CO Extrusion Research Articles

Bioorthogonal Cycloadditions with Sub-Millisecond Intermediates.

Tetrazine- and sydnone-based click reactions have emerged as important bioconjugation strategies with fast kinetics and N2 or CO2 as the only byproduct. Mechanistic studies of these reactions have focused on the initial rate-determining cycloaddition steps. The subsequent N2 or CO2 release from the bicyclic intermediates has been approached mainly through computational studies, which have predicted lifetimes of femtoseconds. In the present study, bioorthogonal cycloadditions involving N2 or CO2 extrusion have been examined experimentally at the single-molecule level by using a protein nanoreactor. At the resolution of this approach, the reactions appeared to occur in a single step, which places an upper limit on the lifetimes of the intermediates of about 80 μs, which is consistent with the computational work.

Bioorthogonal Cycloadditions with Sub‐Millisecond Intermediates

AbstractTetrazine‐ and sydnone‐based click reactions have emerged as important bioconjugation strategies with fast kinetics and N2 or CO2 as the only byproduct. Mechanistic studies of these reactions have focused on the initial rate‐determining cycloaddition steps. The subsequent N2 or CO2 release from the bicyclic intermediates has been approached mainly through computational studies, which have predicted lifetimes of femtoseconds. In the present study, bioorthogonal cycloadditions involving N2 or CO2 extrusion have been examined experimentally at the single‐molecule level by using a protein nanoreactor. At the resolution of this approach, the reactions appeared to occur in a single step, which places an upper limit on the lifetimes of the intermediates of about 80 μs, which is consistent with the computational work.

Decarbonylative cross-coupling of amides.

Cross-coupling reactions are among the most powerful C-C and C-X bond forming tools in organic chemistry. Traditionally, cross-coupling methods rely on the use of aryl halides or pseudohalides as electrophiles. In the past three years, decarbonylative cross-couplings of amides have emerged as an attractive method for the construction of a wide variety of carbon-carbon and carbon-heteroatom bonds, allowing for the synthetically-valuable functional group inter-conversion of the amide bond. These previously elusive reactions hinge upon selective activation of the N-C(O) acyl amide bond, followed by CO extrusion, in a formal double N-C/C-C bond activation, to generate a versatile aryl-metal intermediate as an attractive alternative to traditional cross-couplings of aryl halides and pseudohalides. In this perspective review, we present recent advances and key developments in the field of decarbonylative cross-coupling reactions of amides as well as discuss future challenges and potential applications for this exciting field.

Rearomatization of trifluoromethyl sulfonyl dihydropyridines: Thermolysis vs photolysis

AbstractIn this article, we describe the static gas‐phase pyrolysis, microwave‐induced pyrolysis, and photolysis reactions of trifluoromethyl sulfonyl dihydropyridines. The goal of this work was to find a methodology that allows obtaining of substituted pyridines—which are known to be difficult to synthesize—to be reused in a new substitution reaction. We demonstrated that it is possible to achieve the rearomatization process by the elimination of the trifluoromethyl sulfonyl moiety through the 3 processes, with the static pyrolysis being the best method to obtain the substituted pyridines. In addition, we propose the 1,4‐elimination (CF3SO2 + H) as the first step, since it is the less energetic process, as has also been corroborated by calculations. A competitive reaction (CO2 extrusion) also occurs, yielding undesired products.

Following the Molecular Mechanism of Decarbonylation of Unsaturated Cyclic Ketones Using Bonding Evolution Theory Coupled with NCI Analysis.

The synergetic use of bonding evolution theory (BET) and noncovalent interaction (NCI) analysis allows to obtain new insight into the bond breaking/forming processes and electron redistribution along the reaction path to understand the molecular mechanism of a reaction and recognize regions of strong and weak electron pairing. This viewpoint has been considered for cheletropic extrusion of CO from unsaturated cyclic ketones cyclohepta-3,5-dien-1-one CHD, cyclopent-3-en-1-one CPE, and bicyclo[2.2.1]hept-2-en-7-one BCH by using hybrid functional MPWB1K in conjugation with aug-cc-pVTZ basis set. Decarbonylation of CHD, CPE, and BCH are nonpolar cyclo-elimination reactions that are characterized by the sequence of turning points (TPs) as CHD, 1-11-C[CC]C†C†FFFTSC†C†C†-0:HT + CO; CPE, 1-8-CC[C†C†F†][FF][FF]FTS[C†C†]-0:BD + CO; and BCH, 1-8-CC[C†C†]F[FF]FTS[C†C†]-0:CD + CO. Breaking of C-C bond between the terminal carbon atoms of diene/triene framework and carbon atom of CO fragment starts at a distance of ca. 1.9-2.0 Å in the vicinity of the transition structure where the transition states are not reached yet. NCI analysis explains that the noncovalent interactions between two fragments appeared after the breaking of C-C bonds.

Copper-Mediated SN2' Allyl-Alkyl and Allyl-Boryl Couplings of Vinyl Cyclic Carbonates.

A method for the copper-catalyzed borylmethylation and borylation of vinyl cyclic carbonates through an SN2' mechanism is reported. These singular reactions involve selective SN2' allylic substitutions with concomitant ring opening of the cyclic carbonate and with extrusion of CO2 and formation of a useful hydroxyl functionality in a single step. The stereoselectivity of the homoallylic borylation and allylic borylation processes can be controlled, and synthetically useful unsaturated (E)-pent-2-ene-1,5-diols and (E)-but-2-ene-1,4-diols are accessed.

Substituent effects in the decarboxylation reactions of coordinated arylcarboxylates in dinuclear copper complexes, [(napy)Cu2(O2CC6H4X)]+ †.

A combination of gas-phase ion trap mass spectrometry experiments and density functional theory (DFT) calculations have been used to examine the role of substituents on the decarboxylation of 25 different coordinated aromatic carboxylates in binuclear complexes, [(napy)Cu2(O2CC6H4X)]+, where napy is the ligand 1,8-naphthyridine (molecular formula, C8H6N2) and X = H and the ortho ( o), meta ( m) and para ( p) isomers of F, Br, CN, NO2, CF3, OAc, Me and MeO. Two competing unimolecular reaction pathways were found: decarboxylation to give the organometallic cation [(napy)Cu2(C6H4X)]+ or loss of the neutral copper benzoate to yield [(napy)Cu]+. The substituents on the aryl group influence the branching ratios of these product channels, but decarboxylation is always the dominant pathway. Density functional theory calculations reveal that decarboxylation proceeds via two transition states. The first enables a change in the coordination mode of the coordinated benzoate in [(napy)Cu2(O2CC6H4X)]+ from the thermodynamically favoured O, O-bridged form to the O-bound form, which is the reactive conformation for the second transition state which involves extrusion of CO2 with concomitant formation of the CO2 coordinated organometallic cation, [(napy)Cu2(C6H4X)(CO2)]+, which then loses CO2 in the final step to yield [(napy)Cu2(C6H4X)]+. In all cases the barrier is highest for the second transition state. The o-substituted benzoates show a lower activation energy than the m-substituted ones, while the p-substituted ones have the highest energy, which is consistent with the experimentally determined normalised collision energy required to induce fragmentation of [(napy)Cu2(O2CC6H4X)]+.

Process optimization for antioxidant enriched sorghum flour and apple pomace based extrudates using liquid CO2 assisted extrusion

Extrusion as a thermal process renders nutritional and bioactive compounds losses in final extruded products. To overcome this issue, in the present study, blend of corn flour and hydrodynamic cavitated sorghum flour, apple pomace was extruded in the presence of CO2 to retain total phenolic content and antioxidant activity in extrudates. CO2 extruded products had 12% and 7% more total phenolic content and antioxidant activity, respectively as compared with control extrudates at optimum conditions of 30% apple pomace ratio, 25 g/100 g feed moisture, 97 °C temperature and 100 rpm screw speed. Though expansion ratio decreased after CO2 extrusion, 59% less hardness and 70% more crispness were observed in CO2 extrudates compared with control extrudates. Low water absorption index and higher water solubility index were observed in CO2 extrudates, however extrusion with CO2 decreased in-vitro starch digestibility and total dietary fiber by 3% and 4%, respectively. The color (L*, a*, b*) results proved that the Maillard's reaction rates decreased when CO2 was injected during extrusion process. The high inclusion of apple pomace using CO2 injection produced extrudates with improved antioxidant, textural and functional properties below 100 °C extrusion temperature.

Synthesis of Amidines from Amides Using a Nickel-Catalyzed Decarbonylative Amination through CO Extrusion Intramolecular Recombination Fragment Coupling.

A catalytic synthesis of amidines from amides has been established for the first time. The newly developed CO extrusion recombination process takes advantage of an inexpensive nickel(II) catalyst and provides the corresponding amidines with high efficiency. The intramolecular fragment coupling shows excellent chemoselectivity, starts from readily available amides, and provides a valuable alternative amidine synthesis protocol.

Mechanistic Study for the Photochemical Reactions of d6 M(CO)5(CS) (M = Cr, Mo, and W) Complexes.

The mechanisms of photoextrusion reactions are determined theoretically for the model system of six-coordinated M(CO)5(CS) (M = Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD basis set. Three types of elimination reaction pathways (i.e., path I, path II, and path III for axial CO extrusion, equatorial CO extrusion, and CS ligand extrusion, respectively) are considered in this study. Theoretical findings show that the photoextrusion mechanism for Cr and Mo complexes proceeds as follows: M-S0-Rea + hν → M-S1-FC → M-CI → M-Pro + CO. This study shows that when the reactant, M(CO)5(CS) (M-S0-Rea), is photoirradiated by UV light, it is excited vertically to many low-lying singlet excited states. It then relaxes to the first singlet excited state from the Franck–Condon point (M-S1-FC). After passing through a conical intersection point (M-CI), this species eliminates a CO group to yield a five-coordinated product, M(CO)4(CS) (M-S0-Pro). However, for the W analogue, the photolysis mechanism is represented as W-S0-Rea + hν → W-T1-Min → W-T1-TS → W-T1/S0 → W-S0-Pro + CO. That is to say, when the reactant, W(CO)5(CS) (W-S0-Rea), absorbs UV light, it is excited to its several low-lying excited states by a vertical excitation. This species may then return to an intermediate at the first triplet excited state (W-T1-Min) by means of intersystem crossings or conical intersections. After passing through a triplet transition state (W-T1-TS) and a subsequent intersystem crossing (W-T1/S0), this molecule finally loses a CO ligand to produce a photoproduct at the ground singlet state (W-S0-Pro). In other words, conical intersections and intersystem crossings play a decisive role in these photoextrusion reactions for M(CO)5(CS). Theoretical evidence from a kinetic viewpoint strongly supports the theory that the photolysis of M(CO)5(CS) only produces CO-loss photoproducts rather than the CS-loss photoproduct. Theoretical analysis using the results of this study allows a good interpretation of the available experimental observations.

Main-Group Metallomimetics: Transition Metal-like Photolytic CO Substitution at Boron.

The carbon monoxide adduct of an unhindered and highly reactive CAAC-bound arylborylene, [(CAAC)B(CO)Ar] (CAAC = cyclic (alkyl) (amino)carbene), has been prepared using a transfer reaction from the linear iron borylene complex [(PMe3) (CO)3Fe=BAr]. [(CAAC)B(CO)Ar] is a source of the dicoordinate [(CAAC)ArB:] borylene that can be liberated by selective photolytic CO extrusion and that, although highly reactive, is sufficiently long-lived to react intermolecularly. Through trapping of the borylene generated in this manner, we present, among others, the first metal-free borylene(I) species containing a nitrogen-based donor, as well as a new boron-containing radical.

Application of supercritical carbon dioxide extrusion in food processing technology

Extrusion process is one of the most important innovations of the 20th century applied in many industries. Extrusion is a technology that is increasingly used for the production of various food products, especially snacks and breakfast cereals. Supercritical carbon dioxide (CO2) as a non-toxic, non-flammable and inexpensive, is applied in many processes, including the extrusion technology. Supercritical CO2 extrusion process (SCFX) found its application primarily in the processing and manufacturing plastic, but recently more and more begins to be applied in food production and processing. Scientific researches in this area are based in production of extrudates with improved properties compared to conventional extrusion process without the addition of CO2. A number of applications of SCFX in food processing technology will be reviewed and numerous advantages over the conventional process will be described in this paper.

Syntheses of Combretastatins D-1, D-2, and D-4 via Ring Contraction by Flash Vacuum Pyrolysis.

We report the syntheses of combretastatins D-2 and D-4 as well as a formal synthesis of combretastatin D-1 by a conceptually new route harnessing a ring-contracting flash vacuum pyrolytic extrusion of sulfur dioxide from the respective 16-membered sulfone precursors. Via flash vacuum pyrolysis, even metaparacyclophanes as small and strained as the hitherto unknown oxa[1.5]metaparacyclophane could be prepared as a side product en route to combretastatin D-2 by synchronous extrusion of SO2 and CO2.

Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring-Opening Reactions.

Irradiation of a 1,3-enyne tethered to a 2-pyridone, in the presence of oxygen, leads to formation of a seven-membered ring product, an overall [4+4-1] reaction. This transformation involves two unstable intermediates and a sequence of unusual reactions. An initial [4+4] photocycloaddition of the enyne with the pyridone yields a 1,2,5-cyclooctatriene. Photooxidation of this triene forms a cyclopropanone and subsequent photoextrusion of carbon monoxide gives the observed 1,4-cycloheptadiene product. The first-formed cyclooctatriene and the cyclopropanone could be observed and characterized spectroscopically. The cyclopropanone underwent CO extrusion both photochemically and thermally to give the cycloheptadiene product. Addition of fluoride or acetylide to the most stable cyclopropanone occurred chemoselectively at the two different silicon groups rather than the carbonyl group. The resulting cyclopropanone ring openings gave unsaturated aldehydes.

Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

AbstractIrradiation of a 1,3‐enyne tethered to a 2‐pyridone, in the presence of oxygen, leads to formation of a seven‐membered ring product, an overall [4+4−1] reaction. This transformation involves two unstable intermediates and a sequence of unusual reactions. An initial [4+4] photocycloaddition of the enyne with the pyridone yields a 1,2,5‐cyclooctatriene. Photooxidation of this triene forms a cyclopropanone and subsequent photoextrusion of carbon monoxide gives the observed 1,4‐cycloheptadiene product. The first‐formed cyclooctatriene and the cyclopropanone could be observed and characterized spectroscopically. The cyclopropanone underwent CO extrusion both photochemically and thermally to give the cycloheptadiene product. Addition of fluoride or acetylide to the most stable cyclopropanone occurred chemoselectively at the two different silicon groups rather than the carbonyl group. The resulting cyclopropanone ring openings gave unsaturated aldehydes.

The role of substituents in retro Diels–Alder extrusion of CO2 from 2(H)-pyrone cycloadducts

An experimental and computational investigation is conducted into the role of substituents in retro Diels–Alder extrusion of CO2 from 2-oxa-bicyclo[2.2.2]oct-5-en-3-ones. We provide the first experimental evidence that loss of CO2 from the cycloadducts significantly depends on the nature and position of the substituents. For example, we show that whilst 5-carboethoxy-2-pyrone undergoes a more facile cycloaddition that 3-carboethoxy-2-pyrone, the cycloadduct from the latter pyrone undergoes a more facile loss of CO2 than the cycloadduct from the former pyrone.

Ligand-induced substrate steering and reshaping of [Ag2(H)](+) scaffold for selective CO2 extrusion from formic acid.

Metalloenzymes preorganize the reaction environment to steer substrate(s) along the required reaction coordinate. Here, we show that phosphine ligands selectively facilitate protonation of binuclear silver hydride cations, [LAg2(H)]+ by optimizing the geometry of the active site. This is a key step in the selective, catalysed extrusion of carbon dioxide from formic acid, HO2CH, with important applications (for example, hydrogen storage). Gas-phase ion-molecule reactions, collision-induced dissociation (CID), infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. [Ag2(H)]+ and [Ph3PAg2(H)]+ react with formic acid yielding Lewis adducts, while [(Ph3P)2Ag2(H)]+ is unreactive. Using bis(diphenylphosphino)methane (dppm) reshapes the geometry of the binuclear Ag2(H)+ scaffold, triggering reactivity towards formic acid, to produce [dppmAg2(O2CH)]+ and H2. Decarboxylation of [dppmAg2(O2CH)]+ via CID regenerates [dppmAg2(H)]+. These gas-phase insights inspired variable temperature NMR studies that show CO2 and H2 production at 70 °C from solutions containing dppm, AgBF4, NaO2CH and HO2CH.

Use of thin film of a Co15Ti40N35 alloy for CVD catalytic growth of carbon nanotubes

It is shown that multiwalled carbon nanotubes can be grown on the catalytic surface of a Co–Ti–N alloy with low (~10 at %) cobalt content by the conventional method of chemical deposition from acetylene. Adding nitrogen to the composition of the Co–Ti contributes the formation of the TiN compound and extrusion of Co onto the surface where it makes a catalytic effect for CNT growth. It was found that the tubes begin growth at a temperature of 400°C. It is shown by studies using Raman spectroscopy that the quality of CNT improves with increasing temperature.

Cooperative Reductive Elimination: The Missing Piece in the Oxidative‐Coupling Mechanistic Puzzle

AbstractThe reaction between benzoic acid and methylphenylacetylene to form an isocoumarin is catalyzed by Cp*Rh(OAc)2 in the presence of Cu(OAc)2(H2O) as an oxidant and a leading example of oxidative‐coupling reactions. Its mechanism was elucidated by DFT calculations with the B97D functional. The conventional mechanism, with separate reductive‐elimination and reoxidation steps, was found to yield a naphthalene derivative as the major product by CO2 extrusion, contradicting experimental observations. The experimental result was reproduced by an alternative mechanism with a lower barrier: In this case, the copper acetate oxidant plays a key role in the reductive‐elimination step, which takes place through a transition state containing both rhodium and copper centers. This cooperative reductive‐elimination step would not be accessible with a generic oxidant, which, again, is in agreement with available experimental data.

Cooperative Reductive Elimination: The Missing Piece in the Oxidative-Coupling Mechanistic Puzzle.

The reaction between benzoic acid and methylphenylacetylene to form an isocoumarin is catalyzed by Cp*Rh(OAc)2 in the presence of Cu(OAc)2 (H2 O) as an oxidant and a leading example of oxidative-coupling reactions. Its mechanism was elucidated by DFT calculations with the B97D functional. The conventional mechanism, with separate reductive-elimination and reoxidation steps, was found to yield a naphthalene derivative as the major product by CO2 extrusion, contradicting experimental observations. The experimental result was reproduced by an alternative mechanism with a lower barrier: In this case, the copper acetate oxidant plays a key role in the reductive-elimination step, which takes place through a transition state containing both rhodium and copper centers. This cooperative reductive-elimination step would not be accessible with a generic oxidant, which, again, is in agreement with available experimental data.