Artificial Metalloenzyme-Catalyzed Enantioselective Amidation via Nitrene Insertion in Unactivated C(sp3)-H Bonds

Design and Evolution of an Artificial Friedel-Crafts Alkylation Enzyme Featuring an Organoboronic Acid Residue

Creating artificial enzymes by the genetic incorporation of noncanonical amino acids with catalytic side chains would expand the enzyme chemistries that have not been discovered in nature. Here, we report the design of an artificial enzyme that uses p-boronophenylalanine as the catalytic residue. The artificial enzyme catalyzes Michael-type Friedel-Crafts alkylation through covalent activation. The designer enzyme was further engineered to afford high yields with excellent enantioselectivities. We next developed a practical method for preparative-scale reactions by whole-cell catalysis. This enzymatic C-C bond formation reaction was combined with palladium-catalyzed dearomative arylation to achieve the efficient synthesis of spiroindolenine compounds.

Binding and Stabilization of a Semiquinone Radical by an Artificial Metalloenzyme Containing a Binuclear Copper (II) Cofactor

A binuclear Cu(II) cofactor was covalently bound to a lauric acid anchor. The resulting conjugate was characterized then combined with beta-lactoglobulin (βLG) to generate a new biohybrid following the so-called "Trojan horse" strategy. This biohybrid was examined for its effectiveness in the oxidation of a catechol derivative to the corresponding quinone. The resulting biohybrid did not exhibit the sought after catecholase activity, likely due to its ability to bind and stabilize the semiquinone radical intermediate DTB-SQ. This semi-quinone radical was stabilized only in the presence of the protein and was characterized using optical and magnetic spectroscopic techniques, demonstrating stability for over 16 hours. Molecular docking studies revealed that this stabilization could occur owing to interactions of the semi-quinone with hydrophobic amino acid residues of βLG.

C-H functionalization reactions catalyzed by artificial metalloenzymes

CH functionalization, a promising frontier in modern organic chemistry, facilitates the direct conversion of inert CH bonds into many valuable functional groups. Despite its merits, traditional homogeneous catalysis, often faces challenges in efficiency, selectivity, and sustainability towards this transformation. In this context, artificial metalloenzymes (ArMs), resulting from the incorporation of a catalytically-competent metal cofactor within an evolvable protein scaffold, bridges the gap between the efficiency of enzymatic transformations and the versatility of transition metal catalysis. Accordingly, ArMs have emerged as attractive tools for various challenging catalytic transformations. Additionally, the coming of age of directed evolution has unlocked unprecedented avenues for optimizing enzymatic catalysis. Taking advantage of their genetically-encoded protein scaffold, ArMs have been evolved to catalyze various CH functionalization reactions. This review delves into the recent developments of ArM-catalyzed CH functionalization reactions, highlighting the benefits of engineering the second coordination sphere around a metal cofactor within a host protein.

Recent advances in the design and optimization of artificial metalloenzymes

Embedding a catalytically competent transition metal into a protein scaffold affords an artificial metalloenzyme (ArM). Such hybrid catalysts display features that are reminiscent of both homogeneous and enzymatic catalysts. Pioneered by Whitesides and Kaiser in the late 1970s, this field of ArMs has expanded over the past two decades, marked by ever-increasing diversity in reaction types, cofactors, and protein scaffolds. Recent noteworthy developments include i) the use of earth-abundant metal cofactors, ii) concurrent cascade reactions, iii) synergistic catalysis, and iv) in vivo catalysis. Thanks to significant progress in computational protein design, ArMs based on de novo-designed proteins and tailored chimeric proteins promise a bright future for this exciting field.

Carbamoylphosphinidene: A Phosphorus Analogue of Carbonyl Nitrene

Carbonyl nitrenes are versatile intermediates that have been extensively characterized; however, their phosphorus analogues remain largely unknown. Herein, we report the observation of a rare example of carbonyl phosphinidene NH2C(O)P, which was generated through the photolytic (193 nm) dehydrogenation of phosphinecarboxamide (NH2C(O)PH2) in a solid N2-matrix at 12 K. The characterization of NH2C(O)P in the triplet ground state with matrix-isolation IR and ultraviolet-visible (UV-vis) spectroscopy is supported by comprehensive isotope labeling experiments (D and 15N) and quantum chemical calculations. Upon visible-light irradiation at 680 nm, NH2C(O)P inserts into dihydrogen by the reformation of NH2C(O)PH2 with concomitant isomerization to the more stable aminophosphaketene (NH2PCO). Additionally, the photoisomerization of NH2C(O)PH2 to NH2C(OH) = PH along with decomposition by yielding hydrogen-bonded complexes HNCO···PH3 and HPCO···NH3 has been observed in the matrix.

Chiral Osmium(II)/Salox Species Enabled Enantioselective γ-C(sp3)-H Amidation: Integrated Experimental and Computational Validation For the Ligand Design and Reaction Development

Herein, multiple types of chiral Os(II) complexes have been designed to address the appealing yet challenging asymmetric C(sp3)-H functionalization, among which the Os(II)/Salox species is found to be the most efficient for precise stereocontrol in realizing the asymmetric C(sp3)-H amidation. As exemplified by the enantioenriched pyrrolidinone synthesis, such tailored Os(II)/Salox catalyst efficiently enables an intramolecular site-/enantioselective C(sp3)-H amidation in the γ-position of dioxazolone substrates, in which benzyl, propargyl and allyl groups bearing various substituted forms are well compatible, affording the corresponding chiral γ-lactam products with good er values (up to 99 : 1) and diverse functionality (>35 examples). The unique performance advantage of the developed chiral Os(II)/Salox system in terms of the catalytic energy profile and the chiral induction has been further clarified by integrated experimental and computational studies.

Enzymatic Assembly of Diverse Lactone Structures: An Intramolecular C-H Functionalization Strategy

Lactones are cyclic esters with extensive applications in materials science, medicinal chemistry, and the food and perfume industries. Nature's strategy for the synthesis of many lactones found in natural products always relies on a single type of retrosynthetic strategy, a C-O bond disconnection. Here, we describe a set of laboratory-engineered enzymes that use a new-to-nature C-C bond-forming strategy to assemble diverse lactone structures. These engineered "carbene transferases" catalyze intramolecular carbene insertions into benzylic or allylic C-H bonds, which allow for the synthesis of lactones with different ring sizes and ring scaffolds from simple starting materials. Starting from a serine-ligated cytochrome P450 variant previously engineered for other carbene-transfer activities, directed evolution generated a variant P411-LAS-5247, which exhibits a high activity for constructing a five-membered ε-lactone, lactam, and cyclic ketone products (up to 5600 total turnovers (TTN) and >99% enantiomeric excess (ee)). Further engineering led to variants P411-LAS-5249 and P411-LAS-5264, which deliver six-membered δ-lactones and seven-membered ε-lactones, respectively, overcoming the thermodynamically unfavorable ring strain associated with these products compared to the γ-lactones. This new carbene-transfer activity was further extended to the synthesis of complex lactone scaffolds based on fused, bridged, and spiro rings. The enzymatic platform developed here complements natural biosynthetic strategies for lactone assembly and expands the structural diversity of lactones accessible through C-H functionalization.