Oxidative addition cationic route

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

Cationic ruthenium complexes of the type [Cp Ru(MeCN)3]PF6 have been shown to provide unique selectivities for inter- and intramolecular reactions that are difficult to reconcile with previously proposed mechanistic routes.29-31 These observations led to a computational study and a new mechanistic proposal based on concerted oxidative addition and alkyne insertion to a stable ruthenacyclopropene intermediate.32 This proposal seems to best explain the unique selectivities. A similar mechanism in the context of C-H activation has recently been proposed from a computational study of a related ruthenium(ll) catalyst.33... 

Since plumbylene is a stable state in lead chemistry, oxidative addition of organo-metallics to plumbylene is an attractive way to lead-centered anions. Following this route, multidecker anions can be prepared for lead, if crown or cryptand ligands coordinate the alkali metal cations (equation 62)70. 

In route A, one electron is removed from one double bond to generate a cation radical, and subsequent transarmular reaction of the cation radical widi die odier double bond forms a new carbon-carbon bond. On the odier hand, in route B, allylic substitution or oxidative addition at one double bond takes place widiout intramolecular interaction between the double bonds. As exemplified by the anodic oxidation of... 

The oxidative addition of allylic esters to paUadium(O) complexes is a common route to allylpalladium complexes. This reaction occurs to form cationic allylpalladium(II) complexes containing an acetate or other carboxylate derivative as the counterion. These reactions were described in Chapter 7. They likely occur by coordination of the olefinic unit of the allylic ester to the metal center, followed by ionization of the coordinated allylic ester to form the allylpalladium intermediate. These reactions occur with inversion of configuration, as shown explicitly by the reaction in Equation 20.20. ... 

A useful metalation approach that has little precedent in cyclic carbene chemistry [18] is the use of 2-chloroamidinium or chloroiminium ions as precursors for acyclic carbene ligands. Fiirstner and coworkers prepared cationic Pd complexes of acyclic diamino-, aminooxy-, aminoarjd, and aminothiocarbenes by oxidative addition of chloroiminium precursors to Pd(PPhs)4 (route d. Scheme 16.1), an approach that was also effective for ADC-Ni complexes [19]. This route permits complexation of sterically nonhindered acyclic carbenes that would not be stable in the free state. Chloroamidinium precursors can be meta-lated without a change in metal oxidation state via lithium-halogen exchange followed by transmetalation (route e). This strategy has been successfully employed with Pd", Rh, and Ir [20]. 

Oxidative addition of allylic compounds to Ni(0) precursors is a reliable route to Ni-allyl complexes, with allyl halides being the most commonly used substrates for this purpose. For example, addition of BrGH2G(R)=GH2 (R = Me or H) to Ni(cod)2, followed by reaction with NaBPh4 and dippe, has given the cationic species [Ni(77 -GH2G(R)=GH2)(dippe)]BPh4. Other substrates such as allylic nitriles can also be versatile precursors for the formation of interesting allyl species. Thus, the reaction of Ni(cod)2 with 2-methyl-3-butenenitrile has been reported to proceed by the oxidative activation of the allyl-GN bond to form an allyl intermediate, which has been trapped as the cyano complex 69 in the presence of l,4-bis(diphenylphosphino)butane (dppb), as shown in Scheme 20. The closely related complex of dippe, 71, has been prepared by the reaction of the cationic species 70 " with various sources of cyanide ion. ... 

Another route to catalytically active ir-allyl intermediates proceeds via oxidative addition to a coordinatively unsaturated palla-dium(O) center (Fig. 15). This usually requires a good leaving group such as acetate in the allylic position [28]. A cationic ir-allyl complex has been isolated and shown to be a reaction intermediate. Vlhen the leaving group is H, as it is in 1-alkene, the reaction is also disfavored thermodynamically unless the hydrogen which would be produced is oxidatively converted to water. 

Halide addition to a cationic carbyne complex, [L M=CR]+, or halogen oxidation of a low oxidation state carbyne complex are both potential routes to monohalocarbene species. Examples of the first process are well known for carbyne complexes from Groups 6 and 7 of the periodic table (120), e.g.,... 

Oxidative Alkoxylation of Nitrones to a-Alkoxy Nitrones and a-Alkoxy Substituted Nitroxyl Radicals The first direct experimental evidence of the possibility to carry out radical cation nucleophilic addition to nitrones with the formation of nitroxyl radicals has been cited in Section 2.4. Further, such a reaction route was referred to as inverted spin trapping this route is an alternative to a conventional spin trapping (508-512). Realization of either mechanism depends on the reaction conditions namely, on the strength of both nucleophile and oxidant. The use of strong oxidants in weak nucleophilic media tends to favour the radical cation mechanism... 

---