Methylaluminum Bis(2,6-diphenylphenoxide)

[118496-00-7]  · C37H29AlO2  · Methylaluminum Bis(2,6-diphenylphenoxide)  · (MW 532.64)

(designer Lewis acid for Claisen rearrangements,1,3 aza-Cope rearrangements,4 cycloadditions,5 epoxide rearrangement,6 stabilization of reactive aldehydes,7 and aldehyde recognition8,9)

Alternate Name: MAPH.

Solubility: sol toluene, CH2Cl2.

Form Supplied in: prepared from commercially available reagents and used in situ.

Preparative Method: treatment of 2,6-diphenylphenol (2 equiv) in dry CH2Cl2 with a 1 M hexane solution of Trimethylaluminum at 25 °C for 1 h.1

Handling, Storage, and Precautions: the dry solid and solutions are highly flammable and must be handled in the absence of oxygen and moisture. The solution should be used as prepared for best results. Use in a fume hood.

Claisen Rearrangement.

The modified organoaluminum reagent MAPH was developed for effecting the stereoselective Claisen rearrangement of allylic vinyl ethers.1 Thus reaction of 1-isobutyl-2-propenyl vinyl ether with MAPH in toluene at -20 °C affords, after reduction of the Claisen products with Sodium Borohydride, (E)-7-methyl-4-octen-1-ol with high stereoselectivity (eq 1), superior to that observed in the thermal rearrangement (E/Z ratio = 92:8). The Claisen rearrangement of the optically active substrate with MAPH reveals a chairlike transition-state, with the isobutyl substituent equatorial.2

The organoaluminum-promoted Claisen rearrangement of bisallylic vinyl ethers exhibits unprecedented regiochemical control not observed in the thermal Claisen rearrangement (eq 2).3

Aza-Cope Rearrangement.

The advantage of MAPH over other Lewis acids is exemplified by the Lewis-acid promoted 3-aza-Cope rearrangement of N-alkyl-N-allyl enamine substrates.4 Here, MAPH shows highest diastereoselectivity in the C-C bond forming process (eq 3).

Tandem [4 + 2]/[3 + 2] Cycloaddition.

The [4 + 2] cycloaddition of a nitro alkene with a chiral (+)-camphor vinyl ether, promoted by MAPH, followed in tandem by a [3 + 2] cycloaddition of the intermediate nitronate with an intramolecularly tethered electron-deficient alkene, allows for the construction of five stereogenic centers with excellent stereoselectivity. The stereochemistry is opposite to that formed with a titanium reagent (eq 4).5

Catalytic Rearrangement of Epoxides.

MAPH mixed with silanized silica gel can be utilized as a stationary phase for immobilized organoaluminum columns. These can be used as a workup-free reactor for the catalytic rearrangement of various epoxides to carbonyl compounds.6

Stabilization of Reactive Aldehydes.

Reactive aldehydes such as formaldehyde and a-chloro aldehydes can be successfully generated by treatment of readily available trioxane and a-chloroacetaldehyde trimers, respectively, with MAPH and stabilized as their 1:1 coordination complexes with MAPH. The resulting CH2=O.MAPH complex reacts with a variety of alkenes to furnish ene reaction products with excellent regio- and stereoselectivities (eq 5).7 In addition, these complexes can be used as stable sources of formaldehyde and a-chloroacetaldehyde, respectively, for use in nucleophilic addition reactions with various carbanions (organometallics, enolates, etc.) (eq 6).8

Chemoselective Functionalization.

The high recognition and stabilization ability of MAPH enables the chemoselective alkylation of sterically more hindered aldehydes using alkyllithiums as the nucleophile (eq 7).9

1. (a) Maruoka, K.; Nonoshita, K.; Banno, H.; Yamamoto, H. JACS 1988, 110, 7922. (b) Nonoshita, K.; Banno, H.; Maruoka, K.; Yamamoto, H. JACS 1990, 112, 316.
2. Nonoshita, K.; Maruoka, K.; Yamamoto, H. BCJ 1992, 65, 541.
3. Maruoka, K.; Banno, H.; Nonoshita, K.; Yamamoto, H. TL 1989, 30, 1265.
4. Barta, N. S.; Cook, G. R.; Landis, M. S.; Stille, J. R. JOC 1992, 57, 7188.
5. (a) Denmark, S. E.; Schnute, M. E. JOC 1991, 56, 6738. (b) Denmark, S. E.; Senanayake, C. B. W. JOC 1993, 58, 1853. (c) Denmark, S. E.; Schnute, M. E.; Senanayake, C. B. W. JOC 1993, 58, 1859.
6. Nagahara, S.; Maruoka, K.; Yamamoto, H. CL 1992, 1193.
7. (a) Maruoka, K.; Concepcion, A. B.; Hirayama, N.; Yamamoto, H. JACS 1990, 112, 7422.
8. Maruoka, K.; Concepcion, A. B.; Murase, N.; Oishi, M.; Hirayama, N.; Yamamoto, H. JACS 1993, 115, 3943.
9. Maruoka, K.; Saito, S.; Concepcion, A. B.; Yamamoto, H. JACS 1993, 115, 1183.

Keiji Maruoka & Hisashi Yamamoto

Nagoya University, Japan

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