Hydrocarbon activation has become a powerful tool in molecular science research and great progress has been made in molecular science research in recent years. In particular, weak O-coordinated carboxylic acid-assisted oxidative C-H cyclization was achieved by continuous breaking of C-H and O-H bonds of aromatic carboxylic acids (Figure 1, a). Modular assembly of five – or six-member heterocycles has made great progress under previous studies, but these reactions usually require additional use of stoichiometric chemical oxidants, such as copper salts or silver salts, to facilitate reoxidation of the catalyst.
Instead, oxidation chemical conversion is allowed through C-H bond activation, enabling reoxidation of key catalysts and avoiding the use of expensive or toxic chemical oxidants. For example, C-H cyclization via electrooxidation catalyzed by ruthenium, rhodium, and iridium (Figure 1, b). However, these catalytic processes are greatly affected by inadequate product selectivity.
Lutz Ackermann’s group first reported the activation of hydrocarbons by weak O coordination of benzoic acid catalyzed by osmium (II). The advantages of osmium catalysis were highlighted through detailed reaction analysis of similar transition metal catalytic systems.
Osmium complexes are rarely used in organometallic catalysis as compared with their counterparts of iron and ruthenium, probably due to their lower kinetic activity. As a result, the electrooxidation of osmium catalyzed hydrocarbon activation process is difficult to control.
They hypothesized that metal centers could achieve selective control of chemical reactions by repelling steric hindrance interactions. Osmium complexes have unique physicochemical characteristics compared to other metals, so they have constructed a novel osmium (II) catalyzed electrooxidized hydrocarbon ring using weak O-oriented groups, giving the osmium complexes unique regional and chemical selectivity. According to their hypothesis, they set up feasible reaction conditions for C-H cyclization of o-toluene I with olefin II in undivided cells.
The experimental results show that [OsCl2(P-Cymene)]2 combined with a Graphite felt (GF) Anode and a Platinum plate is a feasible cathode catalyst, and 29% C-H cyclization product 1 can be obtained. On this basis, they explored the influence of additives on the conductivity and reaction performance in the reaction.
They added various additives, such as alkali metal base salts, organic salts and iodine, to the osmotic electrocatalytic hydrocarbon ring. Through osmotic electrocatalysis, non-toxic and inexpensive potassium iodide increased the yield of cyclated product 1 to 75%.
In addition, they conducted a set of experiments in the presence of air or common chemical oxidants such as AgOAc, Cu(OAc)2, Mn(OAc)3, PhI(OAc)2, or K2S2O8. It shows that electricity not only plays a key role in sustainable oxidation, but also provides the best and unique efficiency for C-H cyclization of osmium catalyzed oxidation (Figure 4), highlighting the great advantages of electrochemical methods.
On this basis, they conducted a study on its stability (Figure 5). To this end, they performed a series of metallaelectrocatalyzed reactions with benzoic acid III possessing two accessible ortho-C-H bonds. Interestingly, osmaelectrocatalysis showed unique chemoselectivity (35.5:1), while rhodium,iridium, and ruthenium catalysis gave significantly lower selectivities (5.2:1, 7.4:1, and 6.3:1 respectively). This aspect was also displayed in H/D scrambling experiments in the presence of an isotopically labeled solvent mixture (Figure 5b). With the present osmium electrocatalysis regime, deuterations occurred at the C6 position in a highly selective fashion. In sharp contrast, comparable C2 and C6 deuterations were observed in the reaction with the other Group 8 metal, ruthenium. These findings hence represent a novel tool for selective hydrogen isotopic exchange.