The 2020 discovery by Higashi, Kusumoto, and Nozaki that an alkoxycarbonyl-substituted cyclopentadienone iridium(I) hydride complex cleaves the B–H bonds of pinacolborane (HBpin) and 1,8-naphthalenediaminatoborane (HBdan) with inverted polarity — boron as an anionic boryl ligand on iridium and hydrogen as a proton on the ligand's oxygen — provided the first isolable metal–ligand cooperative B–H umpolung product and a nucleophilic boryliridium species that transfers boryl to allylic halides. The source work left three questions open: (1) the scope and selectivity rules of the umpolung across hydroboranes, ligands, and electrophiles, and what controls the competition between deprotonation and deboration; (2) whether the activation mode can be developed into a genuinely useful catalytic borylation — a proof-of-concept catalytic run gives only about 2.7 turnovers with substantial competing hydroboration — rather than stoichiometric reactivity; and (3) the mechanism of the computed bimetallic deprotonation step. The authors' own follow-ups extended the cooperative-umpolung concept to C–H and Si–H bonds but left all three B–H strands unresolved, and no external research program has taken them up; the problem remains open.

Background

In classical transition-metal boryl chemistry, the B–H bonds of hydroboranes are polarized with boron as the electrophilic (δ+) end and hydrogen as the hydridic (δ−) end. Pinacolborane (HBpin, 4,4,5,5-tetramethyl-1,3,2-dioxaborolane) and HBdan (1,8-naphthalenediaminatoborane) are accordingly used almost exclusively as electrophilic boron sources in hydroboration. Three activation modes were known before 2020. First, oxidative addition places both B and H on the metal as anionic ligands. Second, metal–ligand cooperation (MLC) — a design in which a reactive ligand site participates directly in bond cleavage alongside the metal — had been shown by several groups (for example ruthenium thiolate systems, palladium carbene complexes, and Casey and Clark's boron-substituted analogue of the Shvo catalyst) to cleave B–H bonds heterolytically with the conventional polarity: boron ends up as a cationic borate-type substituent on the ligand's heteroatom and hydrogen as a hydride on the metal. Third, nucleophilic boryl reagents (boryllithium, base-stabilized boryl anions) exist as discrete main-group species but are prepared from diboron reagents or haloboranes, not from simple hydroboranes. The cyclopentadienone/hydroxycyclopentadienyl ligand platform, first popularized by Shvo's ruthenium hydrogenation catalyst, is redox-active: the metal and the ligand's carbonyl/hydroxy pair shuttle between two oxidation/protonation states during heterolytic H–H (and, as later shown, C–H, B–H, and Si–H) cleavage. In 2020 Higashi, Kusumoto, and Nozaki prepared a family of penta-coordinate cyclopentadienone iridium(I) hydride complexes — 4a (diphenyl-substituted ligand), 4b (diethyl), and 4c (bearing alkoxycarbonyl substituents) — and reacted them with hydroboranes. The outcome depends sharply on both ligand and borane: 4a with HBpin gives the conventional product 6a (73%), 4b gives a mixture, and 4c gives the umpolung product 5c — umpolung being the German term for polarity reversal, here meaning that the boron, normally the electrophilic end, instead becomes the nucleophilic anionic end — a hydroxycyclopentadienyl boryliridium(III) complex, in 96% NMR yield (55% isolated); HBdan behaves like HBpin, whereas the bulkier or more Lewis-acidic 9-BBN (9-borabicyclo[3.3.1]nonane) and HBhex (4,4,6-trimethyl-1,3,2-dioxaborinane) give only conventional products. The connectivity of 5c — iridium–boron bond plus an O–H proton on the ligand — was established by NMR and single-crystal X-ray diffraction. Density functional theory calculations (the rM06 meta-GGA exchange–correlation functional with the def2-TZVP triple-zeta valence basis set, on truncated models) reproduce the selectivity through the following sequence: the B–H bond coordinates after acetonitrile dissociation, undergoes near-barrierless oxidative addition to a dihydrido boryl iridium(III) intermediate (Int-II), and is then resolved by intermolecular deprotonation of an Ir–H by a second iridium complex through the bimetallic transition state TSII-IIITS_{II\text{-}III}(cis-H) at 16.6 kcal/mol, versus 25.8 kcal/mol for the competing deboration path from the same cis intermediate; the umpolung product is kinetically controlled and computed to lie 20.3 kcal/mol above the conventional isomer. The acidic ligand proton of 5c can be removed with sodium hydride to give the anionic cyclopentadienone boryliridium(I) complex 7c, whose boron is nucleophilic: with allyl iodide or allyl bromide at room temperature it forms allyl-Bpin in 58% and 42% yield, while benzyl bromide and 3,3-dimethylallyl bromide are unreactive. A later account by the same group summarizes this chemistry and reports a proof-of-concept catalytic borylation of allyl iodide with HBpin and base in the presence of the iridium complex (about 9.5 mmol allyl-Bpin from 3.5 mmol catalyst, i.e. roughly 2.7 turnovers), with the C=C hydroboration product formed as a substantial byproduct and explicitly described as leaving room for improvement. What remains unresolved is how general this polarity-inverting activation mode is, whether it can be made into a practical catalytic method for converting hydroboranes into boryl-nucleophile transfer reagents, and how the unusual bimetallic deprotonation step actually operates.

Problem Statement

The problem is to establish metal–ligand cooperative B–H umpolung with cyclopentadienone iridium complexes as a general, mechanistically understood, and catalytically useful method for converting hydroboranes — traditionally electrophilic boron sources — into nucleophilic boryl-transfer reagents. Three linked questions, all flagged as open in the source work, constitute the objective. (1) Scope and selectivity rules: determine, across a systematic range of hydroboranes, cyclopentadienone ligand substitutions, and electrophilic substrates, when heterolytic B–H cleavage gives the umpolung product (boryl on iridium, proton on the ligand oxygen) and when it gives the conventional product (boron on the ligand, hydride on the metal) — the known data are that HBpin and HBdan give umpolung with the alkoxycarbonyl-substituted ligand while 9-BBN and HBhex do not — and establish the design principles, in terms of borane Lewis acidity and steric bulk and ligand electronic and secondary-interaction capability (such as the stabilizing hydrogen bond of the alkoxycarbonyl group in the deprotonation transition state), that govern the competition between the deprotonation and deboration pathways. (2) Catalytic implementation: develop a catalytic borylation of electrophiles by hydroboranes through this activation mode with a turnover number and selectivity that reproducibly improve on the existing proof-of-concept (about 2.7 turnovers for allyl-Bpin from allyl iodide, HBpin, and base, with substantial competing hydroboration of the C=C bond) — which entails suppressing the hydroboration side reaction and widening the electrophile scope beyond unhindered allylic halides. (3) Mechanism of the bimetallic deprotonation step: elucidate experimentally and computationally the kinetics and intermediates of the intermolecular deprotonation of the iridium hydride by a second iridium complex invoked in the computed pathway (TSII-IIITS_{II\text{-}III}). The three strands are interrelated — a scope map and mechanistic understanding inform the rational development of a practical catalytic borylation — and all three are required parts of the objective; each remains an open research direction in the literature.

References

  1. Takuya Higashi, Shuhei Kusumoto, Kyoko Nozaki, 'Umpolung of B−H Bonds by Metal–Ligand Cooperation with Cyclopentadienone Iridium Complexes', Angewandte Chemie International Edition 2021, 60(6), 2844–2848. DOI 10.1002/anie.202011322, https://doi.org/10.1002/anie.202011322
  2. Shuhei Kusumoto, 'Heterolytic Bond Cleavage and Formation based on the Metal–Ligand Cooperative Function of Cyclopentadienone Metal Complexes' (シクロペンタジエノン金属錯体の金属-配位子協働を基軸とする不均等結合切断・形成反応, in Japanese), Bulletin of the Japan Society of Coordination Chemistry 2025, 85, 37–48. DOI 10.4019/bjscc.85.37, https://doi.org/10.4019/bjscc.85.37 (Scheme 12b reports the proof-of-concept catalytic borylation of allyl iodide)
  3. Takuya Higashi, Shuhei Kusumoto, Kyoko Nozaki, 'Metal-Ligand Cooperative Bond Cleavage/Formation and Their Catalytic Applications with Cyclopentadienone Metal Complexes' (in Japanese), Yuki Gosei Kagaku Kyokaishi (Journal of Synthetic Organic Chemistry, Japan) 2022, 80(5), 459–468. DOI 10.5059/yukigoseikyokaishi.80.459, https://doi.org/10.5059/yukigoseikyokaishi.80.459
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  9. Liza Koren-Selfridge, Hannah N. Londino, Jessica K. Vellucci, Bryan J. Simmons, Charles P. Casey, Timothy B. Clark, 'A Boron-Substituted Analogue of the Shvo Hydrogenation Catalyst: Catalytic Hydroboration of Aldehydes, Imines, and Ketones', Organometallics 2009, 28(7), 2085–2090. DOI 10.1021/om801228m, https://doi.org/10.1021/om801228m
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