A-Hydride abstraction

Reactions of the tetrahedral Ni° complex (994) have already been discussed in Section 6.3.5.4.2397 A nickel carbonyl cation (1062) containing a cyclophosphenium ligand has been assembled through a hydride abstraction reaction according to Equation (37).2552... 

Numerous examples have been reported of transition metal alkyl complexes which can be converted into carbene complexes by a-hydride abstraction [429-431], This process can also proceed intramolecularly by oxidative insertion of the metal into the a-C-H bond. Figure 3.8. shows some illustrative examples of iron, rhenium, and... 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Of the numerous proposals, the metal hydride route with a-hydride abstraction (p elimination) [Eq. (12.18)] has gained substantial experimental support 7... 

As early as in 1956, Braude et al.92 suggested that the selective oxidation of unsaturated alcohols with the quinone o-chloranil (82), can be explained by the intermediacy of a resonance-stabilized cation resulting from a hydride abstraction. Later, detailed mechanistic studies confirmed this hypothesis94c,95e in oxidations performed with the more common quinone DDQ. 

The second class of TAM RE AC s inventory includes the reactions between the coordinated ligands and external organic reagents. We divide these reactions into nucleophilic and electrophilic attacks and consider them as acid-base interactions. Table III presents their general description. The nucleophilic attacks are either addition reactions to unsaturated coordinated ligands (Reactions 44-46) or abstraction reactions (usually a proton abstraction, Reactions 47-50). The electrophilic attacks are similarly addition reactions (Reactions 51 and 52) and abstraction reactions (usually a hydride abstraction, Reactions 53-59). Reactions 60 to 63 represent some other intermolecular reactions. 

Metal hydride species can undergo an insertion reaction with an olefinic double bond, resulting in the formation of a metal alkyl species which then generates the requisite carbenoids by a-hydride abstraction [110-112] ... 

For oxidation of ethylbenzene in FeCl3-catalysed reaction with ferf-butyl hydroxyperoxide isotope effect fcHn//C id = 2.02(4) was found in analysis of deuterium content in ethylbenzene at natural abundance. Mixture of ethylbenzene and 1,1-dideuteroethylbenzene (45 55) was used for the determination of (hh/ fcDD = 5.0(1). Isotope effects KIEr- = 3.5(2) and KIE2- = 1.41(6) were calculated from formulas (26). The value of primary isotope effect is consistent with a C-H bond breakage in the rate-limiting step, but its value does not allow distinguishing among a hydrogen radical abstraction, C-H insertion or a hydride abstraction processes. Secondary isotope effect is consistent with formation of benzylic radical. 

The mechanism of this reaction proceeds via an a-hydride abstraction with formation of an intermediate chloroxonium ion pair, under conditions where the free-radical chlorination is suppressed by the lack of light855. The above cyclic ethers are smoothly isomer-ized in presence of aluminum chloride to 2,2,4-trichlorobutanal856 and 2,2,5-trichlorohep-tanal857, respectively. Methylthio(thiocarbonyl) compounds have also been chlorinated by... 

Methylbenzenes can readily be transformed into benzoic acids at room temperature upon exposure to aqueous sodium hypochlorite at pH 9-10 in the presence of RuC13 and tetra-n-butylammonium bromide (TBAB) catalysts in a two-phase system (Eq. 6) [39]. The proposed mechanism for the Ru-NaOCl phase transfer catalyst system is a hydride abstraction from the substrate by Ru04 to form a carbonium ion which is promptly hydrolysed. A major... 

Moreau et al.56 obtained unexpected results in the alkylation of naphtalene with 2-propanol over H-Beta in the liquid phase at 200°C. Here a cyclic compound 1 was formed with a selectivity around 40% at 28.5% conversion. When applying HY as the catalyst alkylation to di- and trialkylnaphthalenes was faster but the cyclic compound was not observed. These results illustrate the more confined space within the zeolites Beta channels. The cyclic compound is assumed to be formed through iso-propylation of naphthalene followed by a hydride abstraction giving a carbenium ion, reaction with a propylene and finally ring-closure. 

A systematic study showed that the main ionizing reactions of molecules containing heteroatoms occurred through acid-base reactions with C2H5+ and C3H5+. If, however, the sample is a saturated hydrocarbon RH, the ionization reaction will be a hydride abstraction ... 

Ion of the molecular species ion originating from the molecule by the abstraction of a proton [M - H] or by a hydride abstraction [M - H]+, or by interaction with a proton or a cation to form a protonated molecule [M + H]+ or a cationized molecule [M + Cat]+. These ions allow the molecular weight to be deduced. The ambiguous and obsolete terms quasimolecular ion or pseudomolecular ion should be avoided. 

In the cases of tungsten and molybdenum, the complexes are believed to be intermediates in the oxidation of alcohols to carbonyl compounds (a hydride abstraction reaction).85 Figure 2.28 illustrates some of the oxygen transfer... 

The final step of a synthesis of the alkaloid Monocrotaline [Scheme 3.94] required liberation of a hindered 1,2-diol encased in the methylene acetal 941.173 The task was accomplished by a hydride abstraction reaction using triphenyicar-benium tetrafluoroborate to give an intermediate of unknown structure whose hydrolysis afforded Monocrotaline in 86% yield. Triphenylcarbenium tetrafluoroborate can also be used to deprotect dioxolane derivatives of ketones174 and benzylidene derivatives.1755... 

Although in a few tetranuclear complexes fragmentation is the primary result of photolysis , and others, e.g., (h -CjHp Fe fCO), are inert with respect to both photoinduced M—M cleavage and carbonyl loss , substitution without fragmentation is the dominant photoreaction of tetranuclear metal carbonyls, especially of those containing second and third-row metals. Both Ir (CO),2 and H Os (CO),2 " lead to tetranuclear olefin-substituted products when irradiated with alkenes, both with some rearrangement of the Ir moiety in the former case, and a hydride abstraction in the latter. Irradiation of H Ru/CO),j in the presence of P(OMe)j or PPhj leads to stepwise formation of the substituted clusters H Ru/CO), (n = 1-4) with low quantum efficiency . Carbonyl substitution also dominates the photochemistry of HjM CCO) (M = Ru, FeRuj, FeOsj) . 

Metathesis is the best route to these complexes. The preparation of W alkylidehes by a-hydride abstraction has had only limited success , and the neopentylidene substrate is prepared by an unusual heterometallic exchange reaction ... 

Fe( / -C5H5XCO)2CHCH2CH2 a-Hydride abstraction 5.8.225 Fe( / -C5H5XCO)2CH2CH=CH2 Reaction with Fp(olefin) S.8.2.3.4 Reaction with [PhjClCBF ] 5.8.2.2.S Reaction with TCNE 5.8.23.4... 

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