Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2001-09-07
2003-06-03
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C549S512000, C549S513000, C549S533000, C549S535000
Reexamination Certificate
active
06573394
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the catalytic activation of molecular oxygen for alkene epoxidation using transition metal substituted polyoxofluorometalates as catalysts. More specifically, the present invention relates to a process for the catalytic epoxidation of alkene and to novel transition metal substituted polyoxofluorometalates utilizable therein.
BACKGROUND ART
Epoxidation of alkenes is an important chemical transformation whereby an oxygen atom is added to a carbon-carbon double bond to form an epoxide. Epoxides are often utilized as intermediate compounds which can then be transformed to final products. Examples include but are certainly not limited to ethylene glycol and polyethylene glycol from ethylene oxide, propylene glycol from propylene oxide, phenylacetaldehyde from styrene oxide and propranolol from 2R-glycidol.
Epoxidation of alkenes can be carried out using numerous techniques. The oldest and probably most common method is to react the alkene with an organic peracid, equation (1).
Typical peracids used in the art include perbenzoic acid, peracetic acid, performic acid, perphthalic acid and substituted perbenzoic acids such as 3-chloroperbenzoic acid. The salts of such acids may also be effective oxidants as in the case of magnesium monoperoxophthalate. The acids may be used as pure compounds or as prepared in situ in the reaction mixture by for example adding hydrogen peroxide to acetic anhydride to form peracetic acid. Although processes based on the reaction as described in equation (1) are known, there are certain drawbacks that are associated with such reactions. Among these one may cite (a) the propensity for formation of by-products such as glycols and glycol esters by reaction of the epoxide with water and/or acid in the reaction medium, (b) the necessity of recovering and/or recycling the acid co-product and (c) the necessity for stringent reaction control because of the safety danger involved in use of organic peracids (acyl hydroperoxides).
In order to minimize the danger in using peroxides as oxidants the use of alkyl and aryl hydroperoxides in place of acyl hydroperoxides has been suggested. and applied. These oxidants do not normally react with alkenes and the addition of a catalyst is required as described in equation (2).
Some hydroperoxides commonly used in such reactions are tert-butylhydroperoxide, cumene hydroperoxide and ethylbenzene hydroperoxide. The catalysts used are most commonly based on compounds containing Ti(IV), V(V), Mo(VI) or W(VI) although many compounds based on other metals have been described as being effective catalysts. These reactions are safer because of the lower reactivity of alkyl and aryl hydroperoxides compared to organic peracids, however, the other disadvantages associated with the use of acyl hydroperoxides remain. Thus, reactions are not necessarily more selective for the presence of catalysts and often lead to additional side reactions, for example substitution and oxidation at the allylic carbon of the alkene instead of oxygen addition to the double bond. Similarly to the use of acyl hydroperoxides, the alcohol co-product must be recovered, recycled and/or otherwise utilized.
A further method to epoxidize alkenes is to use aqueous hydrogen peroxide as oxidant as shown in equation (3).
Such a reaction representss a conceptual improvement compared to the use of organic hydroperoxides in that the co-product is water and therefore is environmentally benign and need not be recovered or recycled. As in the use of alkyl- and aryl hydroperoxides the presence of a catalyst is necessary, which are again often compounds containing Ti(IV), V(V), Mo(VI) or W(VI) among others. In only certain cases has high selectivity been reported for alkene epoxidation. Some effective and selective catalysts include titanium silicalite-1 and other titanium substituted zeolites, and polyoxometalates such as [WZnMn
2
(ZnW
9
O
19
)
2
]
12−
and {PO
4
[WO(O
2
)
2
]
4
}
3−
. In many cases, the use of hydrogen peroxide represents an ideal oxidant provided reactions are selective. An exception is in cases where the low price of the epoxide makes the use of hydrogen peroxide prohibitively expensive.
An additional important method for synthesis of epoxides from alkenes is via formation of a halohydrin, preferably a chlorohydrin, using hypochlorous acid in the first step, followed by use of base, eg NaOH, for ring closure in the second step, as described in equation (4).
This is a very simple procedure which has, however, two problems. First, usually the presence of molecular chlorine in hypochlorous acid leads to formation of dichlorinated organics which are undesirable by-products and must be disposed of. Second, the process also forms large amounts of salts as co-product which also must be treated or recycled.
The ideal oxidant for alkene epoxidation both from an ecological and economic point of view would be molecular oxygen (dioxygen) as found in air. The addition of dioxygen to an alkene is disfavored kinetically, thus catalytic procedures need to be applied. In cases where there is no allylic carbon to the double bond, oxygen may be added to the double bond using a silver catalyst at elevated temperatures. In this way, ethylene oxide is manufactured from ethylene. For similar procedures with other alkenes, such as 1-butene, propene etc. this reaction fails to give epoxide in significant amounts. The basic problem in use of dioxygen for epoxidation of alkene lies in the radical nature of the molecular oxygen molecule. In homogeneous reactions, this radical nature always leads to a preferred radical reaction via substitution of hydrogen at an allylic carbon atom. Therefore, the common mode of utilization of dioxygen in liquid phase catalyzed reactions does not yield epoxide as major product. The situation in gas phase reactions is similar whereby activation of alkenes leads to allylic type carbocations, carbanions or carbon radicals again preventing formation of epoxides as a significant product.
Conceptually, in order to use dioxygen for alkene epoxidation, activation of dioxygen should be via formation of a high valent metal oxo compound formed after scission of the oxygen-oxygen bond. These high valent metal-oxo intermediates are effective epoxidizing agents. Most commonly this is carried out in nature by use of monooxygenase type enzyme such as cytochrome P-450 or methane monooxygenase. Such enzymes may be mimicked for example by using manganese and iron porphyrins as catalysts. The monooxygenase mechanism, however, requires two electrons from a reducing agent in order to cleave the oxygen-oxygen bond leading to formation of the high valent metal-oxo intermediate active in alkene epoxidation. From a process point of view the reducing agent becomes the limiting reagent instead of dioxygen and negates the attractivity of such a process.
The alternative is activation of dioxygen in a dioxygenase type mechanism. In such a reaction, dioxygen is cleaved using two metal centers leading to formation of two high valent metal-oxo species. This type of reaction has been until recently only realized using a ruthenium substituted tetramesitylporphyrin (RuTMP). Turnover rates to epoxide are very low and the catalyst has limited stability.
The limited stability of porphyrin ligands has led to the suggestion that transition metal substituted polyoxometalates be important alternative catalysts to metalloporphyrins as disclosed and discussed in Hill, U.S. Pat. No. 4,864,041. Transition metal substituted polyoxometalates are compounds of the general formula X
x
(TM)
y
M
m
O
z
q−
where the heteroatom, X, if present (x−0) may be main group or transition metals, the addenda atoms, M, are molybdenum, tungsten, niobium or vanadium or a combination thereof, and TM is one or several different transition metals. Several different structure types are known. These catalysts would retain the high activity of their metalloporphyrin counterparts, however, are significantly more thermally
Ben-Daniel Revital
Khenkin Alexander
Neumann Ronny
Morgan & Lewis & Bockius, LLP
Reyes Hector M
Rotman Alan L.
Yissum Research Development Company of Hebrew University of Jeru
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