Calix[n]arene/Ti(IV) complexes as active catalysts in aldol reaction of Chan's diene

Study on an Aldol Reaction Catalyzed by Ti(IV)/Calix[n]arene Complexes

“…p-tert-Butylcalix[6]arene (5) was chosen as ligand of the Ti(IV) catalytic complex because it gave the best results when benzaldehyde was the substrate. [23] The catalyst was prepared in situ by stirring, for 1 h at room temperature, a 0.02 M solution of the ligand 5 in anhydrous tetrahydrofuran and Ti(O-i-Pr) 4 , in a molar ratio of 1 : 1. The catalyst solution was then cooled at À 78 8C and the aldehyde was added, followed after 30 minutes by the Chans diene 1.…”

“…In addition, these new reaction conditions seemed to be better than the previous ones also for heteroaromatic aldehyde 3-furfural (2d) (entry 8, Table 3) and for a,b-unsaturated cinnamaldehyde (2e) (entry 9, Table 3). p-tert-Butylcalix [4]arene (4) was also examined as ligand of the Ti(IV) catalytic complex, because in the previous work it showed a good yield (77%) [23] of adduct in the aldol reaction of benzaldehyde with Chans diene (1). However, its screening with several aldehydes generally gave low chemical yields (Table 4), the best result (56%) being with p-nitrobenzaldehyde (entry 1, Table 4).…”

“…[23] It was there evidenced that these complexes act as efficient Lewis acids, enhancing the reactivity of the aldehyde towards the silyloxydiene. In addition, it was found that the catalytic efficiency was affected by the reaction temperature and by the structure of the calixarene ligand 4 -6.…”

“…[20] Very recently, as a part of our program devoted to efficient approaches to compounds of type 3 (Scheme 1), useful intermediates in the total synthesis of complex molecules, [21] we have found that calix[n]arene/Ti(IV) complexes, formed in situ, can be used as efficient catalysts in the aldol reaction of Chans diene 1 [22] with benzaldehyde (2a). [23] It was there evidenced that these complexes act as efficient Lewis acids, enhancing the reactivity of the aldehyde towards the silyloxydiene. In addition, it was found that the catalytic efficiency was affected by the reaction temperature and by the structure of the calixarene ligand 4 -6.…”

The synthesis of new calix[n]arene quaternary ammonium salts and investigation of their catalytic affinities for three component Mannich-type reactions in water

“…8,9 In addition, calixarenes have been used as catalysts because of their varied cavity sizes, they serve as a convenient platform for the design of multifunctional catalysts and can be selectively functionalized for a desired goal. [10][11][12][13][14] Organic reactions in aqueous media have drawn a great deal of attention because reactions in water allay environmental concerns and serve the goal of "greening" global chemical processes. 15,16 The discovery of water-soluble calixarenes as catalysts in organic reactions such as C-C bond formations, has attracted the attention of researchers with the goal of accelerating the reaction and decreasing the amount of catalyst required for a reaction to occur in aqueous media, developing this green chemistry.…”