A Novel Synthesis of Phenyleneethynylenes by Using Ph₂P(O)-protected Ethynes

Orita and Otera et al. have developed a new terminal alkyne protecting group, Ph₂P(O).^1,2) This protecting group can be introduced readily to terminal alkynes by CuI catalyzed phosphination using chlorodiphenylphosphine and the subsequent oxidation with H₂O₂. The high polarity of the phosphoryl group enables easy separation of the desired products from the lower polar hydrocarbon byproducts. While Ph₂P(O)-protected ethynes remain under Sonogashira coupling conditions without a deprotection, when they are treated with t-BuOK, the facile deprotection proceeds to provide the corresponding deprotected ethynes. The resulting ethynes can be transformed to more expanded arylethynes by the Sonogashira coupling with aryl halides. This synthetic approach can be applied to a one-pot synthesis of phenyleneethynylenes by the deprotection followed by coupling reactions such as the Sonogashira, Migita–Kosugi–Stille and intramolecular Eglinton couplings.²⁾