Bifunctional Iridium Pincer Catalysis for (De)Hydrogenation of N‐Heterocycles

ABSTRACT

Base‐free, reversible, acceptorless dehydrogenation of 1,2,3,4‐tetrahydroisoquinoline (THIQ), 1,2,3,4‐tetrahydroquinoline (THQ), and other N‐heterocycles catalyzed by an Ir(III) pincer complex, [Ir(H)(Cl)( ^tBu4 PNCNP)] ( 1 ), has been demonstrated. Quantitative yields were obtained in the cases of THIQ, THQ, and 1‐Ph‐THIQ. The catalyst (2.0 mol%) was found to be efficient for five cycles of the reversible process for THIQ. The dehydrogenation of THIQ and THQ proceeded with the rate constants of 1.54 × 10 ⁻¹ and 0.79 ×10 ⁻¹ h ⁻¹ , respectively. The hydrogenation of isoquinoline (IQ) and quinoline (Q) gave rate constants of 1.31 × 10 ⁻¹ and 0.53 × 10 ⁻¹ h ⁻¹ , respectively. IQ and Q coordinate to the metal center in complex 1 to form cis and trans isomers, [Ir(H)(Cl)(L)( ^tBu4 PNCNP)] (L = IQ, cis‐ and trans‐1‐IQ ; L = Q, cis‐ and trans‐1‐Q ) at low temperatures. The reversible transformation was also investigated using trans ‐2,5‐dimethylpiperazine. In the dehydrogenation of THIQ, no intermediate was observed experimentally prior to the complete dehydrogenation of THIQ to form IQ. Acyclic 2°‐amines such as N‐ benzylaniline, dibenzylamine, N‐ methylbenzylamine, N‐ cyclohexylethylamine, diisobutylamine, and dibutylamine underwent transamination except N‐ isopropylaniline, in addition to the dehydrogenation reaction.