Ruthenium-Catalyzed C–H Activation/Cyclization
for the Synthesis of Phosphaisocoumarins
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Abstract
An
efficient and cost-effective ruthenium-catalyzed oxidative cyclization
of phosphonic acid monoesters or phosphinic acids with alkynes has
been developed for the synthesis of a wide range of phosphaisocoumarins
in good to excellent yields under aerobic conditions. A multitude
of arylphosphonic acid monoesters and arylphosphinic acids having
electron-donating and -withdrawing groups were oxidatively cyclized.
Various diarylacetylenes, dialkylacetylenes, and alkylarylacetylenes
effectively underwent the ruthenium-catalyzed oxidative cyclization.
A substrate possessing benzoic acid as well as a phenylphosphonic
monoester moiety was smoothly cyclized with hex-3-yne to afford a
compound having both isocoumarin and phosphaisocoumarin moieties.
Alkenylphosphonic monoester afforded phosphorus 2-pyrone through oxidative
cyclization with alkyne. Competition experiments between diaryl- and
dialkylalkynes and between diarylacetylenes having <i>p</i>-methoxy and <i>p</i>-chloro groups gave results which
showed that the present oxidative cyclizations were not affected by
the electronic effects of alkynes. Mechanistic studies revealed C–H
bond metalation to be the rate-limiting step