A abstraction

The sources of data for this report were associated with NUREG-0737, (c) Diesel generator data submitted to the USNRC in response to a questionnaire prepared as part of the study. 

Table 2.2 Heat of formation Ahf in kJ/mole (at 25 °C and 1 atm)a (abstracted from Reference [2])...

Darwish, D. McLaren, R. A. Abstracts, 14th National Meeting of the American Chemical Society, Chicago September 1964 p. 445. 

A further general route to heteroatom-substituted carbene complexes is based on the a-abstraction of nucleophiles from alkyl complexes (electrophilic abstraction Figure 2.13). 

Experimental Procedure 2.1.3. Preparation of an Iron Carbene Complex by a-Abstraction of Hydride Dicarbonyl(T 5-cyclopentadienyl)(phenylthiocarbene) iron hexafluorophosphate [179]... 

Table 2.9. Preparation of heteroatom-substituted carbene complexes by a-abstraction.

Many non-heteroatom-substituted carbene complexes have been prepared from alkyl complexes by a-abstraction, for which two mechanistically different pathways must be considered (Figure 3.3) ... 

Fig. 3.3. Possible mechanisms of carbon-metal double-bond formation by a-abstraction.

In complexes in which the metal has free coordination sites a-abstractions can also proceed via 1,1-insertion of the metal M into the C-X bond (intramolecular oxidative addition) [394,395]. Such insertion can be followed by reductive elimination of LX (Figure 3.3). 

Normally, hydride can be readily a-abstracted from alkyl complexes when the metal is in a low oxidation state and bound to good n-accepting ligands. In complexes with p-hydrogen, P-elimination might become the main reaction, specially when highly substituted alkenes are formed (Figure 3.10). 

Fig. 3.11. Generation of cationic iron alkylidene complexes by acid-catalyzed a-abstraction of alkoxide [441,447],...

Also non-heteroatom-substituted tungsten [440,443,444,451,452,461], molybdenum [437], and chromium carbene complexes [440] have been prepared by a-abstraction of alkoxide. 

Some a-alkoxy- or a-(acyloxy)alkyl complexes undergo thermal a-abstraction without the need for additional catalysts (Figure 3.12). 

Fig. 3.12. Generation of alkylidene complexes by thermal a-abstraction of oxygen-bound leaving groups [53,463J.

Generation of Non-Heteroatom-Sub. itituted Carbene Complexes 89 3.1.2.4 a-Abstraction of Halides... 

Halides are, not surprisingly, also suitable leaving groups for the generation of carbon-metal double bonds by a-abstraction. a-Haloalkyl complexes can be converted into carbene complexes either thermally [459,483] or by treatment with Lewis acids [180], The vinylogous variant of this reaction has also been reported (Figure 3.17). 

Fig. 3.17. Generation of carbon-metal double bond.s by a-abstraction of halides [459 490 -492].

Electrophilic transition metal complexes can react with organic ylides to yield alkylidene complexes. A possible mechanism would be the initial formation of alkyl complexes, which are converted into the final carbene complexes by electrophilic a-abstraction (Figure 3.18). This process is particularly important for the generation of acceptor-substituted carbene complexes (Section 4.1). 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

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