Oxidation carbon-palladium bonds

As described in the preceding sections, many domino reactions start with the formation of vinyl palladium species, these being formed by an oxidative addition of vinylic halides or triflates to Pd°. On the other hand, such an intermediate can also be obtained from the addition of a nucleophile to a divalent palladium-coordinated allene. Usually, some oxidant must be added to regenerate Pd11 from Pd° in order to achieve a catalytic cycle. Lu and coworkers [182] have used a protonolysis reaction of the formed carbon-palladium bond in the presence of excess halide ions to regenerate Pd2+ species. Thus, reaction of 6/1-386 and acrolein in the presence of Pd2+ and LiBr gave mainly 6/1-388. In some reactions 6/1-389 was formed as a side product (Scheme 6/1.98). 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

A second important palladium-catalyzed process is cross coupling of an aryl or alkenyl tiaUcyltin reagent with an aUcyl or aryl halide. In this reaction, one carbon-palladimn bond is generated by transmetalation of Pd -X with BU3 Sn-R, while the other carbon-palladium bond is formed by oxidative addition of an organic halide with a Pd°L species. This reaction, which is tolerant of many organic functional groups, is normally known as the Stille Reaction (Scheme 4). ... 

The carbon-metal systems considered appropriate for this section (and not covered elsewhere in this series) are carbon-tin, carbon-mercury and, to a restricted extent, carbon-palladium bonds. Although or-ganomercurials and organostannanes have been studied for over a century, the former continue to attract attention because of their controlled formation by the Markovnikov oxymercuration reaction (OM) and its variants, and the latter because of newer and regiospecific methods for forming C—Sn bonds and oxidatively cleaving them. In both cases, signiticant examples of synthetically useful oxidations have... 

TetraaUcylsilanes are stable compounds. Therefore, it is necessary to activate the compounds for the conpling reaction. Since dialkylsilacyclobutanes have high strain energy, palladium complexes can easily insert into the carbon-silicon bond oxidatively, and the resultant complexes conple with acyl chlorides to give l-sila-2-oxa-3-cyclohex-ene derivatives (Scheme 14). 

The key point in divalent Pd-catalyzed cyclization of allylic alkynoates is the method of quenching of the carbon-paUadium bond formed after cyclization. In order to develop a divalent Pd-catalyzed reaction, a divalent palladium species must be regenerated in the quenching of the carbon-paUadium bond. Lu and co-workers developed different quenching methods for the carbon-palladium bonds, for example, /3-heteroatom elimination, copper halide mediated oxidative cleavage, and carbonylation and protonolysis of... 

Another important type of reactivity of palladium, namely oxidative addition to Pd(0), is the foundation for several methods of forming carbon-carbon bonds. Aryl126 and alkenyl127 halides react with alkenes in the presence of catalytic amounts of palladium to give net substitution of the halide by the alkenyl group. The reaction, known as the Heck reaction,128 is quite general and has been observed for simple alkenes, aryl-substituted alkenes, and substituted alkenes such as acrylate esters, vinyl ethers, and A-vinylamides.129... 

Initially we tried the standard approach, reduction of NiL, NiB, or NiC with 2.0 equivalents of potassium in refluxing THF. Finely divided black nickel powders were obtained however, they showed rather limited reactivity toward oxidative addition with carbon-halogen bonds. Similar results were found for palladium and platinum. 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

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. 

The use of 1,6-diene systems usually does not result in cyclization reactions with palladium ) salts. For example, with 1,6-heptadiene a /i-elimination takes place from the cqjr-intermediate to give diene 22 as the major product (equation 10)27. However, more recently Trost and Burgess21 have shown that with a 4,4-bis(phenylsulfonyl) derivative of 1,6-heptadiene (23) an insertion takes place to give a 5-membered ring product (24, equation 11). The final step of the latter reaction is oxidative cleavage of the palladium-carbon bond by CuCl2 to produce a carbon-chlorine bond. 

An example of an intramolecular palladium-catalyzed oxidation of an allene involving carbonylation was used in the synthesis of pumilotoxin 251 D (equation 32)65. Intramolecular aminopalladation of the allene followed by carbonylation of the palladium-carbon bond and subsequent oxidative cleavage of the acylpalladium intermediate by CuCE afforded pyrrolidine 72 in which the chirality at the carbon at the 2-position was established. 

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 ... 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

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