Concise Synthesis of Isocoumarins through Rh-Catalyzed Direct Vinylene Annulation: Scope and Mechanistic Insight

Transition-metal-catalyzed activation
of inert C–H bonds
and subsequent C–C bond formation have emerged as powerful
synthetic tools for the synthesis of elaborate cyclic molecules. In
this report, we introduce an efficient synthetic method of 3,4-unsubstituted
isocoumarins adopting an electron-deficient CpERh complex
as the catalyst. The use of vinylene carbonate as a vinylene transfer
reagent enables the direct construction of isocoumarins from readily
available benzoic acids, without any external oxidants as well as
bases. The reaction mechanism is evaluated by computational analysis
to find an unprecedented “rhodium shift” event within
the catalytic cycle.