbonyl Compounds in Heterocyclic Synthesis, 30, 1 Claisen Rearrangements in Heteroaromatic Systems, 42, 203. [Pg.296]

Claisen rearrangements in heteroaromatic systems, 42, 203 in nitrogen heterocyclic systems, 8, 143 Complex metal hydrides, reduction of nitrogen heterocycles with, 6, 45 39, 1 Concept of aromaticity in heterocyclic chemistry, 56, 303 Condensed heterocyclo[n,m- , b, or cjquinazolines, 52, 1 Condensed 4-thiazolidinones, 49, 1... [Pg.305]

The rate of success falls whenever these conditions are not met. For example, condition (1) is not always satisfied in aromatic substitution reactions. The FOs of polycyclic aromatic hydrocarbons are not well separated from the other MOs,66 so subjacent orbital control may intervene. Particular care should be taken with non-alternant hydrocarbons because they tend to react under charge control. This is even truer in heteroaromatic systems (cf. Exercise 15, p. 119, and Exercise 16, p. 121), where Hiickel... [Pg.129]

Calculated reactivity indices are at one with qualitative electronic theory in accounting for ready nucleophilic attack at C-6 in the phenanthridine molecule. However, the limited data available on positional reactivities in electrophilic substitutions is not accounted for satisfactorily by any of the available treatments (see later) and it has been pointed out that the simple Hiickel treatment used by some authors,223, 220 is generally inapplicable in heteroaromatic systems.228... [Pg.371]

A tandem carbonylation-cyclization radical process in heteroaromatic systems bearing electron-attracting substituents such as l-(2-iodoethyl)indoles and pyrroles 970 result in the formation of 2,3-dihydto-l//-pyrrolo[l,2- ]indol-1-ones and 2,3-dihydro-l//-pyrrolizin-l-ones 974 (Scheme 188). The AIBN-induced radical reaction of compounds 970 with Bu3SnH under pressure of CO suggests that the acyl radical 972, derived from radical 971 and CO, would undergo intramolecular addition to C-2 of heteroaromatic system, and the benzylic radical 973 so obtained, upon in situ oxidation would produce final product 974 <1999TL7153>. [Pg.186]

C. Transmission of the Electronic Effects of Substituents IN Heteroaromatic Systems... [Pg.68]

Chapter 3 is concerned with Claisen rearrangements in heteroaromatic systems and updates an article that appeared in Volume 8 of this series 20 years ago. [Pg.426]

Moody, C. J. Claisen rearrangements in heteroaromatic systems. Adv. Heterocycl. Chem. 1987,42, 203-244. [Pg.560]

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