Cyclohexene, protonated

Both cis and tran -cyclohexene have been synthesized, but only one of them can be isolated. Electrophilic addition of ROH to one isomer occurs spontaneously, while addition to the other isomer occurs only in the presence of a strong acid, such as sulfuric acid. Calculate the energy of protonation for each isomer cyclohexene protonated cyclohexene, trans-cyclohexene protonated trans-cyclohexene), and identify the more reactive isomer. Also examine electrostatic potential maps. Suggest an explanation to account for both the reactivity difference and the structural changes. (See also Chapter 7, Problem 5.)... 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

Once cyclization has occurred, the formed carbocation can lose a proton, and a cyclohexene derivative is obtained. This reaction is aided by the presence of an olefin in the vicinity (R-CH=CH2). 

In the present instance, protonation of the C1-C2 double bond gives a carbo-cation that can react further to give the 1,2 adduct 3-chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-L-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure 6-chloro-l-methyl-cyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation. 

Similar stereochemical results were obtained from the addition of the potassium and lithium ions of ethyl acetate, /V,V-dimethylacetamide, acetonitrile, acetophenone and pinacolone to 3-(/erf-butyldimethylsilyloxy)-T-phenylsulfonyl-1-cyclohexene followed by protonation or methylation of the resulting sulfonyl carbanion intermediates7. 

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). 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

ENDOR lines predicted by improved DFT calculations for methyl protons at 03(13 ) located at 16.5 MHz were broad and not observable due to incomplete averaging of the methyl proton couplings due to a hindering environment. Thus, the methyl groups at the C5(5 ) and C9(9 ) are located away from the surface of the pore and rapidly rotate, while those at 03(13 ) interact with the surface. Steric hindrance by the terminal bulky trimethyl cyclohexene rings preclude attainment of the requisite distance between Cu2+ and the C7=C8 and C8 =C7 bonds. 

Unlike cyclohexene, its oxa analog, 3,4-dihydro-2//-pyran, undergoes facile reduction to tetrahydropyran in yields ranging from 70 to 92% when treated with a slight excess of triethylsilane and an excess of either trifluoroacetic acid or a combination of hydrogen chloride and aluminum chloride (Eq. 69).146 This difference in behavior can be understood in terms of the accessibility of the resonance-stabilized oxonium ion intermediate formed upon protonation. 

The proton NMR spectra corresponding to the cyclohexene oxides 229 and 230, obtained in the reaction with cA-dideuterioethylene, and to cyclohexene 231, obtained in the... 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

FIGURE 46. Retinal chromophore in bR is attached via a protonated Schiff base to Lys-216 on helix G and is tilted toward the extracellular side. To determine its detailed structure, retinal was selectively deuteriated on the three methyl groups on the cyclohexene ring and incorporated into bR from H. Halobium. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

The cis/trans ratio of 1,4-disubstituted cyclohexanes formed from activated cyclohexenes at a Hg cathode depends on the solvent and proton source and shows a low diastereoselectivity. Protonation of the first formed radical anion is kinetically... 

A resolution into two or more components always occurs if the solvent has a high proton affinity, so that a solvent molecule can form a particularly stable association with a phenol molecule as a result of an energetically favourable mutual orientation. This is the case, for example, if benzene and toluene are used as the solvents. However, this effect is even more pronounced in the case of cyclohexene. Dielectric constant measurements for phenol in various solvents agree with this observation. In particular, the dipole moments in benzene and cyclohexene (1-45 and 1-79 D respectively), are considerably greater than the value of 1-32 in cyclohexane. Liittke and Mecke (1949) attributed this effect to the ability of this unsaturated solvent to act as a proton acceptor, i.e. to form 7r-complexes. 

The table shows the effect on product ratio of ultrasonic irradiation (Kerry Pulsatron cleaning bath 35 kHz 50 W) during electrolysis. Here there is only 8% of the bicyclohexyl dimeric one-electron product, with approximately 41 % of the two-electron product from nucleophilic capture of the intermediate carbocation. The preponderance of cyclohexene (32 %) over cyclohexane (> 3 %) shows its formation is by proton loss from the carbocation intermediate, since free-radical routes to cyclohexene (i. e. hydrogen atom abstraction) also produce cyclohexane in equal if not greater amounts... 

Gao et al. (2006) considered the data on an electron double resonance spectra of the cation-radical in conjunction with the results of calculation within the DFT. The authors established that the methyl group at the double bond of the cyclohexene ring is responsible for deprotonation of the P-carotene cation-radical. This route of proton elimination produces the most stable radical leaving the Jt-conjugation system to be intact. Deprotonation at the polyene methyl groups would... 

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. 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

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