Highly selective and eco-friendly dihydroisoquinoline synthesis via Cu/Co synergistic catalysis in Cu NPs@MOFs catalyst under mild conditions

3,4-Dihydroisoquinoline (DHIQ) is an important precursor used in the production of drugs for treating cancer, HIV, Alzheimer's disease, etc. Major studies on DHIQ synthesis show low catalytic selectivity due to the susceptible over-oxidation feedstock of 1,2,3,4-tetrahydroisoquinoline (THIQ), which often requires alkali co-catalysts. Therefore, it is desirable yet challenging to explore a highly selective and efficient oxydehydrogenation capacity for DHIQ synthesis under eco-friendly reaction conditions. Herein, a novel framework 1 was synthesized, exhibiting 1D channels with the size of 4.6 angstrom x 9.6 angstrom and high solvent/pH/thermal stability. A stable framework allows it to encapsulate Cu nanoparticles (NPs) to form Cu NPs@1-x (x = 1, 2, 3, and 4) with varying loading amounts of Cu NPs at 2.0 wt%, 3.0 wt%, 4.0 wt%, and 6.0 wt%, respectively. Cu NPs@1-3 could selectively catalyze the reaction from THIQ to DHIQ with a high selectivity of 98% and a recorded turnover frequency (TOF) of 22.1 h-1 under eco-friendly mild conditions. The corresponding catalytic activity can maintain at least five recyclings and can be further applied to gram-scale experiments. Additionally, the efficient preparation of DHIQ catalyzed by Cu NPs@1-3 could be realized even under air conditions. Importantly, the anticancer molecule precursor synthesis of 6,7-dimethoxy-3,4-dihydroisoquinoline was also selectively catalyzed by Cu NPs@1-3. Mechanism investigations revealed that high catalytic performance can be attributed to the stable framework and the synergistic catalytic effect of the loaded Cu NPs and Co metal centers. More importantly, this work represents the first example of MOF catalysts for selectively thermo-catalytic DHIQ synthesis and demonstrates a simple approach to obtain efficient catalysts for selective oxydehydrogenation in the production of unsaturated compounds.