Synthesis, structure and spectroscopic properties of calix[4]phloroglucinarene dodecamethyl ether and its trifluoroacetic acid complex

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 Δ G ≠ 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. A novel family of metacyclophanes, monikered phloroglucinarene, in free and acid-complex states is presented. The stereochemistry and electronic properties are reported permitting the first understanding of the influence of intercavity moieties on conformation in the liquid phase.