A new titanium catalyst using an ethylene diamine backbone with pyrrolyl-α-methyl groups on the diamine nitrogens has been developed for the hydrohydrazination of alkynes with monosubstituted hydrazines. The catalyst cleanly hydrohydrazinates terminal and internal alkynes with monosubstituted hydrazines bearing both alkyl and aryl substituents. One-pot Fischer indole cyclization led to NH-indoles in moderate to good yield. For example, 2-methyltryptamine was synthesized directly from 5-chloropent-1-yne with phenylhydrazine by hydrohydrazination and Fischer indole cyclization in a one-pot procedure. A variety of heterocycles, including pyrazoles and various hydropyridazines, can be generated using titanium-catalyzed hydrohydrazination. One of the products, 1,2-bis(2-methyl-3-indolyl)ethane, was structurally characterized.
Reaction of 1 equiV each of Ti(NMe 2 ) 2 (dap) 2 ( 1), 1-hexyne, tert-butyl isocyanide, and 1,1-dimethylhydrazine at room temperature results in the formation of Ti( NNMe 2), where one of the ligands is formed through iminohydrazination of the alkyne. A possible pathway for the generation of 2 and the crystal structure of this rare terminal hydrazido(2-) complex of titanium are discussed.
Titanium and zirconium (triphenylsilyl)imido complexes are available through transimination from the tert-butylimido complexes with H2NSiPh3. The useful starting material
Ti(NSiPh3)Cl2(py)2 (1) is prepared in 88% yield by treatment of Ti(NBut)Cl2(py)3 with
H2NSiPh3. Imido 1 is a dimer in the solid state with bridging chlorides; however, solution
molecular weight studies indicate that 1 is a monomer in CH2Cl2. 1 reacts with 4,4‘-di-tert-butyl-2,2‘-bipyridine (bpy) to generate the pseudo-octahedral Ti(NSiPh3)(bpy)(py)Cl2 (2) in
74% recrystallized yield. Replacement of the chloride ligands of 1 with 2 equiv of Lidap, 1
equiv of Libap, or 1 equiv of Lipap afforded the pyrrolylimido complexes Ti(NSiPh3)(dap)2
(3), Ti(NSiPh3)(bap)Cl (4), and Ti(NSiPh3)(pap)Cl (5), respectively. The reaction of 2 equiv
of neophyl (Nph) magnesium bromide with 1 provided [Ti(μ-NSiPh3)(Nph)2]2 (6) in 59% yield.
The pseudo-octahedral complex Ti(NSiPh3)(dpma)(bpy) (8) was available through addition
of Li2dpma to 1, forming Ti(NSiPh3)(dpma)(py)2 (7) followed by treatment with bpy.
Alternatively, 8 was prepared by addition of bpy and H2NSiPh3 to Ti(NMe2)2(dpma). The
zirconium tert-butylimido complex Zr(NBut)(dpma)(bpy) (9) was available in 43% yield by
treatment of Zr(NMe2)2(py)(dpma) with bpy and H2NBut. Transimination on 9 with H2NSiPh3
provided the (triphenylsilyl)imido complex Zr(NSiPh3)(dpma)(bpy) (10) in 34% yield. In an
unusual transformation, 10 reacts with excess sodium 2,6-dimethylphenoxide in THF to
afford [Na(bpy)][Zr(dpma)(OAr)3] (11), where the Na coordinates to the bpy and both pyrrole
rings of the dpma in an η5 fashion. Compounds 1, 5, 6, 8, and 11 were structurally
characterized.
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