Organoaluminum Compounds as Catalysts for Monohydroboration of Carbodiimides

Abstract The effective catalytic activity of organoaluminum compounds for the monohydroboration of carbodiimides has been demonstrated. Two aluminum complexes, 2 and 3, were synthesized and characterized. The efficient catalytic performances of four aluminum hydride complexes L1AlH2 (L1=HC(CMeNAr)2, Ar=2,6‐Et2C6H3; 1), L2AlH2(NMe3) (L2=o‐C6H4F(CH=N‐Ar), Ar=2,6‐Et2C6H3; 2), L3AlH (L3=2,6‐bis(1‐methylethyl)‐N‐(2‐pyridinylmethylene)phenylamine; 3), and L4AlH(NMe3) (L4=o‐C6H4(N‐Dipp)(CH=N‐Dipp), Dipp=2,6‐iPr2C6H3; 4), and an aluminum alkyl complex L1AlMe2 (5) were used for the monohydroboration of carbodiimides investigated under solvent‐free and mild conditions. Compounds 1–3 and 5 can produce monohydroborated N‐borylformamidine, whereas 4 can afford the C‐borylformamidine product. A suggested mechanism of this reaction was explored, and the aluminum formamidinate compound 6 was characterized by single‐crystal X‐ray, also a stoichiometric reaction was investigated.


Experimental General procedures
All manipulations were carried out under a purified nitrogen atmosphere using Schlenk techniques or inside an Etelux MB 200G glovebox. All solvents were refluxed with the appropriate drying agent and distilled prior to use. Commercially available chemicals were purchased from J&K chemical or VAS and used as received. Compounds 1, [1] 4, [2] and 5 [3] were prepared as previously described in the literature. L 2 was obtained from the reaction of 2, 6-diethylbenzenamine and o-fluorobenzaldehyde according to previously reported procedures. [2,4] Elemental analyses were performed by the Analytical Instrumentation Center of the Beijing Institute of Technology. NMR spectra were recorded on Bruker AM 400 spectrometer. Melting points were measured in sealed glass tubes. Synthesis of L 3 2, 6-bis(1-methylethyl)-N-(2-pyridinyl methylene)phenyl amine: [5,6] A solution of 2-pyridinecarboxaldehyde (20 mmol), 2, 6-diisopropylaniline (20 mmol), and a catalytic amount of formic acid in methanol (15 mL) was stirred at room temperature overnight. The solvent was evaporated on a rotary evaporator to remove volatile components, crude product was washed with water to remove residual formic acid, the product was recrystallized from n-hexane to yellow crystals, which were filtered off and washed with cold n-hexane.

Synthesis of L 2 AlH2(NMe3) (2) [2]
H3Al· NMe3 (1 M in toluene, 1 mL, 1 mmol) was added drop by drop into a toluene solution (20 mL) of L 2 (0.255 g, 1 mmol) at 0 °C. After the addition was complete, the reaction mixture was allowed to warm to room temperature and stirring was continued for 24 h. The solution was evaporated to dryness in vacuo, and the residue was dissolved in n-hexane. Then the solution was filtered. The filtrate was concentrated and stored at -20 °C in a freezer for 3 days to afford compound 2 as colorless crystals. An additional crop of 2 was obtained from the mother liquor. Total yield 0.302 g (88%); m.p. 63-66 °C. 1

Hydroboration of carbodiimides
General procedure for hydroboration of carbodiimides HBpin (0.14 g, 1.1 mmol) and carbodiimide (1 mmol) were loaded in a dried sealable J-Young Tube under nitrogen atmosphere inside the glove box, followed by the addition of the desired catalyst (0.04 mmol). Samples were taken out of the glove box and stirred and heated to a certain temperature in an oil bath. The reaction time depends on the nature of the starting materials. The reaction was terminated by exposing the mixture to air. The products were analyzed by using 1 H NMR, 13 C NMR, 11 B NMR.
The yield of hydroborated products was calculated from the ratio of starting material and the target product by 1 H NMR and using the internal standard PhOMe shows the same results. For example, the hydroboration of DippNCNDipp catalyzed by compound 1 at 80℃ was conducted, after 1 h to the reaction mixture was added equimolar amount of PhOMe, and dissolved in CDCl3. Figure S1  Figure S1. Calculation of the yield of hydroborated products.

N-{B(OCMe2)2}-isopropylformamidinate (a) [7]
The hydroboration reaction between N, N'-diisopropylcarbodiimide (1 mmol, 0.126 g) and pinacolborane (1.1 mmol, 0.14 g) was carried out according to the general procedure. The yields of the product are given in Table 1 [7] The hydroboration reaction between N,N'-dicyclohexylcarbodiimide (1 mmol, 0.226 g) and pinacolborane (1.1 mmol, 0.14 g) was carried out according to the general procedure. The yields of the product are given in Table 1, 2 of the manuscript. The same yields were observed using 1 mmol of PhOMe. 1

N-{B(OCMe2)2}-tert-butylformamidinate (c) [7]
The hydroboration reaction between N,N'-di-tert-butylcarbodiimide (1 mmol, 0.154 g) and pinacolborane (1.1 mmol, 0.14 g) was carried out according to the general procedure. The yields of the product are given in Table 1 [7] The hydroboration reaction between N, N'-2,6-diisopropylphenylcarbodiimide (1 mmol, 0.362 g) and pinacolborane (1.1 mmol, 0.14 g) was carried out according to the general procedure. The yields of the product are given in Table 1 of the manuscript. The same yields were observed using 1 mmol of PhOMe. 1

Stoichiometric reactions
In a J. Young tube, 0.5 mmol aluminum complex 1 (0.1965 g) and 1.0 equivalent DIC (0.063 g) was diluted with 0.5 mL CDCl3. The reaction mixture was stirred at 60 °C for 12h. And appearance of a characteristic singlet of aluminum formamidinate compound 6 at δ= 7.458 ppm was observed after 3h. 1.0 equivalent HBpin was added after 5h. After heating for 6 hours, the resonance appeared at δ = 7.68, which was compared to product a, assigned to the N=CHN unit of the expected final formamidinate product. With the extension of heating time, 6 gradually decomposed to produce the final product. The splitting of the resonance signal of compound 1 at δ = 5.33 and 1.64 ppm also indicates the occurrence of hydroalumination and the generation of aluminum formamidinate compound.

Figure S2. Stoichiometric reactions of compound 1 with DIC
In a J. Young tube, 0.5 mmol aluminum complex 3 (0.281 g) and 1.0 equivalent DIC (0.063 g) was diluted with 5 ml Tol. The reaction mixture was stirred at 60 °C for 6h. And a characteristic singlet at δ= 8.560 ppm was observed. We suppose that it might be N=CHN unit, which may indicate that compound 3 reacts with DIC to produce an aluminum formamidinate via hydroalumination. Then in a J. Young tube, 0.5 mmol aluminum complex 5 (0.209 g) and 1.0 equivalent DIC (0.063 g) was diluted with 5 ml Tol. The reaction mixture was stirred at 60 °C for 6 h. The splitting of the resonance signal of compound 5 at δ = 1.151 and 1.636 ppm might indicate the formation of compound 5 with DIC.