Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1H

Jan 5, 2018 - *E-mail for S.S.-E.: [email protected]. ... key important processes: hydrogenation of nitriles, hydroboration of polar bonds...
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Research Article Cite This: ACS Catal. 2018, 8, 939−948

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Mechanistic Studies on the Catalytic Synthesis of BN Heterocycles (1H‑2,1-Benzazaboroles) at Ruthenium Marion Beguerie,†,‡ Chiara Dinoi,§,∥ Iker del Rosal,§,∥ Charly Faradji,†,‡ Gilles Alcaraz,†,‡,⊥ Laure Vendier,†,‡ and Sylviane Sabo-Etienne*,†,‡ †

CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France § Université de Toulouse, INSA, UPS, LPCNO (IRSAMC), 135 avenue de Rangueil, F-31077 Toulouse, France ∥ CNRS, UMR 5215 (IRSAMC), F-31077 Toulouse, France ‡

S Supporting Information *

ABSTRACT: We had recently disclosed a catalyzed transformation toward the synthesis of BN molecules under an H2 atmosphere under mild conditions. We now report an in-depth mechanistic study to understand how a substrate featuring two different functional groups, CN and B−H, namely the 2cyanophenyl(amino)borane HB(NiPr2)C6H4(CN) (2), can be transformed into the BN heterocycle 1H-2,1-benzazaborole (3). Such a complex transformation has direct links with three key important processes: hydrogenation of nitriles, hydroboration of polar bonds, and B−N bond formation. A combination of in situ monitoring of the catalytic reaction, stoichiometric experiments, and variable-temperature multinuclear NMR and DFT studies allowed us to decipher the catalytic cycle. We show that the catalyst precursor [RuH2(η2-H2)2(PCy3)2] (1) is regenerated at the end of the transformation. We intercepted the transformation of the starting substrate 2, in the form of a 1H-2,1benzazaborolyl ligand coordinated to the metal center by the formed BN cycle. The corresponding benzazaboryl complex [Ru{(η5-C(H)N(H)B(NiPr2)(C6H4)}{(η3-C6H8)PCy2}] (9) was independently prepared and fully characterized by X-ray diffraction and multinuclear NMR. We also showed that complex 9 undergoes stepwise hydrogenation, followed by haptotropic rearrangement before release of the final product 3 and regeneration of the catalyst precursor 1. We were able to provide a fairly good view of the activation of the CN and B−H bonds. So far, it appears that nitrile hydroboration with metal hydrides starts with nitrile reduction but subsequent steps are highly dependent on the system. In our case, after the first hydrogen transfer to the nitrile, a boron−nitrogen interaction is highly favored, B−H bond cleavage occurring at a later stage. This field needs further investigation for promising developments of BN molecules. Prospects on reactions involving at least two different intramolecular reactive functions should be encouraged for future development in catalysis. KEYWORDS: hydrogenation, hydroboration, nitriles, ruthenium, hydride, BN compounds, cyclization, boranes

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mild conditions (eq 1).2 For that, we designed as precursors a series of aryl-substituted 2-cyanophenyl(amino)boranes, HB-

INTRODUCTION Activity and selectivity are key issues in catalysis, and a great deal of information can be gained from a mechanistic investigation to improve the catalytic performances. Moreover, when a new transformation is disclosed, a better understanding of the pathways taken to create new bonds may provide novel directions to develop a specific field. Controlling important parameters at a fundamental level proved to be crucial in the development of many processes and remains an active area of research.1 Recently, we have disclosed the synthesis of BN heterocycles, i.e. 1H-2,1-benzazaboroles,2 which could give access to novel BN-embedded molecules and materials.3−6 The process is catalyzed by the bis(dihydrogen) ruthenium complex [RuH2(η2-H2)2(PCy3)2] (1) under an H2 atmosphere under © XXXX American Chemical Society

(NiPr2)C6H4(CN) (2), for the parent compound, with the aim of benefiting from two different functional groups, CN and Received: October 10, 2017 Revised: December 19, 2017

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DOI: 10.1021/acscatal.7b03461 ACS Catal. 2018, 8, 939−948

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ACS Catalysis

levels. NMR monitoring in low-pressure NMR tubes proved to be an efficient way to identify intermediates involving several stages of hydrogen transfer processes. Our in-depth study allowed the identification of several catalytic intermediates and, in particular, the isolation of a benzazaborolyl ruthenium complex which caught out the BN product in an intermediate stage before final release into 1H-2,1-benzazaborole (3).

B−H. These two functional groups were selected on the basis of previous findings in the group and of their link to three important catalytic transformations. (i) Hydrogenation of nitriles: this is an important industrial process when referring to the synthesis of Nylon-6,6, which involves the hydrogenation of adiponitrile into hexamethylenediamine. Recent advances in this area have been achieved using in particular first-row pincer complexes.7−12 Complex 1 was known to be a catalyst precursor for nitrile hydrogenation,13 and we had shown that the analogous bis(dihydrogen) complex 1Cyp, featuring cyclopentylphosphine in place of cyclohexylphosphine substituents, was also an excellent catalyst for the hydrogenation of nitriles under mild conditions.14 (ii) Hydroboration: we also showed that complex 1 is an active catalyst for hydroboration of alkenes and cyclic alkenes and, depending on the experimental conditions, that dehydrogenative borylation can be favored.15−17As we will see in more detail in the last section of the mechanistic discussion, catalyzed nitrile hydroboration leading mainly to the corresponding bis(borylated) amines is attracting more and more interest.18−25 (iii) Dehydrogenation of amine− boranes: this reaction is an extremely active domain of research due to the potential applications in the area of energy (hydrogen storage) or for the design of new materials with various properties, including main-group-containing polymers.26−30 Here also, complex 1 proved to be a versatile precursor able to catalyze the dehydrogenation of diamine− monoboranes leading to the formation of the corresponding cyclic diaminoboranes: namely, 1,3,2-diazaborolidines.31,32 Moreover, at a more fundamental level, we had shown that boranes can either coordinate to the ruthenium center by forming the corresponding σ-complexes, incorporating one B− H or two geminal B−H bonds,33 or coordinate through the establishment of agostic B−H bonds.34−36 One prototypical example is the (o-phosphinophenyl)(amino)borane compound HB(NiPr2)C6H4(o-PPh2), directly related to the present work, as it features a phosphino function in place of the cyano substituent.35 On this basis, one can postulate three main pathways as illustrated in Scheme 1: pathway a involving hydrogenation and

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Scheme 1. Postulated Pathways for the RutheniumCatalyzed Transformation of 2 into 3

RESULTS AND DISCUSSION We have recently reported a general catalytic procedure performed in a Fisher−Porter bottle based on the addition of 5 mol % of complex 1 to a pentane solution of 2cyanophenyl(amino)boranes (2) under 1 bar of dihydrogen for 5 h. The corresponding 1H-2,1-benzazaboroles (3) could be isolated in moderate to high yields with F, Me, or OMe substituents on the phenyl ring.2 Catalytic Conditions and in Situ NMR Monitoring. Characterization of the Bis(nitrile) Complex [RuH2{NC(C6H4)BH(NiPr2)}2(PCy3)2] (4). By sampling the catalytic mixture, we found that the catalyst precursor 1 is recovered at the end of the reaction (see the Supporting Information and Figure S1). When the starting cyano-borane 2 is still present, the 31P{1H} NMR spectrum shows mainly one signal at 10 ppm corresponding to free PCy3. Remarkably, after total conversion of 2, the 31P{1H} NMR spectrum shows one major signal at 76 ppm characteristic of 1, together with the signal for free PCy3. We were pleased to see that, at the end of the first batch, addition of a second batch of 2 leads again to total conversion with similar selectivity into benzazaborole 3, the catalyst precursor 1 being once again recovered with slightly more free PCy3 in the mixture (1:PCy3 in a 2:0.33 ratio by 31P INVGATED 1H NMR). When the reaction was performed in a pressure NMR tube, using 5 mol % of catalyst 1 in C7D8 solution, we noticed, before adding any H2, a color change to red as well as gas evolution. At this point, NMR analysis of the crude mixture indicated the consumption of the catalyst precursor 1 and already the formation of 1H-2,1-benzazaborole 3, despite the fact that we had not yet introduced H2 gas (Figure 1, spectrum 3). A new hydride species was detected and in particular was characterized by a triplet at −14 ppm and a signal at 64 ppm in the 1H and

dehydrogenation sequences, pathway b via hydroboration and hydrogenation steps, and pathway c via hydrogenation followed by hydroboration. At this point it should be noted that the net result is the consumption of 1 equiv of H2, pathway a requiring first 2 equiv of H2 before a subsequent dehydrogenation step. In order to gain information on the mechanism of the complex transformation depicted in eq 1, we have conducted a series of experiments at both the stoichiometric and catalytic

Figure 1. Comparison of 1H NMR spectra in deuterated toluene showing (1) isolated complex 1, (2) isolated compound 2, (3) 15 min after mixing 1 and 2, before the addition of exogenous H2, (4) end of the catalytic reaction after pressurizing with H2, (5) isolated compound 3. 940

DOI: 10.1021/acscatal.7b03461 ACS Catal. 2018, 8, 939−948

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ACS Catalysis 31

P{1H} NMR spectra, respectively (vide infra). At the end of the catalytic reaction, 2 was converted into 3 and the precursor catalyst 1 was recovered (Figure 1, spectrum 4). For comparison, the 1H NMR spectra of 1−3 are also displayed in Figure 1 (spectra 1, 2, and 5, respectively). We repeated the same experiment but introduced 1 bar of dihydrogen and froze the tube right after the mixing in liquid N2. The first NMR spectrum recorded at −80 °C showed the same hydride species depicted in Figure 1 spectrum 3, as the sole ruthenium complex. In-depth NMR characterization allowed us to formulate it as the bis(nitrile) [RuH2{NC(C6H4)BH(NiPr2)}2(PCy3)2] (4), resulting formally from the substitution of the dihydrogen ligands in 1 by two cyano ligands (see the Supporting Information and Figures S2−S4). In addition to DOSY experiments, a cross peak in the 2DHMBC 13C{31P}/1H NMR indicates a correlation between the hydrides and the quaternary carbon of the nitrile at 120.3 ppm. The B−H resonance (+5.7 ppm) is only slightly shifted in comparison to 2 (+5.5 ppm), discarding any agostic interaction. The analogous bis(benzonitrile) complex [RuH2(NCPh)2(PCyp3)2] had been previously characterized in the case of tricyclopentylphosphine, and an X-ray structure of the corresponding bis(acetonitrile) [RuH2(NCMe)2(PCyp3)2] had also been obtained.14 When the temperature is increased to −30 °C, one can see the catalytic reaction proceeding. In addition to the signals for 4 and free PCy3, which remain the major ones, the 31P and 1H NMR spectra indicate a multitude of small signals, rendering any assignment quite difficult. It is however worth noting that a few resonances can be assigned to imine functions (as an example, a deshielded 1H doublet at 8.25 ppm correlating with a deshielded 13C NMR signal at 161 ppm) as we had previously observed during our studies concerning benzonitrile hydrogenation.14 Over a longer reaction time, and with the temperature increased to room temperature, ruthenium species were no longer detected by 31P{1H} NMR, the spectrum only showing one signal for free PCy3. At the end of the reaction, characteristic 1H and 31P NMR signals for 1 and 3 were observed as the main products together with free PCy3 and minor amounts of diisopropylamine resulting from cyclotrimerization of 3. This side reaction is consistently reproduced in all our experiments (7% of HNiPr2 in standard catalytic conditions) and will not be mentioned further throughout the discussion. Stoichiometric Experiments and NMR Monitoring in the Absence of H2 Pressure. In order to determine how the final product could be obtained in the absence of exogenous H2, we performed several stoichiometric reactions by varying the ratio between 1 and 2 from 1:1 up to 1:6. It was known that 1 can lose dihydrogen upon thermal or photochemical activation, producing the dinuclear species [Ru2(H)4(H2)(PCy3)4] (5).37 Previous work in the group had also shown that 1 can provide up to 10 hydrogen atoms in the presence of an olefin acting as a hydrogen acceptor. In addition to the two dihydrogen and hydride ligands, C−H activation within the tricyclohexylphosphine gives rise to two stages of dehydrogenation and the partially dehydrogenated complexes [RuH(H2){(η3-C6H8)PCy2}(PCy3)] (6) and [RuH{(η3-C6H8)PCy2}{(η2C6H9)PCy2}] (7) can be isolated (Scheme 2).38 A series of reactions were conducted in THF-d8 solution using anisole as standard (the corresponding 31P{1H} NMR spectra are depicted in Figure S2 in the Supporting Information). When a 1:1 ratio was used, complex 1 was still

Scheme 2. Known Dehydrogenated Species from the Bis(dihydrogen) Complex 1

present but the major species was the dinuclear complex 5. Increasing the amount of compound 2 led to full consumption of 1, a decrease in 5, and the appearance of 6. Traces of 5 were detected for a 1:3 ratio, 6 being the major species. At that stage, we noticed an increase of the signal for free PCy3 and a tiny broad signal at 115 ppm, later assigned to the new complex 9. For a 1:5 ratio, the latter signal was still present together with free PCy3. The starting borane was fully converted up to 5 equiv, and no more hydride signal was detected by 1H NMR. Other unidentified minor signals were always detected by 31 1 P{ H} NMR, as a result of partial decomposition. Thus, this series of experiments demonstrate that the starting catalyst precursor 1 indeed acts as a hydrogen reservoir, being able to convert up to 5 equiv of compound 2. Both the dihydrogen ligands and the cyclohexyl groups are involved in the hydrogen transfer. Indeed when the analogous bis(dinitrogen) complex [RuH2(N2)2(PCy3)2] (8) is used, the addition of 1 equiv of compound 2 gives a stoichiometric mixture of 3 and complex 6 (eq 2).

One key observation came from the addition of compound 2 to complex 6. When a 1:1 ratio was used, NMR data evidenced the formation of 0.5 equiv of compound 3, 0.5 equiv of 6, and 0.5 equiv of a new species 9 with a 31P{1H} NMR signal at 115 ppm we had already detected (vide supra). Adding 2 equiv of compound 2 to complex 6 resulted in clean transformation into complex 9 together with 1 equiv of compound 3 and free PCy3, as shown in Scheme 3. We are confident of the stoichiometry thanks to reaction monitoring by 1H and 31P INVGATED 1H NMR. Synthesis and Characterization of the Benzazaborolyl Ruthenium Complex [Ru{(η5-C(H)N(H)B(NiPr2)(C6H4)}{(η3C6H8)PCy2}] (9). After workup of the crude solution obtained from the reaction depicted in Scheme 3, complex 9 was isolated in modest yield (45%) due to similar solubility of the three products of the reaction. Single crystals were grown from a diethyl ether/acetonitrile mixture, and X-ray diffraction analysis provided the structure shown in Figure 2. The asymmetric unit contains two molecules, but for clarity, only the species with the carbon in the S configuration is represented. Remarkably, the ruthenium center is surrounded by a benzazaborolyl ligand and only one partially dehydrogenated phosphine to complete its coordination sphere. The 941

DOI: 10.1021/acscatal.7b03461 ACS Catal. 2018, 8, 939−948

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ruthenium the N-attached CH resonates at higher field for a sp2 carbon (59.2 ppm) in the 13C{1H} NMR spectrum. Reactivity of Complex 9 with H2. Haptotropic Rearrangement from 10 to 11. We have thus observed facile activation of the starting borane 2 to afford complex 9, in which the nitrile function has been partially hydrogenated and cyclization has occurred, leading to the formation of a new BN ligand. The starting borane has been captured by the metal center into an intermediate stage prior to its final transformation into 3. With complex 9 being isolated and fully characterized, we focused our attention on its reactivity toward a dihydrogen atmosphere. We were delighted to be able to detect two stages of “hydrogenation”. Exposing 9 to 1 bar of H2 in a toluene-d8 solution led to rapid conversion of 9, as monitored by 1H and 31P NMR. Two new species, [RuH{η5CH(NH)(BN i Pr 2 )C 6 H 4 )}{(η 2 -C 6 H 9 )PCy 2 }] (10) and [RuH2{η6-C6H4(CH2(NH)(BNiPr2)}(PCy3)] (11), were identified and fully characterized by 2D-mutinuclear NMR experiments (Figure 3). Complex 10, the first major species

Scheme 3. Synthesis of the Benzazaborolyl Complex 9

Figure 2. X-ray structure of complex 9. Ellipsoids are given at the 50% probability level, and hydrogen atoms are omitted for clarity except for C3H and N2H.

benzazaborolyl coordinates to the metal center in an unsymmetrical way, as was previously reported for the 1,2-azaborolyl complexes Ru(C3H3NtBuB−CCC6H5)X(PPh3)2 with X = Cl, H.39 In 9, ruthenium is tightly connected with C2, C3, and N2 and the boron is even farther away (2.501(3) Å) than in the azaborolyl complexes reported by Wen (2.357(3) Å for X = Cl). The Ru−C1 bond distance (2.370(2) Å) is longer than the two others (Ru−C3 2.121(3) Å and Ru−C2 2.290(2) Å). The Ru−N2 bond distance in 9 (2.271(2) Å) is similar to that in the azaborolyl complex with X = Cl (2.280(2) Å). In 9, the benzene and BN-substituted rings are not coplanar. The exocyclic B−N1 bond distance of 1.429(4) Å is shorter than the endocyclic distance (1.464(4) Å), consistent with BN ring coordination to ruthenium. It should be noted that the chemistry of benzazaborolyl compounds has been poorly developed so far. One example concerns a zirconium catalyst for ethylene polymerization patented in 1999,40 and in 2014, Dostal et al. reported the first X-ray structure of the potassium 1H-2,1 benzazaborolyl salt, the BN analogue of an indenyl ligand.41 Complex 9 is characterized by a deshielded 31P{1H} NMR signal at +115 ppm. The 11B{1H} NMR signal at 16 ppm is, as expected, shifted to higher field in comparison to the final BN compound 3 (30.6 ppm) and is in the same range as the values found for the two azaborolyl ruthenium complexes (5.2 and 8.3 ppm) reported by Wen and for the 1H-2,1-benzazaborolyl potassium salt (23.4 ppm).39,41 2D-multinuclear experiments allowed full characterization of the benzazaborolyl and dehydrogenated phosphine ligands with, in particular, the NH and the N-attached CH resonating at 3.46 and 5.93 ppm, respectively, in 1H NMR. The latter signal resonates as a pseudotriplet due to coupling to both phosphorus (JPH = 4 Hz) and NH proton (JHH = 2.8 Hz). Upon coordination to

Figure 3. Selected NMR data for complexes 10 and 11: (top) 31 1 P{ H}, 31P{1H sel 2 ppm}, and 1H (hydride region) NMR spectra for complex 10; (bottom) 31P{1H sel 2 ppm}, 1H, and 1H{31P} (hydride region) NMR spectra for complex 11.

identified in the reaction mixture, corresponds to the first stage of “rehydrogenation” of the phosphine ligand. One equivalent of H2 is formally added to 9, giving rise to the formation of the monohydride species 10 with the phosphine ligand now coordinated to the ruthenium center with a η2-C6H9 cyclohexenyl group instead of the allylic η3-C6H8 group. 10 is characterized by a singlet at 96.7 ppm in the 31P{1H} NMR spectrum which splits into a doublet (JP−H = 33.5 Hz) upon selective decoupling of the protons of the phosphine (Figure 3, top). This is consistent with the existence of one hydride, as confirmed by the doublet observed at −17.2 ppm in the 1H NMR spectrum and by a 2D-HMQC 31P−1H experiment. The coupling constant is in the typical range for ruthenium hydride 942

DOI: 10.1021/acscatal.7b03461 ACS Catal. 2018, 8, 939−948

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ACS Catalysis in a piano-stool geometry.42 NMR data for the benzazaboryl ligand are quite similar to those for 9. Upon prolonged H2 exposure, a new singlet appeared at 78.2 ppm in the 31P{1H} NMR spectrum which resolved into a triplet upon decoupling of the cyclohexyl protons, a signature for the presence of two hydrides in complex 11, as depicted at the bottom of Figure 3. Indeed, the 1H NMR spectrum displays in the hydride region a doublet of AB patterns at −10.87 and −10.89 ppm, with a large JPH coupling constant value of 43 Hz and a JHH value of 6.8 Hz, the corresponding 1H{31P} spectrum simplifying to one AB pattern. The nonequivalence of the hydrides indicates a slow rotation of the benzazaborole ligand due to either an unsymmetrical benzene bonding to ruthenium or some steric hindrance between the isopropyl and cyclohexyl groups. 1H NMR analysis of the low-field region together with 13C NMR evidenced an important transformation in 11. Not only is the phosphine ligand now fully rehydrogenated into PCy3 but also, remarkably, the benzazaborolyl ligand converted into the final benzazaborole via hydride transfer and an η5 to η6 haptotropic rearrangement.43,44 The coordinated benzazaborole to ruthenium is well identified by NMR. Unambiguous proof of the η6 coordination of the benzene ring came from the upfield shift of the aromatic protons and carbons around 5.5 and 82 ppm in 1H and 13C NMR, respectively. The diastereotopic CH2 protons of the BN cycle now resonate as an AB pattern at ca. 4 ppm, whereas the NH appears at 2.5 ppm. These chemical shifts are thus closer to those for the free compound 2 and indicate a significant change in the BN cycle in complexes 10 and 11. Reactivity of Complex 9 with H2 in the Presence of PCy3. Regeneration of the Bis(dihydrogen) Complex 1. Over the course of the NMR monitoring of the exposure of 9 to a dihydrogen atmosphere, we noted the release of the final product 2, which became the major species at the end. We anticipated that addition of free PCy3 into the mixture containing monophosphine species (9−11) could help in regenerating the catalyst precursor 1. We thus repeated the experiment described in the previous section, pressurizing a toluene-d8 solution of 9 under H2, but this time adding 1 equiv of PCy3. We were delighted to see that complex 1 was regenerated, as shown by 1H and 31P NMR. We can thus summarize this series of experiments by the cycle depicted in Scheme 4. As is already known for various systems, the starting bis(dihydrogen) complex is able to transfer up to 10 hydrogen atoms. It is thus not surprising to identify complex 6 when up to 4 equiv of 2 is reacted with 1. Complex 7 was never detected, and we checked independently that no reaction occurred upon addition of 2 to 7. Monitoring the catalytic reaction had shown that free PCy3 was released in the media. This is consistent with the formation of the benzazaborolyl complex 9 that we were able to isolate and characterize, including by X-ray diffraction. Hydride transfer and η5 to η6 haptotropic rearrangement led then to the final stage of transformation of the BN ligand before its release in the media together with the recovery of the starting precursor 1 upon addition of PCy3. During this series of reactions, the phosphine can accommodate two stages of dehydrogenation. A Few Key Parameters Influencing the Catalysis: Nature of the Catalyst Precursor, H2 Pressure, and Phosphine. The experiments described above have shown that, even under H2 pressure, the bis(nitrile) complex can be formed and that PCy3 is systematically released in the media. We also observed several ruthenium species that could be active

Scheme 4. Characterized Ruthenium Intermediates during the Transformation of 2 into 3

in the catalytic cycle. Three questions thus arise. Are the isolated intermediates catalyzing the reaction? Is the catalytic reaction inhibited by added PCy3? Is H2 pressure influencing the course of the reaction? The main data are gathered in Table 1. Table 1. Screening of a Few Parameters for the Conversion of 2 into 3a entry

catalyst

H2 (bar(g))

conversn of 2 (%)

conversn to 3 (%)

1a 2a 3a 4a 5a 6a 7b 8b 9b 10a 11b

1 1 1 1 1 + 5 equiv PCy3 1 + 10 equiv PCy3 1 6 9 + 1 equiv PCy3 1Cyp 1Cyp (1%)