A-H elimination

Transformation of an alkyl metal species via a-H elimination to the (C, H)-chelating ground state (written as (b) of the scheme)... 

An alcoholic solution of GTN, if protected from light, is stable over many months. Most organic nitrates are only slightly soluble in water and under alkaline conditions [56] hydrolyse to give an alcohol and nitrate (Eq. (8)) or undergo a-H elimination to give nitrite (Eq. (9)). 

CP/MAS NMR study of CH4 activation on [(=SiO)2Ta(H)J shows the formation even at 150 °C of methyUdene and methyUdyne species by an a-H elimination process on several sites that should correspond to the tris-hydride on other sites a methyl group is transferred to the surface, leading to the formation of (=Si-Me) and of [(=SiO)3Ta]. Correlation with EXAFS suggests that the tris-hydride should exist on surface sites (=20%) quite distant from siloxy bridges whereas methyl transfer to the surface should happen on the specific sites (=80%) close to the siloxy bridges. The latter, which are formally 10 electrons species, exhibit a moderate to weak activity in methane C-H activation. To the best of our knowledge, this is the first observation of methyl group transfer on a surface (Scheme 2.18). 

SiO)MNp3 The supported tris(neopentyl)titanium complex easily undergoes decomposition even at low temperatures [42]. Up to 150 °C, only neopentane is released, which can be explained by either a-H elimination, to give an alkylidene species, or y-H elimination, to give a metaUacyclic species (Scheme 11.1). Nevertheless, no such species could be observed by C CP-MAS or H MAS NMR. The... 

The two intermediates depicted above differ fundamentally from each other. The COx-producing intermediate has a direct metal-carbon (M-R) bond whereas the C2-producing intermediate has a metal-oxygen-carbon (M-O-R) bond. From known organic decomposition pathways, the formation of selective oxidation products from the M-O-R intermediate is likely. An a-H elimination produces acetaldehyde and a P-H elimination produces ethylene. 

An Ir-O heterocycle, Fischer-type carbene 69, has been synthesized in 37% yield by Santos et al. <2003NJC107>. The process involved additionally ortV o-metallation of the anisole ring hence, three C-H bonds were sequentially broken, the last one in the course of an a-H elimination reaction (Equation 9). 

Further a-H-elimination and reaction of the products with CpNiCHa leads to the formation of (/7 -cyclopentadienyl)(jU3-methylidyne)trinickel cluster 2, which has been recently obtained and spectroscopically characterized (Eq. (12)). 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

Later it was found that to get a successful activity in alkane metathesis, catalysts need to have a multifunctionality, i.e., (i) activation of the C-H bond resulting in a metal alkyl, (ii) a-H elimination leading to a metallocarbene, (iii) p-H elimination leading to an olefin, (iv) olefin metathesis, and (v) finally successive hydrogenations of the olefins or the carbenes leading to alkanes. The selectivity of products is a consequence of the relative stabilities of metaUacyclobutanes intermediates formed during the olefin metathesis [52]. 

The reaction of electrophilic platinum cations developed to study the oxidative addition of alkanes to platinum center with diethyl ether has been shown to generate carbene complex 163 in high yield the product is thought to arise from the C-H activation of diethyl ether followed by an a-H elimination reaction (Equation (30)). 

For the hydride elimination in Heck reactions of substituted thiophenes, where the iyn-mechanism is structurally precluded (from the 2-position), three mechanistic pathways can be envisaged (i) isomerization followed by syn-/i-H elimination (ii) a-H elimination, and 1,2-H shift and (iii) anti-P-H elimination. According to DFT calculations, the base-assisted anti-P-H elimination (third pathway) is the most energetically... 

The reverse reaction, extrusion of a methylene from a metal-alkyl complex to give a species of the type M(H)(C2H4) is also known, but it is far from being systematic this is the a-H elimination (see section 5.1 and Chap. 18.5) that sometimes plays an important role in the initiation of metathesis reactions (Chap. 15). 

In the absence of p-H atom, other decomposition routes are viable a- and Y-elimination. The a-H elimination is the classic bulk-induced synthetic route to early-transition-metal-alkylidene complexes. Both a- and y-elimination can occur by oxidative addition or a-bond metathesis depending essentially on the presence of NBVEs on the metal. Example ... 

The synthesis of these high oxidation-state complexes of nucleophilic alkylidene ligands has been extensively developed by Schrock s group and usually involves the a-H elimination from an alkyl ligand. The various means to achieve this elimination... 

The ethylidyne fragment is formed at 300 K by ethylene insertion into Pt-Hads and two a-H eliminations. This is followed by successive loss of 3 H atoms by p-H eliminations as the temperature is increased. At high temperature, irreversible formation of graphite inhibits the catalytic reactions. 

The common point that makes the Schrock and Basset catalysts active in alkene metathesis is the alkoxy or aryloxy ligand in Schrock s catalysts and the silyloxy ligand in Basset s catalysts. Also note that Basset s catalysts are in fact Schrock s complexes or catalysts that are grafted on silica in order to link the metal by a Si-O-M bond and make catalysts heterogeneous. Schrock s high oxidation state compounds also give o-bond metathesis upon a-H elimination, but they do not catalyze alkane disproportionation. 

A similar reaction was reported by Zhang el al. (Equation (6.5)), and [V( = NR)(CH2SiMe3)3] (R = adamantyl or 2,6-dimethylphenyl) precursors were treated with free IPr giving the corresponding alkyl-alkylidene complexes 17 and 18 via a-H-elimination from two adjacent alkyl ligands. 

In Group VA alkylidenes, one of the major mechanistic problems concerns the existence of the a-H elimination from alkyls. The transformation shown in equation (23), which occurs also for the analogous deuteriated complex) offered... 

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