Allylic activation

They have also been implicated (45) in the HC1 catalysed allyl activated reinitiation of thermal"-degradation as well as in the "Schlimper" type complexes which have been thought to contribute to the color of degraded PVC (46). 

Other Pd cross-coupling reactions such as Heck [52] and Suzuki [53] reactions have also been used for macrocyclizations. The main drawback for Pd catalyzed macrocylization is the yield, that is often somewhat disappointing if compared with other established methods. Also, the introduction of the required coupling components (e.g., trialkyltin group, vinylic iodide) can be difficult in some compounds. In other cases, Pd-catalyzed side reactions such as double bond migration or allylic activation can occur. 

The allylically activated chiral methanimidamides are more reactive and can be prepared from 2,5-dihydro-l//-pyrrole or 1,2,5,6-tetrahydropyridine by heating with a chiral auxiliary substituted methanimidamide in toluene. Deprotonation of the more acidic 1 -iminomethy 1-2,5-dihydro-1//-pyrrole with butyllithium was complete after a few minutes, even at — 100 °C41. Alkylation afforded a mixture of regioisomers, 2-alkylated 2,5-dihydro-l //-pyrrole 1 (n = 1) and 3-alkylated 2,3-dihydro-l//-pyrrole 2 (n = 1), the former strongly predominating (about 92 8). During hydrazinolysis of the 2-substituted 2,5-dihydro-1 //-pyrrole 1 (n = 1) the minor product decomposed, thus separation of the regioisomers was unnecessary. About 80-85% of the chiral auxiliary (S)-l- m-butoxy-3-methyl-2-butanamine was recovered after hydrazinolysis. 

It is tempting to correlate this with charge distribution at. a and Y positions o.f the delocalized allylic active centers. In NMR... 

Hydrolysis of tosylhydrazones has been achieved by treatment with alkaline hypochlorite [61]. This allylic activation ensures the sp1 hybridized carbon behave as an avid acceptor. 

This olefin oxidative cleavage is in contrast to the allylic oxygenation observed with less oxidizable olefins. Presumably, small amounts of superoxide generated in an inefficient photoreduction initiate a radical chain oxygenation in which allylic activation is ultimately observed, for example, eq. 90 ... 

Thus it can be concluded that traces of chloroform in the reaction mixture and traces of silicic acid from TLC plates catalyze the formation of free radicals and lead to the disproportionation of to and 12 (Scheme II). The radical 3 is stabilized by the benzene ring and the allylic activation of the C-10 ben-zlic hydrogen. 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

The dehydrochlorination reaction causes a color change. Marvel and co-workers (17, 19) attributed the discoloration to a conjugated polyene structure. They suggested that random dehydrochlorination to form an olefinic bond would be followed by allylically activated removal of successive hydrogen chloride units to form the polyene structure. 

The allylic activation of 1,3-dienes by Ru(COD)(COT) makes possible their hydroacylation to form /5,y-unsaturated ketones via C-H activation of aldehydes at the same metal center [117], and their selective coupling with acrylic compounds [18] (Eq. 87). 

The ions at lower m/e values reveal extensive fragmentation of the molecule, at the dotted lines indicated in IX, with formation of the ions XLIIa-d, which still contain the intact /3-carboline ring system, and the ion XLIIe at m/e 156. This last ion is the most intense in the spectrum at lower m/e values and is formed by the comparatively facile rupture of three allylically activated bonds. There are no intense ions in this... 

These four peaks remain invariable at m/e 170, 169, 184, and 156 respectively when alterations are made in ring E as in the series, CCXCV, CCCX, CCCXI, CCCXII, and CCCXIII, and they do. not therefore contain atoms from this ring. The addition of substituents at positions 1,3,10, and 11 results in a corresponding increment in the mass of these fragments as is seen in the spectra of CCCXIV, CCCXV, and CCCVII (Table V). The importance of these peaks is explained by the ready cleavage of the allylically activated 3,14 bond. 

C-3 is present in this fragment (2 above) and from the spectrum of 3,14-dideuterioajmalicine (CCCVIII) it is seen that C-14 is also incorporated. Fragment y only retains two of the deuterium atoms of 3,5,6-trideuterioajmalicine and therefore cannot contain C-5. Fragment y represents the strongest indolic peak in the spectra of the hetero ring E alkaloids because in these the 14,15 bond in the intermediate aa is doubly allylically activated. 

Keywords Dienyl zirconocene Zirconacyclopentadiene Allylic activation ... 

Besides proteases and nucleases some lipoxygenases [140,141] are also located in the cytosol of cells [142]. Lipoxygenases react with polyunsaturated fatty acids (PUFAs), containing one or several double allylically activated CHa groups (-CH=CH-CH2-CH=CH-) [140,141]. PUFAs are mainly localized at position 2 of a phospholipide [143]. This is exactly the site where phospholipases Aa - liberated by cell injury -attack phospholipides. 

The main response of plants to wounding either to mechanically injury but also by pathogen attack is activation of lipases, followed by those of lipoxygenases as outlined above. Lipoxygenases remove from PUFAs a hydrogen atom localized at a double allylically activated methylene group, forming a mesomeric radical (Scheme 2). 

The process is induced by minimal amounts of 13-HPODE [154], which transform the Fe " ion in the active center of lipoxygenase to Fe ion. Thus the enzyme becomes able to react with a double allylically activated methylene group of a LH molecule (representing in this case a PUFA) by abstraction of a hydrogen atom. The hydrogen atom is transformed to a proton by release of an electron from the Fe ion in the... 

Still possess a double allylically activated CH2 group enabling a new attack by lipoxygenases. As a consequence a second reaction is possible resulting in production of 9,16-bishydroperoxy-10E,12Z,14E-octadecatrienoic acid [158] (Scheme 6). 

Compared to linoleic acid linolenic acid has one more double allylically activated CH2 group, therefore 4 regioisomeric hydroperoxides (9-HPOTE, 12-HPOTE, 13-HPOTE and 16-HPOTE) are generated in form of enantiomeric pairs in nonenzymic catalyzed LPO reactions. 

When the a-protons are allylic or benzylic, the selectivity of the deprotonation is attenuated by the competing stereoelectronic requirements of allylic activation and dipole stabilization. Stabilization of a... 

The now-accepted mechanism of epimerization, first proposed by Busico and Cipullo,238 and recently confirmed by Yoder and Bercaw by means of an elegant double-labeling study,237 involves the reaction product of /3-H transfer to the metal, a metal cation-polymeryl olefin complex, and is shown in Scheme 12. Allylic activation on the same metal cation-olefin complex has been proposed to generate internal double bonds in PE2 9 and vinylidene unsaturations in PP (Scheme 13).249... 

Figure 12 shows the most common terminal unsaturations of iPP, generated by chain-release reactions after primary insertion. The internal vinylidene, likely produced by allylic activation, is also shown.217,246... 

In 1989 Oppolzer reported that the enolates of A-acyl sultams derived from camphor afford highly diastereoselective alkylation products with a variety of electrophiles including those which are not allylically activated [88]. The sultam is deprotonated using either butyllithium with a catalytic amount of cyclohexyl isopropyl amine, or butyllithium alone, or sodium hexamethyldisilyl amide.As illustrated in Scheme 3.18, alkylation occurs selectively from the Re face of the Z(( )-enolate to give monoalkylated sultams which can be cleaved by LAH reduction or lithium hydroperoxide catalyzed hydrolysis. Representative examples are listed in Table 3.7. 

Organobarium reagents. 17. 204-21 Allylation. Active barium is gei of Li biphenylide in THF at room tern allylbarium reagents, which can be us stereoselectivities. rlireo-Homoallylie hydes are obtained. ... 

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