Palladium, Stille coupling reaction mediated

Recently, transition metals and particularly palladium have found an increased use in C-synthesis. Reactions such as aromatic cyanation, the Stille-coupling reactions, and palladium-mediated carbonylation reactions using [ C] carbon monoxide are opening up new labeling pathways. 

Palladium-mediated catalysis has only been exploited relatively recently in the synthesis of substituted PPV derivatives. The use of aryl dibromides as monomers is particularly useful as it allows the synthesis of PPVs substituted with alkyl rather than alkoxy sidechains. The Suzuki [53, 54], Heck [55], and Stille [56] reactions have been used in the synthesis of new PPV derivatives, but attaining high molecular weight PPV derivatives by these methodologies has proved problematic. A phenyl-subslilutcd PPV material PPPV 31 was synthesized by a Suzuki coupling (Scheme 1-10) of dibromoethene and fo/.v-boronic acid 30. Its absorption (2ni ix=385 nm) and emission (2max=475 nm) maxima were strongly... 

Stille cross coupling reactions usually proceed under mild neutral conditions. 2-Substituted thiazoUnes can be obtained by the cross coupling reaction of 2-bromothiazolines with various tributylstannyl compounds. Previous attempts at a palladium mediated coupling of 2-trimethylstannylthiazoUne led to only decomposition of the substrate <96TL4857>. 

The synthetic potential of palladium-mediated cross-coupling reactions (Heck, Suzuki, Stille, Sonogashira, Buchwald-Hartwig) led to the search for a practical synthesis of p-[ F]fluoroiodo- and p-[ F]fluorobromobenzene. p-[ F]Fluoroio-dobenzene (G, X = iodine) can be obtained in poor yield from p F]fluoride and a trimethylammonium precursor (P7). p-p F]Fluorobromobenzene can be prepared in a more reproducible way from 5-bromo-2-nitrobenzaldehyde (radiochemical yields > 70%). The synthesis involves a two-step procedure radiofluorination (F for NO2 substitution), then a catalysed decarbonylation [190,191]. Also very efficient is the one-step reaction of p F]fluoride with a suitable diaryliodonium salt (P6) giving >70% radiochemical yield [192-194]. 

An nice example of the formation of a five membered ring in crosscoupling reaction is the hexamethyldistannane mediated ring closure of the dihalogenated diaryl ether shown in 3.1. The first step of the process is the palladium catalyzed exchange of one of the iodides to a trimethylstannyl moiety, followed by the closure of the five membered ring in Stille coupling. 

To expand the diversity of their libraries Brill et al.16 also modified various heterocycles by alkylation, acylation, or metal-mediated coupling reaction prior to resin capture. A remaining chloro substituent was still available for nucleophilic displacement or a palladium-mediated coupling reaction with anilines, phenols, and boronic acids on solid phase [see Fig. 10 for the preparation of purine derivative (62)]. 

Palladium-mediated Stille coupling of bromopyrazines with aryltributylstannanes produces good yields of aryl-pyrazines <2005T9637> and halogenoquinoxalines undergo Stille (and Suzuki) cross-coupling reactions <1999TL4507>. 

Transition-metal-mediated transformations play an important role in combinatorial synthesis on solid supports because they allow C-C bond formation under mild, neutral conditions and tolerate a broad range of functional groups. In particular, the palladium-mediated cross-coupling reactions, more precisely the Mizoroki-Heck, the Suzuki and the Stille reactions received a lot of attention due to their enormous derivatization potential of functional structures [5]. Although Stille coupling was the first to be explored on and... 

While the nickel-catalyzed reaction facilitates oxidative addition of substrates problematic to palladium-mediated processes, there is still a great deal of interest in the palladium-catalyzed Kumada cross-coupling reaction due to its enhanced chemoselectivity. Also, as mentioned previously, one distinct advantage of the palladium-catalyzed procedure, is that it allows versatile organolithium reagents to be used as an alternative to Grignard... 

The reaction between an organic electrophile 1 and an organostannane 2 mediated by a transition metal catalyst (originally palladium) to form a new sigma carbon carbon bond is referred to as the Stille cross-coupling reaction (equation 1). 

The late Professor J. K. Stille pioneered the development of a very effective and versatile palladium-mediated C-C bond forming method - the palladium-catalyzed cross-coupling of organic electrophiles with organostannanes.48 This process continues to enjoy much success in organic synthesis because it proceeds in high yields under mild reaction conditions and because it tolerates a... 

A convenient method for preparing vinylated or arylated pyridines on insoluble supports is palladium-mediated cross-coupling. The Heck, Suzuki, and Stille reactions have been successfully used for this purpose (Table 15.22). The conditions are essentially the same as those used for the related coupling of arenes (see Section 5.4). 

---