Stereoselective Ortho Borylation of Pyridylferrocenes

Pyrimidine-directed metal-free C-H borylation of 2-pyrimidylanilines: a useful process for tetra-coordinated triarylborane synthesis

Convenient, easily handled, laboratory friendly, robust approaches to afford synthetically important organoboron compounds are currently of great interest to researchers. Among the various available strategies, a metal-free approach would be overwhelmingly accepted, since the target boron compounds can be prepared in a metal-free state. We herein present a detailed study of the metal-free directed ortho-C–H borylation of 2-pyrimidylaniline derivatives. The approach allowed us to synthesize various boronates, which are synthetically important compounds and various four-coordinated triarylborane derivatives, which could be useful in materials science as well as Lewis-acid catalysts. This metal-free directed C–H borylation reaction proceeds smoothly without any interference by external impurities, such as inorganic salts, reactive functionalities, heterocycles and even transition metal precursors, which further enhance its importance.

We present the metal-free ortho-C–H borylation of 2-pyrimidylanilines to afford synthetically important boronic esters and tetra-coordinated triarylboranes, which could be useful in materials science as well as Lewis-acid catalysts.

Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C-Cl bond activation

Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd⁰ complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (pSpR, meso-isomer) and 1b (pSpS-isomer) differ in the orientation of the ferrocene moieties relative to the central Ph₂PC₅H₃-Hg-C₅H₃PPh₂ bridging entity. The structurally distinct trigonal LPd⁰(dba) complexes 2a (meso) and 2b (pSpS) are obtained upon treatment with Pd(dba)₂. A competition reaction reveals that 1b reacts faster than 1a with Pd(dba)₂. Unexpectedly, catalytic interconversion of 1a (meso) into 1b (rac) is observed at room temperature in the presence of only catalytic amounts of Pd(dba)₂. Both Pd⁰ complexes, 2a and 2b, readily undergo oxidative addition into the C-Cl bond of CH₂Cl₂ at moderate temperatures with formation of the square-planar trans-chelate complexes LPd^IICl(CH₂Cl) (3a, 3b). Kinetic studies reveal a significantly higher reaction rate for the meso-isomer 2a in comparison to (pSpS)-2b.

Recent Advances in π-Conjugated N^C-Chelate Organoboron Materials

Boron-containing π-conjugated materials are archetypical candidates for a variety of molecular scale applications. The incorporation of boron into the π-conjugated frameworks significantly modifies the nature of the parent π-conjugated systems. Several novel boron-bridged π-conjugated materials with intriguing structural, photo-physical and electrochemical properties have been reported over the last few years. In this paper, we review the properties and multi-dimensional applications of the boron-bridged fused-ring π-conjugated systems. We critically highlight the properties of π-conjugated N^C-chelate organoboron materials. This is followed by a discussion on the potential applications of the new materials in opto-electronics (O-E) and other areas. Finally, attempts will be made to predict the future direction/outlook for this class of materials.

Intramolecular (directed) electrophilic C-H borylation

The intramolecular C-H borylation of (hetero)arenes and alkenes using electrophilic boranes is a powerful transition metal free methodology for forming C-B bonds. These C-H borylation reactions are preceded by intermolecular bond (both dative and covalent) formation, with examples proceeding via initial C-B and N-B bond formation dominating this field thus both are discussed in depth herein. Less prevalent intramolecular electrophilic C-H borylation reactions that proceed by intermolecular O-B, S-B and P-B bond formation are also summarised. Mechanistic studies are presented that reveal two mechanisms for C-H borylation, (i) electrophilic aromatic substitution (prevalent with B-X electrophiles); (ii) σ-bond metathesis mediated (prevalent with B-H and B-R electrophiles). To date, intramolecular electrophilic C-H borylation is utilised mainly for accessing boron containing conjugated organic materials, however recent developments, summarized herein alongside early studies, have highlighted the applicability of this methodology for forming synthetically versatile organo-boronate esters and boron containing bioactives. The multitude of synthetic procedures reported for intramolecular electrophilic C-H borylation contain many common features and this enables key requirements for successful C-H borylation and the factors effecting regioselectivity and substrate scope to be identified, discussed and summarized.

Diferrocenylmercury diphosphine diastereomers with unique geometries: trans-chelation at Pd(ii) with short Hg(ii)Pd(ii) contacts

Diphosphine chelate ligands are essential in many catalytic processes with both the electronic structure and bite angle having a dramatic influence on the coordination behavior and catalytic performance. The synthesis of a new class of diferrocenylmercury-supported diphosphine chelate ligands was accomplished by the reaction of (ortho-diphenylphosphino)ferrocenyl sulfinate (2) with t-BuLi, followed by treatment with mercury(ii) chloride. Two diastereomers, 4a (pSpR-, meso-isomer) and 4b (pSpS-isomer), differ in the orientation of the ferrocene moiety relative to the central Ph₂PC₅H₃-Hg-C₅H₃PPh₂ bridging entity. They were isolated independently and fully characterized in solution and in the solid state by single crystal X-ray diffraction analysis. Key characteristics of these ligands are their exceptionally wide and flexible bite angles and the unique stereochemical environment that is achieved upon coordination to transition metals. Complexation to Pd(ii)Cl₂ gives rise to unusual square-planar trans-chelate complexes 5a (meso) and 5b (pSpS). In competition reactions, 4a and 4b show similar reactivity toward Pd(ii)Cl₂. The molecular structures of 5a and 5b exhibit short PdHg contacts, possibly indicating secondary metallophilic interactions as further evidenced by bond-critical points between Pd and Hg that were identified by AIM analyses.

Functional conjugated pyridines via main-group element tuning

The functional properties arising from a combination of main-group elements with pyridine-based organic conjugated scaffolds are highlighted.

Planar-chiral ferrocenylphosphine-borane complexes featuring agostic-type B–H⋯E (E = Hg, Sn) interactions

Ferrocenylphosphine-borane adducts with Lewis acidic organotin and organomercury substituents inortho-position show rare agostic-type B–H⋯E (E = Sn, Hg) interactions that have been studied by single crystal XRD, multinuclear solution NMR, and computational methods.

A general diastereoselective synthesis of highly functionalized ferrocenyl ambiphiles enabled on a large scale by electrochemical purification

A diastereoselective modification of tert-butylated ferrocenes leads to highly functionalized constrained ambiphiles (P,B or N,B) and hybrids (P,N or P,P′) on a multigram scale by a smart electrochemical purification.

Highly Emissive Far Red/Near-IR Fluorophores Based on Borylated Fluorene-Benzothiadiazole Donor-Acceptor Materials

Stille, Suzuki-Miyaura and Negishi cross-coupling reactions of bromine-functionalised borylated precursors enable the facile, high yielding, synthesis of borylated donor-acceptor materials that contain electron-rich aromatic units and/or extended effective conjugation lengths. These materials have large Stokes shifts, low LUMO energies, small band-gaps and significant fluorescence emission >700 nm in solution and when dispersed in a host polymer.

Luminescent organoboron ladder compounds via directed electrophilic aromatic C–H borylation

Regioselective electrophilic borylation of a pyridyl-flanked fluorene provides highly luminescent, air-stable boron-bridged ladder species.

Facile Arylation of Four-Coordinate Boron Halides by Borenium Cation Mediated Boro-desilylation and -destannylation

The addition of AlCl₃ to four-coordinate boranes of the general formula (C–N-chelate)BCl₂ results in halide abstraction and formation of three-coordinate borenium cations of the general formula [(C–N-chelate)BCl]⁺. The latter react with both arylstannanes and arylsilanes by boro-destannylation and -desilylation, respectively, to form arylated boranes. Catalytic quantities of AlCl₃ were sufficient to effect high-yielding arylation of (C–N-chelate)BCl₂. Boro-destannylation is more rapid than boro-desilylation and leads to double arylation at the boron center, whereas in reactions with arylsilanes either single or double arylation occurs dependent on the nucleophilicity of the arylsilane and on the electrophilicity of the borenium cation. The electrophilicity of the borenium cation derived from 2-phenylpyridine was greater than that of the benzothiadiazole analogues, enabling the boro-desilyation of less nucleophilic silanes and the direct electrophilic borylation of 2-methylthiophene.

Unsolvated Al(C6F5)3: structural features and electronic interaction with ferrocene

Alkyl/aryl ligand exchange between AlEt3 and B(C6F5)3 in hexanes enables the formation and isolation of the unsolvated Al(C6F5)3 as a crystalline solid, the structure of which has been determined by single-crystal X-ray diffraction analysis. Instead of forming the anticipated AlF contacts with the seemingly more accessible meta- and para-F's of -C6F5 groups, two Al(C6F5)3 molecules form a dimeric structure with double AlF interactions between the Al center of one molecule and the ortho-F atom of the -C6F5 group on the other molecule. This mode of interactions is apparently linked to the thermal and shock sensitivity of the unsolvated Al(C6F5)3 in the solid state. To compare with the B(C6F5)3/ferrocene frustrated Lewis pair system, the complexation between Al(C6F5)3 and ferrocene has also been studied, which affords a stable adduct formed through the η(1)-coordination of Al to one of the CCp atoms, similar to the alane-toluene or benzene complex.

Enhancing electron affinity and tuning band gap in donor-acceptor organic semiconductors by benzothiadiazole directed C-H borylation

Electrophilic borylation using BCl3 and benzothiadiazole to direct the C-H functionalisation of an adjacent aromatic unit produces fused boracyclic materials with minimally changed HOMO energy levels but significantly reduced LUMO energy levels. In situ alkylation and arylation at boron using Al(alkyl)3 or Zn(aryl)2 is facile and affords boracycles that possess excellent stability towards protic solvents, including water, and display large bathochromic shifts leading to far red/NIR emission in the solid state with quantum yields of up to 34%. Solution fabricated OLEDs with far red/NIR electroluminescence are reported with EQEs > 0.4%.

Activation of boryl-, borylene and metalloborylene complexes by isonitriles

One or several isonitriles can insert into metalloboryl and metalloborylene complexes to afford cyclic or spiro compounds with bonding controlled by the electron count at the metal and in the ring.

Synthesis, crystal structure, and photoluminescence studies of a ruthenocenyl-decorated Sn/S cluster

Recently, we reported on ferrocenyl-decorated Sn/S clusters; herein, we present the extension of our investigations by attachment of ruthenocenyl units to an according cluster skeleton. The latter was realized upon improvement of the synthesis of acetylruthenocene, its conversion to a hydrazone derivative, and the subsequent reaction with a keto-functionalized Sn/S precursor complex. The report comprises the crystal structures of acetylruthenocene and the ruthenocenyl-terminated Sn/S cluster [(R(Rc)Sn)4Sn2S10] (R(Rc) = CMe2CH2C(Me)═N-N═C(Me)Rc), as well as the discussion of the electrochemical properties of the latter and its behavior during time-resolved photoluminescence investigations.

From C,N- and N,N-chelated chloroboranes to substituted 1H-2,1-benzazaboroles and 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines: a straightforward route to five-membered rings containing the B–N or N–B–N moiety

The reaction of C,N- and N,N-chelated boranes with lithium reagents resulted in the formation of substituted 1H-2,1-benzazaboroles and 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines.