Do Rhodium Bis(σ‐amine‐borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine‐boranes?

The recently synthesized rhodium complex [Rh{P(C₅H₉)₂(η²‐C₅H₇)}(Me₂HNBH₃)₂]BAr^F₄ ( 2 ), which incorporates two amine‐boranes coordinated to the rhodium center with two different binding modes, namely η¹ and η², has been used to probe whether bis(σ‐amine‐borane) motifs are important in determining the general course of amine‐boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine‐borane cyclic dimer [BH₂NMe₂]₂ ( A ) by hydrogen elimination. Sequential concerted, on‐ or off‐metal, intramolecular dehydrogenations provide two coordinated amine‐borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH???NH hydrogen bond between amine‐borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C₅H₉)₂(η²‐C₅H₇)} B ]⁺ to be important for the whole dehydrocoupling process.     

Electronically Driven Regioselective Iridium-Catalyzed C−H Borylation of Donor-π-Acceptor Chromophores Containing Triarylboron Acceptors

We observed a surprisingly high electronically driven regioselectivity for the iridium-catalyzed C−H borylation of donor-π-acceptor (D -π-A) systems with diphenylamino ( 1 ) or carbazolyl ( 2 ) moieties as the donor, bis(2,6-bis(trifluoromethyl)phenyl)boryl ( B(^FXyl)₂ ) as the acceptor, and 1,4-phenylene as the π-bridge. Under our conditions, borylation was observed only at the sterically least encumbered para-positions of the acceptor group. As boronate esters are versatile building blocks for organic synthesis (C−C coupling, functional group transformations) the C−H borylation represents a simple potential method for post-functionalization by which electronic or other properties of D -π-A systems can be fine-tuned for specific applications. The photophysical and electrochemical properties of the borylated ( 1-(Bpin)₂ ) and unborylated ( 1 ) diphenylamino-substituted D -π-A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non-innocence of boronate esters.     

Metal-Free Phosphorus-Directed Borylation of C(sp2)−H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C?H borylation of phosphines as well as directed electrophilic C?H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine–boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂P→BBr₂⁺ species.     

A Study of the Reactivity of Secondary Phosphanes with Radical Sources: A New Dehydrocoupling Reaction

The reactions of secondary phosphanes with radical sources have been investigated. A stoichiometric dehydrocoupling of Ph₂PH with 1,1′‐azobis[cyclohexane‐1‐carbonitrile] (VAZO^® 88) affords tetraphenyldiphosphane in good yields, whilst reduction of the nitrosyl function was observed upon using 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO). Dialkylphosphane–borane adducts also undergo a dehydrocoupling reaction in the presence of VAZO^® 88 to form R₄P₂.     

Lithium aminoborohydrides 17. Palladium catalyzed borylation of aryl iodides, bromides, and triflates with diisopropylaminoborane prepared from lithium diisopropylaminoborohydride

The Alcaraz-Vaultier borylation of aryl halides and triflates is reported utilizing diisopropylaminoborane (BH₂N(ⁱPr)₂) prepared from the corresponding lithium aminoborohydride (LAB reagent). BH₂N(ⁱPr)₂, prepared by reacting lithium diisopropylaminoborohydride with trimethylsilyl chloride, provided the most consistent isolated yields from this reaction. Catalytic amounts of palladium dichloride produced the highest yields from aryl iodides, while catalytic tris(dibenzylideneacetone)dipalladium(chloroform) provided the best yields for aryl bromides and triflates. This route to boronic acids is mild enough to tolerate various functionalities and for the first time employs aryl triflates as substrates for the Alcaraz-Vaultier borylation. In addition, it was found that both boronic acid and ester compounds could be isolated from the reaction mixture utilizing simple work-up procedures. Treatment of the reaction intermediate with an acid/base work-up provided the corresponding boronic acid, while treating the same intermediate with a diol, such as neopentyl glycol, afforded the corresponding boronic ester.     

Vinylic C?H Borylation of Cyclic Vinyl Ethers with Bis(pinacolato)diboron Catalyzed by an Iridium(I)‐dtbpy Complex

Borylation of the vinylic C? H bond of 1,4‐dioxene, 2,3‐dihydrofuran, 3,4‐dihydro‐2H‐pyran and their γ‐substituted analogs was carried out in the presence of bis(pinacolato)diboron (B₂pin₂) and a catalytic amount of Ir^I‐dtbpy (dtbpy=4,4′‐di‐tert‐butyl‐2,2′‐bipyridine) complex. The two boron atoms in B₂pin₂ participated in the coupling, thus giving two equivalents of the coupling product from one equivalent of B₂pin₂. The borylation of 1,4‐dioxene in hexane resulted in 81 % yield at room temperature. The borylation of 2,3‐dihydrofurans at 80 °C in octane suffered from low regioselectivity, and gave a mixture of α‐ and β‐coupling products even for hindered γ‐disubstituted analogs, but γ‐substituted analogs of 3,4‐dihydro‐2H‐pyran achieved high α‐selectivity, giving single coupling products. This protocol was applied to the syntheses of a key precursor of vineomycinone B2 methyl ester and other C‐substituted D ‐glucals by borylation of protected D ‐glucals with B₂pin₂ to give α‐boryl glucal followed by cross‐coupling with haloarenes, benzyl bromide, and allyl bromide. A catalytic cycle that involves the oxidative addition of sp² C? H bond to iridium(III)‐trisboryl intermediate as the rate‐determining step has been proposed.     

Iridium Diphosphine Complexes Featuring Pendant Lewis Acids: Efforts toward Selective Heteroarene Borylation

The selective forging of carbon-boron bonds via C−H borylation stands as a central means to access fine chemical precursors. Notwithstanding, achieving selectivity in this reaction is difficult, calling for the design of molecular catalysts that offer a vector for mechanistic control. This report aims to achieve such through the strategic placement of Lewis acids in the ligand periphery, permitting engagement with a substrate through non-covalent Lewis acid/base interactions. Various diphosphine iridium(I/III) complexes having 1,2-bis(di-n-propylphosphino)ethane) (dnppe), tetrakisallylphosphinoethane (tape) and 1,2-bis(di(3-dicyclohexylboranyl)propylphosphino)ethane (P₂B^Cy₄) ligands were prepared. The P₂B^Cy₄ ligand scaffold boasts four Lewis acidic boron groups in its secondary coordination sphere, which are shown to engage with N-heterocycles, tape is the precursor to P₂B^Cy₄, and dnppe is a saturated n-propyl analogue devoid of boron functionality. Select combinations of such iridium salts/diphosphine ligands were assayed in the catalytic borylation of 2-methylpyridine using B₂Pin₂ (Pin=pinacol).     

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis,Structures and Applications in the Catalytic C−H Borylation of Arenes and Alkanes

Indenyl (Ind) rhodium N-heterocyclic carbene (NHC) complexes [Rh(η⁵-Ind)(NHC)(L)] were synthesised for 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with L=C₂H₄ ( 1 ), CO ( 2 a ) and cyclooctene (COE; 3 ), for 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) with L=CO ( 2 b ) and COE ( 4 ), and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with L=CO ( 2 c ) and COE ( 5 ). Reaction of SIPr with [Rh(Cp*)(C₂H₄)₂] did not give the desired SIPr complex, thus demonstrating the “indenyl effect” in the synthesis of 1 . Oxidative addition of HSi(OEt)₃ to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)₃}(H)(SIPr)] ( 6 ) with loss of COE. Tethered-fluorenyl NHC rhodium complexes [Rh{(η⁵-C₁₃H₈)C₂H₄N(C)C₂H_xNR}(L)] (x=4, R=Dipp, L=C₂H₄: 11 ; L=COE: 12 ; L=CO: 13 ; R=Mes, L=COE: 14 ; L=CO: 15 ; x=2, R=Me, L=COE: 16 ; L=CO: 17 ) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1 , which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B₂pin₂ (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl-tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B₂pin₂ at 120 and 140 °C, respectively, was monitored by ¹¹B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp³ C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C₂H₄ and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained.     

Photoinduced Miyaura Borylation by a Rare‐Earth‐Metal Photoreductant: The Hexachlorocerate(III) Anion

The first photoinduced carbon(sp²)–heteroatom bond forming reaction by a rare‐earth‐metal photoreductant, a Miyaura borylation, has been achieved. This simple, scalable, and novel borylation method that makes use of the hexachlorocerate(III) anion ([Ce^IIICl₆]^3?, derived from CeCl₃) has a broad substrate scope and functional‐group tolerance and can be conducted at room temperature. Combined with Suzuki–Miyaura cross‐coupling, the method is applicable to the synthesis of various biaryl products, including through the use of aryl chloride substrates.     

Visible Light-Induced C-5 Selective C—H Radical Borylation of Imidazo[1,2-a]pyridines with NHC-Boranes

A visible-light-induced C-5 selective C—H borylation of imidazo[1,2-a]pyridines with NHC-BH₃ via Minisci-type radical borylation reaction has been developed for the first time. The present sustainable protocol provides a new family of regioselectively C5-borylated imidazopyridines that would otherwise be difficult to prepare. It is a supplement to site-selective borylation of azines (nitrogen-containing aromatic heterocycles) and the assembly of sp² carbon-boron bond.      

Stoichiometric and Catalytic Aryl−Cl Activation and Borylation using NHC-stabilized Nickel(0) Complexes

NHC-nickel (NHC=N-heterocyclic carbene) complexes are efficient catalysts for the C−Cl bond borylation of aryl chlorides using NaOAc as a base and B₂pin₂ (pin=pinacolato) as the boron source. The catalysts [Ni₂(ICy)₄(μ-(η²:η²)-COD)] ( 1 , ICy=1,3-dicyclohexylimidazolin-2-ylidene; COD=1,5-cyclooctadiene), [Ni(ICy)₂(η²-C₂H₄)] ( 2 ), and [Ni(ICy)₂(η²-COE)] ( 3 , COE=cyclooctene) compare well with other nickel catalysts reported previously for aryl-chloride borylation with the advantage that no further ligands had to be added to the reaction. Borylation also proceeded with B₂neop₂ (neop=neopentylglycolato) as the boron source. Stoichiometric oxidative addition of different aryl chlorides to complex 1 was highly selective affording trans-[Ni(ICy)₂(Cl)(Ar)] (Ar=4-(F₃C)C₆H₄, 11 ; 4-(MeO)C₆H₄, 12 ; C₆H₅, 13 ; 3,5-F₂C₆H₃, 14 ).     

Ni-Catalyzed Borylation of Aryl Sulfoxides

A nickel/N-heterocyclic carbene (NHC) catalytic system has been developed for the borylation of aryl sulfoxides with B₂(neop)₂ (neop=neopentyl glycolato). A wide range of aryl sulfoxides with different electronic and steric properties were converted into the corresponding arylboronic esters in good yields. The regioselective borylation of unsymmetric diaryl sulfoxides was also feasible leading to borylation of the sterically less encumbered aryl substituent. Competition experiments demonstrated that an electron-deficient aryl moiety reacts preferentially. The origin of the selectivity in the Ni-catalyzed borylation of electronically biased unsymmetrical diaryl sulfoxide lies in the oxidative addition step of the catalytic cycle, as oxidative addition of methoxyphenyl 4-(trifluoromethyl)phenyl sulfoxide to the Ni(0) complex occurs selectively to give the structurally characterized complex trans-[Ni(ICy)₂(4-CF₃-C₆H₄){(SO)-4-MeO-C₆H₄}] 4 . For complex 5 , the isomer trans-[Ni(ICy)₂(C₆H₅)(OSC₆H₅)] 5 - I was structurally characterized in which the phenyl sulfinyl ligand is bound via the oxygen atom to nickel. In solution, the complex trans-[Ni(ICy)₂(C₆H₅)(OSC₆H₅)] 5 - I is in equilibrium with the S-bonded isomer trans-[Ni(ICy)₂(C₆H₅)(SOC₆H₅)] 5 , as shown by NMR spectroscopy. DFT calculations reveal that these isomers are separated by a mere 0.3 kJ/mol (M06/def2-TZVP-level of theory) and connected via a transition state trans-[Ni(ICy)₂(C₆H₅)(η²-{SO}-C₆H₅)], which lies only 10.8 kcal/mol above 5 .     

Hydrogen Release from Dialkylamine–Boranes Promoted by Mg and Ca Complexes: A DFT Analysis of the Reaction Mechanism

Mg and Ca β‐diketiminato silylamides [HC{(Me)CN(2,6‐iPr₂C₆H₃)}₂M(THF)_n{N(SiMe₃)₂}] (M=Mg, n=0; M=Ca, n=1) were studied as precatalysts for the dehydrogenation/dehydrocoupling of secondary amine–boranes R₂HNBH₃. By reaction with equimolar quantities of amine–boranes, the corresponding amidoborane derivatives are formed, which further react to yield dehydrogenation products such as the cyclic dimer [BH₂?NMe₂]₂. DFT was used here to explore the mechanistic alternatives proposed on the basis of the experimental findings for both Mg and Ca amidoboranes. The influence of the steric demand of amine–boranes on the course of the reaction was examined by performing calculations on the dehydrogenation of dimethylamine–borane (DMAB), pyrrolidine–borane (PB), and diisopropylamine–borane. In spite of the analogies in the catalytic activity of Mg‐ and Ca‐based complexes in the dehydrocoupling of amine–boranes, our theoretical analysis confirmed the experimentally observed lower reactivity of Ca complexes. Differences in catalytic activity of Mg‐ and Ca‐based complexes were examined and rationalized. As a consequence of the increase in ionic radius on going from Mg²⁺ to Ca²⁺, the dehydrogenation mechanism changes and formation of a key metal hydride intermediate becomes inaccessible. Dimerization is likely to occur off‐metal in solution for DMAB and PB, whereas steric hindrance of iPr₂NHBH₃ hampers formation of the cyclic dimer. The reported results are of particular interest because, although amine–borane dehydrogenation is now well established, mechanistic insight is still lacking for many systems.     

N-heterocyclic carbene enabled rhodium-catalyzed ortho C(sp2)-H borylation at room temperature

We report a rhodium-catalyzed ortho C(sp²)-H borylation of 2-phenylpyridines using commercially available N-heterocyclic carbenes (NHCs) as ligand and pinacolatodiboron (B₂pin₂) as borylating reagent. The reaction could take place at room temperature, tolerating a wide range of functionalities and affording ortho borylated products in moderate to excellent yields. The current method is also applicable to gram-scale reaction with reduced catalyst loading.     

Metal‐Free Addition/Head‐to‐Tail Polymerization of Transient Phosphinoboranes,RPH‐BH2: A Route to Poly(alkylphosphinoboranes)

Mild thermolysis of Lewis base stabilized phosphinoborane monomers R¹R²P? BH₂?NMe₃ (R¹,R²=H, Ph, or tBu/H) at room temperature to 100 °C provides a convenient new route to oligo‐ and polyphosphinoboranes [R¹R²P‐BH₂]_n. The polymerization appears to proceed via the addition/head‐to‐tail polymerization of short‐lived free phosphinoborane monomers, R¹R²P‐BH₂. This method offers access to high molar mass materials, as exemplified by poly(tert‐butylphosphinoborane), that are currently inaccessible using other routes (e.g. catalytic dehydrocoupling).     

Tetrahydroxydiboron (Bis-boric Acid): a Versatile Reagent for Borylation,Hydrogenation, Catalysis,Radical Reactions and H2 Generation

Diboron compounds with a B−B bond, discovered with B₂Cl₄ a century ago, have been developed only since the turn of this century for catalyzed borylation reactions, mostly with the well-know reagent bis(binacolato)diboron, (B₂pin₂). On the other hand, chemistry of tetrahydroxydiboron (THDB), also named bis-boric acid, is relatively underdeveloped. In this review, the properties of THDB as a borylation, reductant (including transfer hydrogenation), catalyst, source of radicals and generator of H₂ from water upon hydrolysis are summarized.     

Rhodium(I) PNN Pincer Complexes with Proton-responsive Ligands: Synthesis,Characterisation, and Catalytic Dehydrocoupling of Amine Boranes

Coordination of a pyridine-pyrazole-based PNN(H) ligand to Rh^I produces a family of neutral ( 1 ) and cationic ( 2Cl ) Rh^I complexes. Deprotonation of the parent Rh chloride complex with LiNiPr₂ results in formation of a dinuclear LiCl bridged species 3 bearing a pyrazolate fragment. Complexes 1 , 2Cl and 3 were tested as precatalyst for the dehydrocoupling of amine boranes. All complexes studied show activity for the formation of cyclic oligomers with N-methylcyclotriborazane as the main product. Base activation of the neutral Rh chloride complex 1 produces catalyst systems that are significantly more active than the parent system, suggesting that dehydrohalogenation of the Rh chloride precatalyst 1 is one of the key steps for catalyst formation.     

Stereodivergent Synthesis of Arylcyclopropylamines by Sequential C?H Borylation and Suzuki–Miyaura Coupling

A step‐economical and stereodivergent synthesis of privileged 2‐arylcyclopropylamines (ACPAs) through a C(sp³) H borylation and Suzuki–Miyaura coupling sequence has been developed. The iridium‐catalyzed C H borylation of N‐cyclopropylpivalamide proceeds with cis selectivity. The subsequent B‐cyclopropyl Suzuki–Miyaura coupling catalyzed by [PdCl₂(dppf)]/Ag₂O proceeds with retention of configuration at the carbon center bearing the Bpin group, while epimerization at the nitrogen‐bound carbon atoms of both the starting materials and products is observed under the reaction conditions. This epimerization is, however, suppressed in the presence of O₂. The present new ACPA synthesis results in not only a significant reduction in the steps required for making ACPA derivatives, but also the ability to access either isomer (cis or trans) by simply changing the atmosphere (N₂ or O₂) in the coupling stage.     

Stereodivergent Synthesis of Arylcyclopropylamines by Sequential CH Borylation and Suzuki–Miyaura Coupling

A step‐economical and stereodivergent synthesis of privileged 2‐arylcyclopropylamines (ACPAs) through a C(sp³)? H borylation and Suzuki–Miyaura coupling sequence has been developed. The iridium‐catalyzed C? H borylation of N‐cyclopropylpivalamide proceeds with cis selectivity. The subsequent B‐cyclopropyl Suzuki–Miyaura coupling catalyzed by [PdCl₂(dppf)]/Ag₂O proceeds with retention of configuration at the carbon center bearing the Bpin group, while epimerization at the nitrogen‐bound carbon atoms of both the starting materials and products is observed under the reaction conditions. This epimerization is, however, suppressed in the presence of O₂. The present new ACPA synthesis results in not only a significant reduction in the steps required for making ACPA derivatives, but also the ability to access either isomer (cis or trans) by simply changing the atmosphere (N₂ or O₂) in the coupling stage.