Synthesis and catalytic activity of group 5 metal amides with chiral biaryldiamine-based ligands

Abstract

A new series of group 5 metal amides have been prepared from the reaction between V(NMe₂)₄ or M(NMe₂)₅ (M = Nb, Ta) and chiral ligands, (R)-2,2′-bis(mesitoylamino)-1,1′-binaphthyl (1H₂), (R)-5,5′,6,6′,7,7′,8,8′-octahydro-2,2′-bis(mesitoylamino)-1,1′-binaphthyl (2H₂), (R)-6,6′-dimethyl-2,2′-bis(mesitoylamino)-1,1′-biphenyl (3H₂), (R)-2,2′-bis(mesitylenesulfonylamino)-6,6′-dimethyl-1,1′-biphenyl (4H₂), (R)-2,2′-bis(diphenylthiophosphoramino)-1,1′-binaphthyl (5H₂), (R)-2,2′-bis[(3-tert-butyl-2-hydroxybenzylidene)amino]-6,6′-dimethyl-1,1′-biphenyl (6H₂), (R)-2,2′-bis[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]-6,6′-dimethyl-1,1′-biphenyl (7H₂), (R)-2,2′-bis[(3-tert-butyl-2-hydroxybenzylidene)amino]-1,1′-binaphthyl (8H₂), (S)-2-(mesitoylamino)-2′-(dimethylamino)-1,1′-binaphthyl (9H), and (R)-2-(mesitoylamino)-2′-(dimethylamino)-6,6′-dimethyl-1,1′-biphenyl (10H), which are derived from (R) or (S)-2,2′-diamino-1,1′-binaphthyl, and (R)-2,2′-diamino-6,6′-dimethyl-1,1′-biphenyl, respectively. Treatment of V(NMe₂)₄ or M(NMe₂)₅ (M = Nb, Ta) with 1 equiv of C₂-symmetric amidate ligands 1H₂, 2H₂, 3H₂, 4H₂, and 5H₂, or Schiff base ligands 6H₂, 7H₂ and 8H₂ at room temperature gives, after recrystallization from a benzene, toluene or n-hexane solution, the vanadium amides (1)V(NMe₂)₂ (11), (2)V(NMe₂)₂ (14), (3)V(NMe₂)₂ (17), (5)V(NMe₂)₂ (22), (6)V(NMe₂)₂ (23) and (7)V(NMe₂)₂ (24), and niobium amides (1)Nb(NMe₂)₃ (12), (2)Nb(NMe₂)₃ (15), (3)Nb(NMe₂)₃ (18), (4)Nb(NMe₂)₃ (20) and [2-(3-Me₃C-2-O-C₆H₃CHN)-2′-(N)-C₂₀H₁₂][2-(Me₂N)₂CH-6-CMe₃-C₆H₃O]NbNMe₂·C₇H₈ (25·C₇H₈), and tantalum amides (1)Ta(NMe₂)₃ (13), (2)Ta(NMe₂)₃ (16), (3)Ta(NMe₂)₃ (19) and (4)Ta(NMe₂)₃ (21) respectively, in good yields. Reaction of V(NMe₂)₄ or M(NMe₂)₅ (M = Nb, Ta) with 2 equiv of C₁-symmetric amidate ligands 9H or 10H at room temperature gives, after recrystallization from a toluene or n-hexane solution, the chiral bis-ligated vanadium amides (9)₂V(NMe₂)₂·3C₇H₈ (27·3C₇H₈) and (10)V(NMe₂)₂ (28), and chiral bis-ligated metallaaziridine complexes (10)₂M(NMe₂)(η²-CH₂NMe) (M = Nb (29), Ta (30)) respectively, in good yields. The niobium and tantalum amidate complexes are stable in a toluene solution at or below 160 °C, while the vanadium amidate complexes degrade viadiemthylamino group elimination at this temperature. For example, heating the complex (2)V(NMe₂)₂ (14) in toluene at 160 °C for four days leads to the isolation of the complex [(2)V]₂(μ-NMe₂)₂ (26) in 58% yield. These new complexes have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of complexes 12, 13, and 15–30 have further been confirmed by X-ray diffraction analyses. The vanadium amides are active chiral catalysts for the asymmetric hydroamination/cyclization of aminoalkenes, affording cyclic amines in moderate to good yields with good ee values (up to 80%), and the tantalum amides are outstanding chiral catalysts for the hydroaminoalkylation, giving chiral secondary amines in good yields with excellent ee values (up to 93%).