HB(C6F5)2 Adduct as a Dormant ... - ACS Publications

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An Enamine/HB(C6F5)2 Adduct as a Dormant State in Frustrated Lewis Pair Chemistry Bao-Hua Xu,†,§ Kathrin Bussmann,† Roland Fröhlich,†,∥ Constantin G. Daniliuc,†,∥ Jan Gerit Brandenburg,‡,⊥ Stefan Grimme,‡,⊥ Gerald Kehr,† and Gerhard Erker*,† †

Organisch-chemisches Institut der Universität Münster, Corrensstrasse 40, 48149 Münster, Germany Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstraße 4, 53115 Bonn, Germany

‡

S Supporting Information *

ABSTRACT: The enamine piperidinocyclopentene reacts with HB(C6F5)2 by formation of the C-Lewis base/B-Lewis acid adduct 10. It shows a zwitterionic iminium ion/ hydridoborate structure. However, this adduct formation is apparently reversible and may generate the “invisible” frustrated Lewis pair 11 as a reactive intermediate by hydroboration of the enamine CC bond in an equilibrium situation at room temperature. Consequently, the FLP 11 was trapped by typical FLP reactions, namely by the reaction with dihydrogen to give the ammonium/hydridoborate 12, the acetylene deprotonation products 13 and 14, and simple borane adducts with pyridine (15) and with an isonitrile (17). The products 10 and 12−15 and the isonitrile adduct 17 were characterized by X-ray diffraction. A DFT study determined the thermodynamic features of the 10 ⇄ 11 equilibrium and of a previously discussed reference system (18 ⇄ 19) derived by reacting piperidinocyclohexene with HB(C6F5)2.

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INTRODUCTION Frustrated Lewis pair chemistry1 has made some remarkable achievements in metal-free small-molecule activation (e.g., heterolytic dihydrogen splitting2,3 and catalytic hydrogenation4,5) and small-molecule binding (e.g., CO2,6 SO2,7 nitrogen oxides8,9). Some rather unusual reactions have been carried out at intramolecular frustrated Lewis pair templates (e.g., anomalous Staudinger reaction,10 formylborane formation11). Frustrated Lewis pairs (FLP) are comprised of pairs of Lewis acids and bases that are hindered from efficient neutralization by steric1 or electronic means.12 This situation often allows for finding interesting synergistic or cooperative reactions with added substrates. However, this does not mean that FLPs are completely devoid of any interaction between their Lewis acid (LA) and base (LB) components. In the reactions of intermolecular FLPs with small molecules, some interaction between the components may even be necessary to avoid the termolecularity “trap”. In many vicinal P/B and N/B FLPs we had found weak intramolecular interactions between the Lewis acid and base components. For the examples 1 and 3 (in Scheme 1) we determined the Gibbs activation energies of the reversible rupture of the LA···LB interaction by dynamic 19F NMR spectroscopy (ca. 12−14 kcal mol−1).13−15 The typical FLP reactions of these systems are likely to proceed via the open isomers (1′, 3′) out of these equilibria. The adduct situation can be quite extreme in a few FLP cases. We recently found that the camphor-derived enamine 5 selectively formed the apparently stable adduct 616 upon treatment with Piers’ borane [HB(C6F5)2] (Scheme 2).17 Nevertheless, when this was exposed to dihydrogen under © 2013 American Chemical Society

Scheme 1

relatively mild reaction conditions (2.5 bar, room temperature) it cleanly gave the dihydrogen splitting product 8 that was derived from the “invisible” FLP 7, which had probably been available from the equilibrium situation. We have now found another (simpler) example for such a behavior, indicating that the role of preceding LA−LB adduct formation needs to be given proper attention in the design of frustrated Lewis pair systems and their reactions. Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Received: May 14, 2013 Published: July 22, 2013 6745

dx.doi.org/10.1021/om4004225 | Organometallics 2013, 32, 6745−6752

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Lewis base adduct 10 and not to the anticipated formation of the respective hydroboration product 11. This behavior is different from that of other enamine/HB(C6F5)2 reactions (see above) that in many cases cleanly resulted in the formation of the vicinal N/B FLPs under comparable reaction conditions. Nevertheless, we decided to expose the adduct 10 to dihydrogen. For that purpose the adduct 10 was generated in situ in pentane solution from the enamine 9 and HB(C6F5)2 (15 min, room temperature). Then the system was exposed to dihydrogen (2.5 bar, 30 min). After 5 min a white precipitate began to appear. After a 30 min reaction time the precipitate was collected to give a 87% yield of the product 12 (see Scheme 3 and Figure 2). The 1H NMR spectrum of compound

Scheme 2

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Scheme 3

RESULTS AND DISCUSSION The special FLP system that we have investigated here is derived from cyclopentanone. It was converted to the enamine piperidinocyclopentene (9). This enamine was then treated with 1 molar equiv of HB(C6F5)2. Workup after 2 h of stirring the dichloromethane solution at room temperature eventually gave the product 10, which was isolated as a white solid material in 94% yield. The adduct 10 was characterized by an X-ray crystal structure analysis (Figure 1). The structure determination showed that

Figure 1. Molecular structure of the zwitterionic product 10.

compound 10 was formed by addition of the intact HB(C6F5)2 Lewis acid to the nucleophilic β-carbon atom of the enamine CC double bond (B1−C2 = 1.692(4) Å, sum of the CBC angles at boron ∑BCCC = 334.3(2)°, angles C1−C2−C3 = 102.6(2)°, C1−C2−B1 = 104.2(2)°, C3−C2−B1 = 111.4(2)°). This has resulted in the formation of an iminium ion unit (C1− N1 = 1.301(3) Å, ∑C1CCN = 360.1°, ∑N1CCC = 359.8(2)°). In solution compound 10 shows a typical iminium ion 13C NMR resonance at δ 206.2 and a 11B NMR doublet at δ −19.9 (1JBH ≈ 95 Hz). Due to the chiral carbon center in the fivemembered carbocyclic ring system the C6F5 substituents at the pseudotetrahedral boron center of 10 are diastereotopic. They, consequently, give rise to two equal-intensity sets of 19F NMR resonances (δ −132.6 (o), −162.5 (p), −166.3 (m, C6F5a); δ −132.8 (o), −161.2 (p), −165.6 (m, C6F5b)). In the 1H NMR spectrum of 10 we have observed the [B]−H resonance as a broad signal at δ 2.79. We conclude that the exposure of the enamine 9 to the reactive borane HB(C6F5)2 at room temperature apparently leads to the formation of the boron Lewis acid/enamine-carbon

Figure 2. Molecular structure of the zwitterionic ammonium/hydrido borate dihydrogen activation product 12.

12 features a broad [N]−H resonance at δ 6.75 and a [B]−H signal at δ 2.63 (1:1:1:1 quartet, 1JBH ≈ 76 Hz). We have also identified the NCH (δ 2.61) and BCH (δ 1.56) resonances (corresponding 13C NMR signals at δ 74.0 (NCH) and δ 33.1 (BCH), respectively). Compound 12 shows a 11B NMR resonance at δ −21.6 (1JBH ≈ 78 Hz), and it features the 19F NMR signals of a pair of diastereotopic C6F5 groups at the boron atom (for further details see the Supporting Information). The X-ray crystal structure analysis of compound 12 shows a central five-membered carbocyclic ring that bears the Nprotonated piperidino substituent and the B(H)(C6F5)2 group trans-1,2-attached to it (C1−N1 = 1.516(3) Å, C1−C2 = 6746


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1.541(3) Å, C2−B1 = 1.633(4) Å, dihedral angle N1−C1− C2−B1 = −66.6(3)°). The protonated nitrogen atom shows a sum of its CNC angles of ∑N1CCC = 337.1°. The borate B atom likewise has a ∑B1CCC = 338.0° value that indicates a distorted pseudotetrahedral coordination geometry. The fivemembered core ring shows a distorted envelope-like conformation,18 and the piperidinium moiety has an equatorially substituted chair conformation. It appears that the ammonium/borate product 12 was formed by means of heterolytic dihydrogen cleavage by the not directly observed FLP 11. It seems to have become populated to some extent, probably by a pathway involving reversibility of the enamine/HB(C6F5)2 adduct formation followed by a regioand stereoselective hydroboration reaction of the enamine C C bond of 9. Consequently, it might be possible to trap the alleged reactive FLP intermediate 11 by other typical FLP reactions as well. This was actually the case. We could show that the adduct 10 reacted slowly with phenylacetylene to eventually yield the typical FLP reaction product 13. It had often been observed that reactive FLPs tend to deprotonate 1alkynes to form ammonium (or phosphonium) alkynyl borate products.19 In our case the respective zwitterionic ammonium/ alkynyl borate product 13 was isolated in 80% yield. It shows a [N]−H 1H NMR resonance at δ 8.41 and the 13C NMR signals of the [B]−CC−Ph unit at δ 110.1 (1JBC ≈ 70 Hz) and δ 99.2, respectively. The NCH and the BCH 1H NMR resonances were located at δ 3.12 (δ(13C) 74.5) and δ 1.91 (δ(13C) 33.4 (1JBC ≈ 55 Hz), respectively. Compound 13 shows a typical borate anion 11B NMR signal at δ −18.1. Again, the attached C6F5 groups are diastereotopic. In the X-ray crystal structure analysis of 13 (see Figure 3) we find the −CCPh group attached at the boron atom (B1−

Figure 4. Molecular structure of compound 14.

added to the boron Lewis acid (B1−C11 = 1.603(3) Å, C11− C12 = 1.208(3) Å, C12−C13 = 1.435(3) Å, C13−C14 = 1.331(3) Å). We find the usual trans-1,2-N,B-substitution pattern at the central five-membered ring of compound 14 (C1−N1 = 1.523(2) Å, C2−B1 = 1.651(3) Å, dihedral angle N1−C1−C2−B1 = 81.9(2)°). Compound 14 features a 1H NMR [N]−H signal at δ 8.34 and a 11B NMR signal at δ −18.2. The 13C NMR NCH signal occurs at δ 74.4 and that of the adjacent BCH at δ 33.3. The conjugated ene-acetylide ligand shows a set of 13C NMR resonances at δ 109.4 (BC), 100.6 (C), 129.2 (C), and 120.1 (CH2). The adduct 10 also reacts with simple donor ligands via equilibration with the hydroboration isomer 11 to yield typical FLP adducts.20 With pyridine it forms the [B]−pyridine adduct 15 (see Scheme 4), which is characterized by a 11B NMR Scheme 4

Figure 3. Projection of the molecular structure of compound 13.

resonance at δ 1.4 and the occurrence of a pair of diastereotopic C6F5 groups at boron. In this case some of the HB(C6F5)2 has also become trapped by the pyridine donor to form the pyridine−HB(C6F5)2 adduct 16 (ca. 3 mol %). The FLP−pyridine adduct 15 was characterized by X-ray diffraction (Figure 5). It features the trans-1,2-attachment of the piperidino substitutent at C1 and the B(C6F5)2(pyridine) group at C2 of the central saturated five-membered carbocycle (N1−C1 = 1.479(2) Å, C2−B1 = 1.639(2) Å, dihedral angle N1−C1−C2−B1 = 99.1(1)°). The boron atom is fourcoordinated, and it has the pyridine ligand bonded to it (B1−N2 = 1.644(2) Å). We have also treated the in situ formed pyridine adduct 15 with tert-butyl isocyanide. This leads to the formation of the new FLP−isonitrile adduct 17 (admixed with the pyridine− HB(C6F5)2 adduct 16 (ca. 6 mol %) and the enamine 9 (ca. 6

C11 = 1.598(8) Å, C11−C12 = 1.200(6) Å, C12−C13 = 1.437(8) Å, angles B1−C11−C12 = 176.5(5)°, C11−C12− C13 = 175.3(5)°). The boron and the nitrogen atom are again trans-1,2-attached at the central five-membered ring (B1−C2 = 1.668(7) Å, C1−N1 = 1.511(7) Å, dihedral angle B1−C2− C1−N1 = 76.7(5)°), and the nitrogen atom is found to be protonated (∑N1CCC = 338.3°). The enamine/HB(C6F5)2 adduct 10 reacts similarly with 2methylbutenyne. In this case the reaction was facilitated by the addition of a catalytic amount of pyridine. Stirring of a solution of 10 with the enyne overnight at room temperature resulted in the formation of the formal trapping product 14 of the alleged FLP 11 with the terminal acetylene. The X-ray crystal structure analysis (see Figure 4) shows that the conjugated acetylide anion, formed by deprotonation by the amine base of 11, was 6747


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effect. We calculated the equilibrium energies at the PW6B95D3//TPSS-D3 level utilizing def2-QZVP AO basis sets and included corrections to free enthalpies in solution with the COSMO-RS model (for details see the Supporting Information). In the case of the cyclopentane-derived system, adduct formation (giving 10) is favored by ca. 15 kcal mol−1 over the hydroboration reaction, giving the FLP 11. In contrast, in case of the cyclohexane-derived system hydroboration of the cyclohexenylamine to give the FLP 18 is slightly favored (by ca. 1.5 kcal mol−1) over the simple Lewis acid−Lewis base adduct 19 formation. This amazing thermodynamic difference is probably due to a sum of effects, but the internal stabilization of the FLP (or the lack of it) is likely a major factor. We had previously shown that the piperidinocyclohexene-derived FLP 18 shows a weak internal amine−borane interaction.15 We determined the Gibbs activation energy of the N···B dissociation of this system by dynamic 19F NMR spectroscopy as ΔG⧧diss =13.2 ± 0.2 kcal mol−1, which marks an upper limit of the N···B bond strength in 18. In the piperidino−cyclopentene hydroboration product 11 the specific geometry probably prohibits any strong internal N···B interaction so that this thermodynamic stabilizing feature is missing for the 10 ⇄ 11 equilibrium system (similar to what must be assumed for the previously described 6 ⇄ 7 equilibrium16 (see Scheme 2)).

Figure 5. Molecular structure of the FLP−pyridine adduct 15.

mol %)). We obtained crystals of compound 17 that were suitable for an X-ray crystal structure analysis (see Figure 6). It

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CONCLUSION We have shown that the enamine piperidino−cyclopentene forms the Lewis acid−Lewis base adduct 10 upon treatment with HB(C6F5)2. The formation of this adduct by attack of the nucleophilic enamino carbon at the strongly electrophilic boron center is thermodynamically favorable (by >15 kcal mol−1 according to our DFT calculations). Nevertheless, the LA− LB adduct formation is reversible. The FLP 11 can be formed in an equilibrium situation. This was shown by the respective trapping reactions that made use of some typical FLP reactions. The situation is different in the piperidino−cyclohexene/ HB(C6F5)2 case. There we have not observed the formation of the respective iminium−hydridoborate LA···LB adduct but instead hydroboration of the enamino CC bond to yield the isolated FLP 18. The main difference between these two superficially closely related systems seems to be that the FLP 18 featuring the chair-shaped cyclohexylene backbone is geometrically ideally set to undergo a stabilizing internal amine−borane LA···LB interaction that is apparently much less favorable for the cyclopentylene-derived analogue 11. It seems that this internal N···B interaction in 18 is important, since it is of a sufficient magnitude to bring some thermodynamic stabilization that helps to shift the equilibrium (19 ⇄ 18) to the FLP side, but it is weak enough still to allow for a high FLP reactivity. These systems underline the importance of weak Lewis acid− Lewis base interactions in frustrated Lewis pair chemistry and show us that a suitable balance of interaction vs free Lewis acid−base systems can very favorably determine the chemistry of specific FLP systems.

Figure 6. Molecular structure of compound 17.

shows that we have trapped the FLP 11 from the unfavorable 10 ⇄ 11 equilibrium by forming the isonitrile addition product to the boron Lewis acid (B1−C11 = 1.616(3) Å, C11−N2 = 1.141(2) Å, angles B1−C11−N2 = 175.4(2)°, C11−N2−C12 = 172.2(2)°). The free nitrogen base of the piperidino unit (∑N1CCC = 336.8°) and the boron substituent are again trans1,2-attached at the central five-membered ring (N1−C1 = 1.466(3) Å, C2−B1 = 1.630(3) Å, dihedral angle N1−C1− C2−B1 = 70.2(2)°). At first sight it may seem surprising that treatment of piperidinocyclohexene with HB(C6F5)2 gives the hydroboration product 1815 whereas the piperidinocyclopentene−HB(C6F5)2 reaction leads to a completely different result, namely to the formation of the Lewis acid−Lewis base adduct 10 (see Scheme 5). A DFT analysis21 showed that this is a thermodynamic Scheme 5

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EXPERIMENTAL SECTION

General Procedure. Reactions with air- and moisture-sensitive compounds were carried out under argon using Schlenk-type glassware or in a glovebox. Solvents and the commercially available substrate were dried and distilled under argon prior to use. Bis(pentafluorophenyl)borane (HB(C6F5)2) was prepared as described in the literature.22 The following instruments were used for physical 6748


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0.31 (−0.21) e Å−3. The hydrogen atom at B1 was refined freely; others were calculated and refined as riding atoms. Preparation of Compound 12. Enamine 9 (30 mg, 0.20 mmol) and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7 mL), and the mixture was stirred for 15 min. After the solution was degassed, H2 (2.5 bar) was introduced for 30 min, whereupon a white powder precipitated after 5 min. The precipitate was collected by cannula filtration after 2 h and washed with pentane (2 × 2 mL). All volatile components were removed in vacuo to yield compound 12 (87 mg, 87%). Crystals suitable for X-ray crystal structure analysis were grown by a saturated CH2Cl2/cyclopentane (1/3 v/v) solution of 12 at −30 °C. Anal. Calcd for C22H20BF10N: C, 52.93; H, 4.04; N, 2.81. Found: C, 52.95; H, 4.13; N, 2.87. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 6.75 (br m, 1H, NH), 3.49, 2.76 (each m, each 1H, 10-H), 3.38, 3.05 (each m, each 1H, 6-H), 2.63 (1:1:1:1 q, 1JHB ≈ 76 Hz, 1H, BH), 2.61 (m, 1H, NCH), 2.19, 0.94 (each m, each 1H, 5-H)t, 2.10, 1.90 (each m, each 1H, 9-H)t, 2.06, 1.72 (each m, each 1H, 7-H)t, 1.96, 1.47 (each m, each 1H, 8-H)t, 1.77, 1.57 (each m, each 1H, 3-H)t, 1.71, 1.52 (each m, each 1H, 4-H)t, 1.56 (m, 1H, BCH)g (a superscript “g” indicates a ghsqc experiment). 13C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ 74.0 (NCH), 55.9 (C6), 46.4 (C10), 33.1 (br 1:1:1:1 q, 1JBC ≈ 52 Hz, BCH), 28.2 (m, C5)t, 25.2 (C9)t, 24.7 (C7)t, 23.3 (br, C3)t, 22.7 (C8)t, 22.0 (br, C4)t (C6F5 not listed). 11B NMR (192 MHz, CD2Cl2, 299 K) δ −21.6 (d, 1JBH = 77.8 Hz). 19F NMR (564 MHz, CD2Cl2, 299 K): δ −133.5 (2F, o-C6F5a), −133.7 (2F, o-C6F5b), −162.6 (t, 3JFF = 20.0 Hz, 1F, p-C6F5a), −164.0 (t, 3JFF = 20.1 Hz, 1F, p-C6F5b),−165.5 (m, 2F, m-C6F5a), −167.1 (m, 2F, m-C6F5b). X-ray crystal structure analysis of compound 12: formula C22H20BF10N, M = 499.20, colorless crystals, 0.17 × 0.13 × 0.03 mm, a = 11.9859(8) Å, b = 10.9740(7) Å, c = 16.7187(6) Å, β = 102.097(3)°, V = 2150.2(2) Å3, ρcalcd = 1.542 g cm−3, μ = 1.337 mm−1, empirical absorption correction (0.804 ≤ T ≤ 0.961), Z = 4, monoclinic, space group P21/n (No. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 17568 reflections collected (±h, ±k, ±l), (sin θ)/λ = 0.60 Å−1, 3654 independent (Rint = 0.063) and 2780 observed reflections (I > 2σ(I)), 315 refined parameters, R1 = 0.054, wR2 = 0.147, maximum (minimum) residual electron density 0.24(−0.20) e Å−3. Hydrogen atoms at B1 and N1 were refined freely; others were calculated and refined as riding atoms. Preparation of Compound 13. Enamine 9 (30 mg, 0.20 mmol) and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in CH2Cl2 (7 mL), and the mixture was stirred for 15 min. Then a solution of phenylacetylene (20 mg, 0.20 mmol) in pentane (3 mL) was added. After the reaction mixture had been stirred for 12 h, the solvent and volatile components were removed under reduced pressure. The resulting residue was washed with pentane (2 × 30 mL) and dried under vacuum to give a light yellow solid (13; 96 mg, 80%). Crystals suitable for X-ray crystal structure analysis were grown by a saturated CH2Cl2/cyclopentane (1/5 v/v) solution of 13 at −30 °C. Anal. Calcd for C30H24BF10N: C, 60.12; H, 4.04; N, 2.34. Found: C, 60.32; H, 4.01; N, 2.15. 1H NMR (500 MHz, CD2Cl2, 299 K): δ 8.41 (br, 1H, NH), 7.36 (m, 2H, o-Ph), 7.29 (m, 2H, m-Ph), 7.26 (m, 1H, p-Ph), 3.75, 2.74 (each m, each 1H, 10-H), 3.40, 2.91 (each m, each 1H, 6H), 3.12 (m, 1H, NCH), 2.08, 1.01 (each m, each 1H, 5-H)t, 1.91 (br, 1H, BCH), 1.87, 1.49 (each m, each 1H, 9-H)t, 1.83, 1.37 (each m, each 1H, 7-H)t, 1.70 (m, 2H, 4-H)t, 1.58, 1.42 (each m, each 1H, 3H)t, 1.57, 1.30 (each m, each 1H, 8-H)t. 13C{1H} NMR (126 MHz, CD2Cl2, 299 K): δ 131.7 (o-Ph), 128.9 (m-Ph), 128.0 (p-Ph), 125.2 (iPh), 123.7 (br, i-C6F5), 110.1 (br 1:1:1:1 q, 1JBC ≈ 70 Hz, BC), 99.2 (PhC), 74.5 (NCH), 55.7 (C6), 46.4 (C10), 33.4 (br 1:1:1:1 q, 1JBC ≈ 55 Hz, BCH), 29.4 (C5)t, 24.9 (C7)t, 24.6 (C4)t, 23.7 (C3, C9)t, 22.5 (C8)t (C6F5 not listed). 11B{1H} NMR (160 MHz, CD2Cl2, 299 K) δ −18.1 (ν1/2 ≈ 40 Hz). 19F NMR (470 MHz, CD2Cl2, 299 K): δ −130.5 (m, 2F, o-C6F5a), −133.7 (m, 2F, o-C6F5b), −162.0 (t, 3JFF = 20.4 Hz, 1F, p-C6F5a), −163.5 (t, 3JFF = 20.3 Hz, 1F, p-C6F5b), −165.3 (m, 2F, m-C6F5a), −166.4 (m, 2F, m-C6F5b). X-ray crystal structure analysis of compound 13: formula C30H24BF10N, M = 599.31, colorless crystals, 0.28 × 0.16 × 0.13 mm, a = 14.2206(10) Å, b = 15.3184(12) Å, c = 16.8620(20) Å, β = 113.912(7)°, V = 3357.9(5) Å3, ρcalcd = 1.185 g cm−3, μ = 0.942 mm−1,

characterization of the compounds. Elemental analyses: Foss-Heraeus CHNO-Rapid. ESI mass spectra: Bruker Daltonics MicroTof. NMR: Varian UNITY plus 600 NMR spectrometer (1H, 599.9 MHz; 13C, 150.8 MHz; 19F, 564.4 MHz; 11B, 192.4 MHz), Varian 500 MHz INOVA (1H, 499.9 MHz; 13C, 125.7 MHz; 19F, 470.3 MHz; 11B, 160.4 MHz); Bruker AV 300 (1H, 300 MHz; 13C, 75 MHz, 31P, 122 MHz; 19 F, 282 MHz, 11B, 96 MHz). In 1H NMR and 13C NMR, the chemical shift δ is given relative to TMS and referenced to the solvent signal; 19 F NMR, chemical shift δ is given relative to CFCl3 (external reference); 11B NMR, chemical shift δ is given relative to BF3·Et2O (external reference). Assignments of the resonances were supported by 2D experiments. In the lists of NMR data, a superscript “t” indicates a tentative assignment. X-ray diffraction: Data sets were collected with a Nonius KappaCCD diffractometer. Programs used: data collection, COLLECT;23 data reduction, Denzo-SMN;24 absorption correction, Denzo;25 structure solution, SHELXS-97;26 structure refinement, SHELXL-97;27 graphics, XP.28 Thermal ellipsoids are shown with 30% (10, 12, 14, 15, 17) or 15% (13) probability, R1 values are given for observed reflections, and wR2 values are given for all reflections. Preparation of Enamine 9. The reactions were carried out under argon. A mixture of cyclopentanone (1.6 mL, 18.2 mmol) and piperidine (5.3 mL, 54.6 mmol, 3 equiv) in pentane was stirred at room temperature for 1 h and was cooled to 0 °C. Then a solution of titanium tetrachloride (1 mL, 9.1 mmol, 0.5 equiv) in pentane was added dropwise over 30 min. The reaction mixture was stirred for 1 h before it was warmed to room temperature and stirred for another 5 h. The resulting gray suspension was filtered under argon, and the residue was washed with pentane (2 × 30 mL). The organic phases (pentane) were combined, and all volatile components were removed under reduced pressure. Pure enamine 9 (2.3 g, 83%) was obtained by distillation of the crude product at 53−55 °C under 2 mbar. 1H NMR (600 MHz, C6D6, 299 K): δ 4.49 (m, 1H, CH), 2.76 (m, 4H, NCH2), 2.47 (m, 2H, 3-H), 2.31 (m, 2H, 5-H), 1.85 (m, 2H, 4-H), 1.40 (m, 4H, 7-H), 1.28 (m, 2H, 8-H). 13C{1H} NMR (151 MHz, C6D6, 298 K): δ 152.5 (CN), 96.9 (CH), 49.7 (NCH2), 32.3 (C5), 31.0 (C3), 25.9 (C7), 24.8 (C8), 23.2 (C4). Preparation of Compound 10. HB(C6F5)2 (79 mg, 0.20 mmol) was dissolved in CH2Cl2 (6 mL) and the solution stirred for 5 min. A solution of 9 (30 mg, 0.20 mmol) in CH2Cl2 (2 mL) was added at room temperature. After the reaction mixture had been stirred for 2 h, all volatile components were removed under reduced pressure. The resulting residue was washed with pentane (2 × 30 mL) and dried under vacuum to give a white solid of 10 (93 mg, 94%). Crystals suitable for X-ray crystal structure analysis were grown by diffusion of cyclopentane into a saturated CH2Cl2 solution of 10 at −30 °C. Anal. Calcd for C22H18BF10N: C, 53.15; H, 3.65; N, 2.82. Found: C, 53.16; H, 3.81; N, 2.88. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 3.76, 3.41 (each m, each 1H, 6-H), 3.63, 2.76 (each m, each 1H, 10-H), 3.44 (br, 1H, 2-H), 3.02, 2.52 (each m, each 1H, 5-H), 2.79 (br m, 1H, BH), 2.07, 1.84 (each m, each 1H, 4-H)t, 2.06, 1.96 (each m, each 1H, 3H)t, 1.77, 1.68 (each m, each 1H, 7-H)t, 1.72, 1.56 (each m, each 1H, 8-H)t, 1.54, 1.43 (each m, each 1H, 9-H)t. 13C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ 206.2 (NC), 123.5 (br, i-C6F5), 54.4 (C10), 54.0 (C6), 46.1 (br, C2), 34.8 (C5), 31.5 (C3)t, 26.5 (C9)t, 26.3 (C7)t, 23.2 (C8)t, 21.1 (C4)t (C6F5 not listed). 11B NMR (192 MHz, CD2Cl2, 299 K): δ −19.9 (d, 1JBH ≈ 95 Hz). 19F NMR (564 MHz, CD2Cl2, 299 K): δ −132.6 (m, 2F, o-C6F5a), −132.8 (m, 2F, o-C6F5b), −161.2 (t, 3JFF = 20.1 Hz, 1F, p-C6F5b), −162.5 (t, 3JFF = 20.2 Hz, 1F, p-C6F5a), −165.6 (m, 2F, m-C6F5b), −166.3 (m, 2F, m-C6F5a). X-ray crystal structure analysis of compound 10: formula C22H18BF10N, M = 497.18 colorless crystals, 0.48 × 0.42 × 0.10 mm, a = 9.4611(2) Å, b = 10.7218(2) Å, c = 11.6021(3) Å, α = 70.181(1)°, β = 82.793(1)°, γ = 81.050(1)°, V = 1090.37(4) Å3, ρcalcd = 1.514 g cm−3, μ = 0.148 mm−1, empirical absorption correction (0.932 ≤ T ≤ 0.985), Z = 2, triclinic, space group P1̅ (No. 2), λ = 0.71073 Å, T = 223(2) K, ω and φ scans, 9370 reflections collected (±h, ±k, ±l), (sin θ)/λ = 0.60 Å−1, 3674 independent (Rint = 0.038) and 3188 observed reflections (I > 2σ(I)), 311 refined parameters, R1 = 0.056, wR2 = 0.139, maximum (minimum) residual electron density 6749


Organometallics

Article

empirical absorption correction (0.778 ≤ T ≤ 0.887), Z = 4, monoclinic, space group P21/n (No. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 16908 reflections collected (±h, ±k, ±l), (sin θ)/λ = 0.60 Å−1, 4567 independent (Rint = 0.100) and 2344 observed reflections (I > 2σ(I)), 382 refined parameters, R1 = 0.088, wR2 = 0.286, maximum (minimum) residual electron density 0.22(−0.18) e Å−3. The hydrogen atom at N1 was refined freely, but with a fixed U value; others were calculated and refined as riding atoms. Preparation of Compound 14. Enamine 9 (30 mg, 0.20 mmol) and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7 mL), and the mixture was stirred for 30 min. Then a solution of pyridine (ca. 3 mg, 0.04 mmol, 0.2 equiv) in pentane (3 mL) was added and the reaction mixture was stirred for another 1 h before a pentane solution (3 mL) of 2-methylbut-1-en-3-yne (20 mg, 0.30 mmol, 1.5 equiv) was added. Upon reaction overnight, the solvent and volatile components were removed under reduced pressure to give a white solid of 14 as a main product together with the pyridine− HB(C6F5)2 adduct 16 in a ratio of 4/1a (overall yield 112 mg; ca. 84% (14)). Anal. Calcd for C27H24BF10N: C, 57.57; H, 4.29; N, 2.49. Found: C, 57.30; H, 4.30; N, 2.41. Crystals suitable for X-ray crystal structure analysis were grown by a saturated CH2Cl2/cyclopentane (1/ 4 v/v) solution of the white solid (mixture of 14 and 16 in a ratio of 4/ 1a) at −30 °C (a determined by NMR in CD2Cl2 solution). 1H NMR (500 MHz, CD2Cl2, 299 K): δ 8.34 (br, NH), 5.14 (dq, 2JHH = 2.2 Hz, 4 JHH = 1.6 Hz, 1H, CH2Z), 5.13 (dp, 2JHH = 2.2 Hz, 4JHH = 1.0 Hz, 1H, CH2E), 3.75, 2.80 (each m, each 1H, 10-H), 3.44, 2.98 (each m, each 1H, 6-H), 3.06 (m, 1H, NCH), 2.04, 1.82 (each m, each 1H, 7H)t, 2.03, 1.79 (each m, each 1H, 9-H)t, 2.03, 0.95 (each m, each 1H, 5-H)t, 1.91, 1.44 (each m, each 1H, 8-H)t, 1.85 (dd, 4JHH = 1.6 Hz, 4 JHH = 1.0 Hz, 3H, CH3), 1.82 (br m, 1H, BCH), 1.71 (m, 2H, 4-H)t, 1.57, 1.40 (each m, each 1H, 3-H)t, [16: 8.57 (m, 2H, o-py), 8.17 (m, 1H, p-py), 7.69 (m, 2H, m-py)]. 13C{1H} NMR (126 MHz, CD2Cl2, 299 K): δ 129.2 (C), 120.1 (CH2), 109.4 (br 1:1:1:1 q, 1JBC ≈ 72 Hz, BC), 100.6 (br, C), 74.4 (NCH), 55.8 (C6), 46.4 (C10), 33.3 (br 1:1:1:1 q, 1JBC ≈ 54 Hz, BCH), 29.3 (C5)t, 25.1 (C7)t, 24.6 (C4)t, 24.2 (CH3), 23.8 (C9)t, 23.7 (C3)t, 22.7 (C8)t (16: 147.5 (opy), 142.4 (p-py), 126.7 (m-py)) (C6F5 not listed). 11B NMR (160 MHz, CD2Cl2, 299 K) δ −8.3 (d, 1JBH ≈ 100 Hz, 16), −18.2 (ν1/2 ≈ 30 Hz, 14). 19F NMR (470 MHz, CD2Cl2, 299 K): δ −130.6 (m, 2F, o-C6F5a), −133.8 (m, 2F, o-C6F5b), −162.2 (t, 3JFF = 20.3 Hz, 1F, pC6F5a), −163.7 (t, 3JFF = 20.3 Hz, 1F, p-C6F5b), −165.4 (m, 2F, mC6F5a), −166.5 (m, 2F, m-C6F5b) (16: −133.9 (m, 2F, o-C6F5), −158.8 (t, 3JFF = 20.0 Hz, 1F, p-C6F5), −164.9 (m, 2F, m-C6F5)). X-ray crystal structure analysis of compound 14: formula C27H24BF10N, M = 563.28, colorless crystals, 0.35 × 0.18 × 0.12 mm, a = 8.8914(4) Å, b = 21.7999(5) Å, c = 13.4296(5) Å, β = 97.589(4)°, V = 2580.28(16) Å3, ρcalcd = 1.450 g cm−3, μ = 1.184 mm−1, empirical absorption correction (0.682 ≤ T ≤ 0.871), Z = 4, monoclinic, space group P21/n (No. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 19179 reflections collected (±h, ±k, ±l), (sin θ)/λ = 0.60 Å−1, 4460 independent (Rint = 0.041) and 3900 observed reflections (I > 2σ(I)), 357 refined parameters, R1 = 0.046, wR2 = 0.122, maximum (minimum) residual electron density 0.18(−0.26) e Å−3. The hydrogen atom at N1 was refined freely; others were calculated and refined as riding atoms. Preparation of Compound 15. Enamine 9 (30 mg, 0.20 mmol) and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7 mL), and the mixture was stirred for 30 min. Then a solution of pyridine (16 mg, 0.20 mmol) in pentane (3 mL) was added to the white suspension and a clear solution was formed immediately. After the reaction mixture had been stirred for 2 h, the solvent and volatile components were removed under reduced pressure to give a white solid of 15 as the main product together with compound 16 (ca. 3 mol %). Anal. Calcd for C27H23BF10N2: C, 56.27; H, 4.02; N, 4.86. Found: C, 56.43; H, 3.98; N, 4.55. Crystals suitable for X-ray crystal structure analysis were grown by a saturated cyclopentane solution of the white solid (mixture of 15 and 16) at −30 °C. 1H NMR (500 MHz, CD2Cl2, 299 K): δ 9.32 (m, 2H, o-py), 8.07 (m, 1H, p-py), 7.59 (m, 2H, m-py), 2.68, 2.24 (each m, each 2H, NCH2), 2.65 (m, 1H, NCH), 2.47 (m, 1H, BCH), 1.80, 1.23 (each m, each 1H, 5-H)t, 1.77, 0.86 (each m,

each 1H, 3-H)t, 1.51 (m, 4H, NCH2), 1.44 (m, 2H, 8-H), 1.38, 1.27 (each m, each 1H, 4-H)t (16: 8.57 (m, 2H, o-py), 8.17 (m, 1H, p-py), 7.69 (m, 2H, m-py)). 13C{1H} NMR (126 MHz, CD2Cl2, 299 K): δ 148.5 (o-py), 141.6 (p-py), 125.7 (m-py), 70.2 (NCH), 50.9 (br, NCH2), 34.0 (br, BCH), 30.5 (C5)t, 27.5 (NCH2), 26.9 (C4)t, 25.5 (C8), 25.2 (C3)t, (16: 147.5 (o-py), 142.4 (p-py), 126.7 (m-py)) (C6F5 not listed). 11B NMR (160 MHz, CD2Cl2, 299 K) δ 1.4 (ν1/2 ≈ 180 Hz, 15), −8.3 (d, 1JBH ≈ 100 Hz, 16). 19F NMR (470 MHz, CD2Cl2, 299 K): δ −129.7 (m, 2F, o-C6F5a), −131.1 (m, 2F, o-C6F5b), −159.3 (t, 3JFF = 20.4 Hz, 1F, p-C6F5b), −159.9 (t, 3JFF = 20.5 Hz, 1F, p-C6F5a), −164.6 (m, 2F, m-C6F5b), −164.9 (m, 2F, m-C6F5a) (16: −134.0 (m, 2F, o-C6F5), −158.8 (t, 3JFF = 20.1 Hz, 1F, p-C6F5), −164.8 (m, 2F, m-C6F5)). X-ray crystal structure analysis of 15: formula C27H23BF10N2, M = 576.28, colorless crystals, 0.40 × 0.32 × 0.10 mm, a = 10.6415(2) Å, b = 10.9495(7) Å, c = 11.2153(7) Å, α = 86.286(3)°, β = 77.514(3)°, γ = 87.659(3)°, V = 1272.73(12) Å3, ρcalcd = 1.504 g cm−3, μ = 1.227 mm−1, empirical absorption correction (0.639 ≤ T ≤ 0.887), Z = 2, triclinic, space group P1̅ (No. 2), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 17127 reflections collected (±h, ±k, ±l), (sin θ)/λ = 0.60 Å−1, 4329 independent (Rint = 0.031) and 4148 observed reflections (I > 2σ(I)), 361 refined parameters, R1 = 0.039, wR2 = 0.106, maximum (minimum) residual electron density 0.19 (−0.20) e Å−3. Hydrogen atoms were calculated and refined as riding atoms. Preparation of Compound 17. Caution! Many isocyanides are toxic and must be handled with due care. Enamine 9 (30 mg, 0.20 mmol) and HB(C6F5)2 (79 mg, 0.20 mmol) were dissolved in pentane (7 mL), and the mixture was stirred for 30 min. Then a solution of pyridine (ca. 3 mg, 0.04 mmol, 0.2 equiv) in pentane (3 mL) was added, and the reaction mixture was stirred for another 1 h before a pentane solution (3 mL) of 2-isocyano-2-methylpropane (17 mg, 0.20 mmol) was added. Upon reaction overnight, the solvent and volatile components were removed under reduced pressure to give a white solid of 17 as the main product together with 16 in a ratio of 9/1a (overall yield 95 mg; ca. 75% (17)). Crystals suitable for X-ray crystal structure analysis were grown by a saturated CH2Cl2/cyclopentane (1/ 5 v/v) solution of the white solid (mixture of 17, 16 (6 mol %), and 9 (6 mol %)) at −30 °C (a determined by NMR in CD2Cl2 solution). Anal. Calcd for C27H27BF10N2: C, 55.88; H, 4.69; N, 4.83. Found: C, 55.58; H, 4.45; N, 4.70. 1H NMR (600 MHz, CD2Cl2, 299 K): δ 2.59 (m, 1H, NCH), 2.42, 2.08 (each m, each 2H, NCH2), 2.11 (m, 1H, BCH), 2.01, 1.15 (each m, each 1H, 5-H)t, 1.66, 1.41 (each m, each 1H, 4-H)t, 1.64 (s, 9H, tBu), 1.59, 1.46 (each m, each 1H, 3-H)t, 1.24 (m, 2H, 8-H), 1.14, 1.09 (each br m, each 2H, NCH2) (16: 8.57 (m, 2H, o-py), 8.18 (m, 1H, p-py), 7.70 (m, 2H, m-py)). 13C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ 128.5 (br, CN), 73.3 (NCH), 60.4 (tBu), 50.8 (br, NCH2), 30.6 (C5)t, 29.6 (br, BCH), 29.4 (tBu), 27.0 (NCH2), 25.8 (C3)t, 25.4 (C8), 23.9 (C4)t (16: 147.5 (o-py), 142.4 (ppy), 126.7 (m-py)) (C6F5 not listed). 11B NMR (192 MHz, CD2Cl2, 299 K): δ −8.3 (d, 1JBH ≈ 100 Hz, 16), −16.9 (ν1/2 ≈ 120 Hz, 17). 19F NMR (564 MHz, CD2Cl2, 299 K): δ −132.6 (m, 2F, o-C6F5a), −133.8 (m, 2F, o-C6F5b), −159.3 (t, 3JFF = 20.2 Hz, 1F, p-C6F5a), −162.1 (t, 3 JFF = 20.3 Hz, 1F, p-C6F5b), −164.9 (m, 2F, m-C6F5a), −166.6 (m, 2F, m-C6F5b) (16: −134.0 (m, 2F, o-C6F5), −158.8 (t, 3JFF = 20.1 Hz, 1F, p-C6F5), −164.8 (m, 2F, m-C6F5)). X-ray crystal structure analysis of compound 17: formula C27H27BF10N2, M = 580.32, colorless crystals, 0.28 × 0.22 × 0.15 mm, a = 14.8527(9) Å, b = 11.7383(4) Å, c = 15.9004(8) Å, β = 95.858(8)°, V = 2757.7(2) Å3, ρcalcd = 1.398 g cm−3, μ = 1.133 mm−1, empirical absorption correction (0.742 ≤ T ≤ 0.848), Z = 4, monoclinic, space group P21/c (No. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 22826 reflections collected (±h, ±k, ±l), (sin θ)/λ = 0.60 Å−1, 4781 independent (Rint = 0.042) and 3986 observed reflections (I > 2σ(I)), 364 refined parameters, R1 = 0.048, wR2 = 0.136, maximum (minimum) residual electron density 0.23(−0.21) e Å−3, hydrogen atoms calculated and refined as riding atoms. 6750


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S Supporting Information *

Text and figures giving further experimental and spectroscopic details, CIF files giving crystallographic data for 10, 12, 13−15, and 17, and tables giving Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*E-mail for G.E.: [email protected]. Present Address §

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, No. 1, Beiertiao, Haidian District, Beijing 100190, People’s Republic of China.

Author Contributions ∥

X-ray crystal structure analyses.

Author Contributions ⊥

DFT calculations.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. REFERENCES

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Organometallics

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Article

NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on July 22, 2013, with an error in Scheme 1. The corrected version was reposted on July 23, 2012.

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