Glycol formate

There are many related compounds, including rhodium carbonyl cluster anions, which are present in the solutions cataly2ing ethylene glycol formation and which may be the catalyticaHy active species or in equiUbrium with them (38). 

Oxidations in the pteridine series comprise (i) replacement of hydrogen by hydroxyl, (ii) glycol formation at the central C=C bond (iii) the removal of hydrogen atoms from dihydro and tetrahydro derivatives. 

Because osmium tetroxide is expensive, and its vapors are toxic, alternate methods have been explored for effecting vic-glycol formation. In the aliphatic series, olefins can be hydroxylated with hydrogen peroxide with the use of only a catalytic amount of osmium tetroxide. Anhydrous conditions are not necessary 30% hydrogen peroxide in acetone or acetone-ether is satisfactory. The intermediate osmate ester is presumably cleaved by peroxide to the glycol with regeneration of osmium tetroxide. When this reaction was tried on a A -steroid, the product isolated was the 20-ketone ... 

The products formed are a complex mixture, but it appears that initially the double bond is split to give aldehydes which suffer subsequent oxidation. Glycol formation may precede C-C fission. 

A number of carboxylic acids other than acetic were investigated as solvents or promoters. All of these acids which were stable to reaction conditions were found to be effective in promoting glycol ester production (e.g., propionic, pivalic, benzoic, etc.). However, other Br nsted acids of non-carboxylic nature were not found to be effective promoters. Thus penta-chlorophenol, although it has a pKa value (4.82) very close to that of acetic acid (4.76), is not a comparable promoter (Table I, reaction 13). Likewise, phosphoric acid (pK 2.15) is not an effective solvent or co-solvent with acetic acid (Table I, reaction 8). Experiments with lower concentrations of these acids in sulfolane solvent also showed that carboxylic acids are unique in promoting glycol formation. The promoter function of carboxylic acids thus appears not to be dependent (only) upon their acidity, but on some other chemical or structural property. 

Solutions of Ru3(CO)i2 in carboxylic acids are active catalysts for hydrogenation of carbon monoxide at low pressures (below 340 atm). Methanol is the major product (obtained as its ester), and smaller amounts of ethylene glycol diester are also formed. At 340 atm and 260°C a combined rate to these products of 8.3 x 10 3 turnovers s-1 was observed in acetic acid solvent. Similar rates to methanol are obtainable in other polar solvents, but ethylene glycol is not observed under these conditions except in the presence of carboxylic acids. Studies of this reaction, including infrared measurements under reaction conditions, were carried out to determine the nature of the catalyst and the mechanism of glycol formation. A reaction scheme is proposed in which the function of the carboxylic acid is to assist in converting a coordinated formaldehyde intermediate into a glycol precursor. 

The reaction takes place in the medium of acetic acid and yields are generally good. This is why the route to obtain aldehydes or ketones from alkenes via glycol formation is preferred over that of ozonolysis. Other compounds which are readily cleaved include those with the groups ... 

Chen, J.-W. and Chen, L.-W., The kinetics of diethylene glycol formation from bishydroxyethyl terephthalate with antimony catalyst in the preparation of PET, J. Polym. Sci., Polym. Chem. Ed., 37, 1797-1803 (1999). 

The distribution of products in these reactions can change very substantially with reaction time, as illustrated in Fig. 1. The rate of ethylene glycol formation remains quite constant, but rates to other products change markedly as the reaction proceeds, indicating that secondary reactions are taking place. Some of the plausible secondary reactions in this system have been listed by Feder et al. (37) ... 

The reaction rates in this system are presumably first-order in catalyst concentration, as implied by the scaling of product formation rates proportionately to rhodium concentration (90, 92, 93). Responses to several other reaction variables may be found in both the open and patent literature. Fahey has reported studies of catalyst activity at several pressures in tet-raglyme solvent with 2-hydroxypyridine promoter at 230°C (43). He finds that the rate to total products is proportional to the pressure taken to the 3.3 power. A large pressure dependence is also evident in the results shown in Table VII. Analysis of these results indicates that the rate of ethylene glycol formation is greater than third-order in pressure (exponents of 3.2-3.5), and that for methanol formation somewhat less (exponents of 2.3-2.8). The pressure dependence of the total product formation rate is close to third-order. A possible complicating factor in the above comparisons is the increased loss of soluble rhodium species in the lower-pressure experiments, as seen in Table VII. Experiments similar to those of Fahey have also been... 

Kaplan has proposed that ion pairing between rhodium complex anions and the positively charged counterions has an adverse effect on catalytic activity for ethylene glycol formation (96, 109, 110). The following scheme ... 

Fig. 18. Effect of pressure on methanol and ethylene glycol formation rates by an iodide-promoted ruthenium catalyst (191). Reaction conditions 75 ml A -methylpyrrolidone solvent, 15 mmol Ru, 45 mmol Nal, H2/CO = 1, 230 C. 1 MPa = 9.87 atm.

Problem 6.34 ( ) Describe the stereochemistry of glycol formation with peroxyformic acid (HCO,H) if cw-2-butene gives a racemic glycol and frans-2-butene gives the meso form. (6) Give a mechanism for cis. 

Problem 6.35 Describe the stereochemistry of glycol formation with cold alkaline aqueous KMnOj if c -2-butene gives the meso glycol and trans-2-butene gives the racemate. -4... 

Two parallel routes for the elimination of glycol formate are suggested, involving either reaction with H2 or with cocatalyst water. The detection of formic acid in the reaction products suggests another mechanism, with initial production of formic acid from H2 and C02, followed by reaction with the oxirane. This mechanism is not favored however since the yields of glycol formates varied substantially when various substituted oxiranes were reacted. This would not have been expected in a mechanism with formic acid as an intermediate. A third mechanism, not considered by the authors, could proceed through initial production of propylene carbonate, followed by reduction to the mono- or di-formate. 

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