[1458-98-6] · C4H7Br · Methallyl Bromide · (MW 135.01)
(alkylating agent attacking C, N, O, and S nucleophiles, often with enantiomeric or diastereomeric selectivity; alkyl-metal reagents provide a-(methallyl) alcohols with various carbonyl systems; provides derivatives leading to polymers and to other reagents)
Alternate Name: 3-bromo-2-methylpropene.
Physical Data: bp 94-95 °C; d 1.313 g cm-3.
Solubility: organic solvents; sparingly sol water.
Form Supplied in: yellow to brown liquid.
Purification: wash with water and aq. NaHCO3 until washings are not acidic. Dry (MgSO4) and fractionally distill.
Handling, Storage, and Precautions: toxic irritant liquid; protect from light in brown glass. Use in a fume hood.
Methallyl derivatives are prepared by halide ion exchange of Methallyl Chloride (eq 1);1 LiN3 gives the azide2 and KCNO provides tris(methallyl) isocyanurate through cyclic trimerization of the ROCN product.3 Methallyl chloride undergoes conventional alkylation reactions with amines, alkoxides, etc.4 (see Allyl Bromide).
Barbier-type reactions occur (eq 2); octanal gives RCH(OH)CH2C(Me)=CH2 (R = heptyl) in the presence of Sb, and a,b-unsaturated aldehydes give only 1,2-addition.5 Indium behaves similarly.6 Mn in EtOAc or THF with Zinc Chloride (catalytic amount) gives homoallylic alcohols with both R1CHO and R1COR2,7 and the analogous Zn-induced reaction in DMF is also successful.8 The reaction with either R1CHO or R1COR2 may be performed electrochemically, using a Cd-modified anode and reversing the polarity of the cell regularly.9
An interesting extension of the reductive alkylation reaction involves the use of the complex between Tin(II) Chloride and the disodio derivative of (+)-diethyl tartarate, which, when treated sequentially with methallyl bromide and then with R1CHO, gives CH2=C(Me)CH2CH(OH)R1 in good yield and with substantial ee (16-65%).10 Samarium(II) Iodide promotes the alkylation of octanoyl chloride in THF. No ketones can be isolated from the reaction, which yields only the tertiary alcohol R2C(C7H15)OH (R = methallyl).11
Lithiated species have been generally used as nucleophiles towards methallyl bromide. Lithium Diisopropylamide in THF-PhH at 0 °C deprotonates isobutyronitrile, which then methallylates in good yield.12 The corresponding reactions of the phenylsulfonylbutenolide (1) (eq 3)13 and of imidothioesters14 proceed regioselectively.
2-Cyclohexenone reacts with methallyl bromide (RBr) and LiC(SPh)3 in THF-NMP to give 2-methallyl-3-[tris(phenylthio)methyl]cyclohexanone (2) regiospecifically (eq 4). Treatment of (2) with Silver(I) Trifluoromethanesulfonate in THF provides 5-phenylthio-7-methyl-1-tetralone (3).15 The diastereoselective allylation of a camphor imine by methallyl bromide has also been reported.16
Addition reactions also occur, and are exemplified by the reaction with CHBr3 and NaOH in the presence of a phase-transfer catalyst. Either addition of CBr3- or of :CBr2 can be achieved specifically;17 the similar addition of :CCl2 has also been reported.18 Pd-catalyzed ethoxycarbonylation of methallyl bromide (NaOEt-Na2PdCl4-[(Ph2P)2CH2]2 at rt) is also readily achieved.19
University of Surrey, Guildford, UK