Olefins protonated

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

In the NMR spectrum of cis-l,2-bis[2-diethylamino-5-nitrothiazol-4-yl] ethylene (17) (1570), the nonequivalence of olefinic protons requires that the rotation of the NO2 group be hindered. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The olefinic proton of the enol form emerges as a sharp singlet in the region 6.2 to 7.5 ppm (DMSO) (386). while the 5-methyl protons appear at approximately 2.2 ppm. 

The intermediacy of ketenes in some enamine acylation reactions using acid chlorides was described above (386,387). Direct addition of ketene to enamines was studied simultaneously by several groups (414-420). The initially formed aminocyclobutanone products could be isolated in some instances, depending on the substitution of the initial enamine. Opening to give either the acylated enamine or the alternative vinylogous amide was found to occur spontaneously or on heating, particularly in adducts derived from enamines with an olefinic proton. 

The Heck reaction is considered to be the best method for carbon-carbon bond formation by substitution of an olefinic proton. In general, yields are good to very good. Sterically demanding substituents, however, may reduce the reactivity of the alkene. Polar solvents, such as methanol, acetonitrile, N,N-dimethylformamide or hexamethylphosphoric triamide, are often used. Reaction temperatures range from 50 to 160 °C. There are various other important palladium-catalyzed reactions known where organopalladium complexes are employed however, these reactions must not be confused with the Heck reaction. 

Figure 5.29 (a) The H-NMR spectrum of ethyl acrylate showing signals for olefinic protons, (b) A phase-sensitive COSYsjiectrum recorded at higher digital resolution, (c) Expansion of the downfield cross-peak identified by a dashed circle in (b). 

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

For example, the double triplet (6 6.16) was obscured due to the poor signal-to-noise and the severe second order coupling at 100 MHz. Simulations (7,8) of the olefinic protons were critical to these assignments (Figure 3). 

Figure 3. (a) The computer-simulated spectrum of the olefinic protons using chemical shifts of 6.3 and 6.1 ppm and a coupling constant (J) of 15 Hz. (b) The olefinic region of the 100 MHz H-NMR spectrum of the originally isolated xenognosin A. The marked resonances correspond exactly with the resonances of the simulated spectrum. 

NMR Spectra - The proton NMR spectrum of poly(N-pheny1-3,4-dimethy-lenepyrroline) (VII) had three singlet absorptions at 6 2.56, 4.81 and 7.60 respectively (Figure 10). The integration of these peaks showed a ratio of 4 4 5. The presence of exocyclic olefinic protons was not observed, indicating that 1,4- addition was predominant in the polymerization with little or no 1,2 addition taking place. 

The IR spectra of the polymer (P) contained two sharp absorptions near 1000 and 1100 cm-- -, indicative of the presence of unsubstituted cyclopentadienyl rings in the products. The 250-MHz - -H-NMR spectrum, shown in Figure 3, contained the expected peaks for the methyl, methylene, and cyclopentadienyl protons, respectively, at 61.52, 1.57 and 4.04 ppm. No olefinic proton resonances were present, and all of the samples of the polymer in Table II exhibited the same - -H-NMR spectrum. 

The chemical shifts of the olefinic protons from the centrosymmetric (all-/ ) zeaxanthin are very similar to the chemical shifts of (all-//) lutein except for proton 7. The resonances of protons 11/11 (6.65 ppm) and of protons 15/15 (6.62 ppm) show a multiplet with an integration value of four. The... 

In all recorded spectra the 3Jee coupling constants between the olefinic protons are on the order of 11-12 Hz, proving the all-E configuration of the investigated carotenoids. Minor differences between the reported chemical shifts and literature data are due to the effect of different solvent compositions. 

Signals of two olefinic protons on a C=C bond, which are isolated from the other double bond and the functional group in a long chain, appear at almost the same chemical shift, and their coupling constant is not clear because of f ailing the first-order approximation. Instead of that, the chemical shift values... 

Olefin protonation on the acid site, to form a carbenium ion, which can undergo... 

That the [19]annulenone 136 is diatropic is evident from the high field resonance of H13 compared to the adjacent external protons Ht2 and H14, and to all the olefinic protons of the homoannulene 137. Further evidence is the significantly lower field resonance of H3, H4, H17, and Hlg when compared with the similar protons H3 and H4 of the atropic ketones 143,144, and 145 (see below). 

The advancement of >400 MHz NMR instruments with spin decoupling and Fourier transform software now allows identification of individual olefinic protons of nanogram carotenoids53. We have shown two examples (lycopene and capsantin) for which the chemical shifts have been employed in the assignment of relative configuration49. As for review of the 13C NMR of carotenoids, Englert in 198154 gave information especially on the position of the cis double bonds in a polyene chain. 

ADMET condensation of 17 is completed using molybdenum catalysis to give the unsaturated polymer 18, which is reduced to 19 using a variant of hydrazine reduction chemistry. Complete saturation of the polymer backbone has been demonstrated and is illustrated by the absence of olefin protons in the 13C NMR of 19a shown in Fig. 5. 

NMR spectra. The use of NMR spectroscopy for distinguishing between the cis and trans isomers is based on the fact that the spin-spin coupling constants of olefine protons in disubstituted alkenes are as a rule different. The spin-spin coupling constant is usually 4-12 cps (7 cps on the average) with cis protons as the double bond and 12-18 cps (15 cps on the average) with the trans isomer and so can be distinguished. 

The power of the HR-MAS method for on-resin analysis has been further underscored in the development of new linkers. Without this method, only indirect analytical data after removal from the resin was available. Direct assessment of the resin-bound linker greatly facilitated the introduction of a 4,5-dibromo octane- 1,8-diol linker that was converted into an octane-1,8-diol linker cleavable by olefin metathesis at the end of the synthesis.6 The disappearance and reappearance of the olefinic protons as well as the growing oligosaccharide chain was clearly visible in the H spectrum (Fig. 8.7).7... 

In the case of 1,3-butadiene, the chemical shifts of inner (H2, H3) protons and outer (HI, H4) is large, while in the case of cycloalkadienes (e.g. 1,3-cyclopentadiene and 1,3-cyclohexadiene), the difference is very small. It is interesting to note that in 1,3,5-cycloheptatriene, the chemical shifts of three kinds of olefinic protons are very diverse. The effect of the ring size and in the chemical shifts of radialenes was also included. 

TABLE 12. Olefinic protons chemical shifts of 15 in the presence of LIS reagents (vs TMS)... 

FIGURE 3. 400 MHz proton NMR spectrum of 15 in CDCI3 (olefinic protons) with (a) no shift reagent, (b) racemic 15 with Eu(tfc)3 and Ag(fod), (c) 15 produced from piperylene with Ni(COD)2 and D-EPHOSNH, with Eu(tfc)3 and Ag(fod). Reproduced by permission of Elsevier Sequoia S.A. from Reference 24... 

TABLE 22. H NMR chemical shift differences (ppm) of olefinic protons of (all-/i)-... 

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