[100-99-2] · C12H27Al · Triisobutylaluminum · (MW 198.37)
Physical Data: mp 0 °C; bp 46 °C/1 mmHg; d 0.781 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 solutions in hexane, heptane, or toluene.
Analysis of Reagent Purity: brochures from manufacturers describe an apparatus and method for assay.
Handling, Storage, and Precautions: the reagent is indefinitely stable under an inert atmosphere. The neat liquid or the dense solutions are highly pyrophoric. Solutions more dilute than a certain concentration are not pyrophoric and are safer to handle. The nonpyrophoric limits are 20 wt % in isopentane, 22 wt % in hexane, and 25 wt % in heptane, respectively. Use of halogenated hydrocarbon as a solvent should be avoided because of possible explosive reactions sometimes observed for mixtures of CCl4 and organoaluminums.
i-Bu3Al acts primarily as a reducing agent rather than an alkylating agent toward carbonyl compounds.2 Although the reactivity is modest and enolization sometimes competes, some unique stereochemical features are noted in the reduction of cyclic3a and acyclic ketones.3b Enones undergo mainly 1,2-reduction, whereas 1,4-reduction occurs in the presence of a Ni catalyst (eq 1).4
Alkylation of allyl acetates5 or glycal derivatives6 proceeds regioselectively (eqs 2 and 3). The 1,4-addition to nitroalkenes gives, after hydrolysis, a-alkylated ketones.7a Application to the synthesis of alkylated dihydro-2(3H)-furanones7b (eq 4) and 1,4-diketones has been reported.7c
Cross couplings of i-Bu3Al with carboxylic acid chlorides or thioesters can be achieved under Pd- or Cu-catalyzed conditions (eq 5).8 A Ni catalyst is effective for coupling with alkynyl bromides (eq 6).9 Reactions with propargyl acetates are effectively catalyzed by Iron(III) Chloride to give substituted allenes (eq 7).10
The [3,3]-sigmatropic rearrangement of allyl vinyl ethers is promoted by i-Bu3Al, which also reduces intermediate aldehydes to give primary alcohols (eq 8).11 Synthesis of a seven-membered ring via ring enlargement has been reported (eq 9).11c When Trimethylaluminum or Triethylaluminum are used, alkylated products are obtained. Reductive ring contraction reactions are effected by i-Bu3Al to deliver cyclobutyl- or cyclopropylmethanols (eqs 10 and 11).12
Alkylative Beckmann rearrangement of oxime sulfonates is achieved by trialkylaluminums to give, after in situ reduction with Diisobutylaluminum Hydride, the rearranged amines (eq 12).13 Related alkylative Beckmann fragmentations have also been reported (eq 13).14
Regioselective ring cleavage of chiral acetals is effected by i-Bu3Al (eq 14).15a Related reactions involving kinetic resolution have also been reported.15b
The reagent combination of CH2I2 and i-Bu3Al (or other organoaluminums) leads to cyclopropanation of alkenes (eq 15).16 The regioselectivity of this reagent is markedly different from the Simmons-Smith reaction.
Keisuke Suzuki & Tetsuya Nagasawa
Keio University, Yokohama, Japan