Pseudobases, N-heterocyclic

Dithiolium —, Metal complex compounds, Tropenium salts Pseudobases, N-heterocyclic... 

Sodium hydroxide N-Condensed heterocyelics via N-heterocycIic pseudobases N-Heterocyclics from carboxylic acid amides Reductive ring opening... 

Pschorr ring closure 15, 561 Pseudoacids s. Hydroxylactones Pseudoaromatic rings s. Di-thiylium salts. Metal complex compounds, ar., Tropenium salts Pseudoazulenes 15, 468 N-Pseudoazulenes 14, 791 0-Pseudoazulenes 14, 338 Pseudobase adducts and their reactions 13, 584 —, dehydrogenation 14, 769 —, p-methylation of quinolines via — 14, 769 Pseudobases 11, 323 Pseudobases, N-heterocyclic (s. a. 0-Alkylpseudobases, N-heterocyclic) 15, 304... 

The increased susceptibility to pseudobase formation for O-heterocyclic relative to the corresponding N-heterocyclic cations that was noted above for the aromatic series is also seen in the l//-isobenzofurylium (25 X = O) and N-phenyl 1 //-isoindolium (25 X = NPh) series in Table II. For these cases ApKR. (NPh - O) 11.5, which may be converted to ApXR. (NMe - O) % 15.5 using the difference noted previously for N-methyl- and N-phenyl-3,4-dihydroisoquinolinium derivatives. This difference is in reasonable agreement with ApXR + (NMe - O) % 18.5 observed for aromatic cations (Table IV). The low stability of the cation relative to the pseudobase for O-heterocycles is also present in the 2H-furylium cations (26). Although... 

N-Condensed heterocyclics via N-heterocyclic pseudobases—N-Condensed l,3-N,N-heterocyclics s. 18, 531... 

The most reasonable interpretation (there have been many) is to consider the hydroxide or cyanide as forming first an sp -hybridized carbon atom (a pseudobase or Reissert-type adduct, respectively) and then being transmitted from carbon to metal ion. In other words, the change in reactivity of an N-heterocycle on coordination to a metal ion is akin to that of the same N-heterocycle on classical quaternization by an organic agent such as methyl iodide. The unusual rate equation [Eq. (67) or (68)] involving the nucleophile s concentration in first- and second-order terms arises because the rates of these reactions (apparently hydrolysis or substitution by cyanide at the metal ion) are actually controlled by rates of reaction at the ligand (27 28). 

Without additional reagents 0-Alkylation of N-heterocyclic pseudobases... 

In Tables 126,37- 1,43,46,56,73-115 and n 57.90.9.4,..6-.34 pKr+ values currently available for heterocyclic cations are listed in order of increasing complexity of the heterocyclic system. Aromatic and nonaromatic heterocyclic cations are considered separately in Tables I and II, respectively. Only data for strictly aqueous solutions or for aqueous solvents containing small proportions of nonnucleophilic organic solvents are included in Tables I and II. A number of data reported for aqueous alcoholic solutions are discussed separately in Section VI,A. Equilibrium constants in such media can only be considered as apparent constants since no allowance is made for the existence of the pseudobases in these solutions as mixtures of the corresponding hydroxide and alkoxide adducts. The presence of an organic solvent is sometimes necessary to promote sufficiently the solubility of the pseudobase for pXR+ determination. In such cases, interpretation of data relative to strictly aqueous solutions is more straightforward if nonnucleophilic cosolvents such as acetonitrile and dioxane, are chosen in preference to alcohols. 

Further benzologation leads to measurable pvalues for the IV-methyl-acridinium (9.8626) and N-methylphenanthridinium (11.9441) cations however, pKR+ > 14 in aqueous solutions of the N-methyl cations of both the 5, 6- and 7, 8-benzoquinolines.41 The loss in resonance energy upon pseudobase formation is expected to be one of the major factors involved in considerations of the relative susceptibilities of heterocyclic cations to pseudobase formation. A rather crude, but informative, calculation of the loss in resonance energy (AR) upon pseudobase formation has been attempted for each of the above cations. 

Heterocycle P. P Cr. for N—Me cation Resonance energy" of heterocycle (kcal mol-1) Pseudobase reference molecule Resonance energy6 of pseudobase reference A R (kcal mol-1)... 

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