Asymmetric alkene epoxidation with chromium oxo salen complexes. Effect of added phosphoryl ligands

Salen ligands derived from trans-1,2-dimethyl-1,2-cyclohexanediamine: preparation and application in oxo-chromium salen mediated asymmetric epoxidation of alkenes

“…The most interesting observation is that, in the epoxidation of E-␤-methylstyrene, the effect of methyl substitution on the cyclohexyl ring is to dramatically decrease the level of asymmetric induction from that achieved with our previously reported [8,10] cyclohexyldiamine derived chromium salen catalysts (72-92% ee). Also, contrary to the effect previously observed with the cyclohexyl diamine analogues [8,12,15], the use of the additive (Ph 3 PO) has a slightly negative or negligible effect on enantioselectivity. It is also notable that the effect of changing salen substitution (from H to CF 3 ) at the 3,3 -positions on the salen ring is rather muted in comparison to the leap in enantioselectivity (57-89% ee) seen for the cyclohexyl diamine analogues [10].…”

“…Iodosylbenzene was used to generate oxo-12 and oxo-13 and the reactions were carried out both in the presence and absence of our standard donor ligand additive, triphenylphosphine oxide [8,12,15]. In all cases the yields did not exceed 20%, which is not unusual for these stoichiometric reactions [10].…”

“…overrides the effect of either substitution on the aromatic ring of the salen ligand [16] or additives [15]. Different effects are seen in the epoxidation of Z-␤-methylstyrene, Table 2.…”

“…In recent years our group has been working on the development of epoxidation catalysts based on a chromium salen template [6,[8][9][10][11][12][13][14][15][16]. These show a contrasting selectivity pattern to their manganese analogues in that they allow the asymmetric epoxidation, with iodosylbenzene as stoichiometric oxidant, of E-1,2-disubstituted with high enantioselectivity (>90% ee with complex 2) [8,12].…”

Racemic and chiral expanded salen-type complexes derived from biphenol and binaphthol: Salbip and salbin

“…Overall the asymmetric epoxidation of cis-olefins with JacobsenÕs catalyst is effective but the optimized conditions rely on a large catalyst loading (6 mol%) and the use of 4-phenylpyridine-N-oxide as an additive in order to obtain high enantiomeric excesses (eeÕs). Gilheany and coworkers [22][23][24][25] reported the synthesis of a series of chiral chromium-salen complexes capable of epoxidizing trans-disubstituted olefins with poor to good eeÕs, but the system suffers from limited substrate tolerance and requires high catalyst loading (10 mol%). Furthermore, it requires an expensive additive and an expensive oxidant (PhIO).…”