Allylic protons

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier... 

A mechanistic interpretation for the formation of 35 is depicted in Scheme 5. Deprotonation of an allylic proton yields ylide intermediate 36. This then adds to methyl acrylate to give intermediate 37, which cyclizes to construct a cyclopropane ring together with the fission of the S-C bond to afford the final adduct 35. 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

The theoretical study of the structure of propene was then used as a model to calculate the effect of the structure on the proton affinity, and later to predict the acidity of similar systems such as cycloalkenes46. Deformation of the CCC angle as a function of the stability of the anion was probed, and the results were in agreement with the acidities of the hydrogens of propene. The allylic protons were found to be more acidic than the vinylic ones, which is in contrast to the results of Grundler47. 

The checkers found that the 1H NMR spectrum of (2-bromoallyl)-diisopropoxyborane is concentration dependent. When the NMR spectrum was measured at a concentration of 20 pL of product in ca. 0.5 mL of CDCI3, (2-bromo-allyl)diisopropoxyborane was observed along with a substantial amount of a second material that had 1H resonances for the vinylic and allylic protons that were very similar in chemical shift to the vinylic and allylic resonances of the desired product. However, when the 1H NMR spectrum of a much more concentrated solution (ca. 250... 

These catalysts have strong basic characteristics such that they are able to abstract allylic protons from the olefins involved. 

In the presence of additional olefin, an exchange of metal for allylic proton or transmetallation may take place, resulting in the isomerized olefin and more of the basic intermediate. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

Products from the electrochemical oxidation of cyclohexene (Scheme 2.1) illustrate the general course of reaction [28, 29]. The radical-cation either undergoes loss of an allylic proton or reacts, at the centre of liighest positive charge density, with a nucleophile. Either reaction leads to a carbon radical, which is oxidised to the carbonium ion. A Wagncr-Meerwein rearrangement then gives the most stable carbonium ion, which subsequently reacts with a nucleophile. 

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... 

As in the case of double bond isomerization of butenes, the double bond isomerization of VBH is considered to be initiated by abstraction of an allylic proton from a tertiary carbon atom to give an allylic anion that may be stabilized by metal ions, yielding the E- and Z-EBH isomers. 

Di-p-chloro-bis(i74-l,5-cyclooctadiene)dirhodium(I) is a yellow-orange, air-stable solid. It can be used directly as obtained for preparative purposes5 or as a precursor for homogeneous catalysts.3,4 It can be recrystallized from dichloro-methane-diethyl ether to give orange prisms. The compound is soluble in dichloro-methane somewhat less soluble in acetone and insoluble in pentane and diethyl ether. Characteristic strong bands occur in the infrared spectrum at 819, 964, and 998 cm 1 (Nujol mull). The cyclooctadiene vinylic protons resonate in the 1H NMR spectrum at t 5.7 and the allylic protons at t 7.4-8.3 (deuteriochloroform solution). Other physical properties are given by Chatt.1... 

The stars were characterized by GPC (LLS) and NMR spectroscopy. The RI traces (see Fig. 5) showed the formation of monomodal narrow dispersity stars. Molecular characteristics of two representative samples are summarized in Table 2. According to NMR evidence (Fig. 6), the arms were quantitatively functionalized with allyl groups. The end-functionality of the stars was calculated by comparing the integrated peak area of the core protons [aromatic (5= 6.82 ppm), -CH2- (6=4.0 ppm)] and chain end allyl protons (-CH2- (5=2.0 ppm), -CH= (5=5.8 ppm), =CH2 (5=5.1 ppm), and was found to be 8.1, after correcting for the presence of -10% linear contaminant. Quantitative allyl-functionalization is direct proof for the formation of the octa-arm stars. 

The diphenylallyl carbanion is conveniently formed by the abstraction of an allylic proton from the corresponding diphenyl propene. Under certain conditions it was found that the initial product formed from trans 1,3 diphenyl-2-methyl propene was the trans,trans anion which isomerised completely into the cis,trans conformation within a few minutes [3]. Clearly, the abstraction reaction proceeds under kinetic and-not thermodynamic control. 

After removal of an allylic proton, a useful contathermodynamic isomerization ... 

Substitution Reactions. The chemistry at alpha positions hinges on the fact that an allylic hydrogen is easy to abstract because of the resonance structures that can be established with the neighboring double bond. The allylic proton is easier to abstract than one on a tertiary carbon these reactions are important in the formation of alkoxybutenes (ethers). 

A particularly strong type of resonance stabilization is found for those compounds which form an aromatic ring upon removal of a proton. The enhanced aromatic stability of the conjugate base translates into a large increase in acidity of the acid. Whereas the doubly ally lie proton of 1,4-pentadiene is predicted to have a pKa % 40 due to resonance stabilization of the anion, the doubly allylic proton in cyclopentadiene has a pKa = 16 because the resulting anion produces an aromatic jt system. 

Would have allylic protons at 1.8 ppm. Also would have vinyl signal at <5 ppm. 

A quantum chemistry study of the reaction of chloroprene with CI2 has revealed two transition states for substitutive chlorination, which is consistent with two consecutive processes chlorination to give a carbocation followed by abstraction of the originally allylic proton. On the other hand, a single transition state was observed for additive chlorination. The potential barriers for the former process lay below that for the latter102. 

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