Umpolung of B−H Bonds by Metal–Ligand Cooperation with Cyclopentadienone Iridium Complexes

Abstract

In contrast to conventional metal–ligand cooperative cleavage of a B−H bond, which provides a B cation on the ligand and an H anion on the metal, we report herein the umpolung of B−H bonds by novel cyclopentadienone iridium complexes. The B−H bonds of 4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane (HBpin) and 1,8‐naphthalenediaminatoborane (HBdan) were cleaved to give a B anion on the metal and an H cation on the phenolic oxygen atom of the ligand. Mechanistic investigation by DFT calculations revealed that the alkoxycarbonyl‐substituted cyclopentadienone ligand facilitated deprotonation from Ir−H after oxidative addition of the B−H bond to give the umpolung product. The generated boryliridium complex was found to undergo borylation of an allyl halide in the presence of base, thus showing the nucleophilic nature of the boron atom.

Heterolytic Oxidative Addition of sp2 and sp3 C–H Bonds by Metal–Ligand Cooperation with an Electron-Deficient Cyclopentadienone Iridium Complex

“…We also found that complex 1 is highly active toward the dehydrogenation of THF when we attempted to record the NMR spectrum of 1 in THF; 74% of the hydroxycyclo­pentadienyl iridium dihydride complex 3 was generated with the concomitant formation of furan when a solution of 1 in THF was kept at 60 °C for 15 h (Scheme a). This dehydrogenation activity was further applied to catalytic transfer-dehydrogenation of THF to furan in the presence of norbornene as the hydrogen acceptor (Scheme b) .…”

“…The key to our success is the use of an electron-deficient cyclopentadienone ligand, despite the oxidative additions of C−H bonds usually occurring at highly electron-rich metal centers. 1 We synthesized iridium complex 1 bearing an ethoxycarbonyl-substituted naphtho-fused cyclopentadienone ligand, 10 which was used in the following experimental studies (see Supporting Information (SI) section VII for the effect of the metal and substituent). The reactivity of the iridium hydride complex 1 in the cleavage of the C−H bonds of a hydrocarbon was first experimentally confirmed by the H/D exchange reaction with C 6 D 6 .…”

“…After prior coordination of a C−H bond to the iridium center, the nitro group of another nitromethane would act as a mediator to deprotonate the C−H bond (TS MeNO2 , Scheme 4) to form ion pair Int-C of the anionic iridium complex and protonated nitromethane. The facile proton transfer from the hydroxy group in the cationic part to the carbonyl oxygen in anionic iridium complex affords complex 2 with regeneration of 1 equiv of nitromethane (the validity of the proposed mechanism is discussed with theoretical calculations and related literature, 9,10,21 in Supporting Information section VII-IV). Because a simple alkane like CH 4 has no basic site to be a mediator, we think complex 1 itself can work as the base.…”

Cyclopentadienone Iridium Bipyridyl Complexes: Acid-Stable Transfer Hydrogenation Catalysts

“…The synthesis of the Ir–cyclopentadienone complexes was carried out by a modification of the procedure described by Nozaki et al Addition of 4,4′-dimethyl-2,2′-bipyridine (dmbpy) to air-stable but insoluble [(C 5 Tol 2 Ph 2 O)­IrCl] n occurs heterogeneously in methanol to generate the orange solid (C 5 Tol 2 Ph 2 O)­(dmbpy)­IrCl ( 1 ), which is indefinitely stable in air, either as a dry solid or in solution. The 1 H NMR spectrum of 1 in aprotic solvents is poorly resolved, with line broadening in the aromatic region (Figure S2).…”

“…Nozaki et al reported a class of cyclopentadienone–iridium complexes bearing phosphine ligands that catalyze hydrogenation and dehydrogenation reactions between 150 and 200 °C. − Related Cp*­(bpy)­IrH + complexes are efficient catalysts for transfer hydrogenation in water at low pH. − In organic solvents, Cp*­(bpy)­IrH + is unreactive toward aldehydes, which was attributed to the modest hydride donating ability (Δ G H‑(MeCN) ≥ 60 kcal/mol) of these cationic Ir hydrides. , However, in the presence of visible light, Cp*­(bpy)­IrH + complexes are effective in hydride transfer reactions, as the hydride donating ability is enhanced by photoexcitation. , We sought to test the hypothesis that modestly hydridic cationic Ir hydrides bearing hydroxycyclopentadienyl ligands might be more generally reactive toward polar substrates in nonaqueous conditions due to bifunctional activation.…”