Reactivity in the nucleophilic aromatic substitution reactions of pyridinium ions.

The "element effect" in nucleophilic aromatic substitution reactions (SNAr) is characterized by the leaving group order, L = F > NO2 > Cl ≈ Br > I, in activated aryl substrates. A different leaving group order is observed in the substitution reactions of ring-substituted N-methylpyridinium compounds with piperidine in methanol: 2-CN ≥ 4-CN > 2-F ∼ 2-Cl ∼ 2-Br ∼ 2-I. The reactions are second-order in [piperidine], the mechanism involving rate determining hydrogen-bond formation between piperidine and the substrate-piperidine addition intermediate followed by deprotonation of this intermediate. Computational results indicate that deprotonation of the H-bonded complex is probably barrier free, and is accompanied by simultaneous loss of the leaving group (E2) for L = Cl, Br, and I, but with subsequent, rapid loss of the leaving group (E1cB-like) for the poorer leaving groups, CN and F. The approximately 50-fold greater reactivity of the 2- and 4-cyano substrates is attributed to the influence of the electron withdrawing cyano group in the deprotonation step. The results provide another example of ß-elimination reactions poised near the E2-E1cB mechanistic borderline.

Proton transfer from C-6 of uridine 5'-monophosphate catalyzed by orotidine 5'-monophosphate decarboxylase: formation and stability of a vinyl carbanion intermediate and the effect of a 5-fluoro substituent.

The exchange for deuterium of the C-6 protons of uridine 5'-monophosphate (UMP) and 5-fluorouridine 5'-monophosphate (F-UMP) catalyzed by yeast orotidine 5'-monophosphate decarboxylase (ScOMPDC) at pD 6.5-9.3 and 25 °C was monitored by (1)H NMR spectroscopy. Deuterium exchange proceeds by proton transfer from C-6 of the bound nucleotide to the deprotonated side chain of Lys-93 to give the enzyme-bound vinyl carbanion. The pD-rate profiles for k(cat) give turnover numbers for deuterium exchange into enzyme-bound UMP and F-UMP of 1.2 × 10(-5) and 0.041 s(-1), respectively, so that the 5-fluoro substituent results in a 3400-fold increase in the first-order rate constant for deuterium exchange. The binding of UMP and F-UMP to ScOMPDC results in 0.5 and 1.4 unit decreases, respectively, in the pK(a) of the side chain of the catalytic base Lys-93, showing that these nucleotides bind preferentially to the deprotonated enzyme. We also report the first carbon acid pK(a) values for proton transfer from C-6 of uridine (pK(CH) = 28.8) and 5-fluorouridine (pK(CH) = 25.1) in aqueous solution. The stabilizing effects of the 5-fluoro substituent on C-6 carbanion formation in solution (5 kcal/mol) and at ScOMPDC (6 kcal/mol) are similar. The binding of UMP and F-UMP to ScOMPDC results in a greater than 5 × 10(9)-fold increase in the equilibrium constant for proton transfer from C-6, so that ScOMPDC stabilizes the bound vinyl carbanions, relative to the bound nucleotides, by at least 13 kcal/mol. The pD-rate profile for k(cat)/K(m) for deuterium exchange into F-UMP gives the intrinsic second-order rate constant for exchange catalyzed by the deprotonated enzyme as 2300 M(-1) s(-1). This was used to calculate a total rate acceleration for ScOMPDC-catalyzed deuterium exchange of 3 × 10(10) M(-1), which corresponds to a transition-state stabilization for deuterium exchange of 14 kcal/mol. We conclude that a large portion of the total transition-state stabilization for the decarboxylation of orotidine 5'-monophosphate can be accounted for by stabilization of the enzyme-bound vinyl carbanion intermediate of the stepwise reaction.

Accelerated hydrolysis of α-halo and α-cyano pyridinium relative to uracil derivatives: a model for ODCase-catalyzed hydrolysis of 6-cyanoUMP.

α-Halo and α-cyano pyridiniums were found to undergo facile hydrolysis, in contrast to the sluggish reactions of corresponding uracils. The greatly enhanced rates found with pyridinium compounds have indicated a possible source of the rate acceleration seen in the hydrolysis of 6-cyanouridine 5'-monophosphate catalyzed by orotidine 5'-monophosphate decarboxylase.

Convenient one-pot synthesis of trans-1,2-diaryloxiranes from the direct coupling of benzyl halides.

trans-1,2-Diaryloxiranes were conveniently prepared in an one-pot reaction by the direct coupling of benzyl halides in the presence of silver oxide and DMSO under mild conditions.

Surprisingly low aqueous acidity at the alpha-positions of pyridiniums and pyrimidinium: the role of solvation.

The p K(a)'s of the 6-CH groups of N-methyl-2-methoxypyridinium, N-methyl-4-methoxypyridinium, and 1-methyl-2,4-dimethoxypyrimidinium ions in aqueous solution were determined to be about 33. The p K(a) values were nearly identical to each other and surprisingly similar to those reported for the neutral pyridones and uracil. Further determination of the enthalpies and entropies of the H-D exchange reactions revealed the role of solvation.

Convenient oxidation of benzylic and allylic halides to aldehydes and ketones.

Benzylic and allylic halides were conveniently oxidized to aldehydes and ketones by pyridine N-oxide in the presence of silver oxide under mild conditions.

Mechanism of rhodium-catalyzed carbene formation from diazo compounds.

[reaction: see text] A large and normal nitrogen-15 kinetic isotope effect of 1.035 +/- 0.003 provides direct support for the proposed mechanism for the rhodium-catalyzed carbene formation from diazo compounds, which involves the fast formation of a metal-diazo complex followed by rate-limiting extrusion of N2. The large magnitude of the KIE indicates extensive C-N bond fission in the transition state.

Stability of the 6-carbanion of uracil analogues: mechanistic implications for model reactions of orotidine-5'-monophosphate decarboxylase.

[Structure: see text] The pKa's of the 6-CH groups of N-methyl-2-pyridone and N-methyl-4-pyridone in aqueous solution were determined. No correlation between the stability of the carbanions and the rate of decarboxylation of the corresponding carboxylic acids was found.

Carbanions from decarboxylation of orotate analogs: stability and mechanistic implications.

The pKa's of the 6-CH groups of 1,3-dimethyluracil, N-methyl-2-pyridone, and N-methyl-4-pyridone were determined through their reactions with bases derived from carbon acids with known pKa and the reactions of their corresponding carbanions with the carbon acids. No correlation between the stability of the carbanions and the rate of decarboxylation of corresponding carboxylic acids was found.

Accelerated decarboxylation of 1,3-dimethylorotic acid in ionic liquid.

The solvent effect of ionic liquids on the decarboxylation of 1,3-dimethylorotic acid and its analogue in ionic was investigated. The rate acceleration observed was proposed to be a result of the stabilization of the zwitterionic intermediates by the charged groups available in these special solvents.