Asymmetric alkene epoxidation with chromium oxo salen complexes

Unsymmetrical chiral salen Schiff base ligands

“…One complex (22d) performs better than the symmetrical with Ph 3 PO additive present (entry 10) but worse in its absence (entry 9). Mostly the effect of additive is beneficial and in one case (complex 23, entries 13/14) it is very substantial giving an ee increase of 46%, the largest we have observed to date [19,22,23].…”

“…The nitrate complexes 20 and 21 were then oxidised with iodosylbenzene to the oxo-Cr(V) species 22 and 23, Scheme 7, which were then used in situ [50,51] as epoxidising agents for our standard substrate (E)-␤-methylstyrene in stoichiometric reaction at 0 • C in acetonitrile. We conducted two series of experiments, one with no additive and one with equimolar amounts of triphenylphosphine oxide, which we had found to be the best additive [19,22,23]. The reactions were slow, taking days to discharge the green colour of the Cr(V) O species.…”

“…The best known and most useful of these is Jacobsen's catalyst 1 [15], whereas the most selective are those developed by Katsuki and co-workers such as 2 [16]. We have reported extensively on the related chromium (III) and (V) complexes [17][18][19][20][21][22][23][24][25][26][27] which are interesting because they have a strikingly different selectivity profile [17,24] from their manganese analogues and also because they allow the study of the stoichiometric variant of the catalytic reaction. The latter is made possible due to the possibility of isolating the chromium(V) complex [28], which is the active oxidant in the catalytic cycle.…”

Oxidation of cyclohexanol and cyclohexene with triazenido complexes of chromium immobilized in biosorption FAU supports

“…TON values are higher for the catalysts based on NaY prepared by SBR method. This might be due to the presence of chromium species in zeolite sites that are accessible to the reactants [49].…”

Ligand tuning in the chromium–salen-mediated asymmetric epoxidation of alkenes

“…[1][2][3] Synthetic methods based on metal(salen) complexes have been developed for both asymmetric epoxidation (AE) of alkenes [2][3][4] and the asymmetric ringopening of epoxides. [5][6][7][8][9][10][11][12][13] We have specifically investigated Cr(salen)-mediated AE, [14][15][16][17][18][19][20][21][22][23] as have others. [24][25][26][27][28] We found it intriguing that the Cr-system shows high enantioselectivity for the conversion of trans-1,2-disubstituted alkenes (trans-alkenes henceforth) in contrast to almost all known Mn(salen) systems.…”