Selective Cleavage Reactions Research Articles

Synthesis and Deconstruction of Polyethylene-type Materials.

Polyethylene deconstruction to reusable smaller molecules is hindered by the chemical inertness of its hydrocarbon chains. Pyrolysis and related approaches commonly require high temperatures, are energy-intensive, and yield mixtures of multiple classes of compounds. Selective cleavage reactions under mild conditions (<ca. 200 °C) are key to improve the efficacy of chemical recycling and upcycling approaches. These can be enabled by introduction of low densities of predetermined breaking points in the polyethylene chains during the step-growth or chain-growth synthetic construction of designed-for-recycling polyethylene-type materials. Alternatively, they can be accomplished by postpolymerization functionalization of postconsumer polyethylene waste via dehydrogenation and follow-up reactions or through oxidation to long-chain dicarboxylates. Deconstruction of litter under environmental conditions via the aforementioned break points can alleviate plastics' persistency, as a backstop to closed-loop recycling.

Water-Promoted Carbon-Carbon Bond Cleavage Employing a Reusable Fe Single-Atom Catalyst.

The development of methods for selective cleavage reactions of thermodynamically stable C-C/C=C bonds in a green manner is a challenging research field which is largely unexplored. Herein, we present a heterogeneous Fe-N-C catalyst with highly dispersed iron centers that allows for the oxidative C-C/C=C bond cleavage of amines, secondary alcohols, ketones, and olefins in the presence of air (O2 ) and water (H2 O). Mechanistic studies reveal the presence of water to be essential for the performance of the Fe-N-C system, boosting the product yield from <1 % to >90 %. Combined spectroscopic characterizations and control experiments suggest the singlet 1 O2 and hydroxide species generated from O2 and H2 O, respectively, take selectively part in the C-C bond cleavage. The broad applicability (>40 examples) even for complex drugs as well as high activity, selectivity, and durability under comparably mild conditions highlight this unique catalytic system.

Water‐Promoted Carbon‐Carbon Bond Cleavage Employing a Reusable Fe Single‐Atom Catalyst

AbstractThe development of methods for selective cleavage reactions of thermodynamically stable C−C/C=C bonds in a green manner is a challenging research field which is largely unexplored. Herein, we present a heterogeneous Fe−N−C catalyst with highly dispersed iron centers that allows for the oxidative C−C/C=C bond cleavage of amines, secondary alcohols, ketones, and olefins in the presence of air (O2) and water (H2O). Mechanistic studies reveal the presence of water to be essential for the performance of the Fe−N−C system, boosting the product yield from &lt;1 % to &gt;90 %. Combined spectroscopic characterizations and control experiments suggest the singlet 1O2 and hydroxide species generated from O2 and H2O, respectively, take selectively part in the C−C bond cleavage. The broad applicability (&gt;40 examples) even for complex drugs as well as high activity, selectivity, and durability under comparably mild conditions highlight this unique catalytic system.

Substituent effects on the gas‐phase fragmentation reactions of protonated peptides containing benzylamine‐derivatized lysyl residues

Motivated by the need for chemical strategies designed to tune peptide fragmentation to selective cleavage reactions, benzyl ring substituent influence on the relative formation of carbocation elimination (CCE) products from peptides with benzylamine-derivatized lysyl residues has been examined using collision-induced dissociation (CID) tandem mass spectrometry. Unsubstituted benzylamine-derivatized peptides yield a mixture of products derived from amide backbone cleavage and CCE. The latter involves side-chain cleavage of the derivatized lysyl residue to form a benzylic carbocation [C(7)H(7)](+) and an intact peptide product ion [(MH(n))(n+) - (C(7)H(7))(+)]((n-1)+). The CCE pathway is contingent upon protonation of the secondary ε-amino group (N(ε)) of the derivatized lysyl residue. Using the Hammett methodology to evaluate the electronic contributions of benzyl ring substituents on chemical reactivity, a direct correlation was observed between changes in the CCE product ion intensity ratios (relative to backbone fragmentation) and the Hammett substituent constants, σ, of the corresponding substituents. There was no correlation between the substituent-influenced gas-phase proton affinity of N(ε) and the relative ratios of CCE product ions. However, a strong correlation was observed between the π orbital interaction energies (ΔE(int)) of the eliminated benzylic carbocation and the logarithm of the relative ratios, indicating the predominant factor in the CCE pathway is the substituent effect on the level of hyperconjugation and resonance stability of the eliminated benzylic carbocation. This work effectively demonstrates the applicability of σ (and ΔE(int)) as substituent selection parameters for the design of benzyl-based peptide-reactive reagents which tune CCE product formation as desired for specific applications.

Grubbs carbene complex-catalyzed cleavage of allyl vic-diols to aldehydes with a co-oxidant: application to the selective cleavage of huge marine molecules

An oxidative cleavage of allyl vic-diols to aldehydes catalyzed by a Grubbs carbene complex using a co-oxidant has been established for the first time, with good yields. The utility of this selective cleavage reaction was demonstrated through the use of two huge marine molecules, symbiodinolide and N- p-BrBz palytoxin.

Bioactive secondary metabolites from symbiotic marine dinoflagellates: symbiodinolide and durinskiols

Symbiotic relationships play critical roles in marine ecosystems. Among symbionts, marine dinoflagellates have attracted the attention of natural products chemists, biologists, and ecologists, since they are rich sources of unique bioactive secondary metabolites. The polyol compound symbiodinolide, which was isolated from the symbiotic dinoflagellate Symbiodinium sp., exhibits significant voltage-dependent N-type Ca(2+) channel-opening activity and may serve as a defense substance to prevent digestion of the host animals. Durinskiols are also unique long carbon-chain polyol compounds that were isolated from the dinoflagellate Durinskia sp. We found a selective cleavage reaction of allylic 1,2-diol using an olefin metathesis catalyst, and developed a fluorescent-labeling method for MS/MS analysis to achieve the structural elucidation of huge polyol compounds. This review highlights recent advances in structural and biological studies on symbiodinolide, durinskiols, and related polyol compounds.

The competition between Si–Si and Si–C cleavage in functionalised oligosilanes: their reactivity with elemental lithium

The reaction of aryl-substituted disilanes with elemental lithium is a common method for the preparation of lithiosilanes and the subsequent synthesis of functionalised oligosilanes, especially of enantiomerically pure compounds. A series of alkyl- and arylsubstituted di- and trisilanes has been investigated with respect to their reactivity against elemental lithium. Thereby, depending on the substituents, silicon-silicon bond cleavage of the central Si-Si unit and/or silicon-carbon bond cleavage to arenes are observed. Quantum chemical studies provide a deeper insight into the ongoing processes. The reactive centre can be estimated by both, bond elongation after electron transfer and frontier orbital interactions (pi-bonding and sigma-antibonding part). Aromatic substituents at the silicon atoms proved to be necessary for the processing of any cleavage reaction in the studied systems by stabilising the radical anion after electron transfer at the corresponding di- or trisilane. Yet, selective cleavage reactions do not depend on the number of arenes.

Divalent and Multivalent Activation in Phosphate Triesters: A Versatile Method for the Synthesis of Advanced Polyol Synthons.

The construction of mono- and bicyclic phosphate trimesters possessing divalent and multivalent activation and their subsequent use in the production of advanced polyol synthons is presented. The method highlights efforts to employ phosphate tethers as removable, functionally active tethers capable of multipositional activation and their subsequent role as leaving groups in selective cleavage reactions. The development of phosphate tethers represents an integrated platform for a new and versatile tether for natural product synthesis and sheds light on new approaches to the facile construction of small molecules.

A selective Sc(OTf) 3-catalyzed trialkylsilyl ether to acetyl ester exchange reaction with β- l-idopyranoside and 3,4- O-isopropylidene-β- d-galactopyranoside derivatives

The selective silylation of monosaccharide building blocks is useful for preparing complex oligosaccharides. We now report that the diol, methyl (dimethylthexylsilyl 3- O-pivaloyl-β- l-idopyranosyl)uronate, can be selectively silylated at the O-2 position by trialkylsilyl triflates. After protection of O-4, the O-2 silyl group can be selectively replaced by acetate by taking advantage of a trialkylsilyl–acetate exchange reaction catalyzed by Sc(OTf) 3 in the presence of acetic anhydride. The high O-2 selectivity is shown for triethylsilyl (TES), tert-butyldimethylsilyl (TBS), and triisopropylsilyl (TIPS). The selective cleavage reaction only worked well for TES and TBS derivatives. A selection of silyl triflates and silyl chlorides were used as silylating reagents with ethyl 3,4- O-isopropylidene-1-thio-β- d-galactopyranoside. In most cases, silylation afforded 2,6-di- O-silylated products in high yields. Studies on the cleavage reaction showed that only the primary silylated protecting groups were replaced by acetyl groups. This reaction worked with a variety of silyl protecting groups but not the tert-butyldiphenylsilyl (TBDPS) protecting group. Unfortunately, the 1-thioethyl group was also sensitive to the Sc(OTf) 3, leading in these conditions to α/β mixtures of the 1-acetates, which compromised the synthetic utility of this reaction for these compounds. The sequence presented here is a useful synthetic route to differentially protected l-iduronic acid building blocks.

Oxidative removal of 1,3-dithiane protecting groups by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ)

A number of 1,3-dithianes have been efficiently converted into the parent carbonyl compounds in good yields by treatment with 1.5 equiv. of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in MeCN–H2O (9:1). The reactions of 2-aryl-substituted 1,3-dithianes bearing electron-donating groups on the benzene ring with DDQ afforded thioesters along with aldehydes. 1,3-Dithiolanes derived from aromatic aldehydes were transformed to thioesters, whereas 1,3-dithiolanes derived from aliphatic and aromatic ketones were stable under these reaction conditions. Diphenyl dithioacetals were stable except for 4-methoxy- and 3,4-dimethoxy-benzaldehyde diphenyl dithioacetals which gave the corresponding aldehydes. Selective cleavage reactions of 1,3-dithiane in the presence of 1,3-dithiolane or diphenyl dithioacetal have been investigated.

Highly selective cleavage of pyroglutamyl-peptide bond in concentrated hydrochloric acid.

Highly selective cleavage reaction of pGlu-peptide bond is described. Two model peptides, pGlu-X-Ala-Phe-OH (X = Ile and Ser), and dog neuromedin U-8 (d-NMU-8) (1-7)-OH (pGlu-Phe-Leu-Phe-Arg-Pro-Arg-OH) were hydrolyzed in concentrated HCl at 0 degree C for from 6 days to 6 weeks to give the predominant cleavage products of pGlu-X linkage of pGlu-X-Ala-Phe-OH and the pGlu-Phe linkage of d-NMU-8 (1-7)-OH, respectively. The ring-opening reaction of the pyrrolidone moiety of the pGlu residue occurred to a considerably lesser extent.

Preparation and selective cleavage reactions of boron-zirconium 1,1-bimetalloalkanes

Hydrozirconation of various vinyl-9-BBN derivatives with Schwartz's reagent, H(Cl)ZrCp 2, provides 1,1-dimetalloalkanes of boron and zirconium. By selective cleavage of carbon-zirconium bond in these 1,1-dimetallics with bromine, a convenient method for preparation of α-bromoboranes was achieved.

Platinum-catalyzed oxidation of aldopentoses to aldaric acids

The liquid-phase oxidation of xylose, arabinose, lyxose and ribose using platinum-on-alumina catalysts is described. The aldopentoses have equal rates of oxidation to the corresponding aldonic acids, whereas the rates of the consecutive oxidation to the aldaric acids follow the order xylo, arabino > lyxo, ribo. The differences are attributed to leaching of active platinum sites from the catalyst by lyxose and ribose, which provide a favourable coordination site for metal ions. This is supported by the oxidation rates of the corresponding pentitols and the pentonic acids. Analysis of the side-products provided insight into selective cleavage reactions accompanying the oxidation.

Quantitation of prenylcysteines by a selective cleavage reaction.

The allylic thioether bond of the prenylcysteines of prenylated proteins has been shown to be cleaved by 2-naphthol under alkaline conditions to yield substituted naphthopyrans. These products are readily resolved from interfering materials by HPLC and have a strongly absorbing chromophore. Thus, this reaction is suitable for quantitative analysis of prenyl substituents of proteins, and we have examined a number of tissues for their content of prenylcysteines. These amino acids are present in mammalian tissues at a concentration of 0.36-1.4 nmol/mg of protein, with a ratio of geranylgeranylcysteine to farnesylcysteine in the range of 4 to 10. Prenylcysteines were also found in the cytosolic fraction of two mouse tissues at about one-third the concentration of the whole organ. The level of these modified amino acids was found to be significantly less in a yeast, a fungus, a brown alga, a higher plant, and an insect. Again, geranylgeranylcysteine is predominant. Prenylcysteines were absent from Escherichia coli but present in an archaebacterium. The prenylcysteine content of mammalian tissue is about 1% of that of cholesterol and about equal to that of ubiquinones and dolichols. Calculations indicate that about 0.5% of all proteins are prenylated.

Thyroid Hormone Residues Are Released from Thyroglobulin with Only Limited Alteration of the Thyroglobulin Structure

We have tried to characterize thyroglobulin (Tg) degradation products in purified pig thyroid lysosomes to determine whether the release of thyroid hormone residues from Tg involves a random proteolytic attack or discrete and selective cleavage reactions. The intralysosomal soluble protein fraction was prepared by osmotic pressure-dependent lysis of lysosomes purified by isopycnic centrifugation on Percoll gradients. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed the presence of a fraction of Tg (5-10% of total lysosomal protein) with the same molecular weight as that of the intact Tg subunit. This high molecular weight Tg was the only intralysosomal species detected by Western blot using antipig Tg antibodies. In nondenaturing conditions, lysosomal Tg (LTg) identified by radioimmunoassay was in the form of a dimer with a sedimentation coefficient lower than that of either iodinated Tg (colloid Tg) or noniodinated Tg (microsomal Tg). LTg had a lower iodine content than colloid Tg:9-12 versus 39-42 iodine atoms/molecule. Pronase hydrolysates of LTg did not contain any 3,5,3',5'-tetraiodo-L-thyronine or 3,3',5-triiodo-L-thyronine residues detectable by reverse-phase high pressure liquid chromatography; iodine present in LTg was in the form of iodotyrosines. Under reducing conditions, LTg almost completely disappeared and gave rise to various polypeptides of smaller size. These results suggest that Tg transferred to lysosomes is subjected to selective proteolytic cleavage reaction(s) that release thyroid hormone residues. This early step would lead to the formation of hormone-depleted Tg molecules that are cleaved at discrete sites, the resulting polypeptides remaining bound through disulfide bonds to yield Tg molecules with an apparently normal size and a slightly altered structure.

Synthesis and Reactivity of Unsaturated Bisphosphonium Salts

Abstract The synthesis of 1, 2 vinylene bisphosphonium salts 1 has now been enlarged to the vinylogous 1, 4-butadienylene bisphosphonium salts 2. The salts 2, a new class of unsaturated disalts, have also been prepared through a two-step isomerisation of acetylenic salts. New aspects of the reactivity of salts 1 and the comparative study of salts 2 (selective cleavage reactions of P-C bonds and reactions with nucleophiles having a mobile hydrogen) are described. This reactivity allows the preparation of new series of phosphonium salts substituted by heteroatomic groups.