B Pairs by Borane

Article pubs.acs.org/JACS

Cite This: J. Am. Chem. Soc. 2018, 140, 3635−3643

Formation of Reactive π‑Conjugated Frustrated N/B Pairs by BoraneInduced Propargyl Amine Rearrangement Tongdao Wang, Constantin G. Daniliuc, Christian Mück-Lichtenfeld, Gerald Kehr, and Gerhard Erker* Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany S Supporting Information *

ABSTRACT: N-Propargyltetramethylpiperidine reacts with a series of trans-alkenyl-B(C6F5)2 compounds to give the substituted alkenyl-bridged frustrated N/B Lewis pairs 5. Their structures and spectroscopic features indicate a pronounced participation of the mesomeric s-trans-iminium/ borata-alkene resonance form. The compounds are thought to be formed in a stepwise addition/rearrangement process which is initiated by a trans-1,2-amine/borane FLP addition to the carbon−carbon triple bond to generate a reactive zwitterionic aziridinium/alkenylborate intermediate. Subsequent alkenylborate attack leads to opening of the activated three-membered heterocycle with clean formation of the products 5a−c. Treatment of the propargyl-TMP substrate with B(C6F5)3 gave a stable example of such an aziridinium/borate betaine, which was isolated and amply characterized. The products 5a−c are active N/B FLPs. They split dihydrogen heterolytically under mild conditions to give the respective NH+/BH− products 9a−c. These contain Z-configurated core CC double bonds, which indicates rotational equilibration around the central C−C bond of 5a−c during this reaction. Structural and chemical features of the 5c system were analyzed by DFT calculations.

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INTRODUCTION Intramolecular hydrocarbyl-bridged phosphane/borane and amine/borane pairs have contributed significantly to the current development of frustrated Lewis pair (FLP) chemistry.1 Some unsaturated C2-bridged examples (e.g., of compound type 1 in Scheme 12) as well as their phenylene-bridged P/B or

Scheme 1, respectively. Borata-alkenes are the dominant mesomeric resonance forms of the α-boryl carbanions.6 The− B(C6F5)2 containing anions profit from an extraordinarily high thermodynamic stabilization. This has become evident by the high CH acidity of, e.g., H3C−B(C6F5)2, which is in the order of cyclopentadiene.7 The easy availability of the RCH B(C6F5)2− anions has found some interesting applications.8,9 In addition to the RCHB(C6F5)2− anions the chemistry of the other essential building block of the mesomeric form 1’ of the unsaturated vicinal P/B FLPs (see Scheme 1), the respective methylene phosphonium cations, had recently been developed and described.10,11 The structural data and chemical reactivities of a variety of examples of the systems 1 were studied.2 However, no strong indication for a marked conjugative interaction between the phosphane nucleophile and the borane electrophile across the bridging organic CC π-linker in the systems had been found so far, maybe with the exception of the respective formally heteroaromatic 1,4-P/B and 1,4-N/B examples.12 In the corresponding unsaturated N/B systems 2, conjugative interaction might become a more serious issue in view of the increased stabilization of the ubiquitous iminium ions13 vs the “phospha-iminium”, i.e., methylene phosphonium ions.10,11 In order to put this to an experimental test, it

Scheme 1. Question of Conjugative Lewis Acid/Lewis Base Interaction in Unsaturated Vicinal Frustrated Lewis Pairs

N/B analogues3−5 have extended the scope of reaction variants in FLP chemistry to some extent. Their typical structural and chemical features posed the question of possible π-conjugation between the borane Lewis acid and the phosphane or amine Lewis base functionalities across the connecting unsaturated organic linker in such unsaturated FLP systems. Conjugative interaction between these π-connected functional groups might be considered reasonable in view of the known chemistry of the pair of heteroatom π-functionalities to be formulated in the zwitterionic mesomeric forms 1’ and 2’ in © 2018 American Chemical Society

Received: November 10, 2017 Published: February 14, 2018 3635

DOI: 10.1021/jacs.7b11958 J. Am. Chem. Soc. 2018, 140, 3635−3643

Article

Journal of the American Chemical Society

We reacted the propargyl amine 3 with the conjugated dienyl borane 4b20 under similar conditions. In this case the borane reagent was generated in situ by HB(C6F5)2 hydroboration of 2-methylbutenyne and then the propargyl amine 3 was added (toluene, 1 h). Workup gave compound 5b, which we isolated in 65% yield. The reaction of 3 with the tBu substituted alkenyl borane 4c was performed similarly. In this case, the propargyl amine was added to a suspension of the isolated alkenyl borane (toluene, 1 h). Removal of the volatiles gave the pure product 5c (Scheme 2). From both of the compounds 5b and 5c we obtained single crystals at −35 °C from dichloromethane solutions layered with n-pentane, that were suited for their characterization by X-ray diffraction (Table 2, Figures 1 and 2).

would be desirable to have the corresponding unsaturated C2bridged N/B FLPs 2/2′ available for an investigation. We had prepared many of the P/B compounds 1 in a straightforward way by 1,1-carboboration14 of the alkynyl phosphanes Ar2PCCR1 with boranes R2B(C6F5)2.2,15 1,1Carboboration reactions of ynamines16 required special migrating groups17 and in view of the uncertainties of developing a suitable nitrogen analogue we decided to search for a principally different synthetic approach to the unsaturated N/B FLP systems 2. This was achieved by a borane induced rearrangement of a bulky propargyl amine. To the best of our knowledge, this transformation seems to represent a new reaction type for borane/amine combinations. We will describe the first examples of the systems 2 made by this propargyl amine rearrangement approach and try to answer the question about the significance and consequence of conjugative interaction between the two functional parts of these systems.

Table 2. Selected Structural Parameters of the N/B Compounds 5b and 5ca

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RESULTS AND DISCUSSION Synthesis and Characterization of the New N/B FLP Systems. We prepared the starting material N-propargyltetramethylpiperidine (3) by base induced treatment of HTMP with propargyl bromide (K2CO3, KI, 150 °C).18 Compound 3 was then reacted with a small series of alkenyl-bis(pentafluorophenyl)boranes (4), which had been obtained by hydroboration of the respective terminal alkynes HCCR (R: Ph, −C(Me)CH2, tBu) with Piers’ borane [HB(C6F5)2].2e,19 Typically, the reaction of the propargyl amine with the alkenyl borane 4a (R = Ph) was carried out at rt toluene solution (1 h) to give a close to quantitative yield of the reaction product 5a (Scheme 2). Compound 5a was characterized by C,H,Nelemental analysis and NMR spectroscopy (see below and Table 1).

N1−C1 C1−C2 C2−B1 C1−C3 C3−C4 C4−C5 N1−C11 N1−C15 B1−C21 B1−C31 N1−C1−C2 C1−C2−B1 N1−C1−C2-B1 ∑ N1CCC ∑ B1CCC a

5b

5c

1.367(3) 1.396(4) 1.479(4) 1.535(4) 1.511(4) 1.335(4) 1.524(3) 1.531(3) 1.612(4) 1.604(4) 122.4(2) 132.6(3) −174.5(3) 360.0 359.9

1.356(3) 1.395(4) 1.480(4) 1.513(4) 1.517(4) 1.272(4) 1.522(4) 1.523(3) 1.597(4) 1.602(4) 122.8(3) 132.0(3) −174.4(3) 359.9 359.9

Bond lengths in angstroms and angles in degrees.

Scheme 2. Synthesis of the N/B FLPs 5

Table 1. Selected NMR Data of Compounds 5a−ca 11

B 19 F: Δδ(19Fm,p) 13 C: C1 C2 C3 1 H: 2-H 3-H 4-H 5-H

5a

5b

5c

47.0 7.8 180.7 120.5 41.0 6.11 3.53 6.00 6.00

47.0 7.7 181.0 120.4 40.9 6.05 3.41 5.43 5.69

46.6 7.6 182.3 120.3 40.9 6.02 3.29 5.12 5.20

Figure 1. Molecular structure of compound 5b/5′b (thermal ellipsoids are shown at the 30% probability level).

The X-ray crystal structure analysis of compound 5b shows that the compound contains a C2-bridged N/B pair.21 The CH2 group originating from the propargyl moiety of the starting material is found attached at the former alkynyl carbon atom C1 of this framework. It has the alkenyl substituent attached to it that originated from the alkenyl borane reagent. The B(C6F5)2 moiety is found attached at the former terminal

Chemical shifts rel TMS, (1H, 13C) δ scale, CD2Cl2 solution (at 299 K).

a

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Journal of the American Chemical Society Scheme 3. Mechanistic Description of the Propargyl Rearrangement Leading to 5

Figure 2. View of the molecular structure of the tBu-alkenylsubstituted N/B FLP 5c/5′c (thermal ellipsoids are shown at the 15% probability level).

subsequent stabilization by nucleophilic attack of the alkenyl group at the borate moiety at the aziridinium CH2 group that leads to ring opening with direct formation of the observed specific rearrangement product 5. This proposed reaction sequence is strongly supported by the outcome of the reaction of the propargyl amine 3 with the strong boron Lewis acid B(C6F5)3. Addition of a solution of the propargyl amine in toluene to a toluene suspension of B(C6F5)3 at rt resulted in the rapid formation of a white precipitate of the zwitterionic addition product 7 (isolated in 59% yield). The product is not stable at rt, neither in solution nor in the solid state, but can be stored at low temperature. Single crystals suited for the X-ray crystal structure analysis were obtained at −35 °C from a dichloromethane solution layered with npentane. It shows the presence of an aziridinium cation moiety25 that had been formed by internal amine addition to the proximal sp-carbon atom of the propargylic alkynyl unit in cooperation with B(C6F5)3 Lewis acid addition to the distal spC atom. Product 7 was formed by selective trans-1,2-N/B addition (Scheme 3 and Figure 3). In solution (CD2Cl2, 238 K) compound 7 shows the 1H NMR signal of the CH2 group inside the three membered ring at δ 2.97 (13C: δ 34.0) and the olefinic 3-H resonance at δ 6.82 ppm. The adjacent exocyclic olefinic unit gives rise to a pair of 13 C NMR signals at δ 120.4 (C2) and δ 127.5 ppm (C3), respectively, and compound 7 shows a typical borate 11B NMR feature at δ −16.6 ppm. Reaction of the N/B Systems with Dihydrogen. The N/ B systems 5a−c are active dihydrogen splitting reagents1,26 under mild conditions. Typically, the phenyl-substituted compound 5a was exposed to H2 at 2.0 bar pressure at rt in dichloromethane solution for 3 h. Workup gave the zwitterionic hydrocarbyl bridged ammonium/hydridoborate product 9a as a white solid in 86% yield. It was characterized by C,H,Nelemental analysis, NMR spectroscopy, and X-ray diffraction. The X-ray crystal structure analysis revealed the presence of the TMP-H and −BH(C6F5)2 moieties (sum of heteroatom bond angles: ΣN1CCC = 349.9° and ΣB1CCC = 331.5°) cis-1,2attached at the connecting C1−C2 carbon−carbon double bond (Figure 4). Both the functional groups are found in conformational orientations that exhibit the bulky substituents arranged toward the outside of the framework with the N−H proton and B−H hydride oriented inside toward each other (N−H···H−B separation 1.688 Å). The framework has a long−

CH acetylene atom from the substrate 3. The core N1−C1− C2−B1 unit shows a coplanar s-trans-arrangement. A close inspection of the bond lengths (Table 2) shows a short−long− short sequence. The N1−C1 bond is somewhere between a N−C single and NC double bond in length and the C1−C2 distance is between a typical single and double bond. The C2− B1 linkage is short. It is much shorter as compared to the adjacent B1−C21/31 single bonds, but it does not completely reach the true borata-alkene (C6F5)2BC distance of about 1.44 Å. We conclude that the structural features of the N1− C1−C2−B1 moiety in compound 5b/5′b can best be described by a contribution of both the iminium/borata-alkene (5′b) and the conventional 1-amino-2-boryl alkene FLP (5b) mesomeric forms (Scheme 2). The X-ray crystal structure analysis of compound 5c is similar. We again found a markedly shortened N1C1 bond, close to but not quite reaching the limiting iminium ion value. The C1−C2 bond is longer than a CC double bond and the B1−C2 length is again approaching but not completely reaching the typical borata-alkene BC double bond value6,8 (Table 2 and Figure 2). The description of the compounds 5/5′a−c by the iminium/ borata-alkene vs trans-alkenediyl bridged N/B FLP mesomeric forms is in accord with the NMR spectra observed for these compounds (Table 1). Compound 5a features a 11B NMR resonance that represents almost the average value of a typical (C6F5)2BC borata-alkene (ca. δ 30 ppm)6,8 and a trigonalplanar RB(C6F5)2 borane Lewis acid (δ 60−70 ppm).22 The 13 C NMR chemical shift of carbon atom C1 is close to an iminium ion value. The compounds 5b/5′b and 5c/5′c show similar NMR features (for details, see Table 1 and the Supporting Information). We assume a reaction pathway of the formation of the compounds 5 that involves a typical trans-1,2-addition reaction of a frustrated N/B Lewis pair to the CC triple bond of the propargyl amine 3.23,24 The special feature of this reaction (Scheme 3) is that the propargyl amine nitrogen serves as the intramolecular nucleophile that then in combination with the external boron electrophile is thought to lead to the formation of the zwitterionic intermediate 6 featuring an aziridinium ion with a borate anion at its olefinic side chain. We assume 3637


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Table 3. Selected NMR Parameters of the Dihydrogen Splitting Products 9a−ca 1

H: NH BH 3-H 2-H 4-H 5-H 3 J(4‑H,5‑H) 13 C: C1 C2 11 B: BH 1 JBH 19 F: Δδ(19Fm,p)

9a

9b

9c

8.88 3.33 3.26 6.93 6.27 6.53 15.7 134.9 146.8 −23.2 78 3.4

8.83 3.31 3.15 6.86 5.73 6.26 15.6 135.1 146.5 −23.3 78 3.4

8.69 3.31 6.06 7.15 6.17 2.06 14.9b 135.6 144.8 −23.2 79 3.4

In CD2Cl2 (299 K), chemical shifts δ-scale, coupling constants in Hz, Δδ 19Fm,p in ppm. b3J(3‑H,4‑H).

Figure 3. Molecular structure of zwitterionic aziridinium/borate product 7 (thermal ellipsoids are shown at the 30% probability level). Selected bond lengths (Å) and angles (deg) (two independent molecules were found in the crystal, the values of molecule B are given in parentheses): N1A−C1A 1.538(4) [1.530(4)], N1A−C2A 1.495(5) [1.480(5)], C1A−C2A 1.445(5) [1.427(5)], C2A−C3A 1.302(5) [1.309(5)], C3A−B1A 1.635(5) [1.635(5)], C1A−N1A−C2A 56.9(2) [56.6(2)], N1A−C2A−C3A 142.2(3) [141.4(3)], C2A−C3A−B1A 127.2(3) [126.4(3)].

a

We have also performed the reaction of the N/B FLP 5a with dideuterium under similar reaction condition (1.0 bar D2, dichloromethane, rt, 3 h). We isolated the respective ND+/BD− product 9a-D2 in 83% yield. In the 1H NMR spectrum, the NH+/BH− positions are empty, whereas the 2H NMR spectrum shows the expected broad ND+ (δ 8.74) and BD− (δ 3.33 ppm) resonances, as expected (for details and the depicted NMR spectra see the Supporting Information). The compounds 5b and 5c split dihydrogen equally well under our standard conditions to give the respective NH+/BH− products 9b and 9c (see Table 3 and the Supporting Information for their characterization). We found that the central C1C2 carbon−carbon double bond in the products 9a−c was Z-configurated, whereas the starting materials 5 featured a transoid geometry. Therefore, we were led to assume an E- to Z-isomerization preceding the actual H2 cleavage reaction. In view of the strong mesomeric iminium/borata-alkene participation, the s-trans-5 to s-cis-5 equilibration can apparently be achieved easily under equilibrium conditions at rt. The assumed transition-state geometry of the H−H splitting reaction features a side on arrangement to boron with an approaching close to linear relationship between the amine base and the H···H vector.3,27,28 This geometric arrangement which had been computationally found for a variety of metal-free FLP H2-splitting reactions would require the cisoid geometry of the N1−C1−C2−B1 framework. Of course, this would then be conserved in the NH+/BH− product which has a normal olefinic CC moiety linking the pair of functionalities in the products 9a−c. We note the special feature that in compound 9c an isomerization has taken place to form the conjugated diene structural subunit. The required s-trans- to s-cis-5′/5 isomerization for the formation of the cis-configured dihydrogen splitting products 9 was analyzed by DFT calculations of the relative Gibbs free energies for the t-butyl substituted 5′/5c system in CH2Cl2 solution (PW6B95-D3//TPSS-D3/def2-TZVP + COSMORS).29 We note (Scheme 4) that the cisoid isomer is about 4.5 kcal mol−1 higher in energy than the observed s-trans-5′c. The DFT calculated Gibbs activation energy of the s-trans-5′c to s-cis-5′c/cis-5c rotation amounts to 21.7 kcal mol−1 at 298 K. The calculated structural parameters of the central framework of s-trans-5′c [1.373 Å (N1−C1), 1.391 Å (C1−C2), 1.500 Å (C2−B1)] are very similar to the observed values in the crystal (see Table 1 above). For the (not experimentally observed) cis-

Figure 4. View of the molecular structure of the zwitterionic NH/BH dihydrogen splitting product 9a (thermal ellipsoids are shown at the 30% probability level).

short−long bond length sequence as expected (N1−C1 1.499(3) Å, C1−C2 1.328(3) Å, C2−B1 1.612(3) Å; dihedral angle N1−C1−C2−B1 1.2(3)°). The carbon atom C1 has the −CH2 (trans-)CHCHPh substituent attached (C1−C3 1.518(3) Å, C3−C4 1.501(3) Å, C4−C5 1.327(3) Å). The C4−C5 carbon−carbon double bond is oriented almost perpendicular to the central framework (C1−C3−C4−C5 115.6(2)°). In solution the NH+/BH− compound shows the 1H NMR signals of the ammonium/hydridoborate pair (Table 3). It features the typical NMR signals of the −CH2CHCHPh group with a vicinal trans-coupling of the −CHCH− moiety. The 11B NMR resonances of the trio of NH/BH compound 9 is in the typical borate range, as is the small chemical shift difference Δδ(19Fm,p) in their 19F NMR spectra. 3638


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Journal of the American Chemical Society Scheme 4. Dihydrogen Splitting Reaction of the N/B FLPs 5 with DFT Calculated Gibbs Energies [ΔG(298 K)] of the Systems 5c/5′c and the Activation Barrier (ΔG⧧(298 K)), Both in kcal mol−1, of the s-trans-5′c to s-cis-5′c Interconversion (R: tert-Butyl)

Figure 5. Molecular structure of compound 10 (thermal ellipsoids are shown at the 15% probability level). Selected bond lengths (Å) and angles (deg): N1−C1 1.495(3), C1−C2 1.322(4), C2−B1 1.617(4), B1−C6 1.595(4), C6−C7 1.204(4), C7−C51 1.447(4), C1−C3 1.518(4), C3−C4 1.499(5), C4−C5 1.312(4), N1−C1−C2 119.0(2), C1−C2−B1 136.4(2), C2−B1−C6 114.5(2), B1−C6−C7 173.2(3), C6−C7−C51 176.2(3), N1−C1−C2−B1 −1.3(5).

intramolecular P/B FLPs are prone to favor this reaction. This posed the question whether the N/B systems 5 would also undergo this often observed FLP addition reaction type, possibly involving the reactive Z-configurated isomer cis-5 featuring the amino-nucleophile and −B(C6F5)2 electrophile in a favorable position to undergo the conjugate ynone addition reaction. Therefore, we treated the N/B system 5a (R = Ph) with two ynones, one bearing a phenyl substituent at the distal sp-carbon atom, the other an ethyl substituent. 4-Phenyl-3-butyn-2-one (11a) rapidly reacted with 5a (dichloromethane, 1 h, rt) to give compound 12a, which we isolated as a white solid in 81% yield. The analysis, including NMR spectroscopy and X-ray diffraction, showed that enolate formation had taken place (Scheme 5). The X-ray crystal structure analysis showed the presence of the oxygen-bonded enolate moiety at boron. The core of the product shows a Z-configurated C1−C2 carbon− carbon double bond that has the TMP-H group bonded to it (Figure 6). In the 1H NMR spectrum, we see the ammonium NH-signal at δ 10.32 and the olefinic 2-H resonance at δ 6.35 ppm. Quite significantly, the pair of enolate CH2 1H NMR signals appear at δ 4.92 and δ 4.79 ppm. The reaction of 3-hexyn-2-one (11b) with s-trans-5′a (rt, 1 h) takes the analogous course. We isolated the zwitterionic ammonium/boron-enolate product 12b in 78% yield after workup. The NMR data and the structural data are similar to those of 12a (see the Supporting Information for details). Compound 5a contains a partial BC double bond, so it might be able to undergo a borata-Wittig olefination reaction, similar to the examples that we had recently described starting from true (C6F5)2BCHR− reagents.8,9 However, the reaction of s-trans-5′a with cyclopentanone did not give the olefination product, but again instead the respective zwitterionic ammonium/boron−enolate product 14, which we isolated in 92% yield. X-ray diffraction showed that the framework was again Z-configurated at the C1C2 carbon−carbon double

5/5c′ isomer the DFT calculation furnished markedly longer N1−C1 (1.401 Å) and C2−B1 (1.516 Å) bonds and a shorter C1−C2 (1.369 Å) linkage, which indicates that the cis-isomer is much closer to a conventional substituted enamine structure than the isolated s-trans-5′c (for further details, see the Supporting Information). We also reacted the N/B FLP 5a (R = Ph) with a terminal alkyne.30 Reaction of 5a with phenylacetylene (rt, dichloromethane, 1 h) gave the NH/BCCPh product 10, which we isolated in 72% yield. The CH cleavage product was characterized by X-ray diffraction. It showed a Z-arrangement of the pair of nitrogen and boron based functional groups at the central CC double bond. Both the nitrogen and the boron atom show distorted tetrahedral geometries. The N−H hydrogen vector as well as the B-acetylide moiety are oriented toward the inside at the core framework. The B1−C6−C7−Ph acetylide unit is close to linear (Figure 5 and Scheme 4). We assume that the amine base in the N/B FLP cis-5a is used for deprotonation of the terminal acetylene with the borane Lewis acid, being ideally positioned in close proximity to capture the acetylide anion. This might account for the observed selective formation of the Z-CH activation product 10 in this reaction. In solution, compound 10 shows the NH+ 1H NMR resonance at δ 9.60 ppm. The [B]−CC−Ph unit shows a 11 B NMR signal at δ −20.6 (Δδ 19Fm,p = 3.5 ppm) typical of a borate anion unit and 13C NMR acetylide resonances at δ 107.3 and δ 104.6 ppm, respectively. The core Z-[N]−C(R)CH− [B] unit shows olefinic 13C NMR features at δ 146.4 (br 1:1:1:1 intensity q, 1JBC ∼ 58 Hz, [B]CH; 1H: δ 6.68) and δ 133.5 ppm, respectively (for details, see the Supporting Information). Other Reactions of the N/B FLP System 5a. Many frustrated Lewis pairs undergo 1,4-addition reactions to conjugated enones and ynones.31,32 In particular, a variety of 3639


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including removal of dimethyl sulfide in vacuo and product extraction with n-pentane eventually furnished compound 15 as a white solid in 83% yield. The same compound was also formed by treatment of s-trans-5′a with Piers’ borane [HB(C6F5)2]. In this case, B(C6F5)3 was consequently formed as well34 and the workup was more complicated (for details, see the Supporting Information). We assume that the formation of compound 15 was initiated by regioselective hydroboration of the pendent styryl substituent of the side chain of compound s-trans-5′a generating the reactive intermediate 16. Here the nucleophilic character of the framework carbon atom in the α-position to boron seems to determine the subsequent fate of this reactive species. Nucleophilic attack at the strongly Lewis acidic boron center B2 with concomitant formation of the B2−(μ-H)−B1 hydride bridge would directly lead to the formation of the observed product 15 (Scheme 6). A DFT calculation29

Scheme 5. Reaction of the N/B FLP 5a with Conjugated Enones and Cyclopentanone

Scheme 6. Reaction of s-trans-5′a with the C6F5BH2·SMe2 Reagent with DFT-Calculated Gibbs Energies [ΔG(298 K)] in kcal mol−1

provided the Gibbs energies of the proposed intermediate 16 and the product 15 relative to the starting material. It also localized an unproductive diastereoisomer at the intermediate stage (see the Supporting Information for details). Consequently, the X-ray crystal structure analysis of compound 15 showed the four-membered C2(B1, B2)(μ-H) structural subunit annulated with the core five-membered ring structure. We note that this bears the exocyclic C1N1 iminium moiety, and it has the trans-oriented hydrogen atoms at the ring carbon atoms C2 and C4 (Figure 7). In solution, compound 15 features a typical iminium 13C NMR resonance at δ 210.5 ppm. We monitored a pair of 11B NMR signals at δ −8.0 and δ −18.0 ppm, respectively, and found the [B]−(μ-H)−[B] 1H NMR resonance as a broad signal at δ 3.02 ppm. Compound 15 shows three sets of 19F NMR C6F5 signals with their p-F resonances at δ −157.0, −157.1, and −157.5 ppm, respectively.

Figure 6. Molecular structure of the enolate product 12a (thermal ellipsoids are shown at the 15% probability level). Selected bond lengths (Å) and angles (deg): N1−C1 1.503(3), C1−C2 1.325(4), C2−B1 1.623(4), B1−O1 1.495(3), O1−C8 1.362(3), C8−C9 1.320(4), C8−C7 1.439(4), C7−C6 1.199(4), C4−C5 1.318(4), C2−B1−O1 107.0(2), B1−O1−C8 124.8(2), O1−C8−C9 127.3(2), C8−C7−C6 178.1(3), N1−C1−C2−B1 7.8(5), C1−C3−C4−C5 − 118.2(3), C3−C4−C5−C21 178.6(3).

bond. Therefore, a deprotonation reaction by the reactive amine base from the cis-5a isomer out of the endergonic s-trans5a/cis-5a equilibrium must be assumed (see the Supporting Information for the characterization of the product 14, including the results of the X-ray crystal structure analysis). We have, however, eventually found some evidence for a possible borata-alkene-type reactivity of compound s-trans-5′a. This observation occurred when we treated the N/B system strans-5′a with the Lancaster reagent [C6F5BH2·SMe2].33 The reaction was carried out in dichloromethane solution at rt. After 18 h reaction time, the product 15 was formed. Workup

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CONCLUSIONS We prepared the unsaturated C2-bridged N/B systems 5 by an unconventional synthetic route. It was based on the often observed trans-1,2-FLP addition reaction to organic π-systems. Here, the internal amine addition to the propargyl carbon− carbon triple bond in concert with the external addition of the alkenyl-B(C6F5)2 electrophiles probably generated the respective aziridinium moiety, which was then opened by the adjacent 3640


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DFT structure (XYZ) X-ray data (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Christian Mück-Lichtenfeld: 0000-0002-9742-7400 Gerald Kehr: 0000-0002-5196-2491 Gerhard Erker: 0000-0003-2488-3699 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

Figure 7. View of the molecular structure of the bis-borane compound 15 (thermal ellipsoids are shown at the 30% probability level). Selected bond lengths (Å) and angles (deg): N1−C1 1.323(3), C1− C2 1.439(4), C1−C3 1.533(3), C3−C4 1.543(4), C4−C5 1.540(4), B2−C4 1.599(4), B1−C2 1.661(4), B1−B2 1.903(4), B1−C2−B1 71.0(2), ∑N1CCC 360.0.

alkenylborate nucleophile to form the observed N/B reactive products 5. So far, the reaction seems to be limited to the use of the alkenyl-B(C6F5)2 reagents. The parent B(C6F5)3 electrophile only undergoes the first part of the reaction sequence and stops at the state of the zwitterionic aziridinium borate stage. Subsequent ring opening by shifting a C6F5 group from boron to carbon was not observed under our typical reaction conditions. The newly formed N/B products 5 are remarkable compounds. A pronounced π-conjugative interaction could be structurally established in the unsaturated FLP between the Lewis base and Lewis acid components across the π-system of the organic linker. The system 5 features a substantial contribution of the mesomeric iminium/borata-alkene form. The stable form shows an s-trans-arrangement between these two antagonistic π-components. Nevertheless, the compounds 5 are rather reactive N/B FLPs. They split dihydrogen rapidly at ambient conditions. The obtained zwitterionic NH+/BH− products, however, show a Z-configurated central CC double bond, which indicates rotation from the s-trans-5′ arrangement35 prior to the actual H2 splitting reaction. Several other reactions exemplified by compound 5a show similar characteristics. The formation of the unusually structural product 15 by treatment of s-trans-5′a with the H2B−C6F5 reagent might possibly even indicate some borata-alkene type reactivity and, thus, might point to a possible involvement of an iminium/ borata-alkene character in some chemistry of these dual faced systems. It may be that these observations might indicate a route to novel structural features as well as reaction modes possible for intramolecular frustrated Lewis pairs.

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B Pairs by Borane

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