Allylmagnesium halide, preparation

Allylmagnesium halides are best prepared from 3-chloropropene and equimolar amounts of magnesium-anthracene THF complex (Table 1, method A)13 -15. Here, usually, the yields arc higher solvent and temperature have only minor influence13 14. A catalytic version is also possible13. 

The allylmagnesium halide in ether is slowly added to a suspension or solution of the metal halide at 0°C or lower. Addition at low T improves yields and is essential for the synthesis of thermally unstable, homoleptic allyl complexes. The complexes are isolated by evaporation of the reaction mixture and extraction into an aromatic or alkane solvent. Purification is effected by crystallization or sublimation. When stable allyl compounds are prepared, an aqueous wash of the reaction mixture can be used to remove Mg salts and xs RMgX or -Li reagent the crude products can then be isolated from the resultant organic layer. 

Allyltin reagents are employed for the synthesis of Ru-allyl compounds. They are less reactive than allylmagnesium halides and can be used for the preparation of halo-substituted derivatives ... 

Allyltitanium complexes (22) readily add to carbonyl compounds with high regio- and stereo-selection. They are prepared by reaction of a chlorotitanium complex (21) with an allyl-magnesium or -lithium derivative (equation 13). Some of these unsaturated Ti complexes, like (23)-(25) in Scheme 2, obtained from allylmagnesium halides or allyllithium by reaction with titanium tetraisopropoxide or titanium tetramides, are known as titanium ate complexes . The structure of these ate complexes, at least from a formal point of view, can be written with a pentacoordinate Ti atom. Some ate complexes have synthetic interest, as is the case of (allyl)Ti(OPr )4MgBr which shows sharply enhanced selectivity towards aldehydes in comparison with the simple (allyl)Ti(OPr )3. ... 

In his doctoral thesis Grignard had come to the conclusion that his reaction could not be applied to allylic halides [19]. Attempts to prepare allylmagnesium halides led to unidentifiable products and the results of their reactions with e.g. aldehydes were... 

Houben was among the first to adapt Barbier s synthetic procedure. Just as Grignard [19] and Arbusov [20] (see [21]) had experienced before him, he was unsuccessful in his attempts to prepare allylmagnesium halides which he required for the synthesis of 3-butenoic acid [22]. 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

Though technically ether formations, the displacement of halides by hydroxyl groups differs from the examples presented earlier in this section in that these reactions are base mediated. As such, C-glycosidations under these conditions compliment the previously described cyclization strategies in that they allow reactions with acid sensitive substrates. As shown in Scheme 7.3.5, Schmid, et af.,18 utilized this approach in the preparation of C-furanosides. Specifically, the illustrated triol was protected with silyl groups. Subsequent treatment of the ketone with allylmagnesium chloride effected formation of a hydroxide which readily displaced the terminal chloride. The final C-glycoside was isolated as a 2.7 1 mixture of diastereomers. 

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