[917-65-7] · CH3AlCl2 · Methylaluminum Dichloride · (MW 112.92)
(strong Lewis acid that can also act as a proton scavenger; reacts with HX to give methane and AlCl2X)
Alternate Name: dichloro(methyl)aluminum.
Physical Data: mp 72.7 °C; bp 94-95 °C/100 mmHg.
Solubility: sol most organic solvents; stable in alkanes or arenes.
Form Supplied in: commercially available as solutions in hexane or toluene.
Analysis of Reagent Purity: solutions are reasonably stable but may be titrated before use by one of the standard methods.1b
Handling, Storage, and Precautions: must be transferred under inert gas (Ar or N2) to exclude oxygen and water. Use in a fume hood.
The general properties of alkylaluminum halides as Lewis acids are discussed in the entry for Ethylaluminum Dichloride. MeAlCl2 is used less frequently than EtAlCl2, since EtAlCl2 is much cheaper and comparable results are usually obtained. In some cases, use of MeAlCl2 is preferable, since the methyl group of MeAlCl2 is less nucleophilic than the ethyl group of EtAlCl2 and EtAlCl2 can transfer a hydride.
MeAlCl2 has been used as a catalyst for Diels-Alder2-4 and retro-Diels-Alder reactions.5 MeAlCl2 is the catalyst of choice for intramolecular Diels-Alder reactions of furans (eq 1) due to the ease of handling and reaction workup.2
MeAlCl2 has been used as a catalyst for ene reactions of trifluoroacetaldehyde6 and for intramolecular ene reactions of trifluoromethyl ketones (eq 2).7 Use of EtAlCl2 leads to the ene adduct in lower yield accompanied by 25% of the saturated analog resulting from hydride delivery to the zwitterion.
1:2 Aldehyde- or ketone-MeAlCl2 complexes add intramolecularly to alkenes to give zwitterions that undergo 1,2-hydride and alkyl shifts to regenerate a ketone (eqs 3 and 4).8-11 EtAlCl2 can transfer a hydride to the zwitterion to give the reduced product (eq 3).8
MeAlCl2 has been used as the catalyst for epoxide-initiated cation-alkene cyclizations.12 A tertiary alcohol has been converted to a hydrazide by reaction with MeAlCl2 in the presence of Mesitylenesulfonylhydrazide.13 a-Trimethylsilyl enones can be prepared by isomerization of 1-(trimethylsilyl)-2-propynyl trimethylsilyl ethers with MeAlCl2.14
MeAlCl2 has occasionally been used to introduce a methyl group. g-Lactones react with MeAlCl2 to give carboxylic acids with a methyl group at the g-position.15 The reagent prepared from MeAlCl2 and Dichlorobis(cyclopentadienyl)titanium adds to trimethylsilylalkynes in a syn fashion.16 Optimal stereoselectivity is obtained using MeAlCl2 to transfer a methyl group to a formyl amide (eq 5).17
Barry B. Snider
Brandeis University, Waltham, MA, USA