Formation of C(sp2)?Boronate Esters by Borylative Cyclization of Alkynes Using BCl3

BCl₃ is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp²) boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2‐carboboration of alkynes is also achieved using BCl₃ to generate trisubstituted vinyl boronate esters.     

BCl3‐Induced Annulative Oxo‐ and Thioboration for the Formation of C3‐Borylated Benzofurans and Benzothiophenes

BCl₃‐induced borylative cyclization of aryl‐alkynes possessing ortho‐EMe (E=S, O) groups represents a simple, metal‐free method for the formation of C3‐borylated benzothiophenes and benzofurans. The dichloro(heteroaryl)borane primary products can be protected to form synthetically ubiquitous pinacol boronate esters or used in situ in Suzuki–Miyaura cross couplings to generate 2,3‐disubstituted heteroarenes from simple alkyne precursors in one pot. In a number of cases alkyne trans‐haloboration occurs alongside, or instead of, borylative cyclization and the factors controlling the reaction outcome are determined.     

La[N(SiMe3)2]3‐Catalyzed Hydroboration of Esters and Other Challenging Unsaturated Groups

The commercially available homoleptic lanthanum amide La[N(SiMe₃)₂]₃ (LaN^TMS) is reported to enable the hydroboration of esters using pinacolborane (HBpin) as the reducing agent. A wide range of substrates including aromatic, aliphatic esters and lactones were applicable to afford corresponding boronic esters in excellent yields under mild and neat conditions with broad functional group compatibility and good chemoselectivity. Furthermore, LaN^TMS is capable to realize the very challenging and rarely reported hydroboration of carbonate esters with low catalyst loading at room temperature. Both cyclic and linear carbonate esters can be easily converted to the corresponding products with satisfactory yields. Besides, the hydroboration of alkynes has been developed by using LaN^TMS as a catalyst.     

Metal promoted asymmetry in the 1,2-diboroethylarene synthesis: diboration versus dihydroboration

Metal catalysed addition of diboranes to vinylarenes produces the desired 1,2-bis(boronate)ester and mono(boronate)esters as by-products. Their relative rate is a sensitive function between the nature of the catalytic system and the electronic effects of the substrate, that influences the mechanistic steps of the catalytic cycle. However, asymmetry is only induced as moderate enantiomeric excess values, providing an enantioface differentiation, between the bis- and mono(boronate)esters. Alternatively, the method based on the catalytic asymmetric dihydroboration/oxidation of alkynes as diphenylacetylene can provide 1,2-diphenyl-1,2-ethanediol (hydrobenzoin) with a selectivity of 68% mainly as the erythro isomer.     

Boron trifluoride facilitated transesterification of dioxaborolanes

The direct transesterification of dioxaborolanes (alkane-1,2-diol based boronate esters) was explored. Using BF₃·OEt₂, alkane-1,2-diol based mono- and bis-boronate esters (i.e., pinacol and ethylene glycol) have been converted quantitatively to either benzene-1,2-diol or alkane-1,3-diol based boronate esters. In the case of pinacol esters, esterification is facilitated by the accompanying pinacol rearrangement, thus shifting the reaction equilibrium.     

Cationic ruthenium complex of the formula [RuCl(2,6-diacetylpyridine)(PPh3)2]BArF and its catalytic activity in the formation of enol esters

A new ruthenium 2,6-diacetylpyridine complex was synthesized and applied in the atom-economic synthesis of enol esters through Markovnikov-directed addition of carboxylic acids to terminal alkynes. The ruthenium complex [RuCl(dap)(PPh₃)₂]⁺BArF^? was synthesized from [RuCl₂(PPh₃)₂] and the corresponding ligand 2,6-diacetylpyridine (dap). The complex was characterized structurally. The new ruthenium complex was utilized under ambient conditions as a catalyst in the Markovnikov addition of carboxylic acids to terminal alkynes to afford the corresponding enol esters in 93% to 52% isolated yields (85?°C, 16?h reaction time, 1?mol% catalyst loading).     

BCl3‐Mediated Ene Reaction of Sulfur Dioxide and Unfunctionalized Alkenes

The first ene reactions of SO₂ and unfunctionalized alkenes are reported. Calculations suggest that the endergonic ene reactions of SO₂ with alkenes can be used to generate β,γ‐unsaturated sulfinyl and sulfonyl compounds. Indeed, in the presence of one equivalent of BCl₃, the unstable sulfinic acid form stable sulfinic acid?BCl₃ complexes that can be reacted in situ with NCS to generate corresponding sulfonyl chlorides, or with a base to generate corresponding sulfinates. The latter can be reacted with electrophiles to generate sulfones, or with silyl chloride to form β,γ‐unsaturated silyl sulfinates. The sulfinic acid?BCl₃ complexes can be reacted with ethers that act as oxygen nucleophiles to produce corresponding sulfinic esters. Thus one‐pot, three‐component synthesis of β,γ‐unsaturated sulfonamides, sulfinyl esters and sulfones have been developed starting from alkenes and sulfur dioxide (reagent and solvent).     

CO2 tea laser-induced photochemical enrichment of boron isotopes

Mixtures of BCl₃ and H₂S are irradiated with 10.55 μm radiation (P(16) line of the 001–100 band of CO₂) from a TEA laser. After several hours of irradiation it is found that the maximum ¹⁰B to ¹¹B ratio of recovered gaseous boron containing material (primarily unreacted BCl₃) is 0.413 ± 0.004. The corresponding ratio of the BCl₃ starting material is 0.242 ± 0.002. The ¹⁰B concentration has therefore been increased from 19.5% to 29.2%. Further, by irradiating similar mixtures with 10.18 μm radiation (R(30) line of the same CO₂ vibrational band) this ratio changes to 0.169 ± 0.002, the ¹⁰B concentration being lowered to 14.4%. All experiments are performed in a small static system and chemical procedures for recovering milligram quantities of BCl₃ selectively enriched in either isotope are described.     

Carbocyclic synthesis via boranes: stereochemical implications of boron annulation

Boron annulation, hydroboration-carbonylation, of substituted 1,5,9-decatrienes was utilized to prepare the cadinane sesquiterpene system. Stereochemical aspects of the triene hydroboration process and the carbonylation reaction have been analyzed through glc separation of boronate esters. Structural assignments for these esters and their oxidation (H₂O₂/NaOH) products were made through high field ¹H-NMR and other physical and spectral considerations.     

Plasma etching of III–V semiconductors in BCl3 chemistries: Part I: GaAs and related compounds

BCl₃/Ar discharges provide rapid, smooth pattern transfer in GaAs, AlGaAs, GaP, and GaSb over a wide range of plasma conditions. At high BCl₃-to-Ar ratio there is significant surface roughening on GaSb, which is correlated with the presence of B- and Cl-containing residues detected by Auger electron spectroscopy. BCl₃/N₂ discharges provide similar etch rates to BCl₃/Ar, but when used with photoresist masks lead to rough morphologies on the semiconductor materials due to enhanced dissociation and redeposition of the resist. Etch rates with electron cyclotron resonance discharges are up to two orders of magnitude higher than for rf-only conditions.     

Tris(2‐methylphenyl)phosphonium tetrachloroborate

The title compound, C₂₁H₂₂P⁺·BCl₄^?, is the first structurally characterized example of the [HP(o‐tolyl)₃]⁺ cation, presented here with BCl₄^? as the counter‐ion. The cation has a near‐tetrahedral P atom and the BCl₄^? anion is near‐tetrahedral at boron. There are no unusually short cation–anion contacts.     

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.     

Synthesis of enol and vinyl esters catalyzed by an iridium complex

Enol and vinyl esters were successfully synthesized by the use of an iridium complex as a catalyst. The reaction of carboxylic acids with terminal alkynes in the presence of catalytic amounts of [Ir(cod)Cl]₂ and Na₂CO₃ gave the corresponding 1-alkenyl esters. The addition of carboxylic acids to alkynes principally took place in the Markovnikov fashion. In addition, by the use of an Ir complex combined with NaOAc various vinyl esters were prepared through the transvinylation between carboxylic acids and vinyl acetate.     

Heterolysis of H2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl3: Synthesis,Characterization, and Mechanism

We report that 2,6‐lutidine?trichloroborane (Lut?BCl₃) reacts with H₂ in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl₂. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl₃ was calculated to react with H₂ and form the ion pair, [LutH⁺][HBCl₃^?], with a barrier of ΔH^≠=24.7 kcal mol^?1 (ΔG^≠=29.8 kcal mol^?1). Metathesis with a second molecule of Lut?BCl₃ produced Lut?BHCl₂ and [LutH⁺][BCl₄^?]. The overall reaction is exothermic by 6.0 kcal mol^?1 (Δ_rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl₂⁺) and the four‐membered boracycle [(CH₂{NC₅H₃Me})BCl₂]. Barriers for addition of H₂ across the Lut/LutBCl₂⁺ pair and the boracycle B?C bond are substantially higher (ΔG^≠=42.1 and 49.4 kcal mol^?1, respectively), such that these pathways are excluded. The barrier for addition of H₂ to the boracycle B?N bond is comparable (ΔH^≠=28.5 and ΔG^≠=32 kcal mol^?1). Conversion of the intermediate 2‐(BHCl₂CH₂)‐6‐Me(C₅H₃NH) to Lut?BHCl₂ may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl₃. Intramolecular protodeboronation, which could form Lut?BHCl₂ directly, is prohibited by a high barrier (ΔH^≠=52, ΔG^≠=51 kcal mol^?1).     

Copper-catalysed photoinduced decarboxylative alkynylation: a combined experimental and computational study

Redox-active esters (RAEs) as alkyl radical precursors have demonstrated great advantages for C–C bond formation. A decarboxylative cross-coupling method is described to afford substituted alkynes from various carboxylic acids using copper catalysts CuCl and Cu(acac)₂. The photoexcitation of copper acetylides with electron-rich NEt₃ as a ligand provides a general strategy to generate a range of alkyl radicals from RAEs of carboxylic acids, which can be readily coupled with a variety of aromatic alkynes. The scope of this cross-coupling reaction can be further expanded to aliphatic alkynes and alkynyl silanes using a catalytic amount of preformed copper-phenylacetylide. In addition, DFT calculations revealed the favorable reaction pathway and that the bidentate acetylacetonate ligand of the copper intermediate plays an important role in inhibiting the homo-coupling of the alkyne.

Redox-active esters (RAEs) as alkyl radical precursors have demonstrated great advantages for Cu-catalysed C–C bond formation.     

Studies of unoccupied orbitals of BF3 and BCL3 by electron transmission spectroscopy and multiple scattering Xα calculations

Electron transmission spectroscopy (ETS ) and bound-state and continuum multiple-scattering Xα (MS -Xα) calculations are employed to characterize the unoccupied a′₂, a′₁ and e′ orbitals of BF₃ and BCl₃. The a′₂ orbital of BF₃, which produces a peak about 7 eV below threshold in the x-ray absorption spectrum (XAS ), generates a scattering resonance at 3.5 eV in ETS. Similarly, the e′ orbital that lies about 2 eV above threshold in XAS occurs about 13?16 eV above threshold in ETS . Dissociation of F^? from BF₃ due to electron attachment is attributed to a core-excited shape resonance involving an e″ → a″₂ excitation and electron capture into the a′₂ orbital. In BCl₃ all the unoccupied orbitals lie at lower energy than in BF₃ and are closely spaced, making definitive spectral assignments difficult. Both Hartree-Fock (HF ) and MS -Xα methods apparently underestimate the stability of the unoccupied e′ orbital of BCl₃. Vibronic coupling due to out-of-plane bending may significantly affect the spectral intensities. Feshbach resonances are observed for BCl₃ at energies close to those observed in the vacum-UV absorption spectrum. The lower energies of the unoccupied orbitals of BCl₃ are consistent with their stronger bonding to nucleophiles.     

Cationic polymerization with boron halides. VI. Polymerization of styrene with BF3, BCl3, and BBr3

The polymerization of styrene with BF₃, BCI₃, and BBr₃ coinitiators and CH₂Cl₂ solvent has been investigated. The effects of temperature, monomer concentration, and the nature of the boron halide on molecular weights, molecular weight distribution, and conversion were determined. Molecular weights were found to decrease in the order BCl₃ >BF₃ >BBr₃. This sequence was discussed in terms of system ionicity and counterion stability. The overall energies of activation for polymerization (ΔE) were ?1.6 ± 0.3, ?1.9 ± 0.8, and ?0.9 ± 0.5 kcal/mole for BF₃, BCI₃, and BBr₃, respectively, which indicated similar overall polymerization mechanisms in the range of ?20 to ?80°C. The predominant molecular-weight-governing event in polymerizations with BCl₃, and BBr₃ was chain transfer to monomer, whereas with BF₃ chain transfer and termination were nearly equal. Chain termination in BCl₃-coinitiated polymerizations involves chlorination of the growing polystyryl cation by BCl₃OH^? and leads to benzylic chlorine termini.     

The vacuum-ultraviolet absorption spectra of boron-trihalides

The vacuum-ultraviolet spectra from 200 to 120 nm of BF₃, BCl₃ and BBr₃ are reported. Tentative assignments are made. The lowest frequency band of BCl₃ and BBr₃ can be assigned to a valence-shell transition of the π^* ← π type of the σ^* ← σ type. The strongest bands found at higher frequencies in the spectrum of BBr₃ can be assigned to transitions to p-type Rydberg orbitals related to the three lowest (close lying) ionization potentials. Some weaker bands are likely to result from spin-orbit coupling.     

Electron impact and chemical ionization mass spectrometry of cyclic boronate derivatives of sphingosines and related compounds

Electron impact and isobutane chemical ionization mass spectrometric data are reported for cyclic boronate esters of sphinganine and 4-sphingenine and of the related N-ethyl, N-dimethylaminomethylene and acetone Schiff base compounds. Fragmentation of the 6-membered boronate ring is promoted by the presence of the 2-amino or 2-imino substituent and by Δ⁴ unsaturation. Evidence is presented for a transannular interaction stabilizing the ion formed by loss of the alkyl or aryl group, attached to the boron atom, in the case of the N-dimethylaminomethylene derivatives. An NH₂ migration is postulated to occur in 4-sphingenine boronates under electron impact. Mass spectrometric data are also reported for 4D-hydroxysphinganine bis-boronates.     

Regio‐ and Stereoselective Chlorocyanation of Alkynes

A variety of terminal and internal alkynes were converted regio‐ and stereoselectively into (Z )‐3‐chloroacrylonitriles by treatment with BCl₃ in the presence of stoichiometric amounts of imidazolium thiocyanates. These products could be readily functionalized to provide useful building blocks, thus demonstrating the synthetic value of the method. Preliminary mechanistic studies suggest initial activation of the cationic thiocyanate by the Lewis acid, followed by electrophilic attack of the alkyne. The syn addition of a chloride to the vinyl cation intermediate and final elimination of the thiourea unit afford the desired chloroacrylonitriles.