Rupe-Type Rearrangement Intercepted by Diels–Alder Cycloaddition on Osmium

An orthometallated 1-naphthylketone has been generated on osmium by coupling of γ-hydroxyalkynyl and diphenylallenylidene ligands. Treatment of Os{CC–C(OH)Ph2}2(CCCPh2){κ3-P,O,P-[xant(PiPr2)2]}(1) with HBF4 leads to [Os{κ2-O,C-[OC(CHPh2)-naphthyl-Ph]}(C–CHCPh2){κ3-P,O,P-[xant(PiPr2)2]}](BF4)2 (2), which gives [Os{κ2-O,C-[OC(CHPh2)-naphthyl-Ph]}(CCCPh2){κ3-P,O,P-[xant(PiPr2)2]}]BF4 (3) by deprotonation with (piperidinomethyl)polystyrene. The formation of the ketone of 2 and 3 is an HF-catalyzed process. The H+ and F– fragments of HF are introduced sequentially with two different HBF4 molecules. The first molecule delivers H+, while the second provides F–. The proton from the first molecule adds to the Cβ atom of the diphenylallenylidene ligand of 1 to form [Os{CC–C(OH)Ph2}2(C–CHCPh2){κ3-P,O,P-[xant(PiPr2)2]}]BF4 (4). The hydroxide group from a γ-hydroxyalkynyl of 4 is removed with the proton of the second HBF4 molecule, whereas the osmium center abstracts a fluoride of [BF4]−, to give [OsF{C[−CC–C(OH)Ph2]–CHCPh2}(CCCPh2){κ3-P,O,P-[xant(PiPr2)2]}]BF4 (5). Once both fragments of HF are strategically located, the alkenyl-(γ-hydroxyalkynyl)alkylidene ligand experiences a Rupe-type rearrangement intercepted by a Diels–Alder cycloaddition, in two steps. A dehydration intercepted by Diels–Alder cycloaddition initially occurs, which affords the fluoroalkenylnaphthyl derivative [Os{κ2-F,C-[FC(CPh2)-naphthyl-Ph]}(CCCPh2){κ3-P,O,P-[xant(PiPr2)2]}]BF4 (7). The subsequent reaction of the latter with water yields the orthometallated 1-naphthylketone of 3, releasing HF. The protonation of 3 with HBF4 leads to 2.