Aluminum Trifluoromethanesulfonate

Introduction

The synthesis of aluminum trifluoromethanesulfonate from aluminum trichloride and triflic acid was published by Olah et al.[1] [2] in 1988. Aluminum triflate is a white solid with a high melting point[2] and acts as a strong, stable, oxophilic[3–5] Lewis acid that can easily be recycled and reused[6] due to its water-tolerant properties.[7] During initial investigations, aluminum trifluoromethanesulfonate was mainly used for Michael and Friedel–Crafts reactions, and it also functioned as a Lewis acid catalyst for the protection of alcohols, phenols, and thiophenols[8] with a variety of different protecting groups (i.e., methyl, ethyl, isopropyl, tert-butyl, acetyl, tetrahydropyranyl and tetrahydrofuranyl).[1] [9] [10] Recently, the utilization of aluminum trifluoromethanesulfonate has been studied in much more diversity, for example as a co-catalyst in metal-catalyzed reactions,[4] [11] in the nucleophilic opening of epoxides,[5] [12] [13] cyclization,[14] substitution,[15] and other reactions.

Abstracts

(A) Williams et al.[12] obtained excellent results (92–98% yield) in the ring opening of epoxides like styrene oxide with alcohols (ROH, R = C1–C4) and aluminum triflate to produce the monoethers of ­phenyl-substituted glycols. The ring opening of cyclohexene oxide also gave 83–89% yield utilizing only 0.002 mol% of triflate.
(B) Fringuelli et al.[5] and Williams et al.[13] extended the Al(OTf)3-­catalysed ring opening of epoxides to include the use of amines instead of alcohols and were able to isolate the aminoalcohols in good yield (75–90%). The opening of cyclohexene oxide with 2-picolylamine could be effected with Al(OTf)3 at concentrations of 5 mol% leading to the product being obtained in 99% yield under solvent-free conditions.
(C) Chaminade et al.[14] published the preparation of 1,2-oxaza heterocycles from unsaturated oximes through treatment of the substrate with different Lewis acid catalyst–solvent combinations. It was found that Al(OTf)3 in nitromethane (MeNO2) gave the best result for this transformation.
(D) Since naturally occurring 1-amino acids display a variety of bio­logical activities and also act as plant-growth regulators and herbicides, Sobhani et al.[16] studied the synthesis of 1-amino-phosphonates and found that Al(OTf)3 could be applied as a catalyst in a one-pot synthesis of primary diethyl 1-aminophosphonates under solvent-free conditions.
(E) Gohain et al.[15] showed Al(OTf)3 to be a highly efficient and relatively inexpensive catalyst for the direct nucleophilic substitution of propargylic alcohols forming carbon–carbon and carbon–heteroatom bonds.
(F) Gohain et al.[17] reported a ‘green’ Friedel–Crafts, Michael-type addition of indole to α,β-unsaturated ketones catalyzed by Al(OTf)3 in the recyclable solvent polyethylene glycol.
(G) Parvanak-Boroujeni and co-workers utilized polystyrene-­supported aluminum triflate in the acylation of a range of aromatic substrates with benzoic acid and were able to obtain the corresponding benzophenones in excellent yields.[18]
(H) Williams and co-workers discovered that aluminum triflate could be used as a highly active non-protic co-catalyst for the palladium-catalysed methoxycarbonylation of alkenes.[4] High conversions of up to 98% were obtained for the methoxycarbonylation of styrene with l/b ratios of up to 3:1. One of the main advantages of the Lewis acid co-catalyst is that it does not lead to phosphonium salt formation and depletion of the ligand, resulting in palladium ‘fall-out’ during these reactions.
(I) During the author’s M.Sc. project, the utilization of Al(OTf)3 as co-catalyst in alkoxycarbonylation was extended to include alkyl-substituted styrene substrates. In this regard, PdCl2 was found to be more reactive than Pd(OAc)2, and trans-β-methylstyrene could be transformed into its respective methyl and ethyl esters with up to 95% conversion, with the major products being linear in this instance.[19]
(J) The Williams group[11] found that the alkoxycarbonylation reaction could be extended to include alkynes when a bidentate ligand is utilized in the process. High conversions (100%) with very high b/l ratios (99:1) were obtained.

• References

• 1 Olah GA, Farooq O, Farnia SM. F, Olah JA. J. Am. Chem. Soc. 1988; 110: 2560
• 2 Olah GA. US Patent 5,110,778, 1992
• 3 Williams DB. G, Simelane SB, Kinfe HH. Org. Biomol. Chem. 2012; 10: 5636
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• 5 Fringuelli F, Pizzo F, Tortoioli S, Vaccaro L. J. Org. Chem. 2004; 69: 7745
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• 7 Gohain M, Marais C, Bezuidenhoudt BC. B. Tetrahedron Lett. 2012; 53: 4704
• 8 Kamal A, Khan MN. A, Reddy KS, Srikanth YV. V, Krishnaji T. Tetrahedron Lett. 2007; 48: 3813
• 9 Kamal A, Khan MN. A, Srikanth YV, Reddy KS. Can. J. Chem. 2008; 86: 1099
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• 11 Williams DB. G, Shaw ML, Hughes T. Organometallics 2011; 30: 4968
• 12 Williams DB. G, Lawton M. Org. Biomol. Chem. 2005; 3: 3269
• 13 Williams DB. G, Lawton M. Tetrahedron Lett. 2006; 47: 6557
• 14 Chaminade X, Chiba S, Narasaka K, Dunach E. Tetrahedron Lett. 2008; 49: 2384
• 15 Gohain M, Marais C, Bezuidenhoudt BC. B. Tetrahedron Lett. 2012; 53: 1048
• 16 Sobhani S, Tashrifi Z. Synth. Commun. 2008; 39: 120
• 17 Gohain M, Jacobs J, Marais C, Bezuidenhoudt BC. B. Aust. J. Chem. 2013; in press
• 18 Parvanak-Boroujeni K, Parvanak K. J. Serbian Chem. Soc. 2011; 76: 155
• 19 Serdyn M.; M.Sc. Thesis; University of the Free State, Bloemfontein, 2013.