Isocyanide insertion chemistry. Synthesis and structural characterization of bridging imidoyl complexes of nickel and amide formation by intramolecular coupling of acyl and imidoyl functionalities

Article abstract of DOI:10.1021/om00034a039

The binuclear pseudoallyl complex trans-(Me₃P)BrNi(μ₂-η³:η ¹-CH₂-o-C₆H₄)NiBr(PMe ₃)₂ (1) inserts selectively 1 equiv of CNXy (Xy = 2,6-dimethylphenyl) into the Ni-benzylic bond with formation of the cyclic complex trans-(Me₃P)₂BrNi(C(NXy)CH₂-o-C ₆H₄)NiBr(PMe₃) (2) that contains a bridging η²-imidoyl ligand. The analogous reaction of 1 with CNBu^t gives a related μ₂,η²-imidoyl species 3a which, in contrast to 2, exists in solution in thermodynamic equilibrium with its isomer trans-(Me₃P)BrNi(η²-C(NBu^t)CH ₂-o-C₆H₄)NiBr(PMe₃)₂ (3b), in which the imidoyl ligand is terminally bonded in a η² fashion to one of the nickel centers. Variable temperature NMR studies show 3b is the kinetic product of the insertion reaction and that the strongly solvent dependent equilibrium 3a ? 3b is attained only at sufficiently high temperatures (ca. -20°C). At even higher temperatures (ca. 40°C), the 3a-3b mixtures convert into a third isomeric compound, trans-(Me₃P)₂Ni(C(NBu^t)C₆H ₄-o-CH₂)NiBr(PMe₃) (4), in a process which formally corresponds to the deinsertion of CNBut from the Ni-benzyl bond followed by reinsertion into the Ni-aryl moiety. In a rather complex reaction 2 interacts with CO to afford the mononuclear complex trans-Ni(C=CHC₆H₄C(O)NXy)Br(PMe₃)₂ (5) whose formation probably involves the intramolecular coupling of acyl and imidoyl functionalities. Hydrolytic cleavage of the Ni-C bond in 5 provides the corresponding N-aryl-1-isoquinolone 6. Compounds 2 and 3a, have been characterized by X-ray studies. 2 crystallizes in the monoclinic space group P2₁/n with a = 12.437(3) A?, b = 16.025(2) A?, c = 16.015(2) A?, β = 101.47(1)°, and Z = 4. Crystals of 3a are also monoclinic, space group P2₁/n, with a = 26.05(1) A?, b = 10.226(1) A?, c = 12.013(2) A?, β = 93.54(12) °, and Z = 4. The μ₂,η²-imidoyl functionalities of both complexes exhibit similar structural characteristics.