Enantioselective Epoxidations of Olefins with Various Oxidants on

Research Article pubs.acs.org/acscatalysis

Enantioselective Epoxidations of Olefins with Various Oxidants on Bioinspired Mn Complexes: Evidence for Different Mechanisms and Chiral Additive Amplification Roman V. Ottenbacher,†,‡ Denis G. Samsonenko,†,§ Evgenii P. Talsi,†,‡ and Konstantin P. Bryliakov*,†,‡ †

Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation § Nikolaev Institute of Inorganic Chemistry, Pr. Lavrentieva 3, Novosibirsk 630090, Russian Federation ‡

S Supporting Information *

ABSTRACT: It has been demonstrated that chiral manganese aminopyridine complexes [LMnII(OTf)2] are able to epoxidize olefins with excellent enantioselectivities (up to 99% ee, several examples) with various terminal oxidants (H2O2, alkyl hydroperoxides, peroxyacids, and iodosylarenes). The mechanisms of enantioselective olefin epoxidations on aminopyridine manganese(II) complexes [LMnII(OTf)2] with different oxidants are considered. The analysis of reaction products and epoxidation enantioselelctivity, Hammett correlations, and isotopic (18O) labeling studies bear evidence in favor of the formation of different oxidizing species in the presence of different terminal oxidants. The addition of cocatalytic additives (carboxylic acid or H2O) dramatically changes the catalytic behavior of catalyst systems Mn complex/hydrogen peroxide and Mn complex/alkyl hydroperoxide; in the presence of additives, the catalyst systems with H2O2 and alkyl hydroperoxides demonstrate very similar behaviors. Remarkably, in the presence of chiral additive Boc-protected (L)-proline, achiral Mn aminopyridine complexes catalyze the epoxidation of chalcone in an enantioselective fashion (with up to 60% ee), representing a rare example of chiral environment amplif ication. KEYWORDS: asymmetric catalysis, enantioselective, epoxidation, hydrogen peroxide, intermediate, manganese, mechanism

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INTRODUCTION Chemo- and stereoselective epoxidations of olefins constitute an important class of challenging predictably selective transformations of complex organic molecules. Chiral epoxides are useful intermediates in organic synthesis, prone to reacting with a variety of nucleophilic reagents. The latter, via regioselective cleavage of the strained epoxide ring, lead to the formation of α,β-functionalized organic compounds bearing one or two asymmetric centers.1,2 The resulting chiral building blocks are used in the manufacture of drugs, vitamins, fragrances, agrochemicals, and many chemicals used in functional materials. Catalytic asymmetric epoxidation of olefins remains the major synthetic tool to access chiral epoxides, the catalysts scope including enzymes, organocatalysts, and metal complexes.3−5 Many transition metal catalysts have been suggested for asymmetric selective epoxidations.1−6 A remarkable trend of the past decade has been the consideration of metal-complex based epoxidation catalysts as synthetic models of naturally © 2015 American Chemical Society

occurring metalloenzymes capable of conducting this challenging oxidative transformation. In early 2000s, Shul’pin and coworkers 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. 7,8 Subsequently, enantioselectivities up to 17% ee were reported for the epoxidation of indene with H2O2 in the presence of Mn complexes with chiral triazacyclononane derived ligands.9 Today, manganese complexes with aminopyridine and structurally related ligands constitute one of the most rapidly developing class of such bioinspired catalysts,6 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.10−24 Received: October 14, 2015 Revised: December 24, 2015 Published: December 29, 2015 979

DOI: 10.1021/acscatal.5b02299 ACS Catal. 2016, 6, 979−988

Research Article

ACS Catalysis

Scheme 1. Examples of Mn(II) Aminopyridine Catalysts and Structures of Olefins, Additives, and Oxidants Used in This Study

Table 1. Asymmetric Epoxidations Catalyzed by Complex 3a

Recently, we reported the catalytic and mechanistic study of a family of manganese(II) aminopyridine complexes (Scheme 1) that catalyzed the epoxidation of olefins with H2O2 with up to 99% ee (in the presence of 2-ethylhexanoic acid). The mechanistic study showed that [LMn V O(OCOR)] 2+ (OCOR − carboxylic moiety) is the most likely active species, responsible for enantioselective oxygen transfer.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 H2O2. 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 18O labeling data.

entry

olefin

conditions

1b 2 3 4b 5 6 7b 8 9

chalcone chalcone chalcone dbpcn dbpcn dbpcn Z-β-Me-Sty Z-β-Me-Sty Z-β-Me-Sty

Ac B C A B C A B C

additive (equiv. vs substrate)

conversion [%]/yield [%]

EHA (1.0) EHA (1.0)

100/100 100/100 51/51 100/100 100/100 100/100 43/41 100/100 94/94

EHA (1.0) EHA (1.0) EHA (1.0) EHA (1.0)

ee [%] (config.) 98 98 98 99 99 99 87 86 87

(2R,3S) (2R,3S) (2R,3S) (3R,4R) (3R,4R) (3R,4R) (2R,3S) (2R,3S) (2R,3S)

a At −30 °C; conditions: (A) [H2O2]/[substrate] = 130 μmol/100 μmol, H2O2 added by a syringe pump over 30 min; (B) catalyst 1 mol %, [tBuOOH]/[substrate] = 110 μmol/100 μmol, tBuOOH added in one portion; (C) [2-ethylhexaneperoxoic]/[substrate] = 110 μmol/ 100 μmol, oxidant added over 3 min. Conversions and yields were calculated based on substrate. bFrom ref 25. c0.2 mol % of the catalyst.

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RESULTS AND DISCUSSION Previously, catalyst 3 (Scheme 1) showed the best enantioselectivities in the series for the epoxidation of conjugated olefins by H2O2 in the presence of 2-ethylhexanoic acid (EHA).25 Moreover, the same complex was found to catalyze the same oxidation by 2-ethylhexaneperoxoic acid and tBuOOH/2-ethylhexanoic acid, demonstrating rather similar epoxidation enantioselectivity (yet with generally lower yields) (Table 1). At first glance, one could suggest that a similar performance (stereoselectivity) is evidence for similar active species at the enantioselectivity-determining step. However, a broader screening of various oxidants in the presence and absence of various additives (Table 2) ruled out this hypothesis, bearing evidence for multiple reaction mechanisms and different active species (see below). To test the possibility of synergistic interplay between the chiralities of the catalyst and the additive,26 N-boc-protected (L)-proline (boc-(L)-proline) was examined as an additive.27

The latter, however, resulted in inferior enantioselectivity as compared to that of AcOH and 2-EHA (cf. Table 2, entries 1− 5) in the epoxidation of chalcone with H2O2. At the same time, boc-(L)-proline dramatically affected the enantioselectivity of epoxidation with TBHP (entries 6, 7): while without the additive it was only 51% ee, the addition of boc-(L)-proline resulted in a much higher optical yield, equal to that by system 2/H2O2/boc-(L)-proline (entries 4, 5). A similar effect of boc(L)-proline on the epoxidation of chalcone with CHP was documented (entries 9, 10). Moreover, when 2-EHA was used as the additive, the oxidations with alkyl hydroperoxides also showed the same enantioselectivity level as the oxidation with H2O2/2-EHA (Table 2, entries 2 vs 8, 11). The addition of boc(L)-proline or 2-EHA also improved the product yield and epoxidation enantioselectivity (entries 6−8 and 9−11). In contrast to the oxidations with alkyl hydroperoxides, boc(L)-proline caused a relatively minor effect on the enantioselectivity of epoxidation with iodosylbenzene: the ee increased 980


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ACS Catalysis Table 2. Asymmetric Epoxidation of Chalcone with Various Oxidants Catalyzed by Complex 2a

entry

catalyst (mol %)

oxidant (equiv. vs substrate)

additive (equiv. vs substrate)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

0.1 0.1 0.1 0.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

H2O2, 1.3 H2O2, 1.3 H2O2, 1.3 H2O2, 1.3 H2O2, 1.3 TBHP, 1.1 TBHP, 1.1 TBHP, 1.1 CHP, 1.1 CHP, 1.1 CHP, 1.1 PhIO, 1.1 PhIO, 1.1 MesIO, 1.1 MesIO, 1.1 AcOOH, 1.1 AcOOH, 1.1 AcOOH, 1.1 mCPBA, 1.1 mCPBA, 1.1 peroxyIBA, 1.1 peroxyIBA, 1.1 peroxyEHA, 1.1 peroxyEHA, 1.1 peroxyEHA, 1.1

AcOH, 14 EHA, 5.0 boc-(L)-proline, 2.0 boc-(L)-proline, 0.3 boc-(L)-proline, 0.3

100/99 71/71 46/46 13/11 94/94 85/85 100/100 100/100 27/27 72/68 76/76 26/22 83/83 75/73 71/69 100/100 58/58 100/100 70/70 73/73 30/30 27/27 100/100 95/95 100/100

boc-(L)-proline, 0.3 EHA, 5.0 boc-(L)-proline, 0.3 EHA, 5.0 boc-(L)-proline, 0.3 boc-(L)-proline, 0.3 boc-(L)-proline, 0.3 AcOH, 14 boc-(L)-proline, 0.3 boc-(L)-proline, 0.3 boc-(L)-proline, 0.3 EHA, 5.0

ee [%] (config.) 80 91 67 79 76 51 79 91 35 79 91 71 77 81 81 76 74 76 75 74 82 81 89 87 89

(2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S) (2R,3S)

At 0 °C, H2O2 was added by a syringe pump over 30 min; peracetic, 2-ethylhexaneperoxoic, and peroxyisobutyric acids were added over 3 min; ArIO and alkyl hydroperoxides were added in one portion. Conversions and yields were calculated based on substrate. a

from 71 to 77% (entries 12, 13). In the case of iodosylmesitylene, the additive did not influence the ee (which remained at the level of 81%) and the yield (entries 14, 15). It is likely that a more sterically demanding mesityl moiety effectively hampers the access of boc-(L)-proline to the metal, thus reducing its effect to a minimum. Finally, the effect of boc-(L)-proline on the epoxidations with peroxyacids was studied (entries 16−25). In the latter cases, the addition of a chiral additive had virtually no effect on the epoxide yield and ee, suggesting that the contribution of the additive-assisted pathway might be negligible. Different enantioselectivity levels for various peroxyacids (cf. entries 16, 19, 21, 23) also bear evidence that the steric bulk of the peroxyacid is the major factor that determines the enantioselectivity. The epoxidations with AcOOH in the presence of additive AcOH (entry 18) and with peroxyEHA in the presence of additive EHA (entry 25) resulted in the same ee’s as those in experiments without additives (entries 16 and 23, respectively). One has to note that testing the effect of the chiral additive with catalyst 2 may not be optimal since the catalyst shows high ee’s both with and without additives. With this consideration in mind, we have performed the oxidation of chalcone on new achiral catalysts 5 and 6. For complex 5, X-ray quality crystals have been obtained; the molecular structure of 5 is presented in Figure 1, along with the structure of its previously reported chiral ((S,S)-bipyrrolidine derived) counterpart 2. In 5, Mn(II) has a distorted octahedral coordination environment built of 4

Figure 1. Molecular structures of catalysts 2[25] and 5. Hydrogen atoms are omitted for clarity.

N atoms of an organic ligand and 2 O atoms of two triflate anions (in cis-positions). Mn−N bond lengths are in the range 2.231(2)−2.293(3) Å, and Mn−O distances are 2.143(3) and 2.157(3) Å (Table S4). A CF3-group of one of the triflate anions is disordered over 2 orientations. Complexes 5 and 6 were tested as catalysts of chalcone epoxidation in the presence of boc-(L)-proline (as the only chiral molecule in the mixture) (Table 3). It is curious that nonzero enantiomeric excess (with the formation of (2R,3S)epoxide) was detected in all cases, ranging from 3 to 60% ee. Previously, a formally similar effect was reported by Katsuki and co-workers for the epoxidations of 2,2-dimethylchromenes with 981


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stereoselective pathway (driven by the achiral Mn-based oxidant which does not contain boc-(L)-proline). Using the values of the effect of the chiral additive (as summarized in Tables 2 and 3) as the selection criterion, we can formally divide our catalyst systems into two types: type I systems are Mn catalyst/H2O2 and Mn catalyst/alkyl hydroperoxides, and type II systems are Mn catalyst/peroxyacids and Mn catalyst/PhIO. The catalytic properties (enantioselectivity in particular) of type I systems are strongly affected by the addition of boc-(L)-proline, while those of type II systems are weakly affected by boc-(L)-proline (and by carboxylic acids in general). Apparently, the reported difference in the catalytic behaviors of those groups of systems stem from the significant fundamental distinctions of their active species responsible for the enantioselective oxygen transfer. One might expect that the oxidation mechanisms should also be substantially different. So far, it has been generally accepted33 that the epoxidations with H2O2 on Mn aminopyridine and structurally related complexes proceed via the [LMnVO] active species,19,22,24,25 similar to that of the previously discovered [LFeVO] intermediates.34−37 In the framework of this paradigm, carboxylic acid serves to convert the initially formed MnIII hydroperoxo species by facilitating the O−O bond heterolysis (Scheme 2). In turn, the enantioselective oxygen transfer was

Table 3. Epoxidation of Chalcone with Various Oxidants in the Presence of 5 and 6a

entry catalyst 1 2 3 4 5 6 7 8 9 10 11

5 5 5 6 6 6 6 6 6 6 6



ee [%] (config.)

H2O2, 2.0 AcOOH, 1.1 peroxyEHA, 1.1 H2O2, 2.0 AcOOH, 1.1 peroxyEHA, 1.1 mCPBA, 1.1 PhIO, 1.1 MesIO, 1.1 TBHP, 1.1 CHP, 1.1

29/26 92/92 100/100 100/100 100/100 100/100 39/36 89/89 51/51 100/91 73/71

32 (2R,3S) 10 (2R,3S) 3 (2R,3S) 50 (2R,3S) 20 (2R,3S) 15 (2R,3S) 27 (2R,3S) 26 (2R,3S) 6 (2R,3S) 60 (2R,3S) 58 (2R,3S)

At 0 °C, H2O2 was added by a syringe pump over 30 min; 2ethylhexaneperoxoic and peroxyisobutyric acids were added over 3 min; ArIO and alkyl hydroperoxides were added in one portion. Conversions and yields were calculated based on the substrate.

a

Scheme 2. Proposed Consensus Mechanism for the “Carboxylic Acid-Assisted” Asymmetric Epoxidation of Olefins with H2O2 in the Presence of Aminopyridine Manganese Complexes

PhIO on achiral (salen)Mn(III) complexes in the presence of enantiomerically pure additives: either (−)-sparteine or axially chiral bipyridine N,N′-dioxide.28−30 This effect (sometimes commonly referred to as “chiral environment amplification”31) is typically explained by either (1) inducing chirality at-themetal by the chiral additive or (2) shifting the equilibrium between the two mirror conformers of the catalyst (originally present in equal amounts) by the chiral additive through the formation of diastereomers.32 In our case, an alternative possibility exists which assumes that the chiral additive selects one conformer preferentially for coordination, thus assisting the predominant formation of one stereoisomer of the active species; in this case, the mixture of the catalyst conformers may even be static (noninterconverting). Taking into account the documented crucial effect of carboxylic acids on the formation of the active (presumably oxomanganese(V)) species,19,25 we speculate that the latter situation may be the case for the epoxidations with 5(6)/H2O2/boc-(L)-proline as well as with 5(6)/TBHP(CHP)/boc-(L)-proline. Indeed, the highest ee’s were documented for the oxidations with H2O2, TBHP, and CHP (50, 60, and 58% ee, respectively), which suggests that in those cases the chiral boc-(L)-proline molecule serves most efficiently to organize the transition state for stereoselective oxygen transfer. In the cases of peroxyacids and iodosylarenes, the asymmetric induction was much smaller (3−27% ee). One can conclude that (1) the molecule of boc(L)-proline may enter into the coordination sphere of the active species formed in the system’s Mn complex/peroxyacid and Mn complex/ArIO and that (2) the positive effect of the chiral additive decreases with increasing steric demand of the oxidant. Moreover, in the cases of carboxylic peracids, boc-(L)-proline has to compete with the molecules of achiral peroxycarboxylic acids (and carboxylic acids resulting therefrom in the course of the oxidation) for the available coordination site of Mn, which apparently should result in a competition between the stereoselective oxidation pathway (driven by boc-(L)-prolinecontaining Mn-based chiral oxidant) and between the non-

concluded25 to proceed by an electron-transfer mechanism with the formation of a short-lived acyclic (possibly carbocationic) intermediate. The latter could undergo Cα−Cβ rotation, which led to a partial loss of stereochemistry in the course of cisstilbene epoxidation, affording a mixture of cis (major)- and trans (minor)-epoxides.25 In this study, we have conducted a series of cis-stilbene epoxidations with various oxidants in the presence and absence 982


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In general, two kinds of reaction mechanisms have been previously discussed for the metal-catalyzed epoxidations with alkyl hydroperoxides: the concerted oxygen transfer and the radical epoxidation pathway.38 The formation of large amounts of trans-epoxide, together with the detection of radical-type oxidation byproducts (Table 4, entries 4 and 6), is indicative of the radical oxidation pathway, the lifetime of the intermediate radical species being long enough to rotate around the single Cα−Cβ bond and isomerize.38−40 In the presence of the carboxylic acid additive (Scheme 3, bottom), the mechanism is diverted to a variety of the “carboxylic acid-assisted” mechanism (see above) to generate eventually the same active species as that in the catalyst systems Mn complex/H2O2/carboxylic acid. In effect, the cis/trans epoxide ratio (Table 4, entries 6 and 9 vs entries 1−3) and ee (Table 2, entries 7 and 10 vs entry 4 and entries 8, 11 vs entry 2, and Table 3, entries 10 and 11 vs entry 4) were very close to those for the oxidations with H2O2/ carboxylic acid. Hammett analyses (SI) corroborate this conclusion. Complementary support for the diverted mechanism in the presence of protic additives comes from isotopic (18O) labeling data. Indeed, when an excess of 18O-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 18O incorporation into the epoxide (32···36% 18O) as the catalyst system 2/H2O2/H218O (Table 5, entry 1), which is indicative of the MnV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 [LMnV18O(OH)] and [LMnVO(18OH)] with styrene, with transfer of both oxygen atoms from manganese to the olefinic double bond (Scheme 3 bottom). The addition of boc-(L)-proline to the system 2/PhIO and 2/MesIO resulted in a relatively small increase of the cis/trans ratio (from 7.0 to 9.6 and from 9.7 to 11.5, respectively, see Table 4, entries 10, 11 and 12, 13). This minor alteration is in full agreement with the generally small effect of carboxylic acids on the performance of catalyst systems Mn complex/ArIO (Tables 2 and 3). It is likely that the addition of carboxylic acid only slightly shifts the balance between two different oxidation pathways. One could expect that the addition of boc-(L)-proline could affect the previously invoked bifurcated catalytic mechanism which is essentially due to the coexistence of the Lewis-acid activation pathway (when the oxygen is directly transferred by a [LMn(OIPh)] adduct) and the oxometal based pathway (when the oxygen is transferred by the high-valent oxometal active species) (Scheme 4).41−43 We speculate that the Lewis-acid activation pathway plays the predominant role in the absence of boc-(L)-proline, while adding boc-(L)-proline may facilitate the partial conversion of [LMn(OIAr)] to the [LMnVO] species (Scheme 4). In turn, the effect of boc-(L)proline on the catalyst system Mn complex/MesIO is smaller than that on Mn complex/PhIO (cf. 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 18O incorporation levels of 82−89% (Table 5, entries 5 and 6) from 18O-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

of boc-(L)-proline (Table 4). While the oxidations with H2O2/ carboxylic acid yielded 11.8···12.7:1.0 mixtures of the cis- and Table 4. Epoxidation of cis-Stilbene with Various Oxidants in the Presence of Mn Complex 2a

entry


1 2 3

H2O2 (1.3) H2O2 (1.3) H2O2 (1.3)

4 5 6

TBHP (1.1) TBHP (1.1)c TBHP (1.1)

7 8 9

CHP (1.1) CHP (1.1)c CHP (1.1)

10 11

PhIO (1.1) PhIO (1.1)

12 13

MesIO (1.1) MesIO (1.1)

14 15 16 17

AcOOH, 1.1 peroxyEHA, 1.1 mCPBA (1.1) mCPBA (1.1)

18

mCPBA (1.1)e

additive (equiv. vs substrate) AcOH (5.0) EHA (5.0) boc-(L)-proline (2.0)

boc-(L)-proline (0.3)

boc-(L)-proline (0.3) boc-(L)-proline (0.3) boc-(L)-proline (0.3)

boc-(L)-proline (0.3)


cis/trans

41/41 65/65 79/79

12.7 12.0 11.8

91/80b 88/74 100/100

1.9 2.0 12.7

55/48b 60/22 61/61

1.4 1.5 13.5

16/16 82/79

7.0 9.6

57/56 44/43

9.7 11.5

100/100 100/100 62/60d 68/61d

10.1 17.9 10.2 15.5

41/40

30

At 0 °C; H2O2 was added by a syringe pump over 30 min; 2ethylhexaneperoxoic and peroxyisobutyric acid were added over 3 min; ArIO and alkyl hydroperoxides were added in one portion. Conversions and yields were calculated based on the substrate. b Major side products: benzaldehyde and phenyl acetaldehyde. cThe reaction was performed under an argon atmosphere. dSide product: phenyl acetaldehyde. eUncatalyzed Prilezhaev reaction (for details see, SI). a

trans-epoxides, TBHP and CHP afforded a much higher portion of the stereoisomeric (trans-) epoxide (entries 1−3, 4, and 7). Control experiments on cis-stilbene epoxidations with alkyl hydroperoxides were conducted under the atmosphere of argon (to exclude the access of dioxygen to the systems) that showed the same cis/trans selectivities as those under standard conditions (entries 5 vs 4 and 8 vs 7). These findings, as well as the reasonably high enantioselectivity for the trans-stilbene epoxide (1R,2R) formation (48% ee for entry 4 and 47% ee for entry 7) apparently rule out hypothetical contribution from diffusion-controlled radical oxidation initiated by tBuO• radicals. The addition of boc-(L)-proline dramatically changed the situation: the catalyst systems 2/alkyl hydroperoxide/boc-(L)proline displayed cis/trans selectivity very similar to that for the system 2/H2O2/carboxylic acid (in the range 11.8−13.5 cf. entries 6, 9 vs 1−3). Summarizing the effect of boc-(L)-proline on the catalytic properties of catalyst systems Mn complex/ alkyl hydroperoxide, the following reaction scheme can be proposed (Scheme 3). 983


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Scheme 3. Proposed Mechanisms for the Asymmetric Epoxidation of Olefins with Alkyl Hydroperoxides on Aminopyridine Manganese Catalysts in the Absence (Top) and in the Presence of Additives (Bottom)a

a18

O is presented in green.

Table 5. Epoxidation of Styrene with Various Oxidants in the Presence of Mn Complex 2 and H218Oa

entry b

1 2 3c 4 5 6 7

oxidant

epoxide (TON)

epoxide (% 18O)

diol (TON)

diol (16O16O/16O18O, %)

diol (α-16O-β-18O/α-18O-β-16O)

H2O2 TBHP TBHP CHP PhIO MesIO mCPBA

22 35 11 22 38 51 48

35 32 33 36 82 89 0

11 39 72 18 16

13/87 3/97 8/92 3/97 15/85

1/1 1/1 1/1 1/1 1/1

13

19/81

1/2

At 0 °C, 10 equiv of H218O (97% in 18O); H2O2 was added by a syringe pump over 30 min; ArIO, mCPBA, and alkyl hydroperoxides were added in one portion. The reaction was conducted for 2 h. Conversions and yields calculated in catalyst turnover numbers (TON). b20 equiv of H218O (cf. ref 25.). cp-CF3-styrene was used as the substrate.

a

mechanism. Previously, studies by Valentine and others44,45 have shown that fast (on the epoxidation reaction time scale) 18 O exchange occurs between H218O and metal-bound PhIO, with the labeled O atom ultimately being transferred to the substrate.46,47 The slightly higher 18O incorporation into the epoxide for the oxidation with MesIO (89% vs 82% for PhIO) supports the lower contribution of the LMnO driven pathway (for which 18O incorporations into the epoxide of 32−36% are typical; see Table 5, entries 1−4) in the case of more sterically demanding MesIO. This conclusion is further supported by the absence of diol in the oxidation with MesIO.

Indeed, a singly 18O-labeled 1,2-diol is a typical product of oxidations via the MVO driven mechanism (M = Fe and Mn) in the presence of H218O25,48−50 (resulting from the direct reaction of [LMV18O(OH)] or [LMVO(18OH)] with styrene); so, the absence of 1,2-diol (Table 5, entry 6) indicates that the LMnO driven pathway is either not the case in the catalyst system 2/MesIO/H218O or that its contribution is negligible. Epoxidation of cis-stilbene with various peroxycarboxylic acids showed high cis/trans ratios ranging from 10.1 to 17.9 depending on the acid (Table 4, entries 12−15). This is higher 984


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Scheme 4. Proposed Bifurcated Mechanism for the Asymmetric Epoxidation of Olefins with Iodosylarenes, and the Minor Pathway for 1,2-Diol Formation in the Presence of Watera

a S is a coordinated molecule of solvent or additive. The proposed isotopic exchange scheme was compiled from refs 44, 45, and 47, (bottom); 18O is presented in green.

Scheme 5. Proposed Mechanism for the Asymmetric Epoxidation of Olefins with Peroxycarboxylic Acidsa

a

S is a coordinated molecule of solvent or additive.

peroxyacid, such as (1) the pronounced increase of epoxidation enantioselectivity by systems Mn complex/peracid with increasing steric demand of the peracid (Table 2, entries 16, 19, 21, and 23) and (2) the strong dependence of the cis/trans ratio in the cis-stilbene epoxidation on the nature of the peroxyacid (Table 4, entries 14−16) suggest that the peroxyacid moiety should be present in the structure of the active species during the oxygen transfer step. The addition of boc-(L)-proline improved the cis selectivity (cf. Table 4, entries 16 and 17). Additional mechanistic data are provided by the 18O-labeling experiments. Spectacularly, epoxidation experiments with mCPBA in the presence of H218O witnessed zero 18O incorporation into the epoxide (Table 5, entry 7); this can be taken as further evidence against the water-assisted LMnO driven mechanism. Indeed, the absence of 18O incorporation allows one to draw a distinction between (1) the present systems aminopyridine Mn complexes/peracid and previously

than that for the oxidations with ArIO and comparable to or higher than those for the oxidations with H2O2/carboxylic acid. We note, however, that the oxidations with H2O2/carboxylic acid and with the corresponding peroxycarboxylic acids demonstrate different cis/trans selectivities (e.g., H2O2/EHA, cis/trans = 12.0 vs peroxyEHA, cis/trans = 17.9, cf. Table 4, entries 2 vs 15, and H2O2/AcOH, cis/trans = 12.7 vs AcOOH, cis/trans = 10.1, cf. Table 4, entries 1 vs 14), which hint at distinct oxidation mechanisms. Such high cis/trans ratios and the absence of aldehyde byproducts may reflect a predominantly nonradical oxidation mechanism for the catalyzed oxidations with peroxyacids. That this mechanism is different from the LMnO driven mechanism typical for the Mn complex/H2O2/carboxylic acid systems also stems from the significant response of Mn complex/H2O2/acid systems on boc-(L)-proline addition (Tables 2 and 3), while for the systems Mn complex/peroxyacids the effect of boc-(L)-proline (or AcOH or 2-EHA) is small. However, the influence of the 985


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with the formation of relatively long-lived acyclic, presumably radical intermediates. In the presence of added carboxylic acid (or added water), the mechanism is diverted to the [LMnV O] driven pathway typical for oxidations with catalyst systems Mn complex/H2O2/carboxylic acid. This result is in agreement with the recent report of Costas and co-workers who predicted that in the presence of a carboxylic acid additive, analogous nonheme iron complexes oxidize olefins via the same oxygen transferring species irrespective of the terminal oxidant (H2O2 and TBHP).62 For oxidations with catalyst systems Mn complex/iodosylarenes, available data provide evidence in favor of a predominant Lewis-acid activation mechanism, the oxygen transfer to the olefin directly occurring from the [LMn(OIAr)] adduct. The addition of carboxylic acids (or water) may affect the oxidation mechanism, most likely by partially opening the possibility of the LMnO driven mechanism; the effect of an additive is relatively small for the oxidations with iodosylbenzene and is reduced to a minimum for more sterically demanding iodosylmesitylene. The epoxidations with peroxycarboxylic acids very likely occur via concerted oxygen transfer from the manganese acylperoxo intermediate to the olefin; zero incorporation of 18O (from added H 218 O) into the epoxide rules out the participation of a high-valent oxomanganese intermediate in the asymmetric epoxidation in this case. The addition of carboxylic acid has a relatively minor effect on the oxidation mechanism, possibly by just serving as a side ligand and thus affecting the configuration of the catalytically active site. This work provides a series of interesting examples of chiral environment amplif ication in the enantioselective epoxidation of olefins. Indeed, the use of chiral additive boc-protected (L)proline has been shown to make achiral catalysts 5 and 6 catalyze the epoxidation of olefins in an enantioselective fashion (with up to 60% ee for chalcone epoxidation). This effect is most pronounced for catalyst systems Mn complex/H2O2/boc(L)-proline and Mn complex/alkyl hydroperoxide/boc-(L)proline, reflecting the common nature of the oxygen-transferring species (apparently [LMnVO(L*)], where L* is the optically pure boc-(L)-proline anion) in those cases. For other oxidants, the effect of boc-(L)-proline was much smaller, in agreement with the generally smaller susceptibility of the oxidations with iodosylarenes and peroxycarboxylic acids to the additives. The present work was intentionally planned and undertaken relying on purely the chemical apparatus and argumentation to gain a preliminary qualitative overview of the catalytic reactivities and peculiarities of Mn aminopyridine catalysts with a variety of different oxidants. A deeper and more detailed understanding of the mechanisms operating therein could be achieved by direct spectroscopic observation of the active species and in situ evaluation of their reactivity. Such studies are currently being planned in our laboratory.

reported systems [M(cyclam)(OTf)2] complexes (M = Fe, Mn)/peracid,44 on the one hand, and (2) systems [M(bpc)Cl(H2O)] and [M(Me2bpb)Cl(H2O)]·CH3OH (M= Fe, Mn) complexes/peracid51,52 and Mn porphyrin/peracid on the other.53 In case 2, the incorporation was nonzero, which was interpreted as evidence for the predominant role of the active high-valent iron oxo species. Transition metal acylperoxo complexes have been previously invoked as direct oxidizing species in the oxidations of various substrates such as triphenylphosphine,54 alkanes,55 and olefins.44,56−60 The detailed mechanism of oxygen transfer in most cases remained unclear, in particular whether the reaction can proceed in a concerted manner or by a stepwise mechanism (via O−O bond cleavage of the Mn acylperoxo species, followed by reaction with the substrate).55,61 In principle, our experimental data (i.e., (1) quantitative epoxide yields (in most cases) in catalyst systems Mn complex/peracids (see Tables 2 and 4) without traces of products of radical type oxidations and (2) high cis/trans ratios (10.1···17.9) in Z-stilbene epoxidations with peracids, much higher than that for the oxidations by alkyl hydroperoxides (1.4···2.0, Table 4, entries 4 and 5 and 7 and 8) yet somewhat lower than that for the uncatalyzed concerted Prilezhaev epoxidation (Table 4, entry 18) tentatively support the prevalent concerted mechanism (Scheme 5). In Scheme 5, S may be a molecule of a chiral additive, which most likely accounts for the nonzero (still not so high) enantioselectivities of chalcone epoxidation by peracids in the presence of achiral catalyst 6 and chiral additive boc-(L)-proline (Table 3, entries 5−7). Significant amounts of the singly labeled 1,2-diol (13 TON, Table 5, entry 7), along with the dramatically different regioselectivity of 18O incorporation are most likely due to the cleavage of the oxirane ring of the initially formed epoxide in acidic media. Indeed, for the oxidation by catalyst system 2/ mCPBA/H218O, the ratio of α-16O-β-18O/α-18O-β-16O diol was 1:2, whereas for other systems it was statistical (1:1), which is indicative of a different mechanism of diol formation in the system 2/mCPBA/H218O (see Scheme S1 and Table S3). An experiment showed that styrene epoxide underwent epoxide ring cleavage under the conditions of epoxidation with mCPBA in the presence of water (Table S3, entry 5). Moreover, the epoxidations of Z-β-methylstyrene and indene with mCPBA in the presence of added water (10 equiv) showed the formation of preferentially trans-1,2-diols, thus corroborating the proposed mechanism for diol formation in the presence of mCPBA. Contrariwise, the epoxidation of indene with H2O2 in the presence of H2O afforded (along with the epoxide) the cisdiol (entry 6 of Table S3), which is indicative of the contribution of direct cis-1,2-dihydroxylation to the oxidation mechanism.

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SUMMARY AND OUTLOOK In summary, this work demonstrates that bioinspired chiral manganese aminopyridine complexes are able to act as efficient and enantioselective catalysts of epoxidation of conjugated olefins with a variety of terminal oxidants, including hydrogen peroxide, alkyl hydroperoxides, peroxycarboxylic acids, and iodosylarenes, demonstrating good to excellent enantioselectivities (up to 98−99% ee in several cases). The present study casts light on the mechanisms of enantioselective oxygenation of the olefinic group in the presence of different oxidants. In particular, it has been demonstrated that oxidations by catalyst systems Mn complex/alkyl hydroperoxide most likely occur via oxygen transfer from [LMn-alkylperoxo] species to the alkene

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02299. CCDC 1429026 (5) contains the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif and from the authors. 986


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List of abbreviations, materials and methods, synthetic procedures for the novel ligands and complexes, catalytic procedures, crystal data, and additional figures and tables (PDF) X-ray structure of complex 5 (CIF)

AUTHOR INFORMATION

Corresponding Author

*Fax: 7 383 3308056. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Russian Academy of Sciences and Federal Agency of Scientific Organizations (project V.44.2.4). Financial support from the Russian Foundation for Basic Research (grant 14-03-00102) is gratefully acknowledged. We thank Dr. M. V. Shashkov for the GC-MS measurements.

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Enantioselective Epoxidations of Olefins with Various Oxidants on

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