Secondary deuterium isotope effects on epoxide methanolysis reactions

The primary and secondary deuterium isotope effects on proton and carbon chemical shifts were measured for a number of natural pigments and their derivatives, which contain the perylenequinone system and present a phenol–quinone tautomerism, cercosporin 1, isocercosporin 2, phleichrome 3, isophleichrome 4, cladochrome E 5, elsinochromes 6–8, hypocrellin 9, noranhydrocercosporin 10 and noranhydrophleichrome 11. Deuterium isotope effects on protons were also measured for 1,4-dihydroxyanthraquinone 12, methyl 4-(1,4-dihydroxy-9,10-dioxo-9,10-dihydroanthracen-2-yl)butanoate 13, N-acetyldaunomycin 15, daunomycinone 16, naphthazarin 17 and a number of intramolecularly hydrogen-bonded enols from β-diketones, β-ketoesters and o-hydroxyacyl aromatic compounds 19–24. The primary isotope effects Δδ(1H,2H) on OH proton shift and 1H chemical shifts of OH groups are correlated and can be used to estimate the strength of the hydrogen bonds in solution. The secondary isotope effects on proton and carbon nuclei are transmitted along the whole extended conjugated perylenequinone system. Long-range effects over eleven bonds and over seven bonds were observed in compounds 1–11 and in 12–18. The perturbation of the equilibrium due to the presence of deuterium was considered and calculations were performed in order to evaluate the amount of this contribution to the isotope effect. The variation with temperature of primary and secondary effects, from 25 °C down to –70 °C, was studied for compounds 1, 3, 6, 12 and 17, and for acetylacetone 22 and benzoylacetone 23. Parameters from a number of X-ray analyses, for example the O · · · O distances (as suggested by Gilli et al., J. Am. Chem. Soc., 1991, 113, 4917), gave evidence of a substantial parallelism between the liquid and the solid phase. The strength of the hydrogen bond in perylenequinones depends on the planarity of the two naphthalene units rather than on the distortion of the polycyclic ring.

The primary and secondary deuterium kinetic isotope effects as well as leaving-group fluorine kinetic isotope effects have been calculated for the base-promoted elimination of hydrogen fluoride from 4-fluoro-4-(4'-nitrophenyl)butane-2-one in 75% aqueous methanol solution. The elimination was studied for both formate and imidazole as the catalytic base; and reactant and transition state structures and vibrational frequencies have been calculated by including the base explicitly and by including the solvent by an implicit solvation model that includes both electrostatics by class IV charges and first-solvation-shell effects by atomic surface tensions. We used the M06-L density functional for all calculations. The optimized stationary points, the geometry changes along the solution-phase minimum free energy path, and the solution-phase free energy profile indicate that the elimination reaction occurs concertedly but asynchronously via an E1cb-like transition state. Reaction rates were calculated by the equilibrium solvation path method, using variational transition state theory with multidimensional tunneling. The primary deuterium kinetic isotope effects are calculated to be large: 1.67 and 5.13 for formate and imidazole, respectively. The corresponding C4-secondary deuterium kinetic isotope effects are 1.044 and 1.044, and the leaving group fluorine kinetic isotope effects are respectively 1.020 and 1.015.

Secondary deuterium isotope effects on olefin epoxidation by cytochrome P-450

In the pyrimidine biosynthetic pathway, N-carbamyl-L-aspartate (CA-asp) is converted to L-dihydroorotate (DHO) by dihydroorotase (DHOase). The mechanism of this important reaction was probed using primary and secondary 15N and 13C isotope effects on the ring opening of DHO using isotope ratio mass spectrometry (IRMS). The reaction was performed at three different temperatures (25, 37, and 45 degrees C for hamster DHOase; 37, 50, and 60 degrees C for Bacillus caldolyticus), and the product CA-asp was purified for analysis. The primary and secondary kinetic isotope effects for the ring opening of the DHO were determined from analysis of the N and C of the carbamyl group after hydrolysis. In addition, the beta-carboxyl of the residual aspartate was liberated enzymatically by transamination to oxaloacetate with aspartate aminotransferase and then decarboxylation with oxaloacetate decarboxylase. The 13C/12C ratio from the released CO2 was determined by IRMS, yielding a second primary isotope effect. The primary and secondary isotope effects for the reaction catalyzed by DHOase showed little variation between enzymes or temperatures, the primary 13C and 15N isotope effects being approximately 1% on average, while the secondary 13C isotope effect is negligible or very slightly normal (>1.0000). These data indicate that the chemistry is at least partially rate-limiting while the secondary isotope effects suggest that the transition state may have lost some bending and torsional modes leading to a slight lessening of bond stiffness at the carbonyl carbon of the amide of CA-asp. The equilibrium isotope effects for DHO --> CA-asp have also been measured (secondary 13K(eq) = 1.0028 +/- 0.0002, primary 13K(eq) = 1.0053 +/- 0.0003, primary 15K(eq) = 1.0027 +/- 0.0003). Using these equilibrium isotope effects, the kinetic isotope effects for the physiological reaction (CA-asp --> DHO) have been calculated. These values indicate that the carbon of the amide group is more stiffly bonded in DHO while the slightly lesser, but still normal, values of the primary kinetic isotope effect show that the chemistry remains at least partially rate-limiting for the physiological reaction. It appears that the ring opening and closing is the slow step of the reaction.

Secondary deuterium isotope effects on the olefinic one-bond 13C, 1H and three-bond 1H, 2H spin–spin coupling constant were determined in a series of specifically deuteriated cis- and trans-stilbene isotopomers. Inverse (1H detected) heteronuclear multiple quantum correlation (HMQC) NMR pulse sequence was used to measure the coupling constants in these π-conjugated systems of a moderate size. Complete spectral analyses were performed in cases where higher order perturbations took place. All secondary isotope effects on 1J(13Cα, 1H)/Hz are negative, i.e. the reduced values from –0.06 to –0.02 were observed. In contrast, the effects on 3J(1Hα, 2Hα′)/HZ are all positive from 0.04 to 0.13, thus being on average relatively higher than those on 1J(13Cα, 1H). Only two primary effects on 1J(13Cα, 1H) were determined to see whether the same trend holds as for secondary effects. The real values of olefinic coupling constants are given with a high accuracy. In order to relate isotope effects with structural parameters, AM1 semiempirical calculations were performed. Subtle conformational changes due to the deuteriation are discussed in terms of rovibrational averaging and overall geometry.

Primary and secondary kinetic isotope effects for methoxy dehydrogenation and Cu(110): Absence of tunnelling corrections

Das durch Cycloaddition des entsprechenden Imids an Quadricyclan und nachfolgende Hydrierung erhaltene Addukt (I) geht durch oxidative Hydrolyse in das N‐Oxid (II) über.

..beta..-Deuterium secondary kinetic isotope effects (..beta..-D KIEs) on the alkaline hydrolysis of the p-nitrophenyl esters of acetic, propanoic, butanoic, and pentanoic acids in pH 10.70, 0.20 M carbonate buffer at 25/sup 0/C tend to increase with increasing chain length of the esters up to the pentanoate. The ..beta..-D KIEs are respectively 0.975 +/- 0.004, 0.960 +/- 0.002, 0.940 +/- 0.001, and 0.948 +/- 0.004. The activation energies of the esterolyses of the isotopically light esters follow a similar pattern, as do the /sup 13/C NMR nuclear shieldings in CDCl/sub 3/ of the isotopically light parent carboxylic acids (20.9, 27.4, 35.9, and 33.8 (ppm)) and /sup 13/C NMR one-bond isotope shifts produced by disubstitution of deuterium for hydrogen at the ..cap alpha..-carbons of the acids (0.45, 0.55, 0.60, and 0.59 (ppm)). Correlation of nuclear shieldings and isotope shifts is known from previous work. The possibility is considered that all of the kinetics-based and NMR relationships are linked through the operation of a common ground-state feature of the ester and acid alkyl chains.

Acetolysis of ferrocenylmethyl benzoate. High secondary α-deuterium kinetic isotope effect for primary carbon-oxygen cleavage

Primary and secondary deuterium isotope effects have been measured by polarimetry, and primary isotope effects have been calculated for the classical bifunctional catalysis: 2-pyridinone-catalyzed mutarotation of 2,3,4,6-tetra-O-methyl-..cap alpha..-D-glucopyranose (..cap alpha..-TMG) in benzene. From the positively curved plot of the specific rate of epimerization vs. the mole fraction of /sup 2/H in the ''pool'' of OH and NH hydrogens, the isotope effects k/sub HH//k/sub DD/ = 3.66 +/- 0.09, k/sub HH//k/sub DH/ = 1.5, and k/sub HH//k/sub HD/ = 2.4 have been calculated. A secondary isotope effect of 1.14 +/- 0.02 has been measured by using ..cap alpha..-TMG and (1-/sup 2/H)-2,3,4,6-tetra-O-methyl-..cap alpha..-D-glucopyranose ((l-/sup 2/H)-..cap alpha..-TMG), the synthesis of which is described in detail, together with those for (N-/sup 2/H)-2-pyridinone and (1-O-/sup 2/H)-2,3,4,6-tetra-O-methyl-..cap alpha..-D-glucopyranose ((1-O-/sup 2/H)-..cap alpha..-TMG). The rate data obtained have also been analyzed by fractionation theory, yielding approximately equal fractionation factors (0.5). The interpretation of the results has been assisted by calculations of the primary deuterium isotope effects using the BEBOVIB IV program. Two models involving small and considerable coupling, respectively, of the transferring protons to heavy-atom motion have been considered. In the favored structure for the transition state of the rate-limiting step, two protons are in transit, and theirmore » motion is governed either by a potential with a barrier or by one without. Their motion is considerably coupled to the heavy-atom motion (i.e., the breakage of the ring C-O bond), and tunnel corrections to the isotope effects are found to be negligible.« less

Secondary deuterium isotope effects on the acidity of glycine

The unusual enzyme, Gracilariopsis alpha-1,4-glucan lyase of the sequence-related glycoside hydrolase family 31, cleaves the glycosidic bond of alpha-1,4-glucans via a beta-elimination reaction involving a covalent glycosyl-enzyme intermediate (Lee, S. S., Yu, S., and Withers, S. G. (2002) J. Am. Chem. Soc. 124, 4948-4949). The classical bell-shaped pH dependence of k(cat)/K(m) indicates two ionizable groups in the active site with apparent pK(a) values of 3.05 and 6.66. Brønsted relationships of log k(cat) versus pK(a) and log(k(cat)/K(m)) versus pK(a) for a series of aryl glucosides both show a linear monotonic dependence on leaving group pK(a) with low beta(lg) values of 0.32 and 0.33, respectively. The combination of these low beta(lg) values with large secondary deuterium kinetic isotope effects (k(H)/k(D) = 1.16 - 1.19) on the first step indicate a glycosylation step with substantial glycosidic bond cleavage and proton donation to the leaving group oxygen at the transition state. Developed oxocarbenium ion character of the transition state is also suggested by the potent inhibition afforded by acarbose and 1-deoxynojirimycin (K(i) = 20 and 130 nM, respectively) and by the substantial rate reduction afforded by adjacent fluorine substitution. For only one substrate, 5-fluoro-alpha-D-glucopyranosyl fluoride, was the second elimination step shown to be rate-limiting. The large alpha-secondary deuterium kinetic isotope effect (k(H)/k(D) = 1.23) at C-1 and the small primary deuterium kinetic isotope effect (k(H)/k(D) = 1.92) at C-2 confirm an E2 mechanism with strong E1 character for this second step. This considerable structural and mechanistic similarity with retaining alpha-glucosidases is clear evidence for the evolution of an enzyme mechanism within the family.

The mechanism of oxidative addition of ethyl iodide to [PtMe2(2,2′-bipyridine)], 1, has been investigated by product analysis and by study of secondary deuterium kinetic isotope effects (KIEs), usi...

The secondary α-deuterium kinetic isotope effect has been measured for the thermal reaction of 2-phenylethyldimethylamine oxide in 90 mol% DMSO–H2O at 60.0 °C. A large secondary α-deuterium isotope effect of 1.158 per α-D was found, which indicates significant [Formula: see text] bond rupture and very little double bond formation at the transition state for this concerted syn-elimination process. The observed large normal value for (kH/kD)α is discussed in terms of the use of secondary α-D isotope effects for the determination of stereochemistry for a concerted elimination process. Various isotope effects found for the syn elimination of the above amine oxide and those for the anti elimination of 2-arylethyltrimethylammonium salts with ethoxide are considered with respect to the relative transition state structures for syn- and anti-elimination reactions.

Kinetic ..cap alpha.. deuterium isotope effects have been measured for acid-catalyzed hydrolysis of inosine and adenosine. For inosine hydrolysis, values of k/sub H/k/sub D/ follow: in 1.0 M HCl, 1.21 and 1.20 at 25 and 50/sup 0/C, respectively; in 0.1 M HCl, 1.19 and 1.18 at 25 and 50/sup 0/C, respectively. For adenosine hydrolysis, k/sub H/k/sub D/ is 1.23 in 0.1 M HCl at 25/sup 0/C. The values require that the transition states for hydrolysis of both the monocation and dication of inosine and the dication of adenosine have marked oxocarbonium ion character. Detailed mechanisms which accord with this and other experimental observations include (1) a classical Al mechanism in which the C--N bond is largely cleaved in the transition state; (2) a mechanism involving some form of nucleophilic participation by solvent in which bond cleavage is advanced relative to bond formation in the transition state; or (3) complete C--N bond cleavage with rate-determining diffusion apart of oxocarbonium ion and purine base. 53 references, 1 figure, 2 tables.