Quaternary Nitrogen Heterocycles. I. Equilibrium and Spectral Data for Pseudobase Formation by the N-Methyl Cations of Diazanaphthalenes

Abstract

Equilibrium constants have been measured at 25° for the formation of pseudobases from the 2-methyl-phthalazinium, 1-methylquinoxalinium, 1-methyl- 1,5-naphthyridinium, 6-methyl-1,6-naphthyridinium, 7-methyl-1,7-naphthyridinium, 1-methyl-1,8-naphthyridinium, 1-methyl-3-nitroquinolinium, and 2-methyl-4-nitroisoquinolinium cations. Ionization constants have also been obtained for ionization of some of these pseudobases to alkoxide anions. For each cation the site of hydroxide attack has been determined by comparison of the u.v. and p.m.r. spectra of the pseudobases, the corresponding methoxide adducts, and the lithium aluminum hydride reduction products. In all cases, except for the 1-methylquinoxalinium cation, only one major pseudobase species is present in solution, and ring-opening does not occur to any appreciable extent. The pseudobase of the 1-methylquinoxalinium cation exists in equilibrium with a considerable amount of its covalent hydrate from addition of water across the C3—N4 bond.

Kinetic and thermodynamic control of pseudobase formation from C-3 substituted 1-methylquinolinium cations

“…[8] and [9] are consistent with the general observation that Bronsted values for such reactions are usually close to 0.5 (25,26). These relationships also indicate that, for a quinolinium cation having C-2 and C-4 pseudobases of equal thermodynamic stabilities, the rate of hydroxide ion attack at C-2 will be 50-fold greater than at C-4.…”

“…Equation [8] may be compared with eq. [9] deduced previously (24) for C-2 pseudobase formation from a variety of substituted quinolinium cations.…”

“…[4]- [7] where steric effects are ignored. Equations [8] and [9] may also be used to predict second-order rate constants of 1000 M-' s-' and 2 M-' s-' for hydroxide ion attack at C-2 and C-4, respectively, in the parent 1 -methylquinolinium cation.…”

“…An alternative interpretation of eqs. [8] and [9] suggests that k:, will only be greater ;ban k;, when p~i , is more than 3.8 units smaller than pK,+. This pKR+ is not approached for any case in Table 3 and, consequently, in all cases the C-2 pseudobase is the kinetically controlled species, even in those cases in which the C-4 pseudobase is of greater thermodynamic stability.…”

“…Methoxide ion addition to the 1-methylquinolinium cation in methanol gives the C-2 adduct (3: R = CH3) (8). By analogy to this latter observation, it is generally assumed that pseudobase formation in aqueous solution would give the C-2 adduct bases rather than the relative thermodynamic stabilities of these pseudobases has never been satisfactorily addressed.…”