N‐Silylation of Amines Mediated by Et3SiH/KOtBu

Silylation of primary and secondary amines is reported, using triethylsilane as the silylating reagent in the presence of potassium tert‐butoxide (KO^tBu). The reaction proceeds well in the presence of 0.2 equiv. of KO^tBu. In competition experiments, aniline is selectively silylated over aliphatic amines. Computational studies support a catalytic mechanism which is initiated by KO^tBu interacting with the silane to form KH and silylated amine. The KH then takes over the role of base in the propagation of the cyclic mechanism and deprotonates the amine. This reacts with R₃SiH to afford the product R₃SiNR′R′′ and regenerate KH.     

Metal-free diastereoselective catalytic hydrogenations of imines using B(C6F5)3

Reductions of chiral ketimines effected under H(2) by catalytic amounts of B(C(6)F(5))(3) result in moderate to excellent diastereoselectivities. In the case of camphor and menthone derived imines, the reductions proceeded with greater than 95% diastereoselectivity.     

B(C6F5)3- vs Al(C6F5)3-derived metallocenium ion pairs. Structural, thermochemical, and structural dynamic divergences

The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[H3CAl(C6F5)3](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.     

B(C6F5)3-Catalyzed E-Selective Isomerization of Alkenes

Herein, we report the B(C₆F₅)₃-catalyzed E-selective isomerization of alkenes. The transition-metal-free method is applicable across a diverse array of readily accessible substrates, giving access to a broad range of synthetically useful products containing versatile stereodefined internal alkenes. The reaction mechanism was investigated by using synthetic and computational methods.     

B(C6F5)3-catalyzed synthesis of benzylic azides

B(C₆F₅)₃ was found to catalyze the reaction between trimethylsilyl azide and benzylic acetates. Secondary and tertiary benzylic acetates were competent substrates in this reaction providing the azide products in moderate to high yields. Mechanistic experiments are consistent with the possible formation of a Lewis acid-base pair between the B(C₆F₅)₃ and trimethylsilyl azide.     

B(C6F5)3-Catalyzed Cascade Reduction of Pyridines

B(C₆F₅)₃ has been found to be an effective catalyst for reduction of pyridines and other electron-deficient N-heteroarenes with hydrosilanes (or hydroboranes) and amines as the reducing reagents. The success of this development hinges upon the realization of a cascade process of dearomative hydrosilylation (or hydroboration) and transfer hydrogenation. The broad functional-group tolerance (e.g. ketone, ester, unactivated olefins, nitro, nitrile, heterocycles, etc.) implies high practical utility.     

Adducts of (C6F5)3B with Selected Nitrogen

The Lewis acid (C₆F₅)₃B was reacted with ICN, NH₂CN, C₃N₃X₃ (X = H, Cl, F). The resulting Lewis acid base adducts ( 1—5 ) were fully characterized by analytic and spectroscopic methods. Additionally, the structures of the adducts 1—4 were determined by single crystal X‐ray analyses. It has been qualitatively shown, that a high field shift of the ¹¹B as well as the ¹⁹F NMR resonances of the o‐F atoms of the C₆F₅‐substituents suggests a longer B—N distance.     

Interaction of Metal Oxido Compounds with B(C6F5)3

Lewis acid–base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C₆F₅)₃. It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.     

B(C6F5)3-Enabled Synthesis of a Cyclic cis-Arsaphosphene

The synthesis and characterization of an (arsino)phosphaketene, As(PCO){[N(Dipp)](CH₂)}₂ (Dipp=2,6-diisopropylphenyl) is reported along with its subsequent reactivity with B(C₆F₅)₃. When reacted in a stoichiometric ratio, B(C₆F₅)₃ drove the insertion of the P=C bond of the phosphaketene into one of the As−N bonds of the arsino functionality, affording an acid-stabilized, seven-membered, cyclic arsaphosphene. In contrast, when catalytic amounts of B(C₆F₅)₃ were employed, dimeric species, which formed through a formal [2+2] cycloaddition of the cyclic arsaphosphene, were generated. The cyclic arsaphosphene product represents the first example of such a compound in which the two substituents are arranged in a cis-configuration.     

B(C6F5)3 adducts of transition metal acyl compounds

The transition metal acyl compounds [Co(L)(CO)3(COMe)] (L = PMe3, PPhMe2, P(4-Me-C6H4)3, PPh3 and P(4-F-C6H4)3), [Mn(CO)5(COMe)] and [Mo(PPh3)(eta(5)-C5H5)(CO)2(COMe)] react with B(C6F5)3 to form the adducts [Co(L)(CO)3(C{OB(C6F5)3}Me)] (L = PMe3, 1, PPhMe2, 2, P(4-Me-C6H4)3, 3, PPh3, 4, P(4-F-C6H4)3), 5, [Mn(CO)5(C{OB(C6F5)3}Me)] 6 and [Mo(eta(5)-C5H5)(PPh3)(CO)2(C{OB(C6F5)3}Me)], 7. Addition of B(C6F5)3 to a cooled solution of [Mo(eta(5)-C5H5)(CO)3(Me)], under an atmosphere of CO gave [Mo(eta(5)-C5H5)(CO)3(C{OB(C6F5)3}Me)] 8. In the presence of adventitious water, the compound [Co{HOB(C6F5)3}2{OP(4-F-C6H4)3}2] 9, was formed from [Co(P(4-F-C6H4)3)(CO)3(C{OB(C6F5)3}Me)]. The compounds 4 and 9 have been structurally characterised. The use of B(C6F5)3 as a catalyst for the CO-induced migratory-insertion reaction in the transition metal alkyl compounds [Co(PPh3)(CO)3(Me)], [Mn(CO)5(Me)], [Mo(eta(5)-C5H5)(CO)3(Me)] and [Fe(eta(5)-C5H5)(CO)2(Me)] has been investigated.     

Reductive Deamination with Hydrosilanes Catalyzed by B(C6F5)3

The strong boron Lewis acid tris(pentafluorophenyl)borane B(C₆F₅)₃ is known to catalyze the dehydrogenative coupling of certain amines and hydrosilanes at elevated temperatures. At higher temperature, the dehydrogenation pathway competes with cleavage of the C?N bond and defunctionalization is obtained. This can be turned into a useful methodology for the transition‐metal‐free reductive deamination of a broad range of amines as well as heterocumulenes such as an isocyanate and an isothiocyanate.     

Synthesis and reactivity of N-heterocycle-B(C6F5)3 complexes. 4. Competition between pyridine- and pyrrole-type substrates toward B(C6F5)3: structure and dynamics of 7-B(C6F5)3-7-azaindole and [7-Azaindolium]+[HOB(C6F5)3]-

Reaction between 7-azaindole and B(C6F5)3 quantitatively yields 7-(C6F5)3B-7-azaindole (4), in which B(C6F5)3 coordinates to the pyridine nitrogen of 7-azaindole, leaving the pyrrole ring unreacted even in the presence of a second equivalent of B(C6F5)3. Reaction of 7-azaindole with H2O-B(C6F5)3 initially produces [7-azaindolium]+[HOB(C6F5)3]- (5) which slowly converts to 4 releasing a H2O molecule. Pyridine removes the borane from the known complexes (C6F5)3B-pyrrole (1) and (C6F5)3B-indole (2), with formation of free pyrrole or indole, giving the more stable adduct (C6F5)3B-pyridine (3). The competition between pyridine and 7-azaindole for the coordination with B(C6F5)3 again yields 3. The molecular structures of compounds 4 and 5 have been determined both in the solid state and in solution and compared to the structures of other (C6F5)3B-N-heterocycle complexes. Two dynamic processes have been found in compound 4. Their activation parameters (DeltaH = 66 (3) kJ/mol, DeltaS = -18 (10) J/mol K and DeltaH = 76 (5) kJ/mol, DeltaS = -5 (18) J/mol K) are comparable with those of other (C6F5)3B-based adducts. The nature of the intramolecular interactions that result in such energetic barriers is discussed.     

Highly efficient B(C(6)F(5))(3)-catalyzed hydrosilylation of olefins

A convenient and highly efficient method for the Lewis acid-catalyzed trans-selective hydrosilylation of alkenes has been developed. The mechanism of this novel protocol operates via direct addition of silylium type species across C=C bond followed by trapping of the resultant carbenium ion with boron-bound hydride. A number of diversely substituted silanes possessing both aryl and alkyl groups at silicon atom were efficiently prepared using this hydrosilylation methodology. The possibility to employ aryl-containing hydrosilanes in this reaction opens broad capabilities for the synthesis of alcohols via a trans-selective hydrosilylation/Tamao-Fleming oxidation sequence, complementary to the existing cis-selective hydroboration/oxidation protocol.     

B(C6F5)3-catalyzed allylation of propargyl acetates with allylsilanes

[reaction: see text] An efficient method for the B(C(6)F(5))(3)-catalyzed allylation of secondary propargylic alcohol derivatives with allylsilanes has been developed. This method allows for the facile synthesis of a variety of 1,5-enynes in good to high yields with a number of functionalities, such as nitro, chloro, ester, and boronic ester, being tolerated under the reaction conditions.     

A facile and chemoselective conjugate reduction using polymethylhydrosiloxane (PMHS) and catalytic B(C6F5)3

A highly chemoselective conjugate reduction of electron-deficient Michael acceptors, including alpha,beta-unsaturated ketones, carboxylic esters, nitriles and nitro compounds with PMHS in the presence of catalytic B(C6F5)3 is described.     

B(C6F5)3-Catalyzed 1,4-Allylation of Azadienes with Allyltrimethylsilanes

A metal-free 1,4-allylation of azadienes with allyltrimethylsilanes catalyzed by B(C₆F₅)₃ has been developed. The high Lewis acidity, good thermal stability, and good hydrolytic stability of the catalyst make this reaction efficient and operationally simple. In contrast to the previously reported 1,2-allylation of α,β-unsaturated imines, the 1,4-allylation of azadienes is described here for the first time. Benzofuran-, indene-, dihydronaphthalene-, chromene-, and benzothiophene-based allyl arylmethanes have been obtained in up to 95 % yield.     

Heterolytic dihydrogen activation by B(C6F5)3 and carbonyl compounds

Aromatic carbonyl compounds in combination with B(C(6)F(5))(3) are able to activate H(2) heterolytically. The reactivity of the carbonyl-B(C(6)F(5))(3) adduct is initiated by its thermal dissociation into components. After H(2) addition, aromatic carbonyl compounds convert into aryl-substituted methanes or alcohols.