A-Hydrogen eliminations

Aliphatic aldehydes and ketones can be converted to gem-dichlorides by treatment with PCI5. The reaction fails for perhalo ketones. " If the aldehyde or ketone has an a hydrogen, elimination of HCl may follow and a vinylic chloride is a frequent side product ... 

The first step in the cycle, analogous to the cross-coupling reactions, is the oxidative addition of an aryl (vinyl) halide or sulfonate onto the low oxidation state metal, usually palladium(O). The second step is the coordination of the olefin followed by its insertion into the palladium-carbon bond (carbopalladation). In most cases palladium is preferentially attached to the sterically less hindered end of the carbon-carbon double bond. The product is released from the palladium in a / -hydrogen elimination and the active form of the catalyst is regenerated by the loss of HX in a reductive elimination step. To facilitate the process an equivalent amount of base is usually added to the reaction mixture. 

Perfluoroacyl fluorides, containing an a-hydrogen. eliminate hydrogen fluoride in the presence of a base to give ketenes,34 82 83 e.g. formation of 11.82... 

El Nemr, A. Tsuchiya, T. a-Hydrogen elimination in some carbohydrate triflates. Tetrahedron Lett. 1995, 36, 7665-7668. 

Figure 4.27 (a) a-Hydrogen elimination, (b) intra- and (c) intermolecular hydrogen abstraction... 

As mentioned above, one of the thermal decomposition pathways of alkylcopper compounds involves a -hydrogen elimination process, and so it is not surprising that the first well characterised alkylcopper compounds lacked such -hydrogens. Treatment of LiCH2SiMe3 with Cul afforded a tetrameric aggregate, the stmcture of which was unambiguously proven by an X-ray crystal structure determination (see Fig. 1.1 B in the previous section). This represented the first example of a well characterized alkylcopper compound [17]. 

Adsorbed carbenes formed by a-hydrogen elimination are the most likely precursors of dicarbynes on iridium. On this metal, further dehydrogenation of metallocarbenes to metallocarbynes might also be easy, thus explaining why the selective Mechanism B for both dehydrocyclization and hydrogenolysis is the only one operating. 

A last question that arises now is the mode of generation of the metallocarbenes. Besides the indirect formation by C-C bond rupture, in metallocyclobutane dismutation, for instance (Scheme 59a), two other possible ways are a-hydrogen elimination (Scheme 59b) and hydrogen shift in ii-adsorbed olefin (Scheme 59c) (see also Scheme 48). 

Scheme 59. Different modes of generation of metallocarbenes. (a) Metallocyclobutane dismutation (b) a-hydrogen elimination (c) a- -hydrogen shift.

Reed found that crystalline prenyltransferase could solvolyze the allylic substrates. This reaction required inorganic pyrophosphate and had a velocity of about 2% of the normal reaction rate [6]. Examination of the allylic product, either dimethylallyl alcohol or geraniol, revealed that C-1 had inverted and the csu-binol oxygen had come from water. Since the normal reaction involves inversion of C-1 and scission of C-O bond, the solvolysis seemed to be mimicking the normal reaction, with H2O replacing the organic portion of isopentenyl pyrophosphate in the catalytic site. This indicates that ionization of the allylic pyrophosphate is the first event, followed by condensation to form a new bond, then a hydrogen elimination from C-2 of the former isopentenyl moiety. Thus, there is an ionization-condensation-elimination sequence of events. 

Since methyl vinyl ketone and not crotonaldehyde is made from 2-butene, Hearne and Adams 87) suggested that the reaction is not necessarily a direct substitution in the allylic position but may also involve a hydrogen elimination from the allyl position and a shift of the double bond. In view of the allylic intermediate discussed below,... 

The mechanism or mechanisms are believed to involve formation of an alkyl followed by selective a hydrogen abstraction to give an alkylidene, as shown in Eq. 25, although it is not yet known what factor or factors tip the energetic balance in favor of the alkylidene in these examples. Selective a hydrogen elimination and abstraction have been observed in other circumstances, especially those in which (3 processes are sterically blocked. [135-138] These... 

Another example of a hydrogen elimination process which is both reversible and exhibits a statistical kinetic energy release distribution is the dissociation of the cyclopentadienyl rhodium isopropyl ion, reaction 13(12). In this case the... 

Transition metal alkyls having o -hydrogen(s) may undergo the a-hydrogen elimination process as shown in Eq. 7.25. 

Further a-hydrogen elimination giving an alkylidyne complex was induced by addition of PMes to an alkylidene complex (Eq. 7.28). 

The propensity toward the a-hydrogen elimination from the alkylidene ligand was ascribed to the steric effect of the bulky neopentyl group pushing the a-hydrogen toward the metal to facilitate the a-hydrogen abstraction as revealed by neutron diffraction studies of the structure of an alkylidene complex, Ta(=CHBuOCl3PMe3 [124]. 

The utility of the Tebbe type complex in carbonyl olefination is discussed in Chapter 4. The bridged complex may be regarded as a special type of a carbene complex where the Cp2Ti=CH2 unit is masked by interaction with the AlMe2Cl entity. Formation of the Tebbe s complex suggests the occurrence of a-hydrogen elimination in the preparation of the Ziegler-Natta and Kaminsky type olefin polymerization catalysts from titanium chlorides and methylaluminum compounds. 

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