N–H/P–H bond activation of ammonia, amines and phosphines at a transient borylene

Abstract

The metal-free activation of amines and phosphines is of great significance due to its broad relevance in both organic synthesis and industrial chemistry. Herein, we report the cooperative 1,2-addition of N–H bonds from ammonia as well as various primary and secondary amines, and the 1,1-addition of P–H bonds from primary phosphines, across the C=B double bond of a transient cyclic (alkyl)(amino)carbene (CAAC)-stabilized arylborylene. This transient borylene species displays remarkable ambiphilicity, enabling selective, metal-free activation of amines and phosphines. Mechanistic studies reveal distinct activation pathways dictated by the electronic and steric properties of the substrates, highlighting the unique reactivity of CAAC-stabilized borylenes.

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Introduction

Amines and phosphines are the foremost classes of compounds of group 15 due to their extensive use as sources of nitrogen and phosphorus for organic synthesis. In particular, the rising global demand for N-containing compounds in the fertilizer, pharmaceutical, food, fuel, cement, paint, and textiles industries augmented the necessity to develop a system for activating various amines.^1–5 However, amine activation remains challenging due to the high energy required to break the N–H bond.⁶ As a result, most transition metals tend to form Werner-type complexes with amines rather than undergo direct N–H bond activation.⁷ Nevertheless, a substantial number of examples of transition metal-assisted amine N–H bond activation have been reported, proceeding via oxidative addition,^8–10 metal–ligand cooperativity,^11–14 deprotonation,^15,16 and bimetallic addition.¹⁷ Notably, both catalytic hydroamination and the Buchwald–Hartwig amination involve N–H activation as a key step in their respective C–N bond formation catalytic cycles, enabled by transition metals.^18–24

Bertrand's cyclic (alkyl)(amino)carbenes (CAACs) have provided an alternative platform to overcome the limitations associated with transition-metal-mediated amine activation.²⁵ Similar to transition metals, these species exhibit ambiphilic character arising from a filled nonbonding orbital and an accessible vacant p orbital. These electronic features enabled the metal-free oxidative addition of ammonia to the carbene carbon center. Following this discovery, a variety of low-valent group 13–15 species have also been employed for ammonia activation, such as stannylenes, gallylenes, silylenes, disilenes, and complexes based on phosphorus(III) or (I).^26–38

The boron analogs of carbenes, free borylenes, of the form [:BR], possess vacant p-orbitals and a lone pair of nonbonding electrons, are highly reactive and have short lifetimes.^39–44 Direct spectroscopic observation of organoborylenes, [:BR], is extremely rare and has only been achieved in matrix-isolation studies.^45–47 In 1984, Pachaly and West reported the pronounced ambiphilicity and reactivity of an in situ-generated borylene, [:B(SiPh₃)], in C–H and C≡C activation reactions (Fig. 1A).⁴⁸ However, due to the synthetic challenges, progress in metal-free borylenes remained limited until the isolation of the doubly base-stabilized borylene [(^CyCAAC)₂BH] (^CyCAAC = 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4,5]decan-1-ylidene), by Bertrand and coworkers in 2011.⁴⁹ Subsequently, the same group reported the singly base-stabilized dicoordinate borylene, [(Me₃Si)₂NB(^CyCAAC)], which was shown to activate H₂ and CO (Fig. 1B).⁵⁰ Recently, we employed transient dicoordinate borylene species of the form [(CAAC)BAr] in the activation (Fig. 1C)⁵¹ and reductive coupling of N₂.^52,53 The resulting bis(borylene)–dinitrogen complex was further shown to provide access to a number of N-heterocycles as well as ammonium chloride.^54,55 Mo and Wang recently reported the bis(silylene)-stabilized-borylene-mediated cleavage of the N–H bond in aniline, in which the hydrogen atom and the PhNH fragment add across the borylene center and one of the silylene silicon atoms, respectively.⁵⁶ These observations, together with the broad small-molecule activation chemistry of base-stabilized aryl borylenes, motivated us to investigate the potential of borylene species in the activation of amines and phosphines.

Fig. 1 Examples of borylene-mediated activation of various small molecules. Dur = 2,3,5,6-tetramethylphenyl; Dip = 2,6-diisopropylphenyl.

In this work, we present the cooperative 1,2-addition of N–H bonds from ammonia as well as primary and secondary amines across the C=B double bond of a transient CAAC-stabilized aryl borylene. In contrast, a similar reaction with a primary phosphine proceeds via 1,1-addition of the P–H bond at the borylene boron center.

Results and discussion

The transient dicoordinate borylene [(CAAC)BDur] (3) is generated in situ either by direct two-electron reduction of [(CAAC)BDurCl₂] (1) or through a stepwise reduction pathway involving the boryl radical intermediate [(CAAC)BDurCl]˙ (2) (Scheme 1).^51,57

Scheme 1 General scheme for the borylene-mediated coordination and N–H activation of ammonia, primary and secondary amines. (i) −90 °C, toluene (for the synthesis of 4, 5, 7, and 9); −100 °C, Et₂O (for the synthesis of 6). (ii) NH₃ (3 bar), −90 °C to RT, 1 h (for 4); NH₂Me (3 bar), −90 °C to RT, 3 h (for 5); p-toluidine (2 equiv.), −100 °C to RT, 3 h (for 6); NMe₂ (1 bar), −90 °C to RT, 3 h (for 7); piperidine (2 equiv.), −90 °C to RT, 2 h (for 9). (iii) 80 °C, 16 h (for the synthesis of 8 from 9).

The reduction of 1 with excess KC₈ (10 equiv.) in toluene under an overpressure of NH₃ (3 bar) afforded a colorless solid 4 in 90% yield (Scheme 1). The ¹¹B NMR spectroscopic resonance at δ = 45.8 ppm, together with other spectroscopic data clearly identified compound 4 as the aminoborane [(CAACH)(Dur)B(NH₂)], which had previously been isolated as an intermediate during the conversion of the bis(borylene)–dinitrogen complex to ammonium chloride.⁵⁵ A singlet at δ = 4.08 ppm in the ¹H NMR spectrum and a resonance at δ = 68.4 ppm in the ¹³C{¹H} NMR spectrum of 4 correspond to the migrated hydrogen atom and its host (formerly carbene) carbon atom, respectively.

Given the strongly reducing conditions employed for the in situ generation of [(CAAC)BDur], the possible in situ formation of potassium amide species was considered. To evaluate this possibility, a control experiment was performed in which [(CAAC)BDurCl₂] was treated independently with a potassium amide under similar conditions. However, no formation of compound 4 was observed. Instead, the reaction afforded unidentified product(s), as indicated by a single ¹¹B NMR resonance at δ = 30.7 ppm. This suggests that potassium amide is unlikely to be responsible for the formation of 4 under the reaction conditions shown in Scheme 1.

To gain insight into the driving force governing the formation of aminoborane 4, we carried out a detailed computational study on the reaction of transient borylene 3 with NH₃ (Fig. 2), which revealed a feasible two-step mechanism. In the first step, the amine lone pair interacts with the vacant p orbital of the borylene boron to form the borylene–NH₃ adduct 4^Int, a highly exergonic process ( = −24.6 kcal mol⁻¹) consistent with the high accepting ability of the borylene boron, as indicated by the low-lying LUMO (E_π* = 0.34 eV) and the positive natural charge at boron (q_B = +0.87) of 3. In the second step, the boron-centered lone pair of the adduct 4^Int polarizes one N–H bond, generating H^δ+ and NH₂^δ− fragments, as revealed by the natural charge on the NH₂ nitrogen atom and the H atom in the transition state (4^TS) (Fig. 2 and Table S1). The H^δ+ migrates to the electron-rich carbene carbon of the CAAC ligand (Table S1), while the NH₂^δ− fragment forms a multiple bond with boron, affording the 1,2-addition product aminoborane 4. The high energy barrier for this step ( = 36.3 kcal mol⁻¹) accounts for the requirement of elevated NH₃ pressure in the synthesis of 4. The formation of 4 is further driven by the pronounced exergonicity of the overall reaction ( = −57.9 kcal mol⁻¹). Notably, a 1,1-addition pathway is not feasible, likely due to the electron deficiency at boron of adduct 4^Int (q_B = +0.53). These computational results, together with a previous theoretical study on various dicoordinate borylenes by Phukan and us,⁵⁸ suggest that this reactivity can be extended to higher amines.

Fig. 2 Calculated reaction profile diagram illustrating the activation of NH₃ by transient borylene 3. The reaction free energies and bond lengths are given in kcal mol⁻¹ and Å, respectively. All hydrogen atoms, except those of the ammonia fragment, are omitted for clarity.

The reaction of 1 with excess KC₈ (10 equiv.) and primary amines, namely methylamine (3 bar) or para-toluidine, yielded colourless solid 5 (67% yield) and light red solid 6 (84% yield), respectively (Scheme 1). The ¹¹B NMR spectra of 5 and 6 showed broad resonances at δ = 44.4 and 44.8 ppm, respectively, slightly upfield shifted compared to that of aminoborane 4. Also, the ¹H NMR spectra of 5 and 6 showed singlets at δ = 4.03 and 4.18 ppm, respectively, similar to the signal for the C_CAAC–H nucleus of aminoborane 4. The ¹³C{¹H} NMR spectra of 5 and 6 showed chemical shifts at δ = 68.6 and 69.3 ppm, respectively, which are slightly downfield compared to the signal corresponding to the former carbene carbon center (the carbon atom bearing the migrated hydrogen) in aminoborane 4. These data collectively indicate borylene-mediated N–H bond cleavage of the primary amines and formation of aminoboranes, confirmed by single-crystal X-ray diffraction (SCXRD) analyses. The SCXRD studies revealed compounds 5 and 6 as aminoboranes [(CAACH)(Dur)B(NHMe)] and [(CAACH)(Dur)B(NH(p-Tol))], respectively (Fig. S30 and 3). The B–N bond distance in aminoborane 6 (1.402(2) Å) is marginally longer than that of aminoborane 4 (1.376(2) Å), yet comparable to that of aminoborane [(tBu)(C₆F₅)BN(B(C₆F₅)₂)(tBu)]⁵⁹ (1.392(2) Å), indicative of partial double-bond character. The HOMO−5 of 5 (Fig. 4) and HOMO−1 of 6 (Fig. 4) display clear π-bonding interactions between boron (B1) and nitrogen (N1), further supported by the calculated Mayer bond orders (MBOs: 1.54 (5), 1.47 (6)) and Wiberg bond indices (WBIs: 1.50 (5), 1.42 (6)). From ammonia to primary amines, an increase in steric bulk leads to a slight elongation of the B–N partial double bond, as evidenced by the calculated WBIs and MBOs (Table S2).

Fig. 3 Crystallographically-determined molecular structures of 6–9 (ellipsoids shown at the 50% probability level). All hydrogen atoms except those attached to the NBC^CAAC core, and the ellipsoids of the ligand periphery, are omitted for clarity.

Fig. 4 Selected frontier molecular orbitals of 4–8 (isovalue 0.043 au), calculated at ωB97X-D/Def2-SVP level of theory.

Likewise, reduction of 1 with excess KC₈ in the presence of the secondary amine dimethylamine led to N–H bond cleavage, generating aminoborane 7 in 62% yield (Scheme 1). The ¹¹B NMR (δ = 45.7 ppm), ¹H NMR (δ = 3.75 ppm for C_CAAC–H) and ¹³C{¹H} NMR (δ = 73.3 ppm for C_CAAC–H) spectra of 7 showed patterns similar to those of aminoboranes 4–6. These spectroscopic similarities, together with SCXRD study (Fig. 3) confirmed the 1,2-N–H addition of dimethylamine to the C=B double bond of transient arylborylene 3. The B–N bond distance of aminoborane 7 (1.3931(17) Å) is comparable with that of aminoborane 6, suggesting its partial double-bond character. Furthermore, the HOMO−5 (Fig. 4), together with the calculated MBO (1.46) and WBI (1.48), supports the presence of a multiple-bonding interaction between boron (B1) and nitrogen (N1) in 7.

Interestingly, reduction of 1 with excess KC₈ (10 equiv.) in the presence of the cyclic secondary amine, piperidine, afforded an orange solid 9 in 53% yield (Scheme 1). The ¹¹B NMR spectrum showed a resonance at δ = 24.7 ppm, significantly upfield shifted compared to those of aminoboranes 4–7. Notably, unlike aminoboranes 4–7, no characteristic chemical shifts for the C_CAAC–H moiety were observed within δ = 3.75–4.18 ppm region in the ¹H NMR spectrum and within δ = 68.4–73.3 ppm region in the ¹³C{¹H} NMR spectrum of 9. An unambiguous explanation eluded us until a SCXRD study revealed compound 9 as a Werner-type CAAC-stabilized durylborylene–piperidine complex, [(CAAC)(Dur)B(Pip)] (Pip = piperidine) (Fig. 3). The B–N distance in 9 (1.608(2) Å) is significantly longer than those in aminoboranes 4–7, lying instead within the typical range for B–N dative bonds (1.565(7)–1.771(3) Å).^60–62

Upon heating a solid sample of durylborylene–piperidyl adduct 9 at 80 °C for 16 h, a colorless solid 8 was obtained in 86% yield (Scheme 1). The ¹¹B NMR resonance of 8 at δ = 44.4 ppm is comparable to those of aminoboranes 4–7, indicating formation of the corresponding piperidine analogue. A singlet at δ = 3.81 ppm in the ¹H NMR spectrum and a chemical shift at δ = 73.5 ppm in the ¹³C{¹H} NMR spectrum for C_CAAC–H moiety further support the identity of 8 as an aminoborane. SCXRD analysis confirmed the structure of 8 as aminoborane [(CAACH)(Dur)B(NPip)] (Fig. 3). The B–N bond distance in 8 (1.3944(17) Å) is comparable to those in aminoboranes 6 and 7. As in aminoboranes 4–7, the HOMO−5 of 8 showed a distinct π-bonding interaction between boron (B1) and nitrogen (N1) (Fig. 4). The calculated MBO (1.48) and WBI (1.50) for the B1–N1 bond 8 are comparable to those of aminoborane 7. The isolation of the Werner-type durylborylene–piperidine coordination complex 9 during the synthesis of aminoborane 8 is consistent with our proposed borylene-mediated adduct formation, followed by 1,2-N–H addition pathway (Fig. 2). It is noteworthy that aminoboranes 4–8 showed ¹⁵N chemical shifts in the range of δ = −246 to −282 ppm for the nitrogen atom involved in the B=N double bond, whereas the single-bonded nitrogen atom of the CAACH ligand shows ¹⁵N chemical shifts in the range of δ = −314 to −326 ppm. These observations suggest that participation of the nitrogen lone pair in π-bonding with boron reduces the local electron density at nitrogen relative to a purely single-bonded nitrogen environment.

We then extended our studies to primary and secondary phosphines, heavier group 15 species with weaker E–H bonds, in order to investigate the prospect of the formation of a Werner-type borylene–phosphine adduct or P–H activation products. Our group recently reported that the use of tertiary phosphine PMe₃ enabled the isolation of borylene–phosphine adducts.^63–65 Herein, we studied the reactivity of borylene 3 with primary and secondary phosphines. The reduction of CAAC-stabilized durylboryl radical 2 with excess KC₈ (10 equiv.) in the presence of a primary phosphine, phenylphosphine, afforded an orange solid 10 in 80% yield (Scheme 2). Note that compound 10 can also be isolated from the reaction of adduct 1 with excess KC₈ (10 equiv.) in the presence of phenylphosphine under the same conditions; however, the yield is significantly lower. Unlike aminoboranes 4–8, the ¹¹B NMR spectrum of 10 showed a high-field doublet of doublets at δ = −16.3 ppm, coupled to both proton (²J_B–H = 84 Hz) and phosphorus (²J_B–P = 24 Hz). Moreover, no characteristic chemical shifts for the C_CAAC–H moiety were found in the ¹H and ¹³C{¹H} NMR spectra of 10, ruling out its possibility of being a 1,2-P–H addition product. Instead, a broad ¹H chemical shift at δ = 3.5 ppm was observed, suggesting the presence of a B–H unit. Further, the ³¹P{¹H} NMR spectrum showed a signal at δ = −91.9 ppm. Collectively, these spectroscopic features indicate a 1,1-addition of the P–H bond across the boron center of the transient borylene 3. Indeed, the SCXRD study confirmed compound 10 as [(CAAC)BH(Dur)(PHPh)] (Scheme 2), a product of 1,1-P–H-bond addition of phenylphosphine to the boron center of the transient durylborylene 3. The B–P bond distance in 10 (2.015(1) Å) is slightly longer than the B–P single covalent bond (1.974(2) Å) in [(Me₃P)(C₆H₄CH₂PCy₂)BrB₂(PHPh)],⁶⁶ but slightly shorter than that in [(PPh₂)B₂H(C₆H₄CH₂PCy₂)₂] (2.051(2) Å).⁶⁶ Furthermore, the WBI and MBO of 0.99 and 0.95, respectively, corroborate a single bonding interaction between B1 and P1 in 10.

Scheme 2 Synthesis of borylene-mediated P–H activated (10) and coordinated (11) species. xs. = 10 equiv. (for the synthesis of 10) and 8 equiv. (for the synthesis of 11). Inset: Crystallographically-determined molecular structure of 10 (ellipsoids shown at the 50% probability level). All hydrogen atoms and ellipsoids of ligand periphery are omitted for clarity.

It is noteworthy that, unlike the 1,2-N–H addition observed for amines, the reaction with phenylphosphine afforded a 1,1-P–H addition product, likely due to the lower electronegativity of phosphorus compared to nitrogen. Therefore, we investigated the possible reaction mechanism for P–H activation of phenylphosphine by the transient borylene 3 (Fig. 5). In the first step, the boron-centered lone pair of borylene 3 interacts with a P–H bond, forming a three-membered B⋯P⋯H ring in the transition state (10^TS1), probably due to the comparable electronegativity of P and H. In 10^TS1, the P–H bond is slightly polarized (Table S1). Thereafter, cleavage of the P–H bond results in migration of the slightly positive PPhH fragment to the filled orbital of boron, while the hydrogen atom transfers to the vacant orbital of boron, affording the 1,1-P–H addition product 10^Int (Fig. 5). In the second step, 10^Int isomerizes to the thermodynamically stable product 10 via rotation of the phenyl group about the P–Ph bond and translation of the hydrogen atom of the P–H unit (Fig. 5). The free energy barriers associated with the formation of 10 ( = 12.4 kcal mol⁻¹, = 16.0 kcal mol⁻¹) are relatively low, consistent with the mild experimental conditions. Another possible reaction mechanism for the formation of 10 involving the Werner-type adduct as an intermediate is less favorable, as it was calculated to have a significantly higher energy barrier for the 1,1-P–H activation step, which is inconsistent with the experimental reaction conditions.

Fig. 5 Reaction profile diagram illustrating the activation of PPhH₂ by transient borylene 3. The reaction free energies and bond lengths are given in kcal mol⁻¹ and Å, respectively. All hydrogen atoms of ligand periphery are omitted for clarity.

On the other hand, the reaction of CAAC-stabilized durylboryl radical 2 with excess KC₈ (8 equiv.) and a secondary phosphine, diphenylphosphine, afforded an orange solid (11) in 58% yield. Interestingly, the ¹¹B NMR spectrum of 11 showed an intense chemical shift at δ = 0.7 ppm, appearing in the region characteristic of CAAC/phosphine-stabilized borylenes, suggesting the formation of a borylene–diphenylphosphine adduct.^63–65 Although SCXRD data obtained for 11 was of poor quality, they provided sufficient evidence to confidently assign the compound as a borylene–diphenylphosphane adduct. Adduct 11 is thermally sensitive and decomposes upon heating to multiple unidentified products. However, unlike in the case of amines, no P–H activation was observed.

Conclusions

In summary, we have demonstrated the ability of transient CAAC-stabilized arylborylenes to activate N–H and P–H bonds of ammonia, amines and phosphines. The borylene engages in 1,2-addition with ammonia and various amines to afford a family of aminoboranes, while reactions with primary phosphines proceed via 1,1-addition at the boron center to yield phosphinoboranes. Computational studies revealed that these divergent reactivities stem from the interplay between boron's ambiphilicity and the substrate's electronegativity, with the 1,2-addition pathway favored for N–H activation and the 1,1-addition pathway for P–H activation. The isolation of a Werner-type intermediate in the amine system further corroborates a stepwise adduct-assisted mechanism. These findings collectively underscore the potential of borylenes as powerful, metal-free reagents for bond activation and functionalization chemistry. The presented study broadens the scope of main-group reactivity, hinting at potentially sustainable future alternatives to transition-metal-mediated transformations.

Author contributions

Supervision, funding acquisition, conceptualization: H. B.; conceptualization, formal analysis, investigation: M. W.; conceptualization, investigation, formal analysis, writing – original draft: S. K.; investigation, formal analysis: P. J.; writing – review & editing: R. D. D.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: synthetic, spectroscopic, X-ray crystallographic and computational details. See DOI: https://doi.org/10.1039/d6sc00526h.

CCDC 2521121–2521126 contain the supplementary crystallographic data for this paper.^67a–f

Acknowledgements

Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged (BR1149/22-2 and 466954611). S. K. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship.

Notes and references

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Footnote

† These authors contributed equally.

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