María Inés Frascaroli

Oxidation of l-rhamnose and d-mannose by chromium(VI) in aqueous acetic acid

Abstract The oxidation of l -rhamnose and d -mannose by Cr(VI) in aqueous acetic acid follows the rate law: −d[Cr(VI)]/d t = ( k 2 + k 3 [H + ] [ aldose ] [Cr(VI)]/{1 + [H + ]/ K a + K 1 [H + ][ aldose ]}, where k 2 = 3.5 +- 0.8 × 10 −3 s −1 and 8.6 +- 1.0 × 10 −4 s −1 , K 3 = 6.8 ± 0.5 × 10 −3 M −1 s −1 and 5.1 ± 0.5 × 10 −3 M −1 s −1 , K a = 1–4 M and K 1 = 13∓_ 2 and 17±5 M −2 for l -rhamnose and d -mannose, respectively. This rate law corresponds to the reaction leading to the formation of l -1,4-rhamnonalactone and d -1,4-mannonalactone. No cleavage to carbon dioxide takes place when a 30-fold or higher excess of aldose over Cr(IV) is employed. The free radicals formed in the slow electron-transfer steps react with Cr(VI) to yield two intermediate Cr(V) complexes with EPR signals at g 1 = 1.978 and g 2 = 1.973. The mechanism and associated reactions kinetics are presented and discussed.

Chromic Oxidation of 2-Deoxy-d-Glucose. Comparative Study with Aldoses. I.

Abstract A rate law for the oxidation of 2-deoxy-d-glucose (2DG) by Cr(VI) in perchloric acid has been derived. This rate law corresponds to the reaction leading to the formation of 2-deoxy-d-gluconic acid (2DGA). No cleavage to carbon dioxide takes place when a twenty-fold or higher excess of aldose over Cr(VI) is employed. Kinetic constants are interpreted in terms of the absence of an hydroxyl group at C-2 on the stability of the chromic ester formed in the first reaction step. Free radicals formed during the reaction convert Cr(VI) to Cr(V). The latter species was detected by EPR measurements.

Comparative study of oxidation by chromium(V) and chromium(VI)

The kinetics and mechanism of the oxidation of 2-deoxy-D-glucose (dGlc) by CrVI which yields 2-deoxy-D-gluconic acid and CrIII as final products when a ten-fold or higher excess of sugar over CrVI is used, have been studied. The redox reactions occur through CrVI→ CrIV→ CrIII and CrVI→ CrV→ CrIII paths. The experimental data were fitted with a multilinear regression program. The complete rate law for the chromium(VI) oxidation reaction is expressed by –d[CrVI]/dt={c[H+]+(d+e[H+]+f[H+]2)[dGlc]}[CrVI], where c=(5 ± 1)× 10–4 dm3 mol–1 s–1, d=(3 ± 2)× 10–4 dm3 mol–1 s–1, e=(115 ± 13)× 10–4 dm6 mol–2 s–1 and f=(402 ± 17)× 10–4 dm9 mol–3 s–1, at 50 °C. Chromium(V) is formed in a rapid step by reaction of the radical dGlc and CrVI and CV reacts with dGlc faster than does CrVI. The chromium(V) oxidation of dGlc follows the rate low –dCrV/dt=(k1+k2[H+])[dGlc][CrV], where k1= 2.52 × 10–4 dm3 mol–1 s–1 and k2= 54.0 dm6 mol–2 s–1, at 25 °C. The EPR spectra show that three 1:1 CrV:dGlc intermediate complexes (g1= 1.9781, g2= 1.9752, g3= 1.9758) are formed in rapid pre-equilibria before the redox steps.

Detection and structural characterization of oxo-chromium(V)-sugar complexes by electron paramagnetic resonance.

This article describes the detection and characterization of oxo-Cr(V)-saccharide coordination compounds, produced during chromic oxidation of carbohydrates by Cr(VI) and Cr(V), using electron paramagnetic resonance (EPR) spectroscopy. After an introduction into the main importance of chromium (bio)chemistry, and more specifically the oxo-chromium(V)-sugar complexes, a general overview is given of the current state-of-the-art EPR techniques. The next step reviews which types of EPR spectroscopy are currently applied to oxo-Cr(V) complexes, and what information about these systems can be gained from such experiments. The advantages and pitfalls of the different approaches are discussed, and it is shown that the potential of high-field and pulsed EPR techniques is as yet still largely unexploited in the field of oxo-Cr(V) complexes. Subsequently, the discussion focuses on the analysis of oxo-Cr(V) complexes of different types of sugars and the implications of the results in terms of understanding chromium (bio)chemistry.

Redox and ligand-exchange chemistry of chromium(VI/V)-methyl glycoside systems

In order to establish a general pattern for the redox and coordination chemistry of glycosides with Cr(VI) and Cr(V), reactions of a series of methyl glycosides with Cr(VI) and Cr(V) have been studied at different acidities. Oxidations of methyl α- and β-D-glucopyranoside (Glc1Me), methyl α- and β-D-mannopyranoside (Man1Me), methyl α- and β-D-galactopyranoside (Gal1Me) and methyl α- and β-D-ribofuranoside (Rib1Me) by Cr(VI) proceed rapidly at pH ≤ 1, and yield Cr(III) and methyl glycofuranuronolactone as final products when an excess of methyl glycoside over Cr(VI) is used. At constant [H+], the reaction follows the rate law −d[Cr(VI)]/dt = kH [Gly1Me] [Cr(VI)]. Relative reactivities of methyl glycosides toward Cr(VI) reduction are: β-Rib1Me > α-Gal1Me > α-Rib1Me ≈ β-Gal1Me > β-Man1Me > α-Man1Me > α-Glc1Me > β-Glc1Me. This sequence is interpreted in terms of the degree of unfavorable steric interactions induced by the nonbonded 1,3-diaxial interactions in the respective Gly1Me-Cr(VI) monochelate, which is formed in rapid equilibrium that precedes the rate determining step. For all the glycosides, the oxidation rate decreases with an increase in pH value and becomes negligible at pH > 5. At pH 5.5 and 7.5, addition of an excess of α-Man1Me or α(β)-Gal1Me to an equimolar cysteine-Cr(VI) mixture, afforded two EPR triplets at giso1 1.9802 and giso2 1.9800/1 with Aiso 16.5(3) × 10−4 cm−1 in a 50 ∶ 50 giso1 ∶ giso2 ratio. The EPR spectral parameters and the superhyperfine pattern of the signal are consistent with the presence of two geometric isomers of the [CrVO(cis-O3,O4-Gal1Me)2]− and [CrVO(cis-O2,O3-Man1Me)2]− complexes. The same final spectral pattern is observed at pH 7.5 for the ligand-exchange reaction of Man1Me and Gal1Me with [CrVO(ehba)2]− (ehba = 2-ethyl-2-hydroxybutanoato(2−)). No EPR signal is observed when an excess of Xil1Me or Glc1Me is added to an equimolar cysteine-Cr(VI) mixture. In the ligand-exchange reactions of [CrVO(ehba)2]− at pH 7.5 with Xil1Me or Glc1Me, a very low intensity EPR singlet is observed at giso 1.9799. These results show that only glycosides with one cis-diolato group (such as Man1Me and Gal1Me) are effective for stabilizing Cr(V) at pH 5.5 and 7.5. The high redox reactivity of methyl glycosides with Cr(V) at high [H+] is attributed to the formation of [CrVO(O,O-glycoside)(OH2)3]+ species (giso 1.9716), which are not observed at pH 5.5–7.5, where only the five-coordinate bis-chelate oxochromate(V) species are observed.