Quinolines, activation nucleophilic substitution

Keywords Quinoline Cyclocondensation Nucleophilic substitution of fluorine atom Cross-coupling reactions Antibacterial activity Enzyme inhibitor... 

To derive the maximum amount of information about intranuclear and intemuclear activation for nucleophilic substitution of bicyclo-aromatics, the kinetic studies on quinolines and isoquinolines are related herein to those on halo-1- and -2-nitro-naphthalenes, and data on polyazanaphthalenes are compared with those on poly-nitronaphthalenes. The reactivity rules thereby deduced are based on such limited data, however, that they should be regarded as tentative and subject to confirmation or modification on the basis of further experimental study. In many cases, only a single reaction has been investigated. From the data in Tables IX to XVI, one can derive certain conclusions about the effects of the nucleophile, leaving group, other substituents, solvent, and comparison temperature, all of which are summarized at the end of this section. 

Quinoxalinyl, 4-cinnolinyl, and 1-phthalazinyl derivatives, which are all activated by a combination of induction and resonance, have very similar kinetic characteristics (Table XV, p. 352) in ethoxylation and piperidination, but 2-chloroquinoxaline is stated (no data) to be more slowly phenoxylated. In nucleophilic substitution of methoxy groups with ethoxy or isopropoxy groups, the quinoxaline compound is less reactive than the cinnoline and phthalazine derivatives and more reactive than the quinoline and isoquinoline analogs. 2-Chloroquinoxaline is more reactive than its monocyclic analog, 2-chloropyrazine, with thiourea or with piperidine (Scheme VI, p. 350). 

Nucleophilic substitution with heteroaryl halides is a particularly useful and important reaction. Due to higher reactivity of heteroaryl halides (e.g. 35, equation 24) in nucleophilic substitution these reactions are widely employed for synthesis of Al-heteroaryl hydroxylamines such as 36. Nucleophilic substitution of halogen or sulfonate functions has been performed at positions 2 and 4 of pyridine , quinoline, pyrimidine , pyridazine, pyrazine, purine and 1,3,5-triazine systems. In highly activated positions nucleophilic substitutions of other than halogen functional groups such as amino or methoxy are also common. 

Quinoline is normally substituted by nucleophiles at position 2 and to a lesser extent at position 4 (see Chapter 2.05), and replacement of a halogen in the benzo ring occurs less readily than replacement of one at positions 2 or 4. The latter process requires, in general, temperatures of 50-100 °C whereas the former requires temperatures of 150-200 °C or more (Scheme 19). There are many examples of replacement of halogens that are activated by electron-withdrawing groups. The topic has been reviewed (77HC(32-1)526). 

Quinoxalines are more reactive towards nucleophilic substitution than quinolines as a result of inductive activation by the additional ring nitrogen. If substituents are present in both the benzenoid and heteroaromatic ring, monosubstitution occurs predominantly in the latter. 

Benzo-annelated nitrogen heterocycles (indoles, quinolines, isoquinolines, etc.) are often found to be a part of biologically active compounds of both natural and synthetic origin. In a considerable body of data on the syntheses of these compounds, which have so far been documented in the literature, the crucial step is vicarious nucleophilic substitution of hydrogen in nitroarenes. Good examples are presented by the synthesis of nordehydrobufotenine [49], eupolauramine [50, 51], damirone [52], and aklavinone [53]. 

The SnCl2-reduction system has also been apphed in the reduetion of S 2 nucleophilic substitution products 583, alfording more functional quinolines, 4-(substituted vinyl)-quinolines 584, in moderate yields, with several exclusions of the formation of dihydroquinoline derivative 585 (Scheme 4.174). However, using compounds 586 as substrates without a ketone moiety, the ester group can also participate in the intramolecular cychzation, but the subsequent dehydrogenation does not occur and, therefore, tetrahydroquinolin-2-ones 587 were obtained in 51-62% yields (frans form only). From this study, the preference of the activated carbonyl group COR for cychzation has the order R = Me > Ph > O-alkyl. 

Nitrogen nucleophiles used to diplace the 3 -acetoxy group include substituted pyridines, quinolines, pyrimidines, triazoles, pyrazoles, azide, and even aniline and methylaniline if the pH is controlled at 7.5. Sulfur nucleophiles include aLkylthiols, thiosulfate, thio and dithio acids, carbamates and carbonates, thioureas, thioamides, and most importandy, from a biological viewpoint, heterocycHc thiols. The yields of the displacement reactions vary widely. Two general approaches for improving 3 -acetoxy displacement have been reported. One approach involves initial, or in situ conversion of the acetoxy moiety to a more facile leaving group. The other approach utilizes Lewis or Brmnsted acid activation (87). 

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