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Direct Selective Oxidative Functionalization of C–H Bonds with H2O2: Mn-Aminopyridine Complexes Challenge the Dominance of Non-Heme Fe Catalysts

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Direct Selective Oxidative Functionalization of C–H Bonds with H2O2: Mn-Aminopyridine Complexes Challenge the Dominance of Non-Heme Fe Catalysts molecules Review Direct Selective Oxidative Functionalization of C–H Bonds with H2O2: Mn-Aminopyridine Complexes Challenge the Dominance of Non-Heme Fe Catalysts Roman V. Ottenbacher 1,2, Evgenii P. Talsi 1,2 and Konstantin P. Bryliakov 1,2,* 1 Chemistry Department, Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia; [email protected] (R.V.O.); [email protected] (E.P.T.) 2 Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russia * Correspondence: [email protected]; Tel.: +7-383-326-9451; Fax: +7-383-330-8056 Academic Editor: Georgiy B. Shul’pin Received: 28 September 2016; Accepted: 20 October 2016; Published: 31 October 2016 Abstract: Non-heme iron(II) complexes are widespread synthetic enzyme models, capable of conducting selective C–H oxidation with H2O2 in the presence of carboxylic acid additives. In the last years, structurally similar manganese(II) complexes have been shown to catalyze C–H oxidation with similarly high selectivity, and with much higher efficiency. In this mini-review, recent catalytic and mechanistic data on the selective C–H oxygenations with H2O2 in the presence of manganese complexes are overviewed. A distinctive feature of catalyst systems of the type Mn complex/H2O2/carboxylic is the existence of two alternative reaction pathways (as found for the oxidation of cumenes), one leading to the formation of alcohol, and the other to ester. The mechanisms of formation of the alcohol and the ester are briefly discussed. Keywords: C–H functionalization; enzyme models; hydrogen peroxide; manganese; mechanism; oxidation 1. Introduction Given increasing demand for efficient and scalable methods of fine organic syntheses (mostly the syntheses of natural products, pharmaceuticals, biologically active molecules), the design of rigorous synthetic approaches, ensuring precise control of the carbon skeleton and oxidation state, continues to be one of the foremost challenges of synthetic chemistry. In the last years, selective catalytic oxidations of unactivated C–H bonds have attracted great attention: this approach, if implemented on preparative scale, could provide easy and efficient methodologies for the directed introduction of oxygen atom into complex organic molecules at late stages of multistep syntheses, on the basis of novel synthetic strategies [1–11]. Selective C–H oxidation reactions may lead to formation of new C–C and C–X (e.g., C–O, C–N, C–S, C–halogen, etc. [10]) bonds; in this paper, we will solely focus on the oxygenation processes, converting alkane groups into C–OH or C=O functionalities. Metal-mediated C–H activation (including C–H bond cleavage and formation of C-metal s-bonds) has been known for decades; one could mention in this context the works of Shilov [1,8,12], Periana [1,8,13], and others [1,8], dating back to the 1960s–1980s. Typically, C–H activation reactions require that the substrate should first coordinate to the catalytic complex of transition metal (Pt, Rh, Ir, etc.), which is relatively easy if the substrate has p electrons. In contrast to unsaturated substrates, alkanes have only s electrons and thus are poor nucleophiles, coordinating only weakly to metals [8], which complicates the C–H activation step. On the other hand, to obtain a functionalized molecule, the activation step should be combined with subsequent functionalization step, which may represent some obstacles [8]. That is why in the last years, direct C–H oxofunctionalization, which does Molecules 2016, 21, 1454; doi:10.3390/molecules21111454 www.mdpi.com/journal/molecules Molecules 2016, 21, 1454 2 of 16 Molecules 2016, 21, 1454 2 of 16 C–H oxofunctionalization, which does not imply the intermediate formation of a metal-carbon bond, has attracted close attention as a promising strategy for the selective oxidative modification notof challenging imply the substrates intermediate like formation alkanes. An of additional a metal-carbon advantage bond, of has the attracted direct C–H close oxidations attention is asthat a promisingmost of them strategy rely foron catalysts the selective based oxidative on comple modificationxes of cheap of challengingand abundant substrates first-row like transition alkanes. Anmetals additional (Fe, Cu, advantage Mn, V, etc.). of the direct C–H oxidations is that most of them rely on catalysts based on complexesNowadays, of cheap the and design abundant of homogeneous first-row transition catalysts metals for (Fe,the Cu, direct Mn, C–H V, etc.). oxygenation largely followsNowadays, the biomimetic the design approach, of homogeneous which catalystsimplies modeling for the direct the C–H catalytic oxygenation functions largely of followsnatural themetalloenzymes biomimetic approach, (like cytochrome which implies P450 or modeling methane the monooxygenase) catalytic functions with of synthetic natural metalloenzymes low-molecular (likeweight cytochrome complexes. P450 It is or not methane surprising monooxygenase) that the major with part synthetic of reported low-molecular examples weight of catalytic complexes. C–H Itoxidations is not surprising deal with thatcatalysts the majorbased on part complexes of reported of Fe—the examples latter of is catalytic the most C–H widespread oxidations metal deal in withnatural catalysts metalloenzymes based on [14–27]. complexes Most ofoften, Fe—the H2O2 latteris used is as the the most “green” widespread oxidant. metal in natural metalloenzymesToday, non-heme [14–27 ].(essentially Most often, nonporphyrinic) H2O2 is used as theFe “green”complexes oxidant. dominate the area, due to the versatilityToday, of non-heme the ligand (essentially structures, nonporphyrinic)and the rich possibilities Fe complexes of their dominate structural the modifications. area, due to First the versatilityexamples of of non-heme-Fe-cataly the ligand structures,zed andoxidations the rich appeared possibilities in early of their 1990s structural [24,28]; modifications.however, the Firstbreakthrough examples was of non-heme-Fe-catalyzedachieved after 2007, when oxidations White appearedwith co-workers in early 1990scontributed [24,28 ];a however,series of themilestone breakthrough works, presenting was achieved the afterbipyrrolidine-derived 2007, when White non-heme with co-workers iron catalyst contributed 1 (Figure a 1) series and its of milestonestructural works,analogs, presenting ensuring reasonably the bipyrrolidine-derived high level of predictability non-heme iron in the catalyst selective1 (Figure oxidation1) and of itsC(sp structural3)-H groups analogs, [29–34]. ensuring In competitive reasonably contributi high levelon, ofCostas predictability and co-workers in the selective showed oxidation that the ofintroduction C(sp3)-H groupsof additional [29–34 ].steric In competitive crowd at the contribution, pyridine moieties, Costas as and well co-workers as manipulating showed with that the introductionsymmetry of ofthe additional chiral ligand steric crowdcan divert at the the pyridine oxidation moieties, selectivity as well from as manipulating3° C(sp3)-H bonds with the to symmetrystronger 2° of C(sp the chiral3)-H bonds, ligand which can divert is critical the oxidation for the selectivityselective oxygenation from 3◦ C(sp 3of)-H complex bonds tomolecules stronger 2such◦ C(sp as3 )-Hnatural bonds, products which is[35–38]. critical The for themechanism selective oxygenationof non-heme of iron complex catalyzed molecules oxidations such ashas natural been productsextensively [35 studied–38]. The experimentally mechanism of [39]; non-heme it has ironnow catalyzed been accepted oxidations that hasthe C–H been extensivelyoxidation proceeds studied experimentallyvia the classical [39 rebound]; it has nowmechanism been accepted [40] (Figure that the 1) C–H, with oxidation participation proceeds of the via theelusive classical oxoperferryl rebound mechanismspecies [41–49]. [40] (Figure1), with participation of the elusive oxoperferryl species [41–49]. Figure 1. The structure of White’s catalyst 1 ((left) and thethe rebound-mechanismrebound-mechanism ((rightright).). InIn contrastcontrast to to non-heme non-heme Fe Fe complexes, complexes, the catalyticthe catalytic activity activity (as well (as as well the mechanismas the mechanism of catalytic of action)catalytic of action) structurally of structurally related Mn related complexes Mn havecomplexes so far beenhave muchso far less been studied. much Nevertheless,less studied. theNevertheless, Mn based catalystthe Mn systemsbased catalyst for C–H systems oxidation, for reportedC–H oxidation, so far, demonstrate reported so goodfar, demonstrate practical promise; good thepractical Mn-catalyzed promise; C–Hthe Mn-catalyzed oxidation mechanism C–H oxidation seems tomechanism be mostly similarseems to to be that mostly for the similar Fe-mediated to that directfor the C–H Fe-mediated oxidation. direct In this C–H contribution, oxidation. weIn th willis contribution, briefly summarize we will the briefly related summarize synthetic andthe mechanisticrelated synthetic data publishedand mechanistic to date, data and discusspublished a new to mechanisticdate, and discuss feature, a previouslynew mechanistic unobserved feature, for Fe-catalyzedpreviously unobserved C–H oxidations, for Fe-catalyzed which seems toC–H be characteristicoxidations, which of Mn-mediated seems to C–Hbe characteristic oxidations. of Mn-mediated C–H oxidations. 2. C–H Oxidations with H2O2 in the Presence of Mn Porphyrins 2. C–H Oxidations with H2O2 in the Presence of Mn Porphyrins In 1986, Mansuy with co-workers
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