P -hydrogen abstraction

Sakamoto et al. provided the first example of hydrogen abstraction by thiocarbonyl sulfur in the solid state [37,38]. Irradiation of AT-diphenylacetyl-AT-iso-propylthioaroylamides 6 in the solid state gave thioketones 7 (P-hydrogen abstraction products by thiocarbonyl sulfur), mercaptoazetidin-2-ones (y-hy-drogen abstraction products by thiocarbonyl sulfur), and aroylthioamides (y-hydrogen abstraction products by thiocarbonyl sulfur followed by cleavage). 

Irradiation of the powdered crystals of 6a-d with a high-pressure mercury lamp under argon at 0 °C showed a different photochemical behavior from that in benzene solution. Contrary to the photochemical results in solution media in which azetidin-2-ones 8 (y-hydrogen abstraction products) were formed as major products, thioketones 7 (P-hydrogen abstraction products) were obtained as major components in the solid-state photoreactions (Scheme 7). The... 

Nonetheless, there are some examples of cyclopropanol formation by straightforward P-hydrogen abstraction. The Diels-Alder adduct 100 prepared from o-quino-... 

Starting from some initial chain Yj, any chain Y can be created by the succession of — 1 insertion steps. Since each of the intermediates can be desorbed by some termination step, for instance, by addition of a hydrogen atom or p hydrogen abstraction, we arrive at the simple scheme... 

Despite the addition of anti-oxidants, di-ester-based lubricants are limited in their high-temperature capabilities by a P-hydrogen abstraction. The structure of the di-ester molecule allows it to form a hydrogen-bonded, six-membered, ring. This results in the breakdown of the molecule into an alkene and aldehyde, which then leads to further chemical breakdown as follows ... 

The second side reaction is the Hofmann degradation, a destruction of the quaternary ammonium alcoholate by p hydrogen abstraction. This reaction takes place when the hydroxyalkyl group linked to the amine is longer, having a minimum of two PO units (reaction 13.13). 

N-Allylthioamides on irradiation yield the corresponding N-vinylthioamides and pyrroles. For example (305) yields (307) and (309) by p-hydrogen abstraction and the intermediate biradical (306). Irradiation of the N-vinylthioamides, for example the enethioamide (308), results in formation of the corresponding isoquinolinethione derivative (310). ... 

Zambelli, A. Longo, R Pellecchia, C. Grasi, A. P-Hydrogen abstraction and regiospecific insertion in syndiotactic polymerization of styrene. Macromolecules 1987, 20, 2035-2037. 

The metallocene-catalyzed polymerizations of Lewis acidic monomers normally proceed well if the electronic influence of the heteroatom (B, Si, Al) is diminished by a long spacer. However, when the heteroatom locates in the vicinity of the double bond, its behavior at the metallocene cation is altered. This was clearly seen in the studies of Guram et al. [147, 148], where the vinyl-Si(CH3)3 was found to coordinate with the cationic metallocene strongly via a secondary 2,1-insertion mechanism. This was explained as being due to the electropositive nature of silicon, which drives the electron density at the internal olefinic carbon and therefore favors the 2,1-insertion (Scheme 3a) [147,148]. In contrast to vinyl-Si(CH3)3, allyl-Si(CH3)3 was found to coordinate with the metallocene catalyst via the primary 1,2-insertion mechanism [147]. The behavior of allylsilanes was rationalized on the basis of the formed polar transition state (Scheme 3b), which was stabilized by the p-effect of silicon [149]. This was also the reason for the promoted p-hydrogen abstraction after allylsUane insertions in the studies of Guram [148], Byun [150], Brandow [151], and Casey [152]. 

Secondary Inserted comonomer + unfinished p-hydrogen abstraction... 

The ease of -hydrogen abstraction depends of the metal, its valency state, and the ligand environment. The metals of the extreme and of the transition series are prone to p-hydrogen abstraction easily from an attached alkyl group. The complexes based on these metals are good catalysts for the dimerization of olefins. 

In addition to propagation with monomer, radicals also terminate by either combination with another radical to produce a CT-bond or by disproportionation via P-hydrogen abstraction. The propensity for disproportionation increases as the steric hindrance surrounding the radical increases. For example, a propagating methyl methacrylate radical would be more likely to undergo disproportionation compared to a propagating acrylate radical. 

Scheme 2.11 Deactivation of Grubbs catalysts via p-hydrogen abstraction.

Analogous studies were carried out by Dall occo et al. [21]. They found that in the absence of styrene, titanium had mainly a trivalent oxidation state. ESR measurements revealed the formation of an unidentified new Ti(III) species, possibly containing a Ti-H bond, suggesting a certain degree of P-hydrogen abstraction of the bonded polymer. 

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