Catalytic oxidations of steroid substrates by artificial cytochrome p-450 enzymes

Precise supramolecular control of selectivity in the Rh-catalyzed hydroformylation of terminal and internal alkenes

In this study, we report a series of DIMPhos ligands L1-L3, bidentate phosphorus ligands equipped with an integral anion binding site (the DIM pocket). Coordination studies show that these ligands bind to a rhodium center in a bidentate fashion. Experiments under hydroformylation conditions confirm the formation of the mononuclear hydridobiscarbonyl rhodium complexes that are generally assumed to be active in hydroformylation. The metal complexes formed still strongly bind the anionic species in the binding site of the ligand, without affecting the metal coordination sphere. These bifunctional properties of DIMPhos are further demonstrated by the crystal structure of the rhodium complex with acetate anion bound in the binding site of the ligand. The catalytic studies demonstrate that substrate preorganization by binding in the DIM pocket of the ligand results in unprecedented selectivities in hydroformylation of terminal and internal alkenes functionalized with an anionic group. Remarkably, the selectivity controlling anionic group can be even 10 bonds away from the reactive double bond, demonstrating the potential of this supramolecular approach. Control experiments confirm the crucial role of the anion binding for the selectivity. DFT studies on the decisive intermediates reveal that the anion binding in the DIM pocket restricts the rotational freedom of the reactive double bound. As a consequence, the pathway to the undesired product is strongly hindered, whereas that for the desired product is lowered in energy. Detailed kinetic studies, together with the in situ spectroscopic measurements and isotope-labeling studies, support this mode of operation and reveal that these supramolecular systems follow enzymatic-type Michaelis-Menten kinetics, with competitive product inhibition.

Chemoselective hydrosilylation of hydroxyketones

A chemoselective method for the hydrosilylation of ketones has been developed, using the combination of triphenylsilane and a catalyst prepared from Ni(COD)₂ and the simple N-heterocyclic carbene IMes. The most notable feature of this method is that free hydroxyls are largely unaffected, thus providing a simple one-step procedure for the conversion of hydroxyketones to mono-protected diols, wherein the protecting group is exclusively installed on the ketone-derived hydroxyl. The process is typically high yielding with both simple ketones and more complex hydroxyketone substrates.

Mimicking P450 processes and the use of metalloporphyrins

Metalloporphyrins (MPs) are known to catalyze in vitro a broad range of cytochrome P450-mediated reactions occurring in vivo. Most of the biomimetic research using MPs in oxidative catalysis has been directed towards the oxidation of organic compounds presenting significant reactivity features in one functional group. Much less effort has been made to imitate the oxidation of more complex molecules, with a range of functionalities, such as drugs or other xenobiotics. By varying the structure of the porphyrin, the metal ion, the oxidant, and the reaction conditions, it is possible to modulate the regioselectivity of the oxidation reactions. Recently, and along with studies on the synthesis and reactivity of porphyrins, chlorins, and phthalocyanines, our group was able to develop an interesting line of research in the field of biomimetic oxidation of organic compounds using environmentally benign hydrogen peroxide as oxidant and Mn(III) or Fe(III) porphyrin complexes as catalysts. The more up to date results obtained in such work are reviewed here.

Designing enzyme-like catalysts: a rhodium(II) metallopeptide case study

Chemists have long been fascinated by metalloenzymes and their chemistry. Because enzymes are essential for biological processes and to life itself, they present a key to understanding the world around us. At the same time, if chemists could harness the reactivity and selectivity of enzymes in designed transition-metal catalysts, we would have access to a powerful practical advance in chemistry. But the design of enzyme-like catalysts from scratch presents enormous challenges. Simplified, designed systems often don't provide the opportunity to mimic the complex features of enzymes such as selectivity in polyfunctional environments and access to reactive intermediates incompatible with bulk aqueous solution. Extensive efforts by numerous groups have led to remarkable designed metalloproteins that contain complex folds, including well-defined secondary and tertiary structure surrounding complex polymetallic centers. These structural achievements, however, have not yet led to general approaches to useful catalysts; continued efforts and new insights are needed. Our efforts have combined the attributes of enzymatic and traditional catalysis, bringing the benefits of polypeptide ligands to bear on completely nonbiological transition-metal centers. With a focus on designing useful catalytic activity, we have examined rhodium(II) carboxylates, bound to peptide chains through carboxylate side chains. Among other advantages, these complexes are stable and catalytically active in water. Our efforts have centered on two main interests: (1) understanding how Nature's ligand of choice, polypeptides, can be used to control the chemistry of nonbiological metal centers, and (2) mimicking metalloenzyme characteristics in designed, nonbiological catalysts. This Account conveys our motivation and goals for these studies, outlines progress to date, and discusses the future of enzyme-like catalyst design. In particular, these studies have resulted in on-bead, high-throughput screens for asymmetric metallopeptide catalysts. In addition, peptide-based molecular recognition strategies have facilitated the site-specific modification of protein substrates. Molecular recognition enables site-specific, proximity-driven modification of a broad range of amino acids, and the concepts outlined here are compatible with natural protein substrates and with complex, cell-like environments. We have also explored rhodium metallopeptides as hybrid organic-inorganic inhibitor molecules that block protein-protein interactions.

Controlled oxidation of remote sp3 C-H bonds in artemisinin via P450 catalysts with fine-tuned regio- and stereoselectivity

The selective oxyfunctionalization of isolated sp(3) C-H bonds in complex molecules represents a formidable challenge in organic chemistry. Here, we describe a rational, systematic strategy to expedite the development of P450 oxidation catalysts with refined regio- and stereoselectivity for the hydroxylation of remote, unactivated C-H sites in a complex scaffold. Using artemisinin as model substrate, we demonstrate how a three-tier strategy involving first-sphere active site mutagenesis, high-throughput P450 fingerprinting, and fingerprint-driven P450 reactivity predictions enabled the rapid evolution of three efficient biocatalysts for the selective hydroxylation of a primary and a secondary C-H site (with both S and R stereoselectivity) in a relevant yet previously inaccessible region of this complex natural product. The evolved P450 variants could be applied to provide direct access to the desired hydroxylated derivatives at preparative scales (0.4 g) and in high isolated yields (>90%), thereby enabling further elaboration of this molecule. As an example, enantiopure C7-fluorinated derivatives of the clinical antimalarial drugs artesunate and artemether, in which a major metabolically sensitive site is protected by means of a C-H to C-F substitution, were afforded via P450-mediated chemoenzymatic synthesis.

Copper-catalyzed oxaziridine-mediated oxidation of C-H bonds

The highly regio- and chemoselective oxidation of activated C-H bonds has been observed via copper-catalyzed reactions of oxaziridines. The oxidation proceeded with a variety of substrates, primarily comprising allylic and benzylic examples, as well as one example of an otherwise unactivated tertiary C-H bond. The mechanism of the reaction is proposed to involve single-electron transfer to the oxaziridines to generate a copper-bound radical anion, followed by hydrogen atom abstraction and collapse to products, with regeneration of the catalyst by a final single-electron transfer event. The involvement of allylic radical intermediates was supported by a radical-trapping experiment with TEMPO.

5,10,15,20-tetrakis(pentafluorophenyl)porphyrin: a versatile platform to novel porphyrinic materials

5,10,15,20-tetrakis(pentafluorophenyl)porphyrin reacts with a range of nucleophiles (amines, alcohols, thiols, nitrogen heterocycles, and others) resulting in the nucleophilic aromatic substitution of the para-F atoms of the pentafluorophenyl groups. This reaction, which was fortuitously discovered by Kadish and collaborators in 1990, is now being extensively used to synthesize porphyrins bearing electron-donating substituents in the para-position of their meso-aryl groups. This mini-review highlights the methods of synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, the use of its metal complexes in catalysis and its reaction with nucleophiles to yield new monomeric porphyrins, porphyrins supported in polymers or new polymeric porphyrin matrices useful for heterogeneous catalysis.

Host-guest interactions of cyclodextrins and metalloporphyrins: supramolecular building blocks toward artificial heme proteins

Cyclodextrins are versatile building blocks for a variety of macromolecules due to the inclusion complexes that are formed with hydrophobic organic molecules. Cyclodextrin-porphyrin interactions are of particular interest since cyclodextrins can serve as a non-covalent binding pocket while metalloporphyrins could serve as the heme analogs in the construction of heme protein model compounds. Various approaches to the design and assembly of biomimetic porphyrin constructs are compared and contrasted in this minireview with a particular emphasis on self-assembled and porphyrin-cyclodextrin systems. Several recent advances from our laboratories are described in this context. A sensitive fluorescent binding probe, 6A-N-dansyl-permethylated-β-cyclodextrin (Dan-NH-TMCD), was found to form 2:1 complexes with the meso-tetraphenylporphyrins Mn(III)TCPP , Mn(III)TPPS and Mn(III)TF 4 TMAP with high binding constants. A perPEGylated cyclodextrin, heptakis(2,3,6-tri-O-2-(2-(2-methoxyethoxy)ethoxy)ethyl)-β-cyclodextrin (TPCD), has been shown by 1 H NMR spectroscopy to form a 1:1 complex with H 2 TCPP with a binding constant above 108M-1. Such a strong binding constant is the largest found for a 1:1 complex between a monomeric cyclodextrin and a guest. TPCD was also found to bind Mn(III)TCPP with a binding constant of 1.2 × 106 M -1. A novel, self-assembled hemoprotein model, hemodextrin is also described. The molecular design is based on a PEGylated cyclodextrin scaffold that bears both a heme-binding pocket and an axial ligand that binds an iron porphyrin. The binding constant for Fe (III) TPPS (iron(III) meso-tetra(4-sulfonatophenyl)porphyrin) by py-PPCD was determined to be 2 × 106 M -1. The pyridyl nitrogen of py-PPCD was shown to ligate to the iron center by observing signal changes in the Fe(II) -porphyrin 1 H NMR spectrum. This hemodextrin ensemble, a minimalist myoglobin, was shown to bind dioxygen reversibly and to form a stable ferryl species.

Molecular complexity via C-H activation: a dehydrogenative Diels-Alder reaction

Traditionally, C-H oxidation reactions install oxidized functionality onto a preformed molecular skeleton, resulting in a local molecular change. The use of C-H activation chemistry to construct complex molecular scaffolds is a new area with tremendous potential in synthesis. We report a Pd(II)/bis-sulfoxide-catalyzed dehydrogenative Diels-Alder reaction that converts simple terminal olefins into complex cycloadducts in a single operation.

Enzymatic functionalization of carbon-hydrogen bonds

The development of new catalytic methods to functionalize carbon-hydrogen (C-H) bonds continues to progress at a rapid pace due to the significant economic and environmental benefits of these transformations over traditional synthetic methods. In nature, enzymes catalyze regio- and stereoselective C-H bond functionalization using transformations ranging from hydroxylation to hydroalkylation under ambient reaction conditions. The efficiency of these enzymes relative to analogous chemical processes has led to their increased use as biocatalysts in preparative and industrial applications. Furthermore, unlike small molecule catalysts, enzymes can be systematically optimized via directed evolution for a particular application and can be expressed in vivo to augment the biosynthetic capability of living organisms. While a variety of technical challenges must still be overcome for practical application of many enzymes for C-H bond functionalization, continued research on natural enzymes and on novel artificial metalloenzymes will lead to improved synthetic processes for efficient synthesis of complex molecules. In this critical review, we discuss the most prevalent mechanistic strategies used by enzymes to functionalize non-acidic C-H bonds, the application and evolution of these enzymes for chemical synthesis, and a number of potential biosynthetic capabilities uniquely enabled by these powerful catalysts (110 references).

Manganese porphyrins catalyze selective C-H bond halogenations

We report a manganese porphyrin mediated aliphatic C-H bond chlorination using sodium hypochlorite as the chlorine source. In the presence of catalytic amounts of phase transfer catalyst and manganese porphyrin Mn(TPP)Cl 1, reaction of sodium hypochlorite with different unactivated alkanes afforded alkyl chlorides as the major products with only trace amounts of oxygenation products. Substrates with strong C-H bonds, such as neopentane (BDE =∼100 kcal/mol) can be also chlorinated with moderate yield. Chlorination of a diagnostic substrate, norcarane, afforded rearranged products indicating a long-lived carbon radical intermediate. Moreover, regioselective chlorination was achieved by using a hindered catalyst, Mn(TMP)Cl, 2. Chlorination of trans-decalin with 2 provided 95% selectivity for methylene-chlorinated products as well as a preference for the C2 position. This novel chlorination system was also applied to complex substrates. With 5α-cholestane as the substrate, we observed chlorination only at the C2 and C3 positions in a net 55% yield, corresponding to the least sterically hindered methylene positions in the A-ring. Similarly, chlorination of sclareolide afforded the equatorial C2 chloride in a 42% isolated yield. Regarding the mechanism, reaction of sodium hypochlorite with the Mn(III) porphyrin is expected to afford a reactive Mn(V)═O complex that abstracts a hydrogen atom from the substrate, resulting in a free alkyl radical and a Mn(IV)-OH complex. We suggest that this carbon radical then reacts with a Mn(IV)-OCl species, providing the alkyl chloride and regenerating the reactive Mn(V)═O complex. The regioselectivity and the preference for CH(2) groups can be attributed to nonbonded interactions between the alkyl groups on the substrates and the aryl groups of the manganese porphyrin. The results are indicative of a bent [Mn(v)═O---H---C] geometry due to the C-H approach to the Mn(v)═O (dπ-pπ)* frontier orbital.

Combined effects on selectivity in Fe-catalyzed methylene oxidation

Methylene C-H bonds are among the most difficult chemical bonds to selectively functionalize because of their abundance in organic structures and inertness to most chemical reagents. Their selective oxidations in biosynthetic pathways underscore the power of such reactions for streamlining the synthesis of molecules with complex oxygenation patterns. We report that an iron catalyst can achieve methylene C-H bond oxidations in diverse natural-product settings with predictable and high chemo-, site-, and even diastereoselectivities. Electronic, steric, and stereoelectronic factors, which individually promote selectivity with this catalyst, are demonstrated to be powerful control elements when operating in combination in complex molecules. This small-molecule catalyst displays site selectivities complementary to those attained through enzymatic catalysis.

Oxyfunctionalization of unactivated C-H bonds in triterpenoids with tert-butylhydroperoxide catalyzed by meso-5,10,15,20-tetramesitylporphyrinate osmium(II) carbonyl complex

A system consisting of meso-5,10,15,20-tetramesitylporphyrinate osmium(II) carbonyl complex [Os(TMP)CO] as a precatalyst and tert-butylhydroperoxide (TBHP) as an oxygen donor is shown to be an efficient, regioselective oxidant system for the allylic oxidation, ketonization and hydroxylation of unactivated C-H bonds in a series of the peracetate derivatives of penta- and tetracyclic triterpenoids. Treatment of the substrates with this oxidant system afforded a variety of novel or scarce oxygenated derivatives in one-step. Structures of the isolated components, after chromatographic separation, were determined by spectroscopic methods including GC-MS and shift-correlated 2D-NMR techniques. Factors governing the regioselectivity and the possible mechanism for the oxyfunctionalization of the unactivated carbons are also discussed.

Selective oxidation of carbolide C-H bonds by an engineered macrolide P450 mono-oxygenase

Regio- and stereoselective oxidation of an unactivated C-H bond remains a central challenge in organic chemistry. Considerable effort has been devoted to identifying transition metal complexes, biological catalysts, or simplified mimics, but limited success has been achieved. Cytochrome P450 mono-oxygenases are involved in diverse types of regio- and stereoselective oxidations, and represent a promising biocatalyst to address this challenge. The application of this class of enzymes is particularly significant if their substrate spectra can be broadened, selectivity controlled, and reactions catalyzed in the absence of expensive heterologous redox partners. In this study, we engineered a macrolide biosynthetic P450 mono-oxygenase PikC (PikC(D50N)-RhFRED) with remarkable substrate flexibility, significantly increased activity compared to wild-type enzyme, and self-sufficiency. By harnessing its unique desosamine-anchoring functionality via a heretofore under-explored "substrate engineering" strategy, we demonstrated the ability of PikC to hydroxylate a series of carbocyclic rings linked to the desosamine glycoside via an acetal linkage (referred to as "carbolides") in a regioselective manner. Complementary analysis of a number of high-resolution enzyme-substrate cocrystal structures provided significant insights into the function of the aminosugar-derived anchoring group for control of reaction site selectivity. Moreover, unexpected biological activity of a select number of these carbolide systems revealed their potential as a previously unrecorded class of antibiotics.

Total synthesis and study of 6-deoxyerythronolide B by late-stage C-H oxidation

Among the frontier challenges in chemistry in the twenty-first century are the interconnected goals of increasing synthetic efficiency and diversity in the construction of complex molecules. Oxidation reactions of C-H bonds, particularly when applied at late stages of complex molecule syntheses, hold special promise for achieving both these goals. Here we report a late-stage C-H oxidation strategy in the total synthesis of 6-deoxyerythronolide B (6-dEB), the aglycone precursor to the erythromycin antibiotics. An advanced intermediate is cyclized to give the 14-membered macrocyclic core of 6-dEB using a late-stage (step 19 of 22) C-H oxidative macrolactonization reaction that proceeds with high regio-, chemo- and diastereoselectivity (>40:1). A chelate-controlled model for macrolactonization predicted the stereochemical outcome of C-O bond formation and guided the discovery of conditions for synthesizing the first diastereomeric 13-epi-6-dEB precursor. Overall, this C-H oxidation strategy affords a highly efficient and stereochemically versatile synthesis of the erythromycin core.

Molecular recognition in Mn-catalyzed C-H oxidation. Reaction mechanism and origin of selectivity from a DFT perspective

Experimental studies have shown that the C-H oxidation of Ibuprofen and methylcyclohexane acetic acid can be carried out with high selectivities using [(terpy')Mn(OH(2))(mu-O)(2)Mn(OH(2))(terpy')](3+) as catalyst, where terpy' is a terpyridine ligand functionalized with a phenylene linker and a Kemp's triacid serving to recognize the reactant via H-bonding. Experiments, described here, suggest that the sulfate counter anion, present in stoichiometric amounts, coordinates to manganese in place of water. DFT calculations have been carried out using [(terpy')Mn(O)(mu-O)(2)Mn(SO(4))(terpy')](+) as a model catalyst, to analyze the origin of selectivity and its relation to molecular recognition, as well as the mechanism of catalyst inhibition by tert-butyl benzoic acid. The calculations show that a number of spin states, all having radical oxygen character, are energetically accessible. All these spin states promote C-H oxidation via a rebound mechanism. The catalyst recognizes the substrate by a double H bond. This interaction orients the substrate inducing highly selective C-H oxidation. The double hydrogen bond stabilizes the reactant, the transition state and the product to the same extent. Consequently, the reaction occurs at lower energy than without molecular recognition. The association of the catalyst with tert-butyl benzoic acid is shown to shield the access of unbound substrate to the reactive oxo site, hence preventing non-selective hydroxylation. It is shown that the two recognition sites of the catalyst can be used in a cooperative manner to control the access to the reactive centre.