Group 13-derived radicals from α-diimines via hydro- and carboalumination reactions.

The mechanochemical synthesis of tertiary and secondary alanes AlR3 (R = Np 1 or Mes 2; HAlR2 R = Np 3 or Mes 4) is described. These species are reacted with several α-diimines to give a series of aluminium-derived radicals of the form [(diimine)AlR2]˙ (6-11). EPR and several crystallographic studies are reported. These species are thought to form via hydro- or carboalumination and subsequent elimination reactions. This view is supported by the structural data for minor products C12H7(NHDipp)(NDipp)AliBu25 and C13H8(C(iBu)[double bond, length as m-dash]N(m-Xy)(NH(m-Xy)))AliBu212. In addition, the characterization of (C6F5)2B(OC(C6F5)OC12H8) indicates that such a carboboration pathway also provides access to related boron-derived radicals.

trans-Hydroboration of Propargyl Alcohol Derivatives and Related Substrates.

The hydroboration of internal alkynes with pinacolborane as the reagent catalyzed by [Cp*RuCl]4 results in good to excellent levels of regio- as well as stereoselectivity, provided that the triple bond bears one linear and one singly-branched substituent. In such cases, the reaction follows an unusual trans-addition mode and places the boron entity distal to the branching point. The resulting alkenyl boronates, which are difficult to make otherwise, can be engaged in numerous enabling downstream processes.

Interception of intermediates in phosphine oxidation by mesityl nitrile-N-oxide using frustrated Lewis pairs.

Phosphine oxidation by MesCNO is rapid; however, an FLP strategy intercepts the 1,3 addition products including [MesC(R3P)NOB(C6F5)3] (R = Ph 1, p-tol 2), [MesC(Mes2PH)NOB(C6F5)3] 3 (MesC(NOB(C6F5)3)Ph2P)2(CH2)n (n = 2: 4, 3: 5) and [MesC(Ph3P)NOB(C6F4H)3] 6. These species are shown to react with tBuOK or [Bu4N]F permitting the oxidation to proceed via a process involving borane dissociation. Similarly, the equilibrium established by 1 with B(C6F4H)3 and 6 with B(C6F5)3 provides experimental support for the "Cummins mechanism" for these phosphine oxidations.

FLP reduction and hydroboration of phenanthrene o-iminoquinones and α-diimines.

Redox active, or non-innocent, ligands containing O or N heteroatoms are frequently used in transition metal complexes, imparting unique catalytic properties, but have seen comparatively limited use in the chemistry of group 13 elements. In this article we report the frustrated Lewis pair (FLP) hydrogenation and hydroboration of an N-aryl-phenanthrene-o-iminoquinone and two N,N'-diaryl-phenanthrene α-diimines. These reactions exploit B(C6F5)3/H2, HB(C6F5)2 and H2BC6F5·SMe2 to give a series of derivatives including 1,3,2-oxaza- and diazaboroles and borocyclic radicals. The reaction pathways leading to these products are outlined and supported by DFT-calculations and experimental insight. The modular and unusual synthetic strategies described herein give access to new boroles as well as air-stable boron-containing radicals, thus extending the chemistry of redox active ligands in main group systems.

Electron spin relaxation of a boron-containing heterocyclic radical.

Preparation of the stable boron-containing heterocyclic phenanthrenedione radical, (C6F5)2B(O2C14H8), by frustrated Lewis pair chemistry has been reported recently. Electron paramagnetic resonance measurements of this radical were made at X-band in toluene:dichloromethane (9:1) from 10 to 293K, in toluene from 180 to 293K and at Q-band at 80K. In well-deoxygenated 0.1mM toluene solution at room temperature hyperfine splittings from 11B, four pairs of 1H, and 5 pairs of 19F contribute to an EPR spectrum with many resolved lines. Observed hyperfine couplings were assigned based on DFT calculations and account for all of the fluorines and protons in the molecule. Rigid lattice g values are gx=2.0053, gy=2.0044, and gz=2.0028. Near the melting point of the solvent 1/Tm is enhanced due to motional averaging of g and A anisotropy. Increasing motion above the melting point enhances 1/T1 due to contributions from tumbling-dependent processes. The overall temperature dependence of 1/T1 from 10 to 293K was modeled with the sum of contributions of a process that is linear in T, a Raman process, spin rotation, and modulation of g anisotropy by molecular tumbling. The EPR measurements are consistent with the description of this compound as a substituted aromatic radical, with relatively small spin density on the boron.

Reactions of Boron-Derived Radicals with Nucleophiles.

Reactions of phenanthrenedione- and pyrenedione-derived borocyclic radicals, CnH8O2B(C6F5)2• (n = 14 (1), 16 (3)), with a variety of nucleophiles have been studied. Reaction of 1 with P(t-Bu)3 affords the zwitterion 3-(t-Bu)3PC14H7O2B(C6F5)2 (5) in addition to the salt [HP(t-Bu)3][C14H8O2B(C6F5)2] (6). In contrast, the reaction of 1 with PPh3 proceeds to give two regioisomeric zwitterions, 1-(Ph3P)C14H7O2B(C6F5)2 (7a) and 3-(Ph3P)C14H7O2B(C6F5)2 (7b), as well as the related boronic ester C14H8O2B(C6F5) (2). In a similar fashion, 3 reacted with PPh3 to give 3-(Ph3P)C16H7O2B(C6F5)2 (8a), 1-(Ph3P)C16H7O2B(C6F5)2 (8b), and boronic ester C16H8O2B(C6F5) (4). Reactions of secondary phosphines Ph2PH and tBu2PH with 3 yield 3-(R2PH)C16H7O2B(C6F5)2 (R = Ph (9), t-Bu (10)). The reaction of 1 with N-heterocyclic carbene IMes afforded 3-(IMes)C14H7O2B(C6F5)2 (11) and [IMesH][C14H8O2B(C6F5)2] (12), while the reactions with quinuclidine and DMAP afforded the species 3-(C7H13N)C14H7O2B(C6F5)2 (13) and [H(NC7H13)2][C14H8O2B(C6F5)2] (14), and the salt [9,10-(DMAP)2C14H8O2B(C6F5)2][C14H8O2B(C6F5)2] (15), respectively. These products have been fully characterized, and the mechanism for the formation of these products is considered in the light of DFT calculations.

Hydroboration of Phosphaalkynes by HB(C6 F5 )2.

The hydroboration of phosphaalkynes with Piers' borane (HB(C6 F5 )2 ) generated unusual phosphaalkenylboranes [RCH=PB(C6 F5 )2 ]2 that persisted as dimers in both solution and the solid state. These P2 B2 heterocycles underwent ring opening when subjected to nucleophiles, such as pyridine and tert-butylisocyanide, to yield monomeric phosphaalkenylborane adducts RCH=PB(C6 F5 )2 (L). DFT calculations were performed to probe the nature of the interaction of phosphaalkynes with boranes.

Stable Borocyclic Radicals via Frustrated Lewis Pair Hydrogenations.

The synthesis and isolation of stable main group radicals remains an ongoing challenge. Here we report the application of frustrated Lewis pair chemistry to the synthesis of boron-containing radicals. H2 activation with polyaromatic diones and B(C6F5)3 leads to radical formation in good yields. These radicals are robust; they do not decompose on silica gel or react with O2 and are stable at 35 °C under N2 indefinitely. The mechanism of formation is explored experimentally, with support from DFT calculations. EPR and UV/vis spectroscopy as well as cyclic voltammetry data are provided, and the radicals are shown to react with cobaltocenes in one-electron chemical reductions to their corresponding borate anions.

B(C6F5)3 mediated arene hydrogenation/transannulation of para-methoxyanilines.

The stoichiometric reaction of para-methoxyanilines and B(C6F5)3 under H2 results in reduction of the N-bound phenyl ring(s), and subsequent transannular ring closure with elimination of methanol, affording the respective 7-azabicyclo[2.2.1]heptane derivatives.

Rhodium(I)-Catalyzed Intermolecular Hydroacylation of α-Keto Amides and Isatins with Non-Chelating Aldehydes.

The application of the bidentate, electron-rich bisphosphine ligand, 1,3-bis(dicyclohexyl)phosphine-propane (dcpp), in rhodium(I)-catalyzed intermolecular ketone hydroacylation is herein described. Isatins and α-keto amides are shown to undergo hydroacylation with a variety of non-chelating linear and branched aliphatic aldehydes. Also reported is the synthesis of new bidentate chiral phosphine ligands, and their application in hydroacylation is discussed.

Optical and electronic properties of air-stable organoboron compounds with strongly electron-accepting bis(fluoromesityl)boryl groups.

Three compounds with phenyl (1), 4-tert-butylphenyl (2) and 4-N,N-diphenylaminophenyl (3) groups attached to bis(fluoromesityl)boryl ((FMes)2B) through B-C bonds have been prepared. The restricted rotation about the B-C bonds of boron-bonded aryl rings in solution has been studied by variable-temperature 19F NMR spectroscopy, and through-space F-F coupling has been observed for 3 at low temperature. Steric congestion inhibits binding of 1 by Lewis bases DABCO and tBu3P and the activation of H2 in their presence. Photophysical and electrochemical studies have been carried out on 2, 3, and an analogue of 3 containing a bis(mesityl)boryl ((Mes)2B) group, namely 4. Both 2 and 3 show bright emission in nonpolar solvents and in the solid-state, very strong electron-accepting ability as measured by cyclic voltammetry, and good air-stability. In addition, 2 displayed unusually long-lived emission (τ = 2.47 s) in 2-MeTHF at 77 K. The much stronger acceptor strength of (FMes)2B than (Mes)2B leads to significantly red-shifted emission in solution and the solid state, stronger emission solvatochromism, and significantly lower reduction potentials. Theoretical calculations confirm that 2 and 3 tend to form highly twisted excited states with good conjugation between one FMes group and the boron atom, which correlate well with their blue-shifted solid-state emissions and low kr values in solution.

Frustrated Lewis pair activation of an N-sulfinylamine: a source of sulfur monoxide.

Inter- and intramolecular P/B frustrated Lewis pairs are shown to react with an N-sulfinylamine to form PNSOB linakages. These species can be regarded as phosphinimine-borane-stabilized sulfur monoxide complexes, and indeed these species act as sources of SO, effecting the oxidation of PPh3 and delivering SO to [RhCl(PPh3)3] and an N-heterocyclic carbene.

Stoichiometric reductions of alkyl-substituted ketones and aldehydes to borinic esters.

A series of alkyl-substituted ketones are shown to activate hydrogen in the presence of B(C6F5)3, affording the corresponding borinic esters RR'CHOB(C6F5)2. The mechanism is shown to proceed via H2 activation, hydride delivery and protonation of a C6F5 group. The aliphatic aldehyde Et2CHCHO reacts with B(C6F5)3 or BPh3 to give boron enolates Et2C[double bond, length as m-dash]CH(OBAr2) (Ar = C6F5, Ph). These latter species are amenable to FLP-catalyzed reduction to the corresponding borinic esters.

Hydrogen activation by an aromatic triphosphabenzene.

Aromatic hydrogenation is a challenging transformation typically requiring alkali or transition metal reagents and/or harsh conditions to facilitate the process. In sharp contrast, the aromatic heterocycle 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene is shown to be reduced under 4 atm of H2 to give [3.1.0]bicylo reduction products, with the structure of the major isomer being confirmed by X-ray crystallography. NMR studies show this reaction proceeds via a reversible 1,4-H2 addition to generate an intermediate species, which undergoes an irreversible suprafacial hydride shift concurrent with P-P bond formation to give the isolated products. Further, para-hydrogen experiments confirmed the addition of H2 to triphosphabenzene is a bimolecular process. Density functional theory (DFT) calculations show that facile distortion of the planar triphosphabenzene toward a boat-conformation provides a suprafacial combination of vacant acceptor and donor orbitals that permits this direct and uncatalyzed reduction of the aromatic molecule.

An improved method for the synthesis of F-BODIPYs from dipyrrins and bis(dipyrrin)s.

An improved methodology for the synthesis of F-BODIPYs from dipyrrins and bis(dipyrrin)s is reported. This strategy employs lithium salts of dipyrrins as intermediates that are then treated with only 1 equiv of boron trifluoride diethyletherate to obtain the corresponding F-BODIPYs. This scalable route to F-BODIPYs renders high yields with a facile purification process involving merely filtration of the reaction mixture through Celite in many cases.

Palladium-catalyzed synthesis of indoles via ammonia cross-coupling-alkyne cyclization.

The synthesis of indoles via the metal-catalyzed cross-coupling of ammonia is reported for the first time; the developed protocol also allows for the unprecedented use of methylamine or hydrazine as coupling partners. These Pd/Josiphos-catalyzed reactions proceed under relatively mild conditions for a range of 2-alkynylbromoarenes.

Comprehensive chemical characterization of complexes involving lead-amino acid interactions.

Complexes of lead with L-phenylalanine, L-isoleucine, L-valine, or L-arginine have been isolated from reaction mixtures containing lead nitrate and the respective amino acid in acidic aqueous solution. The compounds have been comprehensively characterized using X-ray crystallography, solid state NMR spectroscopy and solution state NMR spectroscopy, IR and Raman spectroscopies, and electrospray ionization mass-spectrometry. The solid state structures of lead-phenylalanine, lead-valine, and lead-valine-isoleucine complexes show a lead center coordinated by two amino acid ligands, while the lead-arginine complex is a cluster involving two lead centers and three arginine molecules. The structural, spectroscopic, and spectrometric characterization of the complexes provides a basis to establish a fundamental understanding of heavy metal-amino acid interactions.