Titanium Complexes as Catalysts for the Intermolecular Hydroamination of Alkynes

Abstract

Several titanium complexes can be used as active catalysts for the intermolecular hydroamination of alkynes. The investigated catalysts include various titanocene complexes as well as titanium compounds bearing amido- and chloro-ligands. The activities of the investigated catalysts are compared in two representative hydroamination/reduction sequences.

Key words

alkynes - aminations - amines - catalysis - titanium

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4

General Procedure: Under an inert atmosphere of argon a flame-dried Schlenk tube equipped with a Teflon stopcock and a magnetic stirring bar was charged with the alkyne (2.40 mmol), the amine (2.64 mmol), the catalyst (0.12 mmol, 5.0 mol%) and toluene (1.0 mL). The resulting mixture was heated to 105 °C for 24 h. Then the mixture was cooled to room temperature and a suspension of NaBH₃CN (302 mg, 4.80 mmol) and ZnCl₂ (326 mg, 2.40 mmol) in methanol (10.0 mL) was added. After stirring for 20 h at r.t. saturated Na₂CO₃ solution (20 mL) was added. The resulting mixture was filtered and the solid residue was washed with CH₂Cl₂ (50 mL). After extraction, the organic layer was separated. The aqueous layer was extracted with CH₂Cl₂
(6 × 50 mL). The combined organic layers were dried over Na₂SO₄. Evaporation of the solvent and flash chromatography on silica gel afforded the pure amine derivative 3 (sequence A) or 6 (sequence B).

6

In all cases 3 and 6 were isolated in pure form as determined by ¹H NMR and TLC analysis. Both compounds were identified by comparison of the obtained ¹H NMR spectra with those reported in the literature. [3c]

11

Due to the low boiling point of 3-hexyne 4, an isolation of unreacted alkyne was not possible for sequence B.