Christopher B. Caputo

Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor

The Pull of Phosphorus Lewis acidity is primarily associated with compounds like boranes that lack a full complement of electrons in their coordination sphere and therefore attract electron donors (Lewis bases) to fill the gap. Caputo et al. (p. 1374; see the Perspective by Gabbaï) now show that a class of 4-coordinate phosphonium salts can act as surprisingly potent Lewis acids, despite their electronic saturation. The phosphorus cations, bearing fluorine and fluorinated aromatic substituents, can sever an alkyl carbon-fluorine bond by pulling away its fluoride—a process rendered catalytic through the use of a silane acceptor. Certain four-coordinate phosphorus cations prove sufficiently Lewis acidic to sever carbon-fluorine bonds. [Also see Perspective by Gabbaï] Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation. Here, we report the Lewis acidity and catalytic application of electronically saturated phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula [(C6F5)3–xPhxPF][B(C6F5)4] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of hydrosilane, the cation [(C6F5)3PF]+ is shown to catalyze the hydrodefluorination of fluoroalkanes, affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion affinity of this highly electron-deficient phosphonium center.

Combinations of ethers and B(C6F5)3 function as hydrogenation catalysts.

It works ether way: Labile adducts of dialkyl ethers with the electrophilic borane B(C6F5)3 are shown to scramble HD to H2 and D2 and catalyze the hydrogenation of 1,1-diphenylethylene.

Phosphorus as a Lewis acid: CO2 sequestration with amidophosphoranes.

The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C6F5)3] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH][HB(C6F5)3], which recently was shown to react with CO2 via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH]+[HC(O)OB(C6F5)3]. In the presence of extra B(C6F5)3 (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH][HB(C6F5)3]. The formatosilane in turn is rapidly hydrosilated by the B(C6F5)3/Et3SiH system to CH4, with (Et3Si)2O as the byproduct. At low [Et3SiH], intermediate CO2 reduction products are observed; addition of more CO2/Et3SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO2 to CH4. The utilization of carbon dioxide as a sustainable and nontoxic C1 feedstock for the production of value-added chemical products such as carboxylic acids or fuels such as methanol and methane is of current interest. The high thermodynamic stability of CO2 necessitates its catalytic activation and coupling to a thermodynamic driver for efficient conversion. Transition-metal-based catalysts have played a dominant role in CO2 conversion, but recently, an increasing number of organocatalytic CO2 reduction schemes have emerged. For example, N-heterocyclic carbenes (NHCs) reversibly form zwitterionic adducts NHC ·CO2 that are considered key intermediates in the reductive deoxygenation of CO2 using diphenylsilane as a sacrificial reducing agent, affording CH3OH upon workup. In this context, activation of CO2 by transition-metal-free “frustrated Lewis pairs” (FLPs) has led to the development of stoichiometric reductions of CO2 to CH3OH. Here, the FLPs form bridging carboxylate species that can accept hydrogen from ammonia borane or via a thermally driven, multistep self-reduction in which the key step is a reversible B-H bond addition of hydridotris(pentafluorophenyl)borate to one CdO double bond of CO2, affording the formatoborate anion [HC(O)OB(C6F5)3]. Ultimately, hydrolysis of CH3O-LA (LA ) BX3 or AlX3) is required in order to obtain methanol. Boron-hydrogen bond addition to CO2 mediated by phosphonium or ammonium borate ion pairs formed via FLP hydrogen splitting thus offers a potential entry point into catalytic CO2 fixation in the presence of a suitable reducing agent (oxygen acceptor). We have shown that perfluoroarylboranes are excellent catalysts for the reductive hydrosilylation of carbonyl functions and C-O bonds, a potentially useful reaction for subsequent steps in the reductive deoxygenation of CO2 to CH4. The ammonium hydridoborate ion pair 1 formed by treatment of the FLP B(C6F5)3/2,2,6,6-tetramethylpiperidine (TMP) and hydrogen (32 M, C6D5Br) reacted with CO2 (2-4 atm) in the presence of Et3SiH (18 equiv) at 56 °C to afford the previously reported formatoborate 2 exclusively (see Scheme 1). The reaction was monitored by 1H and 19F NMR spectroscopy, and integration versus an internal standard (C6H5CF3, 9 mM) revealed that no Et3SiH was consumed. Thus, although the formation of 2 is reversible, there does not appear to be sufficient free B(C6F5)3 present under these conditions to activate silane for further reduction of 2. Accordingly, we carried out a reaction under identical conditi ns with an additional 1.0 equiv of B(C6F5)3 (relative to 1) present. This resulted in the immediate and complete conversion of 2 back into 1 at room temperature and the appearance of the products of CO2 hydrosilylation. Further monitoring of the reaction by 1H and 19F NMR spectroscopy at 56 °C sho ed t at silane was gradually consumed and that CH4 along with 2 equiv of (Et3Si)2O were formed as the ultimate reaction products. Minor amounts of bis(triethylsilyl)acetal, (Et3SiO)2CH2, (∼10%) were also present. Interestingly, 1 and B(C6F5)3 were the only boron-containing compounds detectable during the reaction but diminished in favor of a new species, 3, upon complete silane consumption. At the same time, the characteristic signals of HCO2SiEt3, {Et3SiO}2CH2, and Et3SiOCH3 in C6D5Br became evident in the 1H NMR spectra. Upon addition of further silane equivalents and pressurization with fresh CO2, these partially reduced intermediates were depleted and methane formation resumed, indicating a “living” catalytic system. Scheme 1 Published on Web 07/15/2010 10.1021/ja105320c  2010 American Chemical Society 10660 9 J. AM. CHEM. SOC. 2010, 132, 10660–10661 Reaction of FLP with H2/CO2 JACS 2010, 132, 10660. Introduction 3: FLP other than Group 13/15

Olefin Isomerization and Hydrosilylation Catalysis by Lewis Acidic Organofluorophosphonium Salts

Organofluorophosphonium salts of the formula [(C6F5)(3-x)Ph(x)PF][B(C6F5)4] (x = 0, 1) exhibit Lewis acidity derived from a low-lying σ* orbital at P opposite F. This acidity is evidenced by the reactions of these salts with olefins, which catalyze the rapid isomerization of 1-hexene to 2-hexene, the cationic polymerization of isobutylene, and the Friedel-Crafts-type dimerization of 1,1-diphenylethylene. In the presence of hydrosilanes, olefins and alkynes undergo efficient hydrosilylation catalysis to the alkylsilanes. Experimental and computational considerations of the mechanism are consistent with the sequential activation and 1,2-addition of hydrosilane across the unsaturated C-C bonds.

Metal-free transfer hydrogenation of olefins via dehydrocoupling catalysis

Significance For more than a century, hydrogenation has been limited to the use of transition metal-based catalysts. With the emerging focus on the chemistry of earth-abundant elements, the 21st century has seen a renaissance in main-group chemistry. In this work, an electrophilic phosphonium cation is shown to act as main-group catalyst effecting the dehydrocoupling of silane and amines, phenols, thiols, and carboxylic acids with the concurrent release of H2. In addition, performing the reactions in the presence of olefins, dehydrocoupling occurs with simultaneous hydrogenation of the olefin. This chemistry provides an unprecedented avenue to metal-free transfer hydrogenation catalysis of olefins. A major advance in main-group chemistry in recent years has been the emergence of the reactivity of main-group species that mimics that of transition metal complexes. In this report, the Lewis acidic phosphonium salt [(C6F5)3PF][B(C6F5)4] 1 is shown to catalyze the dehydrocoupling of silanes with amines, thiols, phenols, and carboxylic acids to form the Si-E bond (E = N, S, O) with the liberation of H2 (21 examples). This catalysis, when performed in the presence of a series of olefins, yields the concurrent formation of the products of dehydrocoupling and transfer hydrogenation of the olefin (30 examples). This reactivity provides a strategy for metal-free catalysis of olefin hydrogenations. The mechanisms for both catalytic reactions are proposed and supported by experiment and density functional theory calculations.

Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins

The combination of phosphorus(V)-based Lewis acids with diaryl amines and diaryl silylamines promotes reversible activation of dihydrogen and can be further exploited in metal-free catalytic olefin hydrogenation. Combined experimental and density functional theory (DFT) studies suggest a frustrated Lewis pair type activation mechanism.

Hydrosilylation of Ketones, Imines and Nitriles Catalysed by Electrophilic Phosphonium Cations: Functional Group Selectivity and Mechanistic Considerations

The electrophilic phosphonium salt, [(C6 F5 )3 PF][B(C6 F5 )4 ], catalyses the efficient hydrosilylation of ketones, imines and nitriles at room temperature. In the presence of this catalyst, adding one equivalent of hydrosilane to a nitrile yields a silylimine product, whereas adding a second equivalent produces the corresponding disilylamine. [(C6 F5 )3 PCl][B(C6 F5 )4 ] and [(C6 F5 )3 PBr][B(C6 F5 )4 ] are also synthesised and tested as catalysts. Competition experiments demonstrate that the reaction exhibits selectivity for the following functional groups in order of preference: ketone>nitrile>imine>olefin. Computational studies reveal the reaction mechanism to involve initial activation of the Si-H bond by its interaction with the phosphonium centre. The activated complex then acts cooperatively on the unsaturated substrate.

Use of Trifluoromethyl Groups for Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination

The electrophilic organofluorophosphonium catalyst [(C6F5)3PF][B(C6F5)4] is shown to effect benzylation or alkylation by aryl and alkyl CF3 groups with subsequent hydrodefluorination, thus resulting in a net transformation of CF3 into CH2-aryl fragments. In the case of alkyl CF3 groups, Friedel-Crafts alkylation by the difluorocarbocation proceeded without cation rearrangement, in contrast to the corresponding reactions of alkyl monofluorides.

Release of Free and Conjugated Forms of the Putative Pheromonal Steroid 11-Oxo-etiocholanolone by Reproductively Mature Male Round Goby (Neogobius melanostomus Pallas, 1814)

Previous studies of the round goby (Neogobius melanostomus Pallas, 1814), an invasive fish species in the Laurentian Great Lakes of North America, have shown that this species has the ability to both synthesize and smell steroids that have a 5beta-reduced and 3alpha-hydroxyl (5beta,3alpha) configuration. An enzyme-linked immunoassay (EIA) for 3alpha-hydroxy-5beta-androstane-11,17-dione (11-O-ETIO) has been used to show a substantial rise in the rate of release of immunoreactive compounds into the water when males are injected with salmon gonadotropin releasing hormone analogue. Similar increases were noted for 11-ketotestosterone and 17,20beta-dihydroxypregn-4-en-3-one. Partitioning of the extracts between diethyl ether and water showed the presence of both free and conjugated immunoreactive 11-O-ETIO. Only conjugated immunoreactivity was found in urine (implying that free steroid is released via the gills). The identity of the conjugates was probed by using HPLC, EIA, and mass spectrometry and removal of sulfate and glucosiduronate groups. Immunoreactivity in the conjugated fraction was found to be due mainly to 3alpha,17beta-dihydroxy-5beta-androstan-11-one 17-sulfate. However, the evidence was also strong for the presence in water extracts of substantial amounts of 3alpha-hydroxy-5beta-androstane-11,17-dione 3-glucosiduronate (which could be detected only by EIA after removal of the glucosiduronate group with beta-glucuronidase). There were also small amounts of 3alpha-hydroxy-5beta-androstane-11,17-dione 3-sulfate and 3alpha,17beta-dihydroxy-5beta-androstan-11-one 17-glucosiduronate. These studies give some idea of the types, amounts, and ratios of 11-O-ETIO derivatives that are released by reproductive N. melanostomus and will aid further research into the putative pheromonal roles of 5beta,3alpha-reduced androgens in this species.

Ring openings of lactone and ring contractions of lactide by frustrated Lewis pairs

While B(C(6)F(5))(3) forms the adducts (CH(2))(4)CO(2)B(C(6)F(5))(3)1 and (CHMeCO(2))(2)B(C(6)F(5))(3)7 with δ-valerolactone and lactide, the frustrated Lewis pairs derived from B(C(6)F(5))(3) and phosphine or N-bases react with lactone to effect ring opening affording zwitterionic species of the form L(CH(2))(4)CO(2)B(C(6)F(5))(3) (L = tBu(3)P 2, Cy(3)P 3, C(5)H(3)Me(3)N 4, PhNMe(2) 5, C(5)H(6)Me(4)NH 6) while reaction with rac-lactide results in ring contraction to give salts [LH][OCCHMeCO(2)(CMe)OB(C(6)F(5))(3)] (L = tBu(3)P 8, Cy(3)P 9, C(5)H(3)Me(2)N 10, C(5)H(6)Me(4)NH 11). The mechanistic implications of these reactions are discussed.