Process for producing phenolic thiocarboxylic esters

Compounds of the formula I ##STR1## wherein R.sup.1 and R.sup.2 independently of one another are each tert-butyl, R.sup.3 is C.sub.1 -C.sub.20 -alkyl, or C.sub.2 -C.sub.20 -alkyl interrupted by --O-- or --S--, and n is the number 1 or 2, can be obtained, with a very high degree of yield and purity, by reaction of a phenol of the formula II ##STR2## with formaldehyde and a mercaptan of the formula III EQU R.sup.3 OOC--C.sub.n H.sub.2n --SH (III) in the presence of catalytic amounts of a primary or secondary amine. Such compounds are stabilizers for organic materials.

The present invention relates to a novel single-stage process for producing 
phenolic thiocarboxylic esters. 
The production of phenolic thiocarboxylic esters in two stages, either by 
way of the Mannich base, or by reaction of corresponding phenols with 
mercapto acids and formaldehyde and subsequent esterification of the acids 
obtained, is known. It is described for example in the U.S. Pat. No. 
3,832,328 and, as regards the Mannich reaction, also in the European 
Patent Application No. 59,168. These two-stage processes are however 
involved and therefore costly, and the yield and purity of the resulting 
products are unsatisfactory. U.S. Pat. No. 3,553,270 describes a 
single-stage process for producing phenolic thioethers by reaction of the 
corresponding phenol with formaldehyde and a mercaptan in the presence of 
a strong base, such as trimethylamine or particularly an alkali metal 
hydroxide, as catalyst. Attempts to produce also phenolic thiocarboxylic 
esters by this method have failed. It has however been found that, 
surprisingly, phenolic thiocarboxylic esters can be obtained with a high 
level of purity and yield by the use of primary or secondary amines as 
catalysts. 
The present invention accordingly relates to a process for producing 
compounds of the formula I 
##STR3## 
wherein R.sup.1 and R.sup.2 independently of one another are each 
tert-butyl, R.sup.3 is C.sub.1 -C.sub.20 -alkyl, or C.sub.2 -C.sub.20 
-alkyl interrupted by --O-- or --S--, and n is the number 1 or 2, by 
reaction of a phenol of the formula II 
##STR4## 
with formaldehyde and a mercaptan of the formula III 
EQU R.sup.3 OOC--C.sub.n H.sub.2n --SH (III), 
wherein R.sup.1, R.sup.2, R.sup.3 and n have the meanings defined above, at 
a temperature of between 20.degree. and 200.degree. C. and in the presence 
of a catalyst, in which process the catalyst used is a primary or 
secondary amine of the formula IV 
##STR5## 
in which R.sup.4 and R.sup.5 independently of one another are each C.sub.2 
-C.sub.20 -alkyl or hydroxyalkyl, C.sub.5 -C.sub.7 -cycloalkyl, phenyl or 
benzyl, and R.sup.5 can additionally be hydrogen. 
As C.sub.1 -C.sub.20 -alkyl, R.sup.3 is for example: methyl, ethyl, propyl, 
isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, n-octyl, 
2-ethylhexyl, 1,1,3,3-tetramethylbutyl, nonyl, decyl, dodecyl, tridecyl, 
hexadecyl, octadecyl or eicosyl. The preferred meaning of R.sup.3 is 
2-ethylhexyl or iso-tridecyl (tridecyl isometric mixture). 
Examples of R.sup.3 as C.sub.2 -C.sub.20 -alkyl interrupted by --O-- or 
--S-- are: methoxymethyl, 2-ethoxyethyl, 2-n-butoxyethyl, 3-n-butoxyethyl, 
2-octoxyethyl, 2-hexadecyloxyethyl, 2-ethoxymethyl, butoxymethyl, 
methoxypropyl, ethoxypropyl, 3-thiaheptyl or 3-thia-5-methylhexyl. 
n is preferably 1. 
As C.sub.2 -C.sub.20 -alkyl, R.sup.4 and R.sup.5 are for example: ethyl, 
propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, octyl, 
decyl, dodecyl, hexadecyl, octadecyl or eicosyl; and, as C.sub.2 -C.sub.20 
-hydroxyalkyl, they are: 2-hydroxyethyl, 3-hydroxypropyl, 3-hydroxybutyl, 
8-hydroxyoctyl, 2-hydroxybutyl or 3-hydroxypentyl. 
As C.sub.5 -C.sub.7 -cycloalkyl, R.sup.4 and R.sup.5 are for example: 
cyclopentyl, cycloheptyl and especially cyclohexyl. 
R.sup.4 and R.sup.5 are preferably C.sub.2 -C.sub.8 -alkyl, and R.sup.5 in 
addition also hydrogen. More especially preferred, R.sup.4 and R.sup.5 are 
identical and are particularly n-butyl. 
The process according to the invention is performed especially 
advantageously by reaction of 1 mol of the phenol of the formula II with 
1.0 to 2.0 mols of formaldehyde and 0.9 to 1.3 mols of a mercaptan of the 
formula III, in the presence of at least 0.5 mol %, relative to the phenol 
of the formula II, of the catalyst of the formula IV. There are preferably 
used 1 to 15 mol %, particularly 2.5 to 10 mol %, of catalyst, relative to 
the phenol. 
The reaction can be carried out with or without solvent. Suitable solvents 
are those which have a certain solubility with respect to the reactants, 
but which are essentially inert under the reaction conditions. Examples of 
solvents suitable for this purpose are: hydrocarbons, such as toluene, 
xylene, octane and .beta.-terpene; ethers, such as dioxane, diethyl ether, 
dimethyl ether of ethylene glycol, tetrahydrofuran, and so forth. Also 
chlorinated hydrocarbons, such as carbon tetrachloride, chloroform, 
trichloroethane and perchloroethylene, can advantageously be used. Other 
suitable solvents are for example: dimethylformamide, dimethylacetamide or 
dimethyl sulfoxide. Also primary or secondary alcohols having 3 to 6C 
atoms, such as isopropanol, sec-butyl alcohol, tert-butyl alcohol, 
tert-amyl alcohol and hexyl alcohol, can be recommended for a successful 
carrying out of the process according to the invention. 
The formaldehyde can be in the form of an aqueous solution, especially an 
approximately 35% aqueous solution, or in the form of paraformaldehyde. 
When paraformaldehyde is used, it is necessary to add the 5- to 20-fold 
amount, preferably the 8- to 12-fold amount, in weight, relative to the 
paraformaldehyde, of a depolymeriser, for example dimethylformamide, 
dimethylacetamide or dimethyl sulfoxide. 
The reaction temperature varies depending on the reactant. Suitable 
temperatures are between 20.degree. and 200.degree. C. A preferred 
temperature range is between 80.degree. and 140.degree. C. It proves 
advantageous in practice to perform the process at the boiling temperature 
of the reaction mixture. Accordingly, the temperature range of between 
80.degree. C. and the boiling temperature of the reaction mixture is 
particularly preferred. 
The duration of the reaction is likewise dependent on the reactants; in 
general, the reaction is finished after 4 to 8 hours. Isolation of the 
final product is effected by customary separation of the volatile 
constituents, for example by distillation in vacuo, or by the dissolving 
out of optionally nonvolatile amine catalyst residues with dilute acid, 
for example dilute hydrochloric acid or sulfuric acid. On account of the 
high degree of purity of the product obtained, no additional purification 
is necessary. 
The compounds of the formula I are valuable stabilisers for organic 
materials which are subject to degradation by light, heat or oxygen; they 
are valuable stabilisers in particular for elastomers and lubricants.

The following Examples further illustrate the present invention without 
limiting the scope thereof. 
EXAMPLE 1 
A mixture of 206 g of 2,6-di-tert-butylphenol, 45 g of paraformaldehyde, 
204 g of thioglycolic acid-2-ethyl-n-hexyl ester, 7.6 g of di-n-butylamine 
and 150 ml of dimethylformamide is stirred at about 120.degree. C. for 3 
to 4 hours under refluxing conditions and with the feeding in of nitrogen. 
The volatile constituents (residual aldehyde, di-n-butylamine and 
dimethylformamide) are subsequently completely removed by being distilled 
off in vacuo. There are thus obtained 415 g (98% of theory) of 
3,5-di-tert-butyl-4-hydroxybenzylmercaptoacetic acid-2-ethyl-n-hexyl ester 
in the form of a viscous pale-yellow oil, which, on standing in the cold, 
solidifies in the crystalline state (m.p. 20.degree. C.). Without further 
purification steps, the product is shown to be very pure (content &gt;98%) 
after verification by thin-layer chromatography, NMR and phenol titration. 
EXAMPLES 2-4 
When in Example 1 the thioglycolic acid-2-ethyl-n-hexyl ester is replaced 
in each case by the equivalent amount of one of the homologous thioglycol 
esters of the formula 
##STR6## 
wherein R.sup.3 has the meaning given in the following Table, there are 
obtained, with otherwise an analogous procedure, the corresponding 
homologous phenols with practically the same level of purity and yield. 
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Example R.sup.3 Aspect 
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2 n-C.sub.8 H.sub.17 
crystalline, m.p. 35.degree. C. 
3 n-C.sub.12 H.sub.25 
viscous oil 
4 iso-C.sub.13 H.sub.27 
viscous oil 
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EXAMPLE 5 
By using in Example 1, in place of dimethylformamide, the equivalent amount 
of dimethylacetamide (as depolymeriser of the paraformaldehyde), there is 
obtained, with otherwise the same procedure, the product of Example 1 in a 
similar quality and yield. 
EXAMPLE 6 
If there is used in Example 1, instead of paraformaldehyde, the aliquot 
amount of formaldehyde in the form of a 35% aqueous solution, the addition 
of dimethylformamide being dispensed with, there is obtained within 7 to 8 
hours reaction time at about 100.degree. C., with otherwise the same 
procedure, the product of Example 1 in similar quality and yield. 
EXAMPLE 7 
When in Example 1 the di-n-butylamine is replaced by the equivalent amount 
of n-octylamine, there is obtained, the procedure otherwise being 
analogous, the product of Example 1 with a similar degree of yield and 
purity.