Selective Addition of Organolithium Reagents to BF2-Chelates of β-Ketoesters

CuBr2-mediated dehydrogenative [4+2] annulation of 1-naphthyl-1,3-indandiones and alkenes

Intermolecular annulation reactions of 1-naphthyl-1,3-indandiones with alkenes proceed efficiently in the presence of a copper catalyst to generate spirocarbocycle compounds. Various spirocyclic molecules bearing an all-carbon quaternary center could be obtained by this novel method with good yields, excellent regioselectivity, and good functional group tolerance. A radical mechanism is proposed based on the HRMS analysis results of control experiments.

1-Butyl-3-methylimidazolium tetrafluoroborate as suitable solvent for BF3: the case of alkyne hydration. Chemistry vs electrochemistry

In order to replace the expensive metal/ligand catalysts and classic toxic and volatile solvents, commonly used for the hydration of alkynes, the hydration reaction of alkynes was studied in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF4) adding boron trifluoride diethyl etherate (BF3·Et2O) as catalyst. Different ionic liquids were used, varying the cation or the anion, in order to identify the best one, in terms of both efficiency and reduced costs. The developed method was efficaciously applied to different alkynes, achieving the desired hydration products with good yields. The results obtained using a conventional approach (i.e., adding BF3·Et2O) were compared with those achieved using BF3 electrogenerated in BMIm-BF4, demonstrating the possibility of obtaining the products of alkyne hydration with analogous or improved yields, using less hazardous precursors to generate the reactive species in situ. In particular, for terminal arylalkynes, the electrochemical route proved to be advantageous, yielding preferentially the hydration products vs the aldol condensation products. Importantly, the ability to recycle the ionic liquid in subsequent reactions was successfully demonstrated.

1,2-Addition to trifluoromethylated β-enamino diketones: regioselective synthesis of trifluoromethyl-containing azomethine pyrazoles and isoxazoles

Trifluoromethylated β-enamino diketones undergo type 1,2-addition leading to regioselective synthesis of trifluoromethylated azoles containing an azomethine group.

Difluoroboron Chelation to Quinacridonequinone: A Synthetic Method for Air-Sensitive 6,13-Dihydroxyquinacridone via Boron Complexes

This study aims to perform the chelation of difluoroboron (BF₂ ) to quinacridonequinone (QQ). The resulting dark green solid was determined to be QA-BF₂ , which is a BF₂ complex of 6,13-dihydroxyquinacridone (QA-OH), and not QQ-BF₂ , which is a BF₂ complex of QQ. This result indicated that QQ-BF₂ was first generated as an O,O-bidentate chelate, which immediately underwent a two-electron reduction to produce QA-BF₂ . This compound was converted to air-sensitive QA-OH by undergoing hydrolysis in argon. Since QA-OH has a strong electron-donating property, it easily produced QQ via air oxidation in the solution. QA-OH also acts as a reducing reagent for quinones. The crystal packing of QA-OH is a herringbone type with short π⋅⋅⋅π contacts, and a good hole mobility has been suggested by theoretical calculations. Herein, a new synthetic method from QQ to QA-OH using BF₂ chelation and hydrolysis was proposed. QA-BF₂ and QA-OH are useful organic functional pigments and reducing reagents.

Fluorescent boron complexes based on new N,O-chelates as promising candidates for flow cytometry

This study presents the synthesis and optical properties of a new class of bright green-yellow fluorescent dyes with potential applications in bioimaging. A facile synthetic route via the chelation of aryl(hetaryl)aminoacryloylthiophene scaffolds with a BF2 fragment is presented. The photophysical properties of the dyes are attributed to the nature and position of electron-donating and electron-withdrawing substituents. Upon coordination to a BF2 fragment, characteristic emission was observed, with λem ranging from 503 to 543 nm and quantum yields of 0.14-0.42. Compared with parent aryl(hetaryl)aminoacryloylthiophenes, a significant red shift in absorption (up to 480 nm in solution) and emission (up to 543 nm in solution and 610 nm in the solid state) and high chemical stability and photostability were observed. The electron-accepting character of the substituents on the terminal aromatic ring or replacing this fragment with pyridine or pyrazine moieties resulted in increased quantum yields. To gain insight into the electronic structures and optical properties, quantum mechanical calculations were performed. The results of (TD-)DFT calculations supported the structural and spectroscopic data and showed the features of electronic distribution in the frontier molecular orbitals and active electrophilic and nucleophilic sites in the compounds investigated. Synthesized BF2 complexes are promising dyes for cell imaging and flow cytometry owing to their ready penetration and accumulation in cells.

Copper-catalyzed decarboxylative propargylation/hydroamination reactions: access to C3 β-ketoester-functionalized indoles

A copper-catalyzed reaction of ethynyl benzoxazinanones with readily accessible β-ketoesters via a decarboxylative propargylation/hydroamination sequence has been developed. This protocol furnished a diverse range of C3 β-ketoester-functionalized indoles in good to excellent yields.

Local Desymmetrization through Diastereotopic Group Selection: An Enabling Strategy for Natural Product Synthesis

The application of desymmetrization strategies in chemical synthesis has allowed fundamentally new synthetic sequences that efficiently create dense and polyfunctional stereochemical arrays. Enantiotopic group discrimination has become a well-established method of global desymmetrization, while the conceptually unique strategy of local desymmetrization by diastereotopic group discrimination has its own advantages. This microreview focuses on the application of local desymmetrization in natural product synthesis and places a particular emphasis on the efficiency engendered by diastereotopic group discrimination. Local desymmetrization is subdivided into three distinct manifolds; examples under each paradigm are presented and compared.

An expeditious method to synthesize difluoroboron complexes of β-keto amides from β-keto nitriles and BF3·OEt2

An expeditious and convenient method has been developed to synthesize difluoroboron complexes of β-keto amides from β-keto nitriles and BF₃·OEt₂.

Cooperative effects of o- and m-methyl groups on the intramolecular charge-transfer emission properties of dibenzoylmethanatoboron difluorides

Intramolecular charge-transfer character of the p-xylyl derivative of dibenzoylmethanatoboron difluoride is induced by o- and m-methyl groups cooperatively.

Syntheses of (+)-30-epi-, (-)-6-epi-, (±)-6,30-epi-13,14-Didehydroxyisogarcinol and (±)-6,30-epi-Garcimultiflorone A Utilizing Highly Diastereoselective, Lewis Acid-Controlled Cyclizations

The first syntheses of 13,14-didehydroxyisogarcinol (6) and garcimultiflorone A (5) stereoisomers are reported in six steps from a commercially available phloroglucinol. Lewis acid-controlled, diastereoselective cationic oxycyclizations enabled asymmetric syntheses of (-)-6-epi-6 and (+)-30-epi-6. A similar strategy enabled production of the meso-dervied isomers (±)-6,30-epi-6 and (±)-6,30-epi-5. Finally, a convenient strategy for gram scale synthesis was developed utilizing diastereomer separation at a later stage in the synthesis that minimized the number of necessary synthetic operations to access all possible stereoisomers.

Synthesis, characterization and properties of furan-containing difluoroboron complexes

New β-diketonate difluoroboron complexes (F1–F4) containing a furan ring have been developed, and their luminescence properties have been investigated systematically.

Study on the solution and solid-state fluorescence of novel BF2 complexes with (Z)-2-[phenanthridin-6(5H)-ylidene]-1-phenylethanone and its derivatives as ligands

A family of novel N,O-chelated BF₂ complexes with intense solution and solid-state emission were synthesized and their photophysical properties were studied.

General design strategy for aromatic ketone-based single-component dual-emissive materials

Materials with both fluorescence and room-temperature phosphorescence (RTP) can be useful in the field of optoelectronics. Here we present a general strategy, taking advantage of carbonyl compounds, which have been known to possess efficient intersystem crossing with high triplet state yield, as well as a strongly fluorescent intramolecular charge-transfer (ICT) state, to produce materials with both fluorescence and RTP at the same time, or dual-emission. In the presented model systems, in order to generate a suitable ICT state, Lewis acid binding to aromatic ketone derivatives has been proved to be a viable method. We have selected AlCl3, BCl3, BF3, and GdCl3 as binding Lewis acids, in that they exhibit sufficiently strong binding affinity toward the aromatic ketone derivatives to afford stable complexes and yet do not possess low-lying electronic transitions vs the ligands. We have successfully observed dual-emission from these designed complexes in polymers, which act to suppress competitive thermal decay at room temperature. One of the complexes is particularly interesting as it is dual-emissive in the crystalline state. Single-crystal XRD reveals that the molecule forms multiple hydrogen bonds with its neighbors in crystals, which may significantly enhance the rigidity of the environment.

Enantioselective synthesis of (Z)- and (E)-2-methyl-1,5-anti-pentenediols via an allene hydroboration-double-allylboration reaction sequence

Kinetically controlled hydroboration of allenylboronate 5 followed by double allylboration with the resulting allylborane (Z)-7 gave (Z)-2-methyl-1,5-anti-pentenediols 6 in good yield and high enantioselectivity in the presence of 10% BF3·OEt2 as the catalyst in the second allylboration step. Under thermodynamically controlled isomerization conditions, (Z)-7 can readily isomerize to (E)-7. Double allylboration of representative aldehydes with allylborane (E)-7 gave (E)-2-methyl-1,5-anti-pentenediols 4 in good yield and high enantioselectivity without requiring use of the BF3·OEt2 catalyst. Thus, 2-methyl-1,5-anti-pentenediols with either olefin geometry can be synthesized from the same allenylboronate precursor 5. Furthermore, 1,5-pentenediols 4 and 6 can be easily converted to 1,3,5-triols with excellent diastereoselectivity in one step.

Grob fragmentation of norbornyl α-diketones: a route to α-ketoenols and aromatic compounds

An efficient acid-catalyzed Grob fragmentation of symmetrical and asymmetrically substituted norbornyl α-diketones to the corresponding six-membered α-ketoenols is reported. The regio- and stereochemical outcome of the Grob fragmentation of C2 mono- and disubstituted α-diketones was investigated. A single regioisomer resulting from a favorable half-chair intermediate was normally observed. A departure from the normal course was noticed for C2 disubstituted α-diketones possessing an exo-methyl and an endo-methoxycarbonyl derivative, giving the opposite regioisomers due to initial formation of the hemiketal. The bromo analogues of the C2 disubstituted α-diketones furnished an unusual byproduct, which appears to have been formed through highly reactive fused four-membered bicyclo[2.2.0]hexane intermediates. A plausible mechanistic proposal involving the gem-dihalo intermediate, which in one case was actually isolated as its BF(2)-complex, is outlined. The fragmentation protocol was applied to various norbornyl substrates including bis-α-diketone derivatives. The methodology was successfully utilized for the synthesis of substituted aromatic compounds.