Study On Visible Light Induced Cross-dehydrogenative-coupling Between Styrenes And Formamides

Nowadays, in the context of energy crisis and environmental issues, taking more efficient advantage of the non-polluting and renewable resources such as light energy becomes increasingly urgent. In the realm of organic chemistry, it is of great significance to develop light induced reactions especially the ones promoted by the visible light of high natural abundance. By means of photochemistry, certain transformations unfavorable in thermochemistry can also be realized. Visible light induced reactions, which are generally free from drastic conditions such as high temperature and pressure, usually feature satisfactory reactivity and selectivity. In recent years, constructing new C-C linkage directly from two unactivated C-H bonds under oxidative conditions has emerged to be an attractive research goal, which is generally named Cross-Dehydrogenative-Coupling(CDC). Compared with the traditional cross-couplings, CDC features higher efficiency and better atom economy; but at the same time, achieving this kind of methodology with good selectivity is undoubtedly challenging.Herein a novel visible light induced CDC reaction between styrenes and formamides was investigated, providing a brand new access to α,β-unsaturated amides which are considered to possess various biological activities and synthetic potentials. Upon the determination of the model reaction, a series of condition screenings were carried out to reach a optimum set of conditions which were as follows: 0.1 mmol specific styrene dissolved in 1.0 mL corresponding formamide(0.1 M) was treated with 0.4 mmol ammonium persulfate(4 equiv.), 0.05 mmol anhydrous potassium carbonate(0.5 equiv.) and 0.005 mmol photocatalyst Ru(bpy)3Cl2·6H2O(5 mol%), and subsequently irradiated with a 105 W compact fluorescent lamp under N2 atmosphere. Then 31 pairs between 27 stryene derivatives and 4 formamides as well as 1 formate were subjected to the established conditions, among which only 11 sets worked, affording desired products of low to moderate yields.A plausible mechanism was presented with the reference of previous works of other research groups. The persulfate as the oxidative quencher is first reduced by the photo-excited Ru(bpy)32+* to give a sulfate and a sulfate radical anion; the latter then abstracts an H-atom from the formyl group to form a formyl radical, which subsequently interacts with the C=C bond of styrene; after being oxidated by the Ru(bpy)33+, the adduct finally yields the desired product with the loss of a proton.