Sensuke Ogoshi

The reductive synthesis of fluoroalkyl copper phenanthroline complexes from fluoroalkyl iodides, using diboron as a reductant, is reported in this study. The structure of Cu(C₄F₉)(phen), isolated as a phosphine adduct, was confirmed by X-ray crystallography. These copper complexes underwent smooth cross-coupling with aryl iodides under mild conditions to afford fluoroalkylated arenes. Furthermore, a one-pot protocol enabled the reductive coupling of aryl iodides and fluoroalkyl iodides without the need to isolate the intermediate fluoroalkyl copper complexes.

Transition metals are known to work as electron donors toward electron-accepting heavier-group-13 elements (Al, Ga, and In), called Z-type ligands. However, complexes with boron-based Z-type ligands are stable only in the presence of additional coordination units (the so-called "supported-ligand" strategy). Here, we report the synthesis and characterization of square-planar Ni(0) complexes that bear tris(perfluoroaryl)boranes as monodentate Z-type ligands, even though such coordination geometry has been traditionally associated with Ni(II) species based on the well-established ligand-field theory. A combined theoretical and experimental approach revealed a mixed covalent/dative character for the Ni-B bonds. This strategy uses frustrated L/Z-ligand pairs that combine sterically encumbered electron-donating (L-type) and electron-accepting ligands to form noncovalent interactions over L-M-Z units to achieve unprecedented low-valent transition metal species with monodentate Z-type ligands.

The shelf-stable heteroleptic borane B­(2,6-Cl2C6H3)­(3,5-Br2-2,6-F2C6H)2 (B7) efficiently catalyzes the solvent-free hydrogenation of various substituted indoles to indolines with an unprecedented turnover number of 8,500, which is more than 400-fold higher than that reported for B­(C6F5)3 under diluted conditions. Mechanistic studies revealed that this hydrogenation proceeds via an olefin-to-nitrogen switching of Lewis bases involved in the H2-cleavage steps: initially, H2 cleavage is mediated by a frustrated Lewis pair (FLP) comprising the indole C3-carbon and boron atoms, which then switches to an FLP system comprising the indoline nitrogen and boron atoms after formation of the indoline. This study demonstrates the potential of relatively benign main-group elements for the catalytic synthesis of valuable N-containing molecules using H2.

Oxazaphosphetane species were experimentally demonstrated as a key intermediate in the transformation of CO₂ and isocyanates through their reaction with N-borane-substituted cyclic phosphine imides (BCPIs).

Cheminformatics-based machine learning (ML) has been employed to determine optimal reaction conditions, including catalyst structures, in the field of synthetic chemistry. However, such ML-focused strategies have remained largely unexplored in the context of catalytic molecular transformations using Lewis-acidic main-group elements, probably due to the absence of a candidate library and effective guidelines (parameters) for the prediction of the activity of main-group elements. Here, the construction of a triarylborane library and its application to an ML-assisted approach for the catalytic reductive alkylation of aniline-derived amino acids and C-terminal-protected peptides with aldehydes and H₂ is reported. A combined theoretical and experimental approach identified the optimal borane, i.e., B(2,3,5,6-Cl₄-C₆H)(2,6-F₂-3,5-(CF₃)₂-C₆H)₂, which exhibits remarkable functional-group compatibility toward aniline derivatives in the presence of 4-methyltetrahydropyran. The present catalytic system generates H₂O as the sole byproduct.

A detailed electronic study of the N-phosphine oxide functionalized imidazolin-2-ylidenes (PoxIms) and imidazolidin-2-ylidenes (SPoxIms) has been performed experimentally using IR, ¹³C, and ⁷⁷Se NMR spectroscopies. While the net donor/acceptor properties of the (S)PoxIms could not be differentiated via IR spectroscopy (TEP), NMR spectroscopic methods (HEP, Se) reveal that the (S)PoxIms are slightly weaker σ-donors but stronger π-acceptors compared to common NHCs. Moreover, backbone and substituent-effects could also be resolved by the latter, allowing for a ranking of their electronic properties. Finally, the donicities of these well-designed NHC ligands in their bidentate κ²-C,O modes were evaluated using HEP2 and compared to those of classical chelators.

Cheminformatics-based machine learning (ML) has assisted in determining optimal reaction conditions, including catalyst struc-tures, in the field of synthetic chemistry. However, such ML-focused strategies have remained largely unexplored in the context of catalytic molecular transformations using Lewis-acidic main-group elements, probably due to the absence of a candidate library and effective guidelines (parameters) for the prediction of the activity of main-group elements. Here, the construction of a triarylborane library and its application to an ML-assisted approach for the catalytic reductive alkylation of amino acids with aldehydes and H2 is reported. The obtained results suggest that the deformation energy serves as a useful parameter for Gauss-ian process regression to construct adequate models for predicting the turnover frequencies of triarylboranes under the applied model reaction conditions, while Gaussian progress regression based on the energy levels of the lowest unoccupied molecular orbitals (LUMOs) including the empty p-orbitals of boron tend to underestimate the turnover frequency. The optimal borane, i.e., B(2,3,5,6-Cl4-C6H)(2,6-F2-3,5-(CF3)2-C6H)2, effectively catalyzes the reductive functionalization of aniline-derived amino acids and C-terminal-protected peptides in the presence of 4-methyltetrahydropyrane and H2, generating H2O as the sole by-product.

We herein report the formation of fluorinated N-heterocyclic carbenes (NHC^Fs) that bear fluorine atoms at the 4- and 5-positions of the imidazol-2-ylidene ring. Treatment of sodium N,N'-bis(aryl)formamidinates with tetrafluoroethylene followed by the addition of LiBF₄ induced a [3 + 2] cycloaddition to afford 4,5-difluorinated imidazolium salts, which served as the precursors for 4,5-difluorinated NHCs. A key feature of this procedure is its applicability to other perfluorinated compounds, which enabled us to incorporate polyfluorinated functional groups at 4- and 5-positions on the imidazol-2-ylidene skeleton. Thus, employing octafluorocyclopentene and hexafluorobenzene led to the formation of 4,4,5,5,6,6-hexafluoro-1,3-diaryl-3,4,5,6-tetrahydrocyclopenta[d]imidazolium (CypIPr^F·HBF₄) and 4,5,6,7-tetrafluoro-1,3-diarylbenzimidazolium (BIPr^F·HBF₄) salts, respectively. A thorough NMR analysis of these NHC^Fs, their selenium adducts, and their tricarbonyl nickel complexes, (NHC^F)Ni(CO)₃, demonstrated that the fluorine substituents, contrary to expectations, tend to act as electron donors owing to the considerable positive mesomeric effect, while the perfluorocyclopentene-fused and tetrafluorobenzo-fused rings are pure electron acceptors due to their strong negative inductive effect. The unique and increased π-accepting character of the perfluorocyclopentene-fused and tetrafluorobenzo-fused NHC^Fs in both stoichiometric and catalytic reactions is further demonstrated by employing (NHC^F)Ni(CO)₃ and (NHC^F)AuCl species, respectively. Moreover, an analysis of the % buried volume (%V_bur) values clearly suggests that the modification of the NHC backbone with polyfluorinated groups can drastically alter the electronic properties of the NHC ligand without substantially changing its steric properties. Our experimental results were further corroborated by a series of computational calculations.

Herein, we report the insertion of a difluoromethylene into 1,1,2,2-tetrafluoro-2-arylethyl copper complexes to synthesize extended perfluoroalkyl-bridged compounds that have various functional groups on each edge (ArCF₂CF₂(CF₂)_nR, R = arenes, halogens, alkyl, alkenyl, and benzyloxycarbonyl). Further, the one-pot syntheses of perfluoroalkyl-bridged compounds from aryl boronic acid esters were carried out.

Designing and modulating the electronic and spatial environments surrounding metal centers is a crucial issue in a wide range of chemistry fields that use organometallic compounds. Herein, we demonstrate a Lewis-acid-mediated reversible expansion, contraction, and transformation of the spatial environment surrounding nickel(0) centers that bear N-phosphine oxide-substituted N-heterocyclic carbenes (henceforth referred to as (S)PoxIms). Reaction between tetrahedral (syn-κ-C,O-(S)PoxIm)Ni(CO)₂ and Al(C₆F₅)₃ smoothly afforded heterobimetallic Ni/Al species such as trigonal-planar {κ-C-Ni(CO)₂}(μ-anti-(S)PoxIm){κ-O-Al(C₆F₅)₃} via a complexation-induced rotation of the N-phosphine oxide moieties, while the addition of 4-dimethylaminopyridine resulted in the quantitative regeneration of the former Ni complexes. The corresponding interconversion also occurred between (SPoxIm)Ni(η²:η²-diphenyldivinylsilane) and {κ-C-Ni(η²:η²-diene)}(μ-anti-SPoxIm){κ-O-Al(C₆F₅)₃} via the coordination and dissociation of Al(C₆F₅)₃. The shape and size of the space around the Ni(0) center was drastically changed through this Lewis-acid-mediated interconversion. Moreover, the multinuclear NMR, IR, and XAS analyses of the aforementioned carbonyl complexes clarified the details of the changes in the electronic states on the Ni centers; i.e., the electron delocalization was effectively enhanced among the Ni atom and CO ligands in the heterobimetallic Ni/Al species. The results presented in this work thus provide a strategy for reversibly modulating both the electronic and spatial environment of organometallic complexes, in addition to the well-accepted Lewis-base-mediated ligand-substitution methods.