Alkenyl halides, alkylation

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Notice that all the examples m Table 8 1 involve alkyl halides, that is compounds m which the halogen is attached to an sp hybridized carbon Alkenyl halides and aryl halides, compounds m which the halogen is attached to sp hybridized carbons are essentially unreactive under these conditions and the principles to be developed m this chapter do not apply to them... 

Tnfluorometltylation of aryl, alkenyl, and alkyl halides can be accomplished by heating methyl fluorosulfonyldifluoroacetate and the appropriate halide precursor with copper(I) iodide at 60-80 °C in DMF [27 7] (equation 145). Similar trifluoromethylations of aryl, benzyl, and vinyl halides can be carried out with fluorosulfonyldifluoromethyl iodide and copper metal in DMF at 60-80 °C [2 75] (equation 146). 

Sn2 and SN2 Reactions with Halides and Sulfonates. Corey and Posner discovered that lithium dimethylcuprate can replace iodine or bromine by methyl in a wide variety of compounds, including aryl, alkenyl, and alkyl derivatives. This halogen displacement reaction is more general and gives higher yields than displacements with... 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

Another general process involves the reaction of Pd(0) species with halides or sulfonates by oxidative addition, generating reactive intermediates having the organic group attached to Pd(II) by a ct bond. The oxidative addition reaction is very useful for aryl and alkenyl halides, but the products from saturated alkyl halides often decompose by (3-elimination. The a-bonded species formed by oxidative addition can react with alkenes and other unsaturated compounds to form new carbon-carbon bonds. The... 

Reactions of Organozinc Reagents with Alkyl, Aryl, and Alkenyl Halides 405... 

Scheme 1.19. Synthesis of alkyl and alkenyl halides via halogenolysis...

Coupling of aryl or alkenyl halides with trialkylboranes.1 This reaction can be carried out in refluxing THF with a base (NaOH or NaOCH3,1 equiv.) catalyzed by PdCl2(dppf). A B-alkyl-9-BBN is more useful than a trialkylborane, since only one alkyl group is utilized. 

Recently, with a view to overcome the difficulty on the preparation of aryl or alkenyl halides or sulfonates, thioamides and their S-alkyl derivatives have been proposed as a new class of electrophilic partners. This palladium cross-coupling methodology was developed by Liebeskind and mostly applied to heteroaromatic templates.118 121... 

Radical carbonylation reaction serves as a powerful tool for the synthesis of a range of carbonyl compounds. Radical carbonylation has been successfully applied to the synthesis of functionalized ketones from alkyl, aryl, and alkenyl halides.The radical aminocarbonylation reaction of alkynes and azaenynes provided efficient routes to 2-substituted acrylamides, lactams, and pyrrolidinones. For example, the aminocarbonylation of 4-pentyn-l-yl acetate 318 initiated by tributyltin hydride (Bu"3SnH) (30mol%) with AIBN (20mol%) gave acrylamide 325 in 92% yield (Scheme 43).A proposed mechanism starts from the addition of tributyltin radical 319 to alkyne... 

Under these standard reaction conditions, the acetonitrile/pyridine mixture can replace the DMF/pyridine one. This solvent mixture is also quite convenient for running the preparation of arylzinc halides. Yields are good to excellent (60-90%) and even higher than those obtained in DMF. However, with bromophenol, no organozinc species was formed in acetonitrile as observed in DMF. The formation of arylzinc species is also effective in a mixture of solvents of acetonitrile-DMF-pyridine (8/1/1) in the presence of C0CI2 (13%) as the catalyst precursor and zinc bromide (30%). This method has also been applied to the formation of organozinc halides from alkyl and alkenyl halides. So far, only low yields have been obtained using the standard reaction conditions in DMF-pyridine. Results are reported in Table 8. 

TABLE 8. Preparation of arylzinc halides from alkyl and alkenyl halides... 

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