Oxidative Addition and Reductive Elimination of Alkyl Halides

Intramolecular C-H addition in metal cluster complexes is a frequently observed, if poorly understood, phenomenon/ 

Oxidative Addition and Reductive Elimination of Alkyl Halides 

A gas-phase study in this area has appeared recently/Perfluoroethane and several fluoroethylenes were reacted with [Mn(CO)3] in a mass spectrometer. The metal fragment tended to insert into two vinyl fluoride bonds, creating [p2Mn(acety-lene)] species. The same product was observed when the reaction was carried out using perfluoroethane. The two extra fluorines were released as a carbonyl compound, COF2. A common mechanism for the oxidative addition of RX to late transition metal centers is the S /2 type, which results in trans addition. It is outlined in reaction (13). The lifetime of the intermediate species may vary greatly. 

11 Metal-Alkyl and Metal-Hydride Bond Formation and Fission 

Typically, these reactions proceed by a two-step (so-called S v2 ) mechanism involving a charge-separated intermediate [LnMR] X . Three-center transition states have been considered, but these are likely to be highly polar. Lone-pair donation complexes of alkyl halides (e.g., CH3I) are known, but not as intermediates to oxidative addition. Recently reported reaction (9) is the first such 

Addition of MeX to [(tmeda)PdMe2] generates (40), X = I, Br, Tf or (41), X = Tf, S = NCCD3, 00(003)2 in coordinating solvents. Both rapidly and cleanly eliminate ethane. The favored elimination of ethane or p-Br06H4Et from (42) may be controlled by the choice of (L2) = bipy or phen. S7v2-type intermediates were observed at low temperature in this reaction. A complex, but 

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Oxidative Addition and Reductive Elimination of Alkyl Halides 279 Oxidative Addition and Reductive Elimination Involving... 

Particular attention has been devoted to oxidative addition and reductive elimination reactions of [M2(PNNP)(/x-X)L2] with acyl and alkyl halides. Depending on the electron richness of the metals, a complete spectrum of possibilities was observed from reversible single oxidative addition on one of the metals to irreversible double oxidative addition on both metals (2) (Scheme 12). 

The reversibility argument also applies to reductive elimination of alkyl halides for which an Sn2 pathway (Fig. 6.2) applies for the oxidative addition direction. Iodide attacks the coordinated methyl trans to the open site and nucleophili-cally displaces the Pt complex, which is a good leaving group. The reactive 5-coordinate intermediate, which can even be isolated in some cases, can also undergo concerted reductive elimination of ethane if the concentration is low. ... 

Reductive eliminations to form carbon-heteroatom bonds occur by several mechanisms. As would be expected from microscopic reversibility, these mechanisms parallel those for the oxidative addition of carbon-heteroatom bonds. Recall that oxidative additions to form Pt(IV) alkyl halide complexes and oxidative additions to form Pd(II) alkyl and aryl halide complexes occur by different pathways. The former occurs by a stepwise mechanism involving charged intermediates, and the latter occurs by concerted pathways. 

As we have seen, oxidative addition is the inverse of reductive elimination and vice versa. In principle, each reaction is reversible, but in practice the reactions tend to go in the oxidative or reductive direction only. The position of equilibrium in any particular case is governed by the overall thermodynamics this in turn depends on the relative stabilities of the two oxidation states and the balance of the A—B versus the M—A and M—B bond strengths. Alkyl hydride complexes commonly eliminate alkane, but only rarely do alkanes oxidatively add to a metal. Conversely, alkyl halides commonly add to metal complexes, but the adducts rarely reductively eliminate the alkyl halide. Third-row elements, which tend to have stronger metal-ligand bonds, tend to give more stable adducts. Occasionally, an equilibrium is established in which both the forward and back reactions are observed. 

Although in this chapter we shall be restricting coverage to reactions of transition-metal complexes, the phenomenon of oxidative addition is not confined to this type of compound. Such reactions are also well established for non-transition metals—a recently reported example concerns the oxidative addition of methyl bromide to indium(i) bromide to give InBr2Me— and for non-metals, as in the reaction of phosphorus trichloride with chlorine, to cite a very familiar example. Likewise, reductive eliminations are known and studied outside the area of transition-metal complexes. One example has been mentioned in Chapter 1 of Part II of this volume, namely the elimination of alkyl halides from the thallium(iii) compounds TlRXa. ... 

Stille coupling was also developed in tlie early 1980s and is similar to Suzuki coupling in its sequence. It is used to couple aryl or vinyl halides or triflates with organotin compounds via oxidative addition, transmetallation, and reductive elimination. The oxidative addition reaction has tlie same requirements and preferences as discussed earlier for tlie Heck and Suzuki reactions. The reductive elimination results in formation of tlie new carbon-carbon bond. The main difference is that tlie transmetallation reaction uses an organotin compound and occurs readily without the need for an oxygen base. Aryl, alkenyl, and alkyl stannanes are readily available. Usually only one of tlie groups on tin enters into... 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

Carbonylation of alkyl halides is rare. As an exception, AcOH is produced commercially by the Monsanto process from MeOH and CO using Rh as a catalyst in the presence of HI. In this process (Scheme 3.10), Mel is generated in situ from MeOH and HI and undergoes oxidative addition. Insertion of CO generates an acetylrhodium intermediate, and nucleophilic attack of water produces AcOH, regenerating the Rh catalyst and HI (or reductive elimination to give acetyl iodide and hydrolysis). 

Alkylpalladium complexes generated by oxidative addition of Pd(0) to alkyl halides with a /3 hydrogen can undergo /3-elimination to yield an alkene and a Pd-hydrido complex (as in the Heck reaction Scheme8.7). Nevertheless, this process is relatively slow compared with transmetalations and reductive eliminations, and simple alkyl halides or tosylates with /3 hydrogen can be cross-coupled with carbon nucleophiles under optimized conditions if the nucleophile is sufficiently reactive [9, 73-75] (Scheme8.6). 

A. Oxidative Addition of Alkyl Halides or Related Reagents and Reductive Elimination... 

Mechanistically, the Heck reaction can be rationalized as follows (Scheme 2) First, a Pd(II) complex of type 3 is formed by oxidative addition of the halide 1 to a L2Pd(0) species. Complex 3 then reacts with an olefin via the 7J-com-plex 4 to give the -insertion product, i.e. the alkyl complex 5. After -H elimination, the product 2 is released and the active catalyst is regenerated by base-assisted reductive elimination of HX. 

A second important reaction of L-AuR species is the oxidative addition (see Oxidative Addition) of halogen or alkyl halides, which leads, at least in a first step, to organo(dihalo)gold(IIl) or diorgano(halo)gold(III) products. The structures are transformed from linear to square planar, and therefore cis and trans isomers are possible. The products may undergo secondary reactions and/or become subject to a reductive elimination (see Reductive Elimination) of other substituent combinations (equations 41 and 42). ... 

The acyliron(0) complex (102) has been isolated and subjected to the same nucleophilic displacement (or equivalently oxidative addition) with excellent correlation (Scheme 39). The same species is also readily available from acid chlorides (i.e. formation of 103), but the overall process has not been widely used in the synthesis of ketones (Scheme 40). The final step of the process, a reductive elimination of acyliron(II) complex (104) or (105), is quite rapid and it has not been possible to isolate and identify the presumed intermediates in this case (Scheme 41). Since the oxidative addition of the acyliron complex with the alkyl halide is extremely mild, the corresponding ketone formed in the reaction is not subject to attack by organometallic reagents and no tertiary tdcohol is formed. 

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