Cooperativity in Bimetallic Dihydroalkoxylation Catalysts Built on Aromatic Scaffolds: Significant Rate Enhancements with a Rigid Anthracene Scaffold

This work describes investigations into metal-catalyzed sequential reactions using a series of single metal and bimetallic Rh(I) and/or Ir(I) pyrazolyl complexes. Monometallic complexes with bis(1-pyrazolyl)methane (bpm) ligands [M(CO)(2)(bpm)]BAr4F (1), bimetallic complexes [M-2(CO)(2)(L-scaffold)][BAr4F](2) (2-4) where M = Rh(I) or Ir(I) bearing bitopic ligands L-scaffold = bis(1-pyrazolyl)methane-derived ligands, p-C6H4[CH(Pz)(2)](2) (L-p), m-C6H4[CH(Pz)(2)](2) (L-m), and anthracene-bridged 1,8-C14H8[CH(Pz)(2)](2) (L-Ant), [M-2(CO)(4)(L-p)]. [BAr4F](2) (2), [M-2(CO)(4)(L-m)][BAr4F](2) (3), and [M-2(CO)(4)(L-Ant)][BAr4F](2) (4) were used as catalysts. The efficiency of the complexes as catalysts was tested for the dihydroalkoxylation of a series of alkyne diol substrates, 2-(6-hydroxyhex-1-ynyl)benzyl alcohol (5), 1-methyl-3-heptyne-1,7-diol (6), 2-(5-hydroxypent-1-ynyl)benzyl alcohol (7), and 2-(4-hydroxybut-1-ynyl)benzyl alcohol (8), forming spiroketals. All complexes tested were highly effective catalysts for the intramolecular dihydroalkoxylation reaction. The homobimetallic complexes 2-4 showed significant enhancement in activity and selectivity relative to the single metal catalysts (1). The order of catalytic activity of the bimetallic complexes was found to be [M-2(CO)(4)(L-Ant)][BAr4F](2) > [M-2(CO)(4)(L-m)][BAr4F](2) > [M-2(CO)(4)(L-p)][BAr4F](2) for all substrates, and the bimetallic cooperativity index was established for each reaction.