Olefinic carbon atom resonance

What about a nucleophilic carbene, for which negative charge should build up on the olefinic carbon atoms during the carbene addition cf. 5 With ArCH=CH2 substrates, electron-withdrawing aryl substituents would stabilize such a transition state and the p value should be positive. There are several examples of this phenomenon. For example, cycloheptatrienylidene, 10, adds to / -substituted styrenes with p = -t-1.02 - 1.05 (vs. a) consistent with a nucleophilic selectivity that seems to implicate the aromatic resonance form 10a as an important feature of the carbene. [45] It is satisfying to compare this result with p = -0.76 (vs. a) or -0.46 (vs. a" ") for additions to styrenes of cyclopentadienylidene, 11, where contributions of the cyclopentadienide form (11a) would render the carbene electrophilic. [46] However, these conclusions are too facile. There is reason to believe that the chemistry attributed to 10 might in fact be due to its allenic isomer 12. [47] And the electronic structure of 11 is also more complicated than the simple depiction above. [48]... 

The i c-NMR spectra of the resulting polymer dissolved in CDCI3 shows Figure 1. It was recorded with a pulse delay of 3 seconds for better relaxations of the nuclei and is broad band decoupled. The olefinic carbon atoms (1 and 2) resonate at 130.73 ppm indicating the succsessful ring opening polymerization via metathesis also in... 

The spectrum of the TRIEGMA monomer shows the following resonances the methyl substituents at 18 ppm, ester methylene groups at 64 ppm, ether methylene groups at 69 and 71 ppm, olefinic carbon atoms at 126 and 137 ppm (terminal carbon atom), and carboxyl carbon atoms at 168 ppm. 

The NMR spectra of both the cyclopentane ring and the olefinic carbon atoms in poly(norbomene) are sensitive to the stereochemistry of the neighbouring double bonds and, as seen above, this leads to cc/ct tt/tc doublets for the cyclopentane ring carbon atoms. There is, however, the possibility of quartet fine structure for both cis and trans olefinic carbon resonances, owing to the inequivalence of the carbon atoms in a given double bond [64], as in Scheme 11. 

Olefinic carbon atoms normally resonate between 100-165 ppm. Terminal methylene groups resonate about 24 ppm upheld from the other olefinic carbon. In long-chain alkenes, the olefinic carbon atom bearing the longer side chain appears at lower field. 

The off-resonance experiment allows the assignment of the olefinic carbon atoms of the isoprenyl units in butyl rubber. The signal at 129.98 ppm appears as a doublet... 

The H NMR spectram (CDClj, 500 MHz) of 12 showed two singlets (8 0.83 and 8 0.95), each integrating for three protons due to the C-18 and C-19 methyl protons. Three 3H doublets at 8 0.78 (J= 6.5 Hz), 8 0.79 (J= 6.5 Hz) and 8 0.85 (J = 7.0 Hz) were due to the secondary C-26, C-27 and C-21 methyl protons, respechvely. The C-3 methine proton resonated as a one-proton double doublet at 8 3.63 (JJ= 10.5 Hz and J2= 3.5 Hz) and its downfield chemical shift value was indicative of the presence of a geminal hydroxyl funchonality. A one-proton mulhplet at 8 5.21 was ascribed to the C-6 olefinic proton. The C-28 exocyclic methylene protons appeared as two broad singlets at 8 5.40 and 5.58. The C-NMR spectram (CDCl, 125 MHz) showed the resonance of all 28 carbon atoms. The combination of H and C-NMR data suggested that compound 12 has a sterol like structure as most of the H and C-NMR chemical shift values of 12 were similar to those of sterols reported in the literature [19, 20]. The H and C-NMR chemical shift values were assigned with the aid of COSY-45 , HSQC and HMBC spectral data. Compound 12 was found to have modest inhibitory activity against C. xerosis and S. aureus with minimal inhibitory concentration values of 82.35 and 146 pg/ml, respectively. 

The base thus formed from the olefin has a negative charge on a carbon atom, and so may be referred to as a carbanion. The allylic hydrogen is the most acidic because removing it results in a resonance-stabihzed intermediate. 

Fio. 1., 3C-NMR spectrum of n-BuLi + 1,3 butadiene (DP 1.0 to 1.5). Note the presence of the peak at 143 ppm, a nonterminal carbon atom of a 1,2 unit without the appearance of its sister, the terminal methylene carbon atom at 110-114 ppm. Instead, a new resonance at 82 ppm associated with complexed methylene units is present. (Bottom is aliphatic region top is olefin region.)... 

In a recent 13C-NMR study by Halasa et al. (31), the addition of n-BuLi to 1,3-butadiene at DP = 10-12 (Fig. 7) was studied in the absence of polar modifiers. The vinyl carbon at DP 12 showed the usual (=CH2) olefinic position at 110-114 ppm, structure 16, and a new and unusual absorption at 90-100 ppm appeared. The latter resonance was shown by a gated decoupling technique to be split into a doublet. This observation indicates that this carbon is a methine carbon and could be the y-carbon in structure 15 or the y-carbon in structure 16. Since the methine y-carbon atom in structure 15 is an olefinic carbon, the assignment of the... 

To understand the mechanistic basis behind Markovnikov s rule, it is useful to refer to the mechanisms through which acids add across double bonds. Of particular relevance are the resonance forms of the protonated olefins illustrated in Scheme 7.6. Since, for ethylene, the two carbon atoms are both primary, there is no distinction between them. However, as illustrated in Scheme 7.9, in the case of propene, protonation of the olefin results in introduction of cationic character to both a primary carbon atom and a secondary carbon atom. 

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