Unambiguous Determination of the Absolute Configurations of Acetylene Alcohols by Combination of the Sonogashira Reaction and the CD Exciton Chirality Method ? Exciton Coupling between Phenylacetylene and Benzoate Chromophores

Abstract

The CD exciton chirality method was applied to various phenylacetylene alcohols to determine their absolute configurations; the long axis polarized π–π* transition (λmax=252 nm) of the 4-methoxyphenylacetylene chromophore couples with the transition (λmax=257 nm) of the 4-methoxybenzoate group to generate intense exciton split CD Cotton effects, from the signs of which the absolute configurations of phenylacetylene alcohols were unambiguously determined. As an extension of the results, a new methodology for determining the absolute configurations of acetylene alcohols having the HC≡CCH(OH)-moiety by combination of the Sonogashira reaction and the CD exciton chirality method has been developed and applied. Since the π–π* transition of acetylene triple bond is located below 180 nm, it is difficult to observe ideal bisignate CD Cotton effects due to the exciton coupling between acetylene and benzoate chromophores. To observe the ideal exciton split Cotton effects necessary for the unambiguous determination of absolute configuration, the terminal acetylene group was converted, by the Sonogashira reaction, to the 4-methoxyphenylacetylene moiety, which exhibits an intense π–π* absorption band polarized along the long axis of the chromophore at 252 nm. As a partner of exciton coupling, 4-methoxybenzoate showing a π–π* band at 257 nm was introduced into the alcohol moiety, and the benzoates formed showed intense bisignate CD Cotton effects, from the signs of which the absolute configurations of original acetylene alcohols could be determined in an unambiguous manner.