Lanthanide(III) nitrobenzenesulfonates and p-toluenesulfonate complexes of lanthanide(III), iron(III), and copper(II) as novel catalysts for the formation of calix[4]resorcinarene Karen Deleersnyder, a Hasan Mehdi, b Istv an T. Horv ath, b Koen Binnemans a and Tatjana N. Parac-Vogt a, * a Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, B-3001 Leuven, Belgium b E€ otv € os University, Department ofChemical Technology and Environmental Chemistry, P azm any Pe´ter Se´t  any 1/A, H-1117 Budapest, Hungary Received 2 May 2007; revised 18 June 2007; accepted 27 June 2007 Available online 4 July 2007 Abstract—Lanthanide(III) salts of p-toluenesulfonic acid [lanthanide(III) tosylates, Ln(TOS) 3 ] and nitrobenzenesulfonic acid [Ln(NBSA) 3 ], and p-toluenesulfonate complexes of iron(III) and copper(II) were prepared, characterized, and examined as catalysts for the synthesis of resorcinol-derived calix[4]resorcinarenes. The reaction of resorcinol with benzaldehyde yields two isomers, the all-cis isomer (rccc) and the cis-trans-trans isomer (rctt) with the relative isomer ratios depending on the reaction conditions. However, in the reaction of resorcinol with oc- tanal only one isomer, the all-cis isomer, is formed in high yields with less than 0.1 mol % of Yb(TOS) 3 . Examination of lanthanide(III) tosylates and lanthanide(III) nitrobenzenesulfonates revealed that ytterbium(III) 4-nitrobenzenesulfonate [ytterbium(III) nosylate, Yb(4-NBSA) 3 ] and ytterbium(III) 2,4-dinitrobenzenesulfonate [Yb(2,4-NBSA) 3 ] are the most active catalysts. The catalysts could be easily recovered and re- used several times for resorcinarene formation without loss of efficiency. Surprisingly good results were also obtained with iron(III) and copper(II) p-toluenesulfonates. Besides optimizing the reaction conditions, new insights into the reaction mechanism were also obtained. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Adolf von Baeyer reported already in 1872 that the addition of concentrated sulfuric acid to a mixture of benzaldehyde and resorcinol gave a red-colored product. 1 In 1968, Erdtman finally proved the cyclic tetrameric structure of these conden- sation products by single crystal X-ray diffraction analysis. 2 As the official IUPAC name of this type of compounds is very complicated, they are commonly called calixarenes, refer- ring to the resemblance of the shape of one of the conformers to a type of Greek vase called a calix crater. 3 Calix[4]resorcinarenes (resorcinarenes), the resorcinol- derived calixarenes, are versatile host compounds for ions, sugars, and organic molecules. Therefore they serve as starting materials for a variety of cavitands and other macro- cyclic host compounds. 4 They have also found application as stationary phases in HPLC. 5 Furthermore, they can ex- hibit liquid-crystalline behavior by appropriate choice of the R groups on the resorcinarene. 6 Resorcinarenes are tradi- tionally prepared by the mineral acid-catalyzed condensa- tion of resorcinol with an aliphatic or aromatic aldehyde. 7 However, this synthetic procedure requires the use of large quantities of acid, leading to excessive waste streams that are environmentally unfriendly and expensive to deal with. A solvent-free synthesis of resorcinarenes using p-toluene- sulfonic acid as the catalyst has also been reported. 8 In addi- tion, some conventional Lewis acids like BF 3 $OEt 2 , AlCl 3 , and SnCl 4 have been used in the synthesis of aromatic alde- hyde-derived resorcinarenes. 9 Although these Lewis acids are usually efficient catalysts, they have several drawbacks: they need to be used in at least stoichiometric amounts, they are easily deactivated by water, and they cannot be recycled or reused. More recently, trifluoromethanesulfonate salts, the so-called triflates, such as ytterbium(III) triflate 10 and bismuth(III) triflate, 11 have been described as efficient cata- lysts for the synthesis of calix[4]resorcinarenes. An impor- tant property of triflate catalysts is their compatibility with water and other protic solvents. After work-up, the catalysts can easily be recycled and reused without significant loss of activity. However, the triflate salts have some major disad- vantages including their relatively high cost and the fact that one needs to handle for their preparation the highly corrosive triflic acid. In a pursuit of lanthanide(III) catalysts, which do not require the use of hazardous or costly compounds, we recently examined lanthanide(III) salts of non-corrosive aromatic Keywords: Catalysis; Lanthanides; Lewis acids; Macrocycles; Rare earths. * Corresponding author. Fax: +32 16 327992; e-mail: tatjana.vogt@chem. kuleuven.be 0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2007.06.090 Tetrahedron 63 (2007) 9063–9070