Enhancement of Lewis Acidity by Ligand-Defined Metal Geometry: A Catalytic Allylation of Aldehydes with Allyltrimethylsilane

Abstract

A highly Lewis acidic aluminum complex was produced using a tridentate ligand 1. The enhanced Lewis acidity of 1-Al was attributed to the combination of a stereoelectronic effect and an electrostatic effect. Comparison with an unstrained complex 4-Al indicated that the ligand-defined sp³ geometry of the aluminum in 1-Al led to the lower LUMO level and the larger LUMO coefficient on the aluminum. 1-Al promotes a catalytic allylation of aromatic aldehydes using allyltrimethylsilane. A catalytic amount of excess ligand added to the aluminum was important for high chemical yield. The excess ligand might act as a proton source to facilitate ligand exchange on the highly Lewis acidic aluminum.

Lewis acid - metal - ligand - allylation - aluminum

References

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6a

pK_a values in DMSO-CF₃SO₂NH₂ (9.7), PhSO₂NH₂ (16.1), CH₃OH (29.0)

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These calculations were performed using the UNIVERSAL forcefield (v. 1.02) performed on Cerius 2 4.0 (Molecular Simulations Inc.):

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7c Casewit CJ. Colwell KS. Rappé AK. J. Am. Chem. Soc.  1992,  114:  10046 ; An N-methyl analog, instead of 4, was used in these calculations for simplification

8

The chemical yield decreased when catalyst loading of less than 5 mol% was used, <50% with 2 mol% and no reaction with 1 mol%.

9

Trifluorotoluene (CF₃C₆H₅), toluene, and acetonitrile gave the product in 10%, 50%, and 0% yield, respectively.

10

Unfortunately, the desired allylation did not proceed from aliphatic aldehydes or α, β-unsaturated aldehydes. Cyclic trioxanes were the major products from primary and secondary alkyl substituted aldehydes. No reaction occurred from pivalaldehyde and α, β-unsaturated aldehydes.

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12 Side-reaction pathways mediated by a reagent-derived Lewis acidic silicon are problematic, especially in the case of catalytic enantioselective reactions: Carreira EM. In Comprehensive Asymmetric Catalysis   Vol. 3:  Jacobsen EN. Pfaltz A. Yamamoto H. Springer; Heidelberg: 1999.  p.Chap. 29 ; the possibility that the Lewis acidic silicon of 8 is the actual catalyst in the present case is unlikely due to the fact that the control catalyst 4-Al did not promote the reaction