B

Article pubs.acs.org/Organometallics

Contrasting the Reactivity of Ethylene and Propylene with P/Al and P/B Frustrated Lewis Pairs Gabriel Ménard,† Lina Tran,† Jenny S. J. McCahill,‡ Alan J. Lough,† and Douglas W. Stephan*,† †

Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4

‡

S Supporting Information *

ABSTRACT: Frustrated Lewis pairs (FLPs) derived from R3P (R = otol, Mes) and B(C6F5)3 or AlX3 (X = halide, C6F5) add to ethylene. Similarly, the P/B FLP adds across propylene, whereas the P/Al systems react with propylene to effect propylene dimerization via a C−H bond activation and C−C bond formation, affording an Al-bound 2-methylpentene complex.

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INTRODUCTION The observation that frustrated Lewis pairs (FLPs), which are combinations of sterically congested Lewis acids and bases, can effect H2 activation is certainly remarkable.1 This finding has resulted in the advent of metal-free stoichiometric and catalytic hydrogenations for a growing range of substrates2 and extensions to asymmetric reductions.3 FLPs have also been exploited for other applications in materials science4 and in strategies to capture and activate a variety of other small molecules.2i While efforts in this area continue, the application of the reactivity of FLPs to activate other small molecules has also garnered much attention.5 The first paper in which FLP activation of a small molecule other than H2 was reported described the addition of P/B FLPs to terminal olefins, including ethylene and hexene.6 Subsequently, Erker and co-workers described the addition of intramolecular FLPs to other olefins in a similar fashion (Scheme 1).7 In a more recent study, we showed that van der Waals interactions between the olefin and a Lewis acidic borane8 activates the olefin for nucleophilic attack by a variety of nucleophiles, providing an avenue to form new C−P, C−N, C−C, and C−H bonds.9 The impact of variations in the Lewis acid in such reactions with olefins has received limited attention. While combinations of phosphine with the alane Al(C6F5)3 react with ethylene to give addition products analogous to those in the borane case, reaction of the FLP tBu3P/Al(C6F5)3 with isobutylene has been shown to proceed via an alternate route involving C−H bond activation, generating a reactive bridging allyl bis-Al anion salt (Scheme 1).10 In this present study we further explore the differing reactivities of P/B and P/Al FLPs in reactions with ethylene and propylene. In the case of the Al Lewis acids, we exploit simple Al halides and demonstrate that two formula units of these Lewis acids are required to stabilize addition to ethylene. In contrast, while P/B FLPs are shown to add to propylene in a © XXXX American Chemical Society

Scheme 1. Reactions of Olefins with FLPs

“conventional FLP fashion”, the P/Al FLPs effect C−H bond activation and dimerization of propylene.

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RESULTS AND DISCUSSION The combination of a phosphine (R3P, R = Mes, otol) and Al(C6F5)3·tol in a 1/1 ratio in bromobenzene was exposed to an atmosphere of ethylene. In the case of Mes3P/Al(C6F5)3 a new species appears, as evidenced by a 31P{1H} NMR resonance at 19 ppm. In addition, two new 1H NMR resonances in the aliphatic region were also observed that were consistent with a PCH2CH2Al linkage. The 19F{1H} and Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Received: March 16, 2013

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dx.doi.org/10.1021/om400222w | Organometallics XXXX, XXX, XXX−XXX

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Al NMR spectra were consistent with the presence of a fourcoordinate aluminate anion. Collectively, these data indicate the formulation of the product as Mes3P(CH2CH2)Al(C6F5)3. While this species was stable under an ethylene atmosphere, it could not be isolated in its absence. Rapid loss of ethylene and regeneration of the initial components, PMes3 and Al(C6F5)3, indicates that the binding of ethylene is reversible. In contrast, the corresponding reaction of the less sterically hindered phosphine (otol)3P with Al(C6F5)3 and ethylene led to the isolation of the single product 1 in 77% yield. The 31 1 P{ H} NMR resonance at 29 ppm and two distinct 1H resonances at 3.34 and 0.60 ppm suggest the analogous formulation (otol)3P(CH2CH2)Al(C6F5)3 (1), a fact that was confirmed by single-crystal X-ray diffraction (Figure 1). This species is directly analogous to the previously reported species tBu3P(CH2CH2)Al(C6F5)310 and tBu3P(CH2CH2)B(C6F5)3,6 and the metric parameters are very similar.

Figure 2. POV-Ray depiction of the molecular structure of 3: C, black; P, orange; Al, teal; Br, scarlet. Analogous structures are obtained for 2 and 4 (see the Supporting Information). H atoms are omitted for clarity.

1.874(3), 1.862(3), and 1.89(1) Å, respectively. These are significantly longer than those seen in 1 (1.830(7) Å) and tBu3P(CH2CH2)Al(C6F5)3 (1.833(2) Å).10 In contrast, the corresponding C−Al bond lengths in 2−4 (1.950(3), 1.946(3), and 1.95(1) Å, respectively) are also similar to those found in other aluminates derived from Al(C6F5)3.5s,11 The presence of the halide bridge between the Al centers presumably enhances the Lewis acidity of the C-bound AlX3 center and thus stabilizes the zwitterionic form. A related situation was observed in the capture of CO2 with the bis-borane 1,2-C6H4(BCl2)25b where a B−Cl−B interaction enhanced the thermal stability of the bound CO2. The analogous reactions of propylene were also explored. For a point of comparison, tBu3P/B(C6F5)3 was first reacted with propylene. In this case crystals of 5 were isolated in 63% yield. The 11B{1H} and 31P{1H} NMR spectra gave rise to signals at −11.6 and 56.9 ppm, respectively. These data as well as the 1H, 13C{1H}, and 19F NMR spectra were completely consistent with the formation of tBu3P(CH(CH3)CH2)B(C6F5)3 (5), by direct analogy to the previously reported ethylene analog. Crystallographic data further confirmed this observation (Figure 3). It should be noted that alternation of the tBu3P/B(C6F5)3 ratio to 1/2 had no impact on the formation of 5. In sharp

Figure 1. POV-Ray depiction of the molecular structure of 1: C, black; P, orange; Al, teal; F, pink. H atoms are omitted for clarity.

The FLPs derived from Mes3P/AlX3 (X = Cl, Br, I) were also reacted with ethylene (Scheme 2). In these cases, the reactions Scheme 2. Reactions of Ethylene with Mes3P/AlX3 FLPs

were complete in 2 h and each led to a single product, as evidenced by the formation of new signals in the 31P{1H} and 1 H NMR spectra. While the NMR data suggest the formation of the species Mes3P(CH2CH2)AlX3, attempts to obtain pure products were unsuccessful, as redissolved powders revealed a 1 H NMR signal consistent with the liberation of ethylene. However, using a 1/2 PMes3/AlX3 ratio under ethylene afforded the isolation of stable products with the formulation Mes3P(CH2CH2)(AlX3)2 (X = Cl (2), Br (3), I (4)). Single crystals suitable for X-ray diffraction were obtained for each of these compounds (Figure 2; see the Supporting Information). The structures of 2−4 reveal that a second equivalent of AlX3 is halide-bridged to the carbon-bound Al and display similar features, with P−C bond lengths to the ethylene fragment of

Figure 3. POV-Ray depiction of the molecular structure of 5. C, black; P, orange; B, yellow-green; F, pink. H atoms are omitted for clarity. B


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proposed allyl intermediate was derived from the analogous reaction of Mes3P/Al(C6F5)3 with propylene. In this case, 19F NMR spectroscopy revealed the initial formation of a single Fcontaining product concomitant with resonances attributable to the [Mes3PH]+ cation in the 31P NMR spectrum. A triplet of triplets and a broad singlet were observed in the 1H NMR in a 1/4 ratio, together with the resonances arising from the phosphonium cation (Figure S1, Supporting Information). These data are consistent with the generation of the symmetric σ-allyl bis(aluminate) species [Mes3PH][(C6F5)3AlCH2CHCH2Al(C6F5)3]. This species is directly analogous to the previously reported salt generated from the analogous deprotonation of isobutylene (Scheme 1).10 The steric bulk of Al(C6F5)3 is thought to kinetically slow subsequent reaction with propylene, allowing the observation of this species. Nonetheless, attempts to isolate the present allyl product in an analytically pure form were unsuccessful, due to the presence of some dimerization product (Figure S1). In summary, the addition of Al/PMes3 FLPs with ethylene has been shown to be reversible, although in the case of Al halides 2 equiv of AlX3 serves to stabilize the addition products. The corresponding reactivity of these FLPs with propylene further illustrates the divergent reactivity, as the B/P FLP forms a stable addition product whereas the Al/P FLP effects C−H bond activation, resulting in the dimerization of propylene to afford the intramolecular alane−olefin complex 7. We are continuing to study the reactions of various FLPs with olefins and alkynes in an effort to uncover unique reactivity that can be exploited for catalysis and synthetic applications.

contrast, reactions of propylene with 1/2 Mes3P/AlX3 solutions did not lead to addition products. In all cases, the 31P{1H} NMR spectra exhibited a doublet at −26 ppm (1JP−H = 482 Hz) consistent with the formation of the [Mes3PH]+ cation.11 The formation of this product was accelerated using AlI3. After 24 h under propylene, the 27Al NMR spectrum indicated the presence of two separate signals at 130 (broad) and −25 ppm (sharp). This latter signal results from the known [AlI4]− anion,5f and addition of hexanes prompted the precipitation of the salt [Mes3PH][AlI4] (6). The second product was isolated as an oil from the filtrate. The 1H NMR spectrum displayed nine separate resonances. Detailed analysis of the 1D and 2D NMR spectra established this product as CH2CHCH2C(Me)HCH2AlI2 (7) (Figure 4), in which the pendant olefin is

Figure 4. 1H NMR (600 MHz) spectrum of 7 in C6D5Br.

strongly coordinated to Al. This binding together with formation of the chiral center at the carbon β to Al makes all the protons in the fragment inequivalent (except the methyl group), resulting in a rather complex coupling pattern of the 1H NMR spectrum (Figure 4). As an example, the β olefinic CH is seen to couple to the terminal olefinic protons as well as the adjacent inequivalent methylene protons, affording a wellresolved 16-line signal (Figure 4, insert). The strong binding of the olefinic unit to Al also results in a significant shift of the 13C NMR resonances of the olefinic carbons to 123.0 and 163.5 ppm for the terminal and substituted carbons, respectively, indicative of a markedly polarized bond. The corresponding 27 Al NMR spectrum showed a broad signal at 130.0 ppm. The strong binding of the olefin to Al in 7 stands in contrast to previously described B species containing pendant olefin units,11 where the olefin−boron interaction is weak and is described as a “van der Waals” interaction. Presumably the increased Lewis acidity at Al, as well as its more diffuse vacant p orbital, prompts more efficient overlap with the olefinic π orbital. The formation of 6 and 7 is thought to proceed via initial deprotonation of propylene, generating an allyl intermediate. Subsequent insertion of a second equivalent of propylene liberates 6 and generates 7 (Scheme 3). Support for the

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under an atmosphere of dry, oxygen-free N2 by means of standard Schlenk or glovebox techniques (Innovative Technology glovebox equipped with a −38 °C freezer). Hexanes and pentane (Aldrich) were dried using an Innovative Technologies solvent system. Fluorobenzene and bromobenzene were purchased from Aldrich and dried on P2O5 for several days and vacuum-distilled onto 4 Å molecular sieves prior to use. Trimethylaluminum (TMA), (o-tol)3P, and Mes3P were purchased from Strem and used without further purification. AlX3 were purchased from Strem and sublimed three times prior to use under vacuum using a −78 °C cold finger and a 100 °C (X = Cl), 80 °C (X = Br), or 150 °C (X = I) bath. Me2SiHCl was purchased from Aldrich and used without further purification. B(C6F5)3 was purchased from Boulder Scientific, sublimed under vacuum, and then treated with excess Me2SiHCl for 4 h and resublimed after removal of volatiles. Al(C6F5)3·tol was prepared from B(C6F5)3 and TMA in toluene by a known procedure.13 Ethylene (grade 3.0) and propylene (grade 2.5) were purchased from Linde and passed through a Restek oxygen scrubber and a Restek moisture trap prior to use. NMR spectra were obtained on a Bruker Avance 400 MHz, a Varian 400 MHz, or an Agilent 600 MHz NMR spectrometer, and spectra were referenced to the residual solvent of C6D5Br or to an external reference (27Al, Al(NO3)3; 31P, 85% H3PO4; 19F, CFCl3). Chemical shifts listed are in ppm, and absolute values of the coupling constants are in Hz. NMR assignments are supported by additional 2D experiments. Elemental analyses (C, H) were performed in house. Synthesis of (otol)3P(CH2CH2)Al(C6F5)3 (1) or Mes3P(CH2CH2)(AlX3)2 (X = Cl (2), Br (3), I (4)). These compounds were all synthesized in a similar fashion; therefore, only one synthesis is described. A 50 mL Schlenk bomb equipped with a Teflon cap was charged with Mes3P (150 mg, 0.39 mmol) and AlBr3 (206 mg, 0.77 mmol) in 10 mL of fluorobenzene. The bomb was transferred to the Schlenk line equipped with an ethylene outlet. The bomb was degassed at room temperature, filled with ethylene, and sealed. The solution was stirred rapidly overnight (ca. 12 h). The ethylene

Scheme 3. Proposed Reaction Pathway to 6 and 7

C


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para-C6F5), −165.5 (mt, 6F, m-C6F5). 31P{1H} NMR (THF-d8, 121 MHz, 300 K): δ 56.9. Anal. Calcd for C33H33BF15P: C, 52.40; H, 4.40. Found: C, 52.14; H, 4.36. Reaction of PMes3/AlI3 with Propylene To Generate 6 and 7. A 50 mL Schlenk bomb equipped with a Teflon cap was charged with Mes3P (286 mg, 0.74 mmol) and AlI3 (600 mg, 1.47 mmol) in 10 mL of fluorobenzene. The bomb was transferred to a Schlenk line equipped with a propylene outlet. The bomb was degassed at room temperature, filled with propylene, and sealed. The mixture was stirred rapidly for 36 h. The propylene atmosphere and solvent were removed in vacuo. Hexanes (ca. 20 mL) was added to the remaining oily residue, and the mixture was stirred vigorously for 1 h, upon which time a white precipitate formed. The solid was filtered on a glass frit and washed with copious amounts of hexanes. The isolated yield of 6 was 480 mg (0.52 mmol, 71%). The solvent was thoroughly removed from the filtrate to obtain an oil. Pentane (10 mL) was added to this, and a residual precipitate was filtered on Celite. The solvent from this filtrate was thoroughly removed to give 120 mg of 7 as a viscous oil (0.33 mmol, 45%). 6. 1H NMR (400 MHz, C6D5Br): δ 8.19 (d, 1JH−P = 482 Hz, 1H, PH), 6.78 (bs, 3H), 6.73 (bs, 3H), 2.16 (bs, 9H, o-CH3Mes), 2.11 (s, 9H, p-CH3Mes), 1.80 (bs, 9H, o-CH3Mes). 31P{1H} NMR (161 MHz, C6D5Br): δ −26.0. 27Al NMR (104 MHz, C6D5Br): δ −25. 13C{1H} NMR (100 MHz, C6D5Br): δ 146.7 (d, 4JC−P = 3 Hz, p-C6H2), 143.7 (bs, o-C6H2), 142.9 (bs, o-C6H2), 133.1 (bs, m-C6H2), 131.9 (bs, mC6H2), 111.4 (d, 1JC−P = 83 Hz, i-C6H2), 22.3 (bs, o-CH3Mes), 22.1 (bs, o-CH3Mes), 21.6 (s, p-CH3Mes). Anal. Calcd for C27H34AlI4P: C, 35.09; H, 3.71. Found: C, 35.01; H, 3.75. 7. 1H NMR (600 MHz, C6D5Br): δ 6.73 (dddd, 3JHc−Hb = 18, 3JHc−Hd = 11, 3JHc−Ha = 9, 3JHc−He = 4.9, 1H, Hc), 5.62 (dd, 3JHa−Hc = 9, 2JHa−Hb = 2, 1H, Ha), 5.48 (ddd, 3JHb−Hc = 18, 2JHb−Ha = 3, 4JHb−He = 2, 1H, Hb), 1.93 (ddddd, 2JHe−Hd = 12, 3JHe−Hc = 5.1, 3JHe−Hf = 3, 4JHe−Hb = 2, 4JHe−Hh = 2, 1H, He), 1.83−1.71 (m, 1H, Hf), 1.37 (ddd, 2JHd−He = 11, 3JHd−Hc = 11, 3JHd−Hf = 11, 1H, Hd), 0.87 (d, 3JH−Hf = 6.5, CH3, 3H), 0.57 (ddd, 2 JHh−Hg = 14, 3JHh−Hf = 4, 4JHh−He = 2, 1H, Hh), 0.03 (dd, 2JHg−Hh = 14, 3 JHg−Hf = 12, 1H, Hg). 27Al NMR (104 MHz, C6D5Br): δ 130.0 (bs, ν1/2 = 2000 Hz). 13C{1H} NMR (150 MHz, C6D5Br): δ 163.5 (s, C2), 123.0 (s, C1), 45.4 (s, C3), 33.4 (s, C4), 25.7 (s, CH3), 23.8 (bs, C5). A satisfactory elemental analysis for this compound could not be obtained, despite repeated attempts. X-ray Data Collection, Reduction, Solution, and Refinement. Single crystals were coated in Paratone-N oil in the glovebox, mounted on a MiTegen Micromount, and placed under an N2 stream. The data were collected on a Bruker Apex II diffractometer. The data were collected at 150(±2) or 293(±2) K for all crystals. Data reduction was performed using the SAINT software package and an absorption correction applied using SADABS. The structures were solved by direct methods using XS and refined by full-matrix least squares on F2 using XL as implemented in the SHELXTL suite of programs.14 All non-hydrogen atoms were refined anisotropically. Carbon-bound hydrogen atoms were placed in calculated positions using an appropriate riding model and coupled isotropic temperature factors (see the Supporting Information).

atmosphere was removed. Precipitation using hexanes (ca. 5−10 mL) afforded a white solid, which was filtered and dried on a frit. In all cases vapor diffusion of pentane into a bromobenzene solution or slow cooling yielded single crystals suitable for X-ray crystallography. 1: isolated yield 77%. 1H NMR (400 MHz, C6D5Br): δ 7.37−6.91 (m, 12H), 3.34 (dt, 2JH−P = 8 Hz, 3JH−H = 8 Hz, 2H, P−CH2), 1.85 (bs, 9H, o-CH3), 0.63−0.55 (m, 2H, CH2Al). 31P{1H} NMR (161 MHz, C6D5Br): δ 29. 27Al NMR (104 MHz, C6D5Br): δ 142 (bs, ν1/2 = ca. 2300 Hz). 19F{1H} NMR (376 MHz, C6D5Br): δ −121.4 (dd, 3 JF−F = 30 Hz, 4JF−F = 12 Hz, 6F, o-C6F5), −155.9 (t, 3JF−F = 20 Hz, 3F, p-C6F5), −162.2 (m, 6F, m-C6F5). 13C{1H} NMR (100 MHz, C6D5Br): δ 150.0 (dm, 1JC−F = 236 Hz), 142.9 (d, JC−P = 7.5 Hz), 140.4 (dm, 1JC−F = 248 Hz), 136.6 (dm, 1JC−F = 249 Hz), 134.6 (d, JC−P = 2 Hz), 134.4 (d, JC−P = 10 Hz), 133.5 (d, JC−P = 10 Hz), 127.2 (d, JC−P = 12 Hz), 118.9 (m, i-C6F5), 117.2 (d, JC−P = 79 Hz, i-C6H4), 23.4 (d, 1JC−P = 34 Hz, PCH2), 22.2 (d, 3JC−P = 4 Hz, o-CH3), 6.0 (bs, CH2Al). Anal. Calcd for C41H25AlF15P + 0.5C6H5F: C, 58.16; H, 3.05. Found: C, 58.65; H, 2.93. 2: isolated yield 76%. 1H NMR (400 MHz, C6D5Br): δ 6.68 (bs, 6H), 3.28 (dt, 2JH−P = 9 Hz, 3JH−H = 9 Hz, 2H, P−CH2), 2.08 (bs, 9H, o-CH3Mes), 2.05 (s, 9H, p-CH3Mes), 1.73 (bs, 9H, o-CH3Mes), 0.68 (bs, 2H, CH2-Al). 31P{1H} NMR (161 MHz, C6D5Br): δ 20.0. 27Al NMR (104 MHz, C6D5Br): δ 106.0 (bs, ν1/2 = 1200 Hz). 13C{1H} NMR (100 MHz, C6D5Br): δ 144.3 (d, 4JC−P = 3 Hz, p-C6H2), 143.6 (d, 2 JC−P = 10 Hz, o-C6H2), 132.8 (d, 3JC−P = 12 Hz, m-C6H2), 120.2 (d, 1 JC−P = 73 Hz, i-C6H2), 34.8 (d, 1JC−P = 39 Hz, PCH2), 24.9 (bs, oCH3Mes), 24.1 (bs, o-CH3Mes), 21.1 (s, p-CH3Mes), 10.0 (bs, CH2Al). Anal. Calcd for C29H37Al2Cl6P: C, 50.98; H, 5.46. Found: C, 50.76; H, 5.48. 3: isolated yield 79%.1H NMR (400 MHz, C6D5Br): δ 6.69 (bs, 6H), 3.31 (dt, 2JH−P = 9 Hz, 3JH−H = 9 Hz, 2H, P−CH2), 2.11 (bs, 9H, o-CH3Mes), 2.05 (s, 9H, p-CH3Mes), 1.74 (bs, 9H, o-CH3Mes), 0.89 (bs, 2H, CH2-Al). 31P{1H} NMR (161 MHz, C6D5Br): δ 21.0. 27Al NMR (104 MHz, C6D5Br): δ blank (signal lost in the probe signal). 13C{1H} NMR (100 MHz, C6D5Br): δ 144.3 (d, 4JC−P = 3 Hz, p-C6H2), 143.6 (d, 2JC−P = 10 Hz, o-C6H2), 132.8 (d, 3JC−P = 11 Hz, m-C6H2), 120.1 (d, 1JC−P = 74 Hz, i-C6H2), 34.9 (d, 1JC−P = 39 Hz, PCH2), 25.0 (bs, oCH3Mes), 23.9 (bs, o-CH3Mes), 21.0 (s, p-CH3Mes), 13.0 (bs, CH2Al). Anal. Calcd for C29H37Al2Br6P: C, 36.67; H, 3.93. Found: C, 37.24; H, 4.01. 4: isolated yield 71%. 1H NMR (400 MHz, C6D5Br): δ 6.76 (bs, 3H), 6.63 (bs, 3H), 3.34 (dt, 2JH−P = 10 Hz, 3JH−H = 8 Hz, 2H, P− CH2), 2.18 (bs, 9H, o-CH3Mes), 2.05 (s, 9H, p-CH3Mes), 1.74 (bs, 9H, o-CH3Mes), 1.19 (dt, 3JH−P = 10 Hz, 3JH−H = 10 Hz, 2H, CH2-Al). 31 1 P{ H} NMR (161 MHz, C6D5Br): δ 18.8. 27Al NMR (104 MHz, C6D5Br): δ −15.0 (bs, ν1/2 = 1500 Hz). 13C{1H} NMR (100 MHz, C6D5Br): δ 144.4 (d, 4JC−P = 3 Hz, p-C6H2), 143.6 (d, 2JC−P = 9 Hz, oC6H2), 132.9 (d, 3JC−P = 12 Hz, m-C6H2), 120.2 (d, 1JC−P = 73 Hz, iC6H2), 35.6 (d, 1JC−P = 39 Hz, PCH2), 25.6 (bs, o-CH3Mes), 24.0 (bs, o-CH3Mes), 21.1 (s, p-CH3Mes), 14.3 (bs, CH2−Al). Anal. Calcd for C29H37Al2I6P: C, 28.27; H, 3.03. Found: C, 28.41; H, 3.27. Synthesis of tBu3P(CH(CH3)CH2)B(C6F5)3 (5). To a solution of B(C6F5)3 (0.473 g, 0.92 mmol) in C6H5Br (50 mL) under propylene purge was added a solution of tBu3P (0.258 g, 1.28 mmol) in C6H5Br (2 mL). The solution was purged with propylene for 4 h, and the reaction mixture was stirred under 1 atm of propylene at room temperature for 12 h. The solvent was removed in vacuo, the residue was dissolved in CH2Cl2, and hexanes was added to precipitate a white solid. The solid was filtered and washed with hexane several times and dried in vacuo. Yield: 0.436 g (63%). Crystals suitable for X-ray diffraction were grown from a layered CH2Cl2/pentane solution at 25 °C. 1H NMR (THF-d8, 300 MHz, 300 K): δ 2.72 (br, 1H, PCH), 2.30 (br, 2H, BCH2), 1.59 (d, 27H, 3JH−P = 13 Hz, tBu), 1.57 (m, 3H, Me). 11 1 B{ H} NMR (THF-d8, 96 MHz, 300 K): δ −11.6. 13C{1H} NMR (THF-d8, 75 MHz, 300 K): partial δ 149.2 (dm, 1JC−F = 237 Hz, orthoC6F5), 139.1 (dm, 1JC−F = 230 Hz, para-C6F5), 137.57 (dm, 1JC−F = 245 Hz, meta-C6F5), 41.6 (d, 1JC−P = 25 Hz, tBu), 33.7 (d, 1JC−P = 22 Hz, PCH), 31.2 (s, tBu), 18.9 (s, Me). 19F NMR (THF-d8, 282 MHz, 300 K): δ −129.1 (br, s, 6F, o-C6F5), −162.4 (t, 3F, 3JF−F = 20 Hz,

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ASSOCIATED CONTENT

* Supporting Information S

A CIF file giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for D.W.S.: [email protected]. Notes

The authors declare no competing financial interest. D


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(7) (a) Voss, T.; Sortais, J.-B.; Fröhlich, R.; Kehr, G.; Erker, G. Organometallics 2011, 30, 584. (b) Sortais, J. B.; Voss, T.; Kehr, G.; Fröhlich, R.; Erker, G. Chem. Commun. 2009, 7417. (8) Zhao, X. X.; Stephan, D. W. J. Am. Chem. Soc. 2011, 133, 12448. (9) Zhao, X. X.; Stephan, D. W. Chem. Sci. 2012, 3, 2123. (10) Ménard, G.; Stephan, D. W. Angew. Chem., Int. Ed. 2012, 51, 4409. (11) Ménard, G.; Stephan, D. W. Angew. Chem., Int. Ed. 2012, 51, 8272. (12) Timoshkin, A. Y.; Frenking, G. Organometallics 2008, 27, 371. (13) Hair, G. S.; Cowley, A. H.; Jones, R. A.; McBurnett, B. G.; Voigt, A. J. Am. Chem. Soc. 1999, 121, 4922. (14) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

ACKNOWLEDGMENTS D.W.S. is grateful for the financial support of the NSERC of Canada and the award of a Canada Research Chair. G.M. gratefully acknowledges the financial support of the NSERC and Walter C. Sumner Fellowships.

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REFERENCES

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