[96-10-6] · C4H10AlCl · Diethylaluminum Chloride · (MW 120.56)
(strong Lewis acid that can also act as a proton scavenger; reacts with HX to give ethane and EtAlClX)
Alternate Names: chlorodiethylaluminum; diethylchloroalane.
Physical Data: mp -50 °C; bp 125 °C/50 mmHg; d 0.961 g cm-3.
Solubility: sol most organic solvents; stable in alkanes or arenes.
Form Supplied in: commercially available neat or 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.1e
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. Dialkylaluminum halides are less acidic than alkylaluminum dihalides. Et2AlCl is much cheaper than Dimethylaluminum Chloride and is used more frequently than Me2AlCl since comparable results are usually obtained. In some cases, most notably the ene reactions of carbonyl compounds, use of Me2AlCl is preferable since its methyl groups are less nucleophilic than the ethyl groups of Et2AlCl, which can act as a reducing agent.
Et2AlCl has been extensively used as a Lewis acid catalyst for Diels-Alder reactions. N-Acyloxazolidinones can form both 1:1 (eq 1) and 1:2 complexes with Et2AlCl (eq 2).2 The 1:2 complex is ~100 times as reactive as the 1:1 complex and gives greater endo selectivity and higher de. Me2AlCl gives similar selectivity with fewer byproducts.
Et2AlCl has been extensively used as a Lewis acid catalyst for intermolecular3 and intramolecular4 Diels-Alder reactions with a,b-unsaturated ketones and esters as dienophiles. It also catalyzes inverse electron demand Diels-Alder reactions of alkenes with quinone methides5 and Diels-Alder reactions of aldehydes as enophiles.6
Et2AlCl has been used as a catalyst for ene reactions with ethyl propiolate as an enophile,7 for intramolecular ene reactions of aldehydes,8 and for intramolecular ene reactions with a,b-unsaturated esters as enophiles (eq 3).3e,9
Et2AlCl has been used as a catalyst for Claisen rearrangement of aryl allyl ethers.10 The rearrangement of 2-vinylcyclopropanecarboxylate esters to cyclopentenes is catalyzed by Et2AlCl (eq 4).11
Complexation of Et2AlCl to ketones and aldehydes activates the carbonyl group toward addition of a nucleophilic alkyl- or allylstannane or allylsilane.12 Et2AlCl has been used to initiate Beckmann rearrangements of oxime mesylates. The ring-expanded cation can be trapped intermolecularly by enol ethers and cyanide and intramolecularly by alkenes (eq 5).13
Et2AlCl has been used in modified Reformatsky reactions. Aldol adducts are obtained in good yield by reaction of an a-bromo ketone with a ketone in the presence of Zinc and Et2AlCl in THF (eq 6).14 Lithium enolates of esters do not react with epoxides. Reaction of lithium enolates with Et2AlCl affords aluminum enolates that react with epoxides at the less substituted carbon (eq 7).15
Lithium acetylides react with Et2AlCl to form LiCl and diethylaluminum acetylides. The aluminum acetylides are useful reagents for carrying out SN2 reactions on epoxides (eq 8)15c,16 and undergo conjugate addition to enones that can adopt an S-cis conformation (eq 9).17
Et2AlCl reacts analogously to Ethylmagnesium Bromide and transfers an ethyl group to many electrophiles. Since EtMgBr and Ethyllithium are readily available, use of Et2AlCl to deliver an ethyl group is needed only when the stereochemistry of addition is an important issue. High levels of asymmetric induction are obtained in the conjugate addition of Et2AlCl to unsaturated acyloxazolidinones with carbohydrate-derived chiral auxiliaries (eq 10).18 Et2AlCl opens epoxides to chlorohydrins.19
Barry B. Snider
Brandeis University, Waltham, MA, USA