Oxidative addition of polar reagents

Chapter 6 presented the basic principles of oxidative addition. Recall that the term oxidative addition refers to reactions that lead to an increase in the oxidation state of the metal center by two units, an increase in the number of valence electrons on the metal center by two, and an increase in the coordination number of the complex by one or two. This term does not refer to a particular mechanism by which this transformation occurs. 

Oxidative additions of alkyl halides and pseudohalides can occur by an S 2 mechanism in which the metal acts as a nucleophile (Equation 7.1). The first step of this reaction is analogous to the alkylation of an amine. The data in support of an S 2 mechanism for 

The oxidative addition of methyl iodide to Vaska s complex, shown in Equation 7.2, is a classic example of the oxidative addition of alkyl halides by an mechanism. Strong electrophiles that are sterically accessible, such as methyl iodide, benzyl bromide, allyl halides, and chloromethyl ethers, react with Lj(CO)IrX species by this pathway. A series of data supports addition of these electrophiles by an mechanism. For example, the trans stereochemistry of the kinetic product from addition of methyl iodide is inconsistent with a concerted three-centered mechanism radical traps do not affect the rate or products of the reaction the reaction rates are faster in more polar solvents - and the reactions are first order in both metal and electrophile. Higher aUcyl halides add by more complex mechanisms presented below. 

Palladium(O) complexes also add alkyl halides by 5, 2 mechanisms. Oxidative additions of Mel to Pd(0) complexes were some of the early examples of this reaction, and a subsequent study demonstrated that oxidative addition of an alkyl tosylate and a higher alkyl bromide occurs by an S 2 path. - Equation 7.3 shows the stereochemical evidence for an Sj 2 pathway. This equation shows the individual steps that occur during the catalytic addition of an arylborane to a stereochemically defined alkyl tosylate. The product forms with overall inversion of configuration. As is noted in Chapter 8, the final step, reductive elimination to form a C-C bond, occurs with retention of configuration. Thus, the first oxidative addition step must occur with inversion of configuration, and this inversion of configuration signals an S 2 reaction. 

The analogous anionic iridium complex reacts with methyl iodide 150 times faster than the rhodium complex. The iridium complex also reacts 140-200 times faster than the rhodium analog with higher alkyl iodides/ but competing radical mechanisms appear to occur during the addition of the higher alkyl iodides. More details on the mechanism of rhodium and iridium-catalyzed carbonylation of methanol are provided in Chapter 17. 

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The second most common method to prepare metal-alkyl complexes is the alkylation of nucleophilic metal complexes. Like protonations, these alkylations are formal oxidative additions, and are discussed in that context in Chapter 7, which covers oxidative addition of polar reagents. The reaction between an anionic metal-alkyl complex and an alkyl halide is most common (Equations 3.6 and 3.7), but examples of such reactions between neutral metal-alkyl complexes and alkyl halides are also known (Equation 3.8). ... 

A widely used route for the synthesis of Grignard reagents is the oxidative addition of magnesium metal to organic halides in a polar, aprotic solvent like THE or diethyl ether (equation 1). 

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

Oxidative additions of substrates containing H-X bonds that are more polar than those in a hydrocarbon, silane, or borane have been studied recently. These reactions can occur through a pathway involving a three-centered transition state, or they can occur by protonation of a basic metal center. The latter pathway requires a highly acidic reagent when the reactions are conducted in nonpolar media that do not stabilize charged intermediates. 

In oxidative additions via a concerted mechanism (Fig. 1.4), the A-B molecule binds firstly to the metal center and then, the cleavage of the A-B bond and the formation of the new M-A and M-B bonds take place simultaneously through a three-centered transition state. This mechanism is normally found in oxidative additions of non-polar reagents [78-81] and aryl halides [82-85]. Experimental evidences for this mechanism are the retention of configuration at a stereogenic center in the case of chiral A-B reagents, and the relative cis disposition of the ligands A and B after the oxidative addition [86]. The latter, however, may be not observed in the cases in which the cis-to-trans isomerization reaction of the oxidative product is very fast [87],... 

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