Oxidative addition nonpolar

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

Mechanisms for oxidative additions vary according to the nature of X—Y. If X—Y is nonpolar, as in the case of H3. a concerted reaction leading to a three-centered transition state is most likely. 

A number of different polar and nonpolar covalent bonds are capable of undergoing the oxidative addition to M( ). The widely known substrates are C—X (X = halogen and pseudohalogen). Most frequently observed is the oxidative addition of organic halides of sp2 carbons, and the rate of addition decreases in the order C—I > C—Br >> C—Cl >>> C—F. Alkenyl halides, aryl halides, pseudohalides, acyl halides and sulfonyl halides undergo oxidative addition (eq. 2.1). 

In each of these examples, the metal oxidation state is increased by two, the formed complex contains 18e , and the geometry is octahedral. For oxidative addition of nonpolar molecnles, the adding gronps occnpy cis positions, while for polar molecules the adding gronps adopt trans positions. These different stereochemistries are related to mechanistic differences that will be described below. 

Oxidative addition reactions of nonpolar molecnles, snch as H2, show quite different characteristics from the oxidative addition reactions of polar molecules, such as Mel. For H2 addition, the rate is relatively insensitive to the nature of the metal center, although a stable dihydride is formed only for very electron-rich metal centers. For frani -Ir(CO)(X)(PPh3)2, the rate depends on the X group (X = I > Br > Cl).i The very small deuterium isotope effect = 1-22) and... 

In the preparation of 7r-allyl complexes from cychc allylic chlorides, the stereochemistry of chloride displacement has been found to depend on the reaction conditions (Scheme 27). When the allylic chloride (17) is reacted with Pd2(dba)3, the product from syn oxidative addition, (18)-trans, predominates in nonpolar solvents, while polar solvents give the product from inversion, (18)-c/i. When the Pd(PPh3)4 complex is used as the source of Pd , the isomer from anti addition is isolated in essentially quantitative yield. Apparently, more powerfrd donor solvents or ligands favor anti attack. 

When anhyd HCl is bubbled through trans-IrCl(CO)(PPh3)2 in ether, the product HIr(Cl)2(CO)(PPh3)2 is formed rapidly and quantitatively. In nonpolar solvents or in the solid state the cis isomer is obtained. In polar solvents mixtures of cis and trans adducts formT When the phosphine is tri-o-tolylphosphine, HX addition to trans-IrCl(CO)(PR3)2 is slow because the apical sites are blocked. When the sterically bulky but strongly basic tricyclohexylphosphine is incorporated into trans-IrCl(CO)(PR3)2, the HCl adduct is formed easily. Moreover, when the phosphine of the complex is PMCjCo-MeOCjHp, interaction of the methoxy oxygen with the Ir center increases its nucleophilicity and so facilitates oxidative addition. ... 

When nonpolar X-Y is added across a 77 bond, the first step of the catalytic cycle is usually oxidative addition of Pd(0) across the X-Y a bond. However, when a nucleophile adds to a 77 bond, a Pd(II) complex of the 77 bond may be the first intermediate. 

Oxidative addition is complex in terms of the mechanistic possibilities available. Subtle variations in reaction conditions can result in a change in mechanism. Some symmetrical addenda (H2) or those with nonpolar bonds involving electropositive non-metals (C-H or C-C) seem to undergo addition by a concerted... 

In Chapter 7 (Section 7-2-1), we discussed activation of nonpolar bonds, such as C-H, C-C, and H-H via oxidative addition to a metal center, as having... 

The oxidative addition of molecules having nonpolar or marginally polar c-bonds (e.g., H-H, C-H, Si-H) usually requires the presence of an empty metal d orbital for binding of the substrate before the actual bond-breaking step takes place. The intermediate (7-complexes thus generated are sometimes stable and can be isolated but back-donation of electron density from the metal to the a orbital is often sufficient to induce bond scission and the formation of the oxidized addition product via a three-center transition state (Equation (6)). 

Frontier MO diagram for the interaction of a square planar cP metal compound with H2 in a nonpolar oxidative addition reaction. 

The Whalen intermediate in the nonpolar oxidative addition mechanism for arene substrates. 

Oxidative addition of nonpolar reagents requires a site of unsaturation and a d-electron count of 16 or less. 

Table 6.2. Estimates of the enthalpy and free energy for oxidative addition of several nonpolar and polar reagents.

The remainder of this chapter presents the reactions of nonpolar reagents with a range of metals. The reactions of dihydrogen are presented first the reactions of silanes are presented second because they often react like dihydrogen. The additions of C-H bonds are presented third examples of intramolecular additions of C-H bonds are presented before intermolecular examples. This chapter closes with a discussion of the oxidative addition of C-C bonds. 

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