For allyl group

Detailed kinetic measurements have not been carried out on the soluble allylic compounds from Group IV, V and VI transition metals but rates depend on the transition metal, the presence and number of halogen atoms, and the structure of the allylic grouping [231]. Chromium tris(2-methallyl) was found to be among the more active initiators. Halogen substitution for allyl groups reduced the activity of chromium... 

Initially, Pd-catalyzed deprotections were carried out under reductive conditions. The earliest examples of Pd-mediated allyl group deprotection employed formic acid, usually in conjunction with an amine base, at elevated temperature as a hydride source (Scheme 2). Later development of tri- -butyltin hydride as a hydride source has greatly facilitated the reductive deprotection of a wide variety of aUyl-based protecting groups, reacting almost instantaneously at ambient temperature. " Additional hydride sources for allyl group deprotection include phenyltrihydrosilane and several different borohydrides (Scheme Most recently, Guibe and Albericio have used amine-... 

Several 1,4-dicarbonyl compounds are prepared based on this oxidation. Typically, the 1,4-diketone 10 or the 1,4-keto aldehyde 12 can be prepared by the allylation of a ketone[24] or aldehyde[61,62], followed by oxidation. The reaction is a good annulation method for cyclopentenones (11 and 13). Syntheses of pentalenene[78], laurenene[67], descarboxyquadrone[79], muscone (14 R = Me)[80]) and the coriolin intermediate 15[71] have been carried out by using allyl group as the masked methyl ketone (facing page). 

Allyl aryl ethers are used for allylation under basic conditionsfh], but they can be cleaved under neutral conditions. Formation of the five-membered ring compound 284 based on the cyclization of 283 has been applied to the syntheses of methyl jasmonate (285)[15], and sarkomycin[169]. The trisannulation reagent 286 for steroid synthesis undergoes Pd-catalyzed cyclization and aldol condensation to afford CD rings 287 of steroids with a functionalized 18-methyl group 170]. The 3-vinylcyclopentanonecarboxylate 289, formed from 288, is useful for the synthesis of 18-hydroxyestrone (290)[I7I]. 

It is widely recognized that an allyl group is a useful protecting group for acids, amines, and alcohols. Facile formation of a Tr-allylpalladium complex from... 

Another method for deallylation of ally esters is the transfer of the allyl group to reactive nucleophiles. Amines such as morpholine are used[415-417], Potassium salts of higher carboxylic acids are used as an accepter of the allyl group[418]. The method is applied to the protection and deprotection of the acid function in rather unstable /f-lactam 664[419,420]. 

In the case of the aspidosperma structure, the 5-mcmbcrcd C-ring is constructed from the A -acyl substituent. For the kopsine skeleton, an allyl group is installed and used to form the C6-C20 bridge. 

Although allylic brommations and chlormations offer a method for attaching a reactive functional group to a hydrocarbon framework we need to be aware of two important limitations For allylic halogenation to be effective m a particular synthesis... 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

Both symmetrical and unsymmetrical azo compounds can be made, so that a single radical or two different ones may be generated. The energy for the decomposition can be either thermal or photochemical. In the thermal decomposition, it has been established that the temperature at which decomposition occurs depends on the nature of the substituent groups. Azomethane does not decompose to methyl radicals and nitrogen until temperatures above 400°C are reached. Azo compounds that generate relatively stable radicals decompose at much lower temperatures. Azo compounds derived from allyl groups decompose somewhat above 100°C for example ... 

Compute the group charges for the CH and CH2 groups with each method for allyl cation using each of the methods from the previous exercise. 

Protection for the double bond in the allyl protecting group may be achieved by epoxidation. Regeneration of the allyl group occurs upon... 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

By boiling with alcoholic potash for six to eight hours, the allyl group is changed into the propenyl group, and iso-dillapiol results. This body is a crystalline compound, melting at 44° and boiling at 296°. 

Ethers with a tertiary, benzylic, or allylic group cleave by an S l or FI mechanism because these substrates can produce stable intermediate carbocations. These reactions are often fast and take place at moderate temperatures. fcrf-Butyl ethers, for example, react by an El mechanism on treatment with trifluoroacetic acid at 0 °C. We ll see in Section 26.7 that the reaction is often used in the laboratory synthesis of peptides. 

Evidence for this mechanism comes from the observation that the rearrangement takes place with an inversion of the allyl group. That is, allyl phenyl ether containing a 14C label on the allyl ether carbon atom yields o-allylphenol in which the label is on the terminal vinylic carbon (green in Figure 18.1). It would be very difficult to explain this result by any mechanism other than a pericyclic one. We ll look at the reaction in more detail in Section 30.8. 

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