Methylene carbons, linking

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. 

C NMR of linear cross-linked PS. The proton decoupled 13C NMR spectra of linear and 1% cross-linked PS at 75 MHz in chloroform are illustrated in Figure 3. These spectra are similar to those for linear and cross-linked chloromethylated PS previously reported at lower field (14), although we have been able to resolve more structure in tHe" aliphatic and aromatic regions here. The quarternary and methylene carbon resonances at about 146 ppm and between 40 and 50 ppm respectively, are the most strongly affected by stereochemistry (20). The ortho and meta resonances at 128.4 ppm show partially resolved structure in the linear PS, as does the para carbon at 126.1 ppm. The methine resonance at... 

Trifluoromethyl iodide is a poor substrate for SN2 reactions [28], The increased donativity of the methylene carbon as rendered by the fluorine atoms is reflected in its reluctance to enter a bonding relationship with a nucleophile. Similar reactivity patterns are known for chloromethyl phenyl sulfone [28] and chloromethyltrimethyl-silane. In these latter compounds the reactive center is directly linked to an acceptor group. 

The presence of a thiocyanate group can be very easily deduced from the IR spectrum, which contains a sharp, but weaker in intensity, IR band around 2150 cm-1 and an NMR carbon resonance between 112 and 115 ppm. In the case of psamaplin B (172), C-NMR additivity effects applied to the methylene carbon linked to the thiocyanate were used to distinguish between a thio- and isothiocyanate group [142]. Chemical proof can be provided by LAH reduction of the thiocyanate, which gives a thiol rather than a methylamine group as the reaction product of the reduction of the isothiocyanate [195]. 

The most widely known chemotherapeutic agent directed against TS is 5-fluorour-acil (5-FU). 5-FU was first used clinically almost 50 years ago, yet still remains a mainstay for the treatment of carcinoma of the breast and gastrointestinal tract. In cells, 5-FU is metabolized to 5-FdUMP, which forms a stable inhibitory ternary complex with the co-substrate N5N10-methylene-5,6,7,8-tetrahydrofolate (CH2H4-folate) and TS. In this complex, a covalent bond links the thiol of cysteine 195 of human TS to C6 of deoxyuracil monophosphate (dUMP) and the methylene carbon of the co-substrate is joined to C5 of the nucleotide [55]. The fluorine at C5, unlike the proton, cannot... 

The C-l methylene carbon atoms in the cis and trans units showed signal splittings reflecting the geometric isomerism of the unit linked to the C-l methylene carbon atom, that is, reflecting the dyad sequences of cis and trans units. Here, the carbon atoms are designated as follows ... 

Many synthetic routes are utilized to generate 6,12-functionalized dihydroindeno[l,2-h]fluorenes, typically from 22 or 26. One direct method is base-promoted alkylation of the methylene carbons of 26. For example, lithiation of 26 followed by treatment with 1-bromooctane gave tetrasubstituted 101 (Scheme 29) [80]. Subsequent bromination at the 2- and 8-positions with CuBr/ alumina furnished 102, which could then be polymerized to form polyindeno-fluorene 103 linked at the 2- and 8-positions rather than at the 6- and 12-positions as seen in Scherf s polyindenofluorene 85. Muellen and co-workers foxmd that 103 showed a bathochromic shift in fluorescence compared to polyfluorene (into the visible region) and it formed a liquid crystalline phase at high temperature, possibly making a suitable LED component. 

In 1939 Bacon and Farmer showed that the reaction between c/s-1,4-polyisoprene (natural rubber) and maleic anhydride was accelerated by benzoyl peroxide and they originally suggested that the anhydride linked across adjacent double bonds (intramolecular reaction) or across non-adjacent double bonds (usually intermolecular reaction). They also observed that when the concentration of rubber in the solvent used for the reaction exceeded a few per cent the product gelled and became insoluble. In subsequent work in which the model system cyclohexene-butyl maleate was studied Farmer (1942, 1943) revised his earlier suggestions and concluded that reaction was more likely to be at the ally lie (a-methylene) carbon. 

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