CONFORMATIONS AND STRUCTURES OF TETRA-O-ALKYL-PARA-TERT-BUTYLCALIX[4]ARENES - HOW IS THE CONFORMATION OF CALIX[4]ARENES IMMOBILIZED

p-tert-Butylcalix[4]arene (1H4) was tetra-O-alkylated with alkyl halogens (RX: R = Me, Et, n-Pr, and n-Bu) in the presence of NaH as base, and the products (1R4) were analyzed by HPLC and H-1 NMR spectroscopy. It was found that (i) ring inversion is suppressed by R greater Et, (ii) the final conformer distribution in 1Pr4 and 1Bu4 is governed by the kinetic control, the main products being ''cone'' and ''partial cone'' (approximately in a 1:1 ratio), (iii) 1 Me4 mostly exists as a thermodynamically stable partial-cone conformer, and (iv) 1Et4 shows an intermediary behavior between 1Me4 and 1Pr4: it mostly exists as a partial-cone conformer but slowly isomerizes to a ''1,2-alternate'' conformer at high temperature. The X-ray crystallographic analysis of partial-cone-1Et4 was investigated. To clarify where and how the conformation of 1R4 is immobilized, we alkylated 1H4 in a stepwise manner. It was shown that when NaH is used as base, the conformation of 1Et4 is determined at the fourth ethylation step (1HEt3 --> 1Et4), whereas the conformation of 1Pr4 is determined at the third propylation step (1H2Pr2 --> 1HPr3). The conformer distribution was significantly affected by alkali and alkaline earth metal cations used as base; in particular, it is worthy of mentioning that (i) when Cs2CO3 is used as base, 1,2-alternate-1Pr4 is formed in addition to partial-cone-1Pr4 and (ii) when Ba(OH)2 is used as base, cone-1Pr4 is yielded in 100% selectivity. On the basis of these studies, we discuss how the conformation of calix[4]arenes is immobilized.