Boryl Ligand Research Articles

Reactivity of Diruthenium Bisborylene Complexes: Formation of B-C and B-H Bonds via Borylene Ligand Coupling.

Thermal or photoinduced isomerization of diruthenium bridging bisborylene complexes [{Cp*(H)2Ru}2(μ-BAr)2] (1a, Ar = Ph; 1b, Ar = 3,4,5-F3C6H2) led to nido-ruthenacarboranes 2a and 2b with newly formed B-C and B-H bonds. The reaction mechanism was analyzed by deuterium-labeling experiments and density functional theory calculations. Additionally, 2-fold B-H coupling between borylene and two hydrido ligands of 1a can be achieved, assisted by Lewis base IPr2Me2 to generate a dinuclear bridging borylene complex [(Cp*Ru)2(μ-H)2(μ-BPh)] (3). Our results provide new reactivity patterns for borylene-based functionalizations.

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato‐Silanone and Germanone Systems in Ammonia Activation

AbstractWhile the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room‐temperature stable boryl‐substituted amidinato‐silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6‐iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2‐fashion to give an isolable sila‐hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila‐hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi‐lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech‐anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron‐releasing and sterically bulky nature of the ancillary boryl ligand.

Boryl Ancillary Ligands: Influencing Stability and Reactivity of Amidinato-Silanone and Germanone Systems in Ammonia Activation.

While the nucleophilic addition of ammonia to ketones is an archetypal reaction in classical organic chemistry, the reactivity of heavier group 14 carbonyl analogues (R2E=O; E=Si, Ge, Sn, or Pb) with NH3 remains sparsely investigated, primarily due to the synthetic difficulties in accessing heavier ketone congeners. Herein, we present a room-temperature stable boryl-substituted amidinato-silanone {(HCDippN)2B}{PhC(tBuN)2}Si=O (Dipp=2,6-iPr2C6H3) (together with its germanone analogue), formed from the corresponding silylene under a N2O atmosphere. This system reacts cleanly with ammonia in 1,2-fashion to give an isolable sila-hemiaminal complex {(HCDippN)2B}{PhC(tBuN)2}Si(OH)(NH2). Quantum chemical calculations reveal that the formation of this sila-hemiaminal is crucially dependent on the nature of the ancillary ligand scaffold. It is facilitated thermodynamically by the hemi-lability of the amidinate ligand (which allows for the formation of an energetically critical intramolecular N⋅⋅⋅HO hydrogen bond within the product) and is enabled mech-anistically by a process in which the silanone initially acts in umpolung fashion as a base (rather than an acid), due to the strongly electron-releasing and sterically bulky nature of the ancillary boryl ligand.

Catalytic Enantioselective Primary C-H Borylation for Acyclic All-Carbon Quaternary Stereocenters.

Creating a perfect catalyst to operate enzyme-like chiral recognition has been a long-sought aim. A challenging example in this context is constructing acyclic all-carbon quaternary stereogenic centers by transition metal-catalyzed enantioselective C-H activation. We now report highly enantioselective iridium-catalyzed primary C-H borylation of α-all-carbon substituted 2,2-dimethyl amides enabled by a tailor-made chiral bidentate boryl ligand (CBL). The success of the current transformation is attributed to the CBL/iridium catalyst, which has a confined chiral pocket. This protocol provides a diverse array of acyclic all-carbon quaternary stereocenters with excellent enantiocontrol and distinct structural features. Computational study reveals that steric hindrance of CBL could regulate the type of dominant orbital interaction between the catalyst and substrate, which is crucial to conferring high chiral induction.

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study.

In an effort to explore the coordination chemistry of the coordinative sulfur centers in arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)] (arachno-1), we have thermolyzed arachno-1 with group-6 metal carbonyls [M(CO)5·THF] (M = Cr, Mo and W). The reaction of arachno-1 with [Cr(CO)5·THF] resulted in the formation of hetero-trimetallic triply bridging borylene [(Cp*Ru)2(μ-CO)(μ3-CH2S2-κ2S':κ2S''){Cr(CO)3}(μ3-BH)] (2), bridging boryl-borylene [(Cp*Ru)2(μ-CO){(μ3-BH(CH2S2)-κ2B:κ2S':κ1S'')}{Cr(CO)3}(μ3-BH)] (3), and sulfido bridged hetero-trimetallic complex [(Cp*Ru)2(μ-CO)3{Cr(CO)3}(μ3-S)] (4). In 2, one side of Ru2Cr-triangle features a μ3-BH ligand while the other side is quadruply bridged by a methanedithiolato ligand in an unsymmetrical fashion. Unlike 2, in complex 3, one side of the Ru2Cr-triangle has a μ3-BH ligand while the opposite side is bridged by a boryl ligand BH(CH2S2) in an unsymmetrical way (μ3-κ2:κ2:κ1) to the metal centers. Interestingly, when the similar reactions of arachno-1 were performed with heavier group-6 metal carbonyls [M(CO)5·THF] (M = Mo and W), it led to the formation of methanedithiolato bridged hetero-trimetallic chain complexes, [{Cp*Ru(CO)}2(μ-CO)2(μ3-CH2S2-κ2S':κ2S''){M(CO)2}] (5, M = Mo; 6, M = W) and sulfido-bridged hetero-trimetallic complexes [(Cp*Ru)2(μ-CO)3{M(CO)3}(μ3-S)] (7, M = Mo; 8, M = W), analogous to 4. In complexes 5 and 6, a Ru2M-chain is symmetrically bridged by a methanedithiolato ligand. On the other hand, in complexes 4, 7, and 8, a sulfido ligand coordinates to two ruthenium and one group-6 metal atoms in μ3-fashion. All the complexes have been characterized by 1H NMR, 13C NMR, UV-vis, IR spectroscopy, and mass spectrometry and their structural architectures have been unambiguously established by single crystal X-ray diffraction studies. In addition, theoretical investigations provided valuable insights into their electronic structures and bonding properties.

On the Reactivity of a NHC Nickel Bis-Boryl Complex: Reductive Elimination and Formation of Mono-Boryl Complexes.

The synthesis of the first terminal mono-boryl complexes of nickel, which are not stabilized by a pincer ligand, is reported. The reaction of the nickel bis-boryl complex cis-[Ni(iPr2ImMe)2(Bcat)2] 1 (cat = 1,2-O2C6H4) with the small donor ligand PMe3 led to a complete ligand exchange at nickel with reductive elimination of B2cat2, formation of the bisNHC adduct [B2cat2 · (iPr2ImMe)2] 3 and formation of [Ni(PMe3)4] 2 as the metal-containing species. Electrophilic attack of MeI on complex 1 or ligand dismutation of 1 with trans[Ni(iPr2ImMe)2Br2] led to loss of only one boryl ligand of 1 and afforded the nickel mono-boryl complexes trans-[Ni(iPr2ImMe)2(Bcat)Br] 4a and trans-[Ni(iPr2ImMe)2(Bcat)I] 4b.

Iridium‐Catalyzed Site‐ and Enantioselective C(sp2)‐H Borylation of Benzhydryl Ethers: Enantioselectivity Amplification by Kinetic Resolution Relay

Comprehensive SummaryWe herein report a simple ether‐directed iridium‐catalyzed site‐ and enantioselective C(sp2)‐H borylation of benzhydryl ethers for the first time. Various chiral benzhydryl ethers were obtained with high enantioselectivities in the presence of a tailor‐made chiral bidentate boryl ligand. We found that the kinetic resolution relay significantly amplified the enantioselectivity. The synthetic utility of the current method was demonstrated by gram‐scale C—H borylation and C—B bond transformations.

Synthesis of Chiral Sulfoximines via Iridium-Catalyzed Regio-and Enantioselective C-H Borylation: A Remarkable Sidearm Effect of Ligand.

Transition metal-catalyzed non-annulation enantioselective C-H activation of prochiral sulfoximines remains a formidable challenge. We herein report iridium-catalyzed enantioselective C-H borylation of N-silyl protected diaryl sulfoximines using a well-designed chiral bidentate boryl ligand with a bulky side arm. A broad spectrum of functional groups could be well-tolerated, affording a vast array of chiral sulfoximines with high enantioselectivities. We also demonstrated the synthetic utility on a preparative-scale C-H borylation for diverse downstream transformations, including chiral version of bioactive molecules. Computational studies showed the bulky side arm of the ligand confers high regio- and enantioselectivity through steric effect.

Synthesis of Chiral Sulfoximines via Iridium‐Catalyzed Regio‐ and Enantioselective C−H Borylation: A Remarkable Sidearm Effect of Ligand

AbstractTransition metal‐catalyzed enantioselective C−H activation of prochiral sulfoximines for non‐annulated products remains a formidable challenge. We herein report iridium‐catalyzed enantioselective C−H borylation ofN‐silyl diaryl sulfoximines using a well‐designed chiral bidentate boryl ligand with a bulky side arm. This method is capable of accommodating a broad range of substrates under mild reaction conditions, affording a vast array of chiral sulfoximines with high enantioselectivities. We also demonstrated the synthetic utility on a preparative‐scale C−H borylation for diverse downstream transformations, including the synthesis of chiral version of bioactive molecules. Computational studies showed that the bulky side arm of the ligand confers high regio‐ and enantioselectivity through steric effect.

Pincer‐Supported Perfluororhodacyclopentanes: High Nucleophilicity of the M−CF Bond

AbstractPerfluororhodacyclopentane supported by PBP pincer ligand PBP‐RhIII(CF2)4 (2) was prepared from PBP‐RhI‐PPh3 (1) and tetrafluoroethylene (TFE). By scXRD analysis, it was revealed that the Rh−CF bond trans to the boryl ligand was largely elongated compared to the one in the apical position. Although M−CF bonds of perfluorometallacyclopentanes are known to be very stable, 2 shows high reactivity toward hydrogenolysis, iodinolysis and protonolysis, thanks to the strong trans‐effect of the highly σ‐donating boryl ligand. Analogous PCP pincer complex PCP−RhIII(CF2)4 (4) showed slower reaction rate both in oxidative cyclization with TFE and hydrogenolysis of Rh−CF bond. Catalytic production of 1,1,2,2,3,3,4,4‐octafluorobutane from two TFE and hydrogen in one pot was achieved via repetitive oxidative cyclization of TFE and Rh followed by the hydrogenolysis of Rh−CF bonds. This report demonstrates the possible application of PBP‐pincer skeleton to the preparation of fluorinated compounds from TFE.

Pincer-Supported Perfluororhodacyclopentanes: High Nucleophilicity of the M-CF Bond.

Perfluororhodacyclopentane supported by PBP pincer ligand PBP-RhIII (CF2 )4 (2) was prepared from PBP-RhI -PPh3 (1) and tetrafluoroethylene (TFE). By scXRD analysis, it was revealed that the Rh-CF bond trans to the boryl ligand was largely elongated compared to the one in the apical position. Although M-CF bonds of perfluorometallacyclopentanes are known to be very stable, 2 shows high reactivity toward hydrogenolysis, iodinolysis and protonolysis, thanks to the strong trans-effect of the highly σ-donating boryl ligand. Analogous PCP pincer complex PCP-RhIII (CF2 )4 (4) showed slower reaction rate both in oxidative cyclization with TFE and hydrogenolysis of Rh-CF bond. Catalytic production of 1,1,2,2,3,3,4,4-octafluorobutane from two TFE and hydrogen in one pot was achieved via repetitive oxidative cyclization of TFE and Rh followed by the hydrogenolysis of Rh-CF bonds. This report demonstrates the possible application of PBP-pincer skeleton to the preparation of fluorinated compounds from TFE.

Acid/Base-Free Acyclic Anionic Oxoborane and Iminoborane Bearing Diboryl Groups.

Anionic oxoboranes and neutral iminoboranes, which are isoelectronic to ketones and alkynes, respectively, have attracted much attention because of their unique structures and various reactivity. However, acid/base-free oxoboranes and iminoboranes are still limited, and readily accessible examples with diverse electronic and steric characteristics are highly desirable. Herein, we report the first syntheses of the acyclic anionic oxoborane 2 and iminoborane 4 bearing two boryl ligands, both of which are acid/base-free. Spectroscopic analysis, X-ray crystallography, and theoretical calculations reveal that 2 and 4 possess a polarized terminal B═O double bond and central B≡N triple bond, respectively.

Synthesis of P- and S-Stereogenic Compounds via Enantioselective C–H Functionalization

AbstractTransition metal-catalyzed enantioselective C–H functionalization has emerged as an efficient and powerful strategy to access various chiral molecules. Recently, this strategy has also provided a complementary pathway to the construction of P- and S-stereogenic compounds. In this short review, we summarize the development and applications of various catalytic systems: Pd(II)/mono-N-protected amino acids (MPAA), Pd(0)/trivalent phosphorus chiral ligands, chiral cyclopentadienyl-ligated metal catalysts [CpxM(III)] (M = Rh, Ir), half-sandwich d6 Ir(III) and Ru(II) with a chiral carboxylic acid (CCA) ligand, Ir(I)/chiral bidentate boryl ligand, and Ir(I)/chiral cation, for accessing these chiral compounds via enantioselective C–H functionalization. 1 Introduction2 Pd(II)/Mono-N-protected Amino Acids3 Pd(0)/Trivalent Phosphorus Chiral Ligands4 Chiral Cyclopentadienyl-Ligated Metal Catalysts [CpxM(III)] (M = Rh, Ir)5 Half-sandwich d6 Ir(III) and Ru(II) with a Chiral Carboxylic Acid (CCA) Ligand6 Ir(I)/Chiral Bidentate Boryl Ligand7 Ir(I)/Chiral Cation8 Conclusion and Outlook

Electronic and Steric Properties of N-Heterocyclic Boryl Ligands

Electronic and Steric Properties of <i>N</i>-Heterocyclic Boryl Ligands

Ligand properties of boryl ligands in bis-boryl rhodium(III) complexes: a case study.

The oxidative addition of five diborane(4) derivatives, symmetrical and unsymmetrical, to [Rh(PMe3)3Cl] was studied. Only for the more electron poor diboron derivatives, B2cat2, B2pin2 and catB-Bpin the resulting octahedral bis-boryl complexes [(PMe3)3Rh(boryl)2Cl] were obtained, while for the more electron rich congeners only the equilibrium oxidative addition (catB-Bdmab) or no significant reaction (pinB-Bdmab) was observed (pin = (OCMe2)2, cat = 1,2-O2C6H4, dmab = 1,2-(NMe)2C6H4). By abstraction of the chlorido ligand with NaBArF (BArF = tetrakis-[3,5-bis-(trifluormethyl)-phenyl]-borat) in the presence of a neutral ligand (L = PMe3, MeCN, MeNC) the corresponding cationic octahedral complexes [(PMe3)3Rh(boryl)2L]+ were obtained. All isolated complexes were fully characterised including single crystal X-ray diffraction and heteronuclear, temperature dependent NMR spectroscopy. Whilst the complexes [(PMe3)3Rh(boryl)2Cl] and [(PMe3)3Rh(boryl)2L]+ show many similarities, their detailed structural and spectroscopic properties depend crucially on the properties of both boryl ligands.

A Ni0 σ-Borane Complex Bearing a Rigid Bidentate Borane/Phosphine Ligand: Boryl Complex Formation by Oxidative Dehydrochloroborylation and Catalytic Activity for Ethylene Polymerization.

While of interest, synthetically feasible access to boryl ligands and complexes remains limited, meaning such complexes remain underexploited in catalysis. For bidentate boryl ligands, oxidative addition of boranes to low-valent IrI or Pt0 are the only examples yet reported. As part of our interest in developing improved group 10 ethylene polymerization catalysts, we present here an optimized synthesis of a novel, rigid borane/phosphine ligand and its Ni0 σ-borane complex. From the latter, an unprecedented oxidative dehydrochloroborylation, to give a NiII boryl complex, was achieved. Furthermore, this new B/P ligand allowed the nickel-catalyzed polymerization of ethylene, which suggests that Ni0 σ-hydroborane complexes act as masked NiII boryl hydride reagents.

A Ni0 σ‐Borane Complex Bearing a Rigid Bidentate Borane/Phosphine Ligand: Boryl Complex Formation by Oxidative Dehydrochloroborylation and Catalytic Activity for Ethylene Polymerization

AbstractWhile of interest, synthetically feasible access to boryl ligands and complexes remains limited, meaning such complexes remain underexploited in catalysis. For bidentate boryl ligands, oxidative addition of boranes to low‐valent IrI or Pt0 are the only examples yet reported. As part of our interest in developing improved group 10 ethylene polymerization catalysts, we present here an optimized synthesis of a novel, rigid borane/phosphine ligand and its Ni0 σ‐borane complex. From the latter, an unprecedented oxidative dehydrochloroborylation, to give a NiII boryl complex, was achieved. Furthermore, this new B/P ligand allowed the nickel‐catalyzed polymerization of ethylene, which suggests that Ni0 σ‐hydroborane complexes act as masked NiII boryl hydride reagents.

Trans Influence of Boryl Ligands in CO2 Hydrogenation on Ruthenium Complexes: Theoretical Prediction of Highly Active Catalysts for CO2 Reduction

In this work, we study the trans influence of boryl ligands and other commonly used non-boryl ligands in order to search for a more active catalyst than the ruthenium dihydride complex Ru(PNP)(CO)H2 for the hydrogenation of CO2. The theoretical calculation results show that only the B ligands exhibit a stronger trans influence than the hydride ligand and are along increasing order of trans influence as follows: –H &lt; –BBr2 &lt; –BCl2 ≈ –B(OCH)2 &lt; –Bcat &lt; –B(OCH2)2 ≈ –B(OH)2 &lt; –Bpin &lt; –B(NHCH2)2 &lt; –B(OCH3)2 &lt; –B(CH3)2 &lt; –BH2. The computed activation free energy for the direct hydride addition to CO2 and the NBO analysis of the property of the Ru–H bond indicate that the activity of the hydride can be enhanced by the strong trans influence of the B ligands through the change in the Ru–H bond property. The function of the strong trans influence of B ligands is to decrease the d orbital component of Ru in the Ru–H bond. The design of a more active catalyst than the Ru(PNP)(CO)H2 complex is possible.

Synthesis, Electronic Properties, and Lewis Acidity of Rhodium Complexes Bearing X-Type PBP, PAlP, and PGaP Pincer Ligands

Abstract We have synthesized rhodium complexes that bear X-type PBP and PGaP pincer ligands. X-ray diffraction analysis and IR spectroscopy of the PBP– and PGaP–Rh(CO)2 complexes showed that the σ-donicity and trans-influence of the X-type boryl ligand are stronger than those of the X-type gallyl ligand. Moreover, the Lewis acidity of the PEP–Rh complexes (E = B, Al, Ga) was examined. The PBP–Rh complex did not interact with the Lewis base N,N-dimethyl-4-aminopyridine (DMAP), while the PAlP–Rh complex coordinated DMAP more strongly via the Al atom than the PGaP–Rh complex via the Ga atom. DFT calculations of the PEP–Rh complexes revealed the details of the electronic properties and Lewis acidity of the X-type PEP pincer ligands. Their σ-donicity increases in the order Ga &amp;lt; B &amp;lt; Al, and the strength of their trans-influence follows the order Ga &amp;lt; Al &amp;lt; B. The polarity of the Eδ+–Rhδ− bond was found to be the greatest for Al and the smallest for B. The Lewis acidity based on the calculated binding energy increases in the order B ≪ Ga &amp;lt; Al, which is consistent with the experimental results.

Iridium-Catalyzed Regio- and Enantioselective Borylation of Unbiased Methylene C(sp3 )-H Bonds at the Position β to a Nitrogen Center.

Reported herein is the pyrazole-directed iridium-catalyzed enantioselective borylation of unbiased methylene C-H bonds at the position β to a nitrogen center. The combination of a chiral bidentate boryl ligand, iridium precursor, and pyrazole directing group was responsible for the high regio- and enantioselectivity observed. The method tolerated a vast array of functional groups to afford the corresponding C(sp3 )-H functionalization products with good to excellent enantioselectivity.