Triphenylcyclopentadienyl Rhodium Complexes in Catalytic C-H Annulations. Application for Synthesis of Natural Isocoumarins.

Efficient protocols for the synthesis of triphenylcyclopentadienyl rhodium halides [(1,2,4-C 5 Ph 3 H 2 )RhX 2 ] 2 ( 1a , b : X = Cl, I) starting from 1,2,4-triphenylcyclopentadiene or the cyclooctadiene derivative (1,2,4-C 5 Ph 3 H 2 )Rh(cod) ( 2 ) were developed. Iodide abstraction from 1b with thallium or silver salts allowed us to prepare rhodocenium [(1,2,4-C 5 Ph 3 H 2 )RhCp]PF 6 ( 3 PF 6 ) and mesitylene complex [(1,2,4-C 5 Ph 3 H 2 )Rh(mesitylene)](SbF 6 ) 2 ( 4 (SbF 6 ) 2 ). Halides 1a , b (at 0.5 mol % loading) showed high catalytic activity in the construction of C-C, C-O, and C-N bonds via the C(sp 2 )-H activation approach. Their efficiency was demonstrated in the synthesis of more than 40 examples of polycyclic organic compounds (such as isocoumarins and naphthalenes, as well as isoquinolinium and dibenzo[ a , f ]quinolizinium salts). The protocols developed tolerate a wide range of functional groups. In particular, they were successfully used for the atom- and step-economical synthesis of hydroxy-substituted isocoumarins, including the natural product oospalactone 7fe . The 6- or 8-hydroxy-substituted isocoumarins showed moderate antiproliferative activity against several human cell lines in vitro .