Novel B(Ar')2(Ar'') hetero-tri(aryl)boranes: a systematic study of Lewis acidity

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids

The topological uniqueness of the deltahedra found in the boranes BnHn2− (6≤n≤12)

The deltahedra observed experimentally in the borane anions BnHn2− (6≤n≤12) are the only possible n-vertex deltahedra having only degree 4 and 5 vertices. The existence of an 11-vertex deltahedron having only degree 4 or 5 vertices is topologically impossible in accord with the presence of a degree 6 vertex in the observed structure for B11H112−

Exploring structural and electronic effects in three isomers of tris{bis(trifluoromethyl)phenyl}borane: Towards the combined electrochemical-frustrated Lewis pair activation of H2

Three structural isomers of tris{bis(trifluoromethyl)phenyl}borane have been studied as the acidic com- ponent of frustrated Lewis pairs. While the 3,5-substituted isomer is already known to heterolytically cleave H2 to generate a bridging-hydride; ortho-substituents in the 2,4- and 2,5-isomers quench such reactivity through electron donation into the vacant boron pz orbital and steric blocking of the boron centre; as shown by electrochemical, structural and computational studies. Electrochemical studies of the corresponding borohydrides identify that the two-electron oxidation of terminal-hydrides occurs at more positive potentials than observed for [HB(C6F5)3]−, while the bridging-hydride oxidizes at a higher poten- tial still, comparable to that of free H2

Diastereoselective three-component synthesis of beta-amino carbonyl compounds using diazo compounds, boranes, and acyl imines under catalyst-free conditions

Diazo compounds, boranes, and acyl imines undergo a three-component Mannich condensation reaction under catalyst-free conditions to give the anti β-amino carbonyl compounds in high diastereoselectivity. The reaction tolerates a variety of functional groups, and an asymmetric variant was achieved using the (−)-phenylmenthol as chiral auxiliary in good yield and selectivity. These β-amino carbonyl compounds are valuable intermediates, which can be transformed to many potential bioactive molecules.We gratefully acknowledge Philip N. Moquist for editorial review of the manuscript. Preliminary experiments were performed by Y.L. at Boston University. Completion of the work was accomplished under the direction of G.W. at the University of Science and Technology Beijing, China. S.E.S. and Y.L. gratefully acknowledge the NIH for support (NIGMS R01 GM078240). Y.L., J.Y., and G.W. thank the Innovative Foundation from China National Petroleum Corporation (Grant No. 2012D-5006-0504) for financial support. Y.L. also thanks the Beijing Natural Science Foundation (Grant No. 2144052) and China Postdoctoral Science Foundation (2013M540859) for financial support. (NIGMS R01 GM078240 - NIH; 2012D-5006-0504 - Innovative Foundation from China National Petroleum Corporation; 2144052 - Beijing Natural Science Foundation; 2013M540859 - China Postdoctoral Science Foundation)Published versio

Trialkylborane-Assisted CO_2 Reduction by Late Transition Metal Hydrides

Trialkylborane additives promote reduction of CO_2 to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO_2 to give a formateborane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO_2 reduction. The mechanism likely involves a pre-equilibrium hydride transfer followed by formation of a stabilizing formateborane adduct

Photolysis of Diborane at 1849 Å

The photolysis of diborane at 1849 Å has been studied in a specially constructed, internal‐type mercury‐vapor lamp. The products have been found to be H_2, B_(4)H_(10), B_(5)H_(11), and, at low pressures, a —BH— polymer. Reaction orders at 4°C have been obtained from linear plots of reaction products vs time for a range of diborane pressures from 0.08 to 80 cm, and at two light intensities. Linear relations between products and time existed only at very low conversions (∼1%), which required the development of a low‐temperature separation method for manipulating and analyzing the traces of B_(4)H_(10) and B_(5)H_(11). Because of the reactivity of these compounds, a detailed conditioning procedure was employed for the glass system. A mechanism consistant with the kinetic data and suggested by the kinetic results of thermal and photosensitized decomposition of diborane is postulated: the B_(5)H_(11) is assumed to be formed from a dissociation of B_(2)H_6 into BH_3's, the latter arising from an excited molecule. The B_(4)H_(10) and polymer are assumed to be formed from a dissociation of B_(2)H_6 into B_(2)H_5 and H, followed by radical recombination. There is a significant difference between the kinetics of thermal and photochemical B_(5)H_(11) formation, a result which may be due to the considerable energy excess of the 1849 quantum over that needed for dissociation (∼125‐kcal excess). These kinetic results raise a number of interesting questions, questions which can only be resolved through further investigations of effects due to light intensity, added inert gases, and temperature. The primary quantum yield of the step forming B_(2)H_5 and H is about 10 times higher than that of the one forming BH_3's. A rather rough estimate suggests that the former is of the order of magnitude of unity

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Frustrated Lewis pairs are compounds containing both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and have been shown to be capable of heterolysis of molecular hydrogen, a property that has led to their use in hydrogenation reactions of polarized multiple bonds. Here, we describe a general approach to the hydrogenation of alkynes to cis-alkenes under mild conditions using the unique ansa-aminohydroborane as a catalyst. Our approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies

Role of Li_2B_(12)H_(12) for the Formation and Decomposition of LiBH_4

By in situ X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy, the role of Li_2B_(12)H_(12) for the sorption of LiBH_4 is analyzed. We demonstrate that Li_2B_(12)H_(12) and an amorphous Li_2B_(10)H_(10) phase are formed by the reaction of LiBH_4 with diborane (B_2H_6) at 200 °C. Based on our present results, we propose that the Li -2B - (12)H_(12) formation in the desorption of LiBH_4 can be explained as a result of reaction of diborane and LiBH_4. This reaction of the borohydride with diborane may also be observed for other borohydrides, where B_(12)H_(12) phases are found during decomposition

Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide

Structure-function studies on the role of pendent Lewis acids in the reductive coupling of CO are reported. Cationic rhenium carbonyl complexes containing zero, one, or two phosphinoborane ligands (Ph_2P(CH_2)_nB(C_8H_(14)), n=1-3) react with the nucleophilic hydride [HPt(dmpe)_2]^+ to reduce [M-CO]^+ to M-CHO; this step is relatively insensitive to the Lewis acid, as both pendent (internal) and external boranes of appropriate acid strength can be used. In contrast, whether a second hydride transfer and C-C bond forming steps occur depends strongly on the number of carbon atoms between P and B in the phosphinoborane ligands, as well as the number of pendent acids in the complex: shorter linker chain lengths favor such reductive coupling, whereas longer chains and external boranes are ineffective. A number of different species containing partially reduced CO groups, whose exact structures vary considerably with the nature and number of phosphinoborane ligands, have been crystallographically characterized. The reaction of [(Ph -2P(CH_2)_2B(C_8H_(14)))_2Re(CO)4]^+ with [HPt(dmpe)_2]^+ takes place via a “hydride shuttle” mechanism, in which hydride is transferred from Pt to a pendent borane and thence to CO, rather than by direct hydride attack at CO. Addition of a second hydride in C_6D_5Cl at -40 ºC affords an unusual anionic bis(carbene) complex, which converts to a C-C bonded product on warming. These results support a working model for Lewis acid-assisted reductive coupling of CO, in which B (pendent or external) shuttles hydride from Pt to coordinated CO, followed by formation of an intramolecular B-O bond, which facilitates reductive coupling