Cross-coupling reactions leaving groups

The results given above clearly demonstrate the good leaving group ability of the readily introduced benzylsulfanyl moiety in copper-promoted, Pd-catalyzed cross-coupling reactions. Moreover, the use of non-protected polyhydroxylated substrates has proven to be compatible with the cross-coupling conditions. 

The kind of a fluoride ion activator and the leaving group in electrophiles affects the stereochemistry in the cross-coupling reaction of allylsilanes as exemplified with 2-cyclohexenyl(difluoro)phenylsilane (Eq. 31) 135]. 

A cross-coupling reaction can be partially defined by equation (1), where Nu is a carbon (or heteroatom) nucleophile see Nucleophile), R X is an electrophilic substrate, X is a halogen or other appropriate leaving group, and M is a metal or metalloid. At first glance, it would appear that simple nucleophihc substitution reactions should fall under this definition. However, what makes the cross-coupling chemistry special is its ability to perform transformations that cannot be accomplished with simple substitution chemistry. 

While organic chlorides, bromides, and iodides are still the most common substrates, recently the scope has been expanded to include organic fluorides, nitriles, ethers, triflates, phosphates, iodonium salts, and substrates with various chalconide-leaving groups. Of the different cross-coupling reactions covered in this article, the Kumada Corriu protocol has been the least reviewed. However, the topic has been covered in recent historical treatments. ... 

One extreme in a displacement reaction would be where the leaving group has already departed, and a preformed stable carbocation reacts with the nucleophile. In this event, the cross-coupling reaction might be either direct polar combination of the carbocation with the Grignard reagent or electron transfer to the carbocation followed h radical recombination. 

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