Direct synthesis of organorhenium oxides from compounds...

Details Direct synthesis of organorhenium oxides from compounds... Direct synthesis of organorhenium oxides from compounds...

2000-02-10

2001-01-30

Nazario-Gonzalez, Porfirio (Department: 1621)

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

Organic compounds

Heavy metal containing

Reexamination Certificate

active

06180807

ABSTRACT:

The present invention comprises a novel process for preparing organorhenium oxides from rhenium-containing compounds, in particular from perrhenates, and the catalyst recycling, which is made possible for the first time, on use of organorhenium oxides in catalytic processes.
The first report on the parent compound of the organorhenum oxides, methyltrioxorhenium, was that in 1979 by I. A. Beattie et al. (I. A. Beattie, P. J. Jones,
Inorg. Chem.,
18 (1979) 2318). It is produced in up to 50% yield by oxidation of tetramethylrhenium(VI) oxide (CH
3
)
4
ReO or decomposition of trimethyldioxorhenium(VII) (CH
3
)
3
ReO
2
, the starting compounds being exposed to dry air for some weeks in order to effect the oxidative conversion.
This is no longer a significant access route because the precursors are difficult to obtain and the synthesis is time-consuming. Instead, two alternative routes for synthesizing organorhenium(VII) oxides are currently in use and are essentially derived from the studies by J. G. Kuchler (Thesis, Munich TU) in 1987. Both processes are based on commercially available rhenium heptoxid Re
2
O
7
as rhenium source. Re
2
O
7
is highly moisture-sensitive so that the reactions must be carried out under protective gas and with dry solvents.
Direct alkylation (arylation) of rhenium heptoxide Re
2
O
7
with nonreducing transfer reagents such as tetraalkyltin SnR
4
or dialkylzinc ZnR
2
leads, in smooth reactions, to the corresponding organorhenium oxides. The disadvantage of this method is that half of the rhenium results as polymeric trialkyl stannyl perrhenate or as zinc perrhenate. This means that the maximum yield which can theoretically be achieved is only 50% (based on rhenium). The yield actually achieved then is about 45%.
If the alkylation is carried out with mixed esters of perrhenic acid and carboxylic acids, these rhenium-containing byproducts can be avoided (W. A. Herrmann et al.,
Inorg. Chem.,
31 (1992) 4431). In this so-called anhydride route, rhenium heptoxide are reacted successively with carboxylic anhydrides (preferably trifluoroacetic anhydride) and tri (n-butyl)tin compounds. The yields in this case are 80-90%, although—if high purity is required—complete removal of the resulting tri(n-butyl)tin carboxylic anhydrides from the formed MTO is time-consuming and complicated. The described reaction is confined to less reactive tin compounds because the use of more reactive tin compounds gives only unsatisfactory results. Its synthetic scope is therefore very limited. In addition, it requires the use of relatively costly chemicals.
It was an object of the present invention to provide a novel process with which it is possible to obtain organorhenium oxides in a simple and low-cost manner in good yields.
This object is achieved by a process in which a rhenium-containing compound, where the rhenium-containing compound is perrhenate, is reacted with a silylating agent and an organylating agent to give the corresponding organorhenium oxide.
In a preferred embodiment, the invention relates to a process for preparing compounds of the formula (I)
R
a
Re
b
O
c
L
d
  (I)
where
a=an integer from 1 to 6
b=an integer from 1 to 4
c=an integer from 1 to 12
d=an integer from 0 to 4
L=a Lewis base ligand and the total of a, b and c is such as to comply with the penta- to heptavalency of rhenium, with the proviso that c is not greater than 3×b and in which R is identical or different, and is an aliphatic hydrocarbon radical having 1 to 40 and preferably from 1 to 20 carbon atoms, an aromatic hydrocarbon radical having 6 to 40 and preferably from 6 to 20 atoms or arylalkyl radical having 7 to 40 and preferably from 7 to 20 atoms, where the radical R can, where appropriate, be substituted identically or differently, independently of one another.
Examples of substituents are halogen, hydroxyl, alkoxy, aryloxy, alkylamino and arylamino, alkylphosphine, arylphosphine, alkylsulfonyl, arylsulfonyl, alkyl- and aryl(sulfonyl)phosphine.
The novel direct synthesis of organorhenium oxides from rhenium-containing compounds, especially perrhenates such as MReO
4
, M(ReO
4
)
2
, dispenses with the disadvantages mentioned at the outset. Thus, methyltrioxorhenium (MTO) and other moisture-stable organorhenium oxides can be synthesized in good yields without the exclusion of air or moisture (see Table 1).
Suitable rhenium-containing compounds are, in particular, perrhenates such as, for example, MReO
4
and M(ReO
4
)
2
. M can be any suitable counterion. For example, M can be an ion from the group of alkali metal, alkaline earth metal or B group elements. Examples which may be mentioned are AgReO
4
, KReO
4
, NaReO
4
, Zn(ReO
4
)
2
, Ca(ReO
4
)
2
, (CH
3
)
3
SnReO
4
or else NH
4
ReO
4
.
In another embodiment of the invention, the rhenium-containing compound originates from the catalyst residue from spent organorhenium oxide catalysts or from Re-containing solutions in general.
The reaction takes place in a one-pot reaction in organic solvents, in particular donor solvents (for example THF, acetonitrile).
The dissolved or suspended rhenium-containing compound is reacted with a silylating agent and an organylating agent.
In principle, any silylating agent which results in an appropriate reaction as shown in scheme 1 is suitable. A preferred silylating agent is chlorotrimethylsilane because it is easily obtainable. It is also possible to use other chloroalkylsilanes such as chloro-tert-butyidimethylsilane etc. The amount of silylating agent should in general be at least equimolar relative to the rhenium-containing compound. However, an excess is preferred, and an amount corresponding to 2 to 2.5 equivalents is particularly advantageous.
It is also possible to select the organylating reagent and its amount as desired as long as an appropriate reaction as shown in Scheme 1 is ensured. Thus, the use of any organylating reagent known for such purposes is conceivable. Suitable examples of organylating reagents are the organometallic compounds of tin, zinc, aluminum, magnesium, lithium, copper, cadmium and mercury, and tin or zinc alkyls or aryls are preferred.
The amount of organylating reagent should, in a manner corresponding to the silylating agent, be approximately equimolar to the amount of rhenium-containing compound employed, an excess possibly being advantageous. The organylating reagent is preferably employed in an amount which corresponds to 1 to 1.5 equivalents based on the rhenium-containing compound.
If required, however, the organylating reagent can, like the silylating agent, be employed in smaller or larger amounts.
To react perrhenates of low solubility, such as potassium perrhenate KReO
4
or ammonium perrhenate NH
4
ReO
4
, it is advisable to add up to one equivalent of acid (usually sulfuric acid) in order to increase the solubility of the perrhenate, it also being possible for the amount of added acid to be more if required.
The use of zinc alkylating agents is also possible by in situ reaction of ZnCl
2
with the appropriate organolithium compound LiR or the appropriate Grignard compound RMgX (X=halide).
This novel synthetic strategy represents a low-cost and entirely unproblematical way (no inert gas atmosphere, no anhydrous solvents) of obtaining organorhenium oxides from a large number of starting materials.
As already mentioned, it is not in principle necessary according to the invention to ensure that the solvents used are anhydrous. However, it may be advantageous to use the usual laboratory dry solvents. Thus, it has emerged that the yield can be increased further in this way.
The reaction temperature for the process according to the invention can be chosen in a wide range from −100 to +110° C., in particular −78° C. to +80° C. However, the reaction normally takes place under moderate conditions, i.e. from room temperature to refluxing.
It is assumed that the reaction takes place via the following intermediates (scheme 1, example of methyltrioxorhenium). The following scheme serves mer

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