Intramolecular frustrated N/B lewis pairs by enamine hydroboration

Abstract

A series of enamines undergoes anti-Markovnikov hydroboration with Piers' borane HB(C₆F₅)₂ to yield C₂-bridged frustrated N/B Lewis pairs, featuring weak N⋯B interaction. Some examples show typical frustrated Lewis pair reactions with dihydrogen or with 1-alkynes. One such N/B FLP serves as an active catalyst for the hydrogenation of enamines. Many of the new compounds were characterized by X-ray diffraction.

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Introduction

Frustrated Lewis Pairs (FLP) are systems of co-existing active Lewis acids and bases. Mostly, their mutual annihilation is prevented by the presence of sufficient steric hindrance caused by bulky substituents of both the Lewis acid and the Lewis base component of the pair.¹ Meanwhile a number of combinations of the strong boron Lewis acid B(C₆F₅)₃ with various phosphorus,² nitrogen^3,4 or even carbon centered⁵ bulky Lewis bases have been described.⁶ From the onset of FLP chemistry, intramolecular non-quenched Lewis acid/base combinations have received special attention.⁷

We had described the intramolecular phosphine/borane system 1 which so far is one of the most active FLP systems for cooperative heterolytic dihydrogen cleavage.^8,9 The hydrogen activation system 1/2 (see Chart 1) is an active metal-free catalyst for the hydrogenation of enamines or imines (other FLP systems even catalytically hydrogenate silyl enol ethers¹⁰). The frustrated Lewis pair 1 was shown to have a weak Lewis acid–Lewis base interaction.^11,12

Chart 1 Examples of frustrated P/B Lewis pairs.

We had shown that the derivative 3 features a four-membered heterocyclic ring structure in the crystal that contains a long (and probably weak) P–B bond. From the dynamic ¹⁹F-NMR spectra of 3 we determined a Gibbs activation energy of ΔG^≠ (298 K) = 12.1 ± 0.3 kcal mol⁻¹ for P–B bond dissociation in the intramolecular FLP 3 (see Chart 1).¹³

Since amines represent an important class of Lewis bases, we have now prepared a series of related C₂-bridged intramolecular N/B frustrated Lewis pairs and compared their interesting features with those of the established intramolecular P/B Lewis pairs.

Results and discussions

Synthesis and structural characterization

We prepared the new C₂-bridged amine/borane frustrated Lewis pairs (5) by means of enamine routes. As a typical example 1-diethylaminostyrene (4a)¹⁴ was reacted with “Piers' borane” [HB(C₆F₅)₂]¹⁵ by anti-Markovnikov orientated hydroboration to yield the N/B Lewis pair 5a (70%). Compound 5a was characterized spectroscopically, by C,H,N elemental analysis and by X-ray diffraction (see Fig. 1 and Table 1).

Table 1 Selected structural data of the frustrated N/B Lewis pairs 5a, c and d (bond lengths in Å, angles in °)

Compound:	5a	5c	5d
N–B	1.719(3)	1.723(2)	1.824(6)
N–C1	1.564(3)	1.563(2)	1.508(6)
C1–C2	1.516(3)	1.567(3)	1.514(5)
C2–B	1.627(3)	1.682(3)	1.634(7)
C1–N–B	84.8(1)	88.1(1)	82.3(3)
C2–B–N	84.3(2)	85.5(1)	81.2(3)
N–C1–C2	93.7(2)	95.3(1)	96.5(3)
C1–C2–B	89.6(2)	89.4(1)	88.8(3)
C31–B–C41	109.3(2)	102.9(1)	109.6(3)
N–C1–C2–B	−22.0(2)	10.3(1)	−28.2(3)
C1–C2–B–N	20.0(1)	−9.3(1)	23.1(3)

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Fig. 1 A view of the molecular structure of compound 5a.

Compound 5a features a central NC₂B four-membered heterocyclic ring system. The N–B linkage is rather long at 1.719(3) Å.¹⁶ The B–C2 bond (1.627(3) Å) is significantly shorter than the adjacent B–C31 (1.640(3) Å and B–C41 (1.641(3) Å) linkages (angle C31–B–C41: 109.3(2)°). The central four-membered ring is slightly puckered (angle between the C2–C1–N and C2–B–N planes: 29.5°).

In solution, compound 5a shows a ¹¹B NMR signal in the borate region at δ 2.1. It features the ¹H/¹³C NMR signals of diastereotopic ethyl groups at nitrogen [¹³C: δ 50.2, 9.0/40.7, 8.3]. The CH₂ group of each N–C₂H₅ unit also shows diastereotopic splitting in the ¹H NMR spectrum due to the chirality center at C1 of the bridge (for details see the Supporting Information†). The [N]–CHPh–CH₂–[B] bridge shows a typical pattern of ¹H NMR resonances at δ 2.20/1.80 (²J_HH = 13.5 Hz) and 4.55 (³J_HH = 13.5 and 7.1 Hz) [¹³C: δ 17.9 (br, C2), 75.0 (C1)]. The pair of C₆F₅ substituents at boron are also diastereotopic (470 MHz, 298 K, in benzene-d₆). They show Δδ¹⁹F(m,p) differences (see Scheme 1) in a range between typical values of tri- and tetracoordinated boron.¹⁷

Scheme 1 Synthesis and selected physical data of the frustrated N/B Lewis pairs 5a-e

The analogous HB(C₆F₅)₂ hydroboration reaction of the piperidino substituted styrene derivative 4b gave the N/B Lewis pair 5b. It shows similar spectroscopic features as 5a. Due to the chirality center (C1 of the bridging –CHPh–CH₂–unit) the hydrogens of the CH₂ pairs of the piperidino moiety have become diastereotopic, giving rise to a total of nine separate signals in the ¹H NMR and five resonances in the ¹³C NMR spectrum.

Hydroboration of the tetrasubstituted enamine carbon–carbon double bond of 4c gave compound 5c which was characterized by X-ray diffraction. The structural features of 5c are similar to those of 5a (see Table 1). The N–B bond length in the central four-membered heterocyclic subunit of 5c was found at 1.723(2) Å (for a view of the structure and further details see the Supporting Information). In solution compound 5c shows the NMR signals of a pair of diastereotopic methyl substituents at C2 of the bridge [¹H: δ 1.33, 0.95 (each 3H), ¹³C: δ 33.2, 27.3].

Hydroboration of piperidino-cyclohexene (4d) with HB(C₆F₅)₂ gave the FLP system 5d. The X-ray crystal structure analysis revealed the 1,2-trans-attachment of both the –N(C₅H₁₀) and the –B(C₆F₅)₂ substituents at the chair-shaped cyclohexane backbone. The chiral (racemic) N/B frustrated Lewis pair 5d also shows a direct N-B interaction, although the specific attachment at the cyclohexylene carbocycle appears to introduce some strain: the N-B bond in 5d (1.824(6) Å) is by about 0.1 Å longer than the respective bonds in the related non-annulated compounds 5a and 5c (see Table 1 and Fig. 2).

Fig. 2 A view of the molecular structure of the FLP system 5d.

Eventually, we reacted the closely related enamine starting material pyrrolidino-cyclohexene (4e) with HB(C₆F₅)₂ to obtain 5e, which was isolated in 70% yield.

Dynamic features

We were able to determine the Gibbs activation energies ΔG^≠ (T)_diss of the N–B dissociation process for the chiral examples 5b and 5d of this new class of compounds from their dynamic NMR spectra.

The chiral four-membered heterocyclic Lewis pair 5b shows the ¹⁹F NMR spectra of a pair of diastereotopic C₆F₅ groups at boron. Ring opening of 5b by N–B bond dissociation would give the open isomer 5b′ (see Scheme 2) which contains a non-stereogenic trigonal-planar boron center.¹³ Consequently, its C₆F₅ groups are homotopic in the NMR experiment. Therefore, rapid equilibration between the closed global minimum isomer 5b with the not directly observable open local minimum structure 5b′ of higher energy content on the NMR time scale would reveal itself by temperature dependent coalescence of the signals of respective pairs of diastereotopic groups.

Scheme 2 Dynamic behaviour of the N/B FLPs 5.

This is actually observed: At low temperature the ¹⁹F NMR spectrum of compound 5b (564 MHz, 283 K) shows well separated sets of ortho-, para- and meta-signals of the pair of C₆F₅ substituents at boron. Increasing the monitoring temperature rapidly results in a pairwise coalescence of the ortho-, para- and meta- ¹⁹F NMR signals to eventually result in the observation of a single averaged set (see Fig. 3 and the Supporting Information†). From these dynamic ¹⁹F NMR spectra we estimated the Gibbs activation energy of the N-B dissociation process in compound 5b as ΔG^≠(318 K) = 13.8 ± 0.2 kcal mol⁻¹ (based on the p-¹⁹F resonance; for details see the Supporting Information†).

Fig. 3 Dynamic ¹⁹F NMR spectra of compound 5b, only the p- and m-C₆F₅ resonances are depicted.

The analogous investigation was carried out with the cyclohexylene-bridged N/B Lewis pair 5d. From the coalescence of the respective pairs of ¹⁹F NMR resonances we estimated the Gibbs activation barrier of the N-B dissociation process at ΔG^≠ (298 K) = 13.2 ± 0.2 kcal mol⁻¹ (based on the p-19F resonance; for details see the Supporting Information†). We note that this value is slightly lower for 5d than for 5b which probably reflects the increased strain of the four-membered heterocycle in the former due to the ring annulation. This coincides with the observed rather long N–B distance in 5d (see above).

Chemical properties and reactions

The reaction of the FLPs 5a and 5b with pyridine led to the formation of their respective Lewis acid/Lewis base adducts 6a and 6b, respectively (Scheme 3).¹⁸ In the latter case, adduct formation is evidenced by a characteristic changing of the ¹H NMR ABX pattern of the [N]–CHPh–CH₂–[B] signals of the C₂-bridge, the ¹⁹F NMR chemical shifts and the chemical shift of the ¹¹B NMR resonance [5b: δ 2.1; 6b: δ −0.6] (for further details see the Experimental Section and the Supporting Information†).

Scheme 3 Reaction of the N/B FLPs 5 with pyridine.

Compound 5d undergoes a typical FLP reaction with terminal alkynes.¹⁹ Thus, treatment of in situ generated 5d with 1-pentyne takes place rapidly at room temperature in pentane solution to yield the zwitterionic product 7b as a red solid (59% yield, see Scheme 4). The product 7b shows a ¹H NMR N–H resonance (of 1H rel. intensity) at δ 9.42 and a ¹¹B NMR signal at δ −17.8. It features the ¹⁹F NMR signals of a pair of diastereotopic C₆F₅ substituents at boron.

The compound 7b was characterized by X-ray diffraction (see Fig. 4 and Table 2). It shows a non-constrained B/N-substituted cyclohexane chair. The [–B(C₆F₅)₂–C≡C–C₃H₇] borate substituent and the [–NH(C₅H₁₀)] ammonium group are trans-1,2-attached at the ring. Both the borate B and the ammonium N centers are pseudo-tetrahedrally coordinated. The nitrogen atom has a proton attached to it and the remaining acetylide anion in the zwitterionic product 7b is found bonded to boron (see Fig. 4).

Table 2 Selected structural parameters of the zwitterionic FLP/acetylene reaction products 7 and 8 (bond lengths in Å, angles in °)

Compound:	7a	7b	7c	8
R	Ph	n-C3H7	n(CH2)3C≡CH	Ph
N⋯B	3.162(3)	3.158(2)	3.160(4)	3.233(4)
N–C1	1.524(3)	1.528(2)	1.531(4)	1.538(3)
C1–C2	1.539(3)	1.538(2)	1.531(4)	1.536(4)
C2–B	1.658(3)	1.666(2)	1.659(5)	1.646(4)
B–C3	1.601(3)	1.602(2)	1.608(5)	1.594(5)
C3–C4	1.205(3)	1.199(2)	1.201(4)	1.216(4)
C2–B–C3	114.7(2)	114.3(1)	112.9(3)	109.9(2)
C1–C2–B	117.0(2)	116.4(1)	116.7(2)	114.8(2)
N–C1–C2	111.4(2)	111.3(1)	112.5(2)	108.9(2)
N–C1–C2–B	−56.7(2)	−57.2(2)	−54.9(4)	−72.4(3)
C1–C2–B–C3	57.1(3)	54.1(2)	60.7(4)	59.1(3)

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Scheme 4 Reactions of the N/B FLPs 5 with 1-alkynes

Fig. 4 Molecular structure of compound 7b.

The reaction of 5d with phenylacetylene takes an analogous course: deprotonation of the C–H acidic C(sp)–H acetylene terminus of the alkyne by the amine Lewis base component takes place and the remaining phenylacetylide moiety is then accommodated by the adjacent boron Lewis acid functionality^19,20 to give the zwitterionic cyclohexylene-bridged ammonium–alkynyl borate product 7a (isolated in 64% yield).

Compound 7a was characterized spectroscopically, by C,H,N elemental analysis and by X-ray crystal structure analysis (depicted in the Supporting Information,† see also Table 2). Similarly, the reaction of 5d with 1,7-octadiyne gave the mono-C-H activation product 7c, that was also characterized by X-ray diffraction. Its essential structure features are similar to those of 7a and 7b (for details see Table 2 and the Supporting Information†).²¹

We have also reacted the frustrated Lewis pair 5b with phenylacetylene and obtained the zwitterionic product 8 (58% isolated). It shows a N–H ammonium ¹H NMR resonance at δ 8.92 and a ¹¹B NMR signal at δ −19.2, typical of a borate anion system. The X-ray crystal structure analysis (Fig. 5) shows a slightly opened gauche arrangement of the C1–N (1.538(3) Å) and C2–B (1.646(4) Å) vectors at the connecting C1–C2 unit (θ N–C1–C2–B: −72.4(3)°). The ammonium N–H is directed towards the linear B-acetylide unit (angles B–C3–C4: 175.6(3)°, C3–C4–C5: 176.9(4)°).

Fig. 5 A view of the zwitterion 8.

Heterolytic dihydrogen splitting and activation

The frustrated P/B Lewis pairs 1 and 3 very efficiently split dihydrogen heterolytically^1,11,12,13 under mild conditions (see above). In principle, the new frustrated N/B Lewis pairs react similar, although there are a number of differences in detail. We exposed the N/B Lewis pairs 5a–e to 2.5 bars of dihydrogen at room temperature, usually in pentane solution. All the positive reactions were subsequently confirmed by the analogous treatment of the systems 5 with deuterium (D₂) to actually involve heterolytic splitting of the H–H, respectively D–D bond.

Under these typical “ambient” conditions the systems 5a and 5c did not react with hydrogen. In contrast, the apparently less sterically hindered frustrated N/B Lewis pair 5b reacted rapidly with H₂ at these standard conditions to yield the zwitterionic ammonium/hydridoborate product 9b. It was isolated as a colourless solid in 58% yield. The reactivity of the 5b system towards H₂ is quite similar to that of related intramolecular frustrated P/B Lewis pairs. The product 9b (see Scheme 5) was characterized spectroscopically [¹¹B NMR: δ −22.8 (d, ¹J_BH ∼ 80 Hz); ¹H NMR: δ 7.46 (br, 1H, N–H), 3.08 (br 1 : 1 : 1 : 1 q, 1H, ¹J_BH ∼ 80 Hz, B–H) and by X-ray diffraction (see Fig. 6 and Table 3).

Scheme 5 Reaction of some N/B FLPs with dihydrogen

Table 3 Selected structural data of the zwitterionic H₂ activation products 9 (bond lengths in Å, angles in °)

Compound:	9b	9d	9e
N–C1	1.539(3)	1.523(2)	1.520(3)
C1–C2	1.524(3)	1.537(2)	1.536(3)
C2–B	1.638(3)	1.652(2)	1.652(3)
N⋯B	3.039(3)	3.011(2)	2.992(3)
N–H⋯H–B	1.90	1.77	1.80
C1–N–C11	115.0(2)	114.8(1)	116.6(2)
C1–N–C15/(14)	113.4(2)	114.6(1)	116.6(2)
C2–B–C31	114.0(2)	110.9(1)	111.0(2)
C2–B–C41	111.6(2)	115.3(1)	117.5(2)
N–C1–C2	108.0(2)	109.8(1)	109.0(2)
C1–C2–B	113.1(2)	114.7(1)	112.8(2)
N–C1–C2–B	−60.7(2)	50.0(2)	54.0(2)

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Fig. 6 Molecular structure of the zwitterionic hydrogen activation product 9b.

The X-ray crystal structure analysis of compound 9b shows the presence of an ammonium unit (C11–N–C15: 110.6(2)°) and a hydridoborate moiety (C31–B–C41 107.5(2)°) attached at the (–CHPh–CH₂)-bridge. This central unit attains a gauche conformation of the C1–N and C2–B vectors (θ N–C1–C2–B: −60.7(2)°). The N–H and B–H vectors are oriented towards each other.

The reaction of the cyclohexylene-bridged N/B Lewis pair 5d with dihydrogen proceeded analogously to yield the zwitterion 9d [50% isolated; ¹¹B NMR: δ −21.7 (d, ¹J_BH ∼ 78 Hz), ¹H NMR: δ 7.58 (N–H)]. Compound 9d was also characterized by X-ray diffraction (see Fig. 7 and Table 3). The analogous reaction of the pyrrolidine derived N/B Lewis pair (5e) with dihydrogen gave the H₂-splitting product 9e. It was also characterized by an X-ray crystal structure analysis. Its structural features are similar to those of 9d (see the Supporting Information† for a view of the structure of 9e and further structural details).

Fig. 7 Molecular structure of compound 9d.

The hydrogen activation system 5b/9b is a metal-free catalyst for the hydrogenation of enamines. First we hydrogenated the enamines 4b and 4d at “ambient” conditions (i.e. 2.5 bar H₂ pressure, at r.t. in toluene, 20h). For these experiments we used a rather large amount of the 9b catalyst and isolated the corresponding tert. amines in 83% (10b) and 53% (10d), respectively (see Scheme 6). The catalyst performance was better at 60 bar H₂ pressure. Here, reasonable product conversions were obtained with 5 mol% of the 9b catalyst at room temperature.

Scheme 6 Enamine hydrogenation catalysed by the N/B system 9b

Reactions with carbonyl compounds

The ethylene-bridged intramolecular frustrated P/B Lewis pair 1 and its congeners react rapidly with aldehydes by 1,2-addition to yield the respective six-membered heterocyclic products.^19,22,23 We have reacted the cyclohexylene-bridged intramolecular frustrated N/B Lewis pair 5d with benzaldehyde and observed the formation of a different product type. From the reaction mixture we isolated the product 12a as a white crystalline solid. The X-ray crystal structure analysis revealed the formation of a new zwitterionic product type (Fig. 8). It contains an iminium ion functionality inside of the former piperidine ring (N1–C15: 1.277(3) Å, angle N–C15–C14: 124.4(2) Å). Apparently a hydride was abstracted in the α-position to nitrogen and transferred to the benzaldehyde reagent. Consequently, we find its reduced –O–CH₂–Ph benzyl form attached at the intramolecular –B(C₆F₅)₂ moiety of 12a (B–O3: 1.471(2) Å, O3–C4: 1.416(2) Å, angles C2–B–O3: 108.5(2)°, B–O3–C4: 119.6(1)°, O3–C4–C5: 108.7(2)°).

Fig. 8 A projection of the molecular structure of compound 12a.

The NMR spectra confirmed the formation of the product 12a by an intramolecular disproportion reaction. There are several possible mechanistic pathways to explain this result. One is depicted in Scheme 7 which takes into account that strongly electrophilic boranes of the type R–B(C₆F₅)₂ can abstract hydride anion from the α-CH position of amines to give iminium salts.^16,18,24 We observed the ¹H/¹³C NMR feature of the newly formed –CH=N⁺R– functionality of 12a at δ 8.45/173.9. The ¹¹B NMR resonance of 12a is found at δ 1.6. The ¹⁹F NMR spectrum reveals the presence of a pair of diastereotopic C₆F₅ groups at boron and we observe the ¹H NMR features of the newly formed benzyl –CH₂[Ph] group at δ 4.14/3.83 [AB, ²J_HH = 10.8 Hz; ¹³C: 67.8].

Scheme 7 Reactions of the N/B FLP 5d with carbonyl compounds

The reaction of 5d with benzophenone proceeded analogously to give the related trapping product 12b (for details including the X-ray crystal structure analysis see the Supporting Information†).

Conclusions

The series of enamines 4 used in this study cleanly undergo anti-Markovnikov hydroboration with Piers' borane HB(C₆F₅)₂ to yield the intramolecular frustrated N/B Lewis pairs 5. The systems 5 feature a weak intramolecular N⋯B interaction. The chiral examples of 5 show activation barriers of N–B bond dissociation of ca. 13.5 kcal mol⁻¹, which is slightly higher than it was observed for the corresponding intramolecular frustrated P/B Lewis pairs.^13,25 Nevertheless, ring opening of the systems 5 is facile and, consequently, many of these compounds show typical frustrated Lewis pair behaviour, although they have a higher tendency of allowing competing reactions pathways to be followed. When the examples of 5 are not too bulky their nitrogen Lewis base components deprotonate C–H acidic acetylenes with formation of 7 and 8. Sterically non-encumbered examples of 5 rapidly cleave dihydrogen heterolytically to yield the respective ammonium/hydridoborate zwitterions (9). One of these systems (9b) is a reasonable metal-free catalyst for enamine hydrogenation. We conclude that the C₂-bridged N/B Lewis pairs 5 in many respect resemble their corresponding C₂-connected P/B pairs, but that their stability and frustrated Lewis pair reactivity seems to be somewhat lower. Nevertheless, some of the N/B examples 5 are as good FLPs as the extensively studied analogous P/B pairs. Since the N/B pairs should become easily available in a great structural variability by the introduced and described hydroboration route of readily available enamines we expect the chemistry of such hydrocarbylene-bridged N/B systems to become a very useful addition to frustrated Lewis pair chemistry.

Experimental

As a typical example we here describe the preparation of the N/B-FLP 5b and its reaction with dihydrogen to give the zwitterion 9b.

Preparation of 5b

N-(1-phenylethen-1-yl)piperidine (4b) (100 mg, 0.535 mmol) was added to a suspension of bis(pentafluorophenyl)borane (185 mg, 0.535 mmol) in pentane (10 mL). The reaction mixture was stirred for 20 min at room temperature. Then the precipitate was collected, washed with pentane (3 mL) and dried in vacuo to obtain 5b as a white solid (160 mg, 57%). Anal. Calc. C₂₅H₁₈BF₁₀N: C 56.31, H 3.40, N 2.63; found: C 56.74, H 3.76, N 2.60. ¹H NMR (600 MHz, 298 K, benzene-d₆) δ = 7.06(1H)/7.03(2H)/7.02(2H) (Ph), 4.48 (dd, ³J_HH = 13.2 Hz, ³J_HH = 6.7 Hz, 1H, 1-H), 2.98/2.44, 2.85/2.59, 1.19/0.94, 0.77 (2H), 0.47/0.08 (each m, each 1H, C₅H₁₀N), 2.15 (dd, ²J_HH = 13.6 Hz, ³J_HH = 6.7 Hz, 1H, 2-H), 1.97 (dd, ²J_HH = 13.6 Hz, ³J_HH = 13.2 Hz, 1H, 2′-H). ¹³C{¹H} NMR (151 MHz, 298 K, benzene-d₆) δ = 136.6/130.7/129.6/128.7 (Ph), 75.7 (C1), 57.4/47.2/22.3/21.8/21.3 (C₅H₁₀N), 18.5 (C2) [C₆F₅ not listed]. ¹¹B{¹H} NMR (192 MHz, 298 K, toluene-d₈) δ = 2.1. ¹⁹F NMR (564 MHz, 298 K, toluene-d₈) δ = −127.9 (2F, o-C₆F₅^A), −129.4 (2F, o-C₆F₅^B), −156.0 (1F, p-C₆F₅^B), −157.5 (1F, p-C₆F₅^A), −163.1 (2F, m-C₆F₅^B), −163.5 (2F, m-C₆F₅^A).

Preparation of 9b

The reaction of compound 5b [generated in situ by using 4b (57 mg, 0.305 mmol) and HB(C₆F₅)₂ (105 mg, 0.305 mmol) in pentane (5 mL)] with dihydrogen (2.5 bar) was completed after 2h stirring at room temperature. The solvent was removed and the residue was washed with pentane (3 × 5ml) to get 9b as a white solid (93 mg, 58%). ¹H NMR (600 MHz, 298 K, benzene-d₆) δ = 7.46 (br, 1H, N–H), 7.07 (1H)/7.00 (2H)/6.73 (2H) (Ph), 3.52 (d, ³J_HH = 13.6 Hz, 1H, 1-H), 3.08 (br, 1 : 1 : 1 : 1 q, ¹J_BH ∼ 80 Hz, 1H, B-H), 2.66/1.13, 2.21/1.66, 1.03/0.80, 0.89/0.76, 0.77/−0.12 (each m, each 1H, C₅H₁₀N), 1.85/1.75 (each m, each 1H, 2-H). ¹³C{¹H} NMR (151 MHz, 298 K, benzene-d₆) δ = 133.5/130.3/129.9/128.8 (Ph), 73.7 (C1), 53.3/45.7/23.6/23.3/21.4 (C₅H₁₀N), 22.4 (br, C2), [C₆F₅ not listed]. ¹¹B NMR (192 MHz, 298 K, benzene-d₆) δ = −22.8 (d, ¹J_BH ∼ 80 Hz). ¹⁹F NMR (564 MHz, 298 K, benzene-d₆) δ = −132.6 (m, 2F, o-C₆F₅^A), −133.7 (m, 2F, o-C₆F₅^B), −160.6 (m, 1F, p-C₆F₅^B), −161.8 (m, 1F, p-C₆F₅^A), −164.1 (m, 2F, m-C₆F₅^B), −165.6 (m, 2F, m-C₆F₅^A).

Acknowledgements

Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

Notes and references

1. D. W. Stephan and G. Erker, Angew. Chem., 2010, 122, 50; D. W. Stephan and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 46.
2. D. Chen, Y. Wang and J. Klankermayer, Angew. Chem., 2010, 122, 9665; D. Chen, Y. Wang and J. Klankermayer, Angew. Chem., Int. Ed., 2010, 49, 9475; M. Ullrich, A. J. Lough and D. W. Stephan, Organometallics, 2010, 29, 3647; R. C. Neu, E. Y. Ouyang, S. J. Geier, X. Zhao, A. Ramos and D. W. Stephan, Dalton Trans., 2010, 39, 4285; C. Jiang, O. Blacque and H. Berke, Organometallics, 2009, 28, 5233; A. Ramos, A. J. Lough and D. W. Stephan, Chem. Commun., 2009, 1119; M. Ullrich, A. Lough and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 52; D. P. Huber, G. Kehr, K. Bergander, R. Fröhlich, G. Erker, S. Tanino, Y. Ohki and K. Tatsumi, Organometallics, 2008, 27, 5279; S. J Geier, T. M. Gilbert and D. W. Stephan, J. Am. Chem. Soc., 2008, 130, 12632; P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew. Chem., 2007, 119, 8196; P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew. Chem., Int. Ed., 2007, 46, 8050; G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129, 1880; G. C. Welch, L. Cabrera, P. A. Chase, E. Hollink, J. D. Masuda, P. Wei and D. W. Stephan, Dalton Trans., 2007, 3407.
3. C. Jiang, O. Blacque, T. Fox and H. Berke, Organometallics, 2011, 30, 2117; E. Theuergarten, D. Schüns, J. Grunenberg, C. G. Daniliuc, P. G. Jones and M. Tamm, Chem. Commun., 2010, 46, 8561; G. Erós, H. Mehdi, I. Pápai, T. A. Rakob, P. Király, G. Tárkányi and T. Soós, Angew. Chem., 2010, 122, 6709; G. Erós, H. Mehdi, I. Pápai, T. A. Rakob, P. Király, G. Tárkányi and T. Soós, Angew. Chem., Int. Ed., 2010, 49, 6559; S. J. Geier, A. L. Gille, T. M. Gilbert and D. W. Stephan, Inorg. Chem., 2009, 48, 10466; S. J. Geier and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 3476; C. Jiang, O. Blacque and H. Berke, Chem. Commun., 2009, 5518; K. V. Axenov, G. Kehr, R. Fröhlich and G. Erker, Organometallics, 2009, 28, 5148; V. Sumerin, F. Schulz, M. Nieger, M. Atsumi, C. Wang, M. Leskelä, P. Pyykkö, T. Repo and B. Rieger, J. Organomet. Chem., 2009, 694, 2654; V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger, M. Leskelä, T. Repo, P. Pyykkö and B. Rieger, J. Am. Chem. Soc., 2008, 130, 14117; V. Sumerin, F. Schulz, M. Nieger, M. Leskelä, T. Repo and B. Rieger, Angew. Chem., 2008, 120, 6090; V. Sumerin, F. Schulz, M. Nieger, M. Leskelä, T. Repo and B. Rieger, Angew. Chem., Int. Ed., 2008, 47, 5861.
4. In some cases a nitrogen containing substrate in catalytic metal-free FLP hydrogenation can serve as the Lewis base: S. J. Geier, P. A. Chase and D. W. Stephan, Chem. Commun., 2010, 46, 4884; K. V. Axenov, G. Kehr, R. Fröhlich and G. Erker, J. Am. Chem. Soc., 2009, 131, 3454; P. A. Chase, T. Jurca and D. W. Stephan, Chem. Commun., 2008, 1701; D. Chen and J. Klankermayer, Chem. Commun., 2008, 2130.
5. P. A. Chase, A. Gille, T. M. Gilbert and D. W. Stephan, Dalton Trans., 2009, 7179; D. Holschumacher, C. Taouss, T. Bannenberg, C. G. Hrib, C. G. Daniliuc, P. G. Jones and M. Tamm, Dalton Trans., 2009, 6927; P. A. Chase and D. W. Stephan, Angew. Chem., 2008, 120, 7543; P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed., 2008, 47, 7433; D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and M. Tamm, Angew. Chem., 2008, 120, 7538; D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428; G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller and G. Bertrand, Science, 2007, 316, 439.
6. J. Spielmann, F. Buch and S. Harder, Angew. Chem., 2008, 120, 9576; J. Spielmann, F. Buch and S. Harder, Angew. Chem., Int. Ed., 2008, 47, 9434; R. Noyori and T. Ohkuma, Angew. Chem., 2001, 113, 40; R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 2001, 40, 40; R. Noyori, M. Yamakawa and S. Hashiguchi, J. Org. Chem., 2001, 66, 7931; T. Ohkuma, M. Koizumi, H. Ikehira, T. Yokozawa and R. Noyori, Org. Lett., 2000, 2, 659; T. Ohkuma and R. Noyori, Hydrogenation of Carbonyl Groups, Springer, Berlin, 1999; C. Walling and L. Bollyky, J. Am. Chem. Soc., 1964, 86, 3750; C. Walling and L. Bollyky, J. Am. Chem. Soc., 1961, 83, 2968.
7. D. W. Stephan, Chem. Commun., 2010, 46, 8526; D. W. Stephan, Dalton Trans., 2009, 3129; D. W. Stephan, Org. Biomol. Chem., 2008, 6, 1535; G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124.
8. P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme and D. W. Stephan, Chem. Commun., 2007, 5072.
9. S. Schwendemann, T. A. Tumay, K. V. Axenov, I. Peuser, G. Kehr, R. Fröhlich and G. Erker, Organometallics, 2010, 29, 1067; P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fröhlich and G. Erker, Angew. Chem., 2008, 120, 7654; P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fröhlich and G. Erker, Angew. Chem., Int. Ed., 2008, 47, 7543.
10. H. Wang, R. Fröhlich, G. Kehr and G. Erker, Chem. Commun., 2008, 5966.
11. S. Grimme, H. Kruse, L. Goerigk and G. Erker, Angew. Chem., 2010, 122, 1444; S. Grimme, H. Kruse, L. Goerigk and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 1402.
12. For other theoretical FLP studies, see: e.g. B. Schirmer and S. Grimme, Chem. Commun., 2010, 46, 7942; S. Gao, W. Wu and Y. Mo, J. Phys. Chem. A, 2009, 113, 8108; T. A. Rokob, A. Hamza and I. Pápai, J. Am. Chem. Soc., 2009, 131, 10701; T. A. Rokob, A. Hamza, A. Stirling and I. Pápai, J. Am. Chem. Soc., 2009, 131, 2029; T. Privalov, Chem.–Eur. J., 2009, 15, 1825; T. A. Rokob, A. Hamza, A. Stirling, T. Soós and I. Pápai, Angew. Chem., 2008, 120, 2469; T. A. Rokob, A. Hamza, A. Stirling, T. Soós and I. Pápai, Angew. Chem., Int. Ed., 2008, 47, 2435; T. Privalov, Dalton Trans., 2009, 1321; Y. Guo and S. Li, Inorg. Chem., 2008, 47, 6212; A. Stirling, A. Hamza, T. A. Rokob and I. Pápai, Chem. Commun., 2008, 3148.
13. K. Axenov, C. M. Mömming, G. Kehr, R. Fröhlich and G. Erker, Chem.–Eur. J., 2010, 16, 14069.
14. J. Paleček and O. Paleta, Synthesis, 2004, 4, 521; R. Carlson and A. Nilsson, Acta Chem. Scand., Ser. B, 1984, 38, 49; W. White and H. Winegarten, J. Org. Chem., 1967, 32, 213.
15. D. J. Parks, W. E. Piers and G. P. A. Yap, Organometallics, 1998, 17, 5492; W. E. Piers and T. Chivers, Chem. Soc. Rev., 1997, 26, 345; D. J. Parks, R. E.v H. Spence and W. E. Piers, Angew. Chem., 1995, 107, 895; D. J. Parks, R. E.v H. Spence and W. E. Piers, Angew. Chem., Int. Ed. Engl., 1995, 34, 809.
16. For a comparison: B(C₆F₅)₃–NH(C₅H₁₀) B–N: 1.628(2) Å; B(C₆F₅)₃–N (C₂H₅)₃ B–N: 1.633(4) Å. See: F. Focante, P. Mercandelli, A. Sironi and L. Resconi, Coord. Chem. Rev., 2006, 250, 170; A. J. Mountford, D. L. Hughes and S. J. Lancaster, Chem. Commun., 2003, 2148.
17. T. Beringhelli, D. Donghi, D. Maggioni and G. D'Alfonso, Coord. Chem. Rev., 2008, 252, 2292.
18. G. Erker, Dalton Trans., 2005, 1883; W. E. Piers, Adv. Organomet. Chem., 2004, 52, 1.
19. See for a comparison: C. M. Mömming, S. Frömel, G. Kehr, R. Fröhlich, S. Grimme and G. Erker, J. Am. Chem. Soc., 2009, 131, 12280.
20. For other reaction modes of alkynes with FLPs see: e.g. M. A. Dureen, C. C. Brown and D. W. Stephan, Organometallics, 2010, 29, 6594; C. Chen, G. Kehr, R. Fröhlich and G. Erker, J. Am. Chem. Soc., 2010, 132, 13594; C. Chen, F. Eweiner, B. Wibbeling, R. Fröhlich, S. Senda, Y. Ohki, K. Tatsumi, S. Grimme, G. Kehr and G. Erker, Chem.–Asian J., 2010, 5, 2199; T. Voss, C. Chen, G. Kehr, E. Nauha, G. Erker and D. W. Stephan, Chem.–Eur. J., 2010, 16, 3005; S. Geier, M. A. Dureen, E. Y. Ouyang and D. W. Stephan, Chem. Eur. J., 2010, 16, 988; C. Jiang, O. Blacque and H. Berke, Organometallics, 2010, 29, 125; M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396.
21. For FLP reactions with non-conjugated diynes see: e.g. C. Chen, R. Fröhlich, G. Kehr and G. Erker, Chem. Commun., 2010, 46, 3580.
22. C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich and G. Erker, Dalton Trans., 2010, 39, 7556.
23. C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich, B. Schirmer, S. Grimme and G. Erker, Angew. Chem., 2010, 122, 2464; C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich, B. Schirmer, S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2010, 49, 2414.
24. P. Liptau, M. Neumann, G. Erker, G. Kehr, R. Fröhlich and S. Grimme, Organometallics, 2004, 22, 21; S. Guidotti, I. Camurati, F. Focante, L. Angellini, G. Moscardi, L. Resconi, R. Leardini, D. Nanni, P. Mercandelli, A. Sironi, T. Beringhelli and D. Maggioni, J. Org. Chem., 2003, 68, 5445; N. Millot, C. C. Santini, B. Fenet and J.-M. Basset, Eur. J. Inorg. Chem., 2002, 3328.
25. P. Spies, G. Kehr, K. Bergander, B. Wibbeling, R. Fröhlich and G. Erker, Dalton Trans., 2009, 1534.

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Footnotes

† Electronic supplementary information (ESI) available: Synthetic details and characterization of the new compounds. CCDC reference numbers 815781–815791. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1sc00124h

‡ X-ray crystal structure analyses.

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