Allylmagnesium Bromide1

(X = Br)

[1730-25-2]  · C3H5BrMg  · Allylmagnesium Bromide  · (MW 145.29) (X = Cl)

[2622-05-1]  · C3H5ClMg  · Allylmagnesium Chloride  · (MW 100.84)

(allylating agents capable of easy addition to carbonyl compounds,15-23 nitro compounds57 and nitriles,58-61 imines43-50 and aza aromatics,51-56 unactivated alkenes,30-42 and of displacement reactions on halo compounds6,24-28,53,54)

Physical Data: Br: fp -40 °C; d 0.851 g cm-3 (commercial reagent). Cl: fp -17 °C; d 0.995 g cm-3 (commercial reagent).

Solubility: Br: very sol ether. Cl: slightly sol ether; sol THF.

Form Supplied in: Br: 1.0 M solution in Et2O packaged under N2. Cl: 2.0 M solution in THF packaged under N2.

Preparative Methods: allylmagnesium bromide and chloride can be easily prepared by direct reaction of Magnesium with Allyl Bromide and Allyl Chloride, respectively2-5 in diethyl ether and/or tetrahydrofuran. An excess of Mg (preferably 10%) is recommended.6,7 Prior amalgamation of very pure Mg3,4 suppresses the formation of 1,5-hexadiene. An improved preparation8 uses a THF slurry of Mg, obtained by co-condensation in a rotating solution reactor.9 Preactivation of Mg by dry stirring in an inert atmosphere is also highly beneficial.10 Allylpotassium can be readily converted into the magnesium analogs by simply adding a solution of Magnesium Bromide in THF.11,12 Most of the work on allylic Grignards has been carried out with the bromide. Indeed, the bromide can be prepared in slightly higher yield than the chloride and shows a greater solubility in ether.1

Handling, Storage, and Precautions: both moisture-sensitive; highly inflammable; react violently with water; induce burns. Bottles of these reagents should be flushed with N2, kept tightly sealed to preclude contact with oxygen, and stored far from sparks. In case of contact with eyes and skin, wash immediately with abundant water.

Structure of Allylmagnesium Halides.

NMR data3a of several allylic Grignard reagents support the view that allylMgBr exists as a rapidly equilibrating mixture of the two classical structures (1) and (2) (t1/2 <0.001 s) or as the bridged structure (3) (eq 1). This is supported by application of the Saunders isotopic perturbation technique13 as well as by 25Mg NMR studies.14

Reactions with Carbonyls.

Allylmagnesium halides react with carbonyl compounds to produce carbinols; 1,2-addition occurs with a,b-unsaturated carbonyls.1,15d,16-18 Addition takes place also with severely hindered ketones, which with most other Grignards undergo either reduction or enolization. Allylmetallation of carbonyls offers a complementary approach to the aldol reaction for acyclic stereocontrol.15

Complex (4), prepared by reacting the crystalline chiral complex Cp(OR*)2TiCl with allylMgBr, adds to aldehydes to give homoallylic alcohols19 with 85-94% ee (eq 2).

Homoallylic alcohols have also been synthesized by ring opening of vinylic oxetanes20 with copper-catalyzed allylmagnesium halides. C2 homologation of aldehydes and ketones to 2-alkenals uses allylMgBr.21

AllylMgBr adds to the C=O of imides to give different products depending upon the imide.22

A new synthesis of propenyl ketones involves reaction of an ester with 2 equiv of allylMgCl, furnishing a tertiary bis-homoallylic alcohol that fragments at 80 °C to a mixture of b,g- and a,b-unsaturated ketones.23

Displacement Reactions.

The coupling of allylmagnesium halides with halogenated compounds provides a useful one-step method for introducing an alkenic bond directly into a molecule.1,15d Allylic Grignard halides couple with alkyl, allylic, and benzylic halides.6,24 Sometimes, allylMgCl gives higher yields than the more commonly used allylMgBr.24a Alkylation in the presence of Copper(I) Iodide is much faster.25

AllylMgBr couples with fluoroalkenes in a rather unexpected manner.26 Allylmagnesium halides can be hydroxymethylated in a two-step one-pot procedure by 1-chloro-2-(chloromethoxy)ethane.27 A new synthesis of dienes, with the double bonds in predetermined positions, involves reaction of an a-chloro ketone with an allyl Grignard in ether at -60 °C followed by treatment with Lithium powder (eq 3).29

An efficient synthesis of 3-t-aminopropanols is based on the regioselective ring opening of tetrahydro-1,3-oxazines with allylMgCl.28

Allylmagnesium halides have been used to prepare different organometallics.1,15d

Addition to Alkenic Bonds.

Allylmagnesium halides add smoothly to activated and some unactivated double bonds. Intermolecular and uncatalyzed carbomagnesiation of unfunctionalized alkenes, such as Ethylene and 1-octene, has been reported.31-33 AllylMgCl30 reacts with more than 90% regioselectivity with 1,3-Butadiene (Mg at C-2) to afford 2,6-heptadienylmagnesium chloride (5) that, with a second carbomagnesiation, leads to 1:2 adduct (6), which undergoes a fast ring closure to (7) (eq 4).

The intramolecular carbomagnesiation involving reactive allylic Grignard reagents has been extensively studied.34-38 The presence of a neighboring hydroxy, alkoxy, or amino group facilitates the addition of allylmagnesium halides.39-42

Addition to Azomethine Linkages.

Allylmagnesium halides add to the azomethine linkage easier than the alkyl counterparts. Highly asymmetric syntheses of b- and g-amino acids with good stereocontrol have been described;43 the crucial step is the addition reaction of allylMgBr to chiral N-benzylidene-p-toluenesulfinamides followed by the elaboration of the allylic group (eq 5).

Resonance stabilization of the allyl anion results in a greater ionization of the carbon-metal bond and hence greater reactivity. This promotes addition to C=N linkages usually unaffected by nonresonance-stabilized organometallic reagents.44 Thus the reaction of allylmagnesium halides with achiral aldimines affords homoallylamines.45 The stereochemistry of the addition has been studied.46

The addition of allylmagnesium halides to the O-protected a-hydroxy-N-trimethylsilylimines (8), generated in situ, proceeds in a highly anti stereocontrolled manner to produce a-amino alcohols47 that can be oxidized to a-amino acids (eq 6).48

AllylMgBr, complexed with Cerium(III) Chloride, adds to chiral (E)-alkoxymethyl oxime ether (9) to give the corresponding (S)-amine derivative stereoselectively, while allyllithium gives mainly the opposite (R)-isomer (eq 7).49

Allylmagnesium halides also add to imines, iminium salts, N-hetero substituted imines (oximes, sulfenimines, etc.), gem-amino ethers, gem-amino nitriles, ethers, peroxides, disulfides, epoxides, acids and their derivatives, amides, orthoformates, O-alkylated oximes.15d,50

AllylMgBr adds to the C=N double bond of several aza aromatics to form allylated aza aromatics.51

The reaction of 2-hetero substituted benzothiazoles with allylMgBr affords 2-allylbenzothiazole or 2,2-diallylbenzothiazoline or N-triallylmethyl-o-aminobenzenethiol depending upon the experimental conditions;52 a novel ring-opening reaction of the benzothiazole system with allylic Grignards has been reported.53 2-Alkylbenzoxazoles also react with allylic Grignards, undergoing ring opening to N-diallylalkyl-o-aminophenols, while 2-chlorobenzoxazole affords the cross-coupling product.54 Mono and bis addition reactions of the allylic Grignard reagents to the C=N bond of quinoxalines afford high yields of allyldihydroquinoxalines and diallyltetrahydroquinoxalines; dehydrogenation with 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone leads to monoallyl- and diallylquinoxalines.55 Similarly, reaction with pyrimidines furnishes excellent yields of allylpyrimidines.56

Reactions with Nitro Compounds and Nitriles.

Allylmagnesium halides react with both aromatic and aliphatic nitro compounds via addition to the nitro group, leading to allylamines or to N-hydroxyallylamines (eq 8).57

Allylmagnesium halides add to alkyl, aralkyl, and alkenyl cyanides in a preferred 2:1 ratio to yield trisubstituted primary amines.58-60 The allyl groups on the carbinamine products are susceptible to catalytic hydrogenation to give t-alkyl primary amines, most of which are otherwise difficult to synthesize. Aryl nitriles react with allylmagnesium halides, leading to tetrahydropyridines.61

Related Reagents.

Allyllithium; Crotylmagnesium Bromide; Methallylmagnesium Chloride; 3-Methyl-3-buten-1-ynylcopper(I).

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Saverio Florio & Vito Capriati

University of Bari, Italy

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