p-tert-Butylcalixarene (1H 4 ) 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 (1R 4 ) were analyzed by HPLC and 1 H NMR spectroscopy. It was found that (i) ring inversion is suppressed by R greater than Et, (ii) the final conformer distribution in 1Pr 4 and 1Bu 4 , is governed by the kinetic control, the main products being “cone” and “partial cone” (approximately in a 1:1 ratio), (iii) 1Me 4 mostly exists as a thermodynamically stable partial-cone conformer, and (iv) 1Et 4 shows an intermediary behavior between 1Me 4 and 1Pr 4 : 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-1Et 4 was investigated. To clarify where and how the conformation of 1R 4 is immobilized, we alkylated 1H 4 in a stepwise manner. It was shown that when NaH is used as base, the conformation of 1Et 4 is determined at the fourth ethylation step (1HEt 3 → 1Et 4 ), whereas the conformation of 1Pr 4 is determined at the third propylation step (1H 2 PR 2 →1HPr 3 ). 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 Cs 2 CO 3 is used as base, 1,2-alternate- 1Pr 4 is formed in addition to partial-cone-1Pr 4 and (ii) when Ba(OH) 2 is used as base, cone-1Pr 4 is yielded in 100% selectivity. On the basis of these studies, we discuss how the conformation of calixarenes is immobilized.
All Science Journal Classification (ASJC) codes
- Organic Chemistry