Catalytic isomerization of Z-pentol to E-pentol

Details Catalytic isomerization of Z-pentol to E-pentol Catalytic isomerization of Z-pentol to E-pentol

2001-06-11

2002-10-15

Barts, Samuel (Department: 1621)

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

Organic compounds

Oxygen containing

Reexamination Certificate

active

06465698

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns a process for the catalytic isomerization of Z-3-methylpent-2-en-4-yn-1-ol to E-3-methylpent-2-en-4-yn-1-ol, hereinafter referred to for brevity as the isomerization of “Z-pentol” to “E-pentol.”
BACKGROUND OF THE INVENTION
The known acid-catalyzed allylic rearrangement of 3-methylpent-1-en-4-yn-3-ol affords in thermodynamic equilibrium an isomeric mixture of Z- and E-pentol in the ratio Z-:E-pentol of about 85:15. These stereoisomers can if desired be separated from each other by physical means, e.g. by fractional distillation, to afford each stereoisomer in relatively good purity. The stereoisomer produced and isolated in the larger proportion, i.e. Z-pentol, is a useful intermediate, e.g. for the manufacture of vitamin A, and the stereoisomer produced and isolated in the smaller proportion, i.e. E-pentol, is also a useful intermediate, in this case e.g. for the manufacture of astaxanthin, zeaxanthin and further carotenoids. The situation may be schematically illustrated as follows, whereby the formulae are presented by conventional line representation:
According to the relative requirement for one or the other stereoisomer depending on the relative amounts of the carotenoid and vitamin A end products to be produced therefrom, there exists an economical need to shift the above equilibrium of E- and Z-pentol from the thermodynamic one, and to influence the stereoisomeric ratio of these two useful intermediates. It is seldom economically feasible to separate the stereoisomers from a mixture in the thermodynamic equilibrium (about 85:15 Z-:E-pentol) as above. Indeed, since Z-pentol is the thermodynamically more stable pentol product, a shifting of the equilibrium in the direction Z-→E-entails an input of energy which would be justified if the relative requirement for astaxanthin, zeaxanthin and further carotenoids significantly exceeds about 15% of the total of both isomers. In this case, for example, there exists a need for a process for isomerizing a mixture of Z- and E-pentol, e.g. one in thermodynamic equilibrium with a Z-:E-ratio of about 85:15, to one with an increased proportion, i.e. higher than about 15%, of E-pentol.
SUMMARY OF THE INVENTION
This need has been surprisingly achieved by the catalytic isomerization process of the present invention which involves the use of bromine radicals (Br.) as the catalyst for isomerizing Z-pentol to E-pentol in a mixture of both these stereoisomers.
One embodiment of the invention is a process for catalytically isomerizing Z-3-methylpent-2-en-4-yn-1-ol to E-3-methylpent-2-en-4-yn-1-ol is provided. This process includes contacting a stereoisomeric mixture containing Z-3-methylpent-2-en-4-yn-1-ol and E-3-methylpent-2-en-4-yn-1-ol with a source of bromine radicals in a two-phase reaction mixture having an aqueous phase and a stereoisomeric mixture phase, intermixing the reaction mixture, and heating the reaction mixture to a temperature from about −10° C. to about 100° C.
DETAILED DESCRIPTION OF THE INVENTION
In principle any chemical system for generating the bromine radicals necessary for the performance of the catalytic isomerization process of the present invention may be utilized in the process, and each such chemical system gives rise to a particular embodiment of the process.
Common to all the chemical systems for generating bromine radicals is the actual source of bromine radicals, which is suitably an alkali metal or alkaline earth metal bromide, or ammonium bromide. As the alkali metal or alkaline earth metal bromide there comes into consideration particularly sodium or potassium bromide or, respectively, calcium or magnesium bromide. Preferably sodium bromide or potassium bromide is employed as the source of the bromine radicals.
The amount of such bromide salt employed relative to the amount of pentol starting material (mixture of Z- and E-pentols) is suitably about 0.2 mole to about 5 moles/mole, preferably about 0.2 mole to about 1 mole/mole, most preferably about 0.2 mole to about 0.5 mole/mole.
In one embodiment of the present invention, a salt of a heavy metal is used as the catalyst for promoting the generation of bromine radicals from the source thereof. Oxygen is generally used as an auxiliary agent for promoting the bromine radical generation. Examples of the heavy metals (cationic constituents) of these salts are titanium, vanadium, chromium, manganese, cobalt, nickel, zirconium, niobium, praseodymium, hafnium and lead. Examples of the anionic constituents of these salts are chloride, bromide, oxide, sulphate, oxychloride (OCl
2
4−
) and acetate. Specific examples of such heavy metal salts are titanous chloride (TiCl
3
), vanadium trichloride (VCl
3
), vanadium dioxide (V
2
O
4
), vanadium pentoxide (V
2
O
5
), chromic chloride (CrCl
3
), manganous bromide (MnBr
2
), manganese dioxide (MnO
2
), manganous sulphate (MnSO
4
), manganous acetate (Mn(OCOCH
3
)
2
), manganic acetate (Mn(OCOCH
3
)
3
), cobaltous bromide (CoBr
2
), nickelous bromide (NiBr
2
), zirconic oxychloride (ZrOCl
2
) niobium pentoxide (Nb
2
O
5
), praseodymium chloride (PrCl
3
), praseodymium oxide (Pr
6
O
11
), hafnium tetrachloride (HfCl
4
) and plumbous bromide (PbBr
2
). A heavy metal bromide is preferably used as the catalyst. Independently of the nature of the anion, manganese, especially manganous (Mn
2+
) salts, are preferably used as the catalysts.
The amount in moles of the heavy metal salt employed relative to the amount of pentol starting material (mixture of Z- and E-pentols) is suitably about 0.001 mole to about 0.5 mole/mole, preferably about 0.001 to about 0.3 mole/mole, most preferably about 0.01 to about 0.03 mole/mole.
As indicated above, oxygen is also generally used in the promotion of the bromine radical generation. As will be evident from the nature of the heavy metal salts, various oxidation potentials (levels) are represented by the metal ions in the salts, from as low as 2+, e.g. manganese(II) (manganous) in MnBr
2
, MnSO
4
and Mn(OCOCH
3
)
2
, to as high as 4+, e.g manganese(IV) in MnO
2
, or even 5+, e.g. vanadium(V) and niobium(V) in V
2
O
5
and Nb
2
O
5
, respectively. The function of the oxygen, if used, is to raise the oxidation level of the heavy metal cations to render them effective in generating the bromine radicals from the bromide anions present. Thus a relatively low concentration of heavy metal cations with a high oxidation level suffices to generate the bromine radicals. For example, Mn
2+
ions can be elevated to Mn
3+
ions with oxygen, and a relatively small amount of such Mn
3+
ions enables the bromine radicals to be generated. Indeed, if heavy metal cations of a sufficiently high oxidation level are present at the outset, the presence of oxygen may be omitted. As a further example, Mn
2+
ions are not able to generate bromine radicals from the bromide in the absence of oxygen, but Mn
3+
ions can do this.
In those cases where oxygen is used as an auxiliary agent for the bromine radical generation, it can be used alone or in admixture with an inert gaseous component, e.g. with nitrogen in air. The oxygen gas or gas mixture, preferably containing at least 5 vol. % of oxygen, may be continuously passed through the two-phase reaction medium during the isomerization process. The rate of oxygen or oxygen mixture passage is about 5 l/h to about 200 l/h, preferably about 20 l/h to about 50 l/h. The technical means of oxygen passage is unimportant to the present process and may be achieved using conventional technical methodology, such as with a stirrer having jet outlets through which the oxygen is passed and released continuously into the stirred reaction medium. The oxygen gas or gas mixture can be used under pressure, suitably at a pressure up to a maximum of about 50 bar (5 MPa), which serves to accelerate the isomerization.
In the process of the present invention the mixture of pentol stereoisomers may form the pentol phase, or the stereoisomers may be dissolved in an essentially water-

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