Hydride-a elimination

It is also essential that competing radical pathways are excluded. The radical intermediates should therefore be relatively persistent. This is the case here, because tertiary radicals are relatively slowly trapped by hydrogen atom donors, e.g., THF, which is usually applied as solvent in titanocene-mediated or -catalyzed reactions, or a second equivalent of Cp2TiCl. Flowever, in the absence of other pathways this reduction, which was followed by a -hydride elimination, was observed [75,76]. Our results with 10 are summarized in Table 5. 

The mechanism begins with a-hydride elimination to give a benzylidenetitanium complex. A [2 + 2] cycloaddition gives the titanaoxetane, and [2 + 2] retrocycloaddition affords the product and the byproduct. 

The carboalkoxylation of saturated aliphatic halides may give mixtures of isomeric products if carried out above about 75°, at least with tetra-carbonylcobalt anion as catalyst. Isomerization occurs because the intermediate alkylcobalt complex isomerizes competitively with the carbonylation at the higher temperatures. The isomerization probably involves stepwise loss of carbon monoxides to the tricarbonylalkylcobalt(I) stage. This complex then may reversibly rearrange by a hydride elimination to a hydride-olefin-71 complex. The hydride may also add back in the reverse direction and produce an isomeric alkyl. Subsequent readdition of carbon monoxides and alcoholysis would produce isomerized ester ... 

In their pioneering work on the catalytic carbopalladation reaction of 1,2-heptadiene with phenyl iodide in the presence of a suitable base, Shimizu and Tsuji observed the formation of the corresponding substituted 1,3-dienes 62 via a / -hydride elimination from the 7z>allyl intermediate 61 [61]. Based on these observations, a three-component Heck-Diels-Alder cascade process has been developed by Grigg and co-workers [73]. A wide variety of aryl and heteroaryl iodides were used for the intermolecular reaction with dimethylallene to afford the corresponding 1,3-dienes. These subsequently react in situ with N-methylmaleimide to give the bicyclic adducts 63 (Scheme 8.30). 

To unlock its full potential, C-H activation has to be coupled with a functionalization event (e.g., 3—>4). For instance, a hydride elimination occurring after the formation of metal complexes such as 3 furnishes olefins, versatile intermediates for further modification reactions. Transition metal-catalyzed atom- or atom group-transfer reactions that permit the introduction of oxygen-, carbon-, and boron-containing groups are also presented. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

However, studies on the scope of this sequence revealed that the substrate has to be an N-tosyl sulfonamide and that certain boronic acids are not trans-metallated but rather give rise to the formation of the pyrrole 21 or a pyridine derivative 22 (Scheme 7). The peculiar outcome as a carbopalladation-Suzuki sequence is rationalized by co or dinative stabilization of the insertion intermediate 18 by the sulfonyl oxygen atom, as represented in structure 19, now suppressing the usual /3-hydride elimination. If the transmetallation is rapid the Suzuki pathway is entered leading to product 17. However, if the transmetallation is slow, as for furyl or ferrocenyl boronic acid, either /i-hydride elimination or a subsequent cyclic carbopalladation occurs. The former leads to the formation of the diene 20 that is isomerized to the pyrrole 21. The latter furnishes the cyclopropylmethyl Pd species 23, which rearranges with concomitant ring expansion to furnish piperidyl-Pd intermediate 24 that suffers a -hydride elimination to give the methylene tetrahydro pyridine 22. 

AGE474> react with ethane to yield allyl complexes 82 (R = H, COOMe) by insertion into the Ir-C bond. Propene inserts into an Ir-C bond of compound 81 (R = COOMe) in deuterochloroform at room temperature to yield 83, whereas in methylene chloride at elevated temperatures iridabenzene 84 (R = COOMe, R =Et, R = H) is the product. This process may involve isomerizations of propene to propylidene followed by insertion and a-hydride elimination. Compound 81 (R = H) reacts differently and forms iridabenzene 84 (R = H, R = Me) in methylene chloride both at room and moderate temperatures. The process additionally might involve migration of the alkenyl carbon. 

In the presence of trityl perchlorate the ethyliron complex (C2H5)Fe(CO)2(C5H5) undergoes a hydride elimination reaction to afford the corresponding ethylene complex cation (253). Molecular... 

Two mechanistic possibilities were discussed for the rearrangement reactions (Scheme 2). The 16-electron hydride addition to 25, may undergo a-hydride elimination (for R = H) to generate a hydrido vinyli-... 

Oxidative additions are a special class of insertion reactions. In addition to the categories mentioned in Section 10, which covers this topic, insertions of alkylidenes, silylenes, etc., into M-H bonds fall into an ambiguous domain they are insertion reactions of the unsaturated species into the M-H bond, yet oxidative additions at the C, Si, etc., atom. A similar ambiguity exists regarding the reverse reactions, namely /i-hydride and a-hydride eliminations from element-alkyls compounds to yield hy-drido-olefin and hydrido-alkylidene complexes, respectively. The former reaction is a reverse insertion if the product is viewed as an olefin complex, but an oxidative addition if it is viewed as a three-membered metallocycle. The latter reaction is a reverse insertion if the alkylidene is viewed as neutral, but an oxidative addition of a C-H bond to the metal centre. The tautomerization of phosphorous acid and of dialkylphosphites ... 

Cyclometallation in an iridium complex was used to form an unusual alkylidene complex in the conformationally restricted backbone of certain diphos-phines. Such a transformation occurs via a-hydride elimination [Eq. (6.76)] ... 

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