Palladium nucleophilic addition

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

The palladium catalyst is essential in this reaction, as was shown in control experiments to make sure that this was not a direct nucleophilic addition of the amine to the electron-poor (regarding the low lying LUMO ) cyclobutadiene ligand. A series of amino-substituted cyclobutadiene complexes have been synthesized by this methodology [29]. 

Kostic et al. reported the use of various palladium(II) aqua complexes as catalysts for the hydration and alcoholysis of nitriles,435,456 decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.457 Labile aqua or other solvent ligands can be displaced by a substrate. In many cases, the coordinated substrate thus becomes activated toward nucleophilic addition of water or alcohols. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

As mentioned above nonconjugated dienes give stable complexes where the two double bonds can form a chelate complex. A common pathway in palladium-catalyzed oxidation of nonconjugated dienes is that, after a first nucleophilic addition to one of the double bonds, the second double bond inserts into the palladium-carbon bond. The new (cr-alkyl)palladium complex produced can then undergo a /(-elimination or an oxidative cleavage reaction (Scheme 2). An early example of this type of reaction, although not catalytic, was reported by Tsuji and Takahashi (equation 2)12. 

Preparation of palladium enolates and their reactions (/3-hydride elimination to enones, migratory insertion to C-C multiple bonds, reductive coupling with allyl or aryl groups, etc.) have been reported. However, the nucleophilic addition of palladium enolates to C=0 and C=N bonds has been little investigated.463... 

B. Akermark, B. Krakenberger, S. Hansson, Ligand Effects and Nucleophilic Addition to (T)3-Allyl)palladium Complexes. A Carbon-13 Nuclear Magnetic Resonance Study, Organometallics, 1987, 6, 620-628. 

The Wacker-Hoechst process has been studied in great detail and in all textbooks it occurs as the example of a homogeneous catalyst system illustrating nucleophilic addition to alkenes. Divalent palladium is the oxidising agent and water is the oxygen donor according to the equation ... 

Figure 15.7. Palladium catalysed addition of nitrogen nucleophiles to alkenes...

A a-5-bonded r-alkene (r] ) intermediate (325) has been invoked to account for the hydrogenation of the thiaplatinacycle (324) to the complex (326) in which two hydrogens have been added and a hydrogen shift has occurred." When coordinated to neutral and cationic palladium(II) and platinum(II) centres, the diphosphine 2,3-bis(diphenylphosphino)propene, on treatment with benzylamine, was found to undergo isomerization to coordinated c/i-l,2-bis(diphenylphosphino)propene rather than the expected nucleophilic addition to the double bond. 

The linear telomerization reaction of dienes was one of the very first processes catalyzed by water soluble phosphine complexes in aqueous media [7,8]. The reaction itself is the dimerization of a diene coupled with a simultaneous nucleophilic addition of HX (water, alcohols, amines, carboxylic acids, active methylene compounds, etc.) (Scheme 7.3). It is catalyzed by nickel- and palladium complexes of which palladium catalysts are substantially more active. In organic solutions [Pd(OAc)2] + PPhs gives the simplest catalyst combination and Ni/IPPTS and Pd/TPPTS were suggested for mnning the telomerizations in aqueous/organic biphasic systems [7]. An aqueous solvent would seem a straightforward choice for telomerization of dienes with water (the so-called hydrodimerization). In fact, the possibility of separation of the products and the catalyst without a need for distillation is a more important reason in this case, too. 

Palladium-catalyzed addition of oxygen nucleophiles to alkenes dates back to the Wacker process and acetoxylation of ethylene (Sects. 1 and 2). In contrast, catalytic methods for intermolecular oxidative amination of alkenes (i.e., aza-Wacker reactions) have been identified only recently. Both O2 and BQ have been used as oxidants in these reactions. 

As an alternative to addition of anionic nucleophiles followed by reoxidation, rhodium(l)-catalyzed C-H activation allowed the nucleophilic addition of alkenes to the intermediate Rh(i) carbene complex <2002JA13964, 2004JOC7329>. Purine behaved anomalously compared to other heterocycles, for which selective monoalkylation was observed, and underwent sequential substitution first at C-8 and then at C-6 (Equation 8). Caffeine was monoalkylated at C-8 in low yield (15%). Selectivity for C-8-arylation was also observed in the palladium-catalyzed C-H activation of 6-phenyl-9-benzylpurine (aryl iodides, 0.05 equiv Pd(OAc)2, 3 equiv Cul, 2.5 equiv CS2CO3, DMF, 160 °C, 60 h, 48-95% yields) <2006OL5389>. 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

Figure 1-4. Palladium(II) assisted nucleophilic addition onto double bond...

Alike olefins, allenes also undergo palladium mediated addition in the presence of N-H or O-H bonds. Although these reactions show some similarity to Wacker-type processes, from the mechanistic point of view they are quite different. Allenes, such as the cr-aminoallene in 3.69., usually undergo addition with palladium complexes (e.g. carbopalladation in 3.69. and 3.70., or hydropalladation in 3.71.), which leads to the formation of a functionalized allylpalladium complex. Subsequent intramolecular nucleophilic attack by the amino group leads to the closure of the pyrroline ring.87... 

The palladium catalysed addition of N-H or O-H bonds onto allenes has successfully been exploited in the preparation of oxazepines, diazepines, oxazocines and diazocines. The nucleophilic attack of the pendant alcohol or sulphonamide function on the allene moiety was followed by the incorporation of the alcohol, used as solvent, to give the desired cyclic products in good yield (5.15. and 5.16.). The bromoallene in these processes is the synthetic equivalent of an allylic dication.15... 

Palladium (II)-Nucleophile Addition across Olefins. Adding palladium complexes to olefins, either in the presence of an external nucleophile or a ligand which is attached to palladium, produces a palladium-carbon sigma-bonded complex which is not usually isolated in the case of monoolefins. Instead it decomposes and in doing so oxidizes the olefin to an organic carbonyl compound or a vinyl compound, exchanges a substituent group on the olefin, isomerizes the double bond, arylates (alkylates) the olefin, or carboxylates the olefin (2, 3). 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

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