Double bonds protonation

The H—NMR spectra indicate displacements of chemical shifts for the double bond protons of MA and TBSM and protons of the CH2—Sn group which suggest that two complex types are present in the mixture of the monomer pairs under study ... 

You may think that is the end of the problem but, since we have an unsymmetrical diene, it is also necessary to consider protonation of the other double bond. Protonation on C-4 also gives a favourable resonance-stabilized allylic carbocation, this time with primary and secondary limiting structures. Protonation on C-3 gives an unfavourable primary carbocation with no resonance stabilization. Since the products formed are related to initial protonation at C-1, it is apparent that, despite the stability associated with an allylic cation, a tertiary limiting structure is formed in preference to that with a secondary limiting structure. 

Because of the small amount of natural anandamide available, we were able to record H NMR spectra only. The peaks attributed to double-bond protons (5 5.30 to 5.45, multiplet) were coupled with those of protons that have the chemical shifts of doubly allylic protons (5 2.75 to 2.90, multiplet). Such doubly allylic protons are typically found in all-af, nonconju-gated polyunsaturated fatty acids such as linoleic and arachidonic acids. Three pairs of protons were observed between 5 2.01 and 2.27, which we attributed to two allylic methylene groups and one methylene group a to a carbonyl moiety. Only one methyl group was observed (0.99, triplet). The peaks observed for two protons at 3.42 (N-CH2, triplet), two protons at 3.72 (O-CH2, triplet), and two protons at 2.20 (COCH2, triplet) were... 

These ligands all form 1 1 complexes with platinum(II) halides which are non-conductors, monomeric and, in the solid state, contain one coordinated and two free double bonds. Proton n.m.r. data indicate that all three double bonds are equivalent in solution due to a rapid equilibrium between bonded and non-bonded —CH=CH2 groups. The phosphine and arsine complexes PtBraL (L = tvpp, tvpa) react with two equivalents of bromine giving what are believed to be platinum(II) species containing one coordinated double bond and two —CHBr-CHaBr groups arising from the saturation by bromine of the two free double bonds. 

The H NMR spectra of two diastereomeric hetero-Diels-Alder adducts 43 has been obtained (Figure 10) <2004JOC7198>. The diastereotopic a-protons of the sulfonamide (C-6) fall in the range of 3.4—3.5 ppm, while the C-3 protons occur around 4.6 ppm. Both and C-5 vinylic hydrogens occur in the characteristic region for double-bond protons. 

In general, a saturated center adjacent to sulfur has a proton shift of 2-3 ppm and a saturated center adjacent to nitrogen 3-4 ppm, whereas double-bond protons are 5.7-7 ppm adjacent to sulfur and about 1 ppm higher adjacent to nitrogen. Unfortunately, no information is available of CH=N proton shifts as the parent compound 3 <1948JA684> has not been analyzed by NMR, as is the case with the only other reported 277-thiazine with an unsubstituted 3-carbon <1954CCC754>. 

Solvolysis of (44) gives the doubly destabilized cation (45), which shows a preference for internal return.89 Compound (44) is 109 times less reactive than is (46), of which 106 is attributed to the antiaromaticity of (45) and 103 to the presence of the electron-withdrawing CF3.89 The similarity of the destabilized cation (47) and the doubly protonated species (48) is demonstrated by the similar electrophilic cyclizations that these species undergo.90 Double-bond protonation was not observed.90 Compound (49) undergoes aniline-catalysed ring closure as shown to give (50), rather than the expected isomer (51).91... 

Evaluation of pATR from measurements of rate and equilibrium constants for the protonation of carbon-carbon double bonds of alkenes suggests the possibility of a similar approach for aromatic double bonds. Protonated aromatic molecules are the parent structures of the arenonium ion intermediates of electrophilic aromatic substitution. For these cations the equilibrium constant Kk refers to equilibria with the corresponding aromatic hydrates, as is illustrated in Scheme 5 for the benzenonium ion (cyclohexadienyl cation) 9 for which the hydrate is cyclohexadienol 10. 

CH=CH Isolated double bond protons (not conjugated) Multiplet... 

In aqueous buffered medium, over the pH range 2-9, cytosine exhibits a single weakly structured reduction wave 33,74. The pH-dependence of E1/2 indicates that this reduction proceeds via the protonated form 33,37 . The basic reduction pattern for cytosine involves rapid protonation at N3 to form the electroactive species, two-electron reduction at the 3,4 double bond, protonation of the latter (5 x 104 sec-1), deamination (10 sec-1) to regenerate the N(3) = C(4) bond and one-electron reduction at the latter site to form a free radical which dimerizes 37). Electrolysis at pH 4.5 and 7.0 demonstrated quantitative liberation of NH3 at the acid pH, but only 60% of the theoretically expected amount at neutral pH 1 84). The foregoing is consistent with a three-electron reduction in acid medium 1 >, but not at neutral pH, where coulometric measurements at potential E = —1.5 V point to a four-electron wave (Table I). 

Proton chemical shifts are also a potential index of aromaticity. Thus, in pyrazine 7 the protons resonate at S = 8.6 ppm, whereas in its antiaromatic 162 and nonaromatic 163 derivatives the double bond protons show signals at 4.64 and 5.38 ppm, respectively <1983AGE171>. There is qualitative agreement with the ring current influencing... 

The mechanism of 4-alkyl- and 3,4-dialkyltetrahydropyridine cyclization proceeds under acid conditions in the anticipated manner during trans addition to the tetrahydropyridine double bond, protonation occurs from the topside (Scheme 4.10). In the case of 2-benzyl-l,3-dimethyl-4-propyl-l,2,5,6-... 

Figure 5.7 Proposed mechanism for the cyclization of geranylgeranyl diphosphate (GGPP) to the diterpene copalyl diphosphate, an example of terpene synthase-catalysed cyclization initiated by double-bond protonation, rather than by hydrolysis of the diphosphate ester. PP indicates a diphosphate moiety.

However, two peaks were quite clear one of them was obviorrsly due to 8 double bond protons, and these were coupled to 10 allylic protons. The spectrum looked very much like that of arachidonic add. For us, this was the breakthrough. We knew we were dealing with a fatty acid derivative. 

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