A study of cationic heteroconjugation equilibria of substituted pyridine N-oxides in acetonitrile

Potentiometric studies of cationic heteroconjugation equilibria in systems involving free and protonated pyridine derivatives in dimethyl sulfoxide

“…This conclusion is compatible with that for systems with substituted pyridine N -oxides. (18) Unfortunately, no linear relationship (correlation coefficient less than 0.3) was found between heteroconjugation constants and basicities of proton acceptors in DMSO in the systems with protonated 4NH 2 Py as proton donor, whereas in the systems with protonated 4Pic acting as proton donor the linear relationship was weak (correlation coefficient less than 0.95).…”

“…For instance, no relationship could be established between cationic standard heteroconjugation constants and solvent basicities, which is characteristic for substituted pyridine N -oxides. (17) Furthermore, in the case of the (OHO) + bridges, the cationic standard heteroconjugation constant values increase with increasing acceptor basicity and decrease with proton donor basicity, (18)(19)(20)(21) a regularity not observed in the case of the (NHN) + bridges. Another feature influencing the tendency towards cationic heteroconjugation in the systems with the N -oxides is the capacity of the constituents to form hydrogen bonds, as defined by the arithmetic mean of their cationic standard homoconjugation constant values.…”

“…Also, the (OHO) + bridges, where different amine N -oxides are bound together with a hydrogen bond, turned out to be less stable. (18)(19)(20)(21) However, it should be emphasised that analysing cationic standard heteroconjugation constants determined in non-aqueous media in systems involving organic N -bases is not straightforward for two reasons. First, there were many methods employed to study the equilibria considered.…”

“…A relationship has been established between logarithms of the cationic standard heteroconjugation constants diminished by the arithmetic mean of the cationic standard homoconjugation constants and pK o a (BH + ) { pK o a (BH + ) of protonated acceptor -pK o a (BH + ) of donor}. (18) The purpose of the present contribution was to extend the investigations of cationic heteroconjugation equilibria in systems involving substituted pyridines in polar protophobic aprotic solvents, acetonitrile (AN), nitromethane (NM), acetone (AC), and the amphiprotic methanol (MeOH), to the polar protophilic aprotic solvent-dimethyl sulfoxide (DMSO). The rationale underlying the choice of this solvent were earlier reports of unexpectedly high cationic standard heteroconjugation constants of the (OHN) + complexes in DMSO, in which, bearing in mind the behaviour of the N -oxide systems, (19) the absence of heteroconjugation equilibria should be expected.…”

A potentiometric study of cationic heteroconjugation equilibria in nitromethane andN,N- dimethylformamide

“…Furthermore, cationic homoconjugation shown in equations (3) and (4) is a reaction whereby cationic acid BH + (B 1 H + ) reacts with conjugate base B (B 1 ) to yield a symmetrical complex cation BHB + (B 1 HB 1 ) + , whereas the cationic heteroconjugation phenomenon shown in equation (5) takes place when cationic acid BH + reacts with base B 1 conjugated with another cationic acid to afford an asymmetric hydrogen-bonded complex cation BHB + 1 . Cationic heteroconjugation equilibria in systems comprising amine N -oxides (mainly substituted pyridine N -oxides, as well as aliphatic trimethylamine N -oxide) have been extensively studied in polar protophobic aprotic solvents, such as nitrobenzene, (7) acetonitrile, (8) propylene carbonate, (9) and acetone, (10) as well as in methanol, (10) which is a polar amphiprotic solvent. These studies have shown that the tendency towards heteroconjugation of substituted pyridine N -oxides depends on solvent basicity and decreases with increasing basicity of solvents in the following order: nitrobenzene > propylene carbonate > acetonitrile > acetone > methanol.…”