High Enantioselectivities in an (E)-Alkene Epoxidation by Catalytically Active Chromium Salen Complexes. Insight into the Catalytic Cycle

Abstract

[structure: see text]. The epoxidation of (E)-beta-methylstyrene mediated by an oxochromium salen complex yields the epoxide in 92% ee in stoichiometric mode, the highest ee yet reported for a metal-mediated epoxidation of an (E)-alkene. The effect of added donor ligands, previously substantial, has reached a ceiling with this complex. In catalytic mode a slightly reduced ee and higher yield is obtained, indicating both the presence of a second oxidation cycle and that the major oxidant reacts with its reduced form.

Cr-salen mediated asymmetric epoxidation of alkenes: Rational complex design and substrate scope of catalyst

“…The very low catalytic yield is probably due to disproportionation of PhIO over the extended reaction times necessary for this slower catalyst. 48 In line with previous results, 41 the use of NaOCl as terminal oxidant resulted in a poor ee (entry 3), however, the sense of the enantioselectivity was reversed suggesting a different oxidation pathway. 43 We have also rationalised the reduction in ee relative to stoichiometric mode in terms of a second oxidation cycle being in operation under catalytic conditions.…”

“…44 The protection, formylation, and deprotection steps proceeded in good yield. 45 Contrary to our previous experience, 41 the use of N,N,N′,N′-tetramethylethylenediamine (TMEDA) was found to be essential to achieve a selective formylation -a complicated mixture resulted in its absence. Care had to be taken also to prevent decomposition of the delicate trifluoromethyl phenol derivative 7 which is sensitive to acid and base.…”

“…However it is still slightly lower than in stoichiometric mode, as had been observed previously with 1a. 41 Also the yields were unexpectedly low, usually the yield in catalytic mode is significantly higher than in stoichiometric mode (71% vs 45% with 1a). The very low catalytic yield is probably due to disproportionation of PhIO over the extended reaction times necessary for this slower catalyst.…”

“…After unsuccessful attempts to introduce the trifluoromethyl group into the benzene ring late in the synthesis, we adopted the methods previously used for the synthesis of 3-trifluoromethylsalicylaldehyde. 41 The synthesis of phenol 5 had been described previously by Stokker et al . 44 The protection, formylation, and deprotection steps proceeded in good yield.…”

Synthesis and elaboration of trans 2,3-diaryloxiranes

“…Under catalytic conditions (10 mol% of 7) in the presence of PhIO, the epoxide was formed in higher yield (71%) but with a slightly lower ee (82%). 14 Many others metals, ligands and terminal oxidants have been used to broaden the scope of the reaction, but examples of metal catalysed asymmetric epoxidation of stilbene type olefins are only occasionally described in the literature. 15 Ruthenium catalysts based on pyridine-2,6-bisoxazoline (pybox) ligands, as 9 (Scheme 2), have been used in combination with iodosyl …”