Carbene Insertion Research Articles

Rhodium-Catalyzed Si–H Bond Insertion Reactions Using Functionalized Alkynes as Carbene Precursors

Enantioselective transition-metal-catalyzed carbene insertion into Si–H bonds is a promising method for preparing chiral organosilicons; however, all the carbene precursors used to date in this rea...

Predictive Model for the [Rh2(esp)2]-Catalyzed Intermolecular C(sp3)–H Bond Insertion of β-Carbonyl Ester Carbenes: Interplay between Theory and Experiment

Intermolecular C(sp3)–H insertions of β-carbonyl ester dirhodium-carbenes are extremely rare. Toward developing efficient reactions of these carbenes, a model for their insertion into C(sp3)–H bonds is described using density functional theory (DFT) calculations. In this study, the relevant electronic and steric components of β-carbonyl ester dirhodium-carbenes that affect intermolecular C(sp3)–H activation energies are explored, parametrized, and used to construct an intuitive model for predicting propensity for C–H insertion. The resulting insights from the theoretical investigation are actualized through experiments to establish reactivity trends for these species and reaction discovery. On the basis of these integrated computational and experimental efforts, examples of intermolecular C(sp3)–H insertions featuring secondary α-diazo-β-amide esters are reported. The resulting carbenes feature an intramolecular 1,6-hydrogen bond that affords increased stability and enhanced reactivity for C(sp3)–H insertion as compared to other β-carbonyl ester dirhodium-carbenes. The reactivity of these carbenes is also highlighted through (1) an example of a cyclopropanation reaction and (2) the use of a chiral dirhodium catalyst system.

Photodissociative Cross-Linking of Diazirine-Tagged Peptides with DNA Dinucleotides in the Gas Phase.

Non-covalent complexes of DNA dinucleotides dAA, dAT, dGG, dGC, and dCG with diazirine-tagged Cys-Ala-Gln-Lys peptides were generated as singly charged ions in the gas phase. Laser photodissociation at 355nm of the diazirine ring in the gas-phase complexes created carbene intermediates that underwent covalent cross-linking to the dinucleotides. The dinucleotides differed in the cross-linking yields, ranging from 27 to 36% for dAA and dAT up to 90-98% for dGG, dGC, and dCG. Collision-induced dissociation tandem mass spectrometry (CID-MS3) of the cross-linked conjugates revealed that fragmentation occurred chiefly in the dinucleotide moieties, resulting in a loss of a nucleobase and backbone cleavages. The CID-MS3 spectra further revealed that cross-links were primarily formed in the 3'-nucleotides for the dAT, dGC, and dCG combinations. Gas-phase and solution structures of dGG complexes with S-tagged CAQK were investigated by Born-Oppenheimer molecular dynamics (BOMD) and density functional theory calculations. The low free-energy complexes had zwitterionic structures in which the peptide was protonated at the N-terminus and in the Lys residue whereas the carboxyl or dGG phosphate were deprotonated, corresponding to the respective (Cys+, Lys+, COO-)+ and (Cys+, Lys+, phosphate-)+ protomeric types. Both types preferred structures in which the peptide N-terminal cysteine carrying the S-photo-tag was aligned with the 3'-guanine moiety. BOMD trajectories at 310K were analyzed for close contacts of the incipient peptide carbene with the positions in dGG that pointed to frequent contacts with the N-1, NH2, and N-7 atoms of 3'-guanine, in agreement with the cross-linking results. Carbene insertion to the guanine N-1-H and NH2 bonds was calculated by density functional and Møller-Plesset perturbational theory to be 350-380kJmol-1 exothermic. Based on calculations, we proposed a mechanism for the carbene reaction with guanine starting with an exothermic attack at N-7 to form a dipolar intermediate that can close an aziridine ring in another exothermic reaction, forming a stable covalent cross link. Graphical Abstract.

Carbene insertion to N–H bonds of 2-aminothiazole and 2-amino-1,3,4-thiadiazole derivatives catalyzed by iron phthalocyanine

Iron(III) phthalocyaninate decorated with crown ether substituents, [(15C5)4PcFe]Cl, efficiently catalyzed the insertion of carbene derived from ethyl diazoacetate to six amines functionalized with thiazole, thiazoline and thiadiazole heterocycles. The reactions were carried out under practical conditions using EDA:amine stoechiometric ratio with 0.05 mol% catalyst loading. Turnover numbers up to 3360 have been achieved. The aminoacid derivatives bearing heterocyclic moieties were obtained under catalytic conditions for the first time with 36–69% yields in the case of single N–H insertion products and up to 77% in the case of double N–H insertion products.

Preparation of Axially Chiral 2,2'-Biimidazole Ligands through Remote Chirality Delivery and Their Application in Asymmetric Carbene Insertion into N-H of Carbazoles.

Axially chiral biimidazole ligands have been rarely synthesized and studied, in contrast to the significant achievements in the synthesis and application of central chiral imidazole ligands. Herein, a series of novel axially chiral 2,2'-biimidazole ligands were synthesized from the reaction of 2,2'-bis(bromomethyl)-1,1'-binaphthalene and 2,2'-biimidazole in one step through the strategy of remote chirality delivery. These ligands have been proven to be efficient for Cu- or Fe-catalyzed asymmetric insertion of α-aryl-α-diazoacetates into the N-H bond of carbazoles with up to 96% ee.

Artificial Metalloenzymes Based on the Biotin-Streptavidin Technology: Enzymatic Cascades and Directed Evolution.

ConspectusArtificial metalloenzymes (ArMs) result from anchoring a metal-containing moiety within a macromolecular scaffold (protein or oligonucleotide). The resulting hybrid catalyst combines attractive features of both homogeneous catalysts and enzymes. This strategy includes the possibility of optimizing the reaction by both chemical (catalyst design) and genetic means leading to achievement of a novel degree of (enantio)selectivity, broadening of the substrate scope, or increased activity, among others. In the past 20 years, the Ward group has exploited, among others, the biotin–(strept)avidin technology to localize a catalytic moiety within a well-defined protein environment. Streptavidin has proven versatile for the implementation of ArMs as it offers the following features: (i) it is an extremely robust protein scaffold, amenable to extensive genetic manipulation and mishandling, (ii) it can be expressed in E. coli to very high titers (up to >8 g·L–1 in fed-batch cultures), and (iii) the cavity surrounding the biotinylated cofactor is commensurate with the size of a typical metal-catalyzed transition state. Relying on a chemogenetic optimization strategy, varying the orientation and the nature of the biotinylated cofactor within genetically engineered streptavidin, 12 reactions have been reported by the Ward group thus far. Recent efforts within our group have focused on extending the ArM technology to create complex systems for integration into biological cascade reactions and in vivo.With the long-term goal of complementing in vivo natural enzymes with ArMs, we summarize herein three complementary research lines: (i) With the aim of mimicking complex cross-regulation mechanisms prevalent in metabolism, we have engineered enzyme cascades, including cross-regulated reactions, that rely on ArMs. These efforts highlight the remarkable (bio)compatibility and complementarity of ArMs with natural enzymes. (ii) Additionally, multiple-turnover catalysis in the cytoplasm of aerobic organisms was achieved with ArMs that are compatible with a glutathione-rich environment. This feat is demonstrated in HEK-293T cells that are engineered with a gene switch that is upregulated by an ArM equipped with a cell-penetrating module. (iii) Finally, ArMs offer the fascinating prospect of “endowing organometallic chemistry with a genetic memory.” With this goal in mind, we have identified E. coli’s periplasmic space and surface display to compartmentalize an ArM, while maintaining the critical phenotype–genotype linkage. This strategy offers a straightforward means to optimize by directed evolution the catalytic performance of ArMs. Five reactions have been optimized following these compartmentalization strategies: ruthenium-catalyzed olefin metathesis, ruthenium-catalyzed deallylation, iridium-catalyzed transfer hydrogenation, dirhodium-catalyzed cyclopropanation and carbene insertion in C–H bonds. Importantly, >100 turnovers were achieved with ArMs in E. coli whole cells, highlighting the multiple turnover catalytic nature of these systems.

Tandem Remote Csp3–H Activation/Csp3–Csp3 Cleavage in Unstrained Aliphatic Chains Assisted by Palladium(II)

We report here a proof-of-concept for the cleavage of unstrained remote Csp3–Csp3 bonds at room temperature assisted by a directing group, opening up new possibilities to use aliphatic carboxylic acids as suitable alkenyl coupling partners. This strategy involves the Pd-mediated Csp3–H activation directed by a tethered 8-aminoquinoline group, followed by a concerted asynchronous carbene insertion into the Pd–C bond, and an unexpected β-carbon–carbon bond splitting. The insertion of a coupling partner into a Pd–C bond is a novel route to promote C–C bond cleavage, which in contrast to most common methodologies does not rely on the use of strained carbocycles.

Regio- and Enantioselective Intramolecular Amide Carbene Insertion into Primary C-H Bonds Using Ru(II)-Pheox Catalyst.

We have established a method for the highly regio- and enantioselective functionalization of tert-butyl groups via intramolecular amide carbene insertion into C-H bonds, yielding γ-lactams with 91% ee in up to 99% yield. This reaction uses a ruthenium(II) phenyl oxazoline (Ru(II)-Pheox) complex. The catalytic intramolecular carbene transfer reaction to the primary C-H bond proceeds rapidly and selectively compared to that with secondary C-H, benzylic secondary C-H, tert-C-H, or sp2C-H bonds in the presence of 1 mol % Ru(II)-Pheox catalyst. This is the first example of a catalytic carbenoid insertion into an unactivated tert-butyl group with enantiocontrol at the carbenoid carbon.

Noble−Metal Substitution in Hemoproteins: An Emerging Strategy for Abiological Catalysis

Enzymes have evolved to catalyze a range of biochemical transformations with high efficiencies and unparalleled selectivities, including stereoselectivities, regioselectivities, chemoselectivities, and substrate selectivities, while typically operating under mild aqueous conditions. These properties have motivated extensive research to identify or create enzymes with reactivity that complements or even surpasses the reactivity of small-molecule catalysts for chemical reactions. One of the limitations preventing the wider use of enzymes in chemical synthesis, however, is the narrow range of bond constructions catalyzed by native enzymes. One strategy to overcome this limitation is to create artificial metalloenzymes (ArMs) that combine the molecular recognition of nature with the reactivity discovered by chemists. This Account describes a new approach for generating ArMs by the formal replacement of the natural iron found in the porphyrin IX (PIX) of hemoproteins with noble metals. Analytical techniques coupled with studies of chemical reactivity have demonstrated that expression of apomyoglobins and apocytochrome P450s (for which "apo-" denotes the cofactor-free protein) followed by reconstitution with metal-PIX cofactors in vitro creates proteins with little perturbation of the native structure, suggesting that the cofactors likely reside within the native active site. By means of this metal substitution strategy, a large number of ArMs have been constructed that contain varying metalloporphyrins and mutations of the protein. The studies discussed in this Account encompass the use of ArMs containing noble metals to catalyze a range of abiological transformations with high chemoselectivity, enantioselectivity, diastereoselectivity, and regioselectivity. These transformations include intramolecular and intermolecular insertion of carbenes into C-H, N-H, and S-H bonds, cyclopropanation of vinylarenes and of internal and nonconjugated alkenes, and intramolecular insertions of nitrenes into C-H bonds. The rates of intramolecular insertions into C-H bonds catalyzed by thermophilic P450 enzymes reconstituted with an Ir(Me)-PIX cofactor are now comparable to the rates of reactions catalyzed by native enzymes and, to date, 1000 times greater than those of any previously reported ArM. This reactivity also encompasses the selective intermolecular insertion of the carbene from ethyl diazoacetate into C-H bonds over dimerization of the carbene to form alkenes, a class of carbene insertion or selectivity not reported to occur with small-molecule catalysts. These combined results highlight the potential of well-designed ArMs to catalyze abiological transformations that have been challenging to achieve with any type of catalyst. The metal substitution strategy described herein should complement the reactivity of native enzymes and expand the scope of enzyme-catalyzed reactions.

Reactions of a BICAAC with hydroboranes: propensity for Lewis adduct formation and carbene insertion into the B-H bond.

The reactivity of a bicyclic (alkyl)(amino)carbene (BICAAC) towards different boranes has been examined in the present work. The reactions with boranes BX3·SMe2 (X = H, Cl, Br), BF3·OEt2 and BCl3 yield Lewis adducts [BICAAC·BH3] (1), [BICAAC·BHCl2] (2), [BICAAC·BH2Cl] (3), [BICAAC·BF3] (4), [BICAAC·BCl3] (5) and [BICAAC·BBr3] (6) respectively, whereas more hydridic boranes, 9-borabicyclo[3.3.1]nonane (9-BBN) and catecholborane (HBcat), enable the insertion of the carbene carbon into the B-H bond to form [BICAAC(H)-(9-BBN)] (7) and [BICAAC(H)-Bcat] (8). These complexes are the first examples of BICAAC-boron compounds and have been characterized using IR, multinuclear NMR spectroscopy, HRMS spectrometry and single crystal X-ray diffraction. Computational analyses were also performed to gain insight into the mechanism of B-H bond activation and adduct formation. Furthermore, the reactions of the BICAAC with boranes have been compared with the known reactions of CAACs and NHCs.

Cascade Rh(ii) and Yb(iii) catalyzed synthesis of substituted naphthofurans via transannulation of N-sulfonyl-1,2,3-triazoles with β-naphthols.

An efficient cascade Rh(ii) and Yb(iii) catalyzed transannulation of N-sulfonyl-1,2,3-triazoles with β-naphthols has been accomplished for the synthesis of substituted naphthofurans in good yields. The reaction involves the initial rhodium catalyzed insertion of azavinyl carbene into the O-H bond followed by ytterbium catalyzed annulative C-C bond formation and concomitant aerial oxidation. The developed reaction was successfully extended to phenols to afford substituted benzofurans. Furthermore, the synthetic utility of the method was demonstrated through the synthesis of important polyhetero aromatic compounds.

Successive carbene insertion into 9-phenyl-9-borafluorene.

The reactions of 9-phenyl-9-borafluorene with trimethylsilyldiazomethane in a 1 : 1 and 1 : 2 stoichiometry furnished the corresponding BC5 and BC6 heterocycles via the formal insertion of one and two carbene units.

Enantioselective carbene insertion into the N-H bond of benzophenone imine.

Efficient enantioselective insertion of α-diazoesters into the N-H bond of N-sp2-hybridized benzophenone imine was realized by using Rh2(esp)2 and chiral guanidine cooperative catalysis. Both aliphatic and aromatic substituted α-amino esters were obtained in high yields (up to 99%) and good enantioselectivities (up to 95.5 : 4.5 er) under mild reaction conditions.

Ag NP-Loaded Cotton Fiber Materials: Preparation, Surface Deposition, and Antibacterial Activity with Different Chemical Structures.

An amino-substituted bis(aryldiazomethane) was synthesized and used to modify cotton fibers by a carbene insertion reaction. A subsequent amidation of the modified cotton fibers introduced thioether groups onto the surface. Thus, three different types of cotton fibers with hydroxyl, amine, or thioether groups were expected to bind silver nanoparticles (Ag NPs), which were characterized by scanning electron microscopy to show the surface morphology, as well as X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and thermogravimetric analysis to quantify the loading of Ag NPs. A bioassay experiment showed antibacterial activity of the modified cotton fibers. The development of these antibacterial cotton fibers not only gave a clear and deep comparison of different silver-binding approaches but also had potential industrial and daily applications for medical textiles.

Reversible Carbene Insertion into a Ge-N Bond and Insights into CO and Carbene Substitution Reactions Involving Amidinatogermylenes and Fischer Carbene Complexes.

The formation of the five-membered-ring germylene complexes [M(CO)5 {Ge(tBu2 bzamC(OEt)Me)tBu}] (3M ; M=Cr, W), which occurs readily at room temperature from the germylene Ge(tBu2 bzam)tBu (1tBu ) and Fischer carbenes [M(CO)5 {C(OEt)Me}] (2M ; M=Cr, W), has been found to be reversible. Upon heating at 60 °C, complexes 3M undergo epimerization to an equilibrium mixture of 3M and 3'M . At that temperature, the chromium epimers (but not the tungsten ones) release CO to end in the mixed germylene-Fischer carbene complexes [Cr(CO)4 {C(OEt)Me}{Ge(tBu2 bzam)tBu}] (cis-4Cr and trans-4Cr ). The latter decompose at 120 °C to [Cr(CO)5 {Ge(tBu2 bzam)tBu}] (6Cr ). Because the formation of cis-4Cr and trans-4Cr from 3Cr or 3'Cr requires the presence of free 1tBu and 2Cr in the reaction solutions, the reactions of 1tBu with 2M to give 3M (and 3'M at 60 °C) should be reversible. This proposal has been proven by germylene-exchange crossover reactions in which free 1tBu and [M(CO)5 {Ge(tBu2 bzamC(OEt)Me)CH2 SiMe3 }] (5'M ; M=Cr, W) were formed when complexes 3M were treated at room temperature with the germylene Ge(tBu2 bzam)CH2 SiMe3 (1tmsm ). A clear differential behavior between N-heterocyclic carbenes (NHCs) and amidinatogermylenes (1tBu and 1tmsm ) in their reactivity against group 6 metal Fischer carbene complexes is demonstrated. The higher electron-donor capacity of amidinatogermylenes with respect to NHCs and the bias of the former to get involved in ring expansion processes are responsible for this differential behavior.

Pd-Catalyzed Three-Component Domino Reaction of Vinyl Benzoxazinanones for Regioselective and Stereoselective Synthesis of Allylic Sulfone-Containing Amino Acid Derivatives.

A Pd-catalyzed, highly regioselective and stereoselective three-component domino allylic substitution/N-H carbene insertion reaction under mild conditions is described. This reaction demonstrates a wide substrate scope and satisfactory functional group tolerance, providing a broad range of allylic sulfone-containing amino acid derivatives. Moreover, DBU mediates highly diastereoselective cross-dehydrogenative coupling annulation of allylic sulfones without using peroxides or any metal oxidants. This developed protocol affords 7-membered ring heterocyclic compounds incorporating both sulfone-containing amino acid esters and one quaternary carbon center. Mechanistic studies indicate that an unusual umpolung of glycine occurred in this annulation.

Desymmetrization of cyclohexanes by site- and stereoselective C-H functionalization.

Carbon-hydrogen (C-H) bonds have long been considered unreactive and are inert to traditional chemical reagents, yet new methods for the transformation of these bonds are continually being developed1-9. However, it is challenging to achieve such transformations in a highly selective manner, especially if the C-H bonds are unactivated10 or not adjacent to a directing group11-13. Catalyst-controlled site-selectivity-in which the inherent reactivities of the substrates14 can be overcome by choosing an appropriate catalyst-is an appealing concept, and substantial effort has been made towards catalyst-controlled C-H functionalization6,15-17, in particular methylene C-H bond functionalization. However, although several new methods have targeted these bonds in cyclic alkanes, the selectivity has been relatively poor18-20. Here we illustrate an additional level of sophistication in catalyst-controlled C-H functionalization, whereby unactivated cyclohexane derivatives can be desymmetrized in a highly site- and stereoselective manner through donor/acceptor carbene insertion. These studies demonstrate the potential of catalyst-controlled site-selectivity to govern which C-H bond will react, which could enable new strategies for the production of fine chemicals.

Photochromic Materials by Postpolymerisation Surface Modification.

Photochromic materials are available by a postpolymerization surface modification of diverse polymers in a multistep sequential process mediated, first, by carbene insertion chemistry, second, by diazonium coupling with a tethered precursor, and finally by coupling to a spiropyran. This three-step sequence is efficient, and surface loading densities of 1013 molecules cm–2 are typically achievable, leading to materials with observable photochromic and wettability behavior, which operate over multiple cycles without significant photobleaching or loss of efficacy. Materials suitable for application in this process include both reactive, but also lower surface energy polymers. Although the process is particularly efficient for high surface area materials, surface modification onto lower surface area substrates, while being intrinsically less efficient, is nonetheless sufficiently effective that changes in macroscopic photochromic properties are readily observable.

Probing arginine-phosphopeptide interactions in non-covalent peptide-peptide ion complexes using gas-phase cross-linking and Born-Oppenheimer molecular dynamics calculations

We report a study of non-covalent complexes of phosphopeptides pXAAAA and N-Ac-pXAAAA (X = Ser, Thr, Tyr) with arginine-containing peptides carrying diazirine 4,4-azipentyl tags at the N-terminus, *LGG(A)nR, or in the photoleucine residue, L*GG(A)nR (n = 3–5). Complexes with *LGG(A)nR were successfully generated as singly charged ions in the gas phase in 0.6–3.5% yields. In contrast, complexes with L*GG(A)nR were formed in negligible (<0.1%) yields. Selective photodissociation at 355 nm of the diazirine ring in [*LGG(A)nR + pXAAAA + H]+ ions resulted in loss of N2, producing large (75–95%) fractions of non-dissociating complexes that were further probed by collision-induced dissociation tandem mass spectrometry (CID-MS3). Covalent cross-links in the complexes produced by photoinduced carbene insertion were identified by specific CID-MS3 dissociations involving H3PO4 transfer and backbone fragmentations. N-acetylation in the phosphopeptides was found to have a substantial negative effect on the formation of covalent cross-links. Amongst the phosphorylated amino acid residues, pTyr showed the highest tendency (up to 92%) to form covalent cross-links. The fractions of covalent cross-links substantially increased with the length of the photopeptide chain. Born-Oppenheimer molecular dynamics (BOMD) calculations of canonical and zwitterionic protomers of the (pYAAAA + *LGGAAAR + H)+ complex indicated multiple close contacts between the incipient carbene of the diazirine ring and the XH bonds (XC, N, O) in the phosphopeptide. BOMD in combination with structural analysis by density functional theory calculations were used to interpret the experimental data and explain the cross-linking efficiencies.

Ru(II)/Ir(III)-Catalyzed C-H Bond Activation/Annulation of Cyclic Amides with 1,3-Diketone-2-diazo Compounds: Facile Access to 8H-Isoquinolino[1,2-b]quinazolin-8-ones and Phthalazino[2,3-a]cinnoline-8,13-diones.

Efficient access to 8H-isoquinolino[1,2-b]quinazolin-8-ones and phthalazino[2,3-a]cinnoline-8,13-diones through cyclic amide-directed Ru(II)/Ir(III)-catalyzed C–H bond activation, has been developed. Consecutive C–H bond activation, carbene insertion, and condensation annulation processes were realized, affording 8H-isoquinolino[1,2-b]quinazolin-8-one and phthalazino[2,3-a]cinnoline-8,13-dione derivatives in good-to-excellent yields under mild conditions, with H2O and N2 being generated as the only byproducts.