Unfunctionalized olefins, epoxidation mechanism

The rigid, chiral salen complexes of Mn(III) shown below catalyze the asymmetric epox-idation of alkenes when treated with commercial bleach (NaOCl). This synthesis of enan-tio-enriched epoxides is particularly powerful since the method is applicable to unfunctionalized olefins. In general, (Z)-l,2-disubstituted alkenes afford higher enantioselectiv-ities than do the ( )-isomers or trisubstituted alkenes. The reaction mechanism is com-plex and proceeds via the formation of a Mn(III,IV) dinuclear species. ... 

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... 

The story of mctai-salen catalyzed olefin epoxidation began in 80s. when Kochi cl al. found that (salen)chromium(111) [2-3] and (salcn)mangancsc(lll) 4] complexes catalyze the epoxidation of unfunctionalized olefins using iodosylbcnzenc (PhlO) as terminal oxidant. Kochi was also the first one to describe the effect of the ligand substituent and donor additives (c.g. pyridinc-N-oxido) on the epoxidation, and to study the reaction kinetics and mechanism of the proposed systems (by UV-Vis). The epoxidizing species in both systems were postulated to be oxo(salen)mctal(V) species. 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

Regardless of the mechanism, the chiral (salen)Mn-mediated epoxidation of unfunctionalized alkenes represents a methodology with constantly expanding generality. Very mild and neutral conditions can be achieved, as illustrated by Adam s epoxidation of chromene derivatives 12 using Jacobsen-type catalysts and dimethyldioxirane as a terminal oxidant [95TL3669]. Similarly, periodates can be employed as the stoichiometric oxidant in the epoxidation of cis- and tram-olefins [95TL319],... 

---