Enzymatic functionalization of carbon-hydrogen bonds

P450-catalyzed intramolecular sp3 C-H amination with arylsulfonyl azide substrates

The direct amination of aliphatic C–H bonds represents a most valuable transformation in organic chemistry. While a number of transition-metal-based catalysts have been developed and investigated for this purpose, the possibility to execute this transformation with biological catalysts has remained largely unexplored. Here, we report that cytochrome P450 enzymes can serve as efficient catalysts for mediating intramolecular benzylic C–H amination reactions in a variety of arylsulfonyl azide compouds. Under optimized conditions, the P450 catalysts were found to support up to 390 total turnovers leading to the formation of the desired sultam products with excellent regioselectivity. In addition, the chiral environment provided by the enzyme active site allowed for the reaction to proceed in a stereo- and enantioselective manner. The C–H amination activity, substrate profile, and enantio/stereoselectivity of these catalysts could be modulated by utilizing enzyme variants with engineered active sites.

Catalyst-controlled aliphatic C-H oxidations with a predictive model for site-selectivity

Selective aliphatic C-H bond oxidations may have a profound impact on synthesis because these bonds exist across all classes of organic molecules. Central to this goal are catalysts with broad substrate scope (small-molecule-like) that predictably enhance or overturn the substrate's inherent reactivity preference for oxidation (enzyme-like). We report a simple small-molecule, non-heme iron catalyst that achieves predictable catalyst-controlled site-selectivity in preparative yields over a range of topologically diverse substrates. A catalyst reactivity model quantitatively correlates the innate physical properties of the substrate to the site-selectivities observed as a function of the catalyst.

Nonheme iron-mediated amination of C(sp3)-H bonds. Quinquepyridine-supported iron-imide/nitrene intermediates by experimental studies and DFT calculations

The 7-coordinate complex [Fe(qpy)(MeCN)2](ClO4)2 (1, qpy = 2,2':6',2″:6″,2''':6''',2''''-quinquepyridine) is a highly active nonheme iron catalyst for intra- and intermolecular amination of C(sp(3))-H bonds. This complex effectively catalyzes the amination of limiting amounts of not only benzylic and allylic C(sp(3))-H bonds of hydrocarbons but also the C(sp(3))-H bonds of cyclic alkanes and cycloalkane/linear alkane moieties in sulfamate esters, such as those derived from menthane and steroids cholane and androstane, using PhI═NR or "PhI(OAc)2 + H2NR" [R = Ts (p-toluenesulfonyl), Ns (p-nitrobenzenesulfonyl)] as nitrogen source, with the amination products isolated in up to 93% yield. Iron imide/nitrene intermediates [Fe(qpy)(NR)(X)](n+) (CX, X = NR, solvent, or anion) are proposed in these amination reactions on the basis of experimental studies including ESI-MS analysis, crossover experiments, Hammett plots, and correlation with C-H bond dissociation energies and with support by DFT calculations. Species consistent with the formulations of [Fe(qpy)(NTs)2](2+) (CNTs) and [Fe(qpy)(NTs)](2+) (C) were detected by high-resolution ESI-MS analysis of the reaction mixture of 1 with PhI═NTs (4 equiv). DFT calculations revealed that the reaction barriers for H-atom abstraction of cyclohexane by the ground state of 7-coordinate CNTs and ground state of C are 15.3 and 14.2 kcal/mol, respectively, in line with the observed high activity of 1 in catalyzing the C-H amination of alkanes under mild conditions.

Whole-cell-based CYP153A6-catalyzed (S)-limonene hydroxylation efficiency depends on host background and profits from monoterpene uptake via AlkL

Living microbial cells are considered to be the catalyst of choice for selective terpene functionalization. However, such processes often suffer from side product formation and poor substrate mass transfer into cells. For the hydroxylation of (S)-limonene to (S)-perillyl alcohol by Pseudomonas putida KT2440 (pGEc47ΔB)(pCom8-PFR1500), containing the cytochrome P450 monooxygenase CYP153A6, the side products perillyl aldehyde and perillic acid constituted up to 26% of the total amount of oxidized terpenes. In this study, it is shown that the reaction rate is substrate-limited in the two-liquid phase system used and that host intrinsic dehydrogenases and not CYP153A6 are responsible for the formation of the undesired side products. In contrast to P. putida KT2440, E. coli W3110 was found to catalyze perillyl aldehyde reduction to the alcohol and no oxidation to the acid. Furthermore, E. coli W3110 harboring CYP153A6 showed high limonene hydroxylation activities (7.1 U g CDW-1). The outer membrane protein AlkL was found to enhance hydroxylation activities of E. coli twofold in aqueous single-phase and fivefold in two-liquid phase biotransformations. In the latter system, E. coli harboring CYP153A6 and AlkL produced up to 39.2 mmol (S)-perillyl alcohol L tot-1 within 26 h, whereas no perillic acid and minor amounts of perillyl aldehyde (8% of the total products) were formed. In conclusion, undesired perillyl alcohol oxidation was reduced by choosing E. coli's enzymatic background as a reaction environment and co-expression of the alkL gene in E. coli represents a promising strategy to enhance terpene bioconversion rates.

The palladium-catalyzed anti-Markovnikov hydroalkylation of allylic alcohol derivatives

A palladium-catalyzed hydroalkylation reaction of protected allylic alcohols using alkylzinc bromide reagents is reported. This account includes numerous allylic, homoallylic, and bishomoallylic alcohol derivatives, all with a uniform selectivity of >20:1 for the anti-Markovnikov product. The reaction features the ability to deliver enantiomerically enriched alcohols in unfunctionalized regions, which results from the catalyst avoiding β-hydride elimination at the allylic position.

First synthesis of (+)-myrrhanol C, an anti-prostate cancer lead

The first synthesis of (+)-myrrhanol C (1), an antitumor polypodane-type bicyclic triterpene with inhibitory activity against androgen insensitive prostate cancers, is reported herein (IC(50) 10 μmolar). A key step in our convergent synthesis of (+)-myrrhanol C and related analogues is the employment of a microbial stereo- and regioselective late stage C-H oxidation. A low-waste and sustainable process has been developed to prepare (+)-myrrhanol C for further biological studies.

Oxidative aliphatic C-H fluorination with fluoride ion catalyzed by a manganese porphyrin

Despite the growing importance of fluorinated organic compounds in drug development, there are no direct protocols for the fluorination of aliphatic C-H bonds using conveniently handled fluoride salts. We have discovered that a manganese porphyrin complex catalyzes alkyl fluorination by fluoride ion under mild conditions in conjunction with stoichiometric oxidation by iodosylbenzene. Simple alkanes, terpenoids, and even steroids were selectively fluorinated at otherwise inaccessible sites in 50 to 60% yield. Decalin was fluorinated predominantly at the C2 and C3 methylene positions. Bornyl acetate was converted to exo-5-fluoro-bornyl acetate, and 5α-androstan-17-one was fluorinated selectively in the A ring. Mechanistic analysis suggests that the regioselectivity for C-H bond cleavage is directed by an oxomanganese(V) catalytic intermediate followed by F delivery via an unusual manganese(IV) fluoride that has been isolated and structurally characterized.

Achieving regio- and enantioselectivity of P450-catalyzed oxidative CH activation of small functionalized molecules by structure-guided directed evolution

Directed evolution of the monooxygenase P450-BM3 utilizing iterative saturation mutagenesis at and near the binding site enables a high degree of both regio- and enantioselectivity in the oxidative hydroxylation of cyclohexene-1-carboxylic acid methyl ester. Wild-type P450-BM3 is 84% regioselective for the allylic 3-position with 34% enantioselectivity in favor of the R alcohol. Mutants enabling R selectivity (>95% ee) or S selectivity (>95% ee) were evolved, while reducing other oxidation products and thus maximizing regioselectivity to >93%. Control of the substrate-to-enzyme ratio is necessary for obtaining optimal and reproducible enantioselectivities, an observation which is important in future protein engineering of these mono-oxygenases. An E. coli strain capable of NADPH regeneration was also engineered, simplifying directed evolution of P450 enzymes in general. These synthetic results set the stage for subsequent stereoselective and stereospecific chemical transformations to form more complex compounds, thereby illustrating the viability of combining genetically altered enzymes as catalysts in organic chemistry with traditional chemical methods.

1,2,3-triazole: unique ligand in promoting iron-catalyzed propargyl alcohol dehydration

A 1,2,3-traizole-promoted iron(III)-catalyzed propargyl alcohol dehydration was developed for the synthesis of conjugated enynes. The desired conjugated enynes were prepared in good to excellent yields (up to 95%) with a large substrate scope and excellent stereoselectivity (only Z-isomers).

Copper-catalyzed aerobic oxidative C-H functionalizations: trends and mechanistic insights

The selective oxidation of C-H bonds and the use of O(2) as a stoichiometric oxidant represent two prominent challenges in organic chemistry. Copper(II) is a versatile oxidant, capable of promoting a wide range of oxidative coupling reactions initiated by single-electron transfer (SET) from electron-rich organic molecules. Many of these reactions can be rendered catalytic in Cu by employing molecular oxygen as a stoichiometric oxidant to regenerate the active copper(II) catalyst. Meanwhile, numerous other recently reported Cu-catalyzed C-H oxidation reactions feature substrates that are electron-deficient or appear unlikely to undergo single-electron transfer to copper(II). In some of these cases, evidence has been obtained for the involvement of organocopper(III) intermediates in the reaction mechanism. Organometallic C-H oxidation reactions of this type represent important new opportunities for the field of Cu-catalyzed aerobic oxidations.

Recent advances in transition metal-catalyzed sp3 C-H amination adjacent to double bonds and carbonyl groups

Transition metal-catalyzed C-H amination at positions adjacent to double bonds and carbonyl groups is discussed in this critical review. While the focus will center on the recent developments of α-oxidative amination, some historical developments and mutually beneficial reports in the broader field of C-H amination will be discussed. C-H amination has become a viable method for the efficient installation of nitrogen atoms en route to target molecules (89 references).