title: Synthesis, structure and spectroscopic properties of calix[4]phloroglucinarene dodecamethyl ether and its trifluoroacetic acid complex.
abstract: The action of trifluoroacetic acid (TFA) on 2,4,6-trimethoxybenzyl alcohol 3 affords the title tetrameric compound in high yield as a maroon TFA complex, 4; trituration of which with acetone gives the free, colorless, calix[4]phloroglucinarene (5) that can also be directly isolated by treating the reaction mixture with base. The novelty of compounds 4 and 5 is that they possess four additional methoxy groups, which occupy the cavity of the known calix[4]resorcinarene octamethyl ether (2). Ultraviolet-visible spectroscopic analysis shows that TFA-complex 4 exhibits transannular charge-transfer interactions between the opposite aromatic rings. The (1)H-NMR spectrum of the TFA-complex 4 does not change over a wide temperature range, strongly suggesting that it adopts a saddle (1,3-alternate) structure. The conformation of the free phloroglucinarene 5 is temperature-dependent, as determined by variable temperature (1)H NMR spectroscopy. Tetramer 5 adopts a partial cone conformation at low temperatures, but at elevated temperatures is similar to that of the TFA complex 4 (saddle). Tetramer 5 is conformationally mobile at ambient temperature, but generally has a flattened cone (boat) conformation. The ΔG(≠) for inversion in 5 between partial cone and boat conformation is 12.5 Kcal mol(-1), while that between boat and saddle conformation is 14.3 Kcal mol(-1). Conformational changes are also dependant on pH.
authors: Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454-9110, USA.
authors: Olusegun M
authors: OM
authors: Falana
authors: Philip M
authors: PM
authors: Keehn
authors: Robert
authors: R
authors: Stevenson
mesh_terms: D047250:Calixarenes / Q000138:chemical synthesis* / Q000737:chemistry; D008968:Molecular Conformation; D010696:Phloroglucinol / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry; D014269:Trifluoroacetic Acid / Q000737:chemistry*
chemical_list: C569131:calix(4)phloroglucinarene dodecamethyl ether; D047250:Calixarenes; D010696:Phloroglucinol; D014269:Trifluoroacetic Acid