Non-heme manganese complexes of C1-symmetric N4 ligands: Synthesis, characterization and asymmetric epoxidations of α,β-enones

Enantioselective Epoxidations of Olefins with Various Oxidants on Bioinspired Mn Complexes: Evidence for Different Mechanisms and Chiral Additive Amplification

“…Table 2, entries 12, 13, 14, and 15, and Table 3, entries 8 and 9), possibly due to a larger steric demand of the mesityl group which blocks the access of boc-(L)-proline to the metal center. 18 O-labeling experiments (see below) bolster the support of this conclusion. Indeed, the high 18 O incorporation levels of 82−89% (Table 5, entries 5 and 6) from 18 O-labeled water are indicative of the predominant Lewis-acid activation oxidation mechanism (the oxygen transfer being performed by the [LMn( 18OIAr)] species) rather than of the oxometal based mechanism.…”

“…In effect, the cis/trans epoxide ratio (Table 4, entries 6 and 9 vs entries 1−3) and ee (Table 2 Complementary support for the diverted mechanism in the presence of protic additives comes from isotopic ( 18 O) labeling data. Indeed, when an excess of 18 O-labeled water was added to styrene epoxidations by catalyst systems 2/TBHP or 2/CHP (Table 5, entries 2−4), the latter reactions demonstrated virtually the same levels of 18 O incorporation into the epoxide (32•••36% 18 O) as the catalyst system 2/H 2 O 2 /H 2 18 O (Table 5, entry 1), which is indicative of the Mn V O driven waterassisted mechanism (Scheme 3 bottom). 6 Formation of singly 18 O-labeled 1,2-diol (Table 5, entries 2−4) also provides evidence for the direct interaction of the active species of the type [LMn V  18 O(OH)] and [LMn V O( 18 OH)] with styrene, with transfer of both oxygen atoms from manganese to the olefinic double bond (Scheme 3 bottom).…”

“…25 At the same time, Mn aminopyridine and related complexes were previously found to catalyze the epoxidation of olefins with other oxidants, such as peracetic acid, tBuOOH, and PhIO with comparably high enantioselectivities. 10,12,14,18 Those findings revealed a rich and extremely underexplored mechanistic landscape for the epoxidations in the presence of bioinspired Mn aminopyridine Mn complexes. It was tempting to discover the mechanistic details of those oxidations with various oxidants and conclude on the nature of the active species conducting the enantioselective oxygen transfer in catalyst systems relying on oxidants other than H 2 O 2 .…”

“…In early 2000s, Shul’pin and co-workers discovered that the 1,4,7-trimethyl-1,4,7-triazacyclononane Mn complex can catalyze the epoxidation of olefins with hydrogen peroxide in the presence of acetic acid. , Subsequently, enantioselectivities up to 17% ee were reported for the epoxidation of indene with H 2 O 2 in the presence of Mn complexes with chiral triazacyclononane derived ligands . Today, manganese complexes with aminopyridine and structurally related ligands constitute one of the most rapidly developing class of such bioinspired catalysts, that demonstrate excellent efficiencies (typical TONs of 1000 to 10000) and high-to-excellent enantioselectivities (up to 99% ee ) in the epoxidations of electron-deficient olefins with hydrogen peroxide and peracetic acid. − …”

“…It was tempting to discover the mechanistic details of those oxidations with various oxidants and conclude on the nature of the active species conducting the enantioselective oxygen transfer in catalyst systems relying on oxidants other than H 2 O 2 . In this article, we contribute to the catalytic and mechanistic studies, particularly focusing on the effect of oxidants and additives on the epoxidation enantio-and stereoselectivity and on 18 O labeling data.…”