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Fluorous Biphase SystemsЧThe New Phase-Separation and Immobilization Technique.

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HIGHLIGHTS
Fluorous Biphase Systems-The New Phase-Separation
and Immobilization Technique
Boy Cornils”
Something new IS afoot in chemistry: FBS (fluorous biphase
systems) is a new phase-separation and immobilization technique which makes use of the restricted and thermally controllable solubility and miscibility of perfluorinated hydrocarbons
(or derivatives such as ethers or tertiary amines) and organic
liquids. The method was first used in reactions with FBS-compatible, homogeneous catalysts,[’. and goes back to the unpublished thesis of M. Vogt,[ll who in 1991 attempted to utilize
the solvophobic properties of perfluorinated ethers of the
Hostinert 216 type (structure 1) and homogeneous catalysts dissolved therein for phase separation after successful homogeneous catalysis.
The actual “inventive” idea was to utilize “. . .the high chemical inertness of such perfluorinated compounds. . . ” [ l l to create
a medium for a variety of homogeneously catalyzed reactions,
and to modify the homogeneous catalysts needed by introducing perfluorinated groups into their ligand sphere in order to
make them soluble in the perfluorinated ethers. At elevated
temperatures the biphase system, which contains the corresponding modified complex catalysts and the organic reactants,
forms a single phase and thus provides the best conditions for
the desired reaction. Since the reaction system is again biphasic
upon subsequent cooling, organic reaction products and the
catalysts dissolved in the FBS can be readily separated (“thermal regulation”, see Figure 2). As with the 0x0 process of
Ruhrchemie/Rh&ie- P ~ u l e n c [ ~or
] the SHOP process of
Shell,[*]this would have made it possible to convert the homogeneous catalyst into a “heterogenized” form and to immobilize it
in an extremely el-fective and elegant way by means of simple
phase separation without having to anchor the catalyst on solid
supports (as has also been described for perfluoroalkyl-substituted supportsi5])
Following the old alchemist’s rule “similia similibus solvuntur”, which he cites, Vogt synthesized ligands based on hexafluoropropene oxide oligomers that gave transition metal complexes of Co, Ni, or Mn (structure 2), which were active as
I*)Prof. Dr. B. Cornils
Hoechst AG
D-65926 Frankfurt a m Main (Germany)
Fax: Int. code +(6‘))305-83128
Angen Chem In/ Ed End 1997,36, N o 19
0
CF3
F il?-CF2-O{AF-C’(
CH2
Ni )CH
/\
\
0
2
CH2
homogeneous catalysts and could be employed in the oxidation
of cyclohexene, the polymerization and telomerization of butadiene, and the oligomerization of ethylene.
German theses which, as in the case of Vogt, do not lead to
publications do not become internationally known. As a result,
the phase properties of amphiphilic, perfluorinated species for
solvophobic perfluorinated head groups and solvophilic hydrocarbon tail groups (“bilayer assemblies”)[61and the separation
behavior of high-boiling, reactant-immiscible Fluorinert solvents[’’ for etherifications, alkylations, and other reactions were
described later, but clearly independent of Vogt. The same applies to work by Horvith et al. since 1994,[2~8-’0]
which has the
following focal points:
-
-
-
Using perfluorinated solvents in FBS or FMS (fluorous multiphase systems)
Solubilizing catalysts for homogeneously catalyzed reactions
by replacing customary ligands with partially fluorinated or
perff uorinated ligands
Carrying out selected reactions, for instance hydroformylation or hydroboration,‘”] as biphasic reactions catalyzed by
immobilized homogeneous catalysts.[31
Quite obviously independent of Vogt, Horvath and Rabai as
well as GIadysz et aI. described the genesis of the FBS methodology very logically and with virtually the same words for
the hydroformylation of n-decene to give undecanals [Eq. (l)].
H3C- ( C H Z )~ CH=CH2CO+H2cat.
CH3(CH2)9CHO + CH3(CH&CIt CHO
I
CH3
Their search for two-phase systems other than water/organic
liquids-which provides the foundation for the industrially important aqueous, two-phase 0x0 process[*]-began with studies
of solubility diagrams and the influence of intermolecular dissolution forces, as shown in Figure 1.[lZ1
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HIGHLIGHTS
water
n-heptane
carbon tetrachloride
methyl ethyl ketone
(butanone)
acetic acid
ethyl ester
Figure 1. Miscibility diagram [12a]. Solvents not connected by a binding line are
immiscible; solvents of unlimited miscibility are connected by a solid line, those of
limited miscibility by a dashed line, and those of low miscibility with a dotted line.
As in the case of Vogt, the substances chosen were perfluorinated hydrocarbons which are largely immiscible with organic
liquids at room temperature (and thus also with the preferably
oxygen-containing reaction products of customary organic syntheses). In addition, as already emphasized by Zhu,I7] they are
stable to hydrolysis but nevertheless have a measurable solubilizing capability for nonpolar reactants (for example the feed
olefins of hydroformylations), especially at elevated temperatures.
Taking the two-phase hydrof0rmylation[~1as a model, the
auxiliary solvent in the reaction (in this case water) should be
able to dissolve the complex catalyst completely in order to
retain it after phase separation and removal of the reaction
products (aldehydes), and to make them immediately available
for a further catalysis cycle. Therefore, as in the case of Vogt, the
ligand-modified rhodium carbonyl hydrides as 0x0 catalysts
had to be made FBS-compatible by introducing alkylene perfluoroalkyl phosphanes. Here the tailor-made “ponytail” ligandsl’] are ascribed particular electronic properties which, as a
result of the thermal and leaching stability of the central atom
Rh, make them more suitable than Vogt’s ligands. In the hydroformylation reaction, complex catalysts of type 3 modified with
these ligands display satisfactory activities in the two-phase system perfluoromethylcyclohexane/toluene and an n : i ratio of
about 3, which is astonishing for the use of higher olefins.[’. l o ]
of the epoxide) and of sulfur compounds (for example, diphenyl
sulfide to sulfone) as well as extractions.
The FBS method of immobilizing homogeneous catalysts is
also used for Rh-cataIyzed hydroboration‘’ (in yields of up to
95 %) and for oxidation reactions. The good solubility of oxygen in the FBS auxiliary liquids, which was earlier an argument
for proposing that they be used as synthetic blood substitutes,“ 31 indicates that they may be particularly suitable for
oxidation reactions.[”* Pozzi et al.[’41 used Co complexes of
perfluorohydrocarbon-soluble tetraalkylporphyrins in substrate:catalyst ratios of up to 1OOO:l for the epoxidation of
alkenes (albeit in the presence of considerable amounts of aldehydes, preferably isobutyraldehyde, as reducing agents). For
cycloolefins yields of up to 100% are reported; for I-dodecene
the yield is still 60%. As desired, the cobalt-porphyrin complex
is present in the FBS phase after the reaction and can be recycled. Knochel et al.[l5]described the oxidation of aldehydes to
carboxylic acids and of sulfides to sulfoxides and sulfones using
the Ni complexes as well as the epoxidation of carbocyclic
olefins (likewise in the presence of aliphatic aldehydes) using the
Ru complexes of perfluorinated diketones of structure 4.
K@
Also in these reactions, use is made of perfluorinated hydrocarbons as the FBS solvent; above 60 “C the reaction solution is
a single phase in which the desired reaction proceeds without
mass-transfer limitation. Upon cooling, the solution reverts to a
two-phase system with the catalysts remaining in the FBS phase
(Figure 2). According to Knochel et al.[15]the catalyst under-
/L
Figure 2. Principle of the FBS oxidation; E = starting material, P = product.
The FBS complex catalysts have the general formula
[M,(L(R),(R,),),]; they contain not only a hydrocarbon group
R but also a fluorine-containing part R,, which is similar to the
requirements of Kunitake et a1.[6J for particular two-phase
structures. Incorporation of CH, groups between the P atom of
the phosphanes and the perfluorinated R, groups (the ponytails)[’, 91 is, according to the authors, necessary for matching
reactivity and compatibility for electronic reasons.[81The corresponding
claims catalyzed and stoichiometric reactions in addition to hydrofonnylations; however, the only other
reactions described are oxidations of cyclohexene (to give traces
2058
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
goes no leaching and displays only little ageing. The extent to
which transition metal/oxygen complexes, as first described by
Horvlth and Gladysz et a1.[’61 for the partially fluorinated
Vaska complex 5, play a role in the oxidation is still unknown.
The importance of FBS for the quick separation of homogeneous catalyst and reaction product is so evident that a number
of other studies have attempted to utilize the phase-separation
effect even for reaction systems which do not contain transition
metal catalysts. For example, Schultz et al.“ 71 proposed carry-
0570-0833/97/3619-2058$17.50+.50/0
Angew. Chem. Int. Ed. Engl. 1997,36, No. 19
HIGHLIGHTS
ing out the photoinduced singlet oxidation of cyclohexenes in
this way [Eq. (2)].
OH H
sensitizer
J
J
,
,
C6H13
CH3
02,hv
c6H13&
OOH
Exchanging the sensitizer tetraphenylporphyrin for
5,10,15,20-tetrakis(,heptafluoropropyl)porphyrinin an FBS
medium produced the desired effects: increase in the chemical
stability of the sensitizer towards oxidative degradation and
separation of the reaction solution from the sensitizer solution,
which prevents further decomposition. The increase in yield and
the reduction in decomposition of the sensitizer are considerable. Curran et a1.!la1 used FBS systems to recover and purify
liquids (typical FBS applications) in specific reactions of organic
chemistry, for example, nitrile oxidations/cycloadditions, Grignard and Ugi reactions, and the Bignelli condensation.
In the wider sense catalysis in supercritical CO, (scC0,) is
also a FBS reaction, although the ability to adjust the dissolution and phase-separation behavior of scC0, by changing the
pressure and temperature provides the actual impetus for their
use in two-phase processes. The FBS aspect becomes involved
because, as Leitner et al.“ showed, the customary arylphosphane ligands which are important for homogeneous catalysis
can only be sufficiently dissolved in scC0, if alkenylperfluoroalkyl chains are incorporated in their periphery. The catalytic
properties of the parent phosphanes, and also those of their
chiral derivatives, are retained (see Noyori et al.,[zolwho proposed the external addition of perfluoroalkyl alcohols). In the
method of Leitner et al. (see 3 and Figure 3) the solubilizers are,
as in the case of Horvath et al., fixed directly to the aryl radical
and thus part of the ligand. The hydroformylation of I-octene
demonstrates the effectiveness of this technique: Comparatively
high conversions and n : i ratios are obtained in the homogeneous phase in scC0,.
scco,
scco,
Figure 3. Catalysts with fluorine-containing ligands for use in scC0, according to
Leitner et al. [19]. The “tails” on the phenyl rings represent m-(CH,),(CF,),F.
Angew. Chem. Int. Ed. Engl. 1997.36, No. 19
The FBS method will be secured a place in preparative organic chemistry and homogeneous catalysis in the future because its
thermoselective phase behavior offers an opportunity to carry
out reactions in one phase and thus homogeneously (and without serious mass transfer problems). Furthermore after cooling
the mixture to separate the reaction products and by-products
(and also decomposition products), the catalyst can be recycled
by simple decantation. The question of the industrial importance of the FBS method is more difficult to answer. Some
things do stand in the way of its wider use in industrial homogeneous catalysis: The complex catalysts are tailor-made and, like
Savile Row suits, expensive; it is not so much the price of the
catalytically active noble metals but rather the cost of the ligands and their recovery which becomes important. Furthermore, since they are inert fluorinated materials, ligands and FBS
solvents can very easily raise questions about their presence in
the atmosphere (ozone-depletion or greenhouse-warming potential of fluorine compounds) and the fate of even traces during
any further processing steps (for example the hydrogenation of
aldehydes to alcohols). Nevertheless, the properties of this particular type of two-phase catalysis leave no doubt that this is a
spectacular but also important new development.
German version: Angew. Chem. 1997,109,2147-2149
Keywords: fluorocarbons . homogeneous catalysis
lization . supercritical fluids
- immobi-
[I] M. Vogt, Dissertation, Technische Hochschule Aachen, August 26, 1991.
[2] I. T. Horvath, oral presentation on August 31, 1994 at the NATO Advanced
Research Workshop on Aqueous Organometallic Chemistry and Catalysis in
Debrecen, Hungary; see Aqueous Organometallic Chemistry and Catalysis
(Eds.: I. T. Horvath, F. Joo), Kluwer, Dordecht, 1995.
[3] B. Cornils, E. Wiebus, CHEMTECH 1995, 25, 33.
[4] W. Keim, Chem. I g . Tech. 1984, 56, 850.
[5] T. Umemoto, Tetrahedron Lett. 1984, 25, 81.
[6] H. Kuwahara, M. Hamada, Y Ishikawa, T. Kunitake, J. Am. Chem. Sac. 1993,
l f 5 , 3002.
[7] D.-W. Zhu, Synthesis 1993, 953
[8] I. T. Horvath, J. Rabai, Science 1994, 266, 7 2 .
[9] J. A. Gladysz, Science 1994, 266, 5 5 .
[lo] Exxon Research and Engineering Comp. (I. T. Horvath, J. Rabai), US
5.463.082 (1995)
1111 J. J. J. Juliette, I. T. Horvath, J. A. Gladysz, Angew. Chem. 1997, 109, 1682;
Angew. Chem. Int. Ed. Engl. 1997, 36, 1610.
1121 a) G. Duve, 0.Fuchs, H. Overbeck, LbsemiftelHoechst,6th ed., p. 49, Hoechst AG, Frankfurt am Main, 1976; b) J. H. Hildebrand, D. R. F. Cochran,
J. Am. Chem. Sac. 1949, 71, 22.
[13] See ref. [l] in [15].
[14] G. Pozzi, F. Montanan, S . Quici, Chem. Commun. 1997. 69.
[lS] 1. Klement, H. Liitjens, P. Knochel, Angew. Chem. 1997, 109, 1605; Angew.
Chem. Int. Ed. Engl. 1997, 36, 1454.
[16] M.-A. Guillevec, A. M. Arif, I. T. Horvith, J. A. Gladysz, Angew. Chem. 1997,
109, 1685; Angew. Chem. Int. Ed. Engl. 1997,36, 1612.
[17] S . G . DiMagno, P. H. Dussault, J. A. Schultz, J. Am. Chem. Sac. 1996, 118,
5312.
[18] A. Studer, S . Hadida, R. Ferritto, S.-Y. Kim, P. Jeger, P. Wipf, D. P. Curran,
Science 1997, 275, 823.
[19] W. Baumann, S . Kainz, D. Koch, W. Leitner, Angew. Chem 1997, 109, 1699;
Angew. Chem. Int. Ed. Engl. 1997,36, 1628.
1201 J. Xiao, S. C. A. Neflcens, P. G . Jessop, T. Ikariya, R. Noyori, Tetrahedron Lett.
1996, 37, 2813.
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