RSS-Feed abonnieren
DOI: 10.1055/s-2002-34366
Enhancement of Lewis Acidity by Ligand-Defined Metal Geometry: A Catalytic Allylation of Aldehydes with Allyltrimethylsilane
Publikationsverlauf
Publikationsdatum:
26. September 2002 (online)
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.
Key words
Lewis acid - metal - ligand - allylation - aluminum
- 1
Lewis
Acids in Organic Synthesis
Yamamoto H. Wiley-VCH; Weinheim: 2000. -
2a
Mikami K.Kotera O.Motoyama Y.Sakaguchi H.Maruta M. Synlett 1996, 171 -
2b
Ishihara K.Kubota M.Yamamoto H. Synlett 1996, 265 -
2c
Marx A.Yamamoto H. Angew. Chem. Int. Ed. 2000, 39: 178 -
2d
Ishihara K.Hiraiwa Y.Yamamoto H. Synlett 2001, 1851 ; and references cited therein - For other interesting strategies to enhance the catalyst activity of Lewis acid complexes, see:
-
3a Lewis acid-Brönsted
acid combination:
Ishihara K.Yamamoto H. J. Am. Chem. Soc. 1994, 116: 1561 -
3b See also:
Corey EJ.Shibata T.Lee TW. J. Am. Chem. Soc. 2002, 124: 3808 -
3c Lewis base coordination:
Denmark SE.Wynn T. J. Am. Chem. Soc. 2001, 123: 6199 -
3d Bimetallic system:
Asao N.Kii S.Hanawa H.Maruoka K. Tetrahedron Lett. 1998, 39: 3729 -
3e Monomer formation using
bulky phenoxy ligands:
Maruoka K.Ooi T.Yamamoto H. J. Am. Chem. Soc. 1989, 111: 6431 - For examples of Lewis acidity enhancement by ligand-defined metal geometry, see:
-
4a
Nelson SG.Kim B.-K.Peelen TJ. J. Am. Chem. Soc. 2000, 122: 9318 -
4b
Denmark SE.Griedel BD.Coe DM.Schnute ME. J. Am. Chem. Soc. 1994, 116: 7026 ; and references cited therein - 5
Fleischer EB.Gebala AE.Levey A.Tasker PA. J. Org. Chem. 1971, 36: 3042 -
6a
pKa values in DMSO-CF3SO2NH2 (9.7), PhSO2NH2 (16.1), CH3OH (29.0)
-
6b
Bordwell FG. Acc. Chem. Res. 1988, 21: 456 - These calculations were performed using the UNIVERSAL forcefield (v. 1.02) performed on Cerius 2 4.0 (Molecular Simulations Inc.):
-
7a
Rappé AK.Casewit CJ.Colwell KS.Goddard WA.Skiff WM. J. Am. Chem. Soc. 1992, 114: 10024 -
7b
Casewit CJ.Colwell KS.Rappé AK. J. Am. Chem. Soc. 1992, 114: 10035 -
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 - 11
Hamashima Y.Sawada D.Nogami H.Kanai M.Shibasaki M. Tetrahedron 2001, 57: 805 ; and references cited therein - 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
References
The chemical yield decreased when catalyst loading of less than 5 mol% was used, <50% with 2 mol% and no reaction with 1 mol%.
9Trifluorotoluene (CF3C6H5), toluene, and acetonitrile gave the product in 10%, 50%, and 0% yield, respectively.
10Unfortunately, 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.