P-cleavage

For 4,5-dialkylthiazoles, the molecular ion decomposes by two competitive pathways, either loss of HCN followed by elimination of the radical R in the position /3 to the double bond of the resulting substituted thiirene, or by p cleavage followed by elimination of HCN (119). 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

A modification of the Hantzsch synthesis of thiazoles has been reported. The reaction of alkoxyoxiranes 16 with A-arylthioureas 17 affords thiazoles such as 20. The mechanism involves the initial P-cleavage of the oxirane to give the hemiacetal... 

P-Cleavage of homoallylic alcohols.1 Homoallylic tertiary potassium alkox-ides undergo cleavage in HMPT (or DMF) of the allylic C—C bond to give the enolate of a ketone (equation I). A rigid bicyclic system is not essential for the cleavage. 

The loss of alkenes from molecular ions of carbonyl compounds has early been noted. [23,82] Soon, a mechanism involving y-H shift and P-cleavage has been proposed and studied in detail. [24-26,83,84] Strictly speaking, the term McLafferty rearrangement only describes an alkene loss from molecular ions of satu-... 

The main products under all variations of R and R are the P2-chlorinated triphosphanes. Side reactions such as SiMe3/Cl exchange, P—P cleavage by LiP(SiMe3)R, or transmetallations take place only to a small extent. MeP(SiMe3)2 is an exception inasmuch as it reacts with P-chlorinated phosphanes to yield tri- and even tetraphosphanes. 

Two-photon excitation at X = 307 nm provides 186 kcal/mol of energy to tetra-hydrofuran (THF) the excited molecule breaks a C—O bond with a characteristic time of 55 15 fs to form an oxytetramethylene diradical. This species, now with 114 kcal/mol of available energy, has a lifetime of 65 15 fs it decays primarily through a C—C p-cleavage reaction, giving the trimethylene diradical (42 amu, t 120 fs). The cleavage to trimethylene is the dominant reaction. ... 

An alternative p-cleavage process contributes to a lesser extent Loss of a hydrogen gives a formylalkyl radical of 71 amu, which loses ethylene to provide another formylalkyl radical of 43 amu. The transient species at 71 and 43 amu have decay times of 120 fs, similar to the time seen for the trimethylene diradical. 

This femtosecond study confirmed the involvement of the oxytetramethylene diradical as a reactive intermediate, and found that the trimethylene formed from it had the same hfetime as the trimethylene generated through the photodecarbonyl-ation of cyclobutanone. For tetrahydropyran, the oxypentamethylene drradical (86 amu) is formed readUy and the 85 amu transient, from the p-cleavage of a C H bond, is the dominant fragmentation product. 

Cyclohexene, upon excitation through a two-photon process providing 186 kcal/ mol, gives two species detected through ionization by a probe pulse and mass spectrometry a species at 82 amu, the parent stmcture or the drradical species formed through p-cleavage, and at 54 amu, a mass corresponding to butadiene. An ion at M-15, at 67, is also recorded. The femtosecond transients show that the 82 amu... 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

Type I initiators are compounds that upon irradiation undergo a homolytic cleavage reaction ( -or p-cleavage) to generate two radicals.18... 

This compound can undergo P-cleavage in two different ways to lose either the methyl group or the 4-hydroxyphenyl group (Scheme 5.11), and we can see peaks for both of the possible P-cleavage products in Figure 5.11 [M-CH J at m/z 121 and [M-C6H4OH] + at w/z 43. 

P-Cleavage next to the ester carbonyl is promoted by the ortho hydroxyl group, through a six-membered ring (Scheme 5.19). The radical cation produced, with m/z 120, is resonance stabilized and is observed as a major fragment in the EI MS. 

In effect it is intramolecular H-abstraction followed by p-cleavage and is also called Type II photoelimination. 

Alkene and alkane formation was suggested to take place through p cleavage and subsequent hydrogenation [Eq. (3.17)] chain branching involves the reaction of 1 with a half-hydrogenated intermediate [Eq. (3.18)] ... 

A radical chain reaction to yield sec-butyl hydroperoxide (105) as intermediate is operative17,870 [Eq. (9.163)]. Metal ions play a role in decomposition of the latter by P cleavage [Eq. (9.164)] ... 

Clearly these examples also are in line with what is happening in hydrocracking. In acid-catalyzed hydrocracking cleavage of the larger saturated hydrocarbon chains can take place via p cleavage induced by carbenium cations formed via C—H protolysis, and also via direct C—C bond protolysis (Scheme 11.13). 

Compare with p-oxo acid and (3-0X0-alcohol which undergo easy p cleavage... 

Another related reaction that goes through a ketimine is the conversion of the amino acid kynurenine to alanine and anthranilic add.225 It presumably depends upon hydration of the carbonyl group prior to P cleavage (Eq. 14-35). An analogous thiolytic cleavage utilizes CoA to convert 2-amino-4-ketopentanoate to acetyl-CoA and alanine.226... 

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