BPMEN ligand

Que and coworkers reported on a similar monomeric iron complex, formed with the BPMEN ligand but without acetic acid [128]. This complex was able to epoxidize cyclooctene in reasonably good yield (75%), but at the same time a small amount of the ris-diol (9 %) was formed. This feature observed with this class of complexes has been further studied and more selective catalysts have been prepared. Even though poor levels of conversion are often obtained with the current... 

In a seminal contribution [42a], Que and coworkers revealed insight into the reaction mechanisms of epoxidation and dihydroxylation using the pentadentate TPA and bpmen ligands. Some of those compounds had earlier shown remarkable... 

Scheme 1 BPMEN and TPA ligands for the iron-catalyzed epoxidation of olefins...

In addition, also nonheme iron catalysts containing BPMEN 1 and TPA 2 as ligands are known to activate hydrogen peroxide for the epoxidation of olefins (Scheme 1) [20-26]. More recently, especially Que and coworkers were able to improve the catalyst productivity to nearly quantitative conversion of the alkene by using an acetonitrile/acetic acid solution [27-29]. The carboxylic acid is required to increase the efficiency of the reaction and the epoxide/diol product ratio. The competitive dihydroxylation reaction suggested the participation of different active species in these oxidations (Scheme 2). 

A linear tetradentate ligand, bpmen [N,N -dimethyl-N,N -bis(2-pyridylmethyl)-l,2-diaminoethane] was reported to allow stereospecific alkane hydroxylation [36, 40], The same holds true for the [Fen(TPA)(CH3CN)2]2+ [TPA = tris(2-pyridylmethyl) amine] catalyst family (Figure 3.3) [35],... 

S. Taktak, S. V. Kryatov, T. E. Haas, E. V. Rybak-Akimova, Diiron(lll) oxo-bridged complexes with bpmen and additional monodentate or bidentate ligands Synthesis and reactivity in olefin epoxidation with H2O2, / Mol. Catal. A Chem. 259 (2006) 24. 

A similar architectural feature is shared by doubly carboxylate bridged di-iron(II) compounds [Fe2(/u-OAc)2(BPMEN)2] + (4) and [Fc2(M-02CPh)2(02CPh)2(C5H5N)4] (5) (Scheme 7),i in which coordinatively saturated iron(II) centers are supported by A-rich ligand sets. 

Figure 11.11 BPMEN type ligands 36 and 37 employed by Costas and coworkers for the epoxidation of alkenes with H202/acetic acid [102].

Costas and coworkers have shown recently that manganese complexes based on BPMEN-type ligands (where BPMEN is bis((pyridin-2-yl)methyl)ethylene-l,2-diamine, Figure 11.11) show enhanced activity in the epoxidation of alkenes with H2O2 in the presence of excess acetic add, albeit with lower activity and with a narrower substrate scope than for the pyridyl-tmtacn based systems reported in the same study (see above. Table 11.7) [102]. 

---