Alkyl oxidative addition

A series of studies of the oxidative addition of C-C bonds in substrates containing a group that coordinates the metal center has been reported. Addition of acyl carbon bonds after such coordination is particularly favorable. Suggs reported the C-C oxidative addition of an unstrained C-C bond in an acyl quinoline (Equation 6.61). Murakami reported a catalytic reaction involving insertion of an alkene into a cyclobutanone that appears to occur by C(acyl)-C(alkyl) oxidative addition even without an accompanying ligating group. ... 

The tripodal ligand PhP(CH2CH2CH2PPh2)2 forms a rhodium complex [95] (shown below) which does not require dissociation of a phosphorous donor during the catalytic hydrogenation of alkene involving coordination of the alkene, hydride transfer to produce a metal alkyl, oxidative addition of H2, a second hydride transfer, elimination of the alkane, and regeneration of the catalyst. 

Organic compounds M—R and hydrides M—H of main group metals such as Mg, Zn, B, Al, Sn, SI, and Hg react with A—Pd—X complexes formed by oxidative addition, and an organic group or hydride is transferred to Pd by exchange reaction of X with R or H. In other words, the alkylation of Pd takes place (eq. 9). A driving force of the reaction, which is called transmetallation, is ascribed to the difference in the electronegativities of two metals. A typical example is the phenylation of phenylpalladium iodide with phenyltributyltin to form diphenylpalladium (16). 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

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. 

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

Chemisorption of alkanethiols as well as of di- -alkyl disulfides on clean gold gives indistinguishable monolayers (251) probably forming the Au(l) thiolate species. A simple oxidative addition of the S—S bond to the gold surface is possibly the mechanism in the formation of SAMs from disulfides ... 

The total syntheses have yielded cobyric acid and thence cyanocobalamin. Routes to other cobalamins, eg, methylcobalamin and adenosylcobalamin, are known (76—79). One approach to such compounds involves the oxidative addition of the appropriate alkyl haUde (eg, CH I to give methylcobalamin) or tosylate (eg, 5 -A-tosyladenosine to yield adenosylcobalamine) to cobalt(I)alamine. 

This synthesis is only one example of a wide range of reactions which involve aryl (or alkyl) radical addition to electron-deficient double bonds resulting in reduction.The corresponding oxidative reaction using aryl radicals is the well known Meerwein reaction, which uses copper(II) salts. 

Alkyl compounds can be synthesized by substitution, oxidative addition and insertion reactions... 

The use of this phosphine facilitates assignment of configuration as virtual coupling is observed when the phosphines are trans (section 2.9.5).) Syntheses follow established routes using methyllithium as an alkylating agent the platinum(iV) complexes can be made by direct alkylation of platinum(IV) compounds or by oxidative addition to platinum(II) species. 

However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diazocompounds according to the schemes ... 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Like dicyclopentadienyltin, it undergoes oxidative addition-reactions with alkyl halides, and, again, there is evidence for a homolytic chain-mechanism (330, 331). 

Although, as has already been mentioned, under matrix conditions between 10 and 77 K, there is no oxidative addition of a chloroolefin to nickel or palladium atoms (141), it is evident that this is simply a function of reaction and processing conditions, as it has been shown (68) that oxidative addition to C-C or C-H bonds by nickel atoms leads to pseudocomplexes having Ni C H ratios of 2-5 1 2. Klabunde and co-workers investigated the oxidative addition-reactions of palladium atoms with alkyl halides (73) and benzyl chlorides (74). 

Unsaturated alkyl halides react first by ir-complexation (141), followed by C-X oxidative addition, probably on matrix warm-up [but see the preceding point 3, and see ref. (81), which suggests that pyrolysis and radical production can occur on the crucible insulating material to cause reaction]. 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

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