Keto–Enol tautomerism and ionisation of 1-phenacylpyridinium ions: a model for carbanion-stabilisation of azomethine ylides

Abstract

Measurement of equilibrium constants for keto-enol tautomerism (KT) and ionisation (K,) of 1phenacylpyridinium and 1 -phenacyl(4-dimethylamino) pyridinium ions gives pK, (-log KT) = 6.1 0 and 5.55 and pK, = 10.90 and 13.2 respectively. The enol content and acidity of the 1phenacylpyridinium ion is lower than that of its 2-, 3and 4-isomers, and the possibility that this reflects impaired -M resonance by a 1 -pyridinium substituent is discussed. Notional (proton) activating factors reflecting the influence of the positive charge of the 1 -pyridinium substituent upon equilibrium ionisation and rates of deprotonation by lutidine and hydroxide bases are estimated from free energy correlations as lo3, 17 and 5 x lo3 respectively. These compare with a (methyl) activating factor of 1 O9 derived from equilibrium ionisations of 4-chlorobenzaldehyde oxime and nitrone and a (notional) value of lo8 for pyridine-N-oxide. The implications of these values for the activating effect of N-protonation of an azomethine group in models for pyridoxalcatalysed azomethine rearrangements are discussed.

Substituent Effects on Electrophilic Catalysis by the Carbonyl Group: Anatomy of the Rate Acceleration for PLP-Catalyzed Deprotonation of Glycine

“…For example, ∼30% (2.1 pK a units) of the similar 7.8-unit overall effect of the protonated pyridine nitrogen on the acidity constant for the ionization of the carbon acid benzyl phenyl ketone is observed for the charge-conservative nitrogen substitution, and the remaining 70% of this substituent effect (5.7 pK a units) is expressed when nitrogen is protonated (Chart 3). 50,51 We estimate a pK a of ∼23 (Table 5) for ionization of DPL-IMH by making the assumption that 70% of the total 8-unit effect of the 4-CH for 4-NH þ substitution on carbon acidity is lost upon deprotonation of nitrogen. acetone to glycine methyl ester to form the iminium ion results in a 7-unit decrease in the carbon acid pK a from 21 to 14 (Table 5), 27 while the addition of acetone to the glycine zwitterion results in a similar 7-unit decrease in the carbon acid pK a from 29 to 22 (Table 5).…”

“…We expect that a large fraction of this 8-unit effect is due to the unit increase in positive charge at the aromatic ring of DPL-IMH 2 . For example, ∼30% (2.1 p K a units) of the similar 7.8-unit overall effect of the protonated pyridine nitrogen on the acidity constant for the ionization of the carbon acid benzyl phenyl ketone is observed for the charge-conservative nitrogen substitution, and the remaining 70% of this substituent effect (5.7 p K a units) is expressed when nitrogen is protonated (Chart ). , We estimate a p K a of ∼23 (Table ) for ionization of DPL-IMH by making the assumption that 70% of the total 8-unit effect of the 4-CH for 4-NH + substitution on carbon acidity is lost upon deprotonation of nitrogen.…”

Reactions of o-Quinone Methides with Pyridinium Methylides: A Diastereoselective Synthesis of 1,2-Dihydronaphtho[2,1-b]furans and 2,3-Dihydrobenzofurans

“…In particular, the action of base rapidly generates ylides and further action results in cleavage to an acid and an alkylpyridinium salt (acid splitting) by a mechanism similar to that involved in the fission of β-dicarbonyl compounds. Protic solvents promote acid splitting of reactive ylides (Scheme ) . For this reason, in the case of reactivity ylides acetonitrile should be used as a solvent instead of ethanol.…”

Spectroscopic and theoretical investigation of capillary‐induced keto–enol tautomerism of phenacyl benzoylpyridinium‐type photoinitiators

“…The methylene protons in N ‐alkyl pyridinium salts of the type ≥ N + CH 2 Y are acidic, where Y is a strong electron‐withdrawing substituent, and their reactivity becomes comparable with that of the methylene group of 1,3‐dicarbonyl compounds. The p K a values of a number of N ‐phenacyl pyridinium salts are in the range 7–10.9 26, 27. Strong bases can abstract one of such protons, which may lead to the formation of ylides 28–33.…”