Carbon-palladium bonds palladacycles

Intramolecular arylation of G-H bonds gives cyclic aromatic compounds. In this intramolecular arylation, the carbon-palladium cr-bond is first formed by the oxidative addition of Pd(0) species and then the resulting electrophilic Pd(n) species undergoes the intramolecular G-H bond activation leading to the formation of the palladacycle, which finally affords the cyclic aromatic compounds via reductive elimination.87 For example, the fluoroanthene derivative is formed by the palladium-catalyzed reaction of the binaphthyl triflate, as shown in Scheme 8.88 This type of intramolecular arylation is applied to the construction of five- and six-membered carbocyclic and heterocyclic systems.89 89 89 ... 

There has been a summary of computational and experimental studies of the use of palladium complexes with A -heterocyclic carbenes (NHCs) in the asymmetric coupling of -hybridized carbon-hydrogen bonds with aryl halides. It has been shown that the electronic and catalytic properties of NHCs fused to porphyrins may be modified by varying the inner metal in the porphyrin. A DPT study of the use of palladium-NHC complexes in the asymmetric intramolecular a-arylation of 2-bromoaryl amides to give 3,3-disubstituted oxindoles (101) has been reported. The likely pathway involves insertion of the palladium into the arene-bromine bond to form a palladacycle which deprotonates to give an (9-enolate. Conversion into the C-enolate followed by reductive elimination gives the product. The intramolecular reaction of 0 a cyclopropane carbon-hydrogen bond in a 2-bromoanilide derivative has been used to form cyclopropyloxindoles, (102), in a palladium-catalysed, silver-mediated reaction. 

Novel Palladium Chloride-Based Catalysts for Carbon-Carbon / Carbon-Heteroatom Bond Formations. The past decade has witnessed the development of novel palladacycles as a new class of catalysts for carbon-carbon/carbon-heteroatom bond-forming reactions. Several types of palladacycles (derived from PdCl2) have appeared in the literature. These include PC type, PCP pincer type, phosphite palladacycles, NC type, NCN pincer type, and sulfur containing palladacycles. Heterogeneous palladacycles have also been reported in the literature. These palladacycles are obtained via direct metallation from appropriate ligands with either PdCl2 or Na2PdCl4. Typical examples are shown in Scheme 1. 

Mono- or di-arylaied products may be formed by a-arylation of -dicarbonyl compounds, such as malonamide esters, with arynes generated by reaction of fluoride with ortho-silylaryltriflates. A similar method of benzyne generation has been used in the a-arylation of -ketoamides in a procedure which may be modified to yield asymmettic products. The reaction of 2-haloacetanilides with arynes in the presence of a palladium catalyst may produceiV-acylcarbazoles such as (26). Possible pathways are initial formation of a palladacycle with aryne followed by oxidative addition of the haloacetanilide, or direct insertion of the palladium into the carbon-halogen bond of the acetanilide followed by carbopalladation of the aryne. ... 

Several carbon-hydrogen bond substitutions are involved in the palladium-catalysed reaction of arylsulfonic acids with arenes to yield aromatic sulfones. A plausible mechanism, shown in Scheme 9, involves the initial formation of the palladacycle (99) which, after eoupling with the arene, yields the biphenyl derivative (100). Further coordination and carbon-hydrogen activation gives the seven-membered palladacycle (101) that affords the sulfone, (102), after reductive elimination. ... 

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%. 

Cyclopalladated sulfur-containing < 1995JOC1005> and oxygen-containing complexes <2003OM3967,2005CEJ3268> have also been synthesized. The insertion of phenylacetylene 122 into the palladium-carbon bond of complex 121 yielded the palladacycle 123 (Equation 43). 

A six-membered cyclic allylic carbonate 102 undergoes a palladium-catalyzed decarboxylative C-C bond cleavage to afford dienic carbonyl compound 104 [122]. Decarboxylation of the allylic carbonate moiety provides the driving force for production of the intermediate five-membered hetero-palladacycle 103, from which formal reductive cleavage takes place. 

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. 

Ramesh et al. synthesized a cyclometallated dimeric palladium) ) catalyst with covalently bonded based on the concept that release slowly the highly active species from structurally more stable catalyst precursors [110]. By utilizing this dimeric oxime-type palladacycle as the catalyst, various aryl iodides were reacted with aliphatic alcohols and phenols in a highly selective manner and gave the corresponding esters in excellent yields. Based on these excellent yields, apparently no by-products were formed. Remarkably, the complex was stable even at high temperatures (120 °C) and under 10 bar of carbon monoxide. 

Catellani and Lautens have independently reported unique palladium/ norbornene-catalyzed reactions of aryl halides, which mechanistically involve a reversible alkene insertion/p-carbon elimination process [11]. For example, iodobenzene reacted with 1-iodobutane and methyl acrylate to form the multiply-alkylated benzene 29 (Scheme 7.9) [12]. The following mechanism is proposed oxidative addition of phenyl iodide onto palladium generates phenylpalladium(ll) iodide. A double bond of norbornene inserts into the C-Pd bond to form an alkylpalladium species, which cleaves a C-H bond nearby to form the palladacycle 25. -Butyl iodide then reacts with 25 to form the Pd(IV) intermediate 26, which undergoes reductive elimination. Repetition of the cyclometalation/alkylation process leads to the formation of 27. Then, P-carbon elimination affords the arylpalladium species 28 together with norbornene. Subsequently, a Heck-type reaction takes place with methyl acrylate, giving rise to 29. 

It has been reported that a retro-oxidative cyclization process proceeds in catalytic C-C bond cleavage reactions. For example, a six-membered cyclic allylic carbonate 38 underwent a palladium-catalyzed decarboxylative C-C bond cleaving reaction to afford dienyl aldehyde 40 (Scheme 7.12) [15]. It is proposed that oxidative addition of the allylic carbonate to palladium(O) followed by elimination of carbon dioxide generates the palladacycle 39. Subsequent retro-oxidative cyclization produces the diene and aldehyde functionalities. 

Model stoichiometric reactions of [PdCH3(CO)(Pr DAB)] [B 3,5-(CF3)2C6H3 4] (Pr DAB = l,4-diisopropyl-l,4-diaza-l,3-butadiene) with alkynes and carbon monoxide have been investigated by NMR spectroscopy and DFT studies to identify the putative intermediates involved in the cydocarbonylation of alkynes [47]. Addition of but-2-yne (R = CH3) or 1-phenylpropyne (R= Ph) results in regioseledive insertion into the Pd-acyl bond to afford a five-membered palladacycle 54 that undergoes rapid cydocarbonylation at low temperature to afford a palladium-coordinated, q -allylic lactone 55. The a,f -unsaturated y-lactone could be liberated either by proton abstraction with a stoichiometric amount of Na[BEt3H] or by nucleophilic addition... 

---