Oxidative addition of alkyl halides

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

The most interesting and difficult cross-coupling is alkyl-alkyl coupling, because oxidative addition of alkyl halides having /i-hydrogen is slow. In addition, easy elimination of /d-hydrogen is expected after the oxidative addition. 

The oxidative addition of alkyl halides can proceed in different ways, although the result is usually atrans addition independent of the mechanism. In certain cases the reaction proceeds as an SN2 reaction as in organic chemistry. That is to say that the electron-rich metal nucleophile attacks the carbon atom of the alkyl halide, the halide being the leaving group. This process leads to inversion of the stereochemistry of the carbon atom (only when the carbon atom is asymmetric can this be observed). There are also examples in which racemisation occurs. This has been explained on the basis of a radical chain... 

Formation of five-coordinate nickel intermediates via oxidative addition of alkyl halides has been reported (33). This type of... 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

This process is similar to the formation of Grignard reagents 4 from alkyl halides and Mg(0). In the preparation of Grignard reagents, Mg(0) is oxidized to Mg(II) by the oxidative addition of alkyl halides to form two covalent bonds. 

Palladacycle 5 reacts in various ways depending on ligands and reaction conditions. In particular it readily undergoes oxidative addition of alkyl halides to form a palladium(IV) complex 6, which has been isolated and characterized with stabilizing nitrogen ligands such as phenanthroline. This palladium(IV) metallacycle... 

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

Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C—H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). 

The rate-determining step in the catalytic cycle is the oxidative addition of CH3I to 4.1. Oxidative additions of alkyl halides are often known to follow Sa2 mechanisms. This appears to be also the case here. The net negative charge on 4.1 enhances its nucleophilicity and reactivity towards CH3I. 

Mechanistic studies have been carried out on the oxidative addition of alkyl halides to Sn[CH(SiMe3)2]2 (79, 80). Radical intermediates were observed by spin trapping, and when an optically active alkyl halide was used racemization occurred. The evidence obtained led to the proposed mechanism shown in Eq. (40). 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

There are quite a number of routes available for the production of iridium(ni) alkyl compounds. In addition to the halide displacement and olefin insertion pathways noted above for iridium(l) compounds, oxidative addition of C-H bonds to iridium(l) to form iridium(in) hydrido alkyl complexes is also a possibihty. This subject will be covered in detail in Section 9 and will not be discussed here. However, there are other oxidative addition routes that lead to the formation of iridium(lll) alkyls. First, oxidative addition of O2 or HCl to some alkyl and aryl iridium(l) complexes can produce iridium(lll) alkyl or aryl compounds. In some cases, HgCl2 can add, but this appears to lead to tractable products only for the very stable pentafluorophenyl complex. Of course, oxidative addition see Oxidative Addition) of alkyl halides such as H3CI will also yield alkyl iridium(lll) compounds. Addition of Mel to Vaska s compound yields a stable iridium(III) complex, but addition of Etl does not produce a stable compound, presumably due to subsequent /J-hydride elimination see fi-Hydride Elimination). A number of mechanistic studies have been done on the oxidative addition of alkyl halides to iridium(l), especially Vaska s complex see Vaska s Complex). 

Group VIII—Ni, Pd, Pt. The Ni triad provides the widest reactivity toward oxidative addition of alkyl halides. It is in this triad that the extreme sensitivity of the oxidative addition reaction to changes in ligands, the organic halide and reaction conditions (solvent and T) are most felt. 

In the oxidative addition of alkyl halides to Ni(0) the number of open coordination sites created by dissociable ligands determines whether one- or two-electron oxidations... 

In view of the fact that oxidative addition of alkyl halides and acyl halides to chlorocarbonylbis (triphenylphosphine) rhodium is possible, it was expected that the carbonylation of alkyl halides could be carried out catalytically with this complex. Actually the carbonylation of certain... 

The reactions of germanium metal with alkyl halides afford dialkyldihalogermanes (eq (85)) [82], Alkyltrihalogermanes are obtained by the similar oxidative addition of alkyl halides to germanium(II) iodide (eq (86)) [83],... 

Fast intramolecular palladation to VII iii) Oxidative addition of alkyl halide forms Pd(IV) complex IV... 

SCHEME 13. Preparation of Au(II) and Au(III) complexes from [Au2 (CH2)2PPh2 2 by oxidative addition of alkyl halides... 

The mechanisms of the usual organic reactions are now clearly established, and the reactions are classified as ionic, radical, and molecular. More detailed classifications have also been made. The mechanisms of many reactions involving non-transition metal compounds are clear enough for example, in the Grig-nard or Reformatsky reaction, the first step is the irreversible oxidative addition of alkyl halides to form Mg-carbon or Zn-carbon bonds, in which the carbon is considered to be a nucleophilic center or carbanion which reacts with various electrophiles. 

Another important process involving oxidative addition of alkyl halide is found in the hrst step in Monsanto process, an important industrial process to convert methanol into acetic acid with carbon monoxide at 150-200°C under 30-60 bar of CO [24]. In the Monsanto process methyl iodide is formed first from methanol and hydrogen iodide, added as the promoter to the catalytic system. The subsequent reaction of methyl iodide with a rhodium(I) species forms a methylrhodium(III) iodide species hy oxidative addition (Eq. 1.2). 

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