Computational studies epoxidation selectivity

R. Prabhakar, K. Morokuma, C. L. Hill, D. G. Musaev, Insights into the mechanism of selective olefin epoxidation catalyzed by [7-(SiO4)VVii,O 2H . A computational study, Inorg. Chem. 45 (2006) 5703. 

Prabhakar, R., Morokuma, K., Hill, C., et al. (2006). Insights into the Mechanism of Selective Olefin Epoxidation Catalyzed by [y-(Si04)Wio032H4] . A Computational Study, Inorg. Chem., 45, pp. 5703-5709. 

A reliable model for rationalizing the stereochemistry of the epoxide products has been developed. For the reaction of aUcenes with dioxiranes two extreme transition states have been proposed, the spiro and planar modes (Figure 19.3). Because these two extreme modes of approach often lead to the prediction of opposite enantiofacial selectivity based on consideration of steric interactions with substrate substituents, knowledge of any spiro/planar preference is important Observed experimental evidence along with computational studies indicate the preference for the spiro mode (ca. 7.4kcal according to B3LYP/6-31G calculations) [30-32]. 

Computational studies on the mechanistic features of the oxidation of ethylene and 1,3-butadiene with HP, catalysed by vanadyl acetylacetonate, showed that under thermodynamic control, no selectivity is observed between epoxidation and double bond oxidative cleavage. Under kinetic control, however, in both systems, the double bond oxidative cleavage is the favoured path. ... 

Freccero, M., Gandolfi, R., Sarzi-Amade, M., Rastelli, A. Facial Selectivity in Epoxidation of 2-Cyclohexen-1-ol with Peroxy Acids. A Computational DFT Study. J. Org. Chem. 2000, 65, 8948-8959. 

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... 

---