Magnesium catalyzed hydroamination of carbodiimides and hydroboration of cyanamides

Abstract

A β-diketiminate magnesium hydride complex, Mg-1 [{LMgH}₂; L = ^DepNacnac = (DepNCMe)₂CH; Dep = 2,6-Et₂-C₆H₃], catalyzes the hydroamination of carbodiimides with primary aryl amines and the hydroboration of cyanamides with HBpin. The reactions proceed efficiently under mild conditions, affording a range of guanidines, N-borylformamidines, and N,N-bis-boryldiamines in high yields. Detailed stoichiometric experiments and the successful isolation of active catalysts/key intermediates, i.e., magnesium anilide [{LMg-NH-C₆H₄-4-Me}₂] (Mg-2), magnesium amidinate [{LMg–N=CH–N(Et₂)}₂] (Mg-3), and magnesium guanidinate [LMg(NⁱPr)(N-p-tolyl)C-NH-ⁱPr] (Int-A2) complexes, provided insights into the reaction mechanisms, enabling the elucidation of plausible catalytic cycles. These results highlight the potential of Earth-abundant magnesium-based catalysts for synthesizing valuable nitrogen-containing compounds.

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Introduction

The catalytic functionalization of organic unsaturated substrates, particularly carbodiimides (CDI) and their isomers, cyanamides, is a cornerstone of modern organic synthesis, offering efficient routes to nitrogen-rich compounds. Representative products, including guanidines, N-silyl amines, and N-borylated amines, have diverse applications spanning pharmaceuticals, agrochemicals, organometallic chemistry, and materials science.¹ Among the numerous transformations involving these substrates, the hydroamination of CDIs and the hydroboration of cyanamides stand out due to their atom economy and their ability to deliver such valuable nitrogen-containing products.

The hydroamination of carbodiimides (RN=C=NR) yields guanidine derivatives (RN=C(NR′R″)NHR), which are fundamental intermediates, largely used in the chemical industries. Early studies demonstrated that primary aliphatic amines could react with carbodiimides without a catalyst under extreme conditions.² Similarly, cyclic secondary amines have been shown to react with carbodiimides under ambient conditions, affording tetra-substituted guanidines without the need for a catalyst.³ However, primary aromatic and acyclic secondary amines, which possess significantly lower nucleophilicity, require catalytic activation.⁴ Richeson and co-workers reported the first catalytic guanylation of primary aromatic amines with unactivated carbodiimides using a titanium amido complex in 2003.⁵ Since then, significant efforts have been made to develop effective catalytic systems to carry out such reactions. Several transition-metal,^5,6 rare-earth element,⁷ and main-group metal^1a–d,4,8 complexes have been extensively studied, focusing on improving the reaction efficiency and broadening the substrate scope.

Given the structural similarities between carbodiimides and their isomeric counterparts, cyanamides (R₂NC≡N), which feature a highly reactive nitrile group, they serve as ideal substrates for hydrofunctionalization reactions. Recently, hydroboration of cyanamides has been introduced as an efficient method to produce N-borylated compounds. These products, such as N-boryl formamidines and bis-borylated diamines, provide versatile synthons for downstream applications.⁹ Although CDI hydrofunctionalization has been extensively explored,^6a,8a,10 the hydrofunctionalization of cyanamides is still in its very early stages. To date, there have been only two reports on the hydrofunctionalization of cyanamides in the literature.¹¹

In recent years, the development of main-group metal-based catalytic systems has garnered increasing attention as less toxic, cost-effective alternatives to transition metal-based catalysts.¹² In particular, highly abundant, bio-compatible magnesium-based reagents and molecular complexes have been established as promising catalysts in hydroelementation of several unsaturated organic functionalities.^1a,8e,12b,13 In magnesium-catalyzed hydroboration, hydroamination, and hydrosilylation reactions, magnesium hydrides are often identified as the active catalytic species.^13a–e,14 However, to the best of our knowledge, magnesium hydride-catalyzed hydroamination of carbodiimides has not been reported to date.

Thus, herein, we report ^DepNacnac stabilized magnesium hydride [{LMgH}₂; L = ^DepNacnac = (DepNCMe)₂CH; Dep = 2,6-Et₂-C₆H₃] (Mg-1) as an efficient catalyst for hydroamination of carbodiimides with primary aryl amines and selective hydroboration of cyanamides using pinacolborane (HBpin). These reactions proceed efficiently under mild conditions, yielding N,N′,N″-trisubstituted guanidines, N-borylated imines, and bis-borylated diamines in high yields. We have performed various stoichiometric experiments to isolate key intermediates involved in the catalytic cycle, and based on those outcomes, we have proposed plausible catalytic mechanisms.

Results and discussion

^DepNacnac stabilized magnesium hydride (Mg-1) [{LMgH}₂; L = ^DepNacnac = (DepNCMe)₂CH; Dep = 2,6-Et₂-C₆H₃], previously reported by Jones and co-workers,¹⁵ was synthesized via a two-step process, i.e., treatment of the free ^DepNacnac ligand with the commercially available di-n-butyl magnesium reagent, which affords a magnesium alkyl compound {LMg-ⁿBu}₂. Subsequent addition of the phenylsilane into a solution of {LMg-ⁿBu}₂ as a hydride source produces the desired magnesium hydride {LMgH}₂.¹⁵

Addition of primary amines to diisopropylcarbodiimide (DIC)

In this study, we have described catalytic hydroamination of carbodiimide with primary aryl amines. We began our investigation with p-toluidine (2a) and diisopropylcarbodiimide (DIC) as standard substrates. An initial study in the presence of 3 mol% of Mg-1 under neat conditions at room temperature produced the corresponding product 1,3-diisopropyl-2-(p-tolyl)guanidine (3a) within 30 minutes, with a quantitative ¹H NMR conversion (Table S1 of the SI, entry 2). A decrease in catalyst loading from 3 to 1 mol% resulted in quantitative conversion within 3 h at ambient temperature, achieving the optimal reaction conditions (Table S1 of the SI, entry 4). The conversions were drastically reduced upon shortening the reaction time or reducing the catalyst load any further (Table S1 of the SI, entries 5–7). A reaction in C₆D₆ under the optimal conditions showed no difference in the conversion to 3a (Table S1 of the SI, entry 8). As stated in the introduction, a catalyst-free reaction showed no conversion to 3a even after 24 h at 80 °C (Table S1 of the SI, entry 1), justifying the requirement of a catalytic environment. With the optimized conditions in hand, we explored the substrate scope. Primary aryl amines with electron-donating (2a and 2b) and electron-withdrawing substituents (2c and 2d), as well as N-heterocyclic aryl amines (2e and 2f), were treated with DIC in the presence of pre-catalyst Mg-1 (Table 1). We observed complete conversion to the corresponding guanidine product within 3 h in every case. Interestingly, despite the limited solubility of 4-aminobenzonitrile (2c) in DIC, the reaction proceeded under neat conditions to yield the corresponding guanidine product, which is likely facilitated by localized dissolution between the solid amine and the liquid carbodiimide. The guanidine products (3a–3f) were confirmed by ¹H and ¹³C{¹H} NMR spectroscopy (see the SI for NMR spectra). After successful hydroamination of DIC, we investigated the reaction with different carbodiimides, such as di-tert-butyl and di-p-tolyl carbodiimides; unfortunately, no product conversion was observed. This can be attributed to their increased steric bulk, which impedes the approach of the nucleophilic anilido species (Mg-2) toward the electrophilic carbon center of the carbodiimide. We also attempted Mg-1 catalyzed hydroalkoxylation of carbodiimides with benzyl alcohol to synthesize N,N′-disubstituted isourea. However, the reaction was unsuccessful, yielding no detectable amount of the desired isourea product.

Table 1 Substrate scope for hydroamination of DIC with primary aryl amines^a,b

a Reaction conditions: DIC (1) (0.1 mmol, 1.0 equiv.), aryl amines 2a–2f (0.1 mmol, 1.0 equiv.), pre-catalyst Mg-1 (1 mol%), neat conditions, rt and 3 h. b Conversion was examined by ¹H and ¹³C{¹H} NMR (400 MHz, 25 °C) spectroscopy based on the formation of the product and the consumption of the starting material. Isolated yield is shown in parentheses.

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Hydroboration of cyanamides

Despite the advancement of metal-catalyzed hydrofunctionalization of unsaturated organic compounds, the hydroboration of cyanamides is rarely explored.¹¹ To overcome this limitation, we investigated the catalytic activity of Mg-1 for this hydroboration. We used dibenzyl cyanamide (4d) as a model substrate, and HBpin as the hydroborating agent. An initial attempt at a catalyst-free reaction of 4d with 1 equivalent of pinacolborane (HBpin) showed no conversion to the corresponding N-borylformamidine product (5d) after 24 h at 70 °C. The reaction with 3 mol% of the Mg-1 catalyst at 70 °C under neat conditions afforded an incomplete conversion, yielding a mixture of unreacted cyanamide (4d), the N-borylformamidine product (5d), and the N,N-bis-boryldiamine product (6d), after 12 h (Table S2 of the SI, entry 2). Pleasingly, high selectivity was achieved by conducting the reaction in C₆D₆, which afforded the desired product 5d in quantitative yield (Table S2 of the SI, entry 3). The reaction produced quantitative conversion under reduced catalyst load of 1 mol% at 60 °C within 12 h, in C₆D₆, achieving the optimal reaction conditions (Table S2 of the SI, entry 5). Further decrease in catalyst loading to 0.5 mol% resulted in a decrease in yield even at a prolonged reaction time (Table S2 of the SI, entry 6).

With the optimized reaction conditions in hand, we expanded the substrate scope using aliphatic acyclic (4a–4d) and cyclic (4e and 4f) cyanamides (Table 2). The ¹H NMR spectra show that dimethyl cyanamide yielded the corresponding N-borylformamidine product 5a with a 67% NMR conversion, and furthermore, diethyl cyanamide showed complete conversion to 5b with 1.5 equiv. of HBpin under the optimized conditions. Bulkier substituted cyanamide (4c) showed full conversion under the optimized reaction conditions. Cyclic cyanamides were also reduced to their corresponding N-borylformamidine products (5e and 5f), yielding up to 92% in 12 h at 60 °C. Longer heating of the same resulted in the formation of trace amounts of dihydroborated products. The N-borylformamidine products (5a–5f) were confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. The ¹H NMR spectra revealed a new singlet peak in the 7.99–8.46 ppm region corresponding to the NCHN proton (see the SI for NMR spectra).

Table 2 Substrate scope for monohydroboration of cyanamides^a,b,c

a Reaction conditions: cyanamides (4a–4f) (0.2 mmol, 1.0 equiv.), HBpin (0.2 mmol, 1.0 equiv.), catalyst Mg-1 (1 mol%), C₆D₆, 60 °C and 12 h. Conversion was examined by ¹H NMR spectroscopy based on the appearance of the new characteristic proton resonance (NCHN) of the product. b Yields were calculated using mesitylene as an internal standard (0.2 mmol, 1.0 equiv.). c 1.5 equiv. HBpin was used.

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After successful monohydroboration of cyanamides, we aimed to fully reduce the cyanamides. A reaction of 2.1 equivalents of HBpin with 4d in the presence of 5 mol% of Mg-1 yielded the corresponding N,N-bis-boryldiamine product (6d) after 18 h at 80 °C under neat conditions (Table S3 of the SI, entry 2). A reduction in the catalyst loading to 2 mol% produced complete NMR conversion to 6d after 18 h at 70 °C (Table S3 of the SI, entry 4), achieving the optimal conditions. Further decreasing the catalyst loading significantly decreased product conversion, resulting in the mixture 6d and the N-borylformamidine 5d (Table S3 of the SI, entries 5 and 6).

With the optimized reaction conditions in hand, we explored the substrate scope using cyanamides 4b–4f (Table 3). The N,N-bis-boryldiamine products (6b–6f) were confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. The ¹H NMR spectra show a new singlet peak in the 3.88–4.37 ppm region corresponding to NCH₂N protons (see the SI for NMR spectra). The cyanamides 4c–4f showed full conversion to their corresponding N,N-bis-borylated products under the optimized conditions. However, for dimethyl (4a) and diethyl (4b) cyanamides, we observed a mixture of mono and dihydroborated products under the optimized conditions. The monohydroboration of cyanamides proceeded smoothly in C₆D₆, whereas under neat conditions, a mixture of mono- and dihydroborated products was obtained, highlighting a distinct solvent effect of C₆D₆ in directing the selectivity of the reaction as the concentration of reactants is lower in the solution compared to neat conditions. However, the complete hydroboration to the dihydroborated product proceeded efficiently under neat conditions, although the reaction of diethyl cyanamide (4b) required 3 equiv. of HBpin in C₆D₆ in the presence of 2 mol% of Mg-1 to afford the corresponding product 6b with a >99% NMR conversion after 18 h at 70 °C. After achieving selective hydroboration of cyanamides under mild reaction conditions, we further investigated hydrosilylation of cyanamides. However, attempts at the Mg-1 catalyzed hydrosilylation of these substrates were unsuccessful.

Table 3 Substrate scope for dihydroboration of cyanamides^a,b,c

a Reaction conditions: cyanamides (4b–4f) (0.1 mmol, 1.0 equiv.), HBpin (0.21 mmol, 2.1 equiv.), catalyst Mg-1 (2 mol%), neat conditions, 70 °C and 18 h. b Conversion was examined by ¹H NMR spectroscopy based on the appearance of the new NCH₂N(Bpin)₂ resonance of the products. c Reaction was performed in C₆D₆ in a J. Young valve NMR tube at 70 °C for 18 h, 3.0 equiv. HBpin. Isolated yield is shown in parentheses.

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Scale-up reactions

To explore the practical application of the magnesium hydride complex (Mg-1), we performed a large-scale reaction of carbodiimide with p-toluidine (2a) under the optimized conditions. A 5 mmol scale reaction of DIC (1) with 1 equiv. of p-toluidine (2a) yielded the product 1,3-diisopropyl-2-(p-tolyl)guanidine (3a) in 83% yield (Scheme 1a).

Scheme 1 Scale-up reactions.

For the dihydroboration of cyanamides, a 2 mmol scale reaction was performed with dibenzyl cyanamide (4d) and 2.1 equiv. of HBpin (Scheme 1b). Under the standard reaction conditions, it produced the corresponding N,N-bis-boryldiamine product (6d) in an isolated yield of 91%.

Attempts to scale up the monohydroboration of cyanamides under the optimized conditions resulted in the formation of a mixture containing the desired monohydroborated product along with a minor amount of the dihydroborated species; hence, further scale-up was not pursued.

Stoichiometric experiments

Hydroamination of carbodiimides. To gain insight into the reaction mechanism, we performed several control experiments. A 1 : 2 stoichiometric reaction between Mg-1 and p-toluidine (2a) in C₆D₆ resulted in the formation of a magnesium anilide complex Mg-2 [{LMg-NH-C₆H₄-4-Me}₂] immediately at room temperature (Scheme 2a). Mg-2 was characterized by ¹H and ¹³C{¹H} NMR spectroscopy and single-crystal X-ray diffraction studies. The ¹H NMR spectrum shows a complete disappearance of the Mg–H signal at 3.85 ppm and the appearance of a new peak at 6.03 ppm corresponding to the NH proton of the anilide moiety. Notably, the γ-CH resonance of the NCCCN backbone of Mg-1 (δ 4.79 ppm in C₆D₆) shows a slightly upfield shift to δ 4.73 ppm, indicating the formation of Mg-2.

Scheme 2 Stoichiometric experiments.

Single crystals suitable for X-ray diffraction studies were grown in a vial through slow evaporation of C₆D₆. X-ray diffraction analysis showed that Mg-2 crystallizes in a triclinic system with a P1̄ space group. The solid-state structure (Fig. 1) exhibits a dimeric structure, in which the magnesium center adopts a distorted tetrahedral geometry, bonded to one Nacnac ligand in an N,N′-chelated fashion, and the other two sites are occupied by N atoms of the anilide moieties. The Mg1–N3 bond distance in Mg-2 is 2.0802(13) Å, which is slightly shorter than the previously reported Mg–N bond distance of a ^DippNacnac magnesium anilide complex (2.1251(16) Å).¹⁶ The N1–Mg1–N2 bite angle is 94.32(5)°, which is slightly wider than the reported bite angle (92.65(6)°).¹⁶ Moreover, Mg-2 was tested as a catalyst for the hydroamination reaction between DIC (1) and p-toluidine (2a), affording the desired guanidine product (3a) in quantitative yield (Table S1, entry 9).

Fig. 1 Solid-state structure of Mg-2. Hydrogen atoms except H1 were omitted for better visibility. The selected bond distances (Å) and bond angles (°) are: Mg1–N1 2.0465(13), Mg1–N2 2.0358(13), Mg1–N3 2.0802(13), N3–C1 1.4178(19), N1–Mg1–N2 94.32(5), N1–Mg1–N3 108.17(5), N2–Mg1–N3 129.39(6), and Mg1–N3–Mg1′ 91.00(5).

Furthermore, the addition of 2.0 equiv. of DIC (1) to Mg-2 in C₆D₆ at 50 °C for 2 h afforded the magnesium guanidinate complex Int-A2 [LMg(NⁱPr)(N-p-tolyl)-C-NH-ⁱPr] (Scheme 2b). In the ¹H NMR spectrum of Int-A2, the γ-CH resonance exhibits a downfield shift to 4.93 ppm (δ 4.73 ppm in Mg-2). The isopropyl methyl protons resonate as two distinct doublets at 0.65 and 0.62 ppm. Meanwhile, the NH signal of Int-A2 appears at 3.22 ppm as a doublet signal, markedly shifted from 6.03 ppm in Mg-2. Furthermore, the peak at 164.4 ppm in the ¹³C{¹H} NMR spectrum corresponding to the NCN carbon confirms the formation of Int-A2. Moreover, the possibility of the formation of another intermediate, Int-A1, has been ruled out by the distinct doublet signal for the NH proton at 3.22 ppm in the ¹H NMR spectrum, together with previously reported literature on aluminum-catalyzed hydroamination of carbodiimides, suggesting that Int-A2 is more favourable.⁴ Additionally, Int-A2 has been confirmed by mass spectrometry. Literature surveys show that the metal-catalyzed synthesis of guanidines from carbodiimide and primary amines involves two steps: first, the formation of a metal anilide complex, and second, the insertion of carbodiimide into the metal–nitrogen bond.^{1a–d,4,6,17} The isolation of Mg-2 and Int-A2 confirms that the hydroamination of carbodiimides follows the same pathway.

From the catalytic reaction and stoichiometric experiments, a notable difference in reaction conditions was observed. In the stoichiometric reaction, external heating was necessary to facilitate the formation of Int-A2. In contrast, during catalysis, the in situ generation of Mg-2 was accompanied by exothermic H₂ evolution, which provided sufficient thermal energy to promote the insertion step even at room temperature. This phenomenon became particularly evident during the scale-up experiment, where noticeable heating of the reaction flask was observed.

Hydroboration of cyanamides. A 1 : 2 ratio stoichiometric reaction was conducted with Mg-1 and diethyl cyanamide (4b) in C₆D₆ (Scheme 3a). This afforded the magnesium amidinate complex Mg-3 [{LMg–N=CH–N(Et₂)}₂] after 15 minutes at room temperature. Mg-3 was characterized by ¹H and ¹³C{¹H} NMR spectroscopy and X-ray diffraction analyses. In the ¹H NMR spectrum, the Mg–H resonance at 3.85 ppm disappeared, and concurrently a new singlet at 7.59 ppm appeared, corresponding to the NCHN moiety. Moreover, the ¹³C{¹H} NMR spectrum shows a peak at 152.8 ppm corresponding to the NCHN carbon, confirming the insertion of the Mg–H bond into the N–C≡N moiety of the cyanamide. Single crystals suitable for X-ray diffraction analyses were grown in C₆D₆ in the J. Young valve NMR tube. The X-ray diffraction analyses show that Mg-3 crystallizes in a monoclinic system with a P2₁/n space group. The molecular structure unfolds a dimeric unit, in which the magnesium center adopts a distorted tetrahedral geometry, bonded to the Nacnac ligand at two sites and the other two sites with N atoms of the amidinate moiety (Fig. 2). The N3–C1 bond length (i.e., 1.267(2) Å) closely resembles the N=C double bond lengths (1.263(2) Å for ^DippNacnacMg-N=CH-C₆H₄-3-OMe and 1.271(2) Å for ^DippNacnacMg-N=CH-C₆H₄-4-OMe) reported in the literature.^13c

Scheme 3 Stoichiometric experiments.

Fig. 2 Solid-state structure of Mg-3. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms except H1 were omitted for better visibility. The selected bond distances (Å) and bond angles (°) are: Mg1–Mg1′ 2.9000(9), Mg1–N1 2.0740(13), Mg1–N2 2.0598(13), Mg1–N3 2.0462(13), N3–C1 1.267(2), C1–N4 1.371(2), N1–Mg1–N2 90.69(5), N1–Mg1–N3 119.43(5), N2–Mg1–N3 120.69(5) and Mg1–N3–C1 146.14(11).

Further reaction of Mg-3 with 2 equiv. of HBpin showed no conversion, with the reactants remaining intact even after 24 h at 80 °C. Addition of 2 equiv. of diethyl cyanamide (4b) to the reaction mixture yielded the monohydroboration product 5b (Scheme 3b) with the regeneration of Mg-3 after 12 h at 80 °C. The ¹H NMR spectrum exhibited two downfield singlets, confirming the presence of both 5b and Mg-3 (Fig. S7 of the SI). The resonance at 8.12 ppm corresponds to the NCHN proton of 5b and the signal at 7.59 ppm corresponds to the Mg–NCHN(Et)₂ proton of Mg-3. Subsequent addition of another 2.0 equiv. of HBpin to the same reaction mixture led to quantitative conversion into a dihydroboration product, 6b, along with Mg-3 under the same reaction conditions (Scheme 3c). The signal at 7.59 ppm in the ¹H NMR spectrum (Fig. S8 of the SI) corresponds to the Mg–NCHN(Et)₂ of Mg-3, and the signal at 4.41 ppm indicates the NCH₂N protons of 6b. Notably, the direct reaction of Mg-3 with 2.0 equiv. of 4b and 4 equiv. of HBpin afforded 6b, accompanied by regeneration of Mg-3 (Scheme 3d). The aforementioned results indicate that magnesium hydride Mg-1 is the pre-catalyst, which forms Mg-3 and then continues the catalytic cycle as the active catalyst.

Recently, Thomas and co-workers demonstrated that BH₃ generated from the decomposition of HBpin can serve as a hidden catalyst for the hydroboration reactions.¹⁸ To rule out this possibility, a reaction of excess HBpin with the catalyst Mg-1 was conducted; however, we did not observe the formation of BH₃ in ¹¹B NMR spectroscopy (Fig. S9 of the SI). Moreover, a catalytic reaction was carried out in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) under the optimized conditions; however, no noticeable decrease in conversion was observed in the ¹H NMR spectrum (Fig. S10 of the SI). The aforementioned result and isolation of Mg-3 indicate that Mg-1 is the primary catalyst, and no hidden boron catalysis is involved in the reactions.

The catalytic hydroboration of 4d was monitored via in situ studies by reacting with 2.1 equiv. of HBpin in the presence of 2 mol% of Mg-1, with the reaction progress being tracked at variable temperatures over different time intervals. Time-stacked variable temperature NMR spectra revealed the stepwise formation of N-borylformamidine (5d) and N,N-bis-boryldiamine (6d) products. Initially, we observed a singlet signal at 8.46 ppm corresponding to the NCHN proton, confirming the formation of 5d. Upon increasing the temperature to 70 °C, a new signal appeared at 4.06 ppm corresponding to the NCH₂N proton of 6d, and the signal at 8.46 ppm disappeared, indicating the complete consumption of 5d (Fig. 3).

Fig. 3 Stacked ¹H NMR spectra (400 MHz) for the reaction of 4d (0.1 mmol, 1.0 equiv.), HBpin (0.21 mmol, 2.1 equiv.) and Mg-1 (2 mol%) in C₆D₆. Spectra recorded at different temperatures and time intervals (T = 25–70 °C and t = 5 min to 18 h).

As mentioned earlier, we attempted Mg-1 catalyzed O–H bond addition to DIC, which was unsuccessful. However, a 1 : 2 ratio stoichiometric reaction between Mg-1 and benzyl alcohol in C₆D₆ yielded the magnesium alkoxide complex Mg-4 [{LMg–O–CH₂–C₆H₅}₂, L = ^DepNacnac = (DepNCMe)₂CH; Dep = 2,6-Et₂-C₆H₃] (Scheme 4). Mg-4 was confirmed by both ¹H and ¹³C{¹H} NMR spectroscopy. ¹H NMR analysis shows a singlet at 4.92 ppm signaling the γ-CH proton, downfield shifted compared to Mg-1 (4.79 ppm in C₆D₆). The sharp singlet at 4.50 ppm, corresponding to the OCH₂ protons, further confirmed the formation of Mg-4. Single crystals suitable for X-ray diffraction analysis were obtained inside the NMR tube. The X-ray diffraction data show that Mg-4 crystallizes in a monoclinic system with a P2₁/n space group (Fig. 4). It exhibits a dimeric unit bridged by the oxygen atom (Mg–O–Mg). The magnesium center adopts a distorted tetrahedral geometry, bonded to one Nacnac ligand in an N,N′-chelated fashion, and the other two sites by O atoms. The Mg–O bond distance, i.e., 1.9535(15) Å, is shorter in comparison with the ^DippNacnac analogue (1.998(6) Å).¹⁹ The N–Mg–N bite angle is 92.42(7)°, which is in good agreement with the previously reported bite angle, i.e., 92.2(2)°.¹⁹

Fig. 4 Solid-state structure of Mg-4. Hydrogen atoms except H1a and H1b were omitted for better visibility. The selected bond distances (Å) and bond angles (°) are: Mg1–N1 2.0403(18), Mg1–N2 2.0595(17), Mg1–O1 1.9535(15), O1–C1 1.406(3), N1–Mg1–N2 92.42(7), O1–Mg1–O1′ 83.86(6), N1–Mg1–O1′ 124.28(7), and N2–Mg1–O1 127.00(7).

Scheme 4 Synthesis of compound Mg-4.

Catalytic cycles

Hydroamination of carbodiimides. Based on the results of the above stoichiometric experiments and previously documented mechanisms in the literature,^1d,7b,f we proposed a plausible catalytic cycle for N–H bond addition to CDIs as shown in Scheme 5.

Scheme 5 Proposed catalytic cycle for hydroamination of DIC.

In this reaction, Mg-1 acts as a pre-catalyst, which reacts with the primary aryl amine to form the magnesium anilide (Mg-2) complex with the release of H₂ gas. A subsequent addition of the carbodiimide to the magnesium anilide complex forms the guanidinate intermediate, Int-A2. Furthermore, intermediate IntA2 independently reacts with an aryl amine, affords the corresponding guanidine product, and closes the cycle with the regeneration of the active catalyst Mg-2.

Hydroboration of cyanamides. Based on the stoichiometric reactions, we proposed a catalytic cycle for the hydroboration of cyanamides, as represented in Scheme 6. In this reaction, magnesium hydride Mg-1 acts as a pre-catalyst, contrary to the reported mechanism where metal (zinc) hydride acts as the active catalyst.^11a In the zinc system, the bis-guanidinate zinc hydride itself participates directly as the active species in B–H bond addition and regenerates at the end of the catalytic cycle, while in this case, Mg-1 first reacts with cyanamide 4b to generate the magnesium amidinate complex Mg-3, which functions as the active catalyst. HBpin reacts with Mg-3 to form the borohydride intermediate Int-B, consistent with the type of borohydride species previously described in magnesium-catalyzed hydroboration reactions.^13a–dInt-B further reacts with diethyl cyanamide (4b) to regenerate the active catalyst Mg-3 with the formation of monohydroboration products 5b.

Scheme 6 Proposed catalytic cycle for hydroboration of cyanamides.

For the dihydroboration reaction, Int-B reacts with cyanamide 4b and another equivalent of HBpin to form the dihydroboration product 6b and completes the cycle with the regeneration of Mg-3.

Conclusions

In conclusion, we have demonstrated a method for hydroamination of CDI and chemoselective hydroboration of cyanamides using a β-diketiminate magnesium hydride (pre-)catalyst under mild conditions. The method accommodates the synthesis of a range of guanidine, N-borylformamidine, and N,N-bis-boryldiamine products with high chemoselectivity and yields. Compared with previously reported magnesium-based catalysts for similar hydroamination reactions,^1a,8e,13f the present system exhibits enhanced efficiency, achieving comparable or superior conversions under milder conditions, including lower catalyst loadings, under solvent-free conditions, and even at ambient temperature. Notably, in cyanamide hydroboration, Mg-1 achieved complete conversions under comparatively lower catalyst loadings than that of the recently developed zinc-based system.^11a However, we noticed that the catalytic activity of Mg-1 is comparable to that of a recently reported bis(guanidinate) magnesium catalyst.^11b Moreover, the solvent was found to play a decisive role in controlling the product selectivity, with C₆D₆ favoring monohydroboration while neat conditions promoted full reduction to N,N-bis-boryldiamines. Additionally, we have isolated active catalysts and key intermediates involved in the reaction mechanism, namely magnesium anilide complex (Mg-2) and magnesium amidinate complex (Mg-3), which were characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction analyses. Furthermore, plausible catalytic cycles have been proposed based on the isolation of key intermediates and stoichiometric experiments.

Experimental section

Methods

Unless stated, all the reactions were performed under an inert nitrogen atmosphere using standard Schlenk and glovebox techniques. Silicone-greased glassware and vessels with J. Young valves were used. NMR spectroscopic data were recorded on a Bruker AV 400 MHz or 700 MHz Bruker DPX spectrometer [¹³C{¹H}, 101 MHz or 176 MHz]. Deuterated benzene (C₆D₆) and chloroform (CDCl₃) were used for NMR measurements; chemical shift values (δ) were reported in parts per million relative to the residual signals of their respective solvents.²⁰ Coupling constants (J) are given in Hz. The multiplicity of the ¹H NMR spectrum is given as follows: s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, dt = doublet of triplets, br = broad, and m = multiplet signal. Mass spectrometry analyses were performed on a Waters XevoG2 XS Q-TOF mass spectrometer. The single crystals of compounds Mg-2, Mg-3, and Mg-4 were grown in vials from C₆D₆ inside the glove box at rt. The single crystal data were collected on a Rigaku Oxford diffractometer with graphite-monochromated Cu-Kα radiation (λ = 1.54184 Å) at 100 K. Selected data collection parameters and other crystallographic results are summarized in Table S4 of the SI.

Materials

All the solvents were purified by distillation over Na/benzophenone and stored in a dinitrogen atmosphere with activated molecular sieves. Deuterated chloroform (CDCl₃) was dried with molecular sieves, and deuterated benzene (C₆D₆) was dried over a Na mirror, distilled, and degassed. The β-diketimine ligand was prepared according to the reported literature procedures. For catalytic reactions, J. Young valve NMR tubes were thoroughly dried in an oven before use. All the chemicals and the reagents were supplied by Sigma-Aldrich Co. Ltd, TCI Chemicals, and Merck India Pvt. Ltd and were utilized without any further purification. The catalyst [^DepNacnacMg(μ-H)]₂ (Mg-1) was prepared according to the reported literature.¹⁵

General procedure for hydroamination of DIC

DIC (0.1 mmol, 1 equiv.), aryl amines (0.1 mmol, 1.0 equiv.) and 0.001 mmol (1 mol%) of catalyst Mg-1 were charged in a vial with a magnetic bead inside a glove box. The vial was closed and stirred at room temperature for 3 h. The progress of the reaction was monitored using ¹H NMR spectroscopy.

General procedure for catalytic monohydroboration of cyanamides

Cyanamides (0.2 mmol, 1 equiv.) and 0.002 mmol (1 mol%) of catalyst Mg-1 were charged in a J. Young valve NMR tube inside a glove box. Then, C₆D₆ (∼0.5 mL) was added, followed by HBpin (0.2 mmol, 1 equiv.). The NMR tube was removed from the glove box and heated at 60 °C for 12 h. The progress of the reaction was monitored using ¹H NMR spectroscopy, which indicated the completion of the reaction by observing the characteristic NCHN(Bpin) peak.

General procedure for catalytic dihydroboration of cyanamides

Cyanamides (0.1 mmol, 1 equiv.), HBpin (0.21 mmol, 2.1 equiv.) and 0.002 mmol (2 mol%) of catalyst Mg-1 were charged in a vial with a magnetic bead or in a J. Young valve NMR tube inside a glove box. The sealed vial or NMR tube was removed from the glove box and heated/stirred at 70 °C for 18 h. The progress of the reaction was monitored using ¹H NMR spectroscopy, which indicated the completion of the reaction by observing a characteristic NCH₂N(Bpin)₂ peak.

Synthesis of Mg-2, Int-A2, Mg-3 and Mg-4

Synthesis of [LMgNH-C₆H₄-4-Me]₂ (Mg-2) {NMR scale}. Addition of 5.6 mg of p-toluidine (0.052 mmol, 2 equiv.) to a solution of Mg-1 (20 mg, 0.026 mmol, 1.0 equiv.) in C₆D₆ (∼0.5 mL) in a J. Young valve NMR tube yielded the magnesium anilide complex (Mg-2) within 15 min at room temperature. Single crystals suitable for X-ray diffraction were grown in a vial inside the glovebox at room temperature. NMR conversion >99%. ¹H NMR (400 MHz, C₆D₆, 298 K) δ = 7.18 (dd, J = 13.9, 6.1 Hz, 8H, Ar–H), 7.10 (d, J = 23.1 Hz, 8H, Ar–H), 6.73 (d, J = 7.4 Hz, 4H, Ar–H), 6.03 (d, J = 5.0 Hz, 2H, NH), 4.73 (s, 2H, CH), 2.52 (bs, 4H, CH₂CH₃), 2.24 (bs, 6H, CH₂CH₃), 2.18 (s, 6H, p-CH₃), 1.79 (bs, 6H, CH₂CH₃), 1.48 (s, 12H, CCH₃), 1.05 (m, 24H, CH₂CH₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 298 K) δ = 168.5, 150.4, 147.6, 137.8, 137.2, 129.7, 126.1, 124.7, 120.6, 95.2, 24.3, 23.7, 20.7, 13.6. Due to the highly air and moisture-sensitive nature of the complex, we were unable to produce HRMS data for the appropriate intensity ratio or an adequate percentage in the elemental analysis report.

Synthesis of [LMg(NⁱPr)(N-p-tolyl)-C-NH-ⁱPr] (Int-A2) {NMR scale}. To the NMR tube containing a solution of Mg-2 (0.26 mmol, 1.0 equiv.) in C₆D₆, DIC (8 µL, 0.52 mmol, 2.0 equiv.) was added and heated at 50 °C for 2 h. It afforded us the magnesium guanidinate complex Int-A2. NMR conversion >99%. ¹H NMR (400 MHz, C₆D₆, 298 K) δ = 7.11 (dd, J = 9.3, 4.0 Hz, 6H, Ar–H), 6.98 (d, J = 8.1 Hz, 2H, Ar–H), 6.81 (d, J = 8.2 Hz, 2H, Ar–H), 4.93 (s, 1H, CH), 3.45–3.33 (m, 2H, ⁱPr–CH), 3.22 (d, J = 9.8 Hz, 1H, NH), 2.79–2.65 (m, 8H, CH₂CH₃), 2.23 (s, 3H, p-CH₃), 1.68 (s, 6H, CCH₃), 1.21 (t, J = 7.3 Hz, 12H, CH₂CH₃), 0.65 (d, J = 6.2 Hz, 6H, ⁱPr-CH₃), 0.62 (d, J = 6.3 Hz, 6H, ⁱPr-CH₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 298 K) δ = 168.8, 164.4, 148.5, 147.1, 137.2, 129.6, 127.5, 125.5, 124.7, 122.0, 94.9, 44.3, 44.3, 24.8, 24.7, 23.3, 23.0, 20.9, 13.7. HRMS (ASAP/Q-TOF) m/z: [M + H]⁺ calcd for C₃₉H₅₆MgN₅ 618.4386, Found: 618.4354.

Synthesis of [LMgNC(H)N(Et₂)]₂ (Mg-3) {NMR scale}. Addition of 5.9 µL of diethyl cyanamide (0.052 mmol, 2 equiv.) to a J. Young valve NMR tube containing a solution of compound Mg-1 (20 mg, 0.026 mmol, 1.0 equiv.) in dry C₆D₆ (∼0.5 mL) resulted in the formation of the magnesium amidinate complex Mg-3 within 15 minutes at room temperature. Then, the contents of the NMR tube were transferred to a vial inside the glove box to grow single crystals suitable for X-ray diffraction. NMR conversion >99%. ¹H NMR (400 MHz, C₆D₆, 298 K) δ = 7.59 (s, 2H, NCHN), 7.14 (s, 4H, Ar–H), 7.12 (s, 2H, Ar–H), 7.10 (s, 4H, Ar–H), 7.08 (d, J = 2.3 Hz, 4H, Ar–H), 4.78 (s, 2H, CH), 3.12 (broad s, 4H, CH₂CH₃), 2.99 (broad s, 4H, CH₂CH₃), 2.49 (dq, J = 15.1, 7.5 Hz, 8H, CH₂CH₃), 2.10 (broad dd, J = 14.0, 6.1 Hz, 8H, CH₂CH₃), 1.50 (s, 12H, CCH₃), 1.12 (t, J = 7.6 Hz, 24H, CH₂CH₃), 0.90 (broad s, 12H, CH₂CH₃). ¹³C{¹H} NMR (176 MHz, C₆D₆, 298 K) δ = 167.5, 152.8, 148.4, 137.3, 125.8, 123.9, 94.6, 45.7, 23.7, 23.7, 14.3, 12.7. Due to the highly air and moisture-sensitive nature of the complex, we were unable to produce HRMS data for the appropriate intensity ratio or an adequate percentage in the elemental analysis report.

Synthesis of [LMg-O-CH₂-C₆H₅]₂ (Mg-4) {NMR scale}. 5.4 µL of benzyl alcohol (0.052 mmol, 2.0 equiv.) was added to a J. Young valve NMR tube containing a solution of Mg-1 (20 mg, 0.026 mmol, 1 equiv.) in C₆D₆ (∼0.5 mL) inside a glovebox. This resulted in the formation of the magnesium alkoxide complex Mg-4 after 2 h at room temperature. Within 48 h, we obtained block-shaped single crystals suitable for X-ray diffraction analysis inside the NMR tube. NMR conversion = 99%. ¹H NMR (400 MHz, C₆D₆, 298 K) δ = 7.22 (t, J = 7.5 Hz, 4H, Ar–H), 7.19 (s, 6H, Ar–H), 7.11 (d, J = 7.3 Hz, 8H, Ar–H), 7.07–7.01 (m, 4H, Ar–H), 4.92 (s, 2H, CH), 4.50 (s, 4H, OCH₂), 2.34 (dd, J = 15.3, 7.6 Hz, 8H, CH₂CH₃), 2.06 (dd, J = 15.3, 7.6 Hz, 8H, CH₂CH₃), 1.47 (s, 12H, CCH₃), 1.04 (t, J = 7.5 Hz, 24H, CH₂CH₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 298 K) δ = 168.7, 147.7, 144.9, 137.5, 128.9, 127.4, 126.7, 126.0, 124.5, 95.7, 66.4, 23.8, 23.8, 14.2. Due to the highly air and moisture-sensitive nature of the complex, we were unable to produce HRMS data for the appropriate intensity ratio or an adequate percentage in the elemental analysis report.

Author contributions

The manuscript was written with contributions from all authors. All authors have approved the final version of the manuscript.

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: ¹H and ¹³C{¹H} NMR spectra of compounds Mg-2, Mg-3, Mg-4, and Int-A2, stoichiometric experiments, and catalytic products. See DOI: https://doi.org/10.1039/d5dt02335a.

CCDC 2490597–2490599 contain the supplementary crystallographic data for this paper.^21a–c

Acknowledgements

The authors thank the National Institute of Science Education and Research (NISER), Homi Bhabha National Institute (HBNI), Bhubaneswar, and the Department of Atomic Energy (DAE), Govt. of India. The Science and Engineering Research Board (SERB), India (CRG/2021/007000) is acknowledged for providing financial support.

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