Alkenes allyl halides

Polycyclic Arenes. Trimethylsilylphenyl triflates have found widespread use as benzyne precursors in the preparation of polycyclic arenes. Triphenylenes are made via paUadium-catalyzed [2 + 2 + 2] cyclotrimerization of arynes, by paUadium-catalyzed annulation of the aryne and 2-halobiaryls, or via carbopalladation/carbocycUzation of arynes with substituted iodobenzenes (eq 2). All three approaches furnish substituted triphenylenes in high reported yields. Substituted phenanthrenes or naphthalenes are similarly prepared using 2-(trimethylsilyl) phenyl triflate as an aryne precursor. Treatment of the reagent with fluoride, a palladium catalyst, and a deactivated alkene, allyl halide, or internal aIk5Tie provides 9-substituted or 9,... 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

Allylic brommations are normally carried out using one of a number of special ized reagents developed for that purpose N Bromosuccimmide (NBS) is the most fre quently used of these reagents An alkene is dissolved m carbon tetrachloride N bromo succimmide is added and the reaction mixture is heated illuminated with a sunlamp or both The products are an allylic halide and succimmide... 

Preparing Alkyl Halides from Alkenes Allylic Bromination 339... 

Conjugated dienes can be prepared by some of the methods previously discussed for preparing alkenes (Sections 11.7-11.10). The base-induced elimination of HX from an allylic halide is one such reaction. 

Monodehydrohalogenation of allylic halides is another classical method for diene synthesis24. This method is complementary to double dehydrohalogenation as both the 1,2-dihalides and allylic halides are readily accessed from alkenes. The commonly employed protocol for diene synthesis, particularly for cyclic 1,3-dienes, is through the allylic monobromination of the alkene with A-bromosuccinimide or related reagents followed by dehydrobromination with hindered bases such as DBN or DBU (equation l)25. 

The reaction starts with an oxidative addition of an allylic compound to palladium(O) and a Tt-allyl-palladium complex forms. Carboxylates, allyl halides, etc. can be used. In practice one often starts with divalent palladium sources, which require in situ reduction. This reduction can take place in several ways, it may involve the alkene, the nucleophile, or the phosphine ligand added. One can start from zerovalent palladium complexes, but very stable palladium(O) complexes may also require an incubation period. Good starting materials are the 7t-allyl-palladium intermediates ... 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

Halogenations may also occur by a free-radical mechanism.121,218 Besides taking place in the gas phase, halogenation may follow a free-radical pathway in the liquid phase in nonpolar solvents. Radical halogenation is initiated by the alkene and favored by high alkene concentrations. It is usually retarded by oxygen and yields substitution products, mainly allylic halides. 

Benzylic halides can be prepared by the same radical-halogenating agents that give allylic halides from alkenes. These include Cl2, Br2, N-bromo-succinimide (Section 14-3A), S02C12, and im-butyl hypochlorite (see Exercise 4-18) ... 

Allyl complexes of the type ri1-allyl-Fp are prepared by reaction of [Cp(CO)2Fe] Na+ with allyl halides or, alternatively, by deprotonation of [Ti2-alkene-Fp]+ complexes. The most important reaction of V-allyl-Fp complexes is the [3+ 2]-cycloaddition with electron-deficient alkenes [33]. The reaction proceeds via a non-concerted mechanism, to afford Fp-substituted cydopentanes (Scheme 1.10). 

When the halide is bonded to an allylic system (CH CH-CH -X) an alkoxide ion will react analogously to the previously described S 2 displacement on an alkyl halide. The most significant difference is the rate enhancing effect of the alkene moiety which has been attributed to a decrease in the activation energy of the reaction (9). A second possible mode of reaction is available with allylic halides. This mode of displacement is usually called S 2 and, in general, will be promoted relative to the normal displacement when there are substituents on the alpha carbon which tend to inhibit the normal SN2 pathway by inductive or steric effects (Reaction VII). 

The traditional oxidation of benzylic and other halides to the aldehyde using DMSO (Kornblum oxidation) has been known for about 35 vears246,362,363. Modified oxidations of this type using selenoxides364,365 and 2-nitropropane366,367 have proved more versatile since other functional groups that are sensitive to the traditional oxidation procedure may be tolerated. In the reaction with 2-nitropropane, allylic halides are readily converted into aldehydes with retention of the alkene geometry. 

Allyltrimethylsilanes. Trimethylsilyllithium reacts with primary (E)-allylic halides to form the corresponding allyltrimethylsilanes. Reaction of the reagent prepared from (CH,),SiLi and Cul in HMPT with these halides results in the isomeric 3-trimethylsilyl-1-alkene. ... 

A simple synthetic method for 1,4-dicaibonyl compounds was introduced, based on the allylation of carbonyl compounds with allyl halide as a C3 component, followed by the palladium-catalyzed oxidation of the terminal alkenes (20) to methyl ketones (21). In this method, the allyl group is a synthetic equivalent of the 2-oxopropyl group (Scheme 5). This is a good anellation method for cyclopentenones. 

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