Cyclohexene addition reactions

Perhaps the most striking difference between conjugated and nonconjugated dienes is that conjugated dienes undergo an addition reaction with alkenes to yield substituted cyclohexene products. For example, 1,3-butadiene and 3-buten-2-one give 3-cycIohexenyl methyl ketone. 

Diels-Alder reaction (Sections 14.4, 30.6) The cycio-addition reaction of a diene with a dienophile to yield a cyclohexene. 

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. 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

In addition to substitution N-bromosuccinimide also forms addition compounds although in small amounts. Such addition reactions are catalysed by tetra-alkyl ammonium salts. Thus while with cyclohexene allylic substitution occurs, in presence of tetramethylammonium bromide, 1 2 dibromocyclohexane is obtained as the main product. 

Alkylation reactions reveal a mechanistic aspect of the cuprate reactions different from that of addition reactions. Theoretical analyses of reactions of alkyl halides (Mel and MeBr) [123, 124] and epoxides (ethylene oxide and cyclohexene oxide) [124] with lithium cuprate clusters (Me2CuLi dimer or Me2CuLi-LiCl, Scheme 10.11) resolved long-standing questions on the mechanism of the alkylation reaction. Density functional calculations showed that the rate-determining step of the... 

Care must be taken in extrapolating the results of laboratory studies to the lower concentrations and generally higher relative humidities (RH) found in ambient air. For example, Izumi et al. (1988) showed that the organic aerosol yield for the cyclohexene-03 reaction decreased in a nonlinear fashion as the initial reactant concentrations decreased from 5 ppm in addition, the concentration of condensation nuclei increased steeply with RH above 30%. This may be at least in part due to the effects of gas-particle partitioning on the measured aerosol yields discussed in more detail below. 

The photo-induced addition reaction of 1,4-dibromopiperazine-2,5-dione with several alkenes has been studied (80BCJ219 83BCJ1705 86BCJ479). In acetonitrile, the reaction with cyclohexene occurs by a radical chain mechanism as shown in Scheme 15 (in a sequence of reactions). 

Exercise 22-37 A side reaction when reducing benzene derivatives to 1,4-cyclo-hexadienes with lithium or sodium in liquid ammonia is over-reduction to give cyclohexenes. Addition of ethanol greatly reduces the importance of this side reaction. Explain what role ethanol plays in preventing over-reduction. 

When Br2 is used as oxidant, the excess can be destroyed by the addition of cyclohexene. The reaction mixture is concentrated at the rotary evaporator and the crude residue purified by silica gel chromatography. Alternatively, a crude material, which may need further purification, can be isolated by filtering the reaction mixture through a pad of silica or Celite and removing the solvent in vacuo. 

As you know, Br2 adds to C=C bonds (Section 3.5.1). On the other hand, Br2 replaces an sp2-bonded H atom on the formal C=C double bond of aromatic compounds. Why is it not the other way around That is, why do cyclohexene and Br2 not give 1-bromocyclohexene via a substitution reaction, and why do benzene and Br2 not give a dibromocyclohexadiene via an addition reaction ... 

In an addition reaction C=C + Br—Br — Br—C—C—Br there is a drop in enthalpy of 27 kcal/mol in the substructure shown, that is, a reaction enthalpy of -27 kcal/mol. This enthalpy decrease equals the heat of reaction liberated when Br2 is added to cyclohexene. However, it does not equal the heat of reaction for the addition of Br2 to benzene. 

Finally, because the addition of Br2 to cyclohexene is 27 kcal/mol - 11 kcaFmol = 16 kcal/ mol more exothermic than the substitution of Br2 on cyclohexene, can we conclude that the first reaction also takes place more rapidly Not necessarily The (fictitious) substitution reaction of Br2 on cyclohexene should be a multistep reaction and proceed via a bromonium ion formed in the first and also rate-determining reaction step. This bromo-niurn ion has been demonstrated to be the intermediate in the known addition reaction of Br2 to cyclohexene (Section 3.5.1). Thus, one would expect that the outcome of the competition of substitution vs. addition depends on whether the bromonium ion is converted— in each case in an elementary reaction—to the substitution or to the addition product. The Hammond postulate suggests that the bromonium ion undergoes the more exothermic (exergonic) reaction more rapidly. In other words, the addition reaction is expected to win not only thermodynamically but also kinetically. 

Figure 5.1 illustrates the key reactions implied in the manufacturing of cyclohexanone by phenol hydrogenation. The reactions are of consecutive type, in which the desired product is an intermediate. Small amounts of cyclohexene might appear at higher temperature by cyclohexanol dehydration. Additional reactions can lead to heavies by polymerization or benzene and cyclohexane by disproportionation. 

In addition reactions of bromine to alkenes, Risbood and Ruthven154 have shown that 5A zeolite preadsorbed with bromine can selectively brominate styrene in presence of cyclohexene. By simple alteration of the order of adsorption of reactants to Pentasil zeolites, Smith and Fry155 have been able to add selectively bromine to the double bond of linear alkenes in presence of cyclic and branched alkenes, or to brominate cyclic or branched alkenes in presence of linear ones. An example is shown in equation 9. 

Strategy Reaction of cyclohexene with HC1 or HBr is an electrophilic addition reaction in which a halogen acid adds to a double bond to produce a haloalkane. 

To 4 g (0.0155 mol) of 2,4-dinitrobenzenesulfenyl acetate are added 100 mL of dry 1,2-dichloroethane and 10 mL of cyclohexene. The reaction mixture is kept at 50 C for 2 d. Then, the solvent and excess cyclohexene are removed and the residue is dried under reduced pressure (10 Torr) at r.t. Chromatography of the crude reaction product dissolved in CC14 on alumina eluting with benzene gives a green cluatc, which on concentration to an oily residue and addition of ligroin (bp 37 — 57 °C) affords the title compound as a greenish-yellow solid yield 3.5 g (66%) mp 82-83 °C. 

Quite recently, a new asymmetric addition reaction has been described by Tiecco. The oxidation of the diselenide 26 with ammonium persulfate produces the camphorselenyl sulfate, which reacts with alkenes in acetonitrile in the presence of water, to afford the hydroxyselenenylation products in good yields and with moderate to good diasteroselectivities [46b]. The results of these experiments are collected in Table 3. Moderate diastereomeric ratios were observed in the hydroxyselenenylation of cyclohexene, styrene and -methylstyrene. Good facial selectivity was observed in all the other cases. The two diastereomeric addition products thus obtained could be separated in most cases. Enantiomerically pure saturated or allylic alcohols... 

Diels-Alder reaction (Section 16.12) An addition reaction between a 1.3-diene and a dienophile to form a cyclohexene ring. 

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