[97-93-8] · C6H15Al · Triethylaluminum · (MW 114.19)
Physical Data: mp -58 °C; bp 62 °C/0.8 mmHg; d 0.835 g cm-3 (25 °C).
Solubility: freely miscible with saturated and aromatic hydrocarbons; reacts violently with H2O and protic solvents.
Form Supplied in: as a neat liquid in a stainless container or as a solution in hydrocarbon solvents (hexane, heptane, toluene).
Analysis of Reagent Purity: brochures from manufacturers, describe an apparatus and method for assay.
Handling, Storage, and Precautions: indefinitely stable under an inert atmosphere. The neat liquid or dense solutions are highly pyrophoric. Solutions more dilute than a certain concentration are not pyrophoric and are safer to handle. The nonpyrophoric limits are 13 wt % in isopentane, 12 wt % in hexane, and 12 wt % in heptane, respectively. Use of halogenated hydrocarbons as solvents should be avoided because of possible explosive reactions sometimes observed for mixtures of CCl4 and organoaluminums.
Et3Al, like other organoaluminums, can act as a Lewis acid to activate Lewis basic functionalities and also as a captor of electrophilic species by ethylation, as illustrated in eq 1.2 Substitution reactions of glycosyl fluorides (eq 2)3 and bromides4 can be effected with Et3Al. g-Lactols react with Et3Al in the presence of Boron Trifluoride Etherate to deliver 2,5-disubstituted tetrahydrofurans stereoselectively (eq 3).5
Conjugate addition of Et3Al to 2-nitrofurans provides, after hydrolysis, dihydro-2(3H)-furanones (eq 4).6 Regioselective addition of Et3Al to unsymmetrical 1,1´-azodicarbonyl compounds has been reported.7
Cross coupling of Et3Al with aryl phosphates8 or alkynyl bromides9 proceeds with a Ni catalyst to provide alkylated arenes or alkynes (eqs 5 and 6). While Pd or Cu complexes are used as the catalyst for the coupling of Et3Al with carboxylic acid chlorides or thioesters (eq 7),10 Iron(III) Chloride is used for reactions of propargyl acetates to give substituted allenes (eq 8).11
Et3Al-Hydrogen Cyanide and Diethylaluminum Cyanide are two optional reagents for conjugate hydrocyanation of a,b-unsaturated ketones.12,13 The results from these two reagents often differ. Due to a rather slow reaction rate between Et3Al and HCN, the former reagent contains a proton source (HCN) which can quench aluminum enolate intermediates. An impressive example is shown in eq 9. Preformed Et2AlCN gives cis-isomer, whereas HCN-Et3Al leads to trans-isomer.14 A variant involves trapping of the enolates as TMS ethers by use of Cyanotrimethylsilane-Et3Al (eq 10).14
The HCN-Et3Al system has been used to cleave oxiranes in steroids15a or carbohydrates (eq 11) to give b-cyano alcohols.15b
The ate complex, formed on metalation of allyl i-propyl ether with s-Butyllithium followed by addition of Et3Al, reacts with carbonyl compounds at the a-position in syn-selective manner (eq 12).16
Allyl vinyl ethers undergo [3,3]-sigmatropic rearrangements promoted by Et3Al, which also effects subsequent ethylation of the resulting aldehydes (eq 13). Use of Triisobutylaluminum leads to primary alcohols by b-hydride reduction.17
Alkylative Beckmann rearrangements of oxime sulfonates are promoted by trialkylaluminums. The rearrangements give the imines, which are reduced with Diisobutylaluminum Hydride to the corresponding amines (eq 14).18 Related alkylative Beckmann fragmentations have also been reported.19
Et3Al promotes stereospecific pinacol-type rearrangements of chiral b-methanesulfonyloxy alcohols. Aryl20a or alkenyl groups20b cleanly take part in the 1,2-migration to provide a range of a-chiral ketones (eq 15). The 1,2-migration of alkyl groups is effected by the more Lewis acidic Diethylaluminum Chloride.20c The reagent combination of DIBAL and Et3Al effects the reductive 1,2-rearrangement of a-mesyloxy ketones (eq 16).20d,e
The reagent combination of Diiodomethane and Et3Al (or other organoaluminums) leads to cyclopropanation of alkenes (eq 17).21
Keisuke Suzuki & Tetsuya Nagasawa
Keio University, Yokohama, Japan