The Active Palladium Catalyst

In 1991, Mandai et al. reported that the palladium-catalyzed reaction of propargyl carbonates with olefins proceeded smoothly in DMF at 70 °C in the presence of triethylamine and potassium bromide to give vinylallenes in good yields [54], The active palladium catalyst was generated in situ from Pd(OAc)2 and PPh3. A typical example is shown in Scheme 3.19. 

The influence of high pressure on the Heck reactions of selected alkenyl and aryl halides, respectively, i.e., 1-iodocyclohex-l-ene, iodobenzene, bromobenzene, with methyl acrylate has been investigated and the activation parameters of these reactions determined [142], Two different catalyst cocktails were used in this study, the classical system (Pd(OAc)2, NEtg, PPhg) and the one reported by Herrmann, Beller and others [16] (la). The temperature-dependent and the pressure-dependent rate coefficients both follow the order PhI/Pd(OAc)2 > 1-iodocyclohexene/Pd(OAc)2 > Phl/la > PhBr/la and the activation enthalpies as well as the activation entropies exhibit the trend 1-iodocyclohexene/Pd(OA)2 < Phl/Pd(OAc)2 < Phl/la < PhBr/la. The absolute values of the activation volumes, which were ascertained from the pressure-dependent rate coefficients, increase as follows l-iodocyclohexene/Pd(OAc)2 < PhI/Pd(OAc)2 Phl/la < PhBr/la. Under high pressure, the lifetime of the active palladium catalyst and thereby the turnover numbers are greatly enhanced [88]. 

Macor also exploited the Mori-Ban indole synthesis to synthesize several anti-migraine analogues of sumatriptan and homo-tiyptamines as potent and selective serotonin reuptake inhibitors (SSRIs). Noticeably, the presence of the second bromine (the bromine passenger ) on the substrate was not significantly deleterious to the reaction although a small amount of the 7-bromoindole might be sacrificed at the end of the reaction to consume the active palladium catalyst. The approach to 7-bromoindole could provide a general method to access 7-bromoindoles (a rare class of indole derivatives), which then could be further manipulated for the synthesis of more complex 7-substituted indoles. 

Mechanism. The insertion of acetylene into a palladium-halogen bond occurs as the first step and subsequently allyl halide inserts into a palladium-vinyl bond. The /3-elimination of PdXj gives a codimer and regenerates the active palladium catalyst. In the case of unsubstituted acetylene, the cotrimer is formed by the successive insertion of acetylene and allyl halide into the palladium-vinyl bond (Scheme 10). 

Despite the obvious qualities of the Mizoroki-Heck reaction, some of the disadvantages of this procedure are that the active palladium catalysts used require stabilization with phosphanes, which are generally sensitive to oxidation, thus necessitating the use of inert atmospheric conditions, and high temperatures are normally required, leading to side reactions and catalyst deactivation. 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

Hydrogenation of cinnamaldehyde has been studied extensively since selectivity has often been an issue. Under mild conditions the carbonyl group is reduced giving cinnamyl alcohol, whereas at elevated temperatures complete reduction to 3-phenylpropanol [122-97 ] results. It is possible to saturate the double bond without concomitant reduction of the carbonyl group through selective hydrogenation with a ferrous chloride-activated palladium catalyst (30), thereby producing 3-phenylpropanol [104-53-0]. 

In the preceding section, it has been shown that considerable attention has been devoted to palladium as a heterogeneous catalyst. The present section describes the homogeneous palladium catalysts developed for hydrogenation of NBR. The main drive behind the development of various catalyst systems is to find suitable substituents of the Rh catalyst. Palladium complexes are much cheaper as compared with Rh and exhibit comparable activity and selectivity to Rh and Ru complexes. 

The most active palladium catalyst system developed for the asymmetric hydrosilylation of cyclopentadiene (Scheme 23) involves the use of the (/ )-MOP-phen ligand (38), which shows significant enhancement of enantioselectivity compared to (R)-MeO-MOP (80% ee from (38), 39% ee from (36a)).114 Other phosphine ligands that afford active palladium catalysts for the same transformation include the /3-7V-sulfonylaminoalkylphosphine (39) and phosphetane ligand (40) (Equation (13)).115-117 A comparison of the enantioselectivities of these ligands for the palladium-catalyzed hydrosilylation of cyclopentadiene is given in Table 8. 

Pd(dba)2 [palladium(O)] generally affords the best results and thus an oxidation to the metal center must occur. The most likely mechanism for this to occur is by net oxidative addition of the acidic phosphonium P-H moiety (Scheme 3). This hypothesis is supported by the observation that the pKa of the phosphonium-hydro-gen bond directly affects the activity of catalysts generated in situ with more basic ligands being inactive. 

The utility of a palladium catalyst in the synthesis of substituted aryl acetylenes is well established.(7,8,9,10) The end-capping agent I was produced by using a standard catalyst system, dichlorobls(triphenylphosphlne)palladlum (II)/copper (I) iodide/triphenylphosphlne mixture, which has been employed in previously developed ethynylation procedures.(10) The copper (I) iodide is believed to act as a cocatalyst, reducing the palladium (II) complex to the active palladium (0) catalyst. The scheme is shown in Figure 3 (diethylamine is the solvent).(11)... 

Okubo et al. (260) reported that Pd(II)/Si02 was a more effective catalyst than Pd/C when two equivalents of triethylamine base were added to the same ionic liquid one equivalent was not sufficient. The reaction was carried out without phosphine ligands. The unreduced Pd(II)/Si02 catalyst with two equivalents of base in [BMIM]PF6 was more active than the supported palladium catalysts in DMF. Furthermore, the stability of [BMIM]PF6 also improved with the addition of triethylamine. 

Pfefferle and Lyubovsky executed types of measurements that yielded critical information between active Pd phases for catalytic combustion using pure ot-alumina plates with zero porosity as a support for the catalyst. This procedure uniformly covers the plate with metal particles on the top surface where they are easily available for the reaction gases and optical analysis. This type of experimental procedure has shown that in high-temperature methane oxidation the reduced form of the supported palladium catalyst is more active than the oxidized form. The temperature at which the PdO Pd... 

A similar approach was taken for the synthesis of 45 by Miyaura. " Shaughnessy and Booth synthesized the water-soluble alkylphosphine 46, and found it to provide very active palladium catalysts for the reaction of aryl bromides or chlorides with boronic acids. The more sterically demanding ligand 47 was shown to promote the reactions of aryl chlorides with better results than 46. Najera and co-workers recently reported on the synthesis of di(2-pyridyl)-methylamine-palladium dichloride complexes 48a and 48b, and their use in the coupling of a variety of electrophiles (aryl bromides or chlorides, allyl chlorides, acetates or carbonates) with alkyl- or arylboronic acids very low catalyst loadings at Palladium-oxime catalysts 8a and 8b) have also been developed. In conjunction with... 

Initial studies showed that the encapsulated palladium catalyst based on the assembly outperformed its non-encapsulated analogue by far in the Heck coupling of iodobenzene with styrene [7]. This was attributed to the fact that the active species consist of a monophosphine-palladium complex. The product distribution was not changed by encapsulation of the catalyst. A similar rate enhancement was observed in the rhodium-catalyzed hydroformylation of 1-octene (Scheme 8.1). At room temperature, the catalyst was 10 times more active. For this reaction a completely different product distribution was observed. The encapsulated rhodium catalyst formed preferentially the branched aldehyde (L/B ratio 0.6), whereas usually the linear aldehyde is formed as the main product (L/B > 2 in control experiments). These effects are partly attributed to geometry around the metal complex monophosphine coordinated rhodium complexes are the active species, which was also confirmed by high-pressure IR and NMR techniques. 

The presumed catalytic cycle for this coupling is the following Once formed from 23, the highly coordinatively unsaturated 14-electron palladium(O) complex 24 participates in an oxidative addition reaction with the aryl or vinyl halide to give the 16-electron palladium(II) complex 25. A copper(I)-catalyzed alkynylation of 25 then furnishes an aryl- or vinylalkynyl palladium(II) complex 27. Finally, a terminating reductive elimination step reveals the coupling prduct 9 and regenerates the active palladium(O) catalyst 24. 

The specific property of nickel nanoparticles deposited onto silicon substrates of different types of doping was also clearly manifested in another reaction we studied, hydrogenation of multiple bonds. It is known that the optimal situation for catalysis of processes of this kind is presence of a positive charge on metallic particles [43]. This can be achieved upon deposition of Ni nanoparticles onto p-type silicon. Indeed, experiments did show that the activity of Ni nanoparticles onto p-type silicon exceeds by nearly two orders of magnitude the activity of catalysts based on ultra-dispersed platinum and palladium, prepared by other methods. 

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