Wilkes’ catalyst

Figure 1 Hydrogen uptake curves for 1-hexene hydrogenations run at 35°C and 50 psig of hydrogen in 10% toluene/EtOH with a stirring rate of 1700 rpm. a) AHC-Wilk catalyst b) Homogeneous Wilkinson s catalyst.

The chiral hydrovinylation catalyst could be effectively immobilized for continuous operation in active and selective form by simply dissolving the Wilke catalyst in [EMIM][BTA] (BTA= (CF3S03)2N) and using compressed C02 as the mobile phase (Eq. 3). High conversion and good enantioselectivity were obtained over... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

At first, the reaction was investigated in batch mode, by use of different ionic liquids with wealdy coordinating anions as the catalyst medium and compressed CO2 as simultaneous extraction solvent. These experiments revealed that the activation of Wilke s catalyst by the ionic liquid medium was clearly highly dependent on the nature of the ionic liquid s anion. Comparison of the results in different ionic liquids with [EMIM] as the common cation showed that the catalyst s activity drops in the order [BARF] > [Al OC(CF3)2Ph 4] > [(CF3S02)2N] > [BFJ . This trend is consistent with the estimated nucleophilicity/coordination strength of the anions. 

Interestingly, the specific environment of the ionic solvent system appears to activate the chiral Ni-catalyst beyond a simple anion-exchange reaction. This becomes obvious from the fact that even the addition of a 100-fold excess of Fi[(CF3S02)2N] or Na[BF4] in pure, compressed CO2 produced an at best moderate activation of Wilke s complex in comparison to the reaction in ionic liquids with the corresponding counter-ion (e.g., 24.4 % styrene conversion with 100-fold excess of Fi[(CF3S02)2N], in comparison to 69.9 % conversion in [EMIM][(CF3S02)2N] under otherwise identical conditions). 

Figure 5.4-3 Lifetime study of Wilke s catalyst in the hydrovinylation of styrene, activated and...

Figure 5.4-3 shows the results of a lifetime study for Wilke s catalyst dissolved, activated, and immobilized in the [EMIM][(CF3S02)2N]/compressed CO2 system. Over a period of more than 61 h, the active catalyst showed remarkably stable activity while the enantioselectivity dropped only slightly. These results clearly indicate - at least for the hydrovinylation of styrene with Wilke s catalyst - that an ionic liquid catalyst solution can show excellent catalytic performance in continuous product extraction with compressed CO2. 

A portion of the Wilk/STA catalyst was re-used for the hydrogenation of successive 10,000 TON batches of 1-hexene with the TOP remaining constant for each run. A separate portion of this catalyst was also used for the 100,000 TON hydrogenation of 1-hexene with a TOP of 4800 hr. In each of these reactions the amount of Rh in the reaction mixture was below the level of detection, less than 1 ppm. 

Another interesting comparison of the homogeneous Wilkinson catalyst with AHC-Wilk is in the high TON hydrogenation of cyclohexene in 10%... 

The AHC Wilkinson catalyst (Rh(Ph3P)3/PTA/Al203) (AHC-Wilk) was prepared using the general procedure described previously (10,12). The catalyst contained 0.5% Rh which corresponds to a 1 1 Rh PTA ratio and about a 4.5% load of the anchored complex. The hydrogenations were ran using the low pressure apparatus previously described (19) under the conditions listed in the discussion. 

Wilke s allyl compounds were found to be very poor catalysts indeed, e.g., Ti(2 Me-allyl)4, was found only to have an activity equal to 0.5 gm/m.M Ti/atm/C2H4/hr. For this reason there has been considerable dispute that transition metal alkyls can be the intermediates in Ziegler polymerization. 

A further example of a reaction which may be optimised in IL/scC02 by selection of the appropriate anion for the IL is catalytic enantioselective hydrovinylation, a synthetically interesting and truly atom economic C-C bond forming reaction [77-79]. The nickel complex below has been developed by Wilke and co-workers as precursor for a highly active and enantioselective catalyst for this process. 

The catalytic cyclo-oligomerization of 1,3-butadiene was first reported by Reed in 1954 using modified Reppe catalysts.4 Wilke et al., however, demonstrated in pioneering, comprehensive and systematic mechanistic investigations, the implications, versatility and the scope of the nickel-catalyzed 1,3-diene cyclo-oligomerization reactions.3,5... 

It should be noted here again that the catalytic reaction does not involve a change of valence of the metal. In general, catalytic olefin addition reactions that involve a hydride transfer do not require change of valence in the metal catalyst. On the other hand, carbon-carbon bond formation by coupling reactions which involve electron shifts, such as in Wilke s Ni°-catalyzed butadiene oligomerization reaction [Eq. (1)], requires a valence change on the metal. 

Very recently, Leitner has reported cationic nickel(n) catalyst systems for the cycloisomerization of diethyl diallyl malonate that shows high activities and regioselectivities for the formation of diethyl 3-methylene-4-methylcyclo-pentane-l,l-dicarboxylate with ee values up to 80% when using Wilke s azaphospholene ligand.355... 

Schemes Hydrovinylation of styrene with Wilke s catalyst 13 in an IL/SCCO2 biphasic system...

Wilke G (1995). In Fink G, Miilhaupt R, Brintzinger HH (eds) Ziegler Catalysts. Springer-Verlag, Berlin, p 3... 

The increased cooling efficiency of thin-walled reactors also has permitted the use of more volatile substrates in near molar quantities. (l-3 6-7 10-12-rj 2,6,10-Dodecatriene-l,12-diyl)nickel has been prepared in multiple gram quantities by cocondensation of nickel vapor and 1,3-butadiene. This method has provided a clean one-step route to this complex, which was first isolated and identified by Wilke et al.1 as an intermediate in the cyclotrimerization of 1,3-butadiene by nickel catalysts. 

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