Reusable, Homogeneous Water Soluble Photoredox Catalyzed Oxidative Dehydrogenation of N-heterocycles in Biphasic System: Applications to Synthesis of Biologically Active Natural Products
Herein, a simple and efficient method for the oxidative dehydrogenation (ODH) of tetrahydro--carbolines, indolines and (iso)quinolines is described using reusable, homogeneous cobalt-phthalocyanine photoredox catalyst in a biphasic medium. Biphasic system offers an advantage of easy separation of product and an efficient reusability of the homogeneous photoredox catalyst. Also, the current system significantly helps to overcome the solubility issue of substrate and catalyst at room temperature. The potential applications to organic transformations are demonstrated by the synthesis of various biologically active N-heterocycles such as indoles, (iso)quinolines and -carbolines and natural products such as eudistomin U, norharmane, harmane and precursor to perlolyrine and flazin. Without isolation and purification, the catalyst solution can be reused up to 5 times with almost comparable reactivity. Further, efficiency of the reaction was demonstrated in gram scale. To the best of our knowledge, this is the first report on ODH reactions using a non noble, reusable and homogeneous cobalt photoredox catalyst under environmetally friendly conditions.