Equilibrium nitrogen isotope effects on the basicity of amines

Abstract

methanolic extract was filtered and mixed with an excess of a saturated solution of NaBPh, in MeOH. After that mixture had aged for several hours at -20 "C, the BPh4-salt of the quaternary ion product was isolated by filtration and recrystallized from MeCOEt or MeCOEt plus hexane.The nitrogen in the product in the reactant amine) was converted to NH, by Kjeldahl digestion and then to N, by reaction with NaOBr." The Kjeldahl digestion procedure was modifiedI8 to achieve quantitative conversion of pyridinium nitrogen, and the quantitative result of each digestion was verified by titration of the NH3 produced. The IsN abundances in the N2 were measured with a Micromass 602E isotope ratio mass (17) Shearer, G. B.: Kohl, D. H.: Commoner, B. Soil Sci. 1974, 218,Fish, U. B.; Collier, P. R. Anal. Chem. 1958, 30, 151-152. 308-316.spectrometer. The abundances, 6, and 6,, respectively, in N, generated from amine reactant and quaternary ammonium product were measured relative to N 2 generated from a standard sample of ammonium sulfate. [6 = 1000(R/RStd -l), where R and Rstd are the observed 1sN'4N/14N2 ratios in sample and standard.] The true value of the ratio of lsNI4N/l4N2 ratios from reactant and product, Ro/R,, was thus given by (lo00 + S,)/(lOOO + 6, ) . Most measurements of 6, were by direct Kjeldahl digestion of the amine, but digestions of product from complete reaction of amine with excess Me1 or MeOTf was shown to give the same values for 6, within the experimental reproducibility. Acknowledgment.The I4N/l5N isotope effect on K, has been measured for the conjugate acids of five tertiary amines: 4-methylpyridine (K,l4/K,l5 = 1.0226 f 0.0012), pyridine (1.021 1 i 0.0008), 3-acetylpyridine (1.0189 f 0.0008), quinuclidine (1.0135 f 0.0007), and N,N-dimethyl-4-methylaniline (1.0125 f 0.0008). These values are smaller than would be expected from the valence force fields for amines and their conjugate acids; this observed diminution of the isotope effect is attributed to hydrogen bonding by solvent water to the nitrogen of the unprotonated amine.

Glucosidase-Catalyzed Hydrolysis of α-d-Glucopyranosyl Pyridinium Salts: Kinetic Evidence for Nucleophilic Involvement at the Glucosidation Transition State

“…The EIE for the ionization constant of isoquinolinium was measured in order to detect possible perturbation of the nitrogen atom by the fused aromatic ring (Scheme ). Within experimental error, the evaluated EIE (K a 14 / K a 15 = 1.0216 ± 0.0005) is identical to that obtained for pyridinium deprotonation, a result that is not surprising considering the similarity of the two ionization constants [p K a (pyridine-H + ) = 5.20, p K a (isoquinoline-H + ) = 5.38 44 ]. At a pH of 6.8, less than 4% of the isoquinoline in solution is protonated, a result that indicates that the observed greater rate of change in absorbance for the 15 N-isotopomer of 2 cannot be attributed to the formation of different percentages of protonated products.…”

“…) 1.0211 ( 0.0004 43 ) for the dissociation constant of the conjugate acid of pyridine in water. This nitrogen-15 KIE value and the measured value for the 15 N-KIE of 1.018 (corrected to 25 °C41 ) on the spontaneous hydrolysis of R-D-glucopyranosyl 4′-bromoisoquinolinium bromide (3) are consistent with extensive C-N bond cleavage at the respective transition states.…”

Isotope Effects in the Solvolysis of Sterically Hindered Arenesulfonyl Chlorides

“…EtOD instead of EtOH (Table II). Sterically hindered substrates 1, 2 show slightly lower G ╪ ß (104-105 kJ·mol −1 ) in comparison with the less sterically hindered 3 [31,36,37], which provides additional evidence against proton transfer in the rate-determining step.…”

“…where φ EtOL is the fractionation factor of the alcoholic hydrogen/deuterium of ethanol, whereas φ ╪ is/are the fractionation factor(s) of the alcoholic hydrogen/deuterium atom(s) involved in the TS. The fractionation factor of any hydrogen/deuterium, which behaves like a hydrogen/deuterium of the solvent, is equal to one, whereas that of the corresponding lyonium ion is lower than unity, for example, φ(L 3 O + ) = 0.69 [30] or φ(MeOL 2 + ) = 0.60 [31], where L designates either H or D. From there, it follows that the more charge on the hydrogen/deuterium, the lower is the value of its fractionation factor, i.e. the value of the Scheme 4 "Early" (a) and "late" (b) TSs for the S N 2 mechanism of arenesulfonyl chlorides solvolysis with general base catalysis by a second solvent molecule.…”

“…When the extreme "early" TS is considered, the hydrogen/deuterium is only starting to be transferred to another solvent molecule, and assuming the fractionation factor of the alcoholic hydrogen/deuterium should be similar to that of MeOL 2 + (φ L + ca. 0.60 relative to MeOL) [31], a SIE = (k EtOH /k EtOD = 1/0.60) ca. 1.6 would be expected; in this case, the hydrogen/deuterium should resemble that of protonated alcohol.…”