Palladium-Catalyzed Direct Synthesis of Phosphole Derivatives from Triarylphosphines through Cleavage of Carbon-Hydrogen and Carbon-Phosphorus Bonds

摘要

Phospholes have recently received much attention as promising organic materials because of their characteristic optical and electronic properties, which are derived from the phosphorus-bridged 1.3-dicnic 31 system.The method most frequently used for the synthesis of phospholes involves the nucleophilic substitution of a P-X bond with a stoichiometric amount of an organometallic species such as organolithium or organomagnesium reagents. The' issue of functional gprap compatibility associated with this classical method has been addressed to some extent by using transition metal catalysis. Catalytic [2+2+2] cycloaddition of dialkynylphosphines with polyynes has been used for the synthesis of helicene analogues of phospholes. More versatile synthesis is enabled by catalytic C-P bond formation reactions. The intramolecular cross-coupling of aryl halides or their equivalents with hydrophosphines has been successfully used in the synthesis of a phosphole skeleton (Scheme 1 a). However, this method still needs considerable improvement in terms of the degree of functionalization of the starting material and the instability of a hydrophosphine group. In this context, Takai and Kuninobu et al. made notable progress by developing a palladium-catalyzed synthesis of dibenzophosphole oxides by dehydrogenative cyclization of hydrophosphine oxides (Scheme 1b). In view of the widespread availability and stability of triarylphosphines, a more synthetically valuable approach would involve intramolecular cross-coupling between triarylphosphine and an arene through simultaneous cleavage of C—P and C—H bonds (Scheme lc). Herein, we report the realization of a catalytic reaction of this type.