Organopalladium complex

The Heck reaction is considered to be the best method for carbon-carbon bond formation by substitution of an olefinic proton. In general, yields are good to very good. Sterically demanding substituents, however, may reduce the reactivity of the alkene. Polar solvents, such as methanol, acetonitrile, N,N-dimethylformamide or hexamethylphosphoric triamide, are often used. Reaction temperatures range from 50 to 160 °C. There are various other important palladium-catalyzed reactions known where organopalladium complexes are employed however, these reactions must not be confused with the Heck reaction. 

Volume 1, Metal Complexes. Describes the organopalladium complexes containing Pd—C a bonds, hydrides, olefins and acetylenes, dienes, w-allylic groups, cyclopentadienyls, and benzenes. 

Reinhoudt and coworkers studied the synthesis of hyper-branched polymers composed of organopalladium complexes with an SCS pincer ligand [11]. Removal of acetonitrile ligands on palladium led to the self-assembly of dinu-clear palladium complex (9) to give hyper-branched polymer (10), which was... 

Figure 91 Rigid, cyclic structure that contains 4,7-phenanthroline and the linear organopalladium complex...

Komatsu and coworkers also carried out mechanistic studies on their new domino process, showing there is an interplay of two reactive intermediates radicals and organopalladium complexes (Scheme 5.25). It is proposed that under the influence of both, the Pd° catalyst and light, radical 5-120 is formed first. Subsequent CO addition furnishes the acyl radical 5-121, which is trapped in a 5-exw-trig cycliza-... 

Catalysis by organopalladium complexes has also been used to effect cyclization of appropriately substituted bromarylamines into indoles (cf. Scheme 44 in Section IV,A,3).73 By extending the side chain by one methylene unit, the method can be applied to the synthesis of isoquinolines (Scheme 147) but the preparative scope has not been evaluated.73... 

Several examples of the cyclization of indole derivatives with alkenic side chains in the 3-position have been reported.6 In these examples, palladium chloride in combination with silver tetrafluoroborate is the cyclizing agent. The palladium tetrafluoroborate, presumably formed, should be a very reactive palladating species and probably is the reason why these reactions proceed at room temperature, although the mechanism is not yet completely clear. These reactions were worked up reductively (by addition of sodium borohydride) in order to reduce the expected alkenic product or any relatively stable organopalladium complexes that may have been formed (equation 4).6... 

Sodium arylsulfinates and palladium salts react to form arylpaUadium salts and sulfur dioxide. When carried out in the presence of some dienes, stable organopalladium complexes have been obtained. For example, 1,5-cyclooctadiene and sodium p-toluenesulfinate with palladium chloride yields the a,it-complex shown in equation (48).109... 

The cydization of unsaturated substrates bearing a carbo- or heteronucleophile promoted by an organopalladium complex has recently emerged as a powerful... 

Organopalladium complexes do usually not transfer a formal carbanion to aldehydes or ketones, but intramolecular Barbier-type reactions of this kind have been observed (Scheme8.2). 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

If the initial organopalladium complex is formed in the presence of an alkene, transfer of the organyl group to the alkene can occur (Heck reaction, Scheme 8.7) [5, 78], Electron-deficient and electron-rich alkenes and even some arenes [79] can be used. In the last step of this process a Pd-hydrido complex is formed by /3-hydride elimination, leading to the formation of the C-C double bond. In the presence of bases the hydrido complex eliminates HX, regenerating the catalytically active Pd(0) complex (Scheme 8.7). 

Organopalladium complexes appear to undergo addition-elimination reactions in the same way as the inorganic complexes except that anti-Markovnikov addition is preferred. The reaction, of course, is limited to organopalladium compounds which lack hydrogens in positions to the palladium atom. If the j8 hydrogens are present, the organopalladium compounds decompose by hydridopalladium elimination more rapidly than they add to olefins and useful reactions are not obtained. 

The kinetics of reactions at Pd centers is an additional reason for the numerous synthetic applications of organopalladium complexes. The stereochemistry of Pd is almost exclusively square planar. When the rates of snbstitution at isostmctural Ni , Pd , and Pt complexes are compared (equation 1), the relative rates of reaction are Ni , 5 x 10 Pd , 1 X 10 andPt , 1. 

Cacchi, S. Heterocycles via cyclization of alkynes promoted by organopalladium complexes. J. Organomet. Chem. 1999, 576, 42-64. 

Cyclizations involving attack of carbo- and heteronucleophiles on carbon-carbon 71-bonds activated by organopalladium complexes in the syntheses of heterocycles 03S2115. 

Chiral organopalladium complexes have been designed as initiators in the asymmetric polymerization of 1,2-diisocyanobenzenes instead of the resolved oligoquinoxalinylpalladium complexes. The first series of chiral initiators examined were the organopalladium complexes 98a-d bearing chiral 1,1 -binaphthyl groups (Scheme 57) [99, 100]. They were prepared from... 

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