Theoretical study of structural patterns in CH<Subscript>2</Subscript>OP<Subscript>2</Subscript> isomers

DFT calculations have been performed on the derivatives of formula CH₂OP₂ to determine their total energy, the relative energy between the isomers and their geometry. Among compounds with a P-C-P linkage, the most stable one is the 2-hydroxy-1,2-diphosphirene II.1, a three-membered heterocycle with a P=C unsaturation. The phosphavinylidene(oxo)phosphorane HP=C=P(O)H IV.5 (which has the same skeleton as the experimentally obtained Mes*P=C=P(O)Mes*) lies 36.30 kcal mol^-1 above it. The least stable compounds are carbenes; the singlet carbenes are more stable than the triplet ones.     

On the electron delocalization in cyclopropenylidenes - An experimental charge-density approach

The extent and nature of cyclic electron delocalization in free and coordinated cyclopropenylidene carbenes has been analyzed by combined experimental and theoretical charge-density studies. The significant asymmetry of the C-C bond lengths in substituted cyclopropenylidene carbenes was identified as cooperative effect which depends on contributions of both σ- and π-bonding. We show that analyses of (i) the topology of the Laplacian of the electron density distribution and (ii) the out-of-plane atomic quadrupole moments - the charge-density analogues of p_π occupation - allow to distinguish between the influence of σ- and π-electrons on cyclic electron delocalization. These studies hint for pronounced electron localization in the carbene lone pair region which dominates the electronic structure of free cyclopropenylidene carbenes and hinders the establishment of true aromaticity. We further investigated the electron donating/withdrawing ability of cyclopropenylidene ligands relative to N-heterocyclic carbenes. The experimental benchmark systems LCr(CO)₅ (L = 2,3-diphenylcyclopropenylidene and 1,2-dimethylimidazol-2-ylidene) show that the cyclopropenylidene ligand clearly displays the higher π-acceptor capability relative to N-heterocyclic carbenes.     

Depth-dependent analysis of membranes using benzophenone-based phospholipids

Any attempt to probe the membrane hydrophobic core with chemical reagents necessitates the use of reactive intermediates like carbenes and nitrenes, which can insert into C-H bonds. Several photoactivable reagents based on carbenes and nitrenes have been reported. However, the high reactivity of these reagents, often leads to very low insertion yields. We report here a high degree of cross-linking (35-40%) achieved with three benzophenone-based phospholipids and analyze the carbon functionalization data using a multiple Gaussian function. These phospholipids are so designed so as to permit depth-dependent labeling in membranes. Single bilayer vesicles were prepared from these phospholipids and dimyristoylphosphatidylcholine. The cross-linked product was isolated and characterized by mass spectroscopy. The results obtained indicated that the cross-linked product was dominated by dimeric product formed by intermolecular cross-linking. The Gaussian analysis used here provides insight into the relative depths of the probes inside the membrane.     

Synthesis of N-heterocyclic carbene ligands and derived ruthenium olefin metathesis catalysts

We describe the synthesis of commonly used free N-heterocyclic carbenes (NHCs), 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), and of the two corresponding ruthenium-based metathesis complexes. The complex containing IMes was found to be highly efficient in macrocyclizations involving ring-closing metatheses (RCM), whereas the complex featuring the IPr ligand shows excellent activity in both RCM and cross metathesis because of its greater stability. The free carbenes IMes and IPr are isolated in four steps, with an overall yield of ～50%. They are then used to replace a labile phosphine in precatalysts belonging to two families of ruthenium-containing complexes, benzylidene and indenylidene types, respectively. Such complexes are isolated as analytically pure compounds with 77% and 95% yield. The total time for the synthesis of the free NHCs is 56 h, and incorporation in complexes requires an additional 4-5 h.     

Recent advances in heme biocatalysis engineering

Heme enzymes have the potential to be widely used as biocatalysts due to their capability to perform a vast variety of oxidation reactions. In spite of their versatility, the application of heme enzymes was long time-limited for the industry due to their low activity and stability in large scale processes. The identification of novel natural biocatalysts and recent advances in protein engineering have led to new reactions with a high application potential. The latest creation of a serine-ligated mutant of BM3 showed an efficient transfer of reactive carbenes into C═C bonds of olefins reaching total turnover numbers of more than 60,000 and product titers of up to 27 g/L⁻¹. This prominent example shows that heme enzymes are becoming competitive to chemical syntheses while being already advantageous in terms of high yield, regioselectivity, stereoselectivity and environmentally friendly reaction conditions. Advances in reactor concepts and the influencing parameters on reaction performance are also under investigation resulting in improved productivities and increased stability of the heme biocatalytic systems. In this mini review, we briefly present the latest advancements in the field of heme enzymes towards increased reaction scope and applicability.     

The inter-relationship between triplet energies and spin chemistry.

Triplet energies play a considerable role in optical spectroscopy, and can be determined from phosphorescence or the quenching thereof. Their role in spin chemistry may not be as obvious, but the triplet state has always had an important function or utility, namely of reaction intermediates such as radical pairs, their precursors, of carbenes, and of the final products. In situ NMR spectroscopy represents a useful tool to explore certain properties of the triplet state, especially in cases with no phosphorescence. The 'phase' of CIDNP resonances, i.e., emission or enhanced absorption, reflects the spin selectivity of electron transfer reactions. In radical ion pairs the spin selectivity is determined by the relation between the change of the standard free enthalpy DeltaG degrees during the electron back transfer and the triplet energies (E(T)) of the products. If triplet recombination is energetically feasible (DeltaG degrees > E(T)), it is typically the more efficient process in agreement with the Marcus theory.     

Pd-N-heterocyclic carbene catalyzed synthesis of piperidine alkene–alkaloids and their anti-cancer evaluation

A facile synthesis of piperidine alkene–alkaloids including natural (+)-Caulophyllumine B in high yields has been developed by Heck cross-coupling reaction catalyzed by simple in situ formed palladium-N-heterocyclic carbenes (Pd-NHCs). Formation of Pd(0) nanoparticles has been noticed during the reaction course. The synthesized piperidine alkene–alkaloids were evaluated for in vitro anti-cancer activity against a panel of human tumor cell lines of lung, breast and ovarian. Several of these piperidine alkene–alkaloids were found to possess highest growth inhibition activity than the standard drug cisplatin and support the concept to modulate drug receptor interaction.     

Therapeutic potential of coumarin bearing metal complexes: Where are we headed?

The successfully application of some metallodrugs such as salvarsan, silver sulfadiazine and cisplatin in modern medicine launched the biological investigation of organometallic and metal-organic complexes. The availability and tunability of various ligands including N-heterocycles, phosphines, N-heterocyclic carbenes present an extended research area to chemists. In recent years, the preparation of the metal complexes of bioactive organic compounds is a new strategy. Coumarin derivatives are one of the classes of compounds used for this purpose, and many complexes of coumarin derivatives were prepared for enhanced biological activity, especially anticancer and antimicrobial. In this paper, we discuss the current situation of this topic.     

Computational comparison of the kinetic stabilities of diamino- and diamidocarbenes in the 1,2-H shift reaction

In this study, we performed several DFT, MP2, and BD(T) calculations on the 1,2-H shift reactions of two diaminocarbenes (1, 2) and a diamidocarbene (3) using the Gaussian 09 program. In Gaussian 09, the BD(T) method keyword requests a Brueckner doubles calculation including a perturbative triples contribution. Although N-heterocyclic carbenes (NHC) are typically known for their exceptional σ-donor abilities, recent studies have indicated that π-interactions also play a role in the bonding between NHCs and transition metals or BX₃ (X = H, OH, NH₂, CH₃, CN, NC, F, Cl, and Br) (Nemcsok et al. Organomet 23:3640–3646, 2004, Esrafili. J Mol Model 18:2003–2011, 2012). In order to study the importance of π-interactions between carbenes and transition metals, Hobbs and co-workers (Hobbs et al. New J Chem 34:1295–1308, 2010) focused on the synthesis of NHCs with reduced-energy lowest unoccupied molecular orbitals. By introducing an oxalamide moiety into the heterocyclic backbone, they found the resulting carbene possessed higher electrophilicity than usual NHCs. According to our results, the N,N'-diamidocarbene should be more stable than the diaminocarbenes with respect to the 1,2-H shift reaction.

A class of imidazolium salts is anti-oxidative and anti-fibrotic in hepatic stellate cells

A class of imidazolium salts (IMSs) is routinely used in organic synthetic chemistry as precursors to generate N-heterocyclic carbenes (NHCs) with catalytic activity. However, their biological properties are largely unknown. The current study investigates the biological activity of a typical NHC precursor DBZIM and its trimer TDBZIM in hepatic stellate cells (HSCs), which is an in vitro model for studying liver fibrosis. The results show that HSCs treated with IMSs have an enhanced GSH/GSSG ratio and a reduced level of reactive oxygen species (ROS), which may consequently contribute to the attenuation in gene expression of fibrogenic molecules such as smooth muscle actin-α (SMAA), transforming growth factor-beta 1 (TGF-β1), procollagen αI(I) and fibronectin. Further, the in vivo experiments demonstrate that DBZIM is an anti-fibrotic agent in a mouse model of liver fibrosis. These findings suggest that the versatile IMSs could be a potential source for developing novel therapeutics to treat liver fibrosis and other fibrogenic disorders caused by oxidative stress and TGF-β1 mal-signalling.     

Radiohalogenation of proteins: an overview of radionuclides, labeling methods, and reagents for conjugate labeling.

Direct labeling of proteins with radionuclides of iodine will continue to be the method of choice to answer questions addressed in many future studies. However, it seems likely that a increasing number of applications of radiohalogenated proteins will require, or benefit from, conjugate labeling. While many radiohalogen conjugates have been studied, a large proportion of them have only undergone preliminary studies to date, leaving a question of their overall utility. Phenolic conjugates give good radioiodination labeling yields, but mixtures of radiohalogenated products and problems with in vivo stability can be expected. This fact, along with the fact that phenolic compounds do not have a general application to radiohalogens, makes them less attractive than other alternatives. Radiohalogen labeling through the use of organometallic intermediates has proven to be facile, resulting in high yields of high specific activity labeled small-molecule conjugates. Although the choice of which organometallic intermediate to use may depend somewhat on the radionuclide employed, arylstannanes appear to have the most general applicability. Fluorine-18 labeling of small-molecule conjugates has been best accomplished by ipso aromatic nucleophilic substitution (exchange) reactions. Radiohalogenated small molecules have been prepared which can be conjugated with specific functional groups (e.g. amines, sulfhydryl groups, and carbohydrates) or conjugated nonspecifically with groups in the proximity of the conjugate when it is photolyzed. On the basis of previous studies, good conjugation yields (i.e. 60-90%) can be expected for reactions with specific groups, whereas low yields (i.e. 1-5%) can be expected for conjugations with reactive nitrenes and carbenes. However, recent developments in the chemistry of conjugates that produce nitrenes and carbenes will likely improve the radiolabeling yields. There have been too few comparative studies to readily assess which is the best approach to take when beginning a study involving radiohalogenation of a protein or peptide. However, it is clear that radiohalogenated conjugates of proteins can offer an advantage over direct labeling in that conjugates may be designed which provide some control over in vivo stability and secondary distribution of metabolites. Conjugates can be prepared which are designed to utilize in vivo biochemical processes to release a radiohalogenated small molecule from a tissue (i.e. kidney or liver) or retain the radioactivity at the target tissue (e.g. tumor). Aside from the designing of conjugates with linking molecules for desired biological effects, the ultimate future goal for the radiolabeling chemical should be to prepare protein conjugates which can be radiohalogenated in a single one-step procedure.     

A theoretical study on the hydrogen adducts of diamidocarbenes and diaminocarbenes

The hybrid-meta GGA DFT functional M06-2X was used to examine the potential of N,N′-diamidocarbenes for use as hydrogen storage materials. We previously discovered that borylene, which is isoelectronic with an Arduengo-type carbene, was a suitable candidate for a hydrogen storage material. We compared the capabilities of N,N′-diamidocarbenes and N-heterocyclic carbenes as hydrogen storage materials. The results indicate that diamidocarbenes are not suitable hydrogen storage materials because the removal of H₂ is more endothermic for diamidocarbenes than for diaminocarbenes.

Identification of dichloromethyl carbene as a metabolite of carbon tetrachloride

Although indirect evidence has suggested that liver microsomal cytochrome P-450 can reductively dehalogenate several compounds to carbene metabolites, there has been no direct proof for the formation of these reactive species. We report in this paper that carbenes can be chemically trapped and identified as metabolites. For example, 1,1-dichloro-2,2,3,3-tetramethylcyclopropane was identified as a metabolite by gas chromatography mass spectrometry when carbon tetrachloride (CCl₄) was incubated anaerobically with rat liver microsomes, NADPH and 2,3-dimethyl-2-butene. The reaction required NADPH and was inhibited by carbon monoxide. These findings show that cytochrome P-450 in rat liver microsomes can reductively metabolize CCl₄ to dichloromethyl carbene (:CCl₂) which can be trapped with 2,3-dimethyl-2-butene to form 1,1-dichloro-2,2,3,3-tetramethylcyclopropane. A similar approach may be used for the identification of carbene metabolites of other compounds.     

Green synthesis of selenium-N-heterocyclic carbene compounds: Evaluation of antimicrobial and anticancer potential

Three benzimidazolium salts (III-V) and respective selenium adducts (VI-VIII) were designed, synthesized and characterized by various analytical techniques (FT-IR and NMR ¹H, ¹³C). Selected salts and respective selenium N-Heterocyclic carbenes (selenium-NHC) adducts were tested in vitro against Cervical Cancer Cell line (Hela), Breast Adenocarcinoma cell line (MCF-7), Retinal Ganglion Cell line (RGC-5) and Mouse Melanoma Cell line (B16F10) using MTT assay and the results were compared with standard drug 5-Fluorouracil. Se-NHC compounds and azolium salts showed significant anticancer potential. Molecular docking studies of compounds (VI, VII and VIII) showed strong binding energies and ligand affinity toward following angiogenic factors: VEGF-A (vascular endothelial growth factor A), EGF (human epidermal growth factor), HIF (Hypoxia-inducible factor) and COX-1 (Cyclooxygenase-1) suggesting that the anticancer activity of adducts (VI, VII and VIII) may be due to their strong anti-angiogenic effect. In addition, compounds III-VIII were screened for their antibacterial and antifungal potential. Adduct VI was found to be potent anti-fungal agent against A. Niger with zone of inhibition (ZI) value 27.01 ± 0.251 mm which is better than standard drug Clotrimazole tested in parallel.     

3-Diazirine-derivatives of bile salts for photoaffinity labeling

New carbene-generating photolabile bile salt derivatives, 3,3-azo-7 alpha,12 alpha-dihydroxy-5 beta [7 beta-3H]cholan-24-oic acid and (3,3-azo-7 alpha,12 alpha-dihydroxy-5 beta [7 beta-3H]cholan-24-oyl)-2- aminoethanesulfonic acid were synthesized with high specific radioactivity. These 3-diazirine-derivatives could be activated to the corresponding carbenes by irradiation with ultraviolet light at 350 nm with a half-life time of 2 min. The 3-diazirine derivatives behaved in enterohepatic circulation like the natural bile salts. The uptake of [3H]taurocholate into isolated hepatocytes was competitively inhibited by (3,3-azo-7 alpha,12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2- aminoethanesulfonic acid indicating that the 3,3-azo-derivative of taurocholate shares the hepatic transport systems for natural bile salts. It was demonstrated that the radioactively labeled 3-diazirine bile salt derivatives are useful probes for photoaffinity labeling of bile salt binding proteins especially in intact cells and tissues.     

Biodehalogenation: reactions of cytochrome P-450 with polyhalomethanes

The products, stoichiometry, and kinetics of the oxidation of the enzyme cytochrome P-450 cam by five polyhalomethanes and chloronitromethane are described. The reactivity of the enzyme is compared with that of deuteroheme and with the enzyme in its native cell, Pseudomonas putida (PpG-786). In all cases, the reaction entails hydrogenolysis of the carbon-halogen bond: 2FeIIP + RCXn----2FeIIIP + RCHXn-1 (P = porphyrin or P-450 cam in vitro and in vivo). Trichloronitromethane was the fastest reacting substrate, and chloroform was the slowest. The results establish that P. putida is a valid whole cell model for the reductase activity of the P-450 complement in these reactions. The reactions of cytochrome P-450 with polyhaloalkanes proceed in a manner quite analogous to other iron(II) proteins in the G conformation. The chemistry observed for the enzyme parallels that of its iron(II) porphyrin active site. Iron-bonded carbenes are not intermediates, and hydrolytically stable iron alkyls are not products of these reactions.     

Photoaffinity labeling of bacteriorhodopsin

14C-Labeled optically pure 3S- and 3R-(diazoacetoxy)-all-trans-retinals were incorporated separately into bacterioopsin to reconstitute functional bacteriorhodopsin (bR) analogues, 3S- and 3R-diazo-bRs. UV irradiation at 254 nm generated highly reactive carbenes, which cross-linked the radiolabeled retinals to amino acid residues in the vicinity of the beta-ionine ring. The 3S- and 3R-diazo analogues were found to cross-link, respectively, to cyanogen bromide fragments CN 7/CN9 and CN 8/CN 9. More specifically, Thr121 and Gly122 in fragment CN 7 were found to be cross-linked to the 3S-diazo analogue. The identification of cross-linked residues and fragments favors assignments of the seven helices A-G-F-E-D-C-B or B-C-D-E-F-G-A to helices 1-2-3-4-5-6-7 in the two-dimensional electron density map (Henderson et al., 1975, 1986; Mogi et al., 1987). The present results show that the chromophore chain is oriented with the ionone ring inclined toward the outside of the membrane (the 9-methyl group also faces the extracellular side of the membrane).     

Ultrafast studies of some diarylcarbenes

The photochemistry of 5-diazo-10,11-dihydrodibenzo[a,d]diazocycloheptene (DDBC) and 9-diazoanthrone (DAN) were studied by ultrafast time resolved techniques. The excited states of these diazo compounds were observed by UV-Vis spectroscopy and were found to decay in 300 fs. The diazo excited state decays led to the appearance and first direct observation of singlet 5-diazo-10,11-dihydrodibenzo[a,d]cycloheptenylidene (DBC) and 9-anthronylidene (AN). The dynamics of DBC and AN were studied in acetonitrile, cyclohexane and methanol. The lifetimes of (1)DBC are 83 ps and 72 ps in acetonitrile and cyclohexane, respectively. The lifetime of (1)DBC shortens to 9 ps in methanol due to rapid reaction with the solvent. The lifetime of (1)AN is 87 ps and 66 ps in acetonitrile and cyclohexane, respectively. In methanol, the lifetime of (1)AN cannot be determined due to spectral overlap of (1)AN and cation ANH+. The decays of (1)DBC and (1)AN are controlled by intersystem crossing (ISC) in acetonitrile and cyclohexane and the rates of ISC of (1)DBC or (1)AN are similar in these two solvents. This differs from the solvent dependence of other diarylcarbene intersystem crossing rates. This is attributed to the relatively large singlet-triplet (S-T) gaps of these carbenes and this factor dominates the influence of solvent.     

Chemical detection of carbon-metal bonds: From distinct sigma-bonding of main-group metals to ambiguous pi-bonding with transition metals

The detection, site of binding and quantification of bonds between carbon and a main-group metal can be readily achieved by a combination of proto- and deuteriodemetallation of such stable organometallics, combined with NMR spectral and mass spectrometric analyses of the organic products. Only with chiral sp³-carbon-metal bonds, with geometrically isomeric sp²-carbon-metal bonds or with allylic carbon-metal bonds will further structural physical data be required to identify the actual 3D-structure of the carbon binding site.The thermal and photochemical lability of carbon bonds to transition metals imposes restrictions on the detection of C−M_t bonds by deuteriodemetallation. Often the homolytic rupture of such bonds competes greatly and will lead to geometric isomers. Results of such apparent protodemetallation should be cross-checked with other structural information.The detection of C=M_t and possibly C≡M_t bonds may prove to be generally achievable by cycloadditions of such metal carbenes or carbynes with nitriles or alkynes.The addition of low-valent transition metal salts to alkenes or alkynes leads to complexes viewable as pi-complexes or as epimetallated adducts. By examination of available structural parameters and the chemical reactions undergone by such adducts, the adducts between ethylene and Ti(OiPr)₂ and between diphenylacetylene and α,α-bipyridyl-nickel(0) can best be considered as titana(IV)cyclopropane and nickel(II)cyclopropene structures, respectively.