Distinction among isomeric unsaturated fatty acids as lithiated adducts by electrospray ionization mass spectrometry using low energy collisionally activated dissociation on a triple stage quadrupole instrument

Features of tandem mass spectra of dilithiated adduct ions of unsaturated fatty acids obtained by electrospray ionization mass spectrometry with low-energy collisionally activated dissociation (CAD) on a triple stage quadrupole instrument are described. These spectra distinguish among isomeric unsaturated fatty acids and permit assignment of double-bond location. Informative fragment ions reflect cleavage of bonds remote from the charge site on the dilithiated carboxylate moiety. The spectra contain radical cations reflecting cleavage of bonds between the first and second and between the second and third carbon atoms in the fatty acid chain. These ions are followed by a closed-shell ion series with members separated by 14 m/z units that reflect cleavage of bonds between the third and fourth and then between subsequent adjacent pairs of carbon atoms. This ion series terminates at the member reflecting cleavage of the carbon-carbon single bond vinylic to the first carbon-carbon double bond. Ions reflecting cleavages of bonds distal to the double bond are rarely observed for monounsaturated fatty acids and are not abundant when they occur. For polyunsaturated fatty acids that contain double bonds separated by a single methylene group, ions reflecting cleavage of carbon-carbon single bonds between double bonds are abundant, but ions reflecting cleavages distal to the final double bond are not. Cleavages between double bonds observed in these spectra can be rationalized by a scheme involving a six-membered transition state and subsequent rearrangement of a bis-allylic hydrogen atom to yield a terminally unsaturated charge-carrying fragment and elimination of a neutral alkene. The location of the beta-hydroxy-alkene moiety in ricinoleic acid can be demonstrated by similar methods. These observations offer the opportunity for laboratories that have tandem quadrupole instruments but do not have instruments with high energy CAD capabilities to assign double bond location in unsaturated free fatty acids by mass spectrometric methods without derivatization.     

Collision Induced Dissociation Products of Disulfide-Bonded Peptides: Ions Result from the Cleavage of More Than One Bond

Disulfide bonds are a post-translational modification (PTM) that can be scrambled or shuffled to non-native bonds during recombinant expression, sample handling, or sample purification. Currently, mapping of disulfide bonds is not easy because of various sample requirements and data analysis difficulties. One step towards facilitating this difficult work is developing a better understanding of how disulfide-bonded peptides fragment during collision induced dissociation (CID). Most automated analysis algorithms function based on the assumption that the preponderance of product ions observed during the dissociation of disulfide-bonded peptides result from the cleavage of just one peptide bond, and in this report we tested that assumption by extensively analyzing the product ions generated when several disulfide-bonded peptides are subjected to CID on a quadrupole time of flight (QTOF) instrument. We found that one of the most common types of product ions generated resulted from two peptide bond cleavages, or a double cleavage. We found that for several of the disulfide-bonded peptides analyzed, the number of double cleavage product ions outnumbered those of single cleavages. The influence of charge state and precursor ion size was investigated, to determine if those parameters dictated the amount of double cleavage product ions formed. It was found in this sample set that no strong correlation existed between the charge state or peptide size and the portion of product ions assigned as double cleavages. These data show that these ions could account for many of the product ions detected in CID data of disulfide bonded peptides. We also showed the utility of double cleavage product ions on a peptide with multiple cysteines present. Double cleavage products were able to fully characterize the bonding pattern of each cysteine where typical single b/y cleavage products could not.     

Dye sensitized photooxygenation of substitute oxime carbamates

Sojvolytic cleavage of oxime carbamates can be achieved by dye sensitized photooxygenation. A study with substituted oxime carbamates has revealed an unusual chemoselectivity of singlet oxygen towards carbon—nitrogen single bonds in preference over carbon—nitrogen double bonds in such compounds.     

Aerobic photo-oxidative cleavage of the C-C double bonds of styrenes

Carbon tetrabromide enables us to carry out oxidative cleavage of the C-C double bonds of styrenes under aerobic photo-irradiation conditions. Oxidative cleavage of the various β-substituted styrenes produced benzoic acid in good yields. Since this reaction is found to be applicable to the α- or β-substituted styrenes, which showed very low reactivity under our previous cleavage reaction condition with FSM-16 and I₂, this reaction can be used complementarily.     

Die β,γ-Spaltung von Carbonylver-Bindungen und derivaten als Allgemeingültige Hauptabbau-Reaktion IM Massenspektrometer

Aliphatic compounds with polarized double bonds (I) suffer in the mass spectrometer preferentially cleavage of C? C bonds in β,γ-position to the functional group. The hetero-atom containing fragment produced in this manner must be formulated with a conjugated double bond (IV). It is the result of a reciprocal hydrogen rearrangement (I → II → III → IV). Deuterium labelling, high resolution and metastable ions indicate that this ion (IV) can be degradated further by McLafferty rearrangement.     

α-Chlor-nitrone III: Ein Weg zur indirekten «carboxolytischen» Spaltung von Olefin-Doppelbindungen. Über synthetische Methoden, 7. (vorläufige) Mitteilung

The enaminoid derivatives formed by deprotonation of the olefin-α-chloro-nitrone cycloadducts (see preceding communications) undergo a beautifully clean cycloreversion at ambient or slightly elevated temperatures. The sequence (2+4)-cycloaddition → deprotonation → (2′+4)-cycloreversion constitutes a potentially useful method for the oxidative cleavage of olefinic double bonds with concomitant extension of the carbon chain at one of the double bond termini (indirect «carboxolytic» cleavage of double bonds; see schemes 1 and 3).     

Ultramicromethods : VIII. Ozonolysis

A procedure for locating the position of double bonds in unsaturated compounds is described. It involves ozonolytic cleavage of the double bonds followed by gas chromatographic identification of the fragments. From 100 to 500 ng of starting material is required, depending on the molecular weight of the fragments expected.     

Thioethers as Dichotomous Electrophiles for Site-Selective Silylation via C−S Bond Cleavage

The development of aryl alkyl sulfides as dichotomous electrophiles for site-selective silylation via C−S bond cleavage has been achieved. Iron-catalyzed selective cleavage of C(aryl)−S bonds can occur in the presence of β-diketimine ligands, and the cleavage of C(alkyl)−S bonds can be achieved by t-BuONa without the use of transition metals, resulting in the corresponding silylated products in moderate to excellent yields. Mechanistic studies suggest that Fe−Si species may undergo metathesis reactions during the cleavage of C(aryl)−S bonds, while silyl radicals are involved during the cleavage of C(alkyl)−S bonds.     

Asymmetric catalytic oxidative cleavage of polycyclic systems: the synthesis of atropisomeric diazonanes and diazecanes

Oxidative cleavage of internal double bonds in polycyclic systems can give access to compounds containing medium- to large-sized rings. In this example, the nine- and ten-membered ring containing compounds that resulted from the mCPBA-mediated (mCPBA=meta-chloroperoxybenzoic acid) oxidative cleavage reaction were shown to exhibit atropisomerism. The reaction of the polycyclic system with catalytic amounts of ruthenium tetraoxide followed by diol cleavage achieved the same synthetic goal. Use of the Nishiyama-Beller ruthenium-based catalysts enabled the synthesis of optically-enriched samples, providing the first example of an atropselective oxidative cleavage reaction.     

Formation of a C–C double bond from two aliphatic carbons. Multiple C–H activations in an iridium pincer complex

The search for novel, atom-economic methods for the formation of C–C bonds is of crucial importance in synthetic chemistry. Especially attractive are reactions where C–C bonds are formed through C–H activation, but the coupling of unactivated, alkane-type C_sp³–H bonds remains an unsolved challenge. Here, we report iridium-mediated intramolecular coupling reactions involving up to four unactivated C_sp³–H bonds to give carbon–carbon double bonds under the extrusion of dihydrogen. The reaction described herein is completely reversible and the direction can be controlled by altering the reaction conditions. With a hydrogen acceptor present a C–C double bond is formed, while reacting under dihydrogen pressure leads to the reverse process, with some of the steps representing net C_sp³–C_sp³ bond cleavage. Mechanistic investigations revealed a conceptually-novel overall reactivity pattern where insertion or deinsertion of an Ir carbene moiety, formed via double C–H activation, into an Ir–C bond is responsible for the key C–C bond formation and cleavage steps.     

A novel direct synthesis of polyol from soybean oil

As the oil crisis and environment concerning deepen,the uses of renewable resources have attracted considerable attention.Polyol intended for synthesis of polyurethane polymers was prepared by a novel direct hydroxylation of soybean oil,alternative to petroleumbased process.The transformation can afford soybean oil polyol in excellent yield with a hydroxyl number up to 467.7 mg KOH/g in the presence of OsO 4 as catalyst and NMO as oxidant.The major advantages of this approach are:two hydroxyl groups can be readily added to one double bond,replacing conventional two-step methods by an epoxidation step and then a ring opening step;a wide range of hydroxyl numbers can be obtained via varying catalyst loadings;the reaction can be performed at room temperature.The chemical structure of the polyol prepared was further characterized by chemical methods(hydroxyl number and iodine number) and spectra(1 H NMR and FTIR spectroscopy),which confirmed the cleavage of the double bonds and the produce of hydroxyl groups.     

Rh(II)-catalyzed skeletal reorganization of enynes involving selective cleavage of C-C triple bonds

Treatment of enynes with a catalytic amount of Rh(II) complex results in skeletal reorganization to give cis-configured 1-vinylcycloalkenes, the formation of which occurs via double cleavage of both C-C double and C-C triple bonds.     

Generation of ketyl radical anions by photoinduced electron transfer (PET) between ketones and amines. Synthetic applications

Photoreduction of ketones in the presence of amines led to ketyl radicals through photoinduced electron transfer (PET). Tertiary amines, such as triethylamine (Et₃N) have frequently been used in these reactions. Different reactions can occur from ketyl radicals such as photoreduction, coupling reactions, additions on activated double bonds, cyclizations, bond cleavage of strained rings, tandem reactions such as cyclization-ring opening or ring opening-cyclization.     

Topographical analyses of homonuclear multiple bonds between main group elements

Recent experiments have resulted in the completion of the series of Group 14 and Group 15 element double-bond systems, R(n)E=ER(n) (E = C - Pb, n = 2; E = N - Bi, n = 1). Furthermore, new families of multiple-bonded species have been discovered, such as the radical anion [RSnSnR](-) , the close ion pairs [RE(mu2Na)2ER] (E = Ge,Sn), and a digallyne [RGa(mu2Na)2GaR] for which a Ga=Ga triple bond was formulated. Some of these compounds show classical multiple bond features (i.e. the dipnictogens RE=ER, E=N-Bi) in the sense that planar structures with short E-E distances are observed. However, many (i.e. R2E=ER2, E = Si - Pb) do not behave as expected for compounds with multiple bonds. They have trans bent structures, show enormous variation in their E-E distances, and some dissociate easily under E-E bond cleavage in solution. These properties raised doubts as to whether these compounds can be formulated as multiple-bonded systems. Using the electron localization function (ELF) it is possible to clearly show the topographical similarities between classical and nonclassical multiple bonds; ELF divides these systems into unslipped (classical) and slipped (nonclassical) systems. ELF can also be employed to confirm the nonexistence of multiple bonds. Therefore, topographical analyses using ELF are useful to categorize a bonding system. In particular, the bonds in the heavier Group 14 double systems and the Ga-Ga bond in digallyne are clearly shown by this method as slipped double and triple bonds, respectively.     

Intramolecular oxycyanation of alkenes by cooperative Pd/BPh3 catalysis

The cooperative catalysis by palladium and triphenylborane effects the intramolecular oxycyanation of alkenes through the cleavage of O-CN bonds and the subsequent insertion of double bonds. The use of 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) as a ligand for palladium is essential for allowing the transformation to proceed with high chemo- and regioselectivity. Variously substituted dihydrobenzofurans with both a tetra-substituted carbon and cyano functionality are accessed by the newly developed methodology.     

Intramolecular single and double hydrogen migrations in the mass spectra of substituted tetrahydro-1,3,2-oxazaphosphorin-2-oxides

The mass spectral fragmentation of substituted tetrahydro-1,3,2-oxazaphosphorin-2-oxides occurs by the cleavage of ring bonds. The ions due to simple cleavage, single and double hydrogen migration, seem to be triggered by C O bond cleavage of the oxazaphosphorin ring. The variations in the relative abundance of ions arising due to similar fragmentation modes have been found to depend on the nature of the substituent and the stability of the particular fragment. The single hydrogen transfer process is supported by metastable ion and shift techniques.     

Controlled Chain‐Scission of Polybutadiene by the Schwartz Hydrozirconation

Controlled chain‐scission of polybutadiene (PB), polyisoprene, and poly(styrene‐co‐butadiene), induced by bis(cyclopentadienyl) zirconium hydrochloride (Cp₂ZrHCl), was revealed at room temperature. The chain‐scission reaction of linear PB was studied by means of GPC, NMR spectroscopy, and MALDI‐TOF‐MS. It was confirmed that the molecular weights of degraded products were quasi‐quantitatively controlled by Cp₂ZrHCl loading, irrespective of the starting PB, whereas the microstructure of PB chains was crucial to the scission reaction. The hydrozirconation of model molecules indicated that the existence of an internal double bond in compounds with multiple double bonds was essential for chain cleavage. The chain‐cleavage mechanism was proposed to involve hydrozirconation of internal double bonds in PB chains and β‐alkyl elimination. Furthermore, metallocene‐catalyzed chain‐scission by a chain‐transfer reaction was developed. It is believed that the reported chain scission offers a promising pathway for end‐group functionalization by chain cleavage and presents a new application of Schwartz’s reagent.     

Visible‐Light‐Promoted Metal‐Free Aerobic Oxidation of Primary Amines to Acids and Lactones

A unique metal‐free aerobic oxidation of primary amines via visible light photocatalytic double carbon–carbon bonds cleavage and multi carbon–hydrogen bonds oxidation was observed. Aerobic oxidation of primary amines could be controlled to afford acids by using dioxane with 18 W CFL, and lactones by using DMF with 8 W green LEDs, respectively. A plausible mechanism was proposed based on control experiments. This observation showed direct evidences for the fragmentation in the aerobic oxidation of aliphatic primary amines.     

The ozonolysis of butyl and halobutyl elastomers

The ozonolysis of butyl elastomer and its halogenated derivatives, as well as their model compounds in hexane, was studied. It was found that while the ozonolysis of butyl and chlorobutyl elastomers involves the normal cleavage of olefinic double bonds, the mechanism of ozonolysis of bromobutyl elastomer is more complicated. In the presence of ozone, bromobutyl elastomer was found to rearrange from the exomethylene structure to the bromomethyl structure, thereby generating more in-chain double bonds. This reaction provides an explanation as to why bromobutyl elastomer degrades faster and to a greater extent than chlorobutyl elastomer containing the same number of olefinic double bonds. Different additives were found to have different effects on the chain scission and rearrangement of bromobutyl elastomer. Diphenylamine and triisopropyl phosphite are much more effective than hexylamine and 2,6-di-tert-butyl cresol in inhibiting the chain scission of the bromobutyl elastomer and diphenylamine is more effective than triisopropyl phosphite in suppressing the rearrangement of this elastomer     

A mild procedure for the oxidative cleavage of substituted indoles catalyzed by plant cell cultures

We have developed a novel procedure for the oxidative cleavage of indole carbon double bonds in the presence of H₂O₂ using plant cell cultures as a catalytic system. The oxidative procedure has some advantages, such as mild reactions, good yields, easy work-up and safety.