Phenylpalladium intermediates

When 4-methylpyrimidine was used as the coupling partner of bromobenzene (7.87.), the attack of the phenylpalladium intermediate took place rather at the relatively acidic methyl substituent than on the electron deficient pyrimidine core, giving rise to 4-diphenylmethyl-pyrimidine.110... 

The unstable phenylpalladium intermediate 164 is formed by palladation of benzene with Pd(OAc)2, and then the following three reactions occur. 

The reaction can be explained by the transmetalation of the borate with PdCl2 giving phenylpalladium intermediate and the subsequent insertion of a C=C double bond of norbornadiene into the Pd-Ph bond. Cationic 7t-allyl-palladium(II) and -rhodium(I) complexes having a BPhJ counter anion induce phenyl group migration from B to the transition metals at elevated temperature to form phenyl complexes [54,55] (Eq. 5.17). 

Scheme 3.1 Formation of phenylpalladium intermediates from organic halides and pseudohalides by oxidative addition.

Scheme 3.4 Trapping of phenylpalladium intermediate 1 with anions and nucleophiles.

We do not know if the vinylic alcohol is actually an intermediate or whether a hydride-71 complex of it rearranges directly to the aldehyde as probably happens in the palladium-catalyzed oxidation of ethylene to acetaldehyde. The formation of 4% 2-methyl-2-phenylpropanal is unexpected. This product must arise from a reversed addition of the phenylpalladium group followed by a hydrogen transfer from the hydroxyl-bearing carbon to the palladium, followed by reductive elimination of a hydridopalladium group. An alkyoxypalladium intermediate has been proposed (39). 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Oxidative cross-coupling with alkenes is possible with Pd(OAc)2 [109], The reaction proceeds by the palladation of benzene to form phenylpalladium acetate (164), followed by alkene insertion and elimination of /1-hydrogen. Heteroaromatics such as furan and thiophene react more easily than benzene [109]. Stilbene (177) is formed by the reaction of benzene and styrene. The complex skeleton of paraberquamide 179 was obtained in 80% yield by the Pd(II)-promoted coupling of the indole ring with the double bond in 178, followed by reduction of the intermediate with NaBELt [110]. 

Another source of phenylpalladium involves aromatic sulfinic acid which evolves SO2 in the presence of Pd(II) to give the desired intermediates (73). 

A phenylpalladium complex also causes ring-opening rearrangement of methylenecyclobutane to a ji-allylpalladium complex, which arises from p-carbon elimination of an intermediate (cyclobutylmethyl)palladium complex [82]. 

CycUzation. The last step in a total synthesis of ibogamine (2) involved cyclization of 1 with bis(acetonitrile)dichloropalladium(II) assisted by silver tetrafiuoroborate and triethylamine. Work-up included reduction of an intermediate palladium species with NaBH. The alkaloid was obtained in 40-45% yield. This cyclization is related to the olefin arylation of Heck, catalyzed with phenylpalladium acetate, CeHsPdOCOCHa. ... 

Unusual Pd-catalyzed fragmentation occurred in the reaction of 1-phenyl-2,2-dimethyl-3,4-pentadien-l-ol (395) with iodobenzene. The arylated diene 397 and benzaldehyde were obtained in high yields. In this reaction, phenylpalladium attacked the central carbon to generate the 7r-allylpalladium intermediate 396. Then decarbopalladation ( -carbon elimination) occurred to provide the arylated diene 397 and a-hydroxybenzylpalladium 398, which collapsed to benzaldehyde and Pd(0) [152]. 

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. 

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