3378-72-1Relevant articles and documents
Correlating electronic and catalytic properties of frustrated Lewis pairs for imine hydrogenation
Dorkó, éva,Kótai, Bianka,F?ldes, Tamás,Gy?m?re, ádám,Pápai, Imre,Soós, Tibor
, p. 258 - 262 (2017)
Combined computational and experimental work to probe Lewis acidity of some boranes to be used in FLP hydrogenation. Gutmann-Beckett method of estimating Lewis acidity has limited capacity for sterically congested boranes. Calculated hydride affinity is a more appropriate tool for gauging Lewis acidity and correlate their FLP hydrogenation utility.
A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis
Farrell, Jeffrey M.,Posaratnanathan, Roy T.,Stephan, Douglas W.
, p. 2010 - 2015 (2015)
This manuscript probes the steric and electronic attributes that lead to "frustrated Lewis pair" (FLP)-type catalysis of imine hydrogenation by borenium ions. Hydride abstraction from (ItBu)HB(C6F5)2 2 prompts intramolecular C-H bond activation to give (CHN)2(tBu) (CMe2CH2)CB(C6F5)2 3, defining an upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene-borane (NHC-borane) adducts ((R′CNR)2C)(HBC8H14) (R′ = H, R = dipp 4a, Mes 5a, Me 8a; R = Me R′ = Me 9a, Cl, 10a) and ((HC)2(NMe)(NR)C)(HBC8H14) (R = tBu, 6a, Ph 7a) are prepared and converted to corresponding borenium salts. These species are evaluated as catalysts for metal-free imine hydrogenation at room temperature. Systematic tuning of the carbene donor for the hydrogenation of archetypal substrate N-benzylidene-tert-butylamine achieves the highest reported turn-over frequencies for FLP-catalyzed hydrogenation at amongst the lowest reported catalyst loadings. The most active NHC-borenium catalyst of this series, derived from 10a, is readily isolable, crystallographically characterized and shown to be effective in the hydrogenation catalysis of functional group-containing imines and N-heterocycles.
Synthesis of a diboryl-N-heterocycle and its conversion to a bidentate cationic Lewis acid
Farrell, Jeffrey M.,Stephan, Douglas W.
, p. 14322 - 14325 (2015)
Sequential reaction of 2-lithio-1-methylimidazole with 9-borabicyclo[3.3.1]nonane (9-BBN) dimer and 9-Cl-9-BBN yields diboryl-N-heterocycle C4H5N2(H)(BC8H14)2 (1). Reaction of 1 with I2 results in the net substitution of chelated hydride for a singly boron-bound iodide to produce C4H5N2(I)(BC8H14)2 (2). Conversely, reaction of 1 with [Ph3C][B(C6F5)4] results in the formation of the bidentate cationic Lewis acid [(C4H5N2)(BC8H14)2][B(C6F5)4] (3). Compound 3 catalyzes the hydrogenation of N-benzylidene-tert-butylamine at room-temperature.
A rare olefin 1,1-carboboration reaction opens a synthetic pathway to an unusually structured frustrated Lewis pair
Chen, Chaohuang,Daniliuc, Constantin G.,Mück-Lichtenfeld, Christian,Kehr, Gerald,Erker, Gerhard
, p. 8806 - 8809 (2020)
(2,6-Dimesitylphenyl)P(vinyl)25d reacts with HB(C6F5)2 in a sequence involving a rare example of a 1,1-carboboration of an olefin to give the borylated tetrahydrophosphole derivative 6d. Compound 6d is an active frustrated Lewis pair that splits dihydrogen under mild conditions and serves as a metal-free hydrogenation catalyst. It also adds to carbon dioxide. Compound 6d serves as an intermediate in the HB(C6F5)2 catalyzed aryl(divinyl)phosphane (5d) to dihydrophosphole conversion. This journal is
Reaction of carbon oxides with an ethylene-bridged PH/B Lewis pair
Sun, Qiu,Daniliuc, Constantin G.,Kehr, Gerald,Erker, Gerhard
, p. 3523 - 3528 (2021)
The reaction of 2,4,6-tri(tert-butyl)phenyl vinyl phosphane with Piers’ borane [HB(C6F5)2] gave the ethylene-bridged PH/B frustrated Lewis pair (FLP) system. It is a monomer at high temperature (>323 K), but exists as an associated 12-membered macrocyclic trimer below 273 K. The PH/B FLP splits dihydrogen and serves as a metal-free hydrogenation catalyst. It adds carbon dioxide. It serves as a PH/B template for the reduction of carbon monoxide by the HB(C6F5)2borane to the formyl stage. The resulting six membered P/B/O containing heterocycle is opened upon treatment with pyridine and it reacts with benzaldehyde in a boron mediated Claisen-Tishchenko reaction.
A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C-X (X=Cl, Br) Bond Activation
Samigullin, Kamil,Georg, Isabelle,Bolte, Michael,Lerner, Hans-Wolfram,Wagner, Matthias
, p. 3478 - 3484 (2016)
The geminal frustrated Lewis pair tBu2PCH2B(Fxyl)2 (1; Fxyl=3,5-(CF3)2C6H3) is accessible in 65 % yield from tBu2PCH2Li and (Fxyl)2BF. According to NMR spectroscopy and X-ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P B distance of 2.900(5) ? and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H2, EtMe2SiH, CO2/CS2, Ph2CO, and H3CCN. Terminal alkynes react with heterolysis of the C-H bond. Haloboranes give cyclic adducts with strong P-BX3 and weak R3B-X bonds. Unprecedented transformations leading to zwitterionic XP/BCX3 adducts occur on treatment of 1 with CCl4 or CBr4 in Et2O. In less polar solvents (C6H6, n-pentane), XP/BCX3 adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph2CO with EtMe2SiH.
Iridium(III) Complexes Bearing Chelating Bis-NHC Ligands and Their Application in the Catalytic Reduction of Imines
Aznarez, Francisco,Iglesias, Manuel,Hepp, Alexander,Veit, Benjamin,Sanz Miguel, Pablo J.,Oro, Luis A.,Jin, Guo-Xin,Hahn, F. Ekkehardt
, p. 4598 - 4603 (2016)
The IrIIIcomplexes 4 and 5 bearing bis-NHC ligands (NHC = N-heterocyclic carbene) composed of one classical NR,NR NHC and one N,NR NHC donor were prepared by the reaction of the azolium/azole compounds 2I and 3Br, respectively, with [{Cp*IrCl(μ-Cl)}2] (Cp=η5-C5Me5) in the presence of NaOAc as base. Most likely, the salts 2I and 3Br were first selectively deprotonated at the C2 position of the disubstituted (NR,NR) diazaheterocycle to generate an NHC donor, which then coordinated to the IrIIIcenter. Subsequently, NaOAc promoted C–H bond activation at the pendant imidazole moiety of the intermediate IrIIImono-NHC complexes led to the formation of the six-membered iridacycles 4 and 5, which bear a chelating, doubly C-metalated C(NHC)^C(NHC′) bis-NHC ligand. The IrIIIcomplexes 4 and 5 were tested as precatalysts for the reduction of imines with molecular hydrogen. Moderate to good activity was observed at a catalyst loading of 5 mol-% and an H2pressure of 3 bar in MeOH.
Autoinduced catalysis and inverse equilibrium isotope effect in the frustrated Lewis pair catalyzed hydrogenation of imines
Tussing, Sebastian,Greb, Lutz,Tamke, Sergej,Schirmer, Birgitta,Muhle-Goll, Claudia,Luy, Burkhard,Paradies, Jan
, p. 8056 - 8059 (2015)
The frustrated Lewis pair (FLP)-catalyzed hydrogenation and deuteration of N-benzylidene-tert-butylamine (2) was kinetically investigated by using the three boranes B(C6F5)3 (1), B(2,4,6-F3-C6H2)3 (4), and B(2,6-F2-C6H3)3 (5) and the free activation energies for the H2 activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol-1) for the H2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect. The FLP-catalyzed reaction of N-benzylidene-tert-butylamine with H2 and D2 was kinetically investigated. The free activation energy for the H2 activation by the FLP consisting of an imine and a less Lewis-acidic borane is 2 kcal mol-1 higher than for B(C6F5)3 , resulting in autoinduced catalysis. For the first time, the free activation enthalpies for the H2 activation by FLPs were experimentally determined and an inverse kinetic isotope effect was observed.
Influence of the lewis acidity of gallium atoms on the reactivity of a frustrated lewis pair: Experimental and theoretical studies
Possart, Josephine,Uhl, Werner
, p. 1314 - 1323 (2018)
The reactivity of the Ga/P-based frustrated Lewis pair (FLP) Mes2P-C[=C(H)-Ph]-GatBu2 (3) is influenced by the relatively weak Lewis acidity of its Ga atom and differs significantly from that of the analogous Al compound 1. The adduct of 3 with CO2 was only detectable at low temperature by NMR spectroscopy. Benzaldehyde was coordinated only via a Ga-O bond; the P atom was not involved. In contrast, a relatively persistent adduct was formed with soft CS2 to yield a five-membered GaCPCS heterocycle. Dehydrocoupling with H3B←NHMe2 afforded the dimeric amidoborane (H2B-NMe2)2, while an adduct with a GaCPBN heterocycle was isolated with the sterically less shielded ammonia-borane H3B←NH3. The latter product was unstable in solution and decomposed by H2 elimination and formation of oligomeric BN compounds. Small quantities of 3 catalyzed hydrogen transfer from H3B←NH3 to an imine. The Lewis acidities of the Al/P- and Ga/P-based FLPs were examined by experiments (Gutmann-Beckett method) and by calculation of the fluoride ion affinity (including the B and In analogues). The Al compound is the strongest Lewis acid; the Ga FLP is significantly weaker but is a stronger F- acceptor in comparison to the unknown analogues of B and In. These results reflect the different reactivities of these FLPs and may help to develop FLPs with finely adjusted properties.
Selective Metal-free HB(C6F5)2Catalyzed Allene Cyclotrimerization: Formation of 1,3,5-Trimethylenecyclohexane and Its Tris-hydroboration Product
Tao, Xin,Kehr, Gerald,Daniliuc, Constantin G.,Erker, Gerhard
, p. 1376 - 1380 (2017)
Allene is cyclotrimerized under metal-free conditions with the borane HB(C6F5)2catalyst to selectively give 1,3,5-trimethylenecyclohexane (3 a). Three-fold hydroboration of the 1,3,5-cyclotrimer with Piers’ borane gives the all-cis 1,3,5-CH2B(C6F5)2substituted cyclohexane product 14.
