Epoxidation and 1,2-dihydroxylation of alkenes by a nonheme iron model system - DFT supports the mechanism proposed by experiment

The Fell complexes of two isomeric pentadentate bispidine ligands in the presence of H2O2 are catalytically active for the epoxiclation and 1,2-dihydroxylation of cyclooctene (bispidine = 3,7-diazabicyclo[3.3.1]nonane; the two isomeric pentadentate bispidine ligands discussed here have two tertiary amine and three pyridine donors). The published spectroscopic and mechanistic data, which include an extensive set of O-18 labeling experiments, suggest that the Fe-IV=O complex is the catalytically active species, which produces epoxide as well as cis- and trans-1,2-dihydroxylated products. Several observations from the published experimental study are addressed with hybrid density functional methods and, in general, the calculations support the proposed, for nonheme iron model systems novel mechanism, where the formation of a radical intermediate emerges from the reaction of the Fe-IV=O oxidant and cyclooctene. The calculations suggest that the S = 1 ground state of the Fe-IV=O complex reacts with cyclooctene in a stepwise reaction, leading to the formation of a carbon-based radical intermediate. This radical is captured by O-2 from air to produce the majority of the epoxide products in an aerobic atmosphere. Under anaerobic conditions, the produced epoxide product is due to the cyclization of the radical intermediate. Several possible spin states (S-T = 3, 2, 1, 0) of the radical intermediate are close in energy. As a result of the substantial energy barrier, calculated for the ST = 3 spin ground state, a spin-crossover during the cyclization step is assumed, and a possible two-state scenario is found, where the S = 2 state of the Fe-IV=O complex participates in, the catalytic mechanism. The 1,2-dihydroxylation proceeds, as suggested by experiment, via an unprecedented pathway, where the radical intermediate is captured by a hydroxyl radical, the source of which is Fe-III-OOH, and this reaction is barrierless. The calculations suggest that dihydroxylation can also occur by a direct oxidation pathway from Fe-III-OOH. The strikingly different reactivities observed with the two isomeric bispidine Fell complexes are rationalized on the basis of structural and electronic differences.