Borenium ion catalyzed hydroboration of alkenes with N-heterocyclic carbene-boranes.
ABSTRACT: Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-1-butene with the N-heterocyclic carbene (NHC)-derived borane 2 and catalytic HNTf(2) (Tf = trifluoromethanesulfonyl (CF(3)SO(2))) effects hydroboration at room temperature. With 3-hexene, surprisingly facile migration of the boron atom from C(3) of the hexyl group to C(2) was observed over a time scale of minutes to hours. Oxidative workup gave a mixture of alcohols containing 2-hexanol as the major product. A similar preference for the C(2) alcohol was observed after oxidative workup of the 3-octene and 1-cyclohexyl-1-butene hydroborations. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations, and the C(2) selective migrations are attributed to the four-center interconversion of borenium cations with cationic NHC-borane-olefin ?-complexes.
Project description: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)22 prompts intramolecular C-H bond activation to give (CHN)2(tBu) (CMe2CH2)CB(C6F5)23, 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.
Project description:The <i>trans</i>-hydroboration of terminal alkynes mediated by borenium cations [NHC(9-BBN)]<sup>+</sup> (NHC = N-heterocyclic carbene, 9-BBN = 9-borabicyclo(3.3.1)nonane) exclusively affords <i>Z</i>-vinylboranes. NHCs and chelating dialkyl substituents on the borenium cation and "non"-basic anions were essential to preclude alternative reactions including dehydroboration. Deuterium labelling studies indicate the mechanism involves addition of the boron electrophile to the alkyne and transfer of hydride to the opposite face of the activated alkyne. <i>trans</i>-Hydroboration proceeds with only catalytic amounts of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> or [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] to activate the (NHC)9-BBN(H) precursor with the borenium regenerated in the hydride transfer step. The NHC can be removed from the <i>trans</i>-hydroborated products by the addition of Et<sub>2</sub>O-BF<sub>3</sub> providing access to vinylBBN species effective for Suzuki-Miyaura couplings to generate <i>Z</i>-alkenes. Combinations of catalytic B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> and stoichiometric [HB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>-</sup> also lead to <i>trans</i>-hydroboration of terminal alkynes to form <i>Z</i>-isomers of [arylCH[double bond, length as m-dash]CHB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>-</sup>.
Project description:The first examples of borylation under conditions of borenium ion generation from hydrogen-bridged boron cations are described. The observable H-bridged cations are generated by hydride abstraction from N,N-dimethylamine boranes Ar(CH(2))(n)NMe(2)BH(3) using Ph(3)C(+) (C(6)F(5))(4)B(-) (TrTPFPB) as the hydride acceptor. In the presence of excess TrTPFPB, the hydrogen-bridged cations undergo internal borylation to afford cyclic amine borane derivatives with n = 1-3. The products are formed as the corresponding cyclic borenium ions according to reductive quenching experiments and (11)B and (1)H NMR spectroscopy in the case with Ar = C(6)H(5) and n = 1. The same cyclic borenium cation is also formed from the substrate with Ar = o-C(6)H(4)SiMe(3) via desilylation, but the analogous system with Ar = o-C(6)H(4)CMe(3) affords a unique cyclization product that retains the tert-butyl substituent. An ortho-deuterated substrate undergoes cyclization with a product-determining isotope effect of k(H)/k(D) 2.8. Potential cationic intermediates have been evaluated using B3LYP/6-31G* methods. The computations indicate that internal borylation from 14a occurs via a C-H insertion transition state that is accessible from either the borenium pi complex or from a Wheland intermediate having nearly identical energy. The Ar = o-C(6)H(4)SiMe(3) example strongly favors formation of the Wheland intermediate, and desilylation occurs via internal SiMe(3) migration from carbon to one of the hydrides attached to boron.
Project description:Various oxirane monomers including alkyl ether or allyl-substituted ones such as 1-butene oxide, 1-hexene oxide, 1-octene oxide, butyl glycidyl ether, allyl glycidyl ether, and 2-ethylhexyl glycidyl ether were anionically copolymerized with CO2 into polycarbonates using onium salts as initiator in the presence of triethylborane. All copolymerizations exhibited a "living" character, and the monomer consumption was monitored by in situ Fourier-transform infrared spectroscopy. The various polycarbonate samples obtained were characterized by 1H NMR, GPC, and differential scanning calorimetry. In a second step, all-polycarbonate triblock copolymers demonstrating elastomeric behavior were obtained in one pot by sequential copolymerization of CO2 with two different epoxides, using a difunctional initiator. 1-Octene oxide was first copolymerized with CO2 to form the central soft poly(octene carbonate) block which was flanked by two external rigid poly(cyclohexene carbonate) blocks obtained through subsequent copolymerization of cyclohexene oxide with CO2. Upon varying the ratio of 1-octene oxide to cyclohexene oxide and their respective ratios to the initiator, three all-polycarbonate triblock samples were prepared with molar masses of about 350 kg/mol and 22, 26, and 29 mol % hard block content, respectively. The resulting triblock copolymers were analyzed using 1H NMR, GPC, thermogravimetric analysis, differential scanning calorimetry, and atomic force microscopy. All three samples demonstrated typical elastomeric behavior characterized by a high elongation at break and ultimate tensile strength in the same range as those of other natural and synthetic rubbers, in particular those used in applications such as tissue engineering.
Project description:Cationic zinc Lewis acids catalyse the C-H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat). An electrophile derived from [IDippZnEt][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) combined with <i>N</i>,<i>N</i>-dimethyl-<i>p</i>-toluidine (DMT) proved the most active in terms of C-H borylation scope and yield. Using this combination weakly activated heteroarenes, such as thiophene, were amenable to catalytic C-H borylation using HBCat. Competition reactions show these IDipp-zinc cations are highly oxophilic but less hydridophilic (relative to B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>), and that borylation proceeds <i>via</i> activation of the hydroborane (and not the heteroarene) by a zinc electrophile. Based on DFT calculations this activation is proposed to proceed by coordination of a hydroborane oxygen to the zinc centre to generate a boron electrophile that effects C-H borylation. Thus, Lewis acid binding to oxygen sites of hydroboranes represents an under-developed route to access reactive borenium-type electrophiles for C-H borylation.
Project description:In order to use H2 as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H2 into 2?H(+) and 2?e(-). Herein, we report a metal-free approach to catalyze the oxidation of H2 by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H2 with the in?situ electrochemical oxidation of the resulting borohydride. The use of the NHC-stabilized borenium cation [(IiPr2)(BC8H14)](+) (IiPr2=C3H2(NiPr)2, NHC=N-heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H2 oxidation by 910?mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6?kJ?mol(-1). The NHC-borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C6F5)3, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach.
Project description:The mesomeric betaine imidazolium-1-ylphenolate forms a borane adduct with tris(pentafluorophenyl)borane by coordination with the phenolate oxygen, whereas its NHC tautomer 1-(2-phenol)imidazol-2-ylidene reacts with (triphenylphosphine)gold(I) chloride to give the cationic NHC complex [Au(NHC)<sub>2</sub>][Cl] by coordination with the carbene carbon atom. The anionic N-heterocyclic carbene 1-(2-phenolate)imidazol-2-ylidene gives the complexes [K][Au(NHC<sup>-</sup>)<sub>2</sub>], [Rh(NHC<sup>-</sup>)<sub>3</sub>] and [Ni(NHC<sup>-</sup>)<sub>2</sub>], respectively. Results of four single crystal analyses are presented.
Project description:The metalated ylide YNa [Y=(Ph<sub>3</sub> PCSO<sub>2</sub> Tol)<sup>-</sup> ] was employed as X,L-donor ligand for the preparation of a series of boron cations. Treatment of the bis-ylide functionalized borane Y<sub>2</sub> BH with different trityl salts or B(C<sub>6</sub> F<sub>5</sub> )<sub>3</sub> for hydride abstraction readily results in the formation of the bis-ylide functionalized boron cation [Y-B-Y]<sup>+</sup> (2). The high donor capacity of the ylide ligands allowed the isolation of the cationic species and its characterization in solution as well as in solid state. DFT calculations demonstrate that the cation is efficiently stabilized through electrostatic effects as well as ?-donation from the ylide ligands, which results in its high stability. Despite the high stability of 2 [Y-B-Y]<sup>+</sup> serves as viable source for the preparation of further borenium cations of type Y<sub>2</sub> B<sup>+</sup> ?LB by addition of Lewis bases such as amines and amides. Primary and secondary amines react to tris(amino)boranes via N-H activation across the B-C bond.
Project description:The expansion of the range of physico-chemical methods in the study of industrially significant ?-olefin oligomers and polymers is of particular interest. In our article, we present a comparative Raman study of structurally uniform hydrogenated dimers, trimers, tetramers, and pentamers of 1-hexene and 1-octene, that are attractive as bases for freeze-resistant engine oils and lubricants. We found out that the joint monitoring of the disorder longitudinal acoustic mode (D-LAM) and symmetric C-C stretching modes allows the quantitative characterization of the number and length of alkyl chains (i.e., two structural characteristics), upon which the pour point and viscosity of the hydrocarbons depend, and to distinguish these compounds from both each other and linear alkanes. We demonstrated that the ratio of the contents of CH2 and CH3 groups in these hydrocarbons can be determined by using the intensities of the bands in the spectra, related to the asymmetric stretching vibrations of these groups. The density functional theory (DFT) calculations were applied to reveal the relations between the wavenumber and bandshape of the symmetric C-C stretching mode and a conformation arrangement of the 1-hexene and 1-octene dimers. We found that the branched double-chain conformation results in the splitting of the C-C mode into two components with the wavenumbers, which can be used as a measure of the length of branches. This conformation is preferable to the extended-chain conformation for hydrogenated 1-hexene and 1-octene dimers.
Project description:We report on the first isolation and structural characterization of an iron phosphinoimino-borane complex Cp*Fe(η<sup>2</sup>-H<sub>2</sub>B[double bond, length as m-dash]NC<sub>6</sub>H<sub>4</sub>PPh<sub>2</sub>) by dehydrogenation of iron amido-borane precursor Cp*Fe(η<sup>1</sup>-H<sub>3</sub>B-NHC<sub>6</sub>H<sub>4</sub>PPh<sub>2</sub>). Significantly, regeneration of the amido-borane complex has been realized by protonation of the iron(ii) imino-borane to the amino-borane intermediate [Cp*Fe(η<sup>2</sup>-H<sub>2</sub>B-NHC<sub>6</sub>H<sub>4</sub>PPh<sub>2</sub>)]<sup>+</sup> followed by hydride transfer. These new iron species are efficient catalysts for 1,2-selective transfer hydrogenation of quinolines with ammonia borane.