Supported palladacycles

Phosphites P(OR)3 are much weaker ligands for Pd, and are not capable of supporting Pd° species in solution for the reactions where oxidative addition is rate-limiting therefore they are very rarely used in cross-coupling reactions. Phosphite-derived palladacycles, however, are among the most effective precatalysts (Section 9.6.3.4.8). 

This process is likely to proceed via a palladacycle intermediate followed by a Pd(ll) to Pd(iv) oxidation. Reductive elimination occurs with C-O bond formation and regeneration of the Pd(ll) catalyst. Evidence for a palladacycle intermediate is supported by the high regioselectivity (8-Me group oxidized) observed for the oxidative functionalization of 5,8-dimethylquinoline, which, in the absence of a possibility of coordination, would otherwise contain two identical methyl groups (Equation (57)). 

Benzofurans and dihydrobenzofurans have been prepared on polymeric supports by the palladium-mediated reaction of 2-iodophenols with dienes or alkynes (Entries 1 and 2, Table 15.9). This reaction is closely related to the synthesis of indoles from 2-iodoanilines, and probably proceeds via an intermediate palladacycle (Figure 15.3). Benzofuran and isobenzofuran derivatives have also been prepared on cross-linked polystyrene by intramolecular addition of aryl radicals to C=C double bonds and by intramolecular Heck reaction. 

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

Polystyrene-supported soluble palladacycle catalyst as recyclable catalyst for Heck, Suzuki and Sonogashira reactions... 

POLYSTYRENE-SUPPORTED SOLUBLE PALLADACYCLE CATALYST AS RECYCLABLE CATALYST FOR HECK,... 

A new type of soluble polystyrene-supported palladium complex was synthesised (Figure 6.1) as an excellent and recyclable palladacycle catalyst for carbon-carbon bond formation in Heck, Suzuki and Sonogashira reactions to give high yields of the desired products. 

SYNTHESIS OF 1 - [4-(2-PHEN YLETHYNYL)PHEN YL] ETHAN -1-ONE VIA SONOGASHIRA REACTION BY THE USE OF POLYMER-SUPPORTED PALLADACYCLE CATALYST... 

Polymer-supported palladacycle catalyst, 30 mg, 2 pmol palladium... 

Polystyrene-supported palladacycle catalyst (30 mg, 2 pmol Pd), l-(4-bromo-phenyl)ethan-l-one (0.20 g, 1 mmol), phenylacetylene (0.16 g, 1.5 mmol), and triethylamine (3 mL) were sequentially added into a 15 mL septum-sealed test tube under nitrogen atmosphere. The mixture was then heated at 90 °C for 72 hours. 

After cooling to the room temperature, to the mixture was added 8 mL of dry ether to precipitate the polystyrene-supported palladacycle catalyst. The catalyst was further removed by centrifugation the upper liquid layer of the reaction mixture was transferred via a syringe into another 20-mL round-bottom flask. 

The newly invented polystyrene-supported palladacycle catalyst was prepared in six steps with high yields. The simple precipitation and filtration process to recycle the catalyst after model reactions for Heck, Suzuki, and Sonogashira reactions is noteworthy.[1]... 

Nowotny and co-workers prepared polystyrene-immobilized complex 309. The supported complex is labile under typical Heck conditions, the active catalyst being transferred to solution. When the reaction is run at 140 °C, the kinetics are sigmoidal, showing that the active species slowly builds up in solution, and then decomposes. In addition, on the second recycle, all activity is in solution, probably as colloidal Pd(0) and no activity is left within the polymer support. This is consistent with 308 and other supported palladacycles being simply a slow source of active colloidal palladium particles. 

Immobilized dicyclohexylphosphine ligand, 310, has been used as the starting point for the preparation of supported palladacycles 313 and 314. Simply stirring 310 and dimeric palladium complexes 311 and 312 for 1 h in dichloromethane gave 313 and 314, respectively (Scheme 104). They are active Suzuki coupling catalysts on the first use but cannot be recycled. 

Also worth mentioning here are studies based around the preparation and use of silica-supported palladacyclic complexes. It was the use of these that gave valuable evidence for the decomposition of half-pincer and SCS pincer " complexes during Heck reactions, generating soluble Pd(0) species that are the true catalysts. 

These complexes show high catalytic activity (0.5 mol% loading) in short reaction times for the aryl amination of aryl chlorides, triflates, and bromides. Primary and secondary amines, both alkyl and aryl, are well tolerated. The mirroring of results can be emphasized if these palladacycles are compared to the (NHC)Pd(allyl)Cl systems, supporting the idea of identical active species [NHC-Pd] during the catalytic cycle. One of the limitations of these catalytic procedures or catalysts is their hmited activity for the couphng of electron-rich heterocycles with aryl hahdes. 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

Primary and secondary alcohols such as benzyl alcohol (483) and 1-phenylethanol (484) are oxidized with O2 efficiently to benzaldehyde and acetophenone using Pd(OAc)2, complexed with pyridine, as a catalyst in the presence of a molecular sieve [199] or supported on hydrotalcite [200]. Also, the palladacycle of 4,5-dihydro-l,3-oxazole 485 is a good catalyst for oxidation of alcohols in DMSO under oxygen [201]. 

The oxidative addition of Pd°(dba)2 to PPh2(o-benzyl acetate) (15) (a related but less-hindered hgand than 11) generates a mononuclear P,C-palladacycle 16 (Scheme 1.42) [28, 61]. The cleavage of the benzyl-0 Ac bond of the ligand 15 by a Pd(0) complex by oxidative addition supports the idea that the formation of the Pd(0) complex 10 in Scheme 1.39 is, indeed, reversible [28]. 

---