Catalytic reactions Stille coupling

A second common class of catalytic reaction is coupling (eq. 24), where A-B is the desired product and the nature of X and Y can be chosen to give the reaction the necessary exothermicity to go forward and facilitate the interaction of the reagents with the catalyst. In this class, we have Stille and Suzuki coupling, as well as more recent versions involving carbon-heteroatom bond formation... 

The mechanism " of the Suzuki reaction is closely related to that of the Stille coupling reaction, and is also best described by a catalytic cycle ... 

Scheme 30. Catalytic cycle for the Stille reaction direct coupling.

The palladium-catalyzed coupling of aiyl and vinyl halides to organotin compounds, known as Stille coupling, is one of the most important catalytic methods of carbon-carbon bond formation. The reaction is generally conducted in polar organic solvents, such as dimethylformamide, with tertiary phosphine complexes of palladium, although phosphine-free complexes or simple Pd-salts are also frequently used as catalysts [8]. 

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

A catalytic arene-arene coupling can occur in the presence of cooxidants such as Cu11, Fe111 and heteropolyacids along with molecular oxygen, but this reaction is still not selective enough for industrial use. 

In 1998, Yamamoto et al. reported the first catalytic enantioselective allylation of imines with allyltributylstannane in the presence of a chiral 7i-allylpalladium complex 23 (Scheme 9) [15]. The imines derived from aromatic aldehydes underwent the allylation with high ee values. Unfortunately, the allylation reaction of aliphatic imines resulted in modest enantioselectivities. They proposed that a bis-Jt-allylpalladium complex is a reactive intermediate for the allylation and reacts with imines as a nucleophile. The bis-Jt-allylpalladium complex seemed the most likely candidate for the Stille coupling [16]. Indeed, the Stille coupling reaction takes place in the presence of triphenylphosphine even if imines are present, whereas the allylation of imines occurs in the absence of the phosphine [17]. They suggested the phosphine ligand played a key role in controlling the... 

The catalytic cycle involved in the Stille coupling reaction is generally accepted to involve the following steps ... 

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