Catalysts palladacycles

Other than the aforementioned ynones, P-iodo-p/y-enone 132 was also converted into a 2,5-disubstituted furan, 133, under Pd-catalyzed cyclization conditions using Herrmann s catalyst, palladacycle catalyst 101 [105]. 

A-Heterocycles were synthesized via insertion of 1,1-dimethylallene into Pd-G bonds of cyclopalladated a-tetralone ketimines. Insertion reactions of alkynes into the Pd-G bond of cyclopalladated ferrocenylimines have also been described.Biomimetic hydrolysis of benzoates has been carried out with a water-soluble cyclopalladated aryl oxime, which was proposed as a potential green catalyst. Ghiral cyclopalladated liquid crystals 88 were obtained from amino acids and could possibly serve as enantioselective catalysts. " Palladacycle 89 has been derived from an anthracene-based Schiff base. 

Palladacycles are defined as compounds with a Pd-C CT-bond with the Pd being stabilized by one or two neutral donor atoms, typically forming 5- or 6-membered rings [51]. Ferrocenyl palladacycles constitute a particularly attractive catalyst class partly due to the element of planar chirality. The first diastereoselective cyclopallada-tion of a chiral ferrocene derivative was reported in 1979 by Sokolov [52, 53]. 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

Anderson CE, Donde Y, Douglas CJ, Overman LE (2005) Catalytic asymmetric synthesis of chiral allylic amines. Evaluation of ferrocenyloxazoline palladacycle catalysts and imidate motifs. J Org Chem 70 648-657... 

Weiss ME, Fischer DF, Xin ZQ, Jautze S, Schweizer WB, Peters R (2006) Practical, highly active, and enantioselective ferrocenyl-imidazoline palladacycle catalysts (FIPs) for the Aza-Claisen rearrangement of A-para-methoxyphenyl trifluoroacetimidates. Angew Chem Int Ed 45 5694-5698... 

Fischer DF, Barakat A, Xin ZQ, Weiss ME, Peters R (2009) The asymmetric Aza-Claisen rearrangement development of widely applicable pentaphenylferrocenyl palladacycle catalysts. Chem Eur J 15 8722-8741... 

The Suzuki-Miyaura synthesis is one of the most commonly used methods for the formation of carbon-to-carbon bonds [7]. As a palladium catalyst typically tetrakis(triphenylphosphine)palladium(0) has been used, giving yields of44—78%. Recently, Suzuki coupling between aryl halides and phenylboronic acid with efficient catalysis by palladacycles was reported to give yields of 83%. 

Scheme 6.12 Scope of catalyst 10 containing a palladacycle derived from W,W-dimethylbenzylamine...

The use of well-defined complexes has been widespread in this reaction, despite intriguing studies by Beller and others that have shown that in situ catalytic systems often give better yields in comparison to isolated carbene-Pd(O) complexes [147-149]. Since the mechanism consists of an oxidative addition on a Pd(0)-monocarbene species, efforts in catalyst synthesis have been directed towards Pd(ll)-monocarbene complexes with other labile groups that can be easily released leading to the formation of Pd(0). This is the case for dimers of the type [Pd( j,-C1)C1(NHC)]2, a family of pre-catalysts effective under aerobic conditions [150], the [Pd(acac)Cl(NHC)] complexes [151] and related palladacycles [152-154],... 

We have shown that the direct arylation of acrolein toward the synthesis of cinnamaldehyde derivatives was an efficient procedure. Using the palladacycle 1 as catalyst, substituted aldehydes 3 were prepared with up to 87% isolated yield from condensed aiyl bromides (Scheme 21.1, Route 1) that was extended successfully to heteroaiyl bromides, like bromoquinolines (6). Alternatively, the acrolein diethyl acetal was used as olefin and a selective formation of the saturated ester 4 was attained under the same reaction conditions (Scheme 21.1, Route 2). The expected aldehydes 3 were, however, obtained from most of the aiyl halides used under modified conditions. It was shown that the addition of n-Bu4NOAc in the medium... 

A stable palladacycle 7 derived from biphenyl is also an active catalyst.1... 

Controlling secondary reactions required using appropriate ratios among reagents. Special attention was devoted to norbomene although it acts as a catalyst its concentration must be kept sufficiently high to enable it to form the palladacycle by insertion, but not so high as to prevent its liberation at the end of the sequence. 

Although only a dozen known metal complexes were tested in this manner, proof of principle was demonstrated. The test revealed Wilkinson s catalyst to be the most active hydrosilylating agent, its use in this type of reaction being known. However, the study also led to the discovery that a palladacycle, [Pd (o-tolyl)2PC6H4 (OAc)]2, which is usually considered to be potent in Heck reactions, is also an excellent hydrosilylation catalyst.37,38 Control experiments showed that the relative order of catalyst activity is the same when conventional substrates are used in place of the dyes (8). 

Exposure of the benzo[b]thiophene derivative 44 to a palladium catalyst in the presence of tri(2-furyl)phosphine (TFP) as the ligand led to the product 45, which incorporated two heterocyclic units from the starting material. The mechanistic aspects of this transformation were also discussed, which appears to involve palladacycle intermediates <06JA722>. 

In a recent article by Botella and Najera, controlled mono- and double-Heck arylations in water catalyzed by an oxime-derived palladacycle were described [22], When the reaction was carried out under microwave irradiation at 120 °C in the presence of dicyclohexylmethylamine with only 0.01 mol% of palladium catalyst (palladium acetate or palladacycle), monoarylation took place in only 10 min with a very high turnover frequency (TOF) of > 40000 (Scheme 6.3). As regards diarylation, 1 mol% of the palladacycle catalyst and 2 equivalents of iodobenzene had to be utilized to obtain moderate to good yields of diarylated product. Whereas microwave heating at 120 °C provided a 31% yield after 10 min, a 66% isolated yield of product was obtained by heating the reaction mixture under reflux for 13 h at 100 °C. Here, the... 

Bis(pyrazolyl)borate copper complex 47 has been employed as a catalyst in the homogeneous and heterogeneous styrene epoxidation reactions <00JCS(CC)1653>. Pyrazole palladacycles 48 have proven to be stable and efficient catalysts for Heck vinylations of aryl iodides <00JCS(CC)2053>. An asymmetric borane reduction of ketones catalyzed by N-hydroxyalkyl-/-menthopyrazoles has been reported <00JHC983>. 

Gruber AS, Zim D, Ebeling G, Monteiro AL Dupont (2000) Sulfur-containing palladacycles as catalyst precursors for the Heck reaction. J Org Lett 2 1287-1290... 

Herrmann WA, Brossmer C, Reisinger CP, Riermaier T, Ofele K, Beller M (1997) Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions. Part 10. Palladacycles efficient new catalysts for the Heck vinylation of aryl halides. Chem Eur J 3 1357-1364 Iyer S, Jayanthi A (2001) Acetylferrocenyloxime palladacycle-catalyzed Heck reactions. Tetrahedron Lett 42 7877-7878 Iyer S, Ramesh C (2000) Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts di- x-chlorobis(benzaldehydeoxime-6-C,AT)dipalla-dium(II), di- x-chlorobis(dimethylbenzylamine-6-C,A)dipalladium(II) for the Heck reaction. Tetrahedron Lett 41 8981-8984 Jeffery T (1984) Palladium-catalysed vinylation of organic halides under solid-liquid phase transfer conditions. J Chem Soc Chem Commun 1287-1289 (b) idem,... 

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)). 

Palladium-catalyzed directed intramolecular activations of aryl C-H bonds have been reported, as in the phenyla-tion of heterocycle analogs. Palladacycles are proposed intermediates, acting as effective catalysts, and the mechanism is likely to proceed via oxidation of Pd(ll) to Pd(iv) by the iodonium salt, as for the Equation (57), which described the activation of benzylic i/-CH bonds (Equations (121)—(123).109... 

Rawal s group developed an intramolecular aryl Heck cyclization method to synthesize benzofurans, indoles, and benzopyrans [83], The rate of cyclization was significantly accelerated in the presence of bases, presumably because the phenolate anion formed under the reaction conditions was much more reactive as a soft nucleophile than phenol. In the presence of a catalytic amount of Herrmann s dimeric palladacyclic catalyst (101) [84], and 3 equivalents of CS2CO3 in DMA, vinyl iodide 100 was transformed into ortho and para benzofuran 102 and 103. In the mechanism proposed by Rawal, oxidative addition of phenolate 104 to Pd(0) is followed by nucleophilic attack of the ambident phenolate anion on o-palladium intermediate 105 to afford aryl-vinyl palladium species 106 after rearomatization of the presumed cyclohexadienone intermediate. Reductive elimination of palladium followed by isomerization of the exocyclic double bond furnishes 102. 

---