Cobalt Boryl Complexes: Enabling and Exploiting Migratory Insertion in Base-Metal-Mediated Borylation

Aerobic Visible-Light-Driven Borylation of Heteroarenes in a Gel Nanoreactor

Heteroarene boronate esters constitute valuable intermediates in modern organic synthesis. As building blocks, they can be further applied to the synthesis of new materials, since they can be easily transformed into any other functional group. Efforts toward novel and efficient strategies for their preparation are clearly desirable. Here, we have achieved the borylation of commercially available heteroarene halides under very mild conditions in an easy-to-use gel nanoreactor. Its use of visible light as the energy source at room temperature in photocatalyst-free and aerobic conditions makes this protocol very attractive. The gel network provides an adequate stabilizing microenvironment to support wide substrate scope, including furan, thiophene, selenophene, and pyrrole boronate esters.

Transition metal-mediated B(4)-H hydroxylation/halogenation of o-carboranes bearing a 2-pyridylsulfenyl ligand

The introduction of the 2-pyridylsulfenyl directing group to o-carboranes allowed either B(3)-Ir or B(4)-Ir bond formation using a steric effect strategy. Moreover, the reactivity of the B(4)-Rh o-carborane complexes with small molecules was probed by reactions with N-bromosuccinimide, N-iodosuccinimide and O₂. Rhodium-mediated B(4)-hydroxylation and B(4)-halogenation which are seldom reported have been achieved under practical and mild conditions.

Reactivity of a magnesium diboranate with organic nitriles

A series of complexes generated through reactions of the β-diketiminato magnesium diboranate species, [(BDI)Mg{(n-Bu)pinB-Bpin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-di-iso-propylphenyl), and a variety of organic nitriles are reported. Although, in every case, the diboranate anion acts as a surrogate source of the {Bpin} nucleophile, resulting in B-C bond formation at the electrophilic sp-hydridised nitrile carbon, the resultant compounds display a variable propensity to undergo subsequent reaction with additional nitrile equivalents. This behaviour is rationalised to be a consequence of substituent-dependent modulation in the basicity and resultant electrophilicity of magnesium-coordinated nitrile intermediates.

Visible-Light-Induced Ni-Catalyzed Radical Borylation of Chloroarenes

A highly selective and general photoinduced C-Cl borylation protocol that employs [Ni(IMes)₂] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical borylation of chloroarenes is reported. This photoinduced system operates with visible light (400 nm) and achieves borylation of a wide range of chloroarenes with B₂pin₂ at room temperature in excellent yields and with high selectivity, thereby demonstrating its broad utility and functional group tolerance. Mechanistic investigations suggest that the borylation reactions proceed via a radical process. EPR studies demonstrate that [Ni(IMes)₂] undergoes very fast chlorine atom abstraction from aryl chlorides to give [Ni^I(IMes)₂Cl] and aryl radicals. Control experiments indicate that light promotes the reaction of [Ni^I(IMes)₂Cl] with aryl chlorides generating additional aryl radicals and [Ni^II(IMes)₂Cl₂]. The aryl radicals react with an anionic sp²-sp³ diborane [B₂pin₂(OMe)]^- formed from B₂pin₂ and KOMe to yield the corresponding borylation product and the [Bpin(OMe)]^•- radical anion, which reduces [Ni^II(IMes)₂Cl₂] under irradiation to regenerate [Ni^I(IMes)₂Cl] and [Ni(IMes)₂] for the next catalytic cycle.

Cobalt-Catalyzed Reductive C-O Bond Cleavage of Lignin β-O-4 Ketone Models via In Situ Generation of the Cobalt-Boryl Species

An efficient and mild method for reductive C-O bond cleavage of lignin β-O-4 ketone models was developed to afford the corresponding ketones and phenols with PDI-CoCl₂ as the precatalyst and diboron reagent as the reductant. The synthetic utility of the methodology was demonstrated by depolymerization of a polymeric model and gram-scale transformation. Mechanistic studies suggested that this transformation involves steps of carbonyl insertion, 1,2-Brook type rearrangement, β-oxygen elimination, and rate-limiting regeneration of the catalytic active Co-B species.

Diversity-Oriented Synthesis of α-Functionalized Acylborons and Borylated Heteroarenes by Nucleophilic Ring Opening of α-Chloroepoxyboronates

The ring-opening reactions of N-methyliminodiacetyl (MIDA) α-chloroepoxyboronates with different nucleophiles allow the modular synthesis of a diverse array of organoboronates. These include seven types of α-functionalized acylboronates and seven types of borylated heteroarenes, some of which are difficult-to-access products using alternative methods. The common synthons, α-chloroepoxyboronates, could be viably synthesized by a two-step procedure from the corresponding alkenyl MIDA boronates. Mild reaction conditions, good functional-group tolerance, and generally good efficiency were observed. The utility of the products was also demonstrated.

Synthetic, structural and reaction chemistry of N-heterocyclic germylene and stannylene compounds featuring N-boryl substituents

This study details the syntheses of N-heterocyclic germylenes and stannylenes featuring diazaborolyl groups, {(HCDippN)₂B} (Dipp = 2,6-ⁱPr₂C₆H₃), as both of the N-bound substituents, with a view to generating electron rich and sterically protected metal centres. The energies of their key frontier orbitals - the group 14-centred lone pair and orthogonal p_π-orbital (typically the HOMO-2 and LUMO) have been probed by DFT calculations and compared with a related acyclic analogue, revealing (in the case of the stannylenes) a correlation with the measured ¹¹⁹Sn chemical shifts. The reactivity of the germylene systems towards oxygen atom transfer agents has been examined, with 2 : 1 reaction stoichiometries being observed for both Me₃NO and pyridine N-oxide, leading to the formation of products thought to be derived from the activation of C-H bonds by a transient first-formed germanone.

Snapshots of magnesium-centred diborane heterolysis by an outer sphere SN2 process

Reactions of a magnesium diboranate as a source of [Bpin]^– anions are initiated by ‘outer sphere’ attack of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bonded substrates.

Reactions of the β-diketiminato magnesium diboranate derivative, [(BDI)Mg{pinB(n-Bu)Bpin}] (BDI = HC{(Me)CNDipp}₂; Dipp = 2,6-i-Pr₂C₆H₃), with N,N′-dialkyl and N,N′-diaryl carbodiimides provided the corresponding C-borylated magnesium borylamidinates. This reactivity occurs with the displacement of n-BuBpin and with the apparent addition of a nucleophilic {Bpin} anion to the electrophilic unsaturated carbodiimide carbon centres. In contrast, while analogous reactions of [(BDI)Mg{pinB(n-Bu)Bpin}] with N-alkyl or N-aryl aldimines and ketimines also resulted in facile displacement of n-BuBpin, they provided the organomagnesium products of {Bpin} addition to the imine nitrogen atom rather than the more electrophilic trigonal imine carbon. Computational assessment by density functional theory (DFT) indicated that, although the energetic differences are marginal, the organomagnesium products may be considered as the kinetic outcome of these reactions with respect to the generation of alternative amidomagnesium regioisomers. This latter deduction was borne out by the thermally-induced conversion of two such organomagnesium species to their C-borylated amidomagnesium isomers, both of which occur with negligible entropies of activation indicative of purely intramolecular processes. Detailed analysis by DFT of the reaction of [(BDI)Mg{pinB(n-Bu)Bpin}] with PhN Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHPh indicated that B–N bond formation is initiated by attack of the imine nitrogen at the three-coordinate boron atom of the diboranate anion rather than the more crowded magnesium centre. Consistent with an effectively spontaneous reaction, the resultant cleavage of the B–B bond of the diboranate unit is accomplished via the traversal of two very modest barriers of only 8.3 and 6.7 kcal mol^–1. This analysis was also supportive of a subsequent intramolecular B–N to B–C isomerisation process. Of greater general significance, however, the addition of the {Bpin}^– anion to the reducible aldimine is best rationalised as a consequence of the electrophilic character of this three-coordinate boron centre rather than any intrinsic nucleophilicity associated with the B–B bond of the [pinBB(n-Bu)pin]^– anion.

Oxidative Addition of Dihydrogen, Boron Compounds, and Aryl Halides to a Cobalt(I) Cation Supported by a Strong-Field Pincer Ligand

Cationic cobalt(I) dinitrogen complexes with a strong-field tridentate pincer ligand were prepared and the oxidative addition of polar and non-polar bonds was studied. Addition of H₂ to [(^iPrPNP)Co(N₂)]⁺ (^iPrPNP = 2,6-bis((diisopropylphosphaneyl)methyl)pyridine) in THF-d8 resulted in rapid oxidative addition and formation of the cis-Co(III) dihydride complex, cis-[(^iPrPNP)Co(H)₂L]⁺ where L = THF or N₂. The addition of H₂ was reversible as evidenced by the dynamics observed by variable temperature ¹H NMR spectroscopy and the regeneration of [(^iPrPNP)Co(N₂)]⁺ upon exposure to dinitrogen. In contrast, addition of HBPin, (Pin = pinacolato) B₂Pin₂ and aryl halides resulted in the formation of net one-electron oxidation products: cationic Co(II)-boryl and Co(II)-halide/aryl complexes, respectively. All products were structurally characterized by X-ray crystallography and the electronic structures were determined by a combination of magnetic moment measurements, EPR spectroscopy and DFT calculations. Monitoring the addition of HBPin to [(^iPrPNP)Co(N₂)]⁺ provided evidence for a transient Co(III) oxidative addition product that likely undergoes comproportionation with the cobalt(I) starting material to generate the observed Co(II) products.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Borylated N-Heterocyclic Carbenes: Rearrangement and Chemical Trapping

This study details attempts to access N-heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp)₂ B}, as one or both of the N-bound substituents, to generate strongly electron-donating and sterically imposing new carbene ligands. Attempts to isolate N-heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N-to-C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N-bound diazaborolyl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half-lives of the free carbenes at room temperature range from 4-50 h (depending on the pattern of ancillary substituents) with N-to-C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first-order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2-shift is mechanistically feasible, with calculated activation energies of the order of 90-100 kJ mol^-1 , reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties through systems of the type LAuCl, LRh(CO)₂ Cl, and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ-donor capabilities (compared with IMes) are revealed.

A redox-active diborane platform performs C(sp3)-H activation and nucleophilic substitution reactions

Targeted C(sp³)–H activation or nucleophilic substitution reactions have been achieved through the interaction of a diborane dianion with haloalkanes.

Organoboranes are among the most versatile and widely used reagents in synthetic chemistry. A significant further expansion of their application spectrum would be achievable if boron-containing reactive intermediates capable of inserting into C–H bonds or performing nucleophilic substitution reactions were readily available. However, current progress in the field is still hampered by a lack of universal design concepts and mechanistic understanding. Herein we report that the doubly arylene-bridged diborane(6) 1H₂ and its B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B-bonded formal deprotonation product Li₂[1] can activate the particularly inert C(sp³)–H bonds of added H₃CLi and H₃CCl, respectively. The first case involves the attack of [H₃C]^– on a Lewis-acidic boron center, whereas the second case follows a polarity-inverted pathway with nucleophilic attack of the B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B double bond on H₃CCl. Mechanistic details were elucidated by means of deuterium-labeled reagents, a radical clock, α,ω-dihaloalkane substrates, the experimental identification of key intermediates, and quantum-chemical calculations. It turned out that both systems, H₃CLi/1H₂ and H₃CCl/Li₂[1], ultimately funnel into the same reaction pathway, which likely proceeds past a borylene-type intermediate and requires the cooperative interaction of both boron atoms.

Completing the series of boron-nucleophilic cyanoborates: boryl anions of type NHC-B(CN)2. Chem Sci 2017;8:6274-6280. [PMID: 28989661 PMCID: PMC5628389 DOI: 10.1039/c7sc02238g]

Since the first seminal report of boron-centred nucleophiles, the area of boryl anions has developed only sporadically and requires further systematisation. The boryl anions of type NHC-B(CN)2- (NHC = N-heterocyclic carbene) described herein complete a consistent series with the known anions cAAC-B(CN)2- [cAAC = cyclic(alkyl)(amino)carbene] and B(CN)32-. A novel approach towards NHC-stabilised cyanoboranes based on alkylthio-cyano exchange at boron is presented, and in contrast to other methods affords the products in better purity and yield. Reduction of suitable NHC-dicyanoboranes gave two unprecedented examples of NHC-B(CN)2- boryl anions. The latter were shown to react as boron-centred nucleophiles with facile formation of B-E bonds, where E = C, Si, Sn, P, Au. Bonding analysis by DFT calculations suggests a systematic variation of the energy of the boron-centred HOMO depending on the carbene, which in turn can control the nucleophilic character.

Easy access to nucleophilic boron through diborane to magnesium boryl metathesis

Organoboranes are some of the most synthetically valuable and widely used intermediates in organic and pharmaceutical chemistry. Their synthesis, however, is limited by the behaviour of common boron starting materials as archetypal Lewis acids such that common routes to organoboranes rely on the reactivity of boron as an electrophile. While the realization of convenient sources of nucleophilic boryl anions would open up a wealth of opportunity for the development of new routes to organoboranes, the synthesis of current candidates is generally limited by a need for highly reducing reaction conditions. Here, we report a simple synthesis of a magnesium boryl through the heterolytic activation of the B–B bond of bis(pinacolato)diboron, which is achieved by treatment of an easily generated magnesium diboranate complex with 4-dimethylaminopyridine. The magnesium boryl is shown to act as an unambiguous nucleophile through its reactions with iodomethane, benzophenone and N,N′-di-isopropyl carbodiimide and by density functional theory.

Organoboranes are widely employed in organic synthesis and typically are prepared using electrophilic boron sources. Here the authors report a route to nucleophilic boryl anions via organomagnesium-driven cleavage of boron-boron bonds.

Convenient and General Zinc-Catalyzed Borylation of Aryl Diazonium Salts and Aryltriazenes under Mild Conditions

A convenient and general zinc‐catalyzed borylation of aryl diazonium salts and aryltriazenes has been developed. With bis‐ (pinacolato)diboron as the borylation reagent, aryldiazonium tetrafluoroborate salts and aryltriazenes were transformed into the corresponding arylboronates in moderate to excellent yields under mild conditions. As a convenient and practical methodology, no additional ligands, base, or any other additives are required here.

Isolation, structure and reactivity of a scandium boryl oxycarbene complex

The reaction of a half-sandwich scandium boryl complex 1 with CO (1 atm) afforded a novel boryl oxycarbene complex 2. The structure of 2 was characterized by 1H, 13C and 11B NMR, X-ray diffraction, and DFT analysis. Further reaction of 2 with CO (1 atm) yielded a phenylamido- and boryl-substituted enediolate complex 3 through C-C bond formation between CO and the carbene unit in 2 and cleavage and rearrangement of the Si-N bond in the silylene-linked Cp-amido ligand. Upon heating, insertion of the carbene atom into a methine C-H bond in the boryl ligand of 2 took place to give an alkoxide complex 4. The reactions of 2 with pyridine and 2-methylpyridine led to insertion of the carbene atom into an ortho-C-H bond of pyridine and into a methyl C-H bond of 2-methylpyridine, respectively. The reaction of 2 with ethylene yielded a borylcyclopropyloxy complex 7 through cycloaddition of the carbene atom to ethylene.