Vinylic protons

Hydrogens that are directly attached to double bonds (vinylic protons) or to aro matic rings (aryl protons) are especially deshielded... 

Chemical shift nonequivalence can occur when two environments are stereochem ically different The two vinyl protons of 2 bromopropene have different chemical shifts... 

Figure 13 20 there are two other vinylic protons Assuming that the coupling constant between the two geminal protons in ArCH=CH2 is 2 Hz and the vicinal coupling constants are 12 Hz (cis) and 16 Hz (trans) describe the splitting pattern for each of these other two vinylic hydrogens... 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

The pyrrolidine enamine of cyclopentanone, in which the double bond is exocyclic to two five-membered rings, shows the vinylic proton at 237 Hz (7S). 

In the NMR spectrum of the N-methylaniline enamine of cyclohexanone (135), the vinylic proton appears at a much lower field, i.e., at 324 Hz (75). Here the electron pair on nitrogen tends to conjugate with the phenyl group thus exhibiting a very small degree of overlap with the enamine double bond. 

The chemical shifts for the vinylic protons of some enamines are given n Table 4. 

Z-Configuration of the initial compounds does not change in the reaction course, as indicated by the coupling constants of the methoxyethenyl group vinyl protons of the triazoles 166 and 167. Adducts of -configuration are present only as admixtures (83DIS). 

I The vinylic proton at C3 (green) is next to the aromatic ring and is therefore shifted downfield from the normal vinylic region. This C3 proton signal appears as a doublet centered at 7.49 8. Because it has one neighbor proton at C2, its signal is split into a doublet, with / = 12 Hz. 

I The C2 vinylic proton signal (blue) appears at 6.73 8 and shows an interesting four-line absorption pattern. It is coupled to the two nonequivalent protons at Cl and C3 with two different coupling constants /. 2 6 Hz and /2.3 - 12 Hz. 

Problem 13.22 3-Bromo-l-phenyl-l-propene shows a complex NMR spectrum in which the vinyli< proton at C2 is coupled with both the Cl vinylic proton (J - 16 Hz) and the C3 methylene protons (/ = 8 Hz). Draw a tree diagram for the C2 proton signal, anc account for the fact that a five-line multiplet is observed. 

Long-range coupling between protons more than two carbon atoms apart is sometimes observed when v bonds intervene. An example is found in l-methoxy-l-buten-3-yne. Not only does the acetylenic proton, Ha, couple with the vinylic proton H, it also couples with the vinylic proton Hc, four carbon atoms away. The data are ... 

Much of the difference in chemical shift between aromatic protons (6.5-8.0 8) and vinylic protons (4.5-6.5 5) is clue to a property of aromatic... 

Vinylic halide, alkynes from, 261 S 2 reaction and, 366-367 Vinylic protons, NMR... 

It should also be considered that in heteroaromatic compounds, and in particular in the presence of chelating groups12, there is a high tendency for the abstraction of vinylic protons before benzylic protons, for example13 ... 

The issue of the acidity of a-hydrogens in thiirene oxides and dioxides is dealt with only in the dioxide series, since neither the parent, nor any mono-substituted thiirene oxide, is known to date. Thus the study of the reaction of 2-methylthiirene dioxide (19c) with aqueous sodium hydroxide revealed that the hydroxide ion is presumably diverted from attack at the sulfony 1 group (which is the usual pattern for hydroxide ion attack on thiirene dioxides) by the pronounced acidity of the vinyl proton of this compound113 (see equation 14). 

Deprotonation of the vinylic proton is a serious side-reaction in the conjugate addition of organometallic reagents to y-siloxy-a, /J-unsaturated sulfones (89)63b. The use of the... 

The reaction can be followed by proton magnetic resonance spectroscopy. The original absorption for the vinyl proton disappears and two new absorption peaks appear, one in the vinyl region (ca. <5 6.5, chloroform-d) and the other in the methine region of the spectrum. There are two products formed, presumably the cis- and tfraws-isomers, in the ratio of 95 5, respectively. The checkers also obtained the same yield when the reaction quantities were doubled. 

The progress of the reaction is followed by proton magnetic resonance spectroscopy. When the absorption for the vinyl proton (ca. <5 6.6, chloroform-d) is completely absent, the reaction is stopped. Several minor products that were not identified are also formed in this step. 

The pure adduct had the following proton magnetic resonance spectrum (chloroform-d) <5, multiplicity, number of protons, assignment 6.75 (singlet, 2, cyclohexene vinyl protons), 6.20 (multiplet, 2, cyclobutene vinyl protons), 3.5 (broad multiplet, 4, cyolobutane protons). 

One potentially important example of CIDNP in products resulting from a radical pair formed by electron transfer involves a quinone, anthraquinone j5-sulphonic acid (23). When irradiated in the presence of the cis-syn dimer of 1,3-dimethylthymine (24), enhanced absorption due to vinylic protons and emission from the allylic methyls of the monomer (25) produced can be observed (Roth and Lamola, 1972). The phase of the polarizations fits Kaptein s rules for intermediate X... 

Polarization also occurs in coupling and disproportionation reactions of Grignard reagents with alkyl halides. The vinyl protons of isobutene produced in the reaction of t-butylmagnesium chloride with t-butyl bromide show A/E polarization as do the methyl protons of isobutane (Ward et al., 1970). Similar results arise in the reaction of diethyl-magnesium with organic halides (Kasukhin et al., 1972). 

Three sets of nmr chemical shifts for the ethynyl proton have been correlated with eq. (2). Of these sets, two gave significant correlations with eq. (2). Nevertheless, as the most extensive collection of substituents is included in the set which did not give significant correlation, it seems likely that chemical shifts of ethynyl protons are not correlated by the extended Hammett equation. This behavior contrasts with that of chemical shifts for trans- and c/s-vinyl protons and is in agreement with the behavior of geminal vinyl protons. 

The electrochemical reduction of 4//-thiopyrans bearing four electron-withdrawing substituents leads to 5,6-dihydro-2//-thiopyrans. Four diastereoisomers are produced, their relative proportions depending on the electrolytic conditions. Their conformations have been established using the vinylic proton as an nmr probe and confirmed in some instances by X-ray analysis <96JCS(P2)2623>. 

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