Allylmagnesium halide

An interesting event in the history of the Barbier reaction was the publication in 1928, by Gilman and McGlumphy, of a new procedure to prepare Grignard reagents from allyhc halides [41]. 

These authors demonstrated that, under carefully controlled reaction conditions, stable allylic-magnesium halides could be formed. The Wurtz-type coupling reaction which leads to the formation of 1,5-hexadiene and which is responsible for the loss of most, if not all, of the allylic halide could be suppressed almost completely  

A comparison was made between the results of reactions of this allylic Grignard reagent and those of one-step Barbier reactions of allyl bromide and magnesium with three different substrates, i.e. benzophenone, acetophenone and carbon dioxide. 

These results (see Table 2.3) seem to indicate that the two-step procedure is to be preferred to the one-step Barbier reaction. 

In all fairness, however, it should be noted that only yields were quoted from previous reactions obtained in the very beginning of the development of Barbier chemistry and that no reexamination had taken place of these reactions under comparable conditions. 

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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 addition of allylmagnesium halides (2 equiv.) to 1,3-azadienes affords after in situ alkylation dihomoallylamines, which are useful intermediates in the synthesis of azepines or related heterocycles. Activation of the C=N moiety of aldimines by 1-benzotriazolyltrimethylsilane minimizes side reactions. The mechanism involves reversible addition of BtTMS (216) to the imine 217 followed by displacement of the benzotriazolyl group by a Grignard reagent (equation 148). ... 

Allylic silanesTrimethylsilyllithium reacts with isoprenyl chloride in THF ( — 78° -> 25°) to give isoprenyltrimethylsilane with high selectivity in 82% yield. This method is superior with respect to yield and selectivity to the reaction of allylmagnesium halides with chlorotrimethylsilane (7,9). 

Primary allylmagnesium halides and allylzinc reagents react similarly, presumably by an ene-type reaction, to give selectively the cyclopropyl derivative in which the more substituted allylic carbon is attached to the ring (equation 24). Allylzinc compounds have been employed in the same manner in carbozincation of cyclopropenone acetals ... 

Allylmagnesium halide and lithio reagents are available from allyl halides by lithiation and Mg reaction techniques. These active reagents combine with binary or complex transition-metal halides to form transition-metal-allyl complexes, including both homoleptic and ligand-substituted tj -allyl complexes with substituted and unsubstituted allyl groups. 

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. 

Table 1. Reactions of Cyclopentadienyltitanium Halides with Allylmagnesium Halide Reagents, RMgX... 

Cyclooctatetraene complexes of Zr and Hf halides also undergo simple metathesis with allylmagnesium halides ... 

A (T-allyl complex is formed from allylmagnesium halide and (> -Cp)Fe(COXPPh3)Br in 90% yield this compound liberates PPh3 when heated, affording the r/ -allyl compound in quantitative yield ... 

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 ... 

However, if the CoClj is reacted with allylmagnesium halide in the presence of 1/2 equiv of allyl chloride, the Co(I) intermediate is oxidized to a Co(III) derivative which is then converted to the desired product by further reaction with allylmagnesium halide ... 

Carbonyl- and phosphine-substituted Rh and Ir allyls are synthesized by the reaction of an allylmagnesium halide with a metal halide precursor. Substituted derivatives are more stable than the homoleptic allyls and can be made at RT or 0 C. Examples are listed in Table 3. 

Cyclopentadienyl)( / -allyl)Ni and -Pt complexes are formed when NiClj and PtClj are treated sequentially or simultaneously with allylmagnesium halides and LiCp or NaCp. The voltatile complexes are isolated by distillation or sublimation from the reaction residue after evaporation of the solvent. Platinum(II) olefin complexes afford the same product when treated similarly ... 

A cyclopentadienyl group is displaced from (>/ -Cp)2Ni when the compound is treated with allylmagnesium halide " ... 

Tertiary phosphine and benzonitrile adducts of PdCl afford allyl complexes when reacted with allylmagnesium halide. The complex (PhCN)Pd( j -C3H5)Cl formed in Eq. (c) is mentioned above. Cationic tertiary phosphine complexes containing a Pd-allyl group are isolated from ... 

Allylmagnesium halides under the influence of ClTi(OiPr)3 add at the more-substituted position of unsymmetrical epoxides, generating quaternary carbon centers <2005T6726>. 

In contrast, bimolecular additions of allylmagnesium halides (5) to nonstrained alkenes (6) (Scheme 4 Table 1) are notoriously inefficient due to low reaction rates and the formation of regio- and stereoisomer mixtures, as well as uncontrolled consecutive reactions of the metallated addition products (7) and (8). Only trimethylsilylethylene showed a useful reactivity towards allylic Grignard reagents ... 

Table 1 Additions of Allylmagnesium Halides to 1-Substituted Alkenes Followed by Protonolysis...

Yield based on allylmagnesium halide (in parenthesis based on alkene). "(9) isolated as a stereoisomer mixture. °2 1 mixture of 1,6- and 1,5-heptadienes. 

Table 6 Influence of the Nature of the Halogen on the Frequency of the Magnesium-Halides Vibrations for Allylmagnesium Halides...

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. ... 

Reduction of ReCls with excess allylmagnesium halide produces orange Re2( r-C3H5)4 dimers . A D2d geometry and no bridging allyl ligands characterize this Rej unit ". A thorough treatment of related Re chemistry is available . 

Bimetallic reagents are available from reaction of allylmagnesium halides and ZnBr2 to alkenyllithiums. ... 

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