Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides

Abstract

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ²-{NHI^RP(Ph)NDipp}AlMe₂] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHI^RP(Ph)NDipp]^- [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

Scheme 1. Synthesis of the protic ligands 1a, 1b, and 1c.

Fig. 1. ORTEP drawing of the molecular structure of 1b, where ellipsoids are drawn to encompass 30% probability, while hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles [°]: P1–N1 1.7346(11), P1–N2 1.6844(12), P1–C24 1.8390(13), N2–C13 1.2944(18), N3–C13 1.3853(18), N4–C13 1.3815(18), N1–P1–N2 102.91(6), N1–P1–C24 93.10(6), N2–P1–C24 104.11(6), N2–C13–N3 130.67(13), N2–C13–N4 125.01(13), N3–C13–N4 104.30(12). CCDC no. 2124041.

Fig. 2. ORTEP drawing of the molecular structure of 1c, where ellipsoids are drawn to encompass 30% probability, while hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles [°]: P1–N1 1.729(2), P1–N2 1.652(2), P1–C24 1.848(3), N2–C13 1.292(3), N3–C13 1.386(4), N4–C13 1.394(3), N1–P1–N2 102.63(12), N1–P1–C24 98.45(13), N2–P1–C24 99.86(13), N2–C13–N3 123.4(2), N2–C13–N4 131.6(3), N3–C13–N4 104.6(2). CCDC no. 2124043.

Scheme 2. Synthesis of Al-alkyl complexes 2a and 2b.

Fig. 3. ORTEP drawing of the molecular structure of 2a, where ellipsoids are drawn to encompass 30% probability, while hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles [°]: Al1–N1 1.8642(19), Al1–N2 2.0022(18), Al1–C1 1.974(3), Al1–C2 1.952(3), N2–C15 1.316(2), N3–C15 1.380(3), N4–C15 1.372(2), N1–P1–N2 88.67(8), N1–Al1–N2 76.96(7), N1–Al1–C1 115.89(11), N1–Al1–C2 114.02(13), C1–Al1–C2 107.79(17), N2–Al1–C1 117.37(13), N2–Al1–C2 122.14(13), N2–C15–N3 130.04(18), N2–C15–N4 124.93(18), N3–C15–N4 105.02(16). CCDC no. 2124040.

Fig. 4. ORTEP drawing of the molecular structure of 2b, where ellipsoids are drawn to encompass 30% probability, while hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles [°]: Al1–N1 1.8679(11), Al1–N2 1.9903(11), Al1–C1 1.9672(17), Al1–C2 1.9698(17), N2–C15 1.3177(17), N3–C15 1.3714(16), N4–C15 1.3688(17), N1–P1–N2 88.41(5), N1–Al1–N2 77.16(5), N1–Al1–C1 114.64(7), N1–Al1–C2 114.21(7), C1–Al1–C2 112.97(8), N2–Al1–C1 117.61(6), N2–Al1–C2 115.69(7), N2–C15–N3 130.10(12), N2–C15–N4 125.01(11), N3–C15–N4 104.89(11). CCDC no. 2124042.

Fig. 5. ORTEP drawing of the molecular structure of 2c, where ellipsoids are drawn to encompass 30% probability, while hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles [°]: Al1–N1 1.8979(12), Al1–N2 1.9401(12), Al1–C1 1.987(2), Al1–C2 1.968(2), N2–C15 1.3479(18), N3–C15 1.3722(18), N4–C15 1.372(2), N1–P1–N2 89.26(6), N1–Al1–N2 78.36(5), N1–Al1–C1 120.36(8), N1–Al1–C2 115.83(8), C1–Al1–C2 108.41(10), N2–Al1–C1 115.77(7), N2–Al1–C2 115.77(8), N2–C15–N3 125.48(13), N2–C15–N4 128.37(13), N3–C15–N4 106.06(12). CCDC no. 2124039.

Fig. 6. Plot of the yield of the guanidine derivative (%) vs. time (min) as monitored by ¹H-NMR spectroscopy for the reaction between 2-bromoaniline and DIC at room temperature.

Scheme 3. Most plausible mechanism for guanylation reaction between carbodiimide and aryl amines mediated by aluminium complex 2a as a precatalyst.