Pyrocarbonic acid diesters and the preparation and use thereof

Details Pyrocarbonic acid diesters and the preparation and use thereof Pyrocarbonic acid diesters and the preparation and use thereof

1998-01-13

2001-04-24

Ambrose, Michael G. (Department: 1626)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C540S130000, C544S340000, C546S049000, C546S056000, C546S173000, C548S470000, C548S472000, C548S484000

Reexamination Certificate

active

06222047

ABSTRACT:

The present invention relates to the preparation of pyrocarbonic acid diesters by an improved process and to the novel pyrocarbonic acid diesters prepared according to said process as well as to the use thereof. The products are obtained in high yield and are of very high purity.
Pyrocarbonic acid diesters themselves are active compounds and are used, for example, as antiseptic agents for food. In addition, specific pyrocarbonic acid diesters, such as di-tert-butyl dicarbonate (DIBOC), are important fine chemicals, e.g. for introducing protective groups such as carbonate into alcohols, thiocarbonate into thiols or, in particular, urethane into amines or amides. Such groups are distinguished in that they are stable under normal conditions but can nevertheless be separated, for example hydrolytically or thermally, with reconversion of the original functions.
Substances which utilise said properties for technical purposes are known from EP 648 770 and EP 648 817. However, in contradistinction to standard tert-butyl urethanes prepared from DIBOC, more precise demands are made in such cases on the properties, in particular on the thermal properties. There is therefore a need for novel pyrocarbonic acid diesters as synthesis building blocks.
In view of the importance as food additives and fine chemicals, the most stringent demands are made on DIBOC and other dicarbonates with respect to purity. Many efforts have therefore been made to prepare DIBOC in ever enhanced quality and yield. This is complicated by DIBOC being thermally instable, as is indicated in J. Org. Chem. 43, 2410 (1978).
DIBOC can be prepared by two fundamentally different processes. In the first process, which is described in Org. Synth. 57, 45 (1975) and JP-88/051358, tert-butyl carbonate is reacted with phosgene to di-tert-butyl tricarbonate which is then decarboxylated in the presence of a tertiary amine (for example 1,4-diazabicyclo[2.2.2]octane) as catalyst, according to JP-91/356445 preferably with the addition of a phase transfer catalyst. In JP-92/310646, the tertiary amine is pyridine in a non-specific amount. According to JP-89/186847 it is also possible to use thionyl chloride instead of phosgene.
In the second, and generally preferred, process according to Zh. Org. Khim. 15/1, 106 (1975), tert-butyl carbonate is reacted with an acid chloride to a mixed carboxylic acid anhydride which is converted into the desired dicarbonate with excess tert-butyl carbonate. This process can be improved by replacing carboxylic anhydrides with sulfochlorides (CS-247845 and CS-247846). Other measures for the improvement of this process have also been proposed, for example the addition of a quaternary ammonium salt (CS-257157), the use of amines with an aliphatically bonded tertiary nitrogen, typically triethylamine, N,N-dimethylbenzylamine or N,N,N′,N′-tetramethylethylenediamine (J P-90/103562), concentrating free sodium hydroxide solution below 10 mol % (JP-92/194326) or concentrating free sodium alcoholate below 3 mol % (JP-92/310648), the use of carbon dioxide under pressure (JP-92/279301) as well as the salt-free washing of the crude product prior to distillation (JP-92/30626 1).
It has been found, however, that in spite of improvements all processes described above are still not entirely satisfactory. High yields, for example, are only achieved at elevated temperature, by prolonged reaction times or in the presence of considerable amounts of polar hydrophilic solvents such as tetrahydrofuran or dimethyiformamide. Such polar hydrophilic solvents, however, which are used in pure or mixed form, cannot be readily recovered, if at all, and residues can enter into the waste water during washing. These processes therefore entail great ecological problems which can only be solved satisfactorily with considerable expenditure of costs. In addition, the quality of the crude products obtained leaves much to be desired, requiring a particularly careful and time-consuming fractional distillation to isolate the pure product. However, owing to the thermal sensitivity of DIBOC, distillation should, if possible, be avoided altogether or be at least carried out very rapidly, for example in a single step falling film evaporator.
CS-260076 proposes carrying out this process in the presence of pyridine and a quaternary ammonium salt. While this permits a reaction at room temperature, the yield is only 38.6% of theory and therefore an elevated temperature of preferably 50° C. is indicated in order to improve the yield. It has been found, however, that this induces the decomposition of the product and that said product contains considerable amounts of unreacted p-toluene sulfochloride, so that this method does not solve the problems described above.
EP 468 404 therefore proposes to replace tosyl chloride with mesyl chloride to improve the reactivity. This process should make it possible to produce good product qualities according to the description and the yield could even be increased by adding phase transfer catalysts, aromatic amines, or mixtures thereof.
In practice, however, it has been found that this latter process is also problematical. It has been found, for example, that very vigorous stirring of the reaction mixture is absolutely essential, which in laboratory practice is only possible with a special stirrer, and the yields decrease drastically in the scale-up. This problem is apparently caused at least partly by the physical properties of mesyl chloride which is a liquid of high specific density. In addition, mesyl chloride has a relatively high vapour pressure and is very caustic; it hydrolyses easily and reacts also with the alcoholate to be reacted under formation of methane sulfonic acid alkyl esters, which have a boiling point similar to that of the desired pyrocarbonic acid diesters so that their traces can hardly be removed at all by distillation. Because of this undesirable side reaction, mesyl chloride is also sometimes used in slight excess. And, finally, the resulting methane sulfonic acid cannot be isolated from the aqueous solution for recycling as easily as might be desired.
Surprisingly, it has now been found that pyrocarbonic acid diesters can in fact be obtained from ester carbonate and tosyl chloride in excellent yield and purity if this reaction is carried out in the presence of an ammonium salt and very small amounts of pyridine in a nonpolar inert solvent. Tosyl chloride and pyridine can also be replaced with structurally similar compounds, giving comparable results.
Accordingly, the invention relates to a process for the preparation of a pyrocarbonic acid diester of formula (I)
wherein R
1
and R
1
′ are each independently of the other branched or straight-chain C
1
-C
24
alkyl, C
3
-C
24
alkenyl, C
3
-C
24
alkynyl, C
4
-C
12
cycloalkyl, C
4
-C
12
cycloalkenyl or C
7
-C
24
aralkyl, each of which is unsubstituted or substituted by one or more than one substituent which is inert under the reaction conditions,
by reacting at least one ester carbonate of formula (II)
wherein M
+
is Na
+
, Li
+
, K
+
or NR
2
R
3
R
4
R
5
+
, and R
2
to R
5
are each independently of one another hydrogen, C
1
-C
18
alkyl, C
5
-C
10
cycloalkyl or C
7
-C
18
aralkyl,
with 40-50 mol % of a sulfochloride of formula (IV)
wherein R
6
is —H, —CH
3
, —CH
2
CH
3
, —Cl, —Br, —OCH
3
or —NO
2
,
in the presence of 0.8-5 mol % of a catalyst of formula (V)
wherein R
2
to R
5
have the meaning cited above, and X
−
is a non-nucleophilic anion,
and with minor amounts of a heterocyclic aromatic amine in a nonpolar inert solvent,
in which process the amount of heterocyclic aromatic amine is 1-5 mol % and the reaction is carried out in the temperature range from −10° C. to +25° C.,
all molar amounts being based on 100 mol % of ester carbonate of formula (II).
C
1
-C
18
Alkyl is typically methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2,2-dimethylpropyl, n-hexyl, n-octyl, 1,1,3,3-tetra-methylbutyl, 2-ethylhexyl, n

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