Allylmetallic reagents

One of the most gentle methods for the generation of reactive allylmetallic reagents was introduced in 1977 by Hiyama and Nozaki1,2,3,33. By the action of two equivalents of chromi-um(II) chloride on allylic halides in tetrahydrofuran at 0°C in the presence of a carbonyl compound, reductive coupling with the formation of a homoallylic alcohol takes place. 

Allylmetallic reagents The ally] anions obtained by reductive metallation of ally I phenyl sulfides with lithium l-(dimethy amino)naphthalenide (LDMAN, 10, 244) react with a, 3-enals to give mixtures of 1,2-adducts. The regioselectivity can be controlled by the metal counterion. Thus the allyllithium or the allyltitanium compound obtained from either 1 or 2 reacts with crotonaldehyde at the secondary terminus of the allylic system to give mainly the adduct 3. In contrast the allylcerium compound reacts at the primary terminus to form 4 as the major adduct. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

Zinc(II) halide mediated Gaudemar/Normant-type allylmetallation to metal acetylide is also an efficient route. As shown in Scheme 38, the obtained. sp2-gcminated organodizinc reagent reacts with various electrophiles83. 

Allylmetallation of alkynes is a useful tool for the synthesis of 1,4-dienes. Various main group allylmetals, as well as allylic transition metals, have been utilized in such transformations. The reaction of allylindium reagents with terminal alkynols was first reported in 1992. The reaction proceeds in DMF giving 1,4-dienes, where the proximal... 

Enantioselective allylmetallation of chiral cyclopropene ketal is also possible [135] as well as allylmetallation of the achiral ketal but with the acid of chiral bisoxazoline (BOX) ligands [136] (Scheme 7-117). The chiral allylzinc reagents were much more reactive than the parent allylzinc bromide. 

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