Dimethylaluminum 2,6-Di-t-butyl-4-methylphenoxide

[86803-85-2]  · C17H29AlO  · Dimethylaluminum 2,6-Di-t-butyl-4-methylphenoxide  · (MW 276.44)

(designer Lewis acid for amphiphilic alkylation,1 amphiphilic reduction,2 stereoselective Diels-Alder reaction;3 oxidative methylation4)

Alternate Name: DAD.

Solubility: sol CH2Cl2, toluene, hexane.

Preparative Method: by reaction of a 1-2 M hexane solution of Trimethylaluminum with 1 equiv of 2,6-di-t-butyl-4-methylphenol in toluene or CH2Cl2 at room temperature for 1 h.1,2

Handling, Storage, and Precautions: the concentrated reagent and solutions are highly flammable and must be handled in the absence of oxygen and moisture. Solutions should be used as prepared for best results.

Amphiphilic Alkylation.

DAD is a bulky, oxygenophilic organoaluminum reagent which, like Methylaluminum Bis(2,6-di-t-butyl-4-methylphenoxide) (MAD), has been utilized for stereoselective activation of carbonyl groups. The combination of DAD with Grignard reagents or organolithium reagents generates an amphiphilic alkylation system which reacts with substituted cyclohexanones to afford equatorial alcohols, frequently the minor product with other organometallics (eq 1).1 In general, MAD gives more reliable results in this amphiphilic alkylation than DAD (eq 2).

Amphiphilic Reduction.

When a bulky nucleophile such as t-Butylmagnesium Chloride is employed for the amphiphilic reaction system using DAD, substituted cyclohexanones afford equatorial alcohols almost exclusively as reduction products with high selectivity (eq 3).2

Stereoselective Diels-Alder Reaction.

The steric effect of DAD on the stereoselectivity in Diels-Alder reactions of cyclic dienes and a,b-unsaturated aldehydes has been studied.3 For example, the Boron Trifluoride Etherate catalyzed Diels-Alder reaction of cyclopentadiene with acrolein gives a mixture of endo- and exo-adducts in a ratio of 9:1. The endo selectivity is improved by use of trimethylaluminum. Introduction of a more bulky 2,6-di-t-butyl-4-methylphenoxy group on aluminum by use of DAD results in a further enhancement in endo selectivity (eq 4). However, use of the even more hindered MAD catalyst lowers the endo selectivity.

In contrast, both DAD and MAD enhance exo selectivity in the Diels-Alder reaction of cyclopentadiene with methacrolein. This is due to steric repulsion between the bulky catalyst and cyclopentadiene, rather than an electronic effect involving the carbonyl group of the dienophile, as observed in the cycloaddition with acrolein (eq 5).

Oxidative Methylation.

A novel and direct transformation of aromatic aldehydes to the corresponding methyl ketones has been accomplished using a DAD/ether complex in toluene at rt (eq 6).4

1. (a) Maruoka, K.; Itoh, T.; Yamamoto, H. JACS 1985, 107, 4573. (b) Maruoka, K.; Itoh, T.; Sakurai, M.; Nonoshita, K.; Yamamoto, H. JACS 1988, 110, 3588.
2. Maruoka, K.; Sakurai, M.; Yamamoto, H. TL 1985, 26, 3853.
3. Maruoka, K.; Nonoshita, K.; Yamamoto, H. SC 1988, 18, 1453.
4. (a) Power, M. B.; Barron, A. R. TL 1990, 31, 323. (b) Power, M. B.; Barron, A. R. Polyhedron 1990, 9, 233.

Keiji Maruoka & Hisashi Yamamoto

Nagoya University, Japan

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