Process for the manufacture of citral

Details Process for the manufacture of citral Process for the manufacture of citral

1999-03-16

2001-03-06

Padmanabhan, Sreeni (Department: 1621)

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

Organic compounds

Oxygen containing

C568S450000, C568S449000, C568S467000, C568S488000, C568S490000, C568S492000

Reexamination Certificate

active

06198006

ABSTRACT:

FIELD OF THE INVENTION
The present invention is concerned with a process for the manufacture of citral. The &agr;,&bgr;-unsaturated aldehyde citral (E/Z-3,7-dimethyl-2,6-octadienal, consisting of the isomers geranial, i.e. E-citral, and neral, i.e. Z-citral) is, as is known, a valuable intermediate for the synthesis of odorants, terpinoids and vitamins.
BACKGROUND OF THE INVENTION
&agr;,&bgr;-Unsaturated carbonyl compounds are generally important intermediates for the manufacture of odorants, vitamins and carotenoids [see, for example, Chem. Ztg. 97, 23-28 (1973) and Chap. VI (“Total Syntheses”) in “Carotenoids”, Ed. Otto Isler, published by Birkhäuser Basel and Stuttgart, 1971]. Their production by acid-catalyzed rearrangement of &agr;-alkynols has already been described in the nineteen twenties by K. H. Meyer and K. Schuster [Ber. deutsch. Chem. Ges. 55, 819-823 (1922)] and H. Rupe and E. Kambli [Helv. Chim. Acta 9, 672 (1926)]; the isomerization of secondary or tertiary &agr;alkynols to &agr;,&bgr;-unsaturated carbonyl compounds has also generally become known as the Meyer-Schuster or Rupe-Kambli rearrangement. In the case of the rearrangement of a carbonyl compound having a terminal alkynyl group there are obtained aldehydes, otherwise ketones are the rearrangement products:
wherein R
1
and R
2
each signify hydrogen or an aliphatic or aromatic residue. In addition to citral, the likewise &agr;,&bgr;-unsaturated aldehydes citronellal and hydroxycitronellal are also of particular industrial interest, namely as intermediates for the manufacture of odorants, terpinoids and vitamins; citral itself can be converted, in each case in several process steps, into the important starting materials for the manufacture of d,l-&agr;-tocopherol (vitamin E) and vitamin A, isophytol or &bgr;-ionone [see, for example, “Vitamine I, Fettlösliche Vitamine”, Ed. Otto Isler and Georg Brubacher, published by Georg Thieme Stuttgart, New York 1982, the Chapter VI “Total Syntheses” in “Carotenoids” (published by Birkhäuser 1971) and the literature references referred to therein].
Depending on the reaction conditions, the rearrangement of dehydrolinalyl acetate catalyzed by silver or copper ions yields, according to G. Saucy et al. [Helv. Chim. Acta 42, 1945-1955 (1959)], a mixture of “allene acetate” (1-acetoxy-3,7-dimethyl-octa-1,2,6-triene) and “diacetate” (1,1-diacetoxy-3,7-dimethyl-octa-2,6-diene), which can hydrolyze to citral:
This rearrangement of dehydrolinalyl acetate is also known as the Saucy-Marbet rearrangement. However, dehydrolinalool can be converted directly into citral using an alkyl, cycloalkyl or aryl orthovanadate or another vanadium catalyst (UK Patent 1,204,754). Disadvantages in the direct conversion are, however, the low yield (about 31-37%) as well as the formation of dark precipitates which lead to the decomposition of the reaction solution. The direct rearrangement of dehydrolinalool is effected substantially more selectively and efficiently using tris(triphenylsilyl)vanadium oxide at about 140° C. [Chimia 27, 383 (1973) as well as Helv. Chim. Acta 59, 1233-1243 (1976)]. In this case yields of about 78% are achieved in paraffin oil as the solvent.
Further publications of the direct rearrangement of dehydrolinalool to citral using vanadium-containing catalysts include the use of polyboroxyvanadoxydiphenylsilane and of polysilylvanadates as the catalysts [Czechoslovakian Patent CS 264, 720/Chem. Abs. 114, 122769a (1991) and, respectively, Mendeleev Commun. 1994, 89]. Whereas in the first process the achieved yield of about 70% is too low commercially, an 80% yield can be achieved with the second process.
A further catalyst for the direct rearrangement of &agr;-alkynyls, such as, for example, dehydrolinalool, to &agr;,&bgr;-unsaturated carbonyl compounds consists of the combination of a titanium compound, e.g. titanium tetrachloride or tetrabutoxide, with a copper or silver halide [Tetr. Lett. 29, 6253-6256 (1988) and European Patent Publication 0 240 431 A]. However, the use of copper compounds is disadvantageous in this process. Moreover, also in this case, the about 64% yield of citral which is achieved is unsatisfactory.
An interesting variant of the aforementioned Meyer-Schuster rearrangement has been described briefly by C. Y. Lorber and J. A. Osborn in Tetr. Lett. 37, 853-856 (1996); this is the rearrangement of methylbutynol to prenal using a molybdenum catalyst. In this case, methylbutynol is rearranged to prenal in ortho-dichlorobenzene as the solvent in the presence of the catalyst system molybdenyl acetylacetonate, dibutyl sulphoxide and 4-tert.butylbenzoic acid. Although the yield in this rearrangement is indicated to be 97%, the prenal was not isolated from the reaction mixture, but the stated yield was obtained by gas-chromatographical analysis of the crude product. Presumably, it was difficult to work up the reaction mixture in order to isolate prenal.
L. A. Kheifits and co-workers found that dehydrolinalool could be converted into citral only in 28% yield and into 2-hydroxymethyl-1-methyl-3-isopropenylcyclopent-1-ene in 12% yield at 170° C. in a reaction period of 14 hours when a molybdenum catalyst produced from molybdenum oxide and triphenylsilanol was used for the rearrangement [Tetr. Lett. 34, 2981-2984 (1976)].
From the above remarks it is evident that the previously known processes for the catalyzed rearrangement of &agr;-alkynols, e.g. dehydrolinalool, to &agr;,&bgr;-unsaturated aldehydes, e.g. citral, have serious disadvantages.
SUMMARY OF THE INVENTION
The process in accordance with the invention is a process for the manufacture of citral by the catalyzed rearrangement of dehydrolinalool to citral, which process comprises carrying out the rearrangement in the presence of a molybdenum compound of the general formula:
MoO
2
X
2
  I
wherein X signifies an acetylacetonate or halide ion, and a dialkyl or diaryl sulphoxide as the catalyst system, in the presence of an organic acid having a pK value in the range of about 4.0 to about 6.5 and in an apolar aprotic organic solvent.
The components are added together and mixed. The reaction mixture is heated to the temperature at which the catalytic rearrangement reaction occurs, to provide a resulting mixture. Citral is then isolated from the resulting mixture.
DETAILED DESCRIPTION IF THE INVENTION
The process of the present invention surprisingly achieves a substantial yield of citral using a catalyst system which includes the known molybdenum compound molybdenyl acetylacetonate [also known as dioxomolybdenum (VI) acetylacetonate] or a molybdenyl halide.
The molybdenum compound of formula I, i.e., molybdenyl acetylacetonate (conventionally denoted as MoO
2
acac
2
) or a molybdenyl halide of the formula MoO
2
(Hal)
2
[X=Hal], wherein Hal signifies chlorine or bromine, is in each case a readily obtainable known compound. The molybdenyl halide is preferably molybdenyl chloride, MoO
2
Cl
2
. However, the preferred molybdenum compound of formula I is molybdenyl acetylacetonate.
The dialkyl or diaryl sulphoxide likewise present in the catalyst system is especially a dialkyl sulphoxide, the alkyl groups of which are each straight-chain or branched and contain up to 8 carbon atoms, or a diaryl sulphoxide, the aryl groups of which in each case are optionally substituted phenyl groups. In the latter case, the substituents which may be present are especially C
1-4
-alkyl groups, with the phenyl groups being in each case mono- or multiply-substituted by alkyl. Examples of both types of sulphoxides are dimethyl sulphoxide and dibutyl sulphoxide and, respectively, diphenyl sulphoxide and di(p-tolyl)sulphoxide. Dimethyl sulphoxide is preferably used as the sulphoxide.
As organic acids having a pK value in the range of about 4.0 to about 6.5 there come into consideration, inter alia, optionally halogenated, saturated and unsaturated aliphatic carboxylic acids, e.g. acetic acid (pK value 4.74), p

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