Oxidation-reduction mechanisms addition-elimination

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

Two possible routes are envisioned for X = B in Scheme 7-21. The authors favored a path involving the oxidative addition of the S-B bond to Pd(0), insertion of the alkyne into the Pd-S bond followed by C-B bond-forming reductive elimination. On the other hand, Morokuma et al. studied the mechanism of the addition of HSB(0CH2)2 (99) to acetylene (C2H2) using Pd(PH3)2 (100) as a catalyst to produce 101 using hybrid density functional calculations (Eq. 7.62) [5]. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

In this book, there have been many references to oxidation and reduction reactions. While these reactions are not within the scope of the discussions of this book, their mechanisms do involve the processes presented herein. In the case of the Swem oxidation, the first step is an addition-elimination reaction between dimethyl sulfoxide and oxallyl chloride. This process, illustrated below using arrow pushing, involves addition of the sulfoxide oxygen to a carbonyl with subsequent elimination of a chloride anion. 

The mechanism of reductive elimination of a hydrido alkyl complex is therefore often approached in an indirect manner. The hydrido-alkyl complex is made not by oxidative addition of the alkane but by some other route. The decomposition of the hydrido-alkyl complex to give alkane is then studied for mechanistic information. Reductive eliminations of an aldehyde from an acyl-hydrido complex, Reaction 2.7, and acetyl iodide from an iodo-acyl complex,... 

Mechanism Oxidative addition of the thioester to Pd(0) complex and then transmetallation followed by reductive elimination gives the final product (Scheme 5.22). 

The author believes that students are well aware of the basic reaction pathways such as substitutions, additions, eliminations, aromatic substitutions, aliphatic nucleophilic substitutions and electrophilic substitutions. Students may follow undergraduate books on reaction mechanisms for basic knowledge of reactive intermediates and oxidation and reduction processes. Reaction Mechanisms in Organic Synthesis provides extensive coverage of various carbon-carbon bond forming reactions such as transition metal catalyzed reactions use of stabilized carbanions, ylides and enamines for the carbon-carbon bond forming reactions and advance level use of oxidation and reduction reagents in synthesis. 

A number of reactions which at first sight appear to be substitution reactions, particularly at sp centres, in fact proceed via an addition-elimination mechanism (e.g. Scheme 1.2). Oxidation and reduction reactions may often be regarded as subsets of elimination and addition reactions, respectively. Other oxidation reactions may involve the substitution of a hydrogen atom by an oxygen atom, while some reductions involve the displacement of a substituent by hydrogen (hydrogenol-ysis). Rearrangement reactions (Scheme 1.3) may be considered as internal substitution reactions. 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases." In the conventionally accepted reaction mechanism," a four-coordinate aryl-Pd(II) intermediate is formed by oxidative addition of the aryl halide to a Pd(0) complex prior to olefin addition. This suggests that cleavage of the dimeric precursor complex, reduction of Pd , and ligand dissociation combine to give a viable catalytic species." If these processes occur on a time scale comparable to that of the catalytic reaction, nonsteady-state catalysis could occur while the active catalyst is forming, and an... 

Some typical examples of cyclooligomerization catalysts other than nickel are listed in Table 5 and 6. Examination of these reactions indicates that the mechanisms are closely related to each other. They all seem to proceed via allylic intermediates in a stepwise oxidative insertion (addition)/reductive coupling (elimination) fashion while the metal center undergoes changes in the formal oxidation state (viz. Fe"-Fe Ti"-Ti Cr -Cr Co -Co" Mn -Mn Mo -Mo" Ni°-Ni [6b]. 

Reductive elimination (X-M-Y — M + X-Y) is the microscopic reverse of oxidative addition. This reaction is usually most facile when the X-Y bond is strong (e.g., H-Ti-Bu —> Bu-H + Ti). Not as much is known about the mechanism of reductive elimination as is known about oxidative addition. It is known that the two groups must be adjacent to each other in the metal s coordination sphere. In square planar Pd complexes ((R3P)2PdR2), if the PR3 groups are forced to be trans... 

EIEs provide invaluable information concerning both molecular structure and the determination of reaction mechanisms. EIEs are traditionally defined as the ratio of equilibrium constants for unlabeled and labeled reactants and products (EIE = A h/A d Figure 2). For oxidative addition and reductive elimination reactions, the presence of intermediates along the reaction coordinate, such as alkane cr-complexes and agostic interactions, make these reactions multistep processes and hence, additional terms are necessary in order to more fully describe the overall mechanism. Thus, reductive elimination may consist of a reductive coupling (rc) step followed by dissociation (d), whereas the microscopic reverse, oxidative addition, could consist of ligand association (a) followed by oxidative cleavage (oc), as illustrated in Scheme 6. 

Organic chemists, especially those engaged in synthesis, are acutely aware of reactions and their mechanisms, including substitutions, eliminations, additions to double bond, rearrangements and oxidation-reductions. These reactions are often classified by functional group for convenience and to illustrate patterns of chemical behaviour. The effect of structure on reactivity is crucial to understand mechanisms in both chemistry and toxicology. 

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. 

Reductive eliminations from nickel(ll) complexes to form carbon-heteroatom bonds in amines and ethers have also been reported. Like the mechanisms for oxidative additions to Ni(0) and Pd(0) that cleave carbon-heteroatom bonds, the mechanisms for reductive elimination from nickel(II) and palladium(II) complexes to form caibon-heteroatom bonds are different from each other. Most reductive eliminations from Ni(II) to form carbon-nitrogen bonds occur after oxidation of the Ni(II) to Ni(III) with ferro-ceruum, oxygen, or iodine (Equations 8.53 and 8.54). Reductive eliminations from Ni(II) to form carbon-oxygen bonds in ethers also requires oxidation of ttie Ni(II) to Ni(III) (Equation 8.55). In contrast, reductive eliminations from Ni(II) to form the ester group of a lactone occurred after a proposed insertion of CO into the nickel-carbon bond of an oxametallacycle without oxidation. Reductive eliminations from isolated arylnickd complexes to form amines and ethers have not been reported. 

The mechanism of hydrocyanation of ethylene catalyzed by the combination of Ni(0) and P(0-o-To1)3, as deduced by Tolman, is shown in Scheme 16.27 The L2Ni(ethylene) complex has been isolated and shown to add HCN. The Ni(0) complexes of higher olefins are less stable. In this mechanism, oxidative addition of HCN to a Ni(0) olefin complex forms a cyanometal-hydride complex. In the presence of ethylene, this complex contains olefin, but in the presence of higher olefins this complex has the composition L3Ni(H)(CN). Insertion of an olefin into the metal hydride occurs by a migratory insertion mechanism initiated by coordination of olefin to the cyanometal hydride. Reductive elimination of the alkyl cyanide completes the cycle, and this step is accelerated by Lewis acids, as presented later in this chapter. 

The mechanism of this process has been studied in detail. The identity of the palladium(O) species that lies on the catalytic cycle/ the effect of anions on the oxidative addition step/ " the effect of amines in the dissociation of chelating ligands from the palladium(O) complex during the oxidative addition/ the mechanism of formation of the amido complex/ and the mechanism of reductive elimination of amine - - have all been studied. The oxidative addition of aryl chlorides and bromides is generally the turnover-limiting step of the catalytic cycle. 

As with oxidative addition, we find several mechanisms for reductive elimination. The most common, and the one we will invoke for all the catalytic cycles discussed at the end of this chapter, is a concerted elimination. However, radical mechanisms and those involving bridging structures are possible. In all cases, the loss of a ligand or the use of electron withdrawing ligands facilitates the reactions due to the formal reduction at the metal center. 

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