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Bifunctional Iridium Pincer Catalysis for (De)Hydrogenation of N-Heterocycles
Selvakumar Selvarasu1, Balaji R Jagirdar1
1Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bengaluru, India.
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)(tBu4PNCNP)] (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)(tBu4PNCNP)] (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.
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