Methylene Group In Α-position Research Articles

Oxidation of Self-Organized Nonionic Surfactants

Nonionic surfactants containing a polyoxyethylene headgroup are known to slowly undergo oxidative degradation when exposed to air. The oxidation, which starts by abstraction of a hydrogen atom from a methylene group in alpha-position to an ether oxygen, is accelerated by metal ions. Silver ion mediated oxidation of a technical grade surfactant of this type, Brij 30, was investigated in two types of self-assembled systems, a water-in-oil microemulsion and a liquid crystalline phase. It was found that in both systems the longer homologues, i.e., the surfactant homologues that carry a longer polyoxyethylene chain, decompose faster than the shorter homologues. This trend was found to be more pronounced when the surfactant is present in a liquid crystal than in a microemulsion. The difference is explained in terms of differences in accessibility of the polyoxyethylene chains to the silver ions.

Mechanism of metal-catalyzed CC-coupling reactions with titanocene vinylidene. A theoretical study

The activation barriers and reaction energies for the [2+2]-cycloaddition of titanocene vinylidene with different reagents with double and triple bonds have been investigated at the B3LYP level of theory, using an effective core potential for Ti with a large valence basis set. For nonpolar reagents like ethylene or acetylene the reaction proceeds via a facile [2+2]-cycloaddition. In contrast to that polar reagents like formaldehyde or HCN react via primary formation of a donor–acceptor complex with the electrophilic titanium atom. This adduct rearranges to the transition state of the [2+2]-cycloaddition yielding the four membered titanacyclus. The analysis of the molecular orbitals of the 2-methylenetitanacyclobutene Cp 2 TiC(CH 2)CHC H, the 2-methyleneazatitanacyclobutene Cp 2 TiC(CH 2)CHN and the 2-methyleneoxatitanacyclobutane Cp 2 TiC(CH 2)CH 2–O with the extended Hückel method makes the different reactivity of these compounds understandable. Subsequent reactions of the titanacyclobutanes and -butenes have been investigated as well: Cycloreversion occurs for titanacyclobutane, and with a substantial higher activation barrier for titanacyclobutene. Electrocyclic ring opening is proposed for azatitanacyclobutene. Metathesis reactions are possible for titanaoxetanes. A mechanism for the rearrangement of titanaoxetanes with the exocyclic methylene group in α-position to Ti into titanaoxetanes with the exocyclic methylene group in β-position has been proposed.

A Novel Route To Poly(ε-caprolactone)-Based Copolymers via Anionic Derivatization

Poly(epsilon-caprolactone) (PCL) is known to biodegrade under composting or water sewage plant conditions. However, as compared with poly(alpha-hydroxy acids) derived from lactic and glycolic acids, PCL is much more resistant to chemical hydrolysis and is achiral, a feature that limits very much the possibility of property modulation through the configurational structure of polymer chains. For the sake of enlarging the family of PCL-type polymers, a novel method is proposed which is based on the anionic activation of PCL chain by the removal of a proton from the methylene group in alpha-position of the ester carbonyl present in the main chain, using a nonnucleophilic base such as lithium diisopropyl amide (LDA). This activation leads to a polycarbanion onto which various electrophile groups can be attached. The feasibility of the process was first shown on poly(methyl acrylate), (PMA), whose polyacrylic main chain is resistant to strong bases. The PMA polycarbanion was modified by various electrophiles, namely benzaldehyde, naphthoyl chloride, benzyl chloroformate, and iodomethane. In a second stage, the same reactions were performed successfully on PCL. The degree of substitution depended on the experimental conditions. PCL underwent main chain degradation during the formation of the polycarbanion whereas the reaction with the electrophiles did not cause any further main chain cleavages. The degradation of PCL chains can be limited enough to give access to novel functional PCL polymers.

Photo-oxidation of aromatic polymers

The photodegradation of aromatic polymers involves several distinctive reactions including both direct photolytic pathways and photo-oxidative processes. To account for such a complexity, the photoreactivity of four aromatic polymers is detailed. The mechanisms of photochemical evolution were probed using various techniques such as FTIR and UV spectroscopies, chemical derivatization reactions, physical treatments of irradiated films and mass spectrometric analysis of the low molecular weight fragments. A dual photochemistry, largely dependent upon the spectral distribution of the excitation light source, is well evidenced with bisphenol-A polycarbonate (PC). Excitation of PC at short wavelengths involves two consecutive photo-Fries rearrangements of the aromatic carbonate units; on irradiation at long wavelengths, photoproducts are shown to result from the photo-induced oxidation on the side chain and from the phenyl rings oxidation. The photo-oxidation of poly(butylene terephthalate) (PBT) largely involves pure photolytic pathways; on the opposite, the dominant photo-oxidative pathway of phenoxy resins (PR) is attributed to the high oxidizability of methylene groups in α-position to the oxygen atom of the ether groups. It is shown at last that the photo-oxidation of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) largely involves the oxidation of phenyl rings.

Photo-oxidation of aromatic polymers

The photodegradation of aromatic polymers involves several distinctive reactions including both direct photolytic pathways and photo-oxidative processes. To account for such a complexity, the photoreactivity of four aromatic polymers is detailed. The mechanisms of photochemical evolution were probed using various techniques such as FTIR and UV spectroscopies, chemical derivatization reactions, physical treatments of irradiated films and mass spectrometric analysis of the low molecular weight fragments. A dual photochemistry, largely dependent upon the spectral distribution of the excitation light source, is well evidenced with bisphenol-A polycarbonate (PC). Excitation of PC at short wavelengths involves two consecutive photo-Fries rearrangements of the aromatic carbonate units; on irradiation at long wavelengths, photoproducts are shown to result from the photo-induced oxidation on the side chain and from the phenyl rings oxidation. The photo-oxidation of poly(butylene terephthalate) (PBT) largely involves pure photolytic pathways; on the opposite, the dominant photo-oxidative pathway of phenoxy resins (PR) is attributed to the high oxidizability of methylene groups in α-position to the oxygen atom of the ether groups. It is shown at last that the photo-oxidation of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) largely involves the oxidation of phenyl rings.

Photooxidation of cured fluorinated polymers—I. Photooxidation of copolymers of fluorinated olefins and allyl or vinyl ethers

The photooxidation mechanisms of two copolymers of fluorinated olefins and allyl or vinyl ethers are studied. The ether groups that constitute the reactive part of both of the polymers are located on short pendant-chains. For this reason, the scissions due to the oxidation of the material lead to notable quantities of low molecular weight photoproducts that can be lost from the solid matrix by migration. The classical analysis of the evolution of the solid polymer matrix then has to be completed by an analysis of the gas phase formed during irradiation. Unanticipated results are obtained, since it is shown from the identification of the whole oxidation photoproducts that the presence of the fluorine strongly influences both the orientation of the reaction and the photooxidation kinetics. Due to the neighboring fluorine atoms, the methylene groups in α-position of the oxygen of the ether groups are not equivalent regarding oxidation and the secondary carbon becomes more oxidizable than the tertiary one. Moreover, these polymers show a very high resistance towards photooxidation, which is quite unusual with regard to polyethers in general.

Chloral–ketone condensations in acetic anhydride. Reaction of chloral with butanone, 3-pentanone, cyclohexanone, and 4-methyl-2-pentanone

In the presence of sodium acetate as catalyst, chloral hydrate undergoes a mixed aldol condensation with aliphatic and alicyclic ketones in acetic anhydride as solvent. Contrary to previous literature reports, reaction occurs at both the methyl and the methylene group in α-position to the carbonyl group of butanone, to give a mixture of 1,1,1-trichloro-2-hydroxy-4-hexanone (1) and 1,1,1-trichloro-2-hydroxy-3-methyl-4-pentanone (2a and 2b, diastereomers). 3-Pentanone, cyclohexanone, and 4-methyl-2-pentanoneyield 1,1,1-trichloro-2-hydroxy-3-methyl-4-hexanone (3a and 3b, diastereomers), 2-(1-hydroxy-2,2,2-trichloroethyl-) cyclohexanone (4a and 4b, diastereomers), and 1,1,1-trichloro-2-hydroxy-6-methyl-4- heptanone (5), respectively. Compound 5 is the exclusive product formed from chloral hydrate and 4-methyl-2-pentanone since attack at the methylene group is sterically hindered. The low-melting diastereomers 2a, 3a, and 4a which have not been characterized before, exhibit strong intramolecular hydrogen bonding and have been assigned the threo configuration on the basis of nuclear magnetic resonance and molecular model studies.