Rhodacycles Activity in P-Directed Rhodium-Catalyzed Synthesis of π-Extended Aromatic Fluoranthenyl Phosphines

Abstract

We report herein novel polyarylated diphenylphosphino-1-fluoranthenyl ligands by rhodium-catalyzed phosphine-directed C−H arylation. The nature of the rhodium precatalysts that is employed plays an essential role for the control of the reaction and its resulting product. By using the dimeric [Rh^(III)Cl₂Cp*]₂ as the catalyst, a “classical” single peri-functionalization of the polycyclic aromatic hydrocarbons phosphine selectively occurs. Conversely, the use of the ionic [Rh^(I)(COD)₂] BF₄ as the catalyst reveals an unexpected post-rearrangement that is characterized by the migration of the directing phosphino-group on the inserted aryl moiety. In this process, a concurrent cascade extension of the functionalization is also achieved by the introduction of a second aryl moiety. We focused our attention on the rhodium catalytic species formed, and the five or seven-membered rhodacycles as intermediates involved in these C−H catalytic cascade processes. Besides the electronic effects attached to Rh(I) and Rh(III) precatalysts, either cationic or neutral, our results clearly suggest the role of the pentamethylcyclopentadienyl substituent in the selectivity of peri-C−H-arylation. Conversely, rhodium stabilized by weakly bonded more fluxional bis-cyclooctadienyl ligands enters more easily into further reactivity, promoting 2-fold arylation reactions or even participate to diphenylphosphino group rearrangement.