Oxidative addition electrophilic attack

Various oxidation reactions represented by Patterns 2 and 3 in Scheme 3 are discussed in Sect. VIII.3. However, the Wacker oxidation and related reactions, in which nucleophilic attack on Pd-alkene 7r-complexes plays an important role, are discussed in Sect. V.3. Most of the allylic substitution reactions represented by Pattern 4 are generally preceded by reduction of allylic electrophiles (via oxidative addition). Consequently, the overall processes do not generally involve net oxidation or reduction. So, they are discussed in Sect. V.II. 

The oxidative addition of palladium(O) to aryl bromide generates the arylpalladium(n) intermediate 126 (Scheme 37). The electrophilic activation of the double bond by palladium facilitates the nucleophilic attack, resulting in cyclization. 

While the alkoxymetallation process has typically been affected by highly electrophilic metal salts, high-valent metal species generated by an oxidative addition have also been used to activate alkynes through the formation of 7r-complexes. In such cases, the metal-carbon emerging from the attack of an oxygen nucleophile may enter a reaction manifold that leads to an additional C-G bond formation rather than a simple protic quench. This approach, pioneered by Arcadi and Cacci, has proved to be a powerful strategy for the synthesis of structurally diverse substituted... 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

Pyridine is a jt-electron-deficient heterocycle. Due to the electronegativity of the nitrogen atom, the a and y positions bear a partial positive charge, making the C(2), C(4), and C(6) positions prone to nucleophilic attacks. A similar trend occurs in the context of palladium chemistry. The a and y positions of halopyridines are more susceptible to the oxidative addition to Pd(0) relative to simple carbocyclic aryl halides. Even a- and y-chloropyridines are viable electrophilic substrates for Pd-catalyzed reactions under standard conditions. 

Thiazole is a jt-electron-excessive heterocycle. The electronegativity of the N-atom at the 3-position makes C(2) partially electropositive and therefore susceptible to nucleophilic attack. In contrast, electrophilic substitution of thiazoles preferentially takes place at the electron-rich C(5) position. More relevant to palladium chemistry, 2-halothiazoles and 2-halobenzothiazoles are prone to undergo oxidative addition to Pd(0) and the resulting o-heteroaryl palladium complexes participate in various coupling reactions. Even 2-chlorothiazole and 2-chlorobenzothiazole are viable substrates for Pd-catalyzed reactions. 

The addition of stabilized ylides, e.g. (63), to nitrile imides or oxides gave pyrazoles or isoxazoles respectively (Scheme 23), in extremely clean reactions which probably proceed by electrophilic attack of the dipole at the y-carbon of the phosphorane.66... 

Electrophilic attack of a metal complex on one of the nitrogen atoms of 1,2-diazines has been reported to occur in the mechanism of new metal mediated methods to prepare C-N bonds. Pyrrolo-fused pyridazines and phthalazines for instance were synthesized via attack of the 1,2-diazine on a palladacyclobutane intermediate 34 formed via oxidative addition of an alkylidenecyclopropane to Pd(PPh3)2 (Equation 7) <2004JOC3202>. 

The reaction in Eq. 13.5 can be thought of as an electrophilic attack by HgtUiotvlhe platinum-carbon bond. The oxidative addition reaction shows oxidation of Pt(II) to Pt(lV) with simultaneous expansion of the coordination number of Pt from A to 6. 

Mercury(II) halides electrophilically attack Pt(CF3C CCF3)(PMePh2)2 at the alkynic carbon (equation 278).8SS Site migrations between PtCl(G=CR)(CO)L and Hg(C=CR )2 involve oxidative addition and reductive elimination sequences.856... 

The ozonolysis of olefins may be analyzed as a sequence of two 1,3-dipolar cycloadditions initial electrophilic attack by ozone 18 to form the first intermediate, which decomposes into a carbonyl compound and a carbonyl oxide 14 followed by nucleophilic 1,3-dipolar addition of the carbonyl ylide 14 to the ketone, yielding the molozonide. 

Since F + cannot be eliminated, electrophilic ipso attack at a fluorinated position and subsequent replacement of fluorine by, for example. N-substituents is not possible. Under these conditions, addition of the electrophile or oxidation to quinoid structures is observed.202... 

In 1,3-benzazoIes (benzoxazoles, benzothiazoies, benzoselenazoles) the heteroatom that is not nitrogen shows very low nucleophilicity and does not serve as a reaction center in electrophilic addition and oxidation-addition reactions. By contrast the tellurium atom in benzotellurazoles is quite susceptible to attack by electrophiles and oxidants. 

---