Preparation of .omega.-hydroxyaldehydes or cyclic hemiacetals thereof

Cyclic hemiacetals thereof of the formula ##STR1## where A is saturated hydrocarbyl of 2 to 16 carbon atoms which may contain --O-- as a bridge member are prepared by subjecting cyclic ethers of the formula ##STR2## where A has the abovementioned meaning, to anodic oxidation in aqueous solution at current densities of over 30 mAcm.sup.-2.

The present invention relates to a process for preparing cyclic hemiacetals 
thereof by anodic oxidation of cyclic ethers. 
.omega.-hydroxyaldehydes, which are .alpha.,.omega.-bifunctional compounds, 
are useful intermediates. In the case of .omega.-hydroxybutanal and 
.omega.-hydroxypentanal, these compounds easily form the cyclic 
hemiacetals, namely .alpha.-hydroxytetrahydrofuran (THF--OH) and 
.alpha.-hydroxytetrahydropyran respectively. While, for example, 
.beta.-hydroxyaldehydes are readily accessible by aldol condensation, 
.omega.-hydroxyaldehydes are less simple to prepare. THF--OH, for example, 
is obtained by chemical oxidation of tetrahydrofuran (THF) with an 
aryldiazonium salt: 
##STR3## 
However, the yields with this process, described in Angew. Chem. 70 (1958), 
211, are low. On repeating this work, we found that little THF-OH is 
formed in addition to a lot of butyrolactone (BL) as well as succinic acid 
(SA). THF--OH can also be prepared by hydration of 2,3-dihydrofuran (Bull. 
Soc. Chim. Fr. 1950, 668): 
##STR4## 
We found that the last stage gives a 60% yield. 
EP-B-129,802 describes an Rh-complex-catalyzed hydroformylation of allyl 
alcohol 
EQU CH.sub.2 =CH-CH.sub.2 OH+H.sub.2 +CO.fwdarw.CHO-(CH.sub.2).sub.3 -OH, 
where, in addition to the desired product (79% selectivity), several 
byproducts are obtained. 
We have now found, surprisingly, that an a cyclic hemiacetal (II) thereof 
of the formula 
##STR5## 
where A is saturated hydrocarbyl of 2 to 16 carbon atoms with or without 
--O-- as a bridge member, can be prepared in high selectivity by 
subjecting a cyclic ether of the formula 
##STR6## 
where A has the abovementioned meaning, to anodic oxidation in aqueous 
solution at current densities of over 30 mAcm.sup.-2. 
In the case of tetrahydrofuran (THF), the electrochemical process can be 
written as follows: 
##STR7## 
Suitable starting materials of the formula III are for example THF, 
tetrahydropyran or 1,4-dioxane. 
The cyclic ethers of the formula III are used in the electrolysis in the 
form of their aqueous solutions. The electrolyte solutions preferably 
contain acids having anodically stable anions, in particular sulfuric acid 
or phosphoric acid. If the cyclic ether has a relatively high number of 
carbons, cosolvents, for example methanol or acetonitrile, are added in 
concentrations of from 10 to 80% by weight. Additionally or independently 
thereof, the starting material can be present in the electrolyte as an 
emulsion. 
Expediently the electrolyte used is a 0.1--0.5M, in particular a 0.5-2M, 
aqueous solution of one of the acids mentioned. However, in place of the 
acids the electrolytes can also contain buffer substances to keep the 
electrolyte at pH 0-6. It is also possible to add conventional conducting 
salts, such as sodium sulfate. The cyclic starting materials are present 
in the electrolyte in about 1-6 mol/dm.sup.3. 
The cathodes are made of material customary for electrolysis, such as 
steel, stainless steel, graphite, graphite-filled plastic or copper. 
Particularly suitable anode materials are the platinum metals or oxides 
thereof. Preference is given to smooth platinum, for example in the form 
of a plate or composite electrode. In principle, graphite and glass/carbon 
are also usable anode materials. 
In batchwise electrolysis, the conversion of the cyclic ether is 
expediently kept within the range from 10 to 80%, preferably from 20 to 
60%. However, it is also possible to electrolyse with higher conversions, 
since for example THF and THF--OH are easily separated by distillation. 
The electrolysis is carried out for example at from 0.degree. to 
50.degree. C., preferably at from 30.degree. to 40.degree. C. 
The current densities employed in the electrolysis are above 30 
mAcm.sup.-2, and range for example from over 30 to 1,000, preferably from 
100 to 300, mAcm.sup.-2. The current densities relate to the true surface 
area, and in the case of smooth platinum are thus virtually identical with 
the current density expressed in terms of the geometric surface area. The 
fact that current yields increase with increasing current densities is a 
welcome effect in the light of the high platinum costs. 
The electrolyte is agitated by forced convection, for example by stirring, 
pumping or vibrating. However, since a little oxygen is also always formed 
at the anode, highly effective convection already exists for that reason 
alone. The electrolysis is preferably carried out in divided cells or 
quasi-divided cells as described in Chem. Ber. 118 (1985), 3771-3779, in 
order to avoid reduction of the aldehyde group. However, it is also 
possible to work in undivided cells, provided cathodes having a small 
hydrogen overvoltage are used. 
The process according to the invention produces for example THF--OH from 
THF in high selectivity. This advantageous result was not foreseeable, 
since the existing electrolytic oxidation of THF, which is described for 
example in British Pat. No. 590,310 and where current densities of 10 
mAcm.sup.-2 are employed, gives succinic acid.

EXAMPLE 1 
The electrolysis cell used comprised a cylindrical 400 ml capacity glass 
vessel with cooling jacket and flat flange lid, which was equipped with an 
internal thermometer and a reflux condenser. The anode, which comprised a 
sheet of smooth platinum measuring 50.times.50.times.0.1 mm, ie. having a 
surface area of 50 cm.sup.2 (both sides together) was arranged in the 
center between two V2A wire cathodes (1.5 mm ID). The spacings between 
sheet anode and wire cathode were in each case 1.5 cm. The electrolyte was 
stirred magnetically. 
The cell was charged with 200 ml of an aqueous electrolyte of 1M strength 
with respect to THF and of 1M strength with respect to H.sub.2 SO.sub.4. 
Electrolysis was carried out with a current of 10.0 A, corresponding to an 
anode current density of 200 mAcm.sup.-2. The electrolyte temperature was 
maintained at 35.degree. C. by water cooling. The cell voltage was 6.5 V. 
The electrolysis gases left the cell through a brine-cooled reflux 
condenser. Discharged THF was made up by replenishing with fresh THF 
(constant electrolyte volume). After 38.6 minutes, corresponding to 6.43 
Ah or a theoretical current conversion (2 F/mol of THF) of 60%, the 
electrolysis was discontinued. After cooling down to 20.degree. C. a 1 ml 
sample of the electrolyte was analyzed by HPLC after 1:5 dilution with the 
eluent (aqueous H.sub.2 SO.sub.4 ; pH 1.7). 
Analysis of the HPLC diagram, which can be seen in Table 1, reveals that 
THF--OH was formed as the dominant product in addition to a little 
butyrolactone (BL) and the hydrolysis product thereof, namely 
.omega.-hydroxybutyric acid. The first two peaks are of unknown origin. 
TABLE 1 
______________________________________ 
Conc/M 
as per 
Retention cali- 
time Area bration 
n 
[min] Product % curve mmol 
______________________________________ 
8.16 ? 2.7 ? ? 
9.41 ? 0.54 ? ? 
11.48 Bu--OH* 
21.67 BL 2.6 0.008 1.6 
15.58 THF--OH 94.1 0.404 80.8 
______________________________________ 
*hydroxybutyric acid 
The abovementioned amount of electric charge (6.43 Ah) would have produced 
120 mmol of THF--OH if the current yield had been 100%. The observed 80.8 
mmol thus correspond to a current yield of 67.3%. Furthermore, 
theoretically 60 mmol of BL would have been formed in the case of a 100% 
current yield. The observed amount (1.6 mmol) thus corresponds to a 2.7% 
current yield. 
The unconverted THF was likewise analyzed by HPLC, except that a 1/4 V/V 
methanol/water mixture was used as eluent (2 ml min.sup.-1). 108 mmol of 
unconverted THF were recovered, ie. 92 mmol of THF were thus converted. 
The material yields were thus 88% (THF--OH) and 1.7% (BL). 
To work up the electrolyzed mixture, an extraction with ether was performed 
for 12.5 h in a continuous extractor (perforator). The ether extract 
(about 200 ml) was stirred with 15 ml of saturated aqueous K.sub.2 
CO.sub.3 solution, dried with anhydrous Na.sub.2 SO.sub.4 and evaporated 
in a rotary evaporator under a waterjet vacuum. This left 6.3 g of crude 
product. 822 mg of the crude product were made up to 10 ml in 1M H.sub.2 
SO.sub.4 ; 1 ml thereof was diluted with 5 ml of eluent (about 0.01M 
H.sub.2 SO.sub.4 of pH 1.7) and subjected to HPLC analysis, which revealed 
a THF--OH concentration of 0.84M. This corresponds to a molar amount of 
64.4 mmol of THF--OH, and to a current yield of 54% for the extracted 
product. 
The crude product was distilled under reduced pressure (1.5 mm Hg). To 
stabilize the crude product, a little 85% strength phosphoric acid or 
cationic exchange membrane pieces in the H.sup.+ form were added. The 
boiling point ranged from 24.degree. to 30.degree. C. (2 mm Hg). 
Gas chromatography analysis of the crude product in ether (capillary 
column, 10 m, polar stationary phase, 100.degree. C., He, 25 ml/min) 
produced 5 peaks (retention times in minutes): 
(1) 0.69+++ 
(2) 0.90+(BL) 
(3) 3.01++ 
(4) 3.23 (+) 
(5) 3.56++ 
The four unknown peaks are probably oligoacetals which are formed in the 
column. In the presence of acid traces in the ether solution, the 1st peak 
increases significantly at the expense of the others (3,4,5). 
GC--MS coupling likewise identified corresponding products of mass 
(88).sub.n where n=1-5. A further portion of the crude product was reacted 
with an approximately 5 times molar excess of 2,4-dinitrophenylhydrazine 
in 2N HCL. The yellow 2,4-dinitrophenylhydrazone was formed in a material 
yield of 90%, based on the THF--OH content, and had a melting point of 
117.6.degree. C. 
______________________________________ 
Elemental analysis revealed: 
found % 
calculated % 
______________________________________ 
C 44.97 44.78 
H 4.47 4.48 
N 20.95 20.90 
______________________________________ 
Another portion, namely 1.0 g, of the crude product was dissolved in 
t-butanol and reacted with a concentrated aqueous NH.sub.2 OH.HCl 
solution. Removal of the solvent by distillation and extraction with ether 
followed by stripping of the ether left THF--OH in the form of a colorless 
oil (0.61 g) having a refractive index of n.sub.D.sup.21.5 
.degree.C.=1.4608. The NMR spectrum of the product gave the following 
values: 
##STR8## 
The loss of platinum which was observed at the anode after the experiment 
amounted to 0.26 mg, corresponding to a specific amount of 0.04 mg/Ah. 
EXAMPLE 2 
In the cell of Example 1, 200 ml of an aqueous electrolyte which was 4M in 
strength with respect to THF and 1M in strength with respect to H.sub.2 
SO.sub.4 were reacted at 75 mA cm.sup.-2, corresponding to 3.75 A, and 
35.degree. C. over smooth platinum. The cell voltage was on average 4.7 V. 
After 6 hours and 54 minutes, corresponding to a theoretical current 
conversion of 60%, the electrolysis was discontinued. Direct determination 
of the products in the electrolyzed mixture revealed: THF--OH: 66.4% CY, 
MY=85%; BL: 13.7% CY, MY=6%, SA in traces. The very slightly yellow 
electrolyte was worked up as in Example 1. 17.6 g of crude product were 
obtained. The specific platinum loss was again only 0.04 mg/Ah. (CY 
denotes the current yield and MY the material yield.) 
EXAMPLE 3 
In the cell of Example 1, 200 ml of the electrolyte mentioned in Example 1 
were electrolyzed over a cylindrical (h=7 cm, ID=5 cm) titanium mesh anode 
(8 mesh holes/cm.sup.2), which had been activated with RuO.sub.2 (4 g of 
Ru/m.sup.2, RuO.sub.2 :TiO.sub.2 =1:1), under the following conditions: 
current 17.9 A (anode surface area=228 cm.sup.2, both sides together), 
current density 75 mA cm.sup.-2, counterelectrode: axial V2A wire 
electrode, 1.5 mm ID, temperature 35.degree. C. 
The cell voltage was on average 7.0 V. After 27 minutes, corresponding to a 
theoretical current yield of 60%, the experiment was discontinued. Direct 
determination of the products in the electrolyzed mixture revealed: 
THF--OH: 3.5% CY; BL in traces; SA in traces. In this case, too, THF--OH 
was formed selectively, but only with a small current yield as a 
consequence of the small oxygen overvoltage across the electrode.