Methyl Shift Research Articles

Ambident nucleophiles. III. N-bonded vs. C-bonded adduct formation on reaction of imidazolide anions with 1,3,5-trinitrobenzene

Imidazolide anions of imidazole, 2- and 4-methylimidazoles, 4,5-dimethyl and 4,5-diphenylimidazoles react with 1,3,5-trinitrobenzene (TNB) in dimethylsulfoxide (DMSO) to yield, initially, N-bonded σ-adducts under kinetic control. These undergo a subsequent conversion to C-adducts, which are the thermodynamically stable products. The formation of N,C-diadducts from the reversible attack of a second TNB molecule on the imidazolide moiety of the C-adducts is also observed. Provided that the corresponding position of the parent imidazolide anion is free, the formation of C-adducts occurs preferentially at the C-4 or C-5 carbon atom. However, C-2 substitution is observed in 4,5-disubstituted derivatives. Structures and preferred conformations of some adducts and diadducts are deduced from nOe experiments. The mechanisms of the various reactions are discussed. Evidence is presented that adduct formation results in a low-field shift of methyl groups located in a position adjacent to the substitution site, while all the other protons move upfield. This unusual deshielding presumably reflects a proximity effect between a methyl group and the hydrogen atom attached to the sp3 carbon of the adducts.

Thermal degradation studies of segmented thermoplastic elastomers

Pyrolysis-gas chromatography (GC)-mass spectrometry (MS) has been carried out on several thermoplastic elastomers. The soft blocks are hydroxy terminated polypropylene oxide (PPO, MW = 2000) and the polyacetal from butyraldehyde and diethylene glycol (PA, MW = 1500). The hard blocks were introduced by reacting the soft block polymers with 4,4′-diphenylmethane diisocyanate (MDI) chain extended with butanediol, pentanediol and N,N′-bis(2-hydroxyethyl)isophthalamide (BI) to form the respective polyurethanes (PU) and with pyromellitic anhydride to give the polyimide. Most of the pyrolysates of the soft segments can be accounted for by the thermolysis of any single backbone bond into various macroradicals and elimination of observed compounds by either intramolecular hydrogen transfer, methyl shift or ring closure. Certain other products were better explained by direct intramolecular rearrangements. These processes are believed to be much more probable, according to energetic considerations, than two bond fragmentation followed by multiple hydrogen abstraction or elimination. Pyrograms do not show any products which may result from coupling or disproportionation of such fragments. The hard segment pyrolysates were probably also formed by intramolecular rearrangements. In the case of PPOPU-BI, pyrolysis at 300°C yielded only products from the hard blocks. Subsequent pyrolysis at 550°C has the same pyrogram as hydroxy terminated PPO. Therefore, these segmented block copolymers decompose in two stages; the hard blocks thermolyze first, followed by thermolysis of the soft blocks in agreement with previous TGA results.

Studies on the constituents of asclepiadaceae plants. LX. Further studies on glycosides with a novel sugar chain containing a pair of optically isomeric sugars, D- and L-cymarose, from Cynanchum wilfordi.

Seven new glycosides named wilfosides D1N (7), K1N (8), M1N (9), F1N (10), W1N (11), W3N (12), and G1G (13) were isolated from Cynanchum wilfordi HEMSLEY (Asclepiadaceae), and their structures were deduced on the basis of spectral and chemical evidence. A unique feature is that 7-11 and 13, as well as wilfosides C1N (2), C2N (3), C1G (5), and C2G (6), each include both D-(22) and L-cymarose (23) in the sugar chain. L-Diginose (24) is also a constituent of their sugar chains. The carbon-13 nuclear magnetic resonance (13C-NMR) chemical shifts of methyl α-(24a) and β-L-diginopyranoside (24b) were assigned, and the conformations of 24a and its acetate were studied.

Thermolysis of gem-dimethyl-2-cyclohexenones. Evidence for [1,5] sigmatropic methyl shifts

The vapour phase thermolyses at 400 °C of three gem-dimethylated 2-cyclohexenones 1, 2, and 3 were studied to determine the effect of blocking the direct conversion of these substrates to aromatic systems. In all three reactions it is proposed the major pathway taken was enolization followed by elimination of methane to yield a substituted phenol. This elimination reaction proceeded more readily with 2 than with either 1 or 3, and HMO calculations which are consistent with this order of reactivity are presented. A general mechanistic scheme is proposed to account for the products formed and it is necessary to include a [1,5] sigmatropic methyl shift to explain the formation of at least one product in each thermolysis.

ChemInform Abstract: CHEMISTRY OF PYRROCORPHINS. SYNTHESIS OF ISOBACTERIOCHLORINS AND PYRROCORPHINS BEARING A METHYL GROUP AT THE MESO POSITION BETWEEN RINGS A AND D

AbstractBeim Erhitzen des Perchlorats (I) entsteht unter Wanderung einer Methylgruppe die Verbindung (IIa).

A proton NMR investigation of the influence of distal glutamine on structural and dynamic properties of elephant metmyoglobin.

The proton NMR spectra of metmyoglobin from the Asian elephant, which has the replacement of glutamine for the usual distal histidine, are reported and analyzed. In the low pH region, we detect two interconvertible forms of the met-aquo-protein whose relative stabilities are independent of pH, but depend strongly on both temperature and solvent isotope composition. As the pH is raised, both species convert to the met-hydroxy form, as found for other myoglobins. The temperature dependence of the heme methyl shifts for both acidic protein forms indicates essentially high spin character for the iron, and the mean heme methyl shifts are interpreted as indicating one form with a very slightly weaker, and the other with a significantly stronger, axial ligand field than for the unique sperm whale met-aquo-myoglobin. The thermodynamic data for the equilibrium between the two species are consistent with differences of one hydrogen bond between coordinated water and the distal glutamine. Models are proposed where one form of the protein has not only the glutamine carboxyl oxygen acting as a hydrogen-bond acceptor, but also the amine group. We conclude that a distal glutamine can act both as a stronger and as a weaker hydrogen-bond acceptor towards coordinated water than the usual distal histidine. The relative rates of conversion of the two met-aquo-myoglobin forms to MetMbOH is found to be consistent with the proposed structures for the two forms.

Loss of viral genomes from hamster tumor cells and nonrandom alterations in patterns of methylation of integrated adenovirus type 12 DNA.

The insertion stability and DNA methylation patterns of integrated adenovirus type 12 (Ad12) genomes were investigated in Ad12-induced tumors and in tumor cell lines established from them as a function of time of passage under culture conditions. Upon subcultivation of cells from some of the tumors, the viral genomes were eliminated, apparently in a stepwise process with segments of the left termini of Ad12 DNAs persisting the longest. Morphological variants of these tumor cells lost all viral DNA and yet retained the oncogenic phenotype. All 13 independently isolated clones from one revertant line were devoid of Ad12 DNA. It could not be ruled out that very short sequence elements of viral DNA, such as promoters or enhancing sequences, could have persisted in these variants. The extent of viral DNA methylation was minimal in Ad12-induced tumors, although the viral genome was not extensively expressed, if at all. Upon passage in culture, the levels of viral DNA methylation increased. It was interesting that establishment of the final methylation pattern of integrated Ad12 DNAs required many cell generations after the fixation of foreign DNA in the host genome. The shift in methylation was nonrandom. The late parts of the inserted viral genomes became methylated more extensively than did the early gene segments.

ChemInform Abstract: GAS‐PHASE HETEROAROMATIC SUBSTITUTION. 3. ELECTROPHILIC METHYLATION OF FURAN AND THIOPHENE BY MEXME+ (X = FLUORO, CHLORO) IONS

A previous radiolytic study on the gas-phase methylation of pyrrole and N-methylpyrrole by CH/sub 3/XCH/sub 3//sup +/ (X = F or Cl) ions, from the ..gamma.. radiolysis of CH/sub 3/X, is extended to furan (3) and thiophene (4). The mechanism of the substitution and of the subsequent isomerization occurring via intramolecular 1,2 methyl-group shifts is discussed and the substrate and positional selectivity of the selected electrophilic species evaluated. As for pyrroles, gas-phase CH/sub 3/FCH/sub 3//sup +/ methylation of furan and thiophene is characterized by a scarce substrate discrimination (k/sub S//k/sub B/ = 1.2 (3), 0.8 (4)), accompanied by an appreciable positional selectivity toward those substrate positions with the highest negative net charge (O:..cap alpha..:..beta.. = 36%:35%:29% for 3; S:..cap alpha..:..beta.. = 19%:43%:38% for 4). On the contrary, CH/sub 3/ClCH/sub 3//sup +/ confirms its inherent affinity toward n-type nucleophilic centers by attacking preferentially the heteroatom of 3 and 4. In light of the previous results concerning CH/sub 3/XCH/sub 3//sup +/ methylation of pyrroles, it is concluded that gas-phase attack of CH/sub 3/XCH/sub 3//sup +/ on simple five-membered heteroaromatics is essentially regulated by the electrostatic interaction established within the encounter pair. A close correspondence does exist between this rationalization of the presentmore » gas-phase results and recent theoretical predictions.« less

Dimethyl 7b-methyl-7bH-cyclopent[cd]indene-1,2-dicarboxylate. A new 10π-electron aromatic system

The title compound (8a) is formed by the acid-catalysed elimination of methanol from the [8 + 2]-cycloadduct (7) of 3-methoxy-3a-methyl-3aH-indene and dimethyl acetylenedicarboxylate. Its physical and chemical properties are consistent with a 10π-electron aromatic periphery which sustains a dia-magnetic ring current. It is photochemically and thermally stable, but rearranges slowly in boiling xylene to the isomer (11) by a [1,5]methyl shift. With copper(II) nitrate and acetic anhydride, it gives a mixture of mononitro-substitution products. Preliminary experiments show that decarbonylation of the dialdehyde (8e) with tris(triphenylphosphine)rhodium(I) chloride provides a route to the unsubstituted tricyclic [10]annulene (8f).

Synthesis of prazolinones from β-N,N-dimethylhydrazinopropenoates: an example of a thermally induced [1,4] alkyl shift

Gas phase thermolysis of β-N,N-dimethylhdrazinopropenoates (1) provides a new route to pyrazolinones (2) involving a [1,4] methyl shift from nitorgen to carbon in the pyrazolinium ylide intermediates (3); the mechanism is supported by the isolation of ylides (3a,b,e), the structures of (3a) and (3e) being confirmed by X-ray crystallography.

A 1H nuclear magnetic resonance study of the protonation of methacrylamide and of the tautomerism of its cation

Chemical shifts of the methyl and the vinyl protons of methacrylamide in sulphuric acid–water mixtures have been measured. The resonance of the methyl protons shifts downfield by 0.075 p.p.m. up to 60% acid and then remains virtually unchanged up to 100% acid. The resonances of the vinyl protons show large downfield shifts (the cis-proton by 0.55 p.p.m. and the trans-proton by 0.52 p.p.m.) up to 60% acid and then a substantial further shift to low field of 0.08 p.p.m. between 60% and 100% acid. This indicates a redistribution of charge in the cation, consequent upon the conversion of the N-protonated cation into the O-protonated cation. Half-conversion occurs in 78.5 ± 1.5% acid.

Chemistry of pyrrocorphins: synthesis of isobacteriochlorins and pyrrocorphins bearing a methyl group at the meso position between rings A and D

A sigmatropic methyl shift from the angular position C-1 in ring A to the meso position C-20 between rings A and D constitutes the crucial step in syntheses leading to a 20-methyl-isobacteriochlorin and to 20-methyl-pyrrocorphins which served as substrates in the investigation presented in the accompanying communication.

Gas-phase heteroaromatic substitution. 3. Electrophilic methylation of furan and thiophene by CH3XCH3+ (X = fluoro, chloro) ions

A previous radiolytic study on the gas-phase methylation of pyrrole and N-methylpyrrole by CH/sub 3/XCH/sub 3//sup +/ (X = F or Cl) ions, from the ..gamma.. radiolysis of CH/sub 3/X, is extended to furan (3) and thiophene (4). The mechanism of the substitution and of the subsequent isomerization occurring via intramolecular 1,2 methyl-group shifts is discussed and the substrate and positional selectivity of the selected electrophilic species evaluated. As for pyrroles, gas-phase CH/sub 3/FCH/sub 3//sup +/ methylation of furan and thiophene is characterized by a scarce substrate discrimination (k/sub S//k/sub B/ = 1.2 (3), 0.8 (4)), accompanied by an appreciable positional selectivity toward those substrate positions with the highest negative net charge (O:..cap alpha..:..beta.. = 36%:35%:29% for 3; S:..cap alpha..:..beta.. = 19%:43%:38% for 4). On the contrary, CH/sub 3/ClCH/sub 3//sup +/ confirms its inherent affinity toward n-type nucleophilic centers by attacking preferentially the heteroatom of 3 and 4. In light of the previous results concerning CH/sub 3/XCH/sub 3//sup +/ methylation of pyrroles, it is concluded that gas-phase attack of CH/sub 3/XCH/sub 3//sup +/ on simple five-membered heteroaromatics is essentially regulated by the electrostatic interaction established within the encounter pair. A close correspondence does exist between this rationalization of the presentmore » gas-phase results and recent theoretical predictions.« less

Paraffin disproportionation catalyzed by aluminum chloride/sulfonic acid resin

The disproportionation of n-butane and of n-pentane was investigated as reactant vapors flowed over beads of a solid superacid catalyst (an aluminum chloride/sulfonic acid resin) at 1 atm and 70–143 °C. n-Butane reacted in the presence of HCl to give equimolar ratios of propane and pentanes; the butane isomerization reaction took place simultaneously, occurring at an order-of-magnitudehigher rate than the disproportionation. The observed i- to n-pentane ratios were less than the equilibrium ratios predicted from literature data. The conversion of n-pentane under similar conditions yielded products ranging from methane to hexanes. The main products were propane and isobutane, which indicates that extensive β-scission occurred. The results are broadly consistent with classical carbenium-ion mechanisms involving hydride and methyl shifts and CC bond-forming and -breaking steps, although some details of the mechanism of the n-butane disproportionation remain unexplained.

Thermodynamic and Hydrodynamic Properties of Cellulose Diacetate–Dimethylacetamide Solutions

Viscometric and light scattering investigations were carried out on cellulose diacetate (degree of substitution, DS=2.46) in dimethylacetamide (DMAc) at 25°C. The following molecular weight dependence of limiting viscosity number [η], radius of gyration ‹S2›z1/2 and Flory’s viscosity parameter Φ were obtained: [η]=1.34×10−2 Mw0.82 (cm3 g−1), ‹S2›z1/2=0.68×10−8 Mw0.53 (cm), and Φ=5.15×1021 Mw0.23. Draining effect was not negligible for this system. The unperturbed chain dimension A was determined by methods 2B, 2C, 2E, 2F, and 2G proposed previously [K. Kamide and Y. Miyazaki, Polym. J., 10, 409 (1978)]. The A values obtained by methods 2B, 2C, and 2G were in good agreement with each other and the most probable A value was found to be 1.98×10−8 (cm). The variation in [η], ‹S2›z1/2, and A with solvent is discussed in terms of the dielectric constant of the solvent and also the chemical shifts of the O-acetyl methyl and hydroxyl groups.

Isomerization of alkane molecular ions

The appearance energies of daughter ions for the major low-energy losses of C/sub 2/H/sub 6/, CH/sub 4/, and CH/sub 3/., of pentane and methylbutane radical cations were measured. The behaviors of a variety of /sup 13/C- and /sup 2/H-labeled compounds were examined. For methylbutane, loss of C/sub 2/H/sub 6/ and CH/sub 3/. yielded ionized propene and 2-butyl cations, at their calculated thermochemical thresholds. Loss of CH/sub 4/ proceeded at an energy greater than that calculated for the production of ionized but-2-ene, methylpropene, but-1-ene, or methylcyclopropane. Ethane elimination is preceded partially (ca. 20 to 25%) by a 1 to 2 methyl shift and a concomitant 2 to 1 H shift; isotope effects associated with this reaction are discussed. For pentane, losses of CH/sub 3/. and CH/sub 4/ take place at the same energy, corresponding to the calculated threshold for production of the secondary cations; /sup 13/C labelling experiments showed that the penultimate C atoms are not lost but that C-3 is lost (46%) in these reactions, with an ease similar to terminal C atom losses (54%). /sup 2/H labelling permitted the separation of the metastable peak for methane loss into components having different kinetic energy releases attributable to the production ofmore » different (C/sub 4/H/sub 8/)/sup +/. daughter ions. Although C/sub 2/H/sub 6/ loss was observed to take place at energies down to the calculated thermochemical threshold for generation of ionized propene, the appearance energy of the metastable peak was the same as that for CH/sub 3/. and CH/sub 4/ losses. Labelling experiments showed that the central C atom is not lost in this reaction. Detailed analysis of the observations leads to the conclusion that the CH/sub 3/., CH/sub 4/, and C/sub 2/H/sub 6/ eliminations from pentane radical cations are preceded by an isomerization to energy-rich ionized methylbutane. 2 figures, 3 tables.« less

Preparation of α and β anomers of various isomeric methyl O- d-galactopyranosyl- d-galactopyranosides. Standards for interpretation of 13C-n.m.r. spectra of d-galactopyranans

The four isomers of methyl O-β- d-galactopyranosyl-β- d-galactopyranoside were prepared by condensation of 2,3,4,6-tetra- O-acetyl-α- d-galactopyranosyl bromide with appropriate, partially O-substituted derivatives of methyl β- d-galactopyranoside. Reaction of 3,4,6-tri- O-acetyl-1,2- O-(1-ethoxyethylidene)-α- d-galactopyranose with the same acceptors, in the presence of mercuric bromide, led to the formation of α and β linkages. Thus, it was possible to assign 13C-n.m.r. resonances of α and β anomers of methyl O- d-galactopyranosyl-β- d-galactopyranosides. In terms of application of these shift values and those of related d-galactobioses to the structural analysis of d-galactopyranans by shift comparisons, some generalizations can be made. For β- d-galactopyranans, the resonances of glycosyloxylated carbon atoms of methyl O-β- d-galactopyranosyl-β- d-galactopyranosides are sensitive to structure and appear to have typical values, whereas limited variation was observed with shifts of C-1′ signals. On the other hand, for assigning structures to d-galactopyranans containing α linkages, the C-1′ shifts (at higher field) of methyl O-α- d-galactopyranosyl-β- d-galactopyranosides are sensitive to linkage position, whereas those of glycosyloxylated carbon atoms vary only a little.

Isomerisation des ions alcools ethyleniques phenyles en phase gazeuse

Ethylenic alcohols isomerise by hydrogen, phenyl and methyl shifts before cleavage.

Conformational analysis XX—13C NMR studies of saturated heterocycles 5—substituent effects on the 13C chemical shifts of methyl substituted 1,3‐dithiolanes

AbstractThe 13C NMR chemical shifts for 1,3‐dithiolane and 13 methyl substituted derivatives are reported. Substituent effects are derived and compared with those for cyclopentanes and 1,3‐dioxolanes. The magnitude and variety of the substituent effects are best explained with the aid of a half‐chair conformation where the S‐1C‐2S‐3 plane passes between C‐4 and C‐5.

Reactions of methylpropene-2- 13C on H-mordenite : I. Reaction mechanisms

The reactions of isobutene on a freshly prepared and outgassed H-mordenite have been investigated at 120 and 170 °C. Initially most of the reactant is retained on the surface and the gaseous products are mainly isopentane and isopentenes. Increasing reaction temperature increases the amounts of saturated hydrocarbons. After aging the catalyst, a stationary regime is reached where the formation of the hydrocarbon residues is no longer apparent. The products, mostly olefinic, include C 8 hydrocarbons with trimethylpentane or dimethylhexane structure and C 5C 7 hydrocarbons, mostly with dimentylbutane and dimethylpentane skeletons. The use of methylpropene-2- 13C demonstrates the existence of two types of products. Trimethylpentenes and 2,5-dimethylhexenes are doubly labeled on carbons 2 and 4 or carbons 2 and 5, respectively. In the other gaseous products C n+4 H 2 n+8 (2 ≤ n ≤ 4: 2,4-dimethylhexenes, 2,3-dimethylbutenes, 2,4-dimethylpentenes), four carbon atoms retain the structure and the label of the reacting isobutene while the n additional carbon atoms are randomly labeled, with an overall 13C content of n 4 . The specific labeling of these compounds is best explained by the complete randomization of all the carbon atoms in isobutene via transient formation of a cyclopropylcarbinyl-cyclobutyl cation. Addition of the resulting scrambled C 4 unit as α-methylallylic cation to isobutene yields partially scrambled 2,4-dimethylhexene- 13C 2. Attachment of the C 4 scrambled unit as a homoallylic cation or α-methylallylic cation to the zeolite framework provides an adsorbed intermediate species which yields, after protonation, olefin addition, possibly hydride and methyl shifts, and β-fission the expected C 5C 7 hydrocarbons. It is suggested that two types of sites are involved in the reactions of isobutene on aged mordenite at 120 °C. Weak Brønsted acid sites account for the formation of trimethylpentenes. Strong acid sites, resulting most probably from the conjunction of Brønsted and Lewis sites, are responsible for the generation of allylic cations by hydride abstraction, yielding the dimethylhexenes and the C 5C 7 olefins.