Conformations and Structures of Tetra-O-alkyl-p-tert-butylcalixarenes. How Is the Conformation of Calixarenes Immobilized?

Article abstract of DOI:10.1021/jo00016a027

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 1H NMR spectroscopy.It was found that (i) ring inversion is suppressed by R greater than 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) 1Me4 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 crystallograghic 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 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 100percent selectivity.On the basis of these studies, we discuss how the conformation of calix<4>arenes is immobilized.