Evaluation of Cobalt Complexes Bearing Tridentate Pincer Ligands for Catalytic C–H Borylation

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Catalyst-Free Regioselective Diborylation of Aryllithium with Tetra(o-tolyl)diborane(4)

A catalyst-free 1,2-diborylation of aryllithium with tetra(o-tolyl)diborane(4) has been achieved, giving a series of 1,2-diborylaryl lithium species in excellent yields under mild reaction conditions, which leads to 1,2-di(tolyl)borylarenes in 60-91 % yields upon treatment with the hydride-abstracting reagent. In these transformations, one sp2 C-H of arene is activated and both boryl units are utilized to build two new (sp2 )C-B bonds. This represents a new strategy for selective arene diborylation. Density functional theory (DFT) calculations suggest that an aromatic nucleophilic substitution is a key step in the formation of the products.

Regioselective Iridium-Catalyzed C8-H Borylation of 4-Quinolones via Transient O-Borylated Quinolines

The quinolone-quinoline tautomerization is harnessed to effect the regioselective C8-borylation of biologically important 4-quinolones by using [Ir(OMe)(cod)]2 as the catalyst precursor, the silica-supported monodentate phosphine Si-SMAP as the ligand, and B2 pin2 as the boron source. Initially, O-borylation of the quinoline tautomer takes place. Critically, the newly formed 4-(pinBO)-quinolines then undergo N-directed selective Ir-catalyzed borylation at C8. Hydrolysis of the OBpin moiety on workup returns the system to the quinolone tautomer. The C8-borylated quinolines were converted to their corresponding potassium trifluoroborate (BF3 K) salts and to their C8-chlorinated quinolone derivatives. The two-step C-H borylation-chlorination reaction sequence resulted in various C8-Cl quinolones in good yields. Conversion to C8-OH-, C8-NH2 -, and C8-Ar-substituted quinolones was also feasible by using this methodology.

Development of a Nickel-Catalyzed N-N Coupling for the Synthesis of Hydrazides

A nickel-catalyzed N-N cross-coupling for the synthesis of hydrazides is reported. O-Benzoylated hydroxamates were efficiently coupled with a broad range of aryl and aliphatic amines via nickel catalysis to form hydrazides in an up to 81% yield. Experimental evidence implicates the intermediacy of electrophilic Ni-stabilized acyl nitrenoids and the formation of a Ni(I) catalyst via silane-mediated reduction. This report constitutes the first example of an intermolecular N-N coupling compatible with secondary aliphatic amines.

Exploring the Effect of Pincer Rigidity on Oxidative Addition Reactions with Cobalt(I) Complexes

Cobalt complexes containing the 2,6-diaminopyridine-substituted PNP pincer (iPrPNMeNP = 2,6-(iPr2PNMe)2(C5H3N)) were synthesized. A combination of solid-state structures and investigation of the cobalt(I)/(II) redox potential established a relatively rigid and electron-donating chelating ligand as compared to iPrPNP (iPrPNP = 2,6-(iPr2PCH2)2(C5H3N)). Based on a buried volume analysis, the two pincer ligands are sterically indistinguishable. Nearly planar, diamagnetic, four-coordinate complexes were observed independent of the field strength (chloride, alkyl, aryl) of the fourth ligand completing the coordination sphere of the metal. Computational studies supported a higher barrier for C-H oxidative addition, largely a result of the increased rigidity of the pincer. The increased oxidative addition barrier resulted in stabilization of (iPrPNMeNP)Co(I) complexes, enabling the characterization of the cobalt boryl and the cobalt hydride dimer by X-ray crystallography. Moreover, (iPrPNMeNP)CoMe served as an efficient precatalyst for alkene hydroboration likely because of the reduced propensity to undergo oxidative addition, demonstrating that reactivity and catalytic performance can be tuned by rigidity of pincer ligands.

Mechanism of Hydroboration of CO2 Using an Fe Catalyst: What Controls the Reactivity and Product Selectivity?

Using a combination of density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations, various elementary steps in the mechanism of the reductive hydroboration of CO₂ to two-electron-reduced boryl formate, four-electron-reduced bis(boryl)acetal, and six-electron-reduced methoxy borane by the [Fe(H)₂(dmpe)₂] catalyst were established. The replacement of hydride by oxygen ligation after the boryl formate insertion step is the rate-determining step. Our work unveils, for the first time, (i) how a substrate steers product selectivity in this reaction and (ii) the importance of configurational mixing in contracting the kinetic barrier heights. Based on the reaction mechanism established, we have further focused on the effect of other metals, such as Mn and Co, on rate-determining steps and on catalyst regeneration.

Direct Transformation of SiH4 to a Molecular L(H)2Co═Si═Co(H)2L Silicide Complex

The synthesis of bimetallic molecular silicide complexes is reported, based on the use of multiple Si-H bond activations in SiH₄ at the metal centers of 14-electron LCo^I fragments (L = Tp″, HB(3,5-diisopropylpyrazolyl)₃^-; [BP₂^tBuPz], PhB(CH₂P^tBu₂)₂(pyrazolyl)). Upon exposure of (Tp″Co)₂(μ-N₂) (1) to SiH₄, a mixture of (Tp″Co)₂(μ-H) (2) and (Tp″Co)₂(μ-H)₂ (3) was formed and no evidence for Si-H oxidative addition products was observed. In contrast, [BP₂^tBuPz]-supported Co complexes led to Si-H oxidative additions with the generation of silylene and silicide complexes as products. Notably, the reaction of ([BP₂^tBuPz]Co)₂(μ-N₂) (5) with SiH₄ gave the dicobalt silicide complex [BP₂^tBuPz](H)₂Co═Si═Co(H)₂[BP₂^tBuPz] (8) in high yield, representing the first direct route to a symmetrical bimetallic silicide. The effect of the [BP₂^tBuPz] ligand on Co-Si bonding in 7 and 8 was explored by analysis of solid-state molecular structures and density functional theory (DFT) investigations. Upon exposure to CO or DMAP (DMAP = 4-dimethylaminopyridine), 8 converted to the corresponding [BP₂^tBuPz]Co(L)_x adducts (L = CO, x = 2; L = DMAP, x = 1) with concomitant loss of SiH₄, despite the lack of significant Si-H interactions in the starting complex. On heating to 60 °C, 8 underwent reaction with MeCl to produce small quantities of Me_xSiH_4-x (x = 1-3), demonstrating functionalization of the μ-silicon atom in a molecular silicide to form organosilanes.

Selective Benzylic CH-Borylations by Tandem Cobalt Catalysis

Metal‐catalyzed C−H activations are environmentally and economically attractive synthetic strategies for the construction of functional molecules as they obviate the need for pre‐functionalized substrates and minimize waste generation. Great challenges reside in the control of selectivities, the utilization of unbiased hydrocarbons, and the operation of atom‐economical dehydrocoupling mechanisms. An especially mild borylation of benzylic CH bonds was developed with the ligand‐free pre‐catalyst Co[N(SiMe₃)₂]₂ and the bench‐stable and inexpensive borylation reagent B₂pin₂ that produces H₂ as the only by‐product. A full set of kinetic, spectroscopic, and preparative mechanistic studies are indicative of a tandem catalysis mechanism of CH‐borylation and dehydrocoupling via molecular Co^I catalysts.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Iron-Catalyzed Highly Enantioselective Hydrogenation of Alkenes

Here, we reported for the first time an iron-catalyzed highly enantioselective hydrogenation of minimally functionalized 1,1-disubstituted alkenes to access chiral alkanes with full conversion and excellent ee. A novel chiral 8-oxazoline iminoquinoline ligand and its iron complex have been designed and synthesized. This protocol is operationally simple by using 1 atm of hydrogen gas and shows good functional group tolerance. A primary mechanism has been proposed by the deuterium-labeling experiments.

Building C(sp3 ) Molecular Complexity on 2,2'-Bipyridine and 1,10-Phenanthroline in Rhenium Tricarbonyl Complexes

The reactions of [Re(N-N)(CO)₃ (PMe₃ )]OTf (N-N=2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) compounds with tBuLi and with LiHBEt₃ have been explored. Addition to the N-N chelate took place with different site-selectivity depending on both chelate and nucleophile. Thus, with tBuLi, an unprecedented addition to C5 of bipy, a regiochemistry not accessible for free bipy, was obtained, whereas coordinated phen underwent tBuLi addition to C2 and C4. Remarkably, when LiHBEt₃ reacted with [Re(bipy)(CO)₃ (PMe₃ )]OTf, hydride addition to the 4 and 6 positions of bipy triggered an intermolecular cyclodimerization of two dearomatized pyridyl rings. In contrast, hydride addition to the phen analog resulted in partial reduction of one pyridine ring. The resulting neutral Re^I products showed a varied reactivity with HOTf and with MeOTf to yield cationic complexes. These strategies rendered access to Re^I complexes containing bipy- and phen-derived chelates with several C(sp³ ) centers.

Co(I) complexes with a tetradentate phenanthroline-based PNNP ligand as a potent new metal-ligand cooperation platform

A series of low spin cobalt(i) complexes bearing a tetradentate phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline), [CoCl(PNNP)] (1), [CoMe(PNNP)] (2) and [Co(CH2SiMe3)(PNNP)] (3), were synthesized and structurally identified. Complex 3 underwent a structural rearrangement of the PNNP skeleton upon heating to form [Co(PNNP')] (4), which is supported by an asymmetrical PNNP' ligand with a dearomatized phenanthroline backbone. Mechanistic studies supported that the transformation from 3 to 4 was initiated by the homolysis of either a Co-CH2SiMe3 bond or a benzylic C-H bond. Complex 4 achieved H-H bond cleavage of H2 (1 atm) at ambient temperature, to form [Co(PNNP'')] (6), in which two H atoms were incorporated into the endocyclic double bond of the PNNP'' ligand backbone.

Direct C-H Borylation of Arenes Catalyzed by Saturated Hydride-Boryl-Iridium-POP Complexes: Kinetic Analysis of the Elemental Steps

The saturated trihydride IrH₃ {κ³ -P,O,P-[xant(PiPr₂ )₂ ]} (1; xant(PiPr₂ )₂ =9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) activates the B-H bond of two molecules of pinacolborane (HBpin) to give H₂ , the hydride-boryl derivatives IrH₂ (Bpin){κ³ -P,O,P-[xant(PiPr₂ )₂ ]} (2) and IrH(Bpin)₂ {κ³ -P,O,P-[xant(PiPr₂ )₂ ]} (3) in a sequential manner. Complex 3 activates a C-H bond of two molecules of benzene to form PhBpin and regenerates 2 and 1, also in a sequential manner. Thus, complexes 1, 2, and 3 define two cycles for the catalytic direct C-H borylation of arenes with HBpin, which have dihydride 2 as a common intermediate. C-H bond activation of the arenes is the rate-determining step of both cycles, as the C-H oxidative addition to 3 is faster than to 2. The results from a kinetic study of the reactions of 1 and 2 with HBpin support a cooperative function of the hydride ligands in the B-H bond activation. The addition of the boron atom of the borane to a hydride facilitates the coordination of the B-H bond through the formation of κ¹ - and κ² -dihydrideborate intermediates.

Single-Site Cobalt-Catalyst Ligated with Pyridylimine-Functionalized Metal-Organic Frameworks for Arene and Benzylic Borylation

We report a highly active single-site heterogeneous cobalt-catalyst based on a porous and robust pyridylimine-functionalized metal-organic frameworks (pyrim-MOF) for chemoselective borylation of arene and benzylic C-H bonds. The pyrim-MOF having UiO-68 topology, constructed from zirconium-cluster secondary building units and pyridylimine-functionalized dicarboxylate bridging linkers, was metalated with CoCl₂ followed by treatment of NaEt₃BH to give the cobalt-functionalized MOF-catalyst (pyrim-MOF-Co). Pyrim-MOF-Co has a broad substrate scope, allowing the C-H borylation of halogen-, alkoxy-, alkyl-substituted arenes as well as heterocyclic ring systems using B₂pin₂ or HBpin (pin = pinacolate) as the borylating agent to afford the corresponding arene- or alkyl-boronate esters in good yields. Pyrim-MOF-Co gave a turnover number (TON) of up to 2500 and could be recycled and reused at least 9 times. Pyrim-MOF-Co was also significantly more robust and active than its homogeneous control, highlighting the beneficial effect of active-site isolation within the MOF framework that prevents intermolecular decomposition. The experimental and computational studies suggested (pyrim^•-)Co^I(THF) as the active catalytic species within the MOF, which undergoes a mechanistic pathway of oxidative addition, turnover limiting σ-bond metathesis, followed by reductive elimination to afford the boronate ester.

C(sp2)-H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp2)-H Activation and Catalytically Relevant Intermediates

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp²)-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and pyridines. Specifically, [( ^Ar SiNSi)CoH₃]·NaHBEt₃ ( ^Ar SiNSi = 2,6-[EtNSi(N^tBu)₂CAr]₂C₅H₃N, Ar = C₆H₅ (1-H ₃ ·NaHBEt ₃ ), 4-MeC₆H₄ (2-H ₃ ·NaHBEt ₃ )) and trans-[( ^Ar SiNSi)Co(H)₂BPin] (Ar = C₆H₅ (1-(H) ₂ BPin), 4-MeC₆H₄ (2-(H) ₂ BPin), Pin = pinacolato) were prepared and employed as single component precatalysts for the C(sp²)-H borylation of 2-methylfuran, benzofuran and 2,6-lutidine. The cobalt(III) precursors, 2-H ₃ ·NaHBEt ₃ and 2-(H) ₂ BPin also promoted C(sp²)-H activation of benzofuran, yielding [(^ArSiNSi)CoH(Bf)₂] (Ar = 4-MeC₆H₄, 2-H(Bf) ₂ , Bf = 2-benzofuranyl). Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by ¹H NMR spectroscopy established the trans-dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversion. At higher conversion two distinct pincer modification pathways were identified, depending on the substrate and the boron source.

Enabling Two-Electron Pathways with Iron and Cobalt: From Ligand Design to Catalytic Applications

Homogeneous catalysis with Earth-abundant, first-row transition metals, including iron and cobalt, has gained considerable recent attention as a potentially cost-effective and sustainable alternative to more commonly and historically used precious metals. Because fundamental organometallic transformations, such as oxidative addition and reductive elimination, are two-electron processes and essential steps in many important catalytic cycles, controlling redox chemistry-in particular overcoming one-electron chemistry-has been as a central challenge with Earth-abundant metals. This Perspective focuses on approaches to impart sufficiently strong ligand fields to generate electron-rich metal complexes able to promote oxidative addition reactions where the redox changes are exclusively metal-based. Emphasis is placed on how ligand design and exploration of fundamental organometallic chemistry coupled with mechanistic understanding have been used to discover iron catalysts for the hydrogen isotope exchange in pharmaceuticals and cobalt catalysts for C(sp²)-H borylation reactions. A pervasive theme is that first-row metal complexes often promote unique chemistry from their precious-metal counterparts, demonstrating that these elements offer a host of new opportunities for reaction discovery and for more sustainable catalysis.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Cobalt Pincer Complexes in Catalytic C-H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans-(^iPrPNP)CoH₂(BPin) as the resting state in the borylation of five-membered heteroarenes, and 4-BPin-(^iPrPNP)Co(N₂)BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-(^iPrPNP)CoBPin (R=H, BPin), were generated by reductive elimination of H₂ in the former, through Berry pseudorotation to the cis isomer, and N₂ loss in the latter. The catalytic mechanism of the resulting Co(I) complex was computed to involve three main steps: C-H oxidative addition of the aromatic substrate (C₆H₆), reductive elimination of PhBPin, and regeneration of the active complex. The oxidative addition product formed through the most favorable pathway, where the breaking C-H bond of C₆H₆ is parallel to a line between the two phosphine atoms, leaves the complex with a distorted PNP ligand, which rearranges to a more stable complex via dissociation and re-association of HBPin. Alternative pathways, σ-bond metathesis and the oxidative addition in which the breaking C-H bond is parallel to the Co-B bond, are predicted to be unlikely for this Co(I) complex. The thermodynamically favorable formation of the product PhBPin via reductive elimination drives the reaction forward. The active species regenerates through the oxidative addition of B₂Pin₂ and reductive elimination of HBPin. In the overall reaction, the flipping (refolding) of the five-membered phosphine rings, which connects the species with two phosphine rings folded in the same direction and that with them folded in different directions, is found to play an important role in the catalytic process, as it relieves steric crowding within the PNP ligand and opens Co coordination space. Metal-ligand cooperation based on the ligand's aromatization/dearomatization, a common mechanism for heavy-metal pincer complexes, and the dissociation of one phosphine ligand, do not apply in this system. This study provides guidance for understanding important features of pincer ligands with first-transition-row metals that differ from those in heavier metal complexes.

Cobalt-Catalyzed C-F Bond Borylation of Aryl Fluorides

A mild and practical cobalt-catalyzed defluoroborylation of fluoroarenes is presented for the first time. The method permits straightforward functionalization of fluoroarenes, with high selectivity for borylation of C-F over C-H bonds, and a tolerance for aerobic conditions. Furthermore, two-step ¹⁸F-fluorination was achieved for expanding the scope of ¹⁸F-positron emission tomography probes.

Synthesis and reactivity of a PCcarbeneP cobalt(i) complex: the missing link in the cobalt PXP pincer series (X = B, C, N)

We report the first example of a cobalt PC_carbeneP pincer complex (1) featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand.

We report the first example of a cobalt PC_carbeneP pincer complex (1) featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand. Complex 1 shares bonding similarities with cobalt PBP and PNP pincer complexes where the donor atom engages in π-bonding with the cobalt centre, and thus completes the PXP (X = B, C, N) pincer ligand series for cobalt (for X donors that partake in M–L π-bonding). As compared to PBP and PNP pincer complexes, which are known to be good hydride and proton acceptors (respectively), complex 1 is found to be an effective hydrogen atom acceptor. Complex 1 partakes in cooperative ligand reactivity, engaging in several small molecule activations with styrene, bromine, carbon disulphide, phenyl acetylene, acetonitrile, hydrogen, benzaldehyde and water (through microreversibility). The mechanism for the formation of complex 1 is studied through the isolation and computational analysis of key intermediates. The formation of 1 is found to avoid C–H activation of the proligand, and instead proceeds through a combination of O–H activation, hydrogen atom transfer, β-hydride elimination and hydrogen activation processes.

B(C6F5)3-Catalyzed C3-Selective C-H Borylation of Indoles: Synthesis, Intermediates, and Reaction Mechanism

Without the addition of any additives and production of any small molecules, C3-borylated indoles and transfer hydrogenated indolines have been simultaneously achieved by a B(C₆F₅)₃-catalyzed disproportionation reaction of a broad range of indoles with catecholborane. This catalyst system exhibits excellent catalytic performance for practical applications, such as easy scale-up under solvent-free conditions and long catalytic lifetime over ten sequential additions of starting materials. A combined mechanistic study, including isolation and characterization of key reaction intermediates, analysis of the disproportionation nature of the reaction, in situ NMR of the reaction, and analysis of detailed experimental data, has led to a possible reaction mechanism which illustrates pathways for the formation of both major products and byproducts. Understanding the reaction mechanism enables us to successfully suppress side reactions by choosing appropriate substrates and adjusting the amount of catecholborane needed. More importantly, with an elevated reaction temperature, we could achieve the convergent disproportionation reaction of indoles, in which indolines were continuously oxidized to indoles for the next disproportionation catalytic cycle. Near quantitative conversions and up to 98% yields of various C3-selective borylated indoles were achieved, without any additives or H₂ acceptors.

Preparation and reactivity of a square-planar PNP cobalt(ii)–hydrido complex: isolation of the first {Co–NO}8–hydride

The synthesis of a square-planar cobalt(ii) hydrido complex supported by a pyrrole-based PNP ligand has been reinvestigated and its reactivity with various small molecules examined.

Insights into Activation of Cobalt Pre-Catalysts for C(sp2)-H Functionalization

The activation of readily prepared, air-stable cobalt (II) bis(carboxylate) pre-catalysts for the functionalization of C(sp2)-H bonds has been systematically studied. With the pyridine bis(phosphine) chelate, iPrPNP, treatment of 1-(O2CtBu)2 with either B2Pin2 or HBPin generated cobalt boryl products. With the former, reduction to (iPrPNP)CoIBPin was observed while with the latter, oxidation to the cobalt(III) dihydride boryl, trans-(iPrPNP)Co(H)2BPin occurred. The catalytically inactive cobalt complex, Co[PinB(O2CtBu)2]2, accompanied formation of the cobalt-boryl products in both cases. These results demonstrate that the pre-catalyst activation from cobalt(II) bis(carboxylates), although effective and utilizes an air-stable precursor, is less efficient than activation of cobalt(I) alkyl or cobalt(III) dihydride boryl complexes, which are quantitatively converted to the catalytically relevant cobalt(I) boryl. Related cobalt(III) dihydride silyl and cobalt(I) silyl complexes were also synthesized from treatment of trans-(iPrPNP)Co(H)2BPin and (iPrPNP)CoPh with HSi(OEt)3, respectively. No catalytic silylation of arenes was observed with either complex likely due to the kinetic preference for reversible C-H reductive elimination rather than product- forming C-Si bond formation from cobalt(III). Syntheses of the cobalt(II) bis(carboxylate) and cobalt(I) alkyl of iPrPONOP, a pincer where the methylene spacers have been replaced by oxygen atoms, were unsuccessful due to deleterious P-O bond cleavage of the pincer. Despite their structural similarity, the rich catalytic chemistry of iPrPNP was not translated to iPrPONOP due to the inability to access stable cobalt precursors as a result of ligand decomposition via P-O bond cleavage.

Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C-H Bond Activation

Two novel frustrated Lewis pair (FLP) aminoboranes, (1-Pip-2-BH₂-C₆H₄)₂ (2; Pip = piperidyl) and (1-NEt₂-2-BH₂-C₆H₄)₂ (3; NEt₂ = diethylamino), were synthesized, and their structural features were elucidated both in solution and in the solid state. The reactivity of these species for the borylation of heteroarenes was investigated and compared to previously reported (1-TMP-2-BH₂-C₆H₄)₂ (1; TMP = tetramethylpiperidyl) and (1-NMe₂-2-BH₂-C₆H₄)₂ (4; NMe₂ = dimethylamino). It was shown that 2 and 3 are more active catalysts for the borylation of heteroarenes than the bulkier analogue 1. Kinetic studies and density functional theory calculations were performed with 1 and 2 to ascertain the influence of the amino group of this FLP-catalyzed transformation. The C-H activation step was found to be more facile with smaller amines at the expense of a more difficult dissociation of the dimeric species. The bench-stable fluoroborate salts of all catalysts (1F-4F) have been synthesized and tested for the borylation reaction. The new precatalysts 2F and 3F are showing higher reaction rates and yields for multigram-scale syntheses.

Cobalt-Catalyzed Stereoretentive Hydrogen Isotope Exchange of C(sp3)-H Bonds

Cobalt dialkyl complexes bearing α-diimine ligands proved to be active precatalysts for the nondirected, C(sp3)-H selective hydrogen isotope exchange (HIE) of alkylarenes using D2 gas as the deuterium source. Alkylarenes with a variety of substitution patterns and heteroatom substituents on the arene ring were successfully labeled, enabling high levels of incorporation into primary, secondary, and tertiary benzylic C(sp3)-H bonds. In some cases, the HIE proceeded with high diastereoselectivity and application of the cobalt-catalyzed method to enantioenriched substrates with benzylic stereocenters provided enantioretentive hydrogen isotope exchange at tertiary carbons.

Mechanistic Studies of Cobalt-Catalyzed C(sp2)-H Borylation of Five-Membered Heteroarenes with Pinacolborane

Studies into the mechanism of cobalt-catalyzed C(sp²)-H borylation of five-membered heteroarenes with pinacolborane (HBPin) as the boron source established the catalyst resting state as the trans-cobalt(III) dihydride boryl, (^iPrPNP)Co(H)₂(BPin) (^iPrPNP = 2,6-(ⁱPr₂PCH₂)₂(C₅H₃N)), at both low and high substrate conversions. The overall first-order rate law and observation of a normal deuterium kinetic isotope effect on the borylation of benzofuran versus benzofuran-2-d₁ support H₂ reductive elimination from the cobalt(III) dihydride boryl as the turnover-limiting step. These findings stand in contrast to that established previously for the borylation of 2,6-lutidine with the same cobalt precatalyst, where borylation of the 4-position of the pincer occurred faster than the substrate turnover and arene C-H activation by a cobalt(I) boryl is turnover-limiting. Evaluation of the catalytic activity of different cobalt precursors in the C-H borylation of benzofuran with HBPin established that the ligand design principles for C- H borylation depend on the identities of both the arene and the boron reagent used: electron-donating groups improve catalytic activity of the borylation of pyridines and arenes with B₂Pin₂, whereas electron-withdrawing groups improve catalytic activity of the borylation of five-membered heteroarenes with HBPin. Catalyst deactivation by P-C bond cleavage from a cobalt(I) hydride was observed in the C-H borylation of arene substrates with C-H bonds that are less acidic than those of five-membered heteroarenes using HBPin and explains the requirement of B₂Pin₂ to achieve synthetically useful yields with these arene substrates.

Benzyltriboronates: Building Blocks for Diastereoselective Carbon-Carbon Bond Formation

A highly diastereoselective carbon-carbon bond-forming reaction involving the tandem coupling of benzyltriboronates, enoates, and alkyl halides is described. This method was enabled by the discovery of α-diimine nickel catalysts that promote the chemoselective triborylation of benzylic C(sp³)-H bonds using B₂Pin₂ (Pin = pinacolate). The C-H functionalization method is effective with methylarenes and for the diborylation of secondary benzylic C-H bonds, providing direct access to polyboron building blocks from readily available hydrocarbons. Combination of the benzylic perborylation with a new deborylative conjugate addition-alkylation method enables a one-pot procedure in which multiple simple precursors are combined to generate diastereopure products containing quaternary stereocenters.