Nucleophiles Electron-rich reactants that

An orbital which extends over two or more atoms A repeating unit in a polymer Electron-rich reactant that donates two electrons to form a covalent bond The relative reactivity of a nucleophilic reagent Amount of a substance that reacts with one mole of another substance... 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the most common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to the electrophile. Radical reactions involve species that have an odd number of electrons. A bond is formed when each reactant donates one electron. 

Clarke recently published the first microwave-accelerated Hiyama coupling [163,164]. It was noted that the availability and nontoxic attributes of the organosilicon reactants make them very attractive in synthesis, but their low nucleophilicity limits their potential. Microwave heating allowed aryl bromides and activated aryl chlorides to react under palladium catalysis using an electron-rich N-methyl piperazine/cyclohexyl phosphine ligand (Scheme 75). A vinylation reaction with vinyltrimethoxysilane was also reported [164],... 

Figure 5.2 A comparison of carbon-carbon single and double bunds. A duuble bond is both more accessible to approaching reactants than a single bond and more electron-rich (more nucleophilic). An electrostatic potential map of ethylene indicates that the double bund is the legion of highest negative charge (red).

Both electron richness and electron accessibility lead to the prediction that a carbon-carbon double bond should be nucleophilic. That is, the chemistry of alkenes should involve reactions of the electron-rich double bond with electron-poor reactants. This is exactly what we find The most important reaction of alkenes is their reaction with electrophiles. 

On the other hand, neutral Os alkylidyne 82 undergoes reaction with methanol to give carbene complex 83 (equation 10.56).96 It would appear that 82 undergoes reaction with nucleophilic methanol at 0 first, which is followed by proton transfer to Os. Such reactivity would be consistent with that associated with Fischer carbyne complexes, yet the metal center is more electron-rich than the group 6 metal complex reactant in equation 10.55. 

All compounds with a particular functional group react similarly. Due to the cloud of electrons above and below its TT bond, an alkene is an electron-rich molecule, or nucleophile. Nucleophiles are attracted to electron-deficient atoms or molecules, called electrophiles. Alkenes undergo electrophilic addition reactions. The description of the step-by-step process by which reactants are changed into products is called the mechanism of the reaction. Curved arrows show which bonds are formed and which are broken and the direction of the electron flow that accompany these changes. 

While die above reactions represent only a small fraction of die reactions known for palladium, they form the basis of a powerful methodology for building carbon structures. Several variations have been developed which utilize certain types of reactants and give particular types of products. All diese variations, however, contain a common theme. In each case an electron-deficient reagent (e.g., a vinyl halide or aromatic triflate) reacts with an election-rich reagent (e.g., an alkene, an organoborane, or an organotin) witii the formation of a new carbon-carbon bond. In that sense diese reactions are related to die reactions between carbon nucleophiles and carbon electrophiles discussed previously in diis chapter. They are quite different, however, because diey proceed only in the presence of Pd(0). In fact they proceed only in die coordination sphere of Pd(0). The ability of Pd(0) to catalyze diese reactions is nearly unique We will now examine some of die more common processes. 

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