Methylene carbons

Table 9. Chemical Shifts of Methylene Carbons in Liquid Resoles...

The attack of cysteine-282 at the less hindered methylene carbon was supported by model studies. 

A paper by Lambert et al. <76JA3778) lists the chemical shifts of the a, (3 and -y-methylene carbon atoms of a very wide range of compounds of type (45) here we summarize the data for the compounds relevant to this volume (see Table 7). 

Figure 1.9. NMR spectra of a mixture of ethanol and hexadeuterioethanol [27 75 v/v, 25 °C, 20 MHz], (a) H broadband decoupled (b) without decoupling. The deuterium isotope effect Sch - d on chemical shifts is 1.1 and 0.85 ppm for methyl and methylene carbon nuclei, respectively...

Acylation of the heterocyclic enamines e.g., l,3,3-trimethyl-2-methyl-eneindoline (126) and 2-benzylidene-3-methylbenzothiazoline (42) takes place normally at the methylene carbon (42,69), with both acyl and aroyl chlorides in the presence of base. 

Finally, display the highest-occupied molecular orbital (HOMO) of triphenylphosphinemethylidene. Is it primarily concentrated on the methylene carbon as would be expected of a fully-developed anion, or is it delocalized over both phosphorous and carbon Does this suggest that the molecule incorporates a n bond ... 

It is thus obvious that the 2-thienyl group is both a better electron donator and electron acceptor than the phenyl group, facilitating both nucleophilic and prototropic reactions at the a-methylene carbon. 

Such methyenepyrans afford still another possibility for obtaining new pyrylium salts, namely, electrophilic alkylation or acylation at the exocyclio methylene carbon atom. Thus, 2,6-diphenyl-4-iso-propylidene-4/I-pyran is converted into 2,6-diphenyl-4-i-butyl-pyrylium iodide on refluxing with methyl iodide (see Scheme 3). Unlike the protonation of methylenepyrans, this reaction is no longer... 

With natural rubber and c/5-polyisoprene the normal cross linking agent used is sulphur. When mixed with natural rubber and heated, the sulphur reacts with the alpha methylenic carbon atoms of adjacent molecules, predominantly by S, and Sj cross links. If a low percentage of sulphur is added, normally about 2% by weight on the rubber, the end product is a soft elastic material. 

Let us recall, for instance, the case of the crystalline form of s-PP [113], or of s-PS [114,115], which contain s(2/l)2 helices. In this kind of helix (Fig. 2) there are two non-equivalent sets of methylene carbons in the main chain, the first one on the chain axis and the second one far from the chain axis. While for the methylene on the axis there are two y-carbons (that is carbons separated by three bonds) in G conformation, for the methylene on the periphery of the helix there are two y-carbons in the T conformation. This generates the so called y-elfect, that is a shift difference between the resonances of the two methylene carbons (8.7 ppm for s-PP, 10 ppm for s-PS). 

At elevated temperatures, methylene carbons cleave from aromatic rings to form radicals (Fig. 7.44). Further fragmentation decomposes xylenol to cresols and methane (Fig. 7.44a). Alternatively, auto-oxidation occurs (Fig. 1.44b ). Aldehydes and ketones are intermediates before decarboxylation or decarbonylation takes place to generate cresols and carbon dioxide. These oxidative reactions are possible even in inert atmospheres due to the presence of hydroxyl radicals and water.5... 

Oxidative degradation begins at lower temperatures in air (<300°C) and oxidation occurs most readily on benzylic methylene carbons since they are the most... 

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

Salvadori et al. [62] tested the same strategy but derived the bis(oxazohne) ligands in such a way that they minimized the steric hindrance at the bridging methylene carbon (structure 53 in Scheme 25). The polymer was used affording enantiomeric excesses superior to 90% and was reused at least five times with almost no loss in enantioselectivity or activity. 

In order to obtain more insights into the biosynthetic mechanism, [Me- C-Me-d ] double-labeled methionine was fed to the organism. The C-NMR spectrum of resulting neosaxitoxin showed a clean triplet for C-13 beside the natural abundance singlet. The result indicated that only one deuterium was left on the methylene carbon. 

For quaternary carbons (C) For methine carbons (CH) For methylene carbons (CH2) For methyl carbons (CHj) ... 

As mentioned in the introduction, the prototype o-QM is a highly reactive intermediates in organic reactions, including cycloaddition chemistry and DNA covalent modification, due to the high electrophilicity at the exocylic methylene carbon (see its dipolar representation in Scheme 2.11). 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

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