Alkylation palladium complexes

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. 

In 1965, Tsuji et al. observed that palladium could catalyze the allylic alkylation reaction [18]. This discovery, which is a very attractive way to expand the scope of the allylic amination reactions mentioned above, has stimulated an intense research in this field, and even though complexes of nickel, platinum, rhodium, iron, ruthenium, molybdenum, cobalt, and tungsten have been found also to catalyze the alkylation, palladium complexes have received by far the greatest attention [19]. 

This combination of reagents h s been used to oxidize terminal vinyl groups to methyl ketones and is known as the Wacker oxidation. The nucleophile is simply water, which attacks the activated alkene at the more substituted end in an oxypalladation step. (3-Hydride elimination from the resulting a-alkyl palladium complex releases the enol, which is rapidly converted into the more stable keto form. Overall, the reaction is a hydration of a terminal alkene that can tolerate a range of functional groups. 

In this reaction the first addition product was isolated, in catalytic reactions this is not the case and in these reactions the first (cr-alkyl)palladium complex formed from the nucleophilic addition reacts further. For example, in the palladium-catalyzed oxidation of 1,5-cyclooctadiene with Pb(OAc)4 in acetic acid the corresponding diacetate 6 was obtained in 76% yield together with some chloroacetate (equation Adduct 7 is the... 

A o,jt-bonded alkyl-palladium complex undergoes P-carbon elimination on protonation of the cyclopentadiene ring [79]. 

The pioneering studies in this area were reported in 1999 by Narasaka, who demonstrated intramolecular heteroatom Heck-type reactions of 0-pentafluorobenzoyl oximes [97]. As shown below, treatment of unsaturated substrate 97 with a catalytic amount of Pd(PPh,y in the presence of triethyl amine provided pyrrole 98 upon workup with chlorotrimethylsilane. The mechanism of this reaction proceeds via oxidative addition of the N—O bond to afford 99, which undergoes alkene insertion into the Pd—N bond to provide alkyl-palladium complex 100. The exo-methylene product 101 is generated by [i-hydride elimination from 100, and isomerization to the desired pyrrole 98 occurs when chlorotrimethylsilane is added. 

This reaction profile, also called carbonylation, governs the reactivity of Pd-carbonyl complexes. Anionic M[Pd(CO)l3], for instance, catalyzes the reductive carbonylation of esters.f On the other hand, Pd(CO)(PPh3)3 was reported to catalyze the carboxymethy-lation of organic halides and the cyclocarbonylation of cinnamyl halides.f " However, the Pd-CO complexes are most often generated in situ from preformed alkyl -palladium complexes and CO under stoichiometric or catalytic conditions, for example, in the copolymerization of alkenes and CO. Decarbonylation reactions also involve the intermediacy of Pd-CO complexes. In this case, migratory deinsertion (Sect, n.3.1), that is, the microscopic reversal of the migratory insertion, takes place. 

Substituent effects on the intramolecular paUadahon were studied with alkyl palladium complexes 6a-c with KOPh as the base to form five-membered ring palladacycles 7a-c (Scheme 11.2) [29]. This reachon was shown to follow the order X = MeO > H > NO2 for substituents meta to the reacting site. The activation of monodeuterated substrates 6a-di and S-di with KOPh or Ag2C03 in MeCN at 23 °C led to the palladacycles 7a and 9, respechvely, which were partially deuterated at C-3. The degree of deuterahon was determined by H NMR as 48 3%, which showed that there was no isotopic effect for the intramolecular C—H bond activation by the alkylpalladium [29]. The five-membered ring palladacycles 7a-c were stable complexes at room temperature, and did not suffer any reductive elimination to form strained 2H-benzoxete [30]. 

Although the results of these experiments suggest that the palladation proceeds by an electrophilic aromatic substitution, the transformations are probably more complex than the above results suggest. Indeed, the reaction of alkyl palladium complex 6a with KOPh in MeCN was almost completely inhibited by the addition of lequiv. of PPhs [29], which indicates that ligand substitution, presumably by an associative mechanism, occurs during the C—H bond-activation process. Biden-... 

In the earliest publications [33], it was proposed that the initiation step in both hydroxycarbonylation and polymerization reactions involved the reaction of the alcohol with the palladium complex to give the catalytically active palladium-methoxy complexes. After chain growing reactions, the termination mechanism was supposed to proceed via protonolysis of the alkyl-palladium complex to give the keto-ester (KE) product (methyl propanoate or polymer) and regenerate the active catalyst (Scheme 1.4). In addition, hydrido palladium species are smoothly formed from palladium(II) salts in methanol (not shown). 

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