Axially Chiral, Electrophilic Fluorophosphonium Cations: Synthesis

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

pubs.acs.org/Organometallics

Axially Chiral, Electrophilic Fluorophosphonium Cations: Synthesis, Lewis Acidity, and Reactivity in the Hydrosilylation of Ketones Lars Süsse,† James H. W. LaFortune,‡ Douglas W. Stephan,*,†,‡ and Martin Oestreich*,† †

Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6

‡

Organometallics Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/28/19. For personal use only.

S Supporting Information *

ABSTRACT: Axially chiral [(C6F5)3PF][B(C6F5)4] analogues based on dihydrophosphepines with a binaphthyl backbone were prepared and structurally characterized by X-ray diffraction analysis. Computational calculations of FIA and GEI values attest that these new fluorophosphonium cations have a higher Lewis acidity compared to the ubiquitous B(C6F5)3. Furthermore, application of these highly electrophilic compounds in the catalytic hydrosilylation of ketones and an investigation of the mechanism lead to a refined picture of the role of highly electrophilic fluorophosphonium cations.

■

INTRODUCTION A few years ago, the group of Stephan reported the synthesis of the highly electrophilic, C6F5-substituted fluorophosphonium salt [(C6F5)3PF][B(C6F5)4] (1, Scheme 1)1 and its application in various Lewis acid-catalyzed reactions.2 In the presence of a hydrosilane, transformations such as hydrodefluorination of fluoroalkanes,1 dehydrocoupling reactions,3 hydrodeoxygenation of ketones4 and phosphine oxides,5 as well as hydrosilylation reactions of unsaturated hydrocarbons6 and ketones7 were accomplished. The mechanistic details for the latter reduction reactions are of particular interest. For the B(C6F5)3catalyzed process developed by Piers,8a,b a counterintuitive reaction pathway was proposed and has been supported experimentally as well as computationally by Oestreich,8c Sakata and Fujimoto,8d as well as Piers and Tuononen.8e The proposed mechanism for the process catalyzed by electrophilic phosphonium cations (EPCs) was thought to parallel the Piers mechanism.7,9 Thus, activation of hydrosilane I by 1 via a η1 coordination of the hydridic Si−H bond prompts silyl transfer to the carbonyl oxygen in III and hydride transfer to the phosphorus atom, affording silylcarboxonium ion IV and transient (C6F5)3P(F)H (2, Scheme 1). Subsequently, hydride transfer affords silyl ether V and regenerates catalyst 1. An asymmetric variant of the above EPC-catalyzed hydrosilylation requires a chiral analogue of 1 that is sufficiently electron deficient to promote the Si−H bond activation and also provides a chiral environment that induces enantioselectivity in the hydride transfer step.10 Inspired by the successful application of the dihydroborepines (S)-3· THF11a,b and (S)-4·SMe211c,d with binaphthyl backbones in the Piers hydrosilylation, we anticipated that a similar scaffold in a fluorophosphonium cation could be promising (Scheme 1). Herein, we report the synthesis of axially chiral fluorophosphonium ions (S)-5 as well as (S)-6 and assess © XXXX American Chemical Society

their efficacy in enantioselective hydrosilylation of ketones. While silyl ethers are obtained in high yields, no enantioselectivity is seen. This prompted a careful reinvestigation of the mechanism. NMR spectroscopic and stoichiometric experiments reveal that EPCs do not serve as actual catalysts but rather initiate the formation of achiral electrophilic species that mediate the hydrosilylation.

■

RESULTS AND DISCUSSION Synthesis of Chiral EPCs with a Binaphthyl Backbone. The phosphepines (S)-712a and (S)-812b (Scheme 2) were prepared according to the method reported by Beller and coworkers. Subsequently, using a slight modification of the established protocol,1 the phosphines (S)-7 and (S)-8 were oxidized with XeF2 in CH2Cl2 at −78 °C. The corresponding difluoro-substituted phosphanes (S)-9 and (S)-10 were obtained in quantitative yields. Subsequent fluoride abstraction with freshly prepared silylium ion [Et3Si(C6D6)][B(C6F5)4]13 afforded (S)-5 and (S)-6 in 66% and 49% yield, respectively. Both fluorophosphonium ions were characterized by multinuclear NMR spectroscopy and showed typical P−F resonances in the 31P{1H} NMR spectra as doublets with 1 JP,F = 1058 Hz at 115.9 ppm for (S)-5 and 1JP,F = 1060 Hz at 123.2 ppm for (S)-6. It was further possible to secure the molecular structures of both compounds by X-ray diffraction analysis (Figure 1), confirming that the stereochemical integrity was retained. Both cations showed typical P−F bond lengths for fluorophosphonium ions of 1.5430(19) Å for (S)-5 and 1.540(3) Å for (S)6.1 For (S)-5, the P−C(sp2) bond length is 1.770(4) Å, and longer distances were obtained for the P−C(sp3) bonds with Received: December 17, 2018

A

DOI: 10.1021/acs.organomet.8b00912 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Proposed Mechanism for the EPC-Catalyzed Hydrosilylation of Ketones and Chiral Congeners of 1 Inspired by the Corresponding Boranesa

Figure 1. Molecular structure of (S)-5 (left) and (S)-6 (right). Thermal ellipsoids represent 50% probability level. Hydrogen atoms and the counteranion [B(C6F5)4]− are omitted for clarity. C: gray; P: orange; F: green.

of (S)-5 in CD2Cl2 was treated with Et3PO (1.0 equiv), and the resulting mixture was analyzed by means of 31P{1H} NMR spectroscopy. Coordination of Et3PO to the phosphonium ion was not evident and instead (S)-5 was transformed into the corresponding phosphine oxide with formation of [Et3PF][B(C6F5)4] (see the Supporting Information). Similar fluorideoxide exchange reactions have been observed for other fluorophosphonium ions, 17 and this affirms that the Gutmann−Beckett method is inappropriate in this case. The FIA and GEI values for (S)-5 and (S)-6 (Table 1) were computed at the MP2/def2-TZVPP//BP86/def2-TZVP level Table 1. Computed FIA and GEI Values of (S)-5, (S)-6, Achiral Fluorophosphonium Salts, and B(C6F5)3a

Counteranion [B(C6F5)4]− is omitted. LB = Lewis base.

a

entry

Lewis acid

FIA (kJ mol−1)

GEI (eV)

1 2 3b 4b 5b 6b 7b

(S)-5 (S)-6 [(C6F5)3PF][B(C6F5)4] [Ph(C6F5)2PF][B(C6F5)4] [Ph2(C6F5)PF][B(C6F5)4] [Ph3PF][B(C6F5)4] B(C6F5)3

688.6 649.6 779.3 742.0 700.1 656.3 452.6

2.555 2.080 3.622 3.187 2.820 2.455 1.408

Scheme 2. Synthesis of Axially Chiral Fluorophosphonium Ions

a

1.782(3) Å and 1.791(3) Å as expected. A slightly longer bond length of 1.759(5) Å for the P−C(sp2) of (S)-6 was observed, whereas the P−C(sp3) bonds were similar to (S)-5 with 1.787(5) Å and 1.796(5) Å. Determination of the Lewis Acidity. We next targeted an assessment of the Lewis acidity of these new phosphonium ions using the Gutmann−Beckett method14 and computation of the fluoride ion affinity (FIA)15 as well as the global electrophilicity index (GEI).16 For the first method, a solution

of theory, using the established protocols.16c,17c,18 The FIA and GEI values of (S)-5 are 688.6 kJ mol−1 and 2.555 eV, which are slightly lower than those computed for [Ph2(C6F5)PF][B(C6F5)4] (entries 1 and 5). As expected, (S)-6 showed lower FIA and GEI values of 649.6 kJ mol−1 and 2.080 eV, respectively. These values are comparable with those reported for [Ph3PF][B(C6F5)4] (entries 2 and 6). Nevertheless, both new chiral phosphonium salts (S)-5 and (S)-6 showed higher values than those obtained for B(C6F5)3 (entry 7). From this point of view, both chiral EPCs seemed promising candidates for the activation of hydrosilanes. Hydrosilylation of Ketones. With the chiral phosphonium ions in hand, these species were tested for their ability to mediate the hydrosilylation of ketones (Table 2). Using 1.0 mol % of (S)-5, ketone 11, and 1.1 equiv of Et3SiH in various solvents (1,2-F2C6H4, C6H6, and CH2Cl2), moderate conversion to the silyl ether 12 was achieved (entries 1−3). The reaction required 70 h in CH2Cl2 for 91% conversion (entry 3). No enantioselectivity was observed. Using 1.2 equiv of the hydrosilane and 1 mol % of (S)-5 or (S)-6, the hydrosilylations were accelerated to completion in 6 h in CD2Cl2 (entries 4 and 5). Again, only racemic products were obtained. A higher catalyst loading as well as a further increase of the amount of

Computed with MP2/def2-TZVPP//BP86/def2-TZVP (see the Supporting Information for details). bFIA and GEI values from ref 16c.

B


Article

Organometallics Table 2. Screening of the EPC-Catalyzed Hydrosilylationa

entry 1 2 3 4 5 6 7 8 9 10

Si−H (equiv) Et3SiH (1.1) Et3SiH (1.1) Et3SiH (1.1) Et3SiH (1.2) Et3SiH (1.2) Et3SiH (7.0) Me2PhSiH (1.2) MePh2SiH (1.2) Ph2SiH2 (1.2) PhSiH3 (1.2)

EPC (mol %) (S)-5 (S)-5 (S)-5 (S)-5 (S)-6 (S)-5 (S)-5 (S)-5 (S)-5 (S)-5

(1.0) (1.0) (1.0) (1.0) (1.0) (5.0) (1.0) (1.0) (1.0) (1.0)

solvent

time (h)

1,2-F2C6H4 C6H6 CH2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2

70 70 70 6 6 5 4 2 2 24

silyl ether 12 12 12 12 12 12 13 14 15 16f

conv.b (%) d

70 85d 91d >95e >95e >95e >95e >95e >95e >95e

(60) (73) (92) (51) (12) (57)

eec (%) 0 0 0 0 0 0 0 0 0 0

All reactions were performed according to General Procedure GP1. bIsolated yield after flash column chromatography in parentheses. Determined by HPLC analysis using a chiral stationary phase. dConversion was determined by GLC analysis using mesitylene as internal standard. e Conversion was determined by NMR analysis. f16 was isolated as the free alcohol after hydrolysis of the silyl ether during flash column chromatography. a c

ketones with electron withdrawing groups as otherwise deoxygenation of the ketone was observed. Mechanistic Considerations. The observation of no enantioinduction with either (S)-5 or (S)-6 prompted us to further study the proposed reaction mechanism for the racemic transformation using [(C6F5)3PF][B(C6F5)4] (1)4,6,7 as a catalyst. 1 was found to be stable in the presence of the ketone 11 as verified by 1H and heteronuclear NMR spectroscopy.19 A 1:1 mixture of 1 and Et3SiH showed traces of a decomposition product after 5 min (Scheme 4B), while

hydrosilane had no effect on the outcome of the reaction. Interestingly, deoxygenation and hydrodefluorination of 11 previously described with [(C6F5)3PF][B(C6F5)4] (1) under the same setup were not observed (entry 6).4 The substitution pattern at the silicon atom had no impact on the outcome of the reaction (entries 7−10). The prochiral ketones 17−21 were also tested under the aforementioned conditions using (S)-5 as catalyst. All the silyl ethers 22−26 were obtained as racemic mixtures (Scheme 3). As previously reported for 1, hydrosilylation was limited to alkyl-substituted ketones or

Scheme 4. Decomposition of 1 in the Presence of Hydrosilanea

Scheme 3. Representative Examples for the Hydrosilylation of Ketones with (S)-5 as the Catalysta,b,c

a

Experiment was performed in a J. Young NMR tube. 31P{1H} NMR spectra (283 MHz, CD2Cl2) of the reaction mixture over time: (A) only 1; (B) 1 and Et3SiH (1:1 equiv) after 5 min; (C) 1 and Et3SiH (1:10 equiv) after 1 h.

addition of excess hydrosilane (10 equiv) caused complete decomposition of 1 after 1 h (Scheme 4C). NMR data revealed that the resulting mixture consists of siloxyphosphonium ion 27, (C6F5)3P (28), Et3SiF (29), and traces of other unknown fluorine-containing compounds. In a previous report, a broadened 1H NMR resonance of the silicon-bound proton was attributed to coordination of the hydrosilane to the

a

All reactions were performed according to General Procedure GP1. Conversion was determined by NMR analysis. cEnantiomeric excesses were determined by HPLC analysis using a chiral stationary phase or by optical rotation and all were found to be 0%. dWith 3.0 mol % of (S)-5. b

C


Article

Organometallics [(C6F5)3PF]+ cation.1,9 However, the 31P and 19F NMR spectra presented herein indicate the reaction of 1 to the mixture of products described above. Based on computations by Kostenko and Dobrovetsky, the oxygen atom in 27 was proposed to emerge from the degradation of (C6F5)3P(F)H (2) through loss of HF along with (C6F5)3P (28).9 Reaction of HF with the glass surface is thought to release H2O which then serves as an oxidant for 1. The species 27 was independently synthesized following a reported procedure.20 Using 2 mol % of 27, the catalytic hydrosilylation of 11 yielded the silyl ether 12 quantitatively after 6 h (Scheme 5, top). It is proposed that 27 acts as a

Table 3. Deoxygenation of Acetophenone

Scheme 5. Siloxyphosphonium-Catalyzed Hydrosilylationa

a

entry

catalyst (mol %)

solvent

30:(31 + 32):33

1a 2a 3b 4b 5b

1 (1.5) (S)-6 (1.5) [Ph3C][B(C6F5)4] (2.0) [Et3Si][B(C6F5)4] (2.0) B(C6F5)3 (2.0)

CD2Cl2 CD2Cl2 toluene toluene toluene

1:1:0 1:1:0 1:1:0 1:1:0 0:0:1

Experiments were performed according to General Procedure GP1. Ratio was determined by 1H NMR spectroscopy. bReference 8b.

The initially proposed mechanism suggested the neutral (C6F5)3P(F)H (2) as the transient hydride source. Efforts to independently prepare this species by the reaction of 1 with traditional hydride sources were futile. Similar attempts to prepare the phosphorane 2 by hydrosilane activation with 1 in the presence of a Lewis base such as phosphines or pyridines (targeting capture of the silyl group) were also unsuccessful. Also, 2 could not be observed by NMR spectroscopy during monitoring of the hydrosilylation catalysis at −60 °C. The reported calculations indicate that 2 can decompose to 28 and HF.9 A HF-mediated hydrosilylation, on the other hand, can be excluded, because the corresponding chloro- and phenoxysubstituted phosphonium ions ([(C6F5)3PCl][B(C6F5)4] and [(C6F5)3POPh][B(C6F5)4]) afforded the silyl ether 12 under the standard reaction conditions. Attempts to absorb HF with added Lewis bases such as phosphines and pyridines were unsuccessful as the EPCs were decomposed by these σ-donors; the corresponding phosphines were seen as part of a complex mixture. The aforementioned results summarize to (1) similar substrate scope compared to the silylium ion-catalyzed carbonyl reduction,25(2) decomposition of 1 in the presence of hydrosilane to a catalytically active siloxyphosphonium ion, (3) discrepancies in the behavior to the borane-catalyzed hydrosilylation, and (4) absence of enantioinduction with chiral EPCs.26 These observations call for two alternative pathways in the catalytic cycle (Scheme 6). In the first case, 1 is not the actual catalyst but rather an initiator to generate the siloxyphosphonium ion 27 via (C6F5)3P(F)H (2) (Scheme 6, highlighted in cyan). 27 might then transfer the silyl group to the carbonyl group in III and lead to a silylcarboxonium ion IV and the phosphine oxide 34.27 The hydrosilane I can then act as hydride transfer reagent. This transfer step produces the desired silyl ether V and the stabilized silylium ion 27. Alternatively, the silylcarboxonium ion IV generated through the initially proposed mechanism (Scheme 6, highlighted in gray) could directly react with hydrosilane I to give the silyl ether V and another donor-stabilized silylium ion VI (Scheme 6, highlighted in orange). This, in turn, might react with ketone III, thereby regenerating the silylcarboxonium ion IV. However, based on the experimental data, both pathways are possible and could be competing. Furthermore, the initially proposed EPC-catalyzed variant with a hydride transfer from 2 to IV as initially proposed cannot be completely excluded.

a

Experiments were performed in a J. Young NMR tube. bRatios determined by 19F{1H} NMR spectroscopy.

stabilized silylium ion. This view was corroborated in the hydrosilylation catalysis of 11 using 20 mol % of 27 and excess Me2PhSiH as the hydrosilane (Scheme 5, bottom). This afforded the silyl ethers 12 and 13 in a 19:81 ratio, consistent with the participation of 27 in transfer of the silyl group. Müller and co-workers studied the Lewis acidity of a library of various siloxyphosphonium ions.21 Gagné and co-workers described the use of arylphosphines in the stabilization of silylium ions,22 wherein addition of (C6F5)3P (28) to a freshly prepared solution of [Et3Si(C6D6)][B(C6F5)4] showed no evidence of adduct formation. A complex mixture suggested phosphine degradation.23 These results show that, even when employing 27, the hydrosilane delivers its hydride to the silylcarboxonium ion. This prompted us to question the role of the neutral phosphorane in the proposed EPC-mediated hydrosilylation. The hydrosilylation of acetophenone (30) using 1 or (S)-6 as catalysts afforded only the products ethylbenzene 31 and hexaethyldisiloxane 32 in a 1:1 ratio (Table 3, entries 1 and 2).4,9 Using 1.0 equiv of Et3SiH, the deoxygenation product was formed in 50% yield. Similar results were reported by Piers using [Ph3C][B(C6F5)4] or [Et3Si][B(C6F5)4] as catalysts (entries 3 and 4), whereas B(C6F5)3 only produced the silyl ether 33 (entry 5).8b In the first two cases, the hydride source is the hydrosilane, whereas an in-situ-formed borohydride transfers the hydride in the latter case. As the experimental results obtained with EPCs strongly resemble silylium ion catalysis and not borane catalysis, we concluded that the hydrosilane could also act as hydride transfer reagent in this case.24 D


Article

Organometallics Scheme 6. Alternative Pathways for the Hydride Transfer by the Hydrosilane Itselfa

Counteranion [B(C6F5)4]− is omitted for clarity. Do = Donor.

a

■

Bruker AV500, or Bruker AV700 instruments. Chemical shifts are reported in parts per million (ppm) and are referenced to the residual solvent resonance as the internal standard (CHDCl2, δ 5.32 ppm for 1 H NMR; CD2Cl2, δ 53.84 ppm for 13C NMR; C6D5H, δ 7.16 ppm for 1H NMR; C6D6, δ 128.06 ppm for 13C NMR). 11B, 19F, 29Si, and 31 P NMR were calibrated according to the IUPAC recommendation using a unified chemical shift scale based on the proton resonance of tetramethylsilane as primary reference.29 Data are reported as follows: chemical shift, multiplicity (sbr = broad singlet, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, mc = centrosymmetric multiplet), coupling constants (Hz), and integration. Infrared (IR) spectra were recorded on an Agilent Technologies Cary 630 FTIR spectrophotometer equipped with an ATR unit and are reported as wavenumbers [cm−1]. Gas liquid chromatography (GLC) was performed on an Agilent Technologies 7820A gas chromatograph equipped with an HP-5 capillary column (30 m × 0.32 mm, 0.25 μm film thickness) using the following program: N2 carrier gas, injection temperature 240 °C, detector temperature 300 °C, flow rate: 1.74 mL/min; temperature program: start temperature 40 °C, heating rate 10 °C/min, end temperature 280 °C for 10 min. High-resolution mass spectrometry (HRMS) was performed by the Analytical Facility at the Institut für Chemie, Technische Universität Berlin. Optical rotation was measured on a Schmidt & Haensch Polatronic H532 polarimeter −1 (° cm2 g−1); concentration c is in with [α]20 λ values reported in 10 g/100 mL and as indicated. Enantiomeric excesses were determined by analytical high performance liquid chromatography (HPLC) analysis on an Agilent Technologies 1290 Infinity instrument with a chiral stationary phase using a Daicel Chiral OJ-H, or a Daicel Chiralcel AD-H column (n-heptane/i-PrOH mixture as solvent), or on an Agilent Technologies 1200 Infinity instrument with a stationary phase using a Daicel Chiralcel OJ-RH column (MeCN/H2O mixture as solvent). The chemical purity of all products was only established by 1H NMR spectroscopy. Data for the single crystal structure determination were collected with an Agilent SuperNova diffractometer equipped with a CCD area Atlas detector and a mirror monochromator by utilizing Cu−Kα radiation (λ = 1.5418 Å). Software packages used: CrysAlis PRO for data collection, cell refinement, and data reduction,30 SHELXS-97 for structure solution,31 SHELXL-97 for structure refinement,32 and Mercury

CONCLUSION Herein, we have prepared axially chiral fluorophosphonium salts with a binaphthyl backbone and determined their Lewis acidity using FIA and GEI values. Both compounds (S)-5 and (S)-6 were active in Si−H bond activation and efficiently mediated the hydrosilylation of ketones, but no enantioinduction was seen. This prompted us to (re)examine the reported mechanism for the hydrosilylation of ketones.7 The results of our experimental investigations suggest the possibility that these reactions may be initiated by fluorophosphonium ion but catalyzed by an achiral silylium ion. Further applications of the new phosphonium ions in reactions avoiding these sidereactions are ongoing in our laboratories.

■

EXPERIMENTAL SECTION

General Information. All reactions were performed in flamedried glassware using an MBraun glovebox or conventional Schlenk techniques under a static pressure of argon (glovebox) or nitrogen. Liquids and solutions were transferred with syringes and cannulas. Solvents were dried and purified using standard procedures. Technical grade solvents for extraction or chromatography were distilled prior to use. C6D6 and CD2Cl2 were degassed and stored over 4 Å molecular sieves. Phosphepine (S)-7,12a phosphepine (S)-8,12b [(C6F5)3PF][B(C 6 F 5 ) 4 ] (1), 1 [(C 6 F 5 ) 3 PCl][B(C 6 F 5 ) 4 ], 7 [(C 6 F 5 ) 3 POPh][B(C6F5)4],18 and tris(pentafluorophenyl)phosphine oxide28 were synthesized according to reported procedures. Et3SiH, Me2PhiH, and MePh2SiH were distilled, degassed, and stored over 4 Å molecular sieves. Ph2SiH2 and PhSiH3 were degassed and stored over 4 Å molecular sieves. Ketones 11, 17−21, and 30 were purified by flash chromatography or distillation prior to use. All other chemicals were purchased from commercial suppliers and used without further purification. Analytical thin layer chromatography (TLC) was performed on silica gel 60 F254 aluminum sheets by Merck using the indicated solvents. Flash column chromatography was performed on silica gel 60 (40−63 μm, 230−400 mesh, ASTM) by Merck using the indicated solvents. 1H, 13C, 11B, 19F, 29Si, and 31P NMR spectra were recorded in CD2Cl2 or C6D6 on Bruker AV400, E


Article

Organometallics 3.1.133 for graphics. Electronic structure calculations, including geometry optimization, frequency calculations, and energy calculations, were performed using Gaussian 16.34 Geometry optimizations and frequency calculations were performed using the BP86 functional and def2-TZVP basis set.35 X-ray coordinates were used as the starting geometries where available. Each geometry was confirmed to be a minimum on its potential energy surface by confirming the Hessian to be positive definite with a frequency calculation. The Cartesian coordinates of the optimized structures are collected in Tables S1−S6 (see the Supporting Information). Orbital and internal energies needed to calculate global electrophilicity indices (GEIs) and fluoride ion affinities (FIAs) were obtained from MP2/def2-TZVPP36 calculations at the BP86/def2-TZVP geometries. FIA and GEI were calculated as previously described.17c,18 General Procedure for EPC-Catalyzed Hydrosilylation of Ketones (GP1). In a glovebox, a GLC vial is charged with the corresponding EPC (1.0−5.0 mol %), the hydrosilane (1.1−7.0 equiv), the indicated ketone (1.0 equiv), and solvent (0.6 mL). The resulting mixture is stirred at room temperature for the indicated time. The progress of the reaction is monitored by GLC analysis or 1H NMR analysis. The volatiles are removed under reduced pressure. Purification of the residue by flash chromatography on silica gel (eluent: n-petane) affords the silyl ether. General Procedure for Preparation of Racemic Silyl Ethers (GP2). In a glovebox, a GLC vial is charged with B(C6F5)3 (3.0 mol %), the hydrosilane (1.0−2.0 equiv), the indicated ketone (1.0 equiv), and CH2Cl2 (0.6 mL). The resulting mixture is stirred at room temperature overnight. The volatiles are removed under reduced pressure. Purification of the residue by flash chromatography on silica gel (eluent: n-petane) affords the racemic silyl ether. (S)-4,4-Difluoro-4-(pentafluorophenyl)-4,5-dihydro-3H-4λ5dinaphtho[2,1-c:1′,2′-e]phosphoran [(S)-9]. C6F5-substituted phosphepine (S)-7 (60 mg, 0.13 mmol, 1.0 equiv) was placed in a 10 mL Schlenk tube, dissolved in CH2Cl2 (2 mL), and the resulting solution was cooled to −78 °C under a nitrogen atmosphere. XeF2 (32 mg, 0.19 mmol, 1.5 equiv) in CH2Cl2 (0.6 mL) was added, and the resulting mixture was stirred for 2 h at −78 °C. After removal of all volatiles in high vacuum, difluorophosphoran (S)-9 (65 mg, 0.13 mmol, >99%) was obtained as a white solid. HRMS (EI): calculated for C28H16F7P+ [M+]: 516.0872; found: 516.0878. 1H NMR (500 MHz, CD2Cl2): δ/ppm 3.19 (mc, 2H), 3.98−4.16 (m, 2H), 7.05 (d, 3 JH,H = 8.4 Hz, 2H), 7.21 (dd, 3JH,H = 7.4 Hz, 3JH,H = 7.3 Hz, 2H), 7.46 (dd, 2JH,H = 7.3 Hz, 2JH,H = 6.9 Hz, 2H), 7.53 (d, 3JH,H = 7.9 Hz, 2H), 7.88−8.02 (m, 4H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ ppm 39.5 (dt, 1JC,P = 117 Hz, 2JC,F = 19.7 Hz, 2C), 115.2 (dm, 1JC,P = 158 Hz), 126.3 (s, 2C), 126.7 (s, 2C), 127.2 (s, 2C), 128.6 (s, 2C), 129.2 (s, 2C), 130.1 (d, 3JC,P = 7.7 Hz, 2C), 132.0 (d, 2JC,P = 12.0 Hz, 2C), 133.2 (s, 2C), 133.5 (s, 2C), 134.9 (d, 3JC,P = 5.9 Hz, 2C), 138.0 (dmc, 1JC,F = 255 Hz, 2C), 142.5 (dmc, 1JC,F = 252 Hz), 144.8 (dmc, 1 JC,F = 246 Hz, 2C). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −17.7 (d, 1JF,P = 650 Hz, 2F), −134.9 (mc, 2F), −152.5 (mc, 1F), −160.9 (mc, 2F). 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm −11.3 (t, 1JP,F = 650 Hz). (S)-4,4-Difluoro-4-phenyl-4,5-dihydro-3H-4λ5-dinaphtho[2,1-c:1′,2′-e]phosphoran [(S)-10]. Phenyl-substituted phosphepine (S)-8 (100 mg, 0.263 mmol, 1.00 equiv) was placed in a 10 mL Schlenk tube, dissolved in CH2Cl2 (2 mL), and the resulting solution was cooled to −78 °C under a nitrogen atmosphere. XeF2 (65 mg, 0.38 mmol, 1.5 equiv) in CH2Cl2 (1 mL) was added, and the resulting mixture was stirred for 1 h at −78 °C. After removal of all volatiles in high vacuum, difluorophosphoran (S)-10 (112 mg, 0.263 mmol, >99%) was obtained as a white solid. HRMS (APCI): calculated for C28H21FP+ [(M − F)+]: 407.1359; found: 407.1353. 1H NMR (500 MHz, CD2Cl2): δ/ppm 3.09−3.27 (m, 2H), 3.68−3.80 (m, 2H), 7.07 (d, 3JH,H = 8.7 Hz, 2H), 7.21 (dd, 3JH,H = 7.8 Hz, 3JH,H = 7.3 Hz, 2H), 7.43−7.51 (m, 4H), 7.51−7.54 (m, 1H), 7.56 (d, 3JH,H = 8.7 Hz, 2H), 7.91 (d, 3JH,H = 7.5 Hz, 1H), 7.94 (d, 3JH,H = 7.5 Hz, 1H), 7.98 (dd, 3 JH,H = 8.3 Hz, 3JH,H = 8.2 Hz, 4H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 40.0 (dt, 1JC,P = 117 Hz, 2JC,F = 24.5 Hz, 2C), 125.9 (s, 2C), 126.4 (s, 2C), 127.2 (s, 2C), 128.5 (s, 2C), 128.7 (s, 2C),

128.8 (s, 2C), 130.3−130.5 (m, 2C), 131.9 (s), 133.2 (s, 2C), 133.3− 133.5 (m, 4C), 133.8 (dt, 2JC,P = 12 Hz, 3JC,F = 4.8 Hz, 2C), 134.4 (d, 3 JC,P = 5.5 Hz, 2C), 138.3 (dt, 1JC,P = 174 Hz, 2JC,F = 33 Hz). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −32.0 (d, 1JF,P = 635 Hz, 2F). 31 1 P{ H} NMR (202 MHz, CD2Cl2): δ/ppm −14.9 (t, 1JP,F = 635 Hz). Fluorophosphonium Ion (S)-5. In a glovebox, difluorophosphorane (S)-9 (20 mg, 39 μmol, 1.0 equiv) was placed in a GLC vial, dissolved in C6D6 (0.2 mL), and the solution was added to a freshly prepared solution of [Et3Si(C6D6)][B(C6F5)4] (34 mg, 43 μmol, 1.1 equiv) in C6D6 (0.3 mL). The resulting two-phase system was stirred for 1 h at room temperature before CH2Cl2 (3 drops) was added. After 5 min, the supernatant was removed and the lower phase was washed with C6D6 (3 0.1 mL). After removal of all volatiles in high vacuum, the fluorophosphonium ion (S)-5 (30 mg, 26 μmol, 66%) was obtained as a brown solid. Single crystals of (S)-5 suitable for Xray diffraction were obtained by slow evaporization of a CD2Cl2/C6D6 mixture. HRMS (APCI): calculated for C28H16F6P+ [(M − B(C6F5)4)+]: 497.0888; found: 497.0879. 1H NMR (500 MHz, CD2Cl2): δ/ppm 4.11−4.20 (m, 1H), 4.20−4.28 (m, 1H), 4.32−4.53 (m, 2H), 7.20 (d, 3JH,H = 8.9 Hz, 1H), 7.25 (d, 3JH,H = 8.6 Hz, 1H), 7.38−7.46 (m, 3H), 7.63−7.74 (m, 3H), 8.03 (d, 3JH,H = 8.3 Hz, 1H), 8.07 (d, 3JH,H = 8.6 Hz, 1H), 8.12 (d, 3JH,H = 8.3 Hz, 1H), 8.26 (d, 3 JH,H = 8.5 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 31.5−32.3 (m), 32.3−33.2 (m), 118.0 (d, 2JC,P = 8.4 Hz), 119.6 (d, 2 JC,P = 11.3 Hz), 126.5 (d, 3JC,P = 5.6 Hz), 127.0 (s), 127.17 (s), 127.23 (s), 128.72 (s), 128.74 (s), 128.8 (s), 129.0 (s), 129.2 (s), 129.3 (s), 132.27 (s), 132.32 (s), 132.4 (s), 132.7 (s), 135.1−135.2 (m, 2C), 135.6 (d, 3JC,P = 4.4 Hz), 135.8 (d, 3JC,P = 5.7 Hz), 136.5 (dmc, 1JC,F = 250 Hz, 8C), 138.5 (dmc, 1JC,F = 242 Hz, 4C), 148.5 (dmc, 1JC,F = 237 Hz, 8C). Signals for the quaternary carbon atoms B(ipso-C6F5) and P(ipso-C6F5) could not be detected. 11B{1H} NMR (161 MHz, CD2Cl2): δ/ppm −16.7 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −123.4 (mc, 2F), −127.4 (mc, 1F), −129.0 (d, 1JF,P = 1098 Hz, 1F), −133.3 (mc, 8F), −151.6 (mc, 2F), −163.6 (t, 3JF,F = 21 Hz, 4F), −167.5 (mc, 8F). 19F/13C HMQC (659/176 MHz, CD2Cl2): δ/ppm −123.4/148.6, −127.4/149.4, −133.3/148.0, −151.6/139.1, −163.6/138.2, −167.5/136.3. 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm 115.9 (d, 1JP,F = 1098 Hz). The crystallographic data are available online in the CCDC database under number CCDC 1882909. Fluorophosphonium Ion (S)-6. In a glovebox, difluorophosphorane (S)-10 (50 mg, 0.12 mmol, 1.0 equiv) was placed in a GLC vial, dissolved in C6D6 (0.4 mL), and the solution was added to a freshly prepared solution of [Et3Si(C6D6)][B(C6F5)4] (0.10 g, 0.13 mmol, 1.1 equiv) in C6D6 (0.3 mL). The resulting two-phase system was stirred for 5 min at room temperature before CH2Cl2 (3 drops) was added. After 5 min, the supernatant was removed and the lower phase was washed with C6D6 (1 0.1 mL) and n-pentane (3 0.1 mL). After removal of all volatiles in high vacuum, the fluorophosphonium ion (S)-6 (62 mg, 53 μmol, 45%) was obtained as a brown solid along with traces of an unknown phosphorus-containing species. Single crystals of (S)-6 suitable for X-ray diffraction were obtained by slow evaporization of a CD2Cl2/n-pentane mixture. HRMS (APCI): calculated for C28H21FP+ [(M − B(C6F5)4)+]: 407.1359; found: 407.1356. 1H NMR (500 MHz, CD2Cl2): δ/ppm. 3.96−4.22 (m, 4H), 7.34 (d, 3JH,H = 8.5 Hz, 1H), 7.68−7.86 (m, 13H), 8.04 (t, 3JH,H = 7.3 Hz, 1H), 8.08 (d, 3JH,H = 8.4 Hz, 1H), 8.24 (d, 3JH,H = 8.4 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 31.4 (dd, 1JC,P = 60 Hz, 2JC,F = 9.1 Hz), 31.9 (dd, 1JC,P = 60 Hz, 2JC,F = 9.0 Hz), 115.1 (dd, 1 JC,P = 86 Hz, 2JC,F = 9.0 Hz, 2C), 119.4 (dd, 2JC,P = 7.8 Hz, 3JC,F = 2.8 Hz), 121.4 (d, 2JC,P = 11.9 Hz), 123.7−125.0 (m), 127.2 (d, 3JC,P = 4.7 Hz), 127.3 (d, 3JC,P = 5.2 Hz), 131.5 (d, 2JC,P = 13 Hz, 2C), 131.8−131.9 (m, 10C), 132.3 (s), 132.7 (s), 134.7−134.8 (m, 2C), 134.9 (d, 3JC,P = 3.9 Hz), 135.8 (d, 3JC,P = 5.5 Hz), 136.6 (dmc, 1JC,F = 245 Hz, 8C), 138.6 (dmc, 1JC,F = 244 Hz, 4C), 139.3 (s), 148.6 (dmc, 1 JC,F = 245 Hz, 8C). A signal for the quaternary carbon atoms B(ipsoC6F5) could not be detected. 11B{1H} NMR (161 MHz, CD2Cl2): δ/ ppm −16.7 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −133.1 (mc, 8F), −138.5 (d, 1JF,P = 1060 Hz, 1F), −163.5 (t, 3JF,F = 21 Hz, F


Article

Organometallics

silica gel (eluent: n-pentane) afforded 15 (5.0 mg, 14 μmol, 12%, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 2137, 1428, 1366, 1268, 1206, 1170, 1121, 1070, 1028, 849, 809, 733, 695. HRMS (EI): calculated for C8H7F3O+ [(M + H − C12H11Si)+]: 176.0444; found: 176.0434. HRMS (EI): calculated for C12H11Si• [(M − C8H7F3O)•]: 183.0630; found: 183.0622. 1H NMR (500 MHz, CD2Cl2): δ/ppm 5.04 (q, 3JH,F = 6.7 Hz, 1H), 5.42 (s, 1H), 7.34−7.51 (m, 11H), 7.53− 7.57 (m, 2H), 7.58−7.62 (m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 75.0 (q, 2JC,F = 32.7 Hz), 124.6 (q, 1JC,F = 285 Hz), 128.2 (s, 2C), 128.5 (s, 2C), 128.6 (s, 2C), 128.8 (s, 2C), 129.8 (s), 131.2 (s), 131.3 (s), 132.67 (s), 132.71 (s), 134.5 (s), 135.0 (s, 2C), 135.1 (s, 2C). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm = −78.3 (s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 5.04/−8.1, 5.42/−8.1, (7.53−7.57)/−8.1, (7.58−7.62)/−8.1. The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-RH column, column temperature 20 °C, solvent MeCN/H2O 50/50, flow rate 0.4 mL/ min, λ 210 nm): tR,1 = 46.9 min, tR,2 = 80.1 min. 2,2,2-Trifluoro-1-phenylethanol (16). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan-1-one (11, 20 mg, 0.12 mmol, 1.0 equiv), PhSiH3 (14 mg, 0.13 mmol, 1.1 equiv), and (S)5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 24 h. Purification of the crude product on silica gel (eluent: npentane/tert-butyl methyl ether 100/0 → 6/1) afforded 16 (12 mg, 68 μmol, 57%, 0% ee) as a yellowish liquid. IR (ATR): ν̃/cm−1 3403, 1456, 1429, 1356, 1262, 1165, 1121, 1058, 922, 865, 832, 758, 700. HRMS (EI): calculated for C8H7F3O+ [M+]: 176.0444; found: 176.0445. 1H NMR (500 MHz, CD2Cl2): δ/ppm 2.75 (d, 3JH,H = 4.7 Hz, 1H), 5.06 (mc, 1H), 7.41−7.45 (m, 3H), 7.45−7.51 (m, 2H). 13 C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 72.9 (q, 2JC,F = 34.4 Hz), 124.8 (q, 1JC,F = 285 Hz), 127.8 (s, 2C), 129.0 (s, 2C), 129.9 (s), 134.6 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm = −78.7 (s). The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel AD-H column, column temperature 20 °C, solvent n-heptane/i-PrOH 98/2, flow rate 0.8 mL/min, λ 210 nm): tR,1 = 22.5 min, tR,2 = 24.1 min. The analytical data are in accordance with those reported.37 (1-Cyclohexylethoxy)triethylsilane (22). Prepared according to GP1 from 1-cyclohexylethan-1-one (17, 15 mg, 0.12 mmol, 1.0 equiv), Et3SiH (29 mg, 0.25 mmol, 2.1 equiv), and (S)-5 (1.4 mg, 12 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 1 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 22 (>95% conversion, 0% ee) in a mixture with disiloxane as a colorless liquid. IR (ATR): ν̃/cm−1 2923, 2875, 1450, 1413, 1372, 1298, 1236, 1188, 1139, 1069, 1003, 967, 910, 840, 775, 722, 672. HRMS (APCI): calculated for C14H31OSi+ [(M + H)+]: 243.2139; found: 243.2140. 1H NMR (400 MHz, CD2Cl2): δ/ppm 0.58 (q, 3 JH,H = 7.8 Hz, 6H), 0.89−0.98 (m, 11H), 1.07 (d, 3JH,H = 6.6 Hz, 3H), 1.11−1.27 (m, 4H), 1.59−1.83 (m, 5H), 3.55 (qd, 3JH,H = 6.1 Hz, 3JH,H = 6.1 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 5.4 (s, 3C), 7.1 (s, 3C), 21.0 (s), 26.86 (s), 26.88 (s), 27.2 (s), 29.1 (s), 29.2 (s), 46.2 (s), 72.9 (s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm (0.55−0.62)/15.5, (0.89− 0.98)/15.5, 3.55/15.5. The enantiomeric excess was determined by HPLC analysis of the corresponding 4-nitrobenzoyl ester38 on a chiral stationary phase (Daicel Chiracel AD-H column, column temperature 20 °C, solvent n-heptane/i-PrOH 100/0, flow rate 0.8 mL/min, λ 210 nm): tR,1 = 35.2 min, tR,2 = 37.6 min. The analytical data are in accordance with those reported.7 Dimethyl[(4-methylpentan-2-yl)oxy](phenyl)silane (23). Prepared according to GP1 from isobutyl methyl ketone (18, 15 mg, 0.15 mmol, 1.0 equiv), Me2PhSiH (28 mg, 0.21 mmol, 1.4 equiv), and (S)-5 (1.8 mg, 15 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 19 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 23 (>95% conversion, 0% ee) as a colorless liquid. HRMS (APCI): calculated for C13H21OSi+ [(M − CH3)+]: 221.1362; found: 220.9857 (no better value obtained). 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.33 (mc, 6H), 0.90 (mc, 3H), 0.91 (mc, 3H), 1.15 (d, 3JH,H = 6.1 Hz, 3H), 1.17−1.24 (m, 1H), 1.33−1.40 (m, 1H), 1.67−1.76 (m, 1H), 3.80−3.88 (m, 1H), 7.32−

4F), −167.4 (mc, 8F). 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm 123.2 (d, 1JP,F = 1060 Hz). The crystallographic data are available online in the CCDC database under number CCDC 1882910. Triethyl(2,2,2-trifluoro-1-phenylethoxy)silane (12). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan-1-one (11, 30 mg, 0.17 mmol, 1.0 equiv), Et3SiH (24 mg, 0.21 mmol, 1.2 equiv), and (S)-6 (2.0 mg, 1.7 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 6 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 12 (39 mg, 0.13 mmol, 73%, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 2957, 2879, 1456, 1365, 1269, 1240, 1205, 1166, 1126, 1072, 1004, 974, 853, 830, 728, 697, 671. HRMS (EI): calculated for C12H16F3OSi+ [(M − C2H5)+]: 261.0917; found: 261.0923. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.55−0.66 (m, 6H), 0.90 (t, 3JH,H = 7.9 Hz, 9H), 4.97 (q, 3JH,F = 6.7 Hz, 1H), 7.37−7.42 (m, 3H), 7.44−7.79 (m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 4.8 (s, 3C), 6.6 (s, 3C), 73.7 (q, 2JC,F = 31.8 Hz), 124.8 (q, 1JC,F = 284 Hz), 128.0 (s, 2C), 128.7 (s, 2C), 129.5 (s), 136.1 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −78.7 (s). 1 H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ ppm 0.55−0.66/24.4, 0.90/24.4. The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-RH column, column temperature 20 °C, solvent MeCN/ H2O 50/50, flow rate 0.4 mL/min, λ 210 nm): tR,1 = 59.7 min, tR,2 = 65.2 min. The analytical data are in accordance with those reported.4 (2,2,2-Trifluoro-1-phenylethoxy)dimethyl(phenyl)silane (13). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan1-one (11, 20 mg, 0.12 mmol, 1.0 equiv), Me2PhSiH (18 mg, 0.13 mmol, 1.1 equiv), and (S)-5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 4 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 13 (34 mg, 0.11 mmol, 95%, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 2960, 1428, 1367, 1267, 1206, 1167, 1117, 1072, 1029, 999, 860, 828, 786, 738, 697. HRMS (EI): calculated for C16H17F3OSi+ [M+]: 310.0995; found: 310.0981. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.33 (s, 3H), 0.38 (s, 3H), 4.90 (q, 3JH,F = 6.7 Hz, 1H), 7.32−7.43 (m, 8H), 7.49− 7.53 (m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −1.5 (s), −1.4 (s), 73.8 (q, 2JC,F = 32.6 Hz), 124.7 (q, 1JC,F = 281 Hz), 128.1 (s, 2C), 128.3 (s, 2C), 128.7 (s, 2C), 129.6 (s), 130.4 (s), 133.9 (s, 2C), 135.5 (s), 136.4 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −78.6 (s). 29Si{1H} DEPT (99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 13.1 (s). The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-H column, column temperature 20 °C, solvent n-heptane/i-PrOH 95/5, flow rate 0.8 mL/min, λ 210 nm): tR,1 = 6.2 min, tR,2 = 8.2 min. (2,2,2-Trifluoro-1-phenylethoxy)methyl(diphenyl)silane (14). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan1-one (11, 20 mg, 0.12 mmol, 1.0 equiv), MePh2SiH (25 mg, 0.13 mmol, 1.1 equiv), and (S)-5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) with a reaction time of 2 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 14 (22 mg, 0.059 mmol, 51%, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 3048, 1454, 1428, 1366, 1267, 1206, 1168, 1115, 1072, 1029, 998, 843, 791, 725, 696, 663. HRMS (EI): calculated for C21H19F3OSi+ [M+]: 372.1152; found: 372.1140. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.61 (s, 3H), 5.00 (q, 3JH,F = 6.7 Hz, 1H), 7.35−7.49 (m, 11H), 7.54−7.59 (m, 4H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −2.7 (s), 73.6 (q, 2 JC,F = 32.9 Hz), 124.8 (q, 1JC,F = 281 Hz), 128.2 (s, 2C), 128.3 (s, 2C), 128.4 (s, 2C), 128.7 (s, 2C), 129.7 (s), 130.6 (s), 130.7 (s), 134.7 (s, 2C), 134.8 (s, 2C), 134.9 (s), 135.1 (s), 135.3 (s). 19F{1H} NMR (471 MHz, CD2Cl2): δ/ppm −78.3 (s). 1H/29Si HMQC (500/ 99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 0.61/1.8, 5.00/1.8. The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-RH column, column temperature 20 °C, solvent MeCN/H2O 60/40, flow rate 0.4 mL/ min, λ 210 nm): tR,1 = 36.3 min, tR,2 = 41.7 min. (2,2,2-Trifluoro-1-phenylethoxy)diphenylsilane (15). Prepared according to GP1 from 2,2,2-trifluoro-1-phenylethan-1-one (11, 20 mg, 0.12 mmol, 1.0 equiv), Ph2SiH2 (23 mg, 0.13 mmol, 1.1 equiv), and (S)-5 (1.4 mg, 1.1 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 2 h. Purification of the crude product on G


Article

Organometallics

optimized for J = 7 Hz): δ/ppm (0.51−0.61)/20.8, 0.89/20.8, 5.26/20.8. The enantiomeric excess was determined by optical rotation: = 0° (c = 1.18, CHCl3, 0% ee). Siloxyphosphonium Ion 27. In a glovebox, tris(pentafluorophenyl)phosphine oxide (34, 10 mg, 18 μmol, 1.0 equiv) was placed in a GLC vial, dissolved in C6D6 (0.4 mL), and was added to a freshly prepared solution of [Et3Si(C6D6)][B(C6F5)4] (17 mg, 22 μmol, 1.2 equiv) in C6D6 (0.3 mL). The resulting two-phase system was stirred for 5 min at room temperature before CH2Cl2 (3 drops) was added. After 5 min, the supernatant was removed and the lower phase was washed with C6D6 (3 0.1 mL). After removal of all volatiles in high vacuum, the siloxyphosphonium ion 27 (17 mg, 13 μmol, 72%) was obtained as a white solid. Selected analytical data: 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.87−1.05 (m, 18H). 13C{1H} NMR (176 MHz, CD2Cl2): δ/ppm 5.6 (s, 3C), 5.9 (s, 3C), 98.0 (dm 1JC,P = 135 Hz, 3C). 11B{1H} NMR (161 MHz, CD2Cl2): δ/ppm −16.6 (s). 19 1 F{ H} NMR (471 MHz, CD2Cl2): δ/ppm −128.4 (sbr), −129.1 (sbr), −133.2 (sbr), −152.0 (sbr), −163.9 (t, 3JF,F = 20 Hz), −167.8 (sbr). 19F/13C HMQC (659/176 MHz, CD2Cl2): δ/ppm −128.4/ 148.4, −129.1/149.4, −133.2/148.6, −152.0/139.5, −163.9/138.7, −167.8/136.8. 31P{1H} NMR (202 MHz, CD2Cl2): δ/ppm 17.7 (sbr). 1 H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ ppm (0.87−1.05)/53.6. The analytical data are in accordance with those reported.20

7.38 (m, 3H), 7.53−7.56 (m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −1.0 (s), −0.9 (s), 22.6 (s), 23.3 (s), 24.3 (s), 25.0 (s), 49.5 (s), 67.5 (s), 128.1 (s, 2C), 129.8 (s), 133.9 (s, 2C), 139.2 (s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 0.33/−0.9. The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-RH column, column temperature 20 °C, solvent MeCN/H2O 50/50, flow rate 0.4 mL/min, λ 210 nm): tR,1 = 40.3 min, tR,2 = 43.3 min. [1-(4-Cyanophenyl)ethoxy]dimethyl(phenyl)silane (24). Prepared according to GP1 from 4-acetylbenzonitrile (19, 20 mg, 0.14 mmol, 1.0 equiv), Me2PhSiH (20 mg, 0.15 mmol, 1.1 equiv), and (S)5 (4.9 mg, 4.2 μmol, 3.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 5 d. Purification of the crude product on silica gel (eluent: npentane) afforded 24 (>95% conversion, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 2958, 2228, 1608, 1427, 1406, 1252, 1206, 1117, 1088, 1027, 956, 826, 785, 727, 698. HRMS (EI): calculated for C16H16NOSi+ [(M − CH3)+]: 266.0996; found: 266.0994. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.32 (s, 3H), 0.36 (s, 3H), 1.39 (d, 3JH,H = 6.5 Hz, 3H), 4.90 (q, 3JH,H = 6.2 Hz, 1H), 7.32−7.40 (m, 3H), 7.40−7.44 (m, 2H), 7.51−7.56 (m, 2H), 7.58−7.62 (m, 2H). 13 C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −1.3 (s), −1.1 (s), 26.8 (s), 70.9 (s), 111.1 (s), 119.3 (s), 126.5 (s, 2C), 128.2 (s, 2C), 130.1 (s), 132.5 (s, 2C), 133.9 (s, 2C), 138.0 (s), 152.2 (s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 0.32/7.9, 0.36/7.9. The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-H column, column temperature 20 °C, solvent n-heptane/i-PrOH 95/5, flow rate 0.5 mL/min, λ 210 nm): tR,1 = 20.3 min, tR,2 = 31.6 min. The analytical data are in accordance with those reported.39 [1-(2-Bromophenyl)ethoxy]triethylsilane (25). Prepared according to GP1 from 1-(2-bromophenyl)ethan-1-one (20, 35 mg, 0.18 mmol, 1.0 equiv), Et3SiH (23 mg, 0.20 mmol, 1.1 equiv), and (S)-5 (2.3 mg, 2.5 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) within a reaction time of 2 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 25 (>95% conversion, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 2953, 2785, 1466, 1438, 1413, 1368, 1237, 1237, 1200, 1130, 1094, 1018, 950, 794, 722, 665. HRMS (EI): calculated for C14H23BrOSi+ [(M − C2H5)+]: 285.0305; found: 285.0308. 1H NMR (500 MHz, CD2Cl2): δ/ppm 0.51−0.64 (m, 6H), 0.91 (t, 9H), 1.38 (d, 3JH,H = 6.3 Hz, 3H), 5.18 (q, 3JH,H = 6.2 Hz, 1H), 7.10 (ddd, 3JH,H = 7.6 Hz, 3JH,H = 7.6 Hz, 4JH,H = 1.7 Hz, 1H), 7.33 (ddd, 3JH,H = 7.6 Hz, 3JH,H = 7.6 Hz, 4JH,H = 1.2 Hz, 1H), 7.47 (dd, 3JH,H = 7.8 Hz, 4JH,H = 1.2 Hz, 1H), 7.63 (dd, 3JH,H = 7.8 Hz, 4 JH,H = 1.8 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm 5.1 (s, 3C), 6.9 (s, 3C), 25.8 (s), 69.9 (s), 121.1 (s), 127.8 (s), 128.0 (s), 128.7 (s), 132.5 (s), 146.5 (s). 1H/29Si HMQC (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm (0.51−0.64)/19.0, 0.91/ 19.0, 5.18/19.0. The enantiomeric excess was determined by HPLC analysis on a chiral stationary phase (Daicel Chiracel OJ-RH column, column temperature 20 °C, solvent MeCN/H2O 50/50, flow rate 0.5 mL/min, λ 210 nm): tR,1 = 71.0 min, tR,2 = 76.6 min. The analytical data are in accordance with those reported.40 Triethyl[1-(pentafluorophenyl)ethoxy]silane (26). Prepared according to GP1 from 1-(pentafluorophenyl)ethan-1-one (21, 36 mg, 0.17 mmol, 1.0 equiv), Et3SiH (23 mg, 0.20 mmol, 1.1 equiv), and (S)-5 (2.0 mg, 2.2 μmol, 1.0 mol %) in CD2Cl2 (0.6 mL) with a reaction time of 2 h. Purification of the crude product on silica gel (eluent: n-pentane) afforded 26 (>95% conversion, 0% ee) as a colorless liquid. IR (ATR): ν̃/cm−1 2957, 2879, 1650, 1499, 1458, 1415, 1374, 1302, 1238, 1151, 1103, 1055, 963, 890, 800, 727, 671. HRMS (EI): calculated for C12H14F5OSi+ [(M − C2H5)+]: 297.0729; found: 297.0730. 1H NMR (700 MHz, CD2Cl2): δ/ppm 0.51−0.61 (m, 6H), 0.89 (t, 3JH,H = 8.0 Hz, 9H), 1.56 (d, 3JH,H = 6.5 Hz, 3H), 5.26 (q, 3JH,H = 6.6 Hz, 1H). 13C{1H} NMR (176 MHz, CD2Cl2): δ/ ppm 4.8 (s, 3C), 6.7 (s, 3C), 24.0 (s), 62.5 (s). 1H/13C HMQC (700/ 176 MHz, CD2Cl2): δ/ppm 1.56/119.6, 5.26/119.6. 19F NMR (659 MHz, CD2Cl2): δ/ppm −142.2 (dd, 3JF,F = 21.5 Hz, JF,F = 8.3 Hz, 2F), −155.6 (t, 3JF,F = 21.0 Hz, 1F), −161.5 (mc, 2F). 19F/13C HMQC (659/176 MHz, CD2Cl2): δ/ppm −142.2/146.6, −155.6/ 142.1, −161.5/139.3. 1H/29Si HMQC (500/99 MHz, CD2Cl2,

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00912. Overview of screened catalysts and figures giving NMR spectra of compounds synthesized in this paper (PDF) Cartesian coordinates (S)-5 (XYZ) Cartesian coordinates (S)-6 (XYZ) Cartesian coordinates (S)-9 (XYZ) Cartesian coordinates (S)-10 (XYZ) Accession Codes

CCDC 1882909−1882910 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.W.S.). *E-mail: [email protected] (M.O.). ORCID

Lars Süsse: 0000-0002-4894-1198 James H. W. LaFortune: 0000-0003-4217-1038 Douglas W. Stephan: 0000-0001-8140-8355 Martin Oestreich: 0000-0002-1487-9218 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS L.S. was supported through an Einstein Visiting Fellowship of the Einstein Foundation Berlin to D.W.S. (2016−2019). D.W.S. and M.O. thank the Einstein Foundation Berlin for generous funding. D.W.S acknowledges the support of NSERC of Canada and is grateful for the award of a Canada Research Chair. We are grateful to Dr. Susanne Bähr for fruitful H


Article

Organometallics

Asymmetric Piers Hydrosilylation. J. Am. Chem. Soc. 2016, 138, 6940−6493. (12) (a) Hagemann, B.; Junge, K.; Enthaler, S.; Michalik, M.; Riermeier, T.; Monsees, A.; Beller, M. A General Method for the Enantioselective Hydrogenation of β-Keto Esters using Monodentate Binaphthosphepine Ligands. Adv. Synth. Catal. 2005, 347, 1978− 1986. (b) Junge, K.; Hagemann, B.; Enthaler, S.; Spannenberg, A.; Michalik, M.; Oehme, G.; Monsees, A.; Riermeier, T.; Beller, M. Synthesis of chiral monodentate binaphthophosphepine ligands and their application in asymmetric hydrogenations. Tetrahedron: Asymmetry 2004, 15, 2621−2631. (13) (a) Lambert, J. B.; Zhang, S. Tetrakis(pentafluorophenyl)borate: a New Anion for Silylium Cation in the Condensed Phase. J. Chem. Soc., Chem. Commun. 1993, 383−384. (b) Lambert, J. B.; Zhang, S.; Ciro, S. M. Silyl Cations in the Solid and in Solution. Organometallics 1994, 13, 2430−2443. (14) (a) Mayer, U.; Gutmann, V.; Gerger, W. The Acceptor Number − A Quantitative Empirical Parameter for the Electrophilic Properties of Solvents. Monatsh. Chem. 1975, 106, 1235−1257. (b) Gutmann, V. Solvent Effects On The Reactivities Of Organometallic Compounds. Coord. Chem. Rev. 1976, 18, 225−255. (c) Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Varma, K. S. A convenient n.m.r. method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity. Polymer 1996, 37, 4629−4631. (15) (a) Christe, K. O.; Dixon, D. A.; McLemore, D.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A. On a quantitative scale for Lewis acidity and recent progress in polynitrogen chemistry. J. Fluorine Chem. 2000, 101, 151−153. (b) Böhrer, H.; Trapp, N.; Himmel, D.; Schleep, M.; Krossing, I. From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinities. Dalton Trans. 2015, 44, 7489−7499. (16) (a) Parr, R. G.; v. Szentpály, L.; Liu, S. Electrophilicity Index. J. Am. Chem. Soc. 1999, 121, 1922−1924. (b) Chattaraj, P. K.; Sarkar, U.; Roy, D. R. Electrophilicity Index. Chem. Rev. 2006, 106, 2065− 2091. (c) Jupp, A. R.; Johnstone, T. C.; Stephan, D. W. The global electrophilicity index as a metric for Lewis acidity. Dalton Trans. 2018, 47, 7029−7035. (17) (a) Holthausen, M. H.; Mehta, M.; Stephan, D. W. The Highly Lewis Acidic Dicationic Phosphonium Salt: [(SIMes)PFPh2][B(C6F5)4]2. Angew. Chem., Int. Ed. 2014, 53, 6538−6541. (b) Holthausen, M. H.; Hiranandani, R. R.; Stephan, D. W. Electrophilic bisfluorophosphonium dications: Lewis acid catalysts from diphosphines. Chem. Sci. 2015, 6, 2016−2021. (c) LaFortune, J. H. W.; Szkop, K. M.; Farinha, F. E.; Johnstone, T. C.; Postle, S.; Stephan, D. W. Probing steric influences on electrophilic phosphonium cations: a comparision of [(3,5-(CF3)2C6H3)3PF]+ and [(C6F5)3PF]+. Dalton Trans. 2018, 47, 11411−11419. (18) LaFortune, J. H. W.; Johnstone, T. C.; Pérez, M.; Winkelhaus, D.; Podgorny, V.; Stephan, D. W. Electrophilic phenoxy-substituted phosphonium cations. Dalton Trans. 2016, 45, 18156−18162. (19) For the decomposition of 1 in the presence of acetophenone (30) in CD2Cl2 after 1 h, see ref 4. (20) vom Stein, T.; Pérez, M.; Dobrovetsky, R.; Winkelhaus, D.; Caputo, C. B.; Stephan, D. W. Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins. Angew. Chem., Int. Ed. 2015, 54, 10178− 10182. (21) Großekappenberg, H.; Reißmann, M.; Schmidtmann, M.; Müller, T. Quantitative Assessment of the Lewis Acidity of Silylium Ions. Organometallics 2015, 34, 4952−4958. (22) Gudz, A.; Payne, P. R.; Gagné, M. R. Phosphines as Silylium Ion Carriers for Controlled C−O Deoxygenation: Catalyst Speciation and Turnover Mechanisms. Organometallics 2017, 36, 4047−4053. (23) For the synthesis of [(Me5C6)3Si−P(C6F5)3]+, see: Reißmann, M.; Schäfer, A.; Jung, S.; Müller, T. Silylium Ion/Phosphane Lewis Pairs. Organometallics 2013, 32, 6736−6744.

discussions and to Vittorio Bonetti for his experimental contributions (both TU Berlin). We thank Dr. Elisabeth Irran (TU Berlin) for the X-ray analysis.

■

REFERENCES

(1) Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W. Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor. Science 2013, 341, 1374−1377. (2) For reviews, see: (a) Stephan, D. W. A Tale of Two Elements: The Lewis Acidity/Basicity Umpolung of Boron and Phosphorus. Angew. Chem., Int. Ed. 2017, 56, 5984−5992. (b) Bayne, J. M.; Stephan, D. W. Phosphorus Lewis acids: emerging reactivity and applications in catalysis. Chem. Soc. Rev. 2016, 45, 765−774. (3) Pérez, M.; Caputo, C. B.; Dobrovetsky, R.; Stephan, D. W. Metal-free transfer hydrogenation of olefins via dehydrocoupling catalysis. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10917−10921. (4) Mehta, M.; Holthausen, M. H.; Mallov, I.; Pérez, M.; Qu, Z.-W.; Grimme, S.; Stephan, D. W. Catalytic Ketone Hydrodeoxygenation Mediated by Highly Electrophilic Phosphonium Cations. Angew. Chem., Int. Ed. 2015, 54, 8250−8254. (5) Mehta, M.; Garcia de la Arada, I.; Pérez, M.; Porwal, D.; Oestreich, M.; Stephan, D. W. Metal-Free Phosphine Oxide Reductions Catalyzed by B(C6F5)3 and Electrophilic Fluorophosphonium Cations. Organometallics 2016, 35, 1030−1035. (6) Pérez, M.; Hounjet, L. J.; Caputo, C. B.; Dobrovetsky, R.; Stephan, D. W. Olefin Isomerization and Hydrosilylation Catalysis by Lewis Acidic Organofluorophosphonium Salts. J. Am. Chem. Soc. 2013, 135, 18308−18310. (7) Pérez, M.; Qu, Z.-W.; Caputo, C. B.; Podgorny, V.; Hounjet, L. J.; Hansen, A.; Dobrovetsky, R.; Grimme, S.; Stephan, D. W. Hydrosilylation of Ketones, Imines, and Nitriles Catalysed by Electrophilic Phosphonium Cations: Funtional Group Selectivity and Mechanistic Considerations. Chem. - Eur. J. 2015, 21, 6491− 6500. (8) (a) Parks, D. J.; Piers, W. E. Tris(pentafluorophenyl)boronCatalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters. J. Am. Chem. Soc. 1996, 118, 9440−9441. (b) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Functions. J. Org. Chem. 2000, 65, 3090− 3098. (c) Rendler, S.; Oestreich, M. Conclusive Evidence for an SN2Si Mechanism in the B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Compounds: Implications for the Related Hydrogenation. Angew. Chem., Int. Ed. 2008, 47, 5997−6000. (d) Sakata, K.; Fujimoto, H. Quantum Chemical Study of B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Group. J. Org. Chem. 2013, 78, 12505−12512. (e) Houghton, A. Y.; Hurmalainen, J.; Mansikkamäki, A.; Piers, W. E.; Tuononen, H. M. Direct observation of a borane-silane complex involved in frustrated Lewis-pair-mediated hydrosilylation. Nat. Chem. 2014, 6, 983−988. (9) Kostenko, A.; Dobrovetsky, R. The Role of the Fluoro− Hydrido−Phosphorane Intermediate in Catalytic Hydrosilylation of Acetophenone: Computational Study. Eur. J. Org. Chem. 2019, 318− 322. (10) For reviews of phosphonium ions in asymmetric catalysis, see: (a) Golandaj, A.; Ahmad, A.; Ramjugernath, D. Phosphonium Salts in Asymmetric Catalysis: A Journey in a Decade’s Extensive Research Work. Adv. Synth. Catal. 2017, 359, 3676−3706. (b) Werner, T. Phosphonium Salt Organocatalysis. Adv. Synth. Catal. 2009, 351, 1469−1481. (11) (a) Mewald, M.; Fröhlich, R.; Oestreich, M. An Axially Chiral, Electron-Deficient Borane: Synthesis, Coordination Chemistry, Lewis Acidity, and Reactivity. Chem. - Eur. J. 2011, 17, 9406−9414. (b) Mewald, M. Ph.D. Thesis, Westfälische Wilhelms-Universität Münster, Münster, Germany, 2012. (c) Hermeke, J.; Mewald, M.; Irran, E.; Oestreich, M. Chemoselective Tin−Boron Exchange Aided by the Use of Dummy Ligands at the Tin Atom. Organometallics 2014, 33, 5097−5100. (d) Süsse, L.; Hermeke, J.; Oestreich, M. The I


Article

Organometallics (24) For a previously observed reactivity difference between the fluorophosphonium ion 1 and B(C6F5)3 in the hydrosilylation of an alkyl-substituted ketone, see ref 4. (25) Müther, K.; Oestreich, M. Self-regeneration of a silylium ion catalyst in carbonyl reduction. Chem. Commun. 2011, 47, 334−336. (26) We are aware that several diastereomers of the P−H species from (S)-5 or (S)-6 can be formed due to a Berry pseudorotation. These might react with different reaction rates and lead to varying ee values. For the Berry pseudorotation of (C6F5)3P(F)H (2), see ref 9. (27) The phosphine oxide was not detected after the hydrosilylation of ketones due to subsequent reduction of the phosphine oxide to the phosphine; see ref 5. (28) Song, L.; Hu, J.; Wang, J.; Liu, X.; Zhen, Z. Novel perfluorodiphenylphosphinic acid lanthanide (Er or Er−Yb) complex with high NIR photoluminescence quantum yield. Photochem. Photobiol. Sci. 2008, 7, 689−693. (29) Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.; Goodfellow, R.; Granger, P. NMR Nomenclature. Nuclear Spin Properties And Conventions For Chemical Shifts. Pure Appl. Chem. 2001, 73, 1795−1818. (30) Agilent CrysAlis PRO; Agilent Technologies: Yarnton, U.K., 2012. (31) Sheldrick, G. M. Phase Annealing in SHELX-90: Direct Methods for Larger Structures. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 467−473. (32) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (33) Cambridge Crystallographic Data Centre: http://www.ccdc. cam.ac.uk/Solutions/CSDSystem/Pages/mercury.aspx. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, Revision A.03; Gaussian, Inc.: Wallingford, CT, 2016. (35) (a) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3000. (b) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (c) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (36) (a) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. MP2 Energy Evaluation By Direct Methods. Chem. Phys. Lett. 1988, 153, 503−506. (b) Sæbø, S.; Almlöf, J. Avoiding The Integral Storage Bottleneck In LCAO Calculations Of Electron Correlation. Chem. Phys. Lett. 1989, 154, 83−89. (c) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. A Direct MP2 Gradient Method. Chem. Phys. Lett. 1990, 166, 275−280. (37) Hevia, E.; Kennedy, A. R.; Klett, J.; Livingstone, Z.; McCall, M. D. New insights into addition reactions of dialkylzinc reagents to trifluoromethyl ketones: Structural authentication of a β-hydride elimination product containing a tetranuclear zinc chain. Dalton Trans. 2010, 39, 520−526. (38) Palmer, M. J.; Kenny, J. A.; Walsgrove, T.; Kawamoto, A. M.; Wills, M. Asymmetric transfer hydrogenation of ketones using amino alcohol and monotosylated diamine derivatives of indane. J. Chem. Soc., Perkin Trans. 1 2002, 416−427.

(39) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste, F. D. Reversing the Role of the Metal−Oxygen π-Bond. Chemoselective Catalytic Reductions with a Rhenium(V)-Dioxo Complex. J. Am. Chem. Soc. 2003, 125, 4056−4057. (40) Teci, M.; Lentz, N.; Brenner, E.; Matt, D.; Toupet, L. Alkylfluorenyl substituted N-heterocyclic carbenes in copper(I) catalysed hydrosilylation of aldehydes and ketones. Dalton Trans. 2015, 44, 13991−13998.

J


Axially Chiral, Electrophilic Fluorophosphonium Cations: Synthesis

Recommend Documents